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Matrix Resins

1 Introduction

Matrix resins are unpolymerized monomer/oligomer blends or polymerized solid materials that may contain different types of fillers (organic or inorganic), initiators, catalysts, stabilizers, pigments or various types of other additives (Fig. 1b).

Unpolymerized matrix resins are more or less viscous materials that are blended with other ingredients in special mixers or kneaders mostly under vacuum and/or warmth. Then the matrix resin is polymerized to solid state to obtain the final product that fulfills the requirements for the respective application.

In case the starting matrix resin is a solid polymer no initiator or catalyst is needed. Such matrices must be thermoplastic to be transformed by heat to a more or less viscous molten mass to admix the requested ingredients. This is also done, sometimes under vacuum, in special kneaders, extruders or injection devices. When cooled down in a mold representing the requested shape the final product is obtained. It is also possible that an intermediate product is received after cooling which is granulated by milling. To obtain the final product the granules are plasticized again in injection devices and are injected into a mold to cool in the final shape (injection molding).

2 Functional Groups and Monomer Links

Polymers or macromolecules are formed when numerous monomer molecules link with each other according to particular chemical principles. To transform the monomeric/oligomeric state into the solid state the monomers/oligomers must provide special molecular groups, so called polymerizable or functional groups, which can perform the polyreaction. Depending on the type of functional group various kinds of polyreactions can be executed and various kinds of links between the reacting molecules are possible to be formed (Figs. 2b-1 and 2b-2).

Fig. 2b-1: Polymers, links and polyreactions.

Fig. 2b-2: Polymers, links and polyreactions.

The functional groups (Fig. 3b) of the reacting molecules determine the type of links that are characteristic for the created macromolecule. Few different polyreactions and their characteristic links give rise to many different polymers. Although the carbon-carbon link is characteristic for very many different polymers these differ significantly in their chemical and physical properties. This reveals that the properties do not to depend only on the type of link but to a greater extent on the molecular structure of the chosen monomers/oligomers.

Fig. 3b: Functional groups, examples of unsaturated molecule structures.

3 Polyreactions

Polyreactions are chemical reactions that link appropriate monomers to polymers under appropriate conditions. There are several types of polyreactions in polymer chemistry but for dental polymers three types are the most important ones

- polymerization (free radical, cationic, anionic)

- polycondensation

- polyaddition

This sounds simple but hundreds of different monomers are known to form thousands of different polymers according to the aforesaid reaction mechanisms. A huge variety of tailor-made resin composite materials not only for dental purposes but also for technical, medical or household applications can be produced.

This textbook considers only the basic principles of the chemical reactions which are of special interest for dental materials. Figure 4b classifies the different polyreactions.

Fig. 4b: Classification of polyreactions.

3.1 Polymerization Reactions

Polymerization means the reaction of unsaturated monomeric molecules to macromolecules. Such unsaturated molecules are for instance olefins, also called alkenes, carbonyls or ring structures such as epoxides or dioxanes (Fig. 5b). Polymerization reactions are started by appropriate initiators or catalysts and principally run according to the same reaction scheme

- chain initiation

- chain propagation

- chain termination

But there are some essential facts in which the polymerization mechanisms differ and, therefore, they are subdivided in

a) free radical polymerization

b) anionic polymerization

c) cationic polymerization

d) special forms of polymerization: ring-opening polymerization and thiol-ene polymerization

All these mechanisms will be discussed because they are important for dental resins.

The tendency of a monomer to react radically, ionically or coordinately on metalorganic complexes depends on its structure and polarity:

- terminal double bonds polymerize fast, centrally located or placed in cyclic systems ones only slow

- non-polar monomers polymerize preferably radically

- carbon-carbon double bonds with electron-attracting substituents polymerize preferably anionically

- carbon-carbon double bonds with electron-repellent substituents polymerize preferably cationically

As already shown in figures 2b-1 and 2b-2 various polymers can be synthesized via polymerization reactions and mostly the carbon-carbon link is created.

Fig. 5b: Examples of unsaturated molecules.

3.1.1 Free Radical Polymerization

The free radical polymerization is a chain reaction (Fig. 6b). This is undoubtedly the most important and most often performed polyreaction to synthesize dental polymers. In a first step, initiator molecules must provide energy-rich free radicals to start the free radical polymerization. The free radicals attack the monomers’ carbon-carbon double bonds and then the monomer molecules become radicals themselves; this is called chain initiation. The newly formed monomeric radicals attack the next monomer molecules creating new but now one monomeric unit longer chain radicals and so on; this is called chain propagation. In case that two chain radicals react with each other - called recombination (Fig. 6b, No. 4.a) - the chain propagation terminates; this is called chain termination. The chain termination also occurs when no further monomeric molecules are available or when their concentration is too low and the chain too long so that the probability of a monomeric molecule to find a chain end-radical tends to zero. The non-converted molecules remain in the polymer and are called residual monomer.

When certain initiator molecules are exposed to certain energy forms they create energy-rich radicals that are able to start the polymerization. This means that thermal or heat initiators decompose when temperature is applied and photoinitiators when light is applied. When radicals are created by “chemical energy” redox-initiators are involved in the process (see chapter “Initiators”).

But there is also a further very important termination reaction called disproportionation. The termination by disproportionation occurs frequently and simultaneously creates new double bonds (Fig. 6b, No. 4.b). Two chain radicals react in such a way that one chain radical is saturated by taking a hydrogen atom from the other chain radical which keeps one electron and forms a carbon-carbon double bond. The double bond can start again a new chain reaction when initiator molecules are available. This is very remarkable, because it makes it possible to graft a new chain onto the surface of an already existing polymer. This reaction is of high importance especially for light-curing resin composites because unpolymerized new material layers are possible to graft (bond) onto already polymerized material (see layering technique).

Atmospheric oxygen is also a very good inhibitor/stabilizer because of its biradical character (Fig. 6b, No. 4c). The unpaired electrons of the oxygen molecule react with chain end-radicals forming non-reactive terminations. The inhibitory effect of oxygen becomes particularly obvious when photopolymerization (light-curing) is performed. A sticky smear layer (also called dispersion or inhibition layer) remains very often on the polymer surfaces after the reaction due to the non-reactive chain ends (for details please see chapter “Oxygen Inhibition”).

Inhibitors (also called stabilizers) also cause chain terminations (Fig. 6, No.4d). They are so-called radical scavangers that catch free radicals and remove them from the reaction mixture by forming new non-reactive radicals that are stabilized by mesomeric dislocation of the electrons. This will be discussed later (see chapter “Stabilizers”).

Fig. 6b: Principal mechanism of the free radical polymerization.

Table 1a presents some advantages and disadvantages of the free radical polymerization according to Peacock and Calhoun [115] and other literature [116].

Tab. 1a: Some advantages and disadvantages of the free radical polymerization.

3.1.1.1 Oxygen Inhibition

Atmospheric oxygen is a very good inhibitor/stabilizer because of its biradical character (Fig. 6b, No. 4c). Its two unpaired electrons react with the chain end-radicals creating new radicals. In the case of methacrylates the monomers copolymerize strictly alternatingly with oxygen during the inhibition phase (Fig. 1a). The polymerization rate of oxygen with a growing chain is significantly higher compared with the rate of the monomer. The chains formed by this copolymerization are significantly shorter than the ones formed by the “regular” polymerization. The termination of these alternating chains occurs predominately by recombination of two macromolecules carrying oxygen radicals (Fig. 1a No. 1) but the resulting copolymer is instable and partially even degrade during the inhibition period [117].

Fig. 1a: Copolymerization of oxygen and monomer radicals. Termination reaction No. 1 predominates.

The inhibitory effect of oxygen becomes particularly obvious when photopolymerization (light-curing) is performed. A sticky smear layer (also called dispersion or inhibition layer) remains on the polymer’s surface after the reaction due to low molecular mass oligomers/polymers and non-reactive chain ends. Inhibition layer thicknesses of 17 to 22 µm were reported [118].

The effect of the inhibition layer is impressively demonstrated by figure 2a. Light-cured microfill resin composite specimens are shown after immersion in aqueous methylene blue solution in the original shade immediately after curing and after their surfaces were treated with different methods. Severe surface discolorations occur in case the inhibition layer has not been completely removed. Therefore, in practice it is very important to remove the inhibition layer after the last processing step to avoid surface discolorations caused by the different media present in the oral cavity (nutrition, bacteria etc.). The best way to remove this layer is to grind off 1 tenth of a millimeter and to polish carefully.

Fig. 2a:1) Microfill resin composite after light-curing, surface untreated.2) No anti-inhibition protection varnish, badly ground and polished after 24h immersion in aqueous methylene blue solution.3) Anti-inhibition protection varnish, not polished after 24h immersion in aqueous methylene blue solution.4) No anti-inhibition protection varnish, not ground and polished after 24h immersion in aqueous methylene blue solution.

Another possibility to avoid the smear layer is the application of anti-inhibition varnishes or gels (Fig. 2a). These varnishes are gel-like preparations of polyvinyl alcohol solved in water or of unsilanized highly dispersed SiO₂ mixed with water or glycerin. When ceramic inlays are inserted with luting resin composites the application of anti-inhibition varnishes is recommended to avoid marginal steps between tooth and ceramic. These steps are caused by the removal of excess material or by abrasive processes during service of the restoration due to the not or badly removed smear layer [119] (see chapter “Resin-based Luting Composites”.

Furthermore, light-curing devices are available for the dental laboratory to light-cure veneer resin composites under vacuum or inert gas (nitrogen) to avoid the smear layer.

The inhibition/smear layer is important to be considered when the layer/incremental technique is performed (see layering/incremental technique).

3.1.2 Cationic Polymerization

Cationic polymerizations are started by Lewis acids (BF₃, AlCl₃), protonic acids (HCl, H₂SO₄, CH₃COOH) as well as by carbenium (RR’R’’C⁺X^-, R = organic rests) and onium (e.g. NH₄⁺X^-, PH₄⁺X^-, H₃O⁺, H₂Cl⁺) salts. The activation energy of ionic polymerizations is very low so that they run very vigorously even at low temperatures (below 0 °C). Lewis acid initiators only start the reaction in the presence of traces of water so that addition compounds are formed which transfer protons to the alkene. The chain initiation is started by the carbenium ion - formed by the initiator - that undergoes addition to the alkene and builds a new carbenium ion (Fig. 7b). The chain propagation proceeds by adding further alkenes. The chain is terminated by bases (e.g. OH^-).

Since the cationic polymerization can also be initiated by photoinitiators it is used for dental filling resin composites, too. Combinations of radically and cationically induced polymerizations are also possible and even realized in filling resin composites.

Product formulations with cationic initiators undergo also very slow, thermally induced polymerization and, therefore, must be stabilized with special inhibitors to avoid premature polymerization and to guarantee good shelf life [120].

Due to their reaction mechanism ionic polymerizations are theoretically not inhibited by oxygen. Therefore, it is assumed that their surfaces always polymerize completely and dry even under air access what is not easily achievable performing the free radical polymerization. But it has to be noted that this does not fully apply to the dental silorane filling resin composites (see silorane inhibition layer).

Fig. 7b: Cationic polymerization mechanism.

Table 2a presents some advantages and disadvantages of the cationic polymerization according to Peacock and Calhoun [115] and other literature [116].

Literature gives deeper knowledge about the cationic polymerization [28, 121-126].

Tab. 2a: Some Advantages and disadvantages of the cationic polymerization.

3.1.3 Anionic Polymerization

Anionic polymerizations are started by Lewis bases (e.g. OH^-, NH₃) or alkali metals (e.g. Na, K). The electron pair donor creates a carbanion which reacts with the double bond of the monomer so that a new carbanion is formed and so on (Fig. 8b). The inhibitors responsible for the termination of the anionic polymerization are proton donors as for instance water, acids or aliphatic halogen compounds. Since anionic polymerizations are difficult to carry out they are only done industrially.

Fig. 8b: Anionic polymerization mechanism.

The anionic polymerization is insensitive to temperature and can be performed at high as well as at low temperatures. A special case of the anionic polymerization is the synthesis of so-called “living polymers” which have dianionic character and are stable at sufficiently low temperatures over a longer period of time. These polymeric dianions can be reactivated by adding appropriate monomers and new polymers can be formed.

Important products for dental purposes which are manufactured by anionic polymerization are cyanoacrylate adhesives.

Table 3a presents some advantages and disadvantages of the anionic polymerization following Peacock and Calhoun [115] and other literature [116] and for a comprehensive treatise it is referred to [36].

Tab. 3a: Some advantages and disadvantages of the anionic polymerization.

3.1.4 Ring-Opening Polymerization

Ring-opening polymerizations are of great and increasing interest not only for scientific reasons or many technical applications [127] but also for dental resins. The great advantage of ring-opening polymerizations is their very low shrinkage because space is gained by the opening of the ring structures during the reaction. It has even been succeeded to synthesize monomeric ring structures that allow the resulting polymer not to shrink at all but even to expand [122, 128, 129]. Ring-opening polymerization can run radically [127, 128], cationically [122, 127, 129] or anionically [127] (Figs. 9b-1 and 9b-2). As above already mentioned and as it will be shown later the radical and cationic ring-opening polymerizations were realized already for resin-based filling composites [130-136].

Fig. 9b-1: Ring-opening cationic polymerizations.

Fig. 9b-2: Ring-opening free radical polymerization.

3.1.5 Thiol-Ene Polymerization

The thiol-ene reaction has also been investigated to polymerize dental resins. But this very special reaction does not find any direct use in dental products up to now. More details about this reaction mechanism will be given in the Expert Level of this book series. But whoever wants to have more information right now is referred to the literature [137-142].

3.1.6 Technical Polymerization Processes

There are many different technical processes to perform polymerization reactions. The most important ones for dental resins are

- mass/bulk polymerization

- suspension polymerization

The mass or bulk polymerization is performed with the pure monomer/monomer blend that contains only the initiator which can be activated by heat or light or is a redox system. This polymerization process is done to manufacture splinter polymers added for instance as filler to dental resins.

The suspension polymerization, also very often called bead polymerization, is a widely used process to produce bead polymers with a certain molecular mass and grain size distribution. The monomer containing the dissolved initiator is finely dispersed to droplets (beads) in a dispersion medium, mostly water. The monomer and its polymer are insoluble in the dispersion medium. To stabilize the dispersion polyvinyl alcohol, tricalcium phosphate or magnesium phosphate are added. The polymerization is initiated by heat or light depending on the employed initiator. The formed bead polymers are important fillers and are also used to blend the powders of powder/liquid denture base resins.

3.2 Polycondensation

The group of polycondensates is rather heterogeneous because there are various functional groups that are able to condensate with each other. The polycondensation is a step reaction. As shown in figures 2b-1 and 2b-2 polyamides, polysulfones, polyethers, polyurethanes, polyesters, polycarbonates or polysiloxanes can be synthesized by polycondensation [143]. Characteristic for condensation reactions is the combination of two or more molecules by eliminating a third molecule (Fig. 10b).

Fig. 10b: Condensation reaction mechanism.

The polycondensation principally follows the mechanism of the condensation reaction [39, 84, 144]. To create macromolecules the only difference is that at least two bifunctional molecules are requested because otherwise no chain propagation would be possible (Fig. 11b). The polycondensation is not started by initiators but by catalysts. These are organometallic compounds, acids or bases. Since the condensation mechanism is not radically the reaction is not influenced by radical scavengers or atmospheric oxygen.

Since the polycondensation is a step reaction the formation of macromolecules happens stepwise (each step has approximately the same activation energy) and higher average molecular masses are only created at the end of the reaction. This means that the reaction rate depends on the amount of unreacted monomeric molecules and, therefore, the reaction rate drops with growing chain length contrarily to the polymerization.

Fig. 11b: Polycondensation reaction mechanism.

3.3 Polyaddition

The most important polymers formed by the polyaddition are polyurethanes, epoxides and polysiloxanes (Figs. 2b-1, 2b-2). Unlike the condensation reaction it is characteristic for the addition reaction that two molecules are combined without the elimination of a third molecule (Fig. 12b).

Fig. 12b: Addition reaction mechanism.

The polyaddition is a step reaction just as the polycondensation and, therefore, the reacting monomers must also be at least bifunctional otherwise no chain propagation is possible (Fig. 13b). Polyaddition reactions are not started by initiators but by catalysts. Since the addition mechanism is not radically the reaction is not influenced by radical scavengers or atmospheric oxygen [84].

Fig. 13b: Polyaddition reaction mechanism.

4 Matrix Resins According to Links

It is common sense in polymer chemistry to classify the matrix resins according to their links as it has already been shown in figures 2b-1 and 2b-2. To get specific links the starting monomers must provide specific functional groups to perform specific polyreactions. This has been discussed in the previous chapters. But it also happens rather often that a polymer chain has more than only one characteristic link. The number of possible monomeric structures and, therefore, the number of possible polymeric structures grows to a huge variety due to the involved number of different functional groups. Based on the selection of monomeric structures and functional groups it is possible to tailor-made appropriate products for each individual demand.

It has to be considered that the polymers or polymeric products which are discussed hereinafter are standard versions representing only a very small exemplary segment of the vast number of possibilities.

4.1 Carbon-Carbon Link

Pure carbon-carbon links, meaning polymeric chains with no heteroatoms (O, N, P, S), are formed when molecules with carbon multiple bonds (e.g. vinyl, acetyl groups) are polymerized. It should be noted that this only applies as long as the starting monomers do not contain heteroatoms themselves. For dental materials the most important polymers with mere C-C links are still the acrylates and methacrylates.

Other important C-C-linked polymers are

- polystyrene

- polyacrylonitrile polymers (trademark: Chloropren, Isopren, Neopren)

- polyethylene polymers

- polypropylene polymers (PP)

- polytetrafluoroethylene polymers (PTFE, trademark: Teflon)

- polyvinyl acetate polymers (PVAC)

- polyvinyl chloride polymers (PVC)

- synthetic rubbers: polyisoprene, polybutylene

Some of the aforesaid polymers are also employed in dentistry as described below.

4.1.1 Acrylates and Methacrylates

Acrylates and methacrylates are the most important groups of olefinic monomers forming carbon-carbon links (Fig. 14b).

Fig. 14b: Basic structures of methacrylates and acrylates.

The simplest representative of the acrylates is methyl acrylate (MA) that is shown in comparison with methyl methacrylate (MMA), also called methacrylic methyl ester, in figure 15b.

Fig. 15b: Structures of methyl methacrylate (MMA) and methyl acrylate (MA).

MMA is the best-known and oldest representative of these compounds. The mechanism of the free radical polymerization shown in figure 6b is more specified in figure 16b by the example of MMA.

Fig. 16b: Free radical polymerization of methyl methacrylate (MMA) to polymethyl methacrylate (PMMA).

Some important acrylates and methacrylates used for dental resins are shown in figures 17b, 18b-1 and 18ba-2. They are easy to synthesize with any desired organic rest “R” and the polymerization is started with thermal, redox or photoinitiators.

Fig. 17b: Some important acrylates.

Fig. 18b-1: Some important bifunctional methacrylates.

Fig. 18ba-2: Some important monofunctional methacrylates.

Due to the wide variety of substituents “R” a numerous linear or cross-linked polymers are synthesized and tailor-made products with properties such as

- water soluble or insoluble

- soluble in solvents or insoluble

- soft or absolutely hard

- very elastic or absolutely rigid

- brittle or tough

- glassy with any refractive index

can be manufactured. The most well-known representative of these polymers is polymethyl methacrylate (PMMA) also known under the trademark Plexiglas. PMMA is characterized by very good mechanical and chemical properties (strength, hardness, toughness, chemical resistance etc.) and excellent optical clarity, transparency and color stability. PMMA is synthesized by free radical polymerization of MMA (Fig. 16b). Pure PMMA is thermoplastic and processed by injection molding.

Poly(meth)acrylates with predominant thermoplastic or duromeric properties are synthesized by adding bi- or multifunctional (meth)acrylates, so called cross-linkers, to MMA. These cross-linkers are available in very many different chemical structures; one representative is 1,4-butanediol dimethacrylate.

4.1.2 Other Important C-C-Linked Polymers

Syntheses and structural formulas of some other important C-C-linked polymers are shown in figures 3a and 4a. Their properties and areas of application in dentistry are described hereinafter.

Fig. 3a: Examples of some important C-C-linked polymers.

Fig. 4a: Examples of some important C-C-linked synthetic rubbers.

4.1.2.1 Polyethylene

Polyethylene (PE) is synthesized by radical polymerization of ethylene (Fig. 3a). It is a thermoplastic polymer. Polyethylene polymers are available depending on the manufacturing process as

- low density polyethylene (LDPE), low material density

- high density polyethylene (HDPE), high material density

- medium density polyethylene (MDPE), medium material density

- linear low density polyethylene (LLDPE), linear polymer structure and low material density

All polyethylene types can be processed with injection molding or extrusion at melt temperatures between 160 and 300 °C depending on the PE-type and processing method. The hardness of polyethylene increases with increasing material density but its permeability for gases and liquids decreases. Depending on the applied pressure during the synthesis the obtained products are more or less branched. The stronger the branching, the less is the material density. The respective PE-type is selected according to the respective application. PE is resistant against water, salt solutions, acids, bases, alcohols or petrol. PE is insoluble < 60 °C in all organic solvents.

In dentistry PE-fibers were found to be a very good reinforcing material [145, 146] for resin composites even superior to glass fibers [147, 148]. Ultrahigh molecular weight polyethylene (UHMWPE) was used in the field of orthopedic implants [149].

4.1.2.2 Polypropylene

Polypropylene (PP) is synthesized by radical polymerization of propylene (Fig. 3a). It is a thermoplastic polymer. PP and PE resemble each other in their properties. The isotactic PP is preferably used for most applications because it is characterized by high material density. It is insoluble and not swellable in all kinds of organic solvents. PP’s deformation temperature is significantly higher than of PE due to differences in their secondary structures. Based on the interactions of the isotactically arranged methyl groups of the C-C-main chain PP crystallizes in form of a helix with the methyl groups outwards oriented. Thus, a very rigid arrangement with very strongly acting van-der-Waals forces occurs. The melt temperature is between 250 to 270 °C, the long-term service temperature is around 100 °C and the crystal melting range is around 160 °C. PP is resistant against water, salt solutions, acids, and bases up to 120 °C.

In dentistry PP is used as suture material in oral surgery [150], as glass-fiber-reinforced material for non-metal clasp dentures [151] or as fiber material to strengthen PMMA denture base resins [152, 153].

4.1.2.3 Polyvinyl chloride

Polyvinyl chloride (PVC) is synthesized by radical polymerization of vinyl chloride (Fig. 3a). It is a thermoplastic polymer. It is differentiated between hard and soft PVC but there are many other modifications. The former is very hard and rigid; the latter is very soft and produced by adding plasticizers. When burned PVC releases toxic hydrogen chloride (HCl) gas. PVC has high strength and high modulus of elasticity but it is brittle at low temperatures. It is stable against acids up to 60 °C. PVC is used for many technical and medical applications.

In dentistry PVC sheets are used to vacuum-form distortion-free orbital prosthesis pattern [154] and for the production of medical gloves [155]. PVC is also used as a blend of a thermoplastic vinyl chloride copolymer and MMA as denture base material called Luxene.

4.1.2.4 Polytetrafluoroethylene

Polytetrafluoroethylene (PTFE, trademark: Teflon) is synthesized by radical polymerization of tetrafluoroethylene (Fig. 3a). It is a thermoplastic polymer with a very high degree of crystallinity (up to 97 %) and, therefore, its softening temperature is extremely high (> 320 °C). The decomposition temperature of PTFE is > 400 °C. Due to its structure and the shielding effect of the fluorine atoms PTFE’s resistance against all types of chemicals is unattainable. Only molten liquid alkali metals can attack PTFE. PTFE is soft but extremely tough.

PTFE is broadly used in dentistry for many applications especially as tapes in conservative or surgical dentistry or as coatings of wires for brackets in orthodontics [156-160].

4.1.2.5 Polyvinyl acetate

Polyvinyl acetate (PVAC) is synthesized by radical polymerization of vinyl acetate (Fig. 3a). It is a thermoplastic polymer. PVAC is a very hydrophilic polymer and easily dispersible in water. It is very good solvable in nearly all solvents but not in aliphatic hydrocarbons and water-free alcohols. Its thermal stability is low so that it cannot be used for molding compounds.

Copolymers of ethylene with vinyl acetate (EVAC) are used in dentistry to manufacture protector devices for the orofacial system [161] such as mouthguards. Also interocclusal splints are made from EVAC [161, 162]. PVAC homopolymer is an ingredient of denture adhesive products [163]. Investigations suggest PVAC nanofibers that are obtained by electrospinning process to be a promising material for toughening of dental composites [164]. PVAC-strips are also used for gingival retraction [165].

4.1.2.6 Polystyrene

Polystyrene (PS) is synthesized by radical polymerization of styrene (Fig. 3a). It is a thermoplastic polymer. Depending on the manufacturing process four types are differentiated

- standard polystyrene (PS)

- styrofoam (trademark: Styropor)

- expandable polystyrene (EPS)

- impact-resistant polystyrene (SB, HIPS)

- acrylonitrile-butadiene polystyrene polymers (ABS)

The aforesaid polystyrene products differ in their physical, chemical and processing properties. The selection is done according to the respective application purpose.

In dentistry polystyrene primers are used to avoid bacterial contamination of the cavity walls before filling resin composites are applied [166]. PMMA/polystyrene/silica nanocomposite membranes were tested as drug delivery systems for dental purposes [167, 168].

4.1.2.7 Synthetic Rubbers

Two very important synthetic rubbers are cis-1,4-polyisoprene (IR) and polybutadiene (BR). These materials have elastomeric character and are not thermoplastic. They are on the market available only as vulcanized finished products. The vulcanization occurs at the free double bonds of the polyisoprene or polybutadiene polymer chains (Fig. 4a). The physical and chemical properties of IR and BR are similar to natural rubber. Besides of the aforesaid polymers a broad variety of other synthetic rubbers is available. They differ only in the side groups of the C-C main chain and are used for many different technical and medical applications. Especially their use for latex-free gloves (nitrile gloves) for medical examinations must be mentioned. All synthetic rubbers provide high tensile/tear strength and elasticity even at low temperatures and are stable against water, alcohol and diluted acids. But only some special types are also stable against hydrocarbons.

Polybutadiene is used for different purposes in dentistry such as

- experimental enamel/dentine adhesives [169]

- rubber-toughening of filling resin composites by polybutadiene/silica combination filler [170]

- reduction of shrinkage of filling resin composites by polybutadiene/silica combination filler [171]

- rubber-toughening of denture base resins [172, 173]

- root canal sealer root improve bond to root canal filling material [174]

Polyisoprene is used for different purposes in dentistry such as

- orthodontic elastics [175]

- component of root canal filling materials [174, 176, 177]

- latex-free rubber dams

4.2 Ester Link

Polyesters are synthesized by condensation reaction of carboxyl and hydroxyl end groups (Fig. 5a). The ester linked polymers are primarily differentiated into

- saturated polyesters

- unsaturated polyesters

- polycarbonates

Fig. 5a: Principle of synthesis of polyesters.

4.2.1 Saturated Polyesters

Saturated polyesters are synthesized by polycondensation. Important saturated polyesters are

- polybutylene terephthalate (PBTP) (Fig. 6a)

- polyethylene terephthalate (PETP) (Fig. 7a)

Both of the aforesaid polymers are partially crystalline thermoplastics, have very good mechanical properties and low water sorption. They are resistant to fats and oils but not to strong acids and alkali. The glass transition range of PBTP is 30 to 50 °C what is significantly lower than of PETP, the temperature range of which is 70 to 80 °C.

Saturated polyester homo- and copolymers are used for several dental applications such as

- separating strips [178-180]

- fibers to reinforce methacrylate resins for denture bases [181, 182]

- denture base resin for injection molding [183]

- matrix systems [184]

A nice compilation is given by a review article [185].

Fig. 6a: Synthesis of polybutylene terephthalate (PBTP).

Fig. 7a: Synthesis of polyethylene terephthalate (PETP).

4.2.2 Unsaturated Polyesters

The unsaturated polyesters (UP) have very good processing properties (e.g. flowability) and, therefore, they are preferably used to manufacture large preforms. They are synthesized by co-condensation of di- or trifunctional alcohols with saturated (Fig. 8a) and unsaturated (Fig. 9a) dicarboxylic acid mixtures or their anhydrides with difunctional alcohols (Fig. 10a). This process creates a certain amount of thermoplastic unsaturated polyesters which are soluble in vinyl monomers such as styrene or MMA. These solutions are compounded with heat, light or redox initiators so that the unsaturated groups of the polyester macromolecules copolymerize with the vinyl groups of the solvent and cross-linking and hardening to duromers occur. The unsaturated polyesters provide very pleasant processing properties (e.g. very good flowability) so that they are preferably used to manufacture large molded parts.

After cross-linking they keep their shape and obtain their final material properties. Their mechanical properties depend on the selection of the components but especially on the amount of vinyl groups available for the cross-linking reaction. The UP resins generally offer very good mechanical properties and very good chemical resistance.

Unsaturated polyester homo- and copolymers are used for several dental applications such as

- substitute for Bis-GMA in filling resin composites [186]

- embedding material for research work [187, 188]

- unsaturated polyester preparation for restoration purposes [189]

Fig. 8a: Some saturated dicarboxylic acids.

Fig. 9a: Some unsaturated dicarboxylic acids.

Fig. 10a: Examples of some dialcohols.

4.2.3 Polycarbonates

Polycarbonates (PC) are usually synthesized by polycondensation of dialcohols and phosgene (Fig. 11a). The dialcohols are very often diphenols such as bisphenol A (Fig. 12a).

The polycarbonates (PC) are thermoplastics and their degree of crystallinity is relatively high. Their mechanical and optical (colorless, transparent) properties are very good but the chemical resistance is limited especially to strong aqueous alkali, ammoniac, amines, esters, and permanent exposure to hot water. They are of low flammability. PCs on the basis of bisphenol A (Fig. 12a) are processed by temperatures of 280 to 320 °C. They are very rigid and highly impact resistant in a temperature range of -150 to +130 °C.

Polycarbonates are used for various dental applications such as

- resin brackets [190-196]

- glass-fiber-reinforced PC as orthodontic wires [197]

- PC-urethane matrix reinforced with PE fibers for shock-absorbing dental implants [198]

- PC crowns [199-202]

- denture bases [203-206]

- see also review article [185]

Fig. 11a: Principle of synthesis of polycarbonates.

Fig. 12a: Principle of synthesis of the most often used polycarbonate (bisphenol A based PC).

4.3 Amide Link

Polyamides (PA) are thermoplastics and are usually synthesized by polycondensation of omega-amino carboxylic acids (Fig. 13a) but they can also be obtained by condensation of a diamine with a dicarboxylic acid (synthesis of Nylon, Fig. 14a). Polyamides are basically differentiated into

- PA 6 (Perlon)

- PA 66 (Nylon)

- PA 11

- PA 12

The polyamides form almost only linear polymeric chains. All PAs offer very high toughness, tear strength, hardness, and abrasion resistance. PA 66 only gets its optimal toughness after some water has been absorbed. The mechanical properties of PA 11 and PA 12 are independent from the water uptake. The PAs chemical resistance is good to most organic solvents but they are soluble in concentrated acids whereby the amide link is slowly hydrolytically split. PAs are processed at temperatures of 250 to 300 °C.

Polyamides are used in dentistry for partial and full dentures [185, 203, 207-219] as well as for resin clasps of cast partial dentures [220, 221] and PA fibers to reinforce resin filling composites [222].

Fig. 13a: Synthesis of polyamides by polycondensation of omega-amino carboxylic acids.

Fig. 14a: Synthesis of Nylon (PA 66) by polycondensation of a diamine with a dicarboxylic acid.

4.4 Urethane Link

Polyurethanes (PU) are synthesized by polycondensation (Fig. 15a) but technically mainly by polyaddition (Fig. 16a) of di- or trifunctional (polyfunctional) alcohols (Fig. 17a) with di- or polyisocyanates (Fig. 18a). The choice of the starting compounds determines if linear or cross-linked polymers are obtained. Little cross-linked PUs provide elastomeric properties and are used as synthetic rubber and higher cross-linked materials as foams, sponges and compact bodies.

Fig. 15a: Principle of synthesis of polyurethanes by polycondensation.

Fig. 16a: Principle of synthesis of polyurethanes by polyaddition.

Fig. 17a: Examples of some difunctional alcohols.

Fig. 18a: Examples of some diisoscyanates.

Today, the most important polyurethanes contain only a low number of the characteristic urethane links due to the fact that the isocyanate components are mostly polyisocyanates (Fig. 19a). The polyfunctional alcohols used for cross-linking of the polyisocyanates are mostly ethylene glycol or 1,4-butane diol but also polyether polyols and polyesters with terminal hydroxyl groups are used (Fig. 20a).

Fig. 19a: Principle structure of polyisocyanates.

Fig. 20a: Principle structures of polyether and polyester polyols.

Furthermore, the urethane link is a very important structural feature of many acrylate and methacrylate monomers (Fig. 18b-1).

Polyurethanes are used in dentistry

- as foam sheets or blocks to evaluate dental implants [223, 224]

- for bone tissue engineering [225, 226]

- to manufacture resin dies [227-231]

- as elastomeric chains and wires in orthodontics [232-235]

- as root canal-obturation material [236-238]

- as tape to seal the proximal surfaces of the teeth [239]

- to manufacture milled casts [240, 241]

- as liners for facial prosthesis [242-244]

4.5 Ether Link

Important polymers of this group are

- aromatic polyethers (e.g. PPO/PPE, PEEK)

- polyoxymethylene (also called polyacetal) (POM)

- epoxy/epoxide polymers (EP)

Polyethers are synthesized by several polyreactions (polyaddition, polycondensation, anionic polymerization, oxidative coupling).

The polyether impression materials are certainly the best known polyethers in dentistry. The polyether impression materials are not “pure” polyethers but have elastic aliphatic polyether macromolecular segments with polyester groups bearing the reactive aziridine terminal groups which form C-C links during cross-linking (Fig. 59b) [114, 245].

4.5.1 Polyphenylene oxide (PPO)/Polyphenylene ether (PPE)

PPO or, more correctly, polyphenylene ether PPE (trademark: Noryl) is one of the most important aromatic polyethers and can be synthesized by polyaddition but technically it is synthesized by oxidative coupling (Fig. 21a). It must be noted that the name PPO is widely used but not quite right because this polymer is not an oxide but ether. Therefore, the more correct name is polyphenylene ether (PPE). The dimensional stability of PPE is very good. The water sorption is extremely low and, therefore, the hydrolytic stability is extraordinary. PPE is thermoplastic and commercially available products are mostly blends of PPE and PS. Such blends have service temperatures up to 100 °C but low impact strength at low temperatures.

PPE was suggested in dentistry

- for use in dental restorative prosthetics [246, 247]

- as dental resin posts [248]

- as resin caps for implants [249]

There are several other patents that claim PPE for use as or as component of dental materials. But this field is so broad and very special that it is not be dealt with this book.

Fig. 21a: Synthesis of PPO/PPE by oxidative coupling.

4.5.2 Poly(aryl-ether-ether-ketone) (PEEK)

Poly(aryl-ether-ether-ketone) is very often only called polyether ether ketone. It is synthesized by polycondensation (Fig. 22a). PEEK’s melt temperature is at approx. 335 °C and its glass transition range at approx. 141 °C. PEEK is a partially crystalline thermoplastic and it provides very high strength, toughness and fatigue strength over a wide temperature range. The continuous service temperature is at approx. 250 °C. PEEK is very resistant to non-oxidizing acids, bases, oils and fats even at high temperatures. Hot water or steam do not hydrolyze or oxidize PEEK. Sulfuric acid is the only known solvent at room temperature.

PEEK is widely used for medical and dental applications such as

- implants [250-254]

- prosthodontic applications [255]

- crowns [256-258]

- removable and fixed dentures [259-267]

- maxillofacial prosthesis [268, 269]

- orthodontics [270]

Very nice and informative reviews are [271, 272].

Fig. 22a: Principle of synthesis of poly(aryl-ether-ether-ketone).

4.5.3 Polyoxymethylene (POM)

Polyoxymethylene is also known under the name polyacetal. POM polymers are synthesized by anionic polymerization. POM homopolymers with terminal hydroxyl groups are thermally rather unstable and depolymerize easily. Therefore, the terminal groups must be stabilized by acylation with acetic acid anhydride (Fig. 23a). POM polymers are thermoplastics and are characterized by very high hardness, toughness, and bending rigidity. They are dimensionally stable even at high temperatures. POM homopolymers have a very high degree of crystallinity of nearly 90 % and, therefore, have higher strength than the copolymers with only approx. 75 % degree of crystallinity. POM is not very stable against hot water, rather stable against many organic solvents but unstable against strong acids and oxidants.

POM is used in dentistry

- for partial and full dentures [267, 273, 274]

- for brackets [275-279]

- in implantology [280]

Fig. 23a: Synthesis of stabilized polyoxymethylene (POM), also called polyacetal.

4.5.4 Epoxide Polymers (EP)

Epoxide polymers are also called epoxy polymers or epoxy resins. EPs are synthesized by polyaddition of bifunctional alcohols (Fig. 10a) and diepoxides (diglycidyl ethers) (Fig. 24a). Some technically important diepoxides are shown in figure 25a. The glycidyl ethers are obtained by addition reaction of epychlorohydrine and dialcohols (Fig. 26a). Linear polymer chains with many hydroxyl groups and free epoxy terminal groups are formed when the polyaddition is done in excess of epychlorohydrine.

Fig. 24a: Principle of synthesis of epoxy resins by polyaddition.

Fig. 25a: Examples of some diepoxides.

Fig. 26a: Principle of synthesis of glycidyl ethers.

EPs are thermoplastics and sometimes liquids and can be cross-linked (Fig. 27a) by diamines, carboxylic anhydrides or their dicarboxylic acids (Fig. 28a). Therefore, the cross-linking bridges are amide or ester bridges. The cross-linking reaction is an addition reaction and is catalyzed by water or by the free hydroxyl groups of the EP. Cross-linked epoxides are mainly duromers. Their big advantage compared with other duromers manufactured via cross-linking reactions (e.g. aminoplasts, phenoplasts) is that their cross-linking is done by polyaddition that means no low molecular compounds are set free. As a consequence, their polymerization shrinkage is very low. The properties of epoxides after cross-linking depend strongly on the structures of the reactants as well as on the type of catalyst and vary in a wide range. The higher the degree of cross-linking, the higher is the temperature resistance and the chemical resistance. Systems cross-linked with dicarbonic acids have higher temperature resistance than the amine cross-linked ones. The resistance against acids and alkali is also strongly influenced by the respective cross-linker. For instance, the ester links formed by cross-linking with dicarbonic acids are unstable to strong acids and bases (hydrolysis). Cross-linked EPs are insoluble but swellable in organic solvents.

EPs are used for many applications in dentistry such as for

- sealers for root canals [281-287]

- bioactive dental sealers [288]

- die materials [230, 289-292]

- post materials [293, 294]

- filling composites [295-300]

- brackets [301]

Fig. 27a: Cross-linking of epoxides by addition reaction.

Fig. 28a: Examples of some important cross-linkers for epoxy resins.

4.6 Siloxane Link

Siloxane linked polymers are called polysiloxanes or silicones. They are elastomers and synthesized by polycondensation of silicon-organic compounds such as

- dimethyl silicone dichloride, (CH₃)₂-Si-Cl₂

- trimethyl silicone chloride, (CH₃)₃-Si-Cl

- methyl silicone trichloride, CH₃-Si-Cl₃

with water or alcohols. Polysiloxanes can form linear (Fig. 19b-a) or cross-linked chains (Fig. 19b-b). Instead of methyl groups phenyl groups are very often used as substituents. The degree of cross-linking of polysiloxanes is determined by the used amounts of dimethyl silicone dichloride, trimethyl silicone chloride, methyl silicone trichloride or tetrachorosilane (SiCl₄).

Polysiloxanes are very often manufactured as more or less viscous oils (silicon oils) or pastes depending on the type of application. These oils or pastes, respectively, can be formed easily to the final shape and then cured by polycondensation (Fig. 19b) or polyaddition cross-linking reactions (Fig. 29a).

Depending on the degree of cross-linking polysiloxane products of varying elasticity (very elastic up to very hard or brittle) are obtained. Polysiloxanes offer very good temperature resistance and are extremely robust and weatherproofed. Due to their low carbon content some are nearly fireproofed. All polysiloxanes are very resistant to fats, oils, water, and aqueous acids but of low resistance to several organic solvents (aromatics, esters, ketones).

In dentistry addition and condensation cross-linking polysiloxanes are mainly used as impression materials [302-309], for alignment blocks and investment materials for processing resin partial or full cast dentures and as soft relining material for dentures [310-314].

Fig. 19b: Synthesis of linear and cross-linked polysiloxanes by condensation reaction.

Fig. 29a: Principle of synthesis of linear and cross-linked polysiloxanes by addition reaction.

4.7 Sulfone Link

Polysulfones (PSU) are synthesized by polycondensation reaction (Fig. 30a). They are thermoplastics with relative low softening temperatures (170 to 180 °C). PSUs are very resistant to oxidation and ionizing radiation.

Polysulfones are mainly used to produce medical or dental equipment. But they are also used as bioactive materials [315], to manufacture denture bases [316, 317] or as attachment vehicle for orthopedic and dental implants [318].

Fig. 30a: Principle of synthesis of polysulfones by condensation reaction.

5 Structures and Properties of Monomers and Oligomers

The structures and properties of monomers and oligomers crucially determine the structures and properties of the resulting polymers. As already explained earlier acrylates and methacrylates - in the following named (meth)acrylates - are still the most important monomeric categories used for dental resins. Important representatives are shown in figures 17b, 18b-1 and 18ba-2.

5.1 Acrylates and Methacrylates

Firstly, it must be differentiated between mono-, bi-, tri- or higher functional (meth)acrylates with the (meth)acrylic group as functional group. Monofunctional (meth)acrylates have only one, bifunctional two, trifunctional three and so forth (meth)acrylic groups (Fig. 20b). Through polyreaction of monofunctional (meth)acrylates, as for instance MMA, threadlike linear polymeric molecules are formed because chain propagation only occurs at one functional group. The cohesion between the threadlike macromolecules is due to intermolecular interactions such as hydrogen bridge bonds or electrostatic forces.

Considering bi- or higher functional (meh)acrylates chain propagation happens simultaneously at two or more places of the molecule (Fig. 20b). This results in chemical cross-linking due to covalent bonds between the separately growing chains and, finally, a three-dimensional polymeric network is formed. Therefore, multifunctional monomers are also called cross-linkers. Depending on the functionality of monomers polymers with totally different properties are synthesized what will be described hereinafter.

Fig. 20b: Schematic drawing of mono-, bi-, tri- and tetrafunctional methacrylates.

Figures 17b, 18b-1 and 18ba-2 show that the monomers can strongly differ in their molecular mass what does not only determine their own properties but also the course of the polymerization process and quite sure the properties of the emerging polymers. With increasing molecular mass boiling point and viscosity of the monomers increase but polymerization shrinkage and odor decrease and biocompatibility improves. For instance, contrarily to MMA which is very odorous at room temperature, evaporates fast (boiling point 100.6 °C) and is irritating to skin and mucous membranes Bis-GMA is nearly odorless, does not boil at atmospheric pressure and is not irritating at all. Therefore, monomers with high molecular masses are more appropriate to prepare pasty resinous preparations which offer long shelf life as it is requested for dental filling composites, luting composites or light-curing crown and bridge veneer resins because their pasty preparations do not dry out during storage. Of special importance is the fact that with increasing molecular mass the polymerization shrinkage decreases. Table 1ba gives the polymerization shrinkages of some monomers compared with methacrylates as they are used in dental resins.

Tab. 1ba: Volumetric polymerization shrinkage in [vol%] of some monomers.

The monomeric structure also significantly influences water sorption and solubility when hydroxyl (-OH) or carboxyl (-COOH) groups are part of the molecule. The amount of these hydrophilic groups determines the monomer’s extent of water miscibility as well as of the resulting polymer’s water uptake or even its complete solubility in water. Acrylic acid is one example of such a monomeric structure. Due to the carboxyl groups polyacrylic acid is very good soluble in water. Polyvinyl alcohol is another example for the total solubility of a polymer in water but now due to its high number of hydroxyl groups.

Polymethyl methacrylate (PMMA) polymerized form methyl methacrylate (MMA) (Fig. 16b) which has neither carboxyl nor hydroxyl groups is insoluble in water and its water sorption is also very low. Although 2-hydroxyethyl methacrylate (HEMA) is miscible with water in any ratio its polymer (Poly-HEMA) is insoluble in water but can incorporate > 30 mass% water in its polymeric network. Water-free Poly-HEMA is hard and rigid like PMMA but the water containing product is soft and elastic. In principle, atoms (e.g. oxygen) or atomic groups (e.g. -OH, -COOH, - NH₂) that are able to form hydrogen bridge bonds determine the monomer’s or polymer’s tendency of water sorption or solubility. Therefore, water sorption or solubility can be controlled by copolymerization of hydrophilic and hydrophobic (cannot form hydrogen bridge bonds) monomers in a specific ratio. This means it is possible to tailor-made the hydrophilicity or hydrophobicity, respectively, of polymers according to the intended use.

5.2 Other Monomers

The aforesaid aspects about the structure/property relation of monomers and polymers do not only apply to (meth)acrylates/poly(meth)acrylates but principally to all other kinds of monomers/polymers. Polysiloxanes, also called silicones, as well as their siloxane monomers, also called silicon oils, are particular cases. The monomeric units of polysiloxanes are silanediols but which are never used in praxi for the syntheses. The starting materials to synthesize polysiloxanes are higher molecular mass compounds so called pre-polymers or silicon oils. Although the pre-polymers/silicon oils might have already molecular masses of around some thousand g mol^-1 they are still more or less liquid (oily to syrupy). This is due to no or only very low molecular interactions between the single pre-polymers. These are inhibited by the molecular structure of the pre-polymers because the Brownian motion (thermal molecular motion) of the oligomeric chains is relatively high even at low temperatures (low glass transition range). The low molecular interactions between the macromolecules are caused by strong shielding effects of tetrahedral methyl (-CH₃) or benzyl (-C₆H₅) groups which commonly serve as side groups so that the main chains (-Si-O-Si-chains) interact only low. The reactive terminal groups of the pre-polymers form high molecular mass elastomers after cross-linking that is performed by polyaddition or polycondensation. Due to the high molecular masses of the pre-polymers the setting shrinkage is significantly reduced particularly when polyaddition is performed.

5.3 Degree of Conversion (DC)

In general, the degree of conversion (DC) is the percentage of monomers that polymerize and form the polymer. The DC depends on various factors such as the type of polyreaction (chain reactions or step reactions), the polyreaction conditions (e.g. temperature, pressure) or the character of the used monomers (e.g. viscosity, structure, functionality). Within the chain reactions different DCs are observed between free radical, anionic and cationic polymerization. This happens also between the step reactions polyaddition and polycondensation.

The DC influences various properties of the resulting polymer such as

- shrinkage

- strength

- hardness

- water sorption and solubility

- residual/leachable monomer

The DC is highly important for the overall quality of the material.

5.3.1 Degree of Conversion of Methacrylate-Based Composites

In case of methacrylate-based composites the DC is considered as the residual number of double bonds (RDB) and related to the ratio of unreacted double bonds (DB_unreacted) before polymerization to reacted double bonds (DB_reacted) afterwards. It is calculated by DC[%]=RDB[%]=(1-(DB_reacted/DB_unreacted))x100. The number of double bonds correlates to the peak area of the respective FTIR spectrum (Fourier-transform infrared spectroscopy) [319-321].

Different monomers have different DCs varying from approx. 35 % to > 90 % (Tab. 4a) [319-321]. The DC depends on several factors [319-322] such as

a) the structure of the monomers

- molecular mass

- reactivity

- number of double bonds (functionality)

- viscosity

- ability to from intermolecular interaction (e.g. hydrogen bridge bonds)

b) the polymerization conditions

- DC heat cured > light cured > self cured

- DC increases with temperature

- DC increases by time

- DC increases by post-heat treatment

c) the comonomers

The DC of monofunctional monomers as for instance MMA is usually significantly higher than of di- or multifunctional (meth)acrylates. This is due to the higher mobility based on the low viscosity but above all on the single double bond of MMA. Once one double has reacted di- or multifunctional monomers lose their mobility because they are part of the polymeric network and are not anymore able to move. Their unreacted double bonds must wait until a “completely free” monomer or macroradical is approaching.

The aforesaid factors explain the low DC of Bis-GMA (Tab. 4a, approximate values, compiled from literature [319-321, 323]). Its high viscosity results from strong intermolecular interactions between the singe monomeric molecules. These interactions are due to the hydroxyl groups forming hydrogen bridge bonds and the electrostatic interactions of the benzene rings. The DC increases significantly when there are no hydrogen bridge bonds even though electrostatic forces act due to the benzene rings but which are not that strong [320]. This was confirmed by studies thinning Bis-GMA with TEGDMA that show DC to be dependent on the mixing ratio [321]. Depending on the monomer the DC also increased by the time and 24 h after polymerization it was approx. 10 to 50 % higher than after 10 min. The influence of the monomers with regard to DC increase was observed to be Bis-GMA > UDMA > TEGDMA > Bis-EDMA4 [320]. But 7 days after polymerization only low further DC increase was found and it remained almost constant up to 30 days [324].

With regard to light-curing filling resin composites significantly different DCs were calculated for different products. The DC also varied with the curing depth as well as with the type of the eluted monomers [325]. It was found that the DC decreased with increasing curing depth. Depending on the product the DC decreased up to 50 % at a depth of 6 mm and at a depth of 4 mm it decreased up to 25 % [326]. The DC depends on the type of light guide tip, too. Fiber optic light guides were found to be superior to polymer guides [327].

A critical review analyzing 73 publications reported that the DC determined the quality of the polymers: the higher the DC, the higher the strength of the resin matrix [320, 328]. It was also found that the higher the DC, the lower water sorption, solubility, shrinkage and the amount of leachable components/monomers [325, 326, 329-331].

Tab. 4a: Degree of conversion (DC) in [%] of some monomers often used in dental resins measured after 24h by Fourier Transformed Near-Infrared (NIR) Spectroscopy. Structure of the monomers see fig. 31a)

Fig. 31a: Structures of the monomers of which the DCs are presented in Tab. 4a.

5.3.2 Degree of Conversion of Silorane-Based Composites

With the introduction of the silorane filling resin composites the cationic ring-opening polymerization has gained increasing interest. Since no double bonds are involved in this polymerization process the DC must be calculated differently. Therefore, the number of the epoxy rings (ER) is considered in the FTIR spectrum before and after the reaction. The number of epoxy rings correlates to the peak area of the respective FTIR spectrum. The DC is calculated as DC[%]=(1-(ER_reacted/ER_unreacted))x100 [332-334].

It was found that the DC of siloranes does not significantly depend on irradiance [mW cm^-2] or radiant exposure [J cm^{- 2}] of modern light-curing devices and it was measured to be between approx. 47 to 57 % [335]. The DC was found to be 40 % for irradiances between 400 mW cm^-2 (60 s irradiation time) and 950 mW cm^-2 (26 s irradiation time) [336]. Some authors reported higher DCs for silorane-based filling resin composites than for methacrylate-based ones and discovered higher DC for standard than for soft start polymerization (Fig. 32a, rounded values following [333]). Contrarily are the results of other authors reporting higher DCs for methacrylate- than for silorane-based composites (Fig. 33a, rounded values following [334]). It must be noted that both of the aforesaid investigations used different curing times and irradiances. It should therefore be noted that the DCs reported by different sources must be critically considered because they strongly depend on the curing conditions and the test setup.

Fig. 32a: DCs (rounded values) of a methacrylate-based and a silorane-based filling resin composite after standard (40 s irradiation at 750 mW cm-2) and soft start (5s irradiation at 200 mW cm-2 increasing up to 750 mW cm-2 for 40 s total irradiation) polymerization with an LED-light. Measuring was done immediately after curing.

Fig. 33a: DCs (approx. values) of two methacrylate-based (20 s irradiation) and one silorane-based (40 s irradiation) filling resin composite polymerized with LED-light (irradiance 1,150 mW cm-2) measured immediately after polymerization and after 24 h.

As already said above, the DCs of siloranes are somewhere between 45 % to 55 % what is significantly lower compared with methacrylate-based composites even when observed over longer time periods after termination of irradiation. The LED-light performed better than the tungsten halogen light (Fig. 34a, rounded values following [337]). Figure 33a and more clearly figure 34a demonstrate that the methacrylate-based as well as the silorane-based composites continue to polymerize for many days after termination of the light irradiation. This post-curing effect may improve the quality and the longevity of the materials.

Despite the lower the DC of the silorane-based composites their physical and chemical properties are not significantly or only a bit worse compared with the methacrylate-based products. This was found for water sorption, solubility, Knoop hardness, flexural strength and microleakage [334, 336-338]. But it must be noted that it is crucial to observe that despite of the siloranes’ lower DC their optimal DC must be achieved to obtain the optimal properties.

Fig. 34a: Degree of conversion (DC) (rounded values) of two methacrylate-based and one silorane-based filling resin composite polymerized with halogen light (40 s, 530 mW cm-)2 and LED-light (20 s, 1,000 mW cm-2) measured after 1 d, 7 d and 30 d.

5.3.3 Degree of Conversion of Anionic Polymerization

Since the anionic polymerization is almost only of technical interest its DC is not subject of this book. But whoever is interested in this matter is referred to the literature [36, 339, 340].

5.3.4 Degree of Conversion of Polyaddition and Polycondensation

No specific literature was found describing the DC of polyaddition and polycondensation elastomeric impression materials. Therefore, general chemical knowledge was used to schematically describe the reaction behavior of the step reactions versus the chain reactions (see also chapter “Polyreactions”).

The speed of the chain polymerization is very high and directly proportional to the molecular mass of the growing macromolecule so that high molecular masses are achieved even at low DC (Fig 35a). During the chain propagation only monomers with existing “active” terminal groups can react with each other (e.g. initiator radicals, macroradicals) and the monomer concentration and mobility decreases steadily with the reaction time. Since the reaction speed depends on the concentration of the initiator high initiator concentrations generate also high-molecular mass polymers throughout the reaction. Furthermore, there are competing reactions steps during the polymerization; these are the initiation, the propagation and the termination steps (see chapter “Polymerization Reactions”). Especially the termination by disproportionation creates new double bonds what counters the DC [29, 31, 35, 36, 115, 144, 341-343].

Contrarily to the chain reaction, the step reaction can only be done with monomers providing two functional groups at least (see chapters “Polycondensation” and “Polyaddition”). Each monomer reacts with another monomer to give a dimer and in the next step the dimer reacts with another dimer to give a tetramer etc. Although the reaction proceeds rapidly at the beginning the molecular mass increases only slowly so that high molecular masses occur only after a relatively long time after start of the reaction and at high DCs. All molecules present (monomers, oligomers, polymers) can react with any other molecule because every molecule always has at least two reactive terminal groups. These terminal groups remain reactive throughout the reaction because there are no termination steps similar to the chain reactions. This means, although the original number of reactive groups decreases during the reaction the resulting macromolecules are still able to grow [29, 31, 35, 36, 115, 144, 341-343]. As a result the DC of step reactions is usually higher than of chain reactions and under optimal processing conditions DCs > 99 % are achievable [344]. With regard to the chain reaction such high DCs are only possible with monofunctional (meth)acrylates (see DC of MMA).

Fig. 35a: Schematic representation of molecular mass growth versus degree of conversion (DC) of step reactions (polyaddition, polycondensation) and chain reactions (free radical, anionic, cationic polymerization). The molecular mass of a polymer is directly proportional to its degree of polymerization (DP).

6 Structures and Properties of Polymers

The polymer properties are not only determined by the properties of the starting monomers but also by the type of link, the molecular mass and above all the emerging structure. The polymer structures are differentiated into

- primary

- secondary

- tertiary structures.

The primary structures are determined by the molecular structure of the monomers and mainly covalent bonds occur. To form secondary or tertiary structures besides of covalent bonds (intramolecular forces) also intermolecular forces must be considered. Such forces are the so-called secondary forces which are

- dipol-dipol forces

- hydrogen bridge bonds

- hydrophobic interactions

- van der Waals forces

Synthetic polymers show usually primary or secondary structures. The arrangement of secondary structures to tertiary structures usually occurs statistically and by chance.

According to their properties and structures the polymers are classified into

- thermoplastics

- elastomers

- duromers

6.1 Types of Chemical Bonds/Forces

In this chapter some short explanations about the occurring chemical bonds

- atomic (covalent) bonds

- dipol-dipol forces

- hydrogen bridge bonds

- hydrophobic interactions/forces

- van der Waals forces

are given [27, 33, 46, 49, 345-347]. Table 5a shows the bond energies of some important chemical bonds acting between polymeric chains. But these bonds are also important regarding adhesion phenomena between different materials such as tooth/resin, metal/resin, resin/resin or ceramic/resin as it will be discussed later (see chapter “Theoretical Aspects of Adhesion”).

Atomic (covalent) bonds are the strongest chemical forces. They occur when two atoms share one common electron pair. This is for example the case during the chain propagation of a polyreaction or when cross-linking between separately growing chains occurs.

Dipole-dipole forces between atoms or molecules arise in case one atom/molecule is slightly positively charged and the other one slightly negatively. The charges are not permanent but accidentally because they are due to the continuous movement of the electrons so that for very short periods of time the charge distribution becomes unsymmetrical. This means the atom/molecule which is slightly negatively charged is attracted by the slightly positively charged atom/molecule. The dipole-dipole forces are not very strong.

Hydrogen bridge bonds (also often only called hydrogen bonds) are also based on dipole-dipole forces. They are usually much stronger than the van der Waals forces because the dipole character is permanent (charge distribution is continuously unsymmetrical) and much stronger. The IUPAC defines the hydrogen bridge bonds as follows: “The hydrogen bond is an attractive interaction between a hydrogen atom from a molecule or a molecular fragment X–H in which X is more electronegative than H, and an atom or a group of atoms in the same or a different molecule, in which there is evidence of bond formation “ [348]. Typical molecules forming hydrogen bonds are molecules with a permanent very strong dipole character such as H₂O or the gases NH₃ or HF. The same applies to molecules, macromolecules, polymers or surfaces with functional groups such as -OH, -NH₂, -COOH or -C=O when in contact with the H-atoms of other molecules or with other chains of the same macromolecule. The hydrogen bridge bonds may also occur between different chains of the same polymers when O- or N-atoms are part of the chain.

Hydrophobic interactions/forces are also known under the expression hydrophobic effects that describes the tendency of nonpolar groups to adhere to each other. These interactions are visualized by the spontaneous tendency of nonpolar molecules to adhere to each other in water to minimize their contact with water molecules (oil drops). These forces are effective within a range of less than 100 angstrom (0.01 µm) between the nonpolar groups [345].

Van der Waals forces are effective between single molecules and increase with increasing molecular mass. They are based on the attraction of opposite electric charges. Following the IUPAC recommendations van der Waals forces can be defined as “attractive forces between molecular entities, or between groups within the same molecular entity, other than those due to bond formation or to the electrostatic interaction of ions or of ionic groups with one another or with neutral molecules” [349]. The term includes all types of dipole-dipole forces and is sometimes used loosely for the totality of nonspecific attractive intermolecular forces [349]. The importance of van der Waals forces is based on their following properties according to [350]

a) they are universal

b) all atoms and molecules attract one another

c) they account for the

- cohesion of inert gases in the solid and liquid state

- physical adsorption of molecules to solid surface, where usually no chemicals bonds are formed

d) they are effective even when the molecules are rather far apart

e) they are additive for large number of molecules

f) they effect cohesion between solid objects even when the objects are separated by a small gap

The van der Waals interactions are highly distance dependent and decrease in proportion to the sixth power of the separation. The bond energy of each interaction is only ca. 4 kJ mol^-1. The van der Waals forces become significant when many interactions are combined. Therefore, bond energies up to approx. 40 kJ mol^-1 can be achieved [347].

For the sake of completeness, it should be mentioned that repellent effects might occur when the interacting atoms get too close. This applies to all forms of the aforesaid interactions but this is not a subject of this book.

Tab. 5a: Bond energies in [kJ mol-1] of some chemical bonds.

6.2 Primary Polymer Structures

The primary structure describes the monomer’s spatial configuration in the polymer. It is determined by the monomer’s character and the type of polyreaction. The chemical bonds responsible for the primary structures are covalent bonds.

Polymerization Reactions

Homopolymers create different primary structures than copolymers what also applies to the polymerization of monofunctional or higher functional monomers (see “functionality of monomers”. Monofunctional monomers grow one-dimensional and usually form unbranched linear threadlike molecules. But in certain cases, branched macromolecules are formed by chain transfer reaction.

Branched or little cross-linked products result when low amounts of bifunctional monomers are added. Increasing concentrations of bifunctional monomers cause significant increase of two-dimensional cross-linking of the macromolecules. The degree of cross-linking rises and a three-dimensional network constitutes when tri- or multifunctional monomers are added. The extent of cross-linking influences significantly the physical and chemical properties of polymers.

Fig. 36a: Schematic drawing of primary structures of polymers.

Polymers with substituents at the 1 and 3 position to the C-C main chain are synthesized using alpha-olefins as monomeric compounds. Two different regular structures, called isotactic and syndiotactic, or one irregular structure, called atactic, can be obtained by this synthesis (Fig. 37a). These different spatial structures of the substituents definitely influence the mechanical properties of the polymers. For instance, isotactic and syndiotactic polymers have higher degrees of crystallinity than atactic materials. Especially isotactic polymers have higher softening temperatures, strength and lower swellability than atactic polymers.

Fig. 37a: Schematic drawing of the primary structures of polymers which can be obtained by synthesis of alpha-olefins.

The polyadducts and polycondensates principally form similar polymeric structures (Fig. 38a). Linear threadlike macromolecules are obtained when only bifunctional monomers are used for synthesis. Branched structures occur by copolymerization of bifunctional and little amounts of trifunctional monomers. Two- or three-dimensional cross-linked polyadducts or polycondensates occur with increasing amounts of tri- or higher functional monomers. Here again, the extent of cross-linking determines the properties of the polymers.

Fig. 38a: Schematic drawing of the primary structures of polyadducts and polycondensates.

But not only the spatial arrangement, the degree of branching or the degree of cross-linking determine the properties of the polymers but also amount and type of monomers used for copolymerization need to be considered. Specific functional groups, as well as aliphatic or aromatic ring systems may influence the polymeric properties significantly. For example, there are monomers to tailor-made macromolecules of high strength, toughness, hardness, elasticity or high or low water sorption.

Polymers with many hydrophilic groups (-OH, -O-, -COO-, -COOH, -NH-, -NH₂) that are able to form hydrogen bridge bonds tend to high water sorption. These hydrophilic groups can either be formed by the type of link (polyester, polyether, polysulfone, polyamide) or are already part of the starting monomers. Without these chemical conditions the polymers have hydrophobic character or in the presence of hydrophobic groups (e.g. -F) they are even water repellent.

Polymers with aliphatic or aromatic ring-systems are mechanically strong and rigid. But polymers with linear C-C chains alternating with urethane links are viscoplastic and mechanically strong.

6.3 Secondary Polymer Structures

The secondary structure describes the polymer’s spatial appearance in all (helices, fold micelles, fringe micelles, coils, networks) as it exists in solutions, molten masses, glasses or other amorphous or partially crystalline structures. It is determined by inter- and intramolecular forces of the polymeric chains. Such interactions can originate from the chemical bonds (cross-linking) or the formation of secondary forces. Since especially hydrogen bridge bonds and van der Waals forces appear in macromolecules in great numbers they are significantly important to create intermolecular forces. But it has to be considered that the influence of the secondary forces decreases with increasing degree of cross-linking. The higher the degree of cross-linking, the lower is the mobility of the polymeric chains and very highly cross-linked polymers can be thought of as “one” macromolecule with an infinite high molecular mass. Therefore, the secondary forces are particularly strong in linear, only slightly branched and cross-linked polymers. This means that regular primary structures and spatial advantageous conditions facilitate (isotactic, syndiotactic) the effect of secondary forces (see Fig. 37a).

Contrarily to covalent bonds, secondary forces can be opened and closed reversible without chemical degradation. This can be done by temperature increase and decrease or by passing the glass transition or melting range. This means that during a certain period of time when no secondary forces are effective the macromolecule’s secondary structure can be rearranged and after it has been rebuilt the polymer freeze keeping the new arrangement (see chspter “Thermoplastics”).

Partially or even highly crystalline structures can be obtained under favorable conditions. In crystalline polymers there are regularly arranged macromolecules (see chapter “Thermoplastics”). The degree of crystallinity describes the extent of these regular regions. Optimal conditions to get crystallinity are suitable primary structures (isotactic, syndiotactic) as well as post heat treatment, tempering or stretching. Crystalline polymers have high mechanical strength and in particular high tear strength and toughness (synthetic fibers such as Nylon, Perlon, Aramid).

6.4 Tertiary Polymer Structures

The tertiary structure is determined by covalent bonds, ionic bonds, or secondary forces and describes the mutual spatial configuration of the helices, coils or networks. Such tertiary structures commonly exist in the living nature (RNA, DNA, collagen, keratin) and they are not interesting for synthetic polymers at least not for dental resins for the time being.

6.5 Thermoplastics

Thermoplastics emerge from polymerizing monofunctional monomers meaning monomers with only one reactive group. Only thus, non cross-linked linear threadlike macromolecules are obtained (e.g. methyl methacrylate polymerizes forming polymethyl methacrylate). Linear threadlike macromolecules can also be formed via polyaddition or polycondensation of bifunctional monomers due to the different reaction mechanisms.

The threadlike high molecular mass macromolecules of thermoplastics are randomly and statistically arranged in coils (Fig. 21b). These threadlike molecules interact only with secondary forces which can reversely be separated or established by temperature increase or decrease or by addition or elimination of solvents. This makes it easy to reshape thermoplastics by injecting the molten mass into a mold. After cooling they keep the new shape unless the service temperature exceeds the heat distortion temperature or the glass transition range. This technical process of “re-shaping” is called injection molding.

Fig. 21b: Schematic drawing of the structure of thermoplastics.

Since threadlike molecules grow statistically and by chance thermoplastics have no definite molecular mass and, therefore, no definite melting point but a melting interval. By passing through this interval they firstly become plastic and with increasing temperature they transform into a more or less viscous molten mass (Fig. 22b). With further increasing temperature the macromolecules start to decompose (decomposition temperature). The glass transition temperature is another temperature characterizing thermoplastics. Since the macromolecules have no uniform molecular mass there is no definite glass transition temperature and, therefore, it is recommended to speak of a glass transition range. Below the glass transition range the entire polymer is hard and brittle that means glassy. Above the glass transition range the entire polymer is plastically deformable or plastically workable, respectively. Between the beginning and the end of the glass transition range the polymer’s plasticity increases with increasing temperature. This behavior is explained by the molecular motions which are, so to say, “frozen” below the glass transition range. Above this region many molecular areas can move easily due to the higher temperatures and coiled macromolecules loosen up so that the polymer becomes deformable without showing brittle fractures (Fig. 22b).

Fig. 22b: Temperature behavior of thermoplastics.TG = glass transition range, TS = melting range.

But thermoplastics may also have partially crystalline areas. These areas are characterized by regularly arranged, folded or stretched, threadlike macromolecules (Fig. 23b). Whether a polymer is able to form crystalline structures or not depends very strongly on its symmetry and the structure of the selected monomers. The single threadlike molecules of the thermoplastics form the primary structure, the statistically arranged coils or crystalline areas, respectively, form the secondary structure. To obtain crystalline polymers the macromolecules must be very evenly arranged so that the single chains can get closer and intermolecular secondary forces can take effect. This can be achieved by tempering or stretching processes. For instance, the degree of crystallinity is low when the melt is rapidly cooled but it is high when the melt is kept longer at high temperatures and cooled down slowly.

In general, thermoplastics are capable of swelling or are even soluble in organic solvents. The swelling causes volume increase and plasticization of the material. The coiled macromolecules are nearly totally dissolved in solutions and are only intertwined loosely. With increasing crystallinity, the capability of swelling and the solubility normally decrease and the polymer gains strength and becomes more rigid. Due to their high degree of crystallinity thermoplastics such as polyethylene, polypropylene or polytetrafluoroethylene (Teflon) are insoluble and do not even swell in water, acids, bases or organic solvents. Very low or no crystallinity is found for polyvinylchloride, polymethyl methacrylate or polystyrene which can easily be attacked by organic solvents.

Fig. 23b: Schematic drawing of crystalline districts in thermoplastic polymers.

6.6 Elastomers

Elastomers are amorphous materials meaning they do no have crystalline districts. It is characteristic for elastomers that they show rubberlike elasticity at room temperature because their glass transition range is at very low temperatures (mostly far below 0 °C). The capability of a material to deform strongly under an acting force and to return to its original state after the force has been removed is called rubberlike elasticity (memory effect). Such behavior of polymers is achieved by synthesizing very slightly cross-linked macromolecules whose chains only interact to a very low extent due to weak secondary forces (Fig. 24b). The polysiloxanes meet this requirement best due to their non-crystalline structure and the very low interactions between the macromolecules. This determines the reduction of the glass transition range of the elastomers between -70 to -130 °C contrarily to thermoplastics as for instance polymethyl methacrylate with a glass transition region between approx. 95 to 110 °C. The very low degree of cross-linking of elastomers allows the macromolecular chains to glide past each other to a certain extent when force is applied. This happens without destruction of the network. The limit value describing this property is called expandability, tensile yield strength or tear strength. This low degree of cross-linking and the thermal motion (Brownian motion) of the molecules cause the network to return to its initial state when the applied force drops. The structure of the elastomers is schematically shown in figure 24b. Usually elastomers are not meltable but dependent on their molecular structure they are very often swellable but not soluble in organic solvents.

Fig. 24b: Schematic drawing of the structure of elastomers.

6.7 Duromers

The properties of duromers (often also called duroplastics) are totally contrasting to the ones of thermoplastics and elastomers. They are highly cross-linked materials (Fig. 25b). The glass transition range of duromers is at very high temperatures and coincides with the decomposition temperature very often. This means that duromers are not meltable. They are very hard and mostly also brittle but they generally offer very good mechanical and chemical properties. Therefore, duromers are used to manufacture products for applications where highest performance is required. Duromeric products must be manufactured in their final shape because they cannot be reshaped afterwards unless it is done by grinding or cutting. They are completely insoluble and nearly not swellable in all kinds of solvents.

Fig. 25b: Schematic drawing of the structure of duromers.

Duromers are synthesized by polyreaction of monomers which form highly cross-linked polymers. These are bi- or higher functional monomers used for polymerization and tri- or higher functional ones used for polyaddition or polycondensation. Many polymeric properties are influenced by the degree of cross-linking (Fig. 25b-1).

Finally, it is important to note that there is no clear distinction between thermoplastics, elastomers and duromers but the transitions are fluid.

Fig. 25b-1: Dependence of the polymeric properties on the cross-linking density.

6.8 Interpenetrating Polymer Networks

Interpenetrating networks are of growing importance in dental material technology. Interpenetrating networks are at least two polymeric networks that are synthesized completely independent from each other in such a way that they interpenetrate homogenously (Fig. 26b). There are no covalent bonds between the two networks but only secondary and mechanical forces. The latter ones are represented by the different networks which are very intensely physically interwoven similar to a fabric. Such interpenetrating networks are obtained when

a) a cross-linked polymer is swollen with a monomer meaning the monomer is able to penetrate the polymeric network. Next, the penetrated monomer is polymerized.

b) two types of monomers which perform different types of polyreaction, for instance one undergoes polymerization and the other one polycondensation, are mixed and the polyreactions are done simultaneously.

This is a possibility to manufacture polymeric products that offer very interesting physical, chemical and processing properties.

Fig. 26b: Interpenetrating network:Possibility a) None or slightly cross-linked polymer A is penetrated by monomer B. After polymerization interpenetrating network C is created.Possibility b) Blend of monomers D; one undergoes polymerization, the other one polycondensation, polyreactions are done simultaneously and interpenetrating network E is created.

6.9 Polymer Blends

Polymer blends are thermoplastic resins consisting of two or more different thermoplastics [351]. They are produced by mixing the melts of the different thermoplastic polymers at defined temperatures and mixing conditions to get a homogenous mass that is cooled to solid state and then granulated. The definition “polymer blend” requests that the part of each of the thermoplastics must be at least 10 mass%. The granules are ready for use and can be further processed immediately. Due to the well-defined manufacturing process polymer blends significantly differ from the common polymeric mixtures made in processing devices (e. g. extruders). They are formed by injection molding.

Polymer blends can be single-phased or multi-phased. They are single-phased when all components are totally miscible independent from temperature, mixing ratio and mixing conditions. With regard to multi-phased systems the morphological properties of the single phases and their physical and chemical interactions are of decisive importance for the properties of the resulting polymer blend. Totally immiscible components are forcibly linked by creating an interpenetrating network.

It is differentiated between polymer blends based on

- polypropylene

- polyamide

- polyphenylene oxide

- polycarbonate

Polymer blends might become of increasing interest in dental material science because it is possible to manufacture products with tailor-made properties provided as blanks that can be computer-controlled machined.

7 Chemical Reactions of Polymers

Not only the monomers can chemically react with each other but also the polymers can perform chemical reactions under certain conditions to form new polymers with different properties. Some of these reactions and the resulting consequences are discussed in the following.

7.1 Grafting

Grafting means the reaction of monomers with reactive sites of a polymeric chain building new side chains. Three possible grafting reactions will be introduced

a) The polymer has reactive groups that are able to polymerize radically with the added monomer. The reactive groups provided by the polymers have already been built-in during its synthesis. These groups are hydroperoxides, azo or perester groups (Fig. 39a).

Fig. 39a: Grafting reaction by polymerization.

But it is also possible to create the reactive groups retroactively. These reactive groups are statistically arranged and are made by oxidation or chloromethylation. Then the grafting is done anionically or cationically on halo, halo methyl groups or polyhalo aromatics in the presence of Lewis acids or bases (Fig. 40a).

Fig. 40a: Subsequent incorporation of reactive groups into the polymeric chain by chloromethylation.

b) Grafting is also possible by radically polymerizing the monomers to be grafted in presence of the polymer. In doing so either the double bonds existent in the polymer are attacked or chain transfer reactions occur. During a chain transfer reaction a macroradical takes a hydrogen atom from the adjacent polymeric chain which then itself becomes a macroradical (Fig. 41a).

Fig. 41a: Schematic illustration of a chain transfer reaction.

c) Furthermore, grafting is also possible performing polyaddition or polycondensation with functional groups that are part of the polymeric chain. Such functional groups are for instance -OH, -NH₂, -COOH, -COOR or -COCl. Figure 42a exemplarily shows grafting by a condensation reaction.

Fig. 42a: Exemplary presentation of grafting by condensation reaction.

7.2 Cross-Linking of Polymers and Vulcanization

During cross-linking the polymers are converted from the plastic/pasty condition into the elastomeric or even the duromeric condition. From the chemical point of view the cross-linking of polymers takes place almost identical to the vulcanization of rubbers. The only difference is that only elastomers are formed by vulcanization but also duromers can be obtained by the cross-linking of unsaturated polyesters. It depends on the number of cross-linking points available in a polymeric chain as well as on the intermolecular forces if elastomers or duromers are obtained. Therefore, the vulcanization is also considered as a cross-linking reaction. However, this expression is still used today for cross-linking of natural rubber or synthetic rubber via sulfur compounds (Fig. 43a).

Fig. 43a: Rubber cross-linking (vulcanization) via sulfur compounds.

Generally, cross-linking reactions of polymers can be conducted via radical polymerization as well as condensation or addition reactions (Fig. 44a).

Fig. 44a: Cross-linking reactions via free radical polymerization, polycondensation and polyaddition.

7.3 Layering/Incremental Technique and Repair of Resin Composites

The layering technique, also known as incremental technique, is a well proven technique to reduce polymerization shrinkage when large restorations are done with filling resin composites. Generally, no layers thicker than 2 mm are applied and cured in one step. But light-curing crown and bridge, denture base or orthodontic resins are also built-up and polymerized layer-wise to perform or to improve the modelling process. Incremental technique means that a first layer of the resin composite is applied and cured. Then the next layer is applied and cured and so on and so forth until a multi-layered body is obtained representing the final restoration.

At present two different types of resin composite materials need to be considered

a) the methacrylate-based products (MB) and

b) the silorane-based products (SB)

The bond strengths between each of the separately cured layers of the combinations MB/MB, SB/SB and MB/SB were investigated [118, 352, 353].

MB/MB Combinations

Methacrylate-based resin composites retain the already described smear layer (also called inhibition, dispersion or oil layer) on their surface after curing. It was thought that the smear layer is important for the subsequent unpolymerized resin to optimally wet the polymerized pervious layer and to bond without an interface after curing so that finally a coherent restoration is formed. Therefore, it was recommended to leave the smear-layer untouched. But investigations showed that this layer actually weakens the bond between the differently applied and cured layers. This might be due to a non-continuous interface very likely attributed to a non-satisfactory interfacial curing or trapped air. Therefore, it is even recommended to remove this layer at least partially [353].

Very good bond strength was reported for freshly built-up MB/MB-combinations and predominantly cohesive failures were found [118, 352, 353]. No significant differences of bond strength seem to occur between layers each of which was either cured in air or nitrogen (no visible inhibition layer) but it decreases with the time the subsequent layer was applied and cured [118].

SB/SB Combinations

Although slightly lower compared with MB/MB combinations very good bond strength was reported for freshly built-up SB/SB-combinations and predominantly cohesive failures were found. The bond strength of SB/SB combinations decreased with the time the subsequent layer was applied and cured [118, 352]. Although no visible inhibition layer occurs due the cationic polymerization of the siloranes SB/SB bond strength significantly decreased when cured in nitrogen in contrast to MB/MB combinations [118].

SB/MB Combinations

SB/MB combinations have shown the lowest bond strength compared with all of the aforesaid combinations. But it was found that bond strength was significantly improved by the use of phosphate-methacrylate adhesive resins as intermediate layer [352].

Repairability of Filling Resin Composites

As described above, independent of the combination (MB/MB, SB/SB, SB/SM) the bond strength between the layers decreased with time. Therefore, in case of repair work certain pretreatments of the “old” filling’s surface are necessary to obtain an acceptable bond. Diamond bur roughening or sandblasting of the “old” filling’s surface followed by the application of an adhesive (silane preparation, phosphate-methacrylate adhesive) result in an acceptable bond. The repair work can then be done on a silorane- or methacrylate-based filling with a silorane or methacrylate composite or vice versa [354-357]. In any case, it is much more preferable to renew the “old” filling completely.

Inhibition Layer & Interfacial Bonding

The publications of Eliades et. al. [353] and Shawkat et al. [118] found no inhibition layers under certain processing conditions. But contrarily to them, we think that, although invisible, the inhibition layer has not been totally removed neither by rinsing with acetone [353] nor by polymerizing in nitrogen atmosphere [118]. It is assumed that the remaining extremely thin layer is essential to allow the subsequently applied unpolymerized composite to wet the polymerized surface. This assumption is supported by the fact that SB/SB layers have significantly less bond strength when processed under nitrogen. The extremely thin inhibition layer on siloranes of 2 to 8 µm thickness [118] when cured in air might probably actually be avoided by polymerization under nitrogen. Therefore, interfacial bond strength decreased. The finding that wetting the polymerized silorane surface with a low viscosity adhesive significantly improves interfacial bond strength [352] further justifies the aforesaid assumption. It is concluded that thick inhibition layers must be avoided but thin ones are advantageous. In case the inhibition layer has really been totally removed, the application of a low viscosity adhesive is requested.

Chemistry of Interfacial Bonding

The chemistry of interfacial composite/composite bonding is easy to understand. The degree of conversion of the reactive groups of methacrylate- as well as of silorane-based composites is only between approx. 45 to 70 % after polymerization. This means that many reactive groups are left for further polymerization and in case of optimally wetting the subsequently applied and cured composite achieves good adhesion to the previous layer. When stored in water or during service in the oral cavity the reactivity of the cured material’s surface was observed to decrease. This may be explained by superficially incorporated water molecules so that quasi a hydrophilic insolation layer is formed that repels the hydrophobic uncured composite. Therefore, amphiphilic adhesives such as phosphate-methacrylate adhesive resins are advantageously used as intermediate products for repair work [352].