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The preparation of PLA from lactic acid by direct condensation can be divided into three principal stages: (a) removal of the free water content; (b) oligomer polycondensation; and (c) melt polycondensation of high‐molecular‐weight PLA:

1 Besides lactic acid, the feedstock also contains the so‐called free water. Due to the equilibrium of lactic acid and water, some low amount of oligomers of lactic acid (linear dimer, linear trimer, etc.) can already be formed in this stage. To convert lactic acid to PLA, first the free water has to be removed. The evaporation of the free water requires a system having good heat transfer and can be carried out in commonly used evaporators, such as falling film evaporators. Flash evaporation can also be used to remove the free water in lactic acid feedstock.

2 In the second stage, the lactic acid is converted into low‐molecular‐weight PLA or oligo(lactic acid). In this step, the removal of water is not critical because of the low viscosity of the reaction mixture. The rate‐determining step in this stage is usually the chemical reaction, which is significantly affected by the catalyst used [22]. Traditional polycondensation catalysts are strong acids, and organometallic compounds are also commonly used catalysts. The low‐molecular‐weight PLA polycondensation can also be carried out in an evaporator or alternatively in a stirred reactor having an agitator that generates good radial and axial mixing. The loss of lactic acid due to entrainment can be overcome by using a reflux condenser, a demister package, or a rectification column. Preferably, this stage is carried out in a system having a narrow residence time distribution (plug‐flow behavior) to obtain a prepolymer of lactic acid of narrow molecular weight distribution (small dispersion).

3  The third stage is the melt‐polycondensation in which the removal of water becomes critical. To enhance the polycondensation reaction, and not the transesterification reactions, the water formed in the reaction mixture should be removed efficiently. The rate‐determining step in this phase is the mass transfer of water. To enhance both mass and heat transfer, the melt‐polycondensation reaction should be applied in an apparatus having an efficient renewal of phase boundary layers. The apparatus should have intensive mixing and kneading in order to homogenize the reaction mixture. The removal of water from the viscous PLA mass can be further enhanced by carrying out the reaction under vacuum conditions in an inert atmosphere. A mathematical model for the polycondensation of lactic acid accounting for water removal by diffusion has been developed [23]. The increasing molecular weight of the PLA requires a system that can handle high‐viscosity mass. Such an apparatus could be a rotating disk type of reactor, generating a good surface renewal to enhance the mass transfer of the water formed. Such an apparatus should also have very good heat transfer to have a homogeneous temperature profile in the reaction mixture. Especially the mechanical heat formed due to mixing and kneading of the highly viscous PLA should be controlled. In this stage also a plug‐flow behavior is preferred to obtain a narrow molecular weight distribution.

Only a few studies have dealt with the influence of the catalyst when preparing PLA of high molecular weight through the direct bulk condensation reaction. In most studies with regard to catalysts, the polycondensations were carried out only to obtain low‐molecular‐weight polymers with an M _w of a few thousands, before they were stopped. PLA having a molecular weight of as high as 130,000 g/mol (gel permeation chromatography (GPC) relative to polystyrene (PS) standards) was synthesized by direct bulk condensation polymerization at 180°C using titanium(IV) butoxide as catalyst [24]. In another study, several metal catalysts based on Ge, Sb, Zn, Fe, Al, Ti, and Sn were employed in the melt‐polycondensation reaction [25]. The most efficient catalyst was SnO with regard to molecular weight of the PLA, but the yield was below 40% when using this catalyst at 180°C (20 h). However, when using p‐toluenesulfonic acid as a co‐catalyst with SnCl₂, the efficiency was drastically improved and molecular weights above 100,000 g/mol (GPC relative to PS standards in chloroform, 35°C) were achieved within 15 h of polycondensation. Sodium carbonate, calcium carbonate, and lanthanum oxide have also been used as catalysts when preparing PLA of high molecular weight [26]. Weight‐average molecular weights ranging from 63,000 to 79,000 g/mol (GPC relative to PS standards in chloroform at 40°C) were obtained by melt‐polycondensation but with poor yields (33–52%).

To achieve an increased molecular weight of the PLA, comonomers with functionality higher than two have been used. A process for making a star‐shaped PLA was described, where the lactic acid is polycondensated in the presence of a polyhydroxyl compound having at least four hydroxyl groups [27]. The PLA obtained has a higher molecular weight than a polymer prepared without the use of comonomer, but the invention has a clear limit in obtainable molecular weight. If the polyhydroxyl compound is used in large amounts, the polymer will be hydroxyl terminated and the condensation reaction cannot continue, thus yielding a low‐molecular‐weight polymer. On the other hand, if the polyhydroxyl compound is used in small amounts, the effect of the polyhydroxyl compound will diminish and the polycondensation reaction will be a blend of star‐shaped PLA and linear PLA. A hyperbranched PLA of high molecular weight was also manufactured by coupling a first prepolymer having at least three functional end groups with a second prepolymer having at least two functional end groups by a condensation reaction between the end groups in the prepolymers [28]. The improvement of the process was that the number of arms and/or molecular weight of the functionalized prepolymers could be accurately adjusted, thus affecting the properties of the resulting hyperbranched polymer in a desired way. Molecular weight in excess of 200,000 g/mol (GPC relative to PS standards in chloroform at 30°C) was obtained for the hyperbranched PLA.

Lactide has been used as a coreactant and yield enhancer in the polycondensation reaction of lactic acid [29]. M _ws in the range of 65,000–83,000 g/mol were obtained in 17–42 h (GPC, 40°C, chloroform), starting from 90 wt% lactic acid, when an inorganic solid acid catalyst (aluminum silicate) was used.

Copolymers with high enough molecular weight for practical use were prepared from succinic acid and 1,4‐butanediol and minor amounts of lactic acid [30]. An increase in the reaction rate was reported when the aliphatic diol and the aliphatic dicarboxylic acid were polycondensated using a few mole percent of lactic acid and a germanium oxide catalyst.