Читать книгу Poly(lactic acid) - Группа авторов - Страница 100

A block copolyester formed by the condensation reaction of PLA with hydroxyl‐terminated four‐armed PCL macroinitiators are shown in Figure 4.17. This reaction was catalyzed by two different catalysts, Sn(Oct)₂ and Fe(OAc)₂. The so‐formed block copolyester poly(ε‐caprolactone‐b‐lactic acid) and its blend with poly(lactic acid) was explored for adhesive application [56]. Further crosslinking reaction of thus‐obtained four‐armed polymer with diisocyanate resulted in a biodegradable polymeric material composed of well‐defined alternating hard and soft domains [188].

Precision synthesis of microstructures in star‐shaped copolymers of CL, LLA, and DXO was accomplished using a spirocyclic tin initiator and Sn(Oct)₂ (cocatalyst) together with pentaerythritol ethoxylate (co‐initiator) [189]. Four‐arm star‐shaped DLLA oligomers of controlled molar mass and low dispersity were synthesized by using ethoxylated pentaerythritol initiator. The terminal hydroxyl group was converted to methacrylate (methacrylic anhydride) or 2‐isocyanatoethyl methacrylate. Photo‐crosslinking of these functional oligomers yielded networks with high gel contents. The T _g of the copolymers depended on the prepolymer molar mass [190].

FIGURE 4.17 Hydroxy end‐functionalized star‐shaped PCL macroinitiators [56].

Star‐shaped PEO–PLA showed a shorter degradation times in comparison to previously reported linear PLA and PLA‐b‐PEG copolymers, along with exceptional amphiphilic characteristics, which may be appealing for their utility as excellent candidates for drug release intracellular carriers [191]. The four‐arm star‐branched block copolymer of LA and EO was investigated for the release of anticancer drugs 5‐FU and paclitaxel. The drug release of paclitaxel from the micellar nanoparticles could be better controlled, as compared with linear block copolymers. The cumulative drug release reaches 60 and 85% by 4th and 14th day, respectively [192]. A rapid and complete release of drug was due to the rapid degradation of micelles from the star‐shaped copolymer, compared to the linear block copolymers. PEGylated copolymers with CL, VL, and LA were amphiphilic in nature and formed micelles with low critical micellar concentration (CMC) values in the range of ~10⁻⁷–10⁻⁸ M [193].

A copolymer having seven arms of poly(LA‐co‐2‐ethyl‐2‐oxazoline) have been successfully prepared. Star‐shaped copolymers were prepared by using tosylated β‐cyclodextrin (β‐CD) as a core while having LA and 2‐ethyl‐2‐oxazoline copolymers as branches. The hydroxyl functional group of (Tosyl)7‐β‐CD was used as the initiator for ROP of LA. The hydroxyl chain end of PLA chain was later used for ROP of 2‐ethyl‐2‐oxazoline [194].

A novel degradable chestnut‐shaped polymer having a PLA shell and hyperbranched D‐mannan (HBM) was synthesized by polymerization of LLA and HBM with DMAP as catalyst. The number of PLA chains on PLA–HBM could be controlled by the ratio of DMAP to sugar [195].

Hyperbranched vs linear polymer structures based on Sn(Oct)₂‐mediated one‐pot copolymerization of glycidol and LA can be achieved by controlling the reaction temperature. Usage of different temperature conditions allowed a controlled occurrence of epoxide ring opening that leads to hyperbranching. Epoxide ring opening was prevented in low‐temperature solution polymerizations, resulted in essentially linear PLA functionalized with an epoxide end‐groups [196].

LA has been polymerized in a star‐shape to poly(amidoamine) dendrimer (PAMAM), the copolymer was synthesized by bulk polymerization of LA with PAMAM. Unlike the linear PLA with similar molar mass, PAMAM‐g‐PLA revealed a higher hydrophilicity and a faster degradation rate. The highly branched structure significantly accelerated the release of water‐soluble bovine serum albumin from these graft copolymers, whereas a time lag was observed in linear PLLA of similar molar mass [197].