Читать книгу Poly(lactic acid) - Группа авторов - Страница 90

There has not been much research considering copolymerization of LA with δ‐valerolactone (VL) and β‐butyrolactone (BL). Anionic block copolymerization of VL and LLA in the presence of potassium methoxide in THF at 20°C gave diblock copolymers with expected compositions and molar mass [49]. Slight racemization of LLA was observed during polymerization due to transesterification reactions.

Block copolymers of LA and BL have been prepared by first preparing a hydroxyl‐terminated poly(β‐butyrolactone) (PBL). The ROP of (R)‐BL or (RS)‐BL with distannoxanes as catalyst in the presence of 1,4‐butanediol as initiator gave optically active poly[(R)‐BL] or atactic poly[(RS)‐BL] with secondary hydroxyl chain ends and oxytetramethylene units in the backbone. These polymers may be used to initiate the copolymerization of LA at the chain ends and form block copolymers. The optically active poly[(R)‐BL] was found to be brittle, whereas atactic poly[(RS)‐BL] showed elastomeric properties. Thus, ROP of [RS]‐βBL with LLA could be used to prepare elastomeric copolymers, which may alleviate brittle behavior of pristine PLLA. However, utility of Sn(IV) compounds is known as active transesterification catalysts and may cause scrambling of monomer units when LLA is used as a comonomer. Therefore, a two‐stage polymerization was carried out. In the first step, telechelic poly[(RS)‐BL] in the molar mass range of 5000–12,000 g/mol was prepared at 100°C by maintaining the desired molar ratios of (RS)‐BL and 1,4‐butanediol and using Sn(IV) as catalyst. In the second stage, the desired ratio of hydroxyl‐terminated poly[(RS)‐BL] and LLA monomer was added and Sn(Oct)₂ was used as a catalyst and polymerization is carried out at 160°C [47]. Hori et al. [45] have also presented research about the synthesis of random copolymers using LLA and (R)‐BL.

Various mono and bisbenzylalkoxy‐bridged dinuclear indium complexes were explored as catalysts to form poly(hydroxybutyrate) (PHB) and star‐shaped block copolymer, PHB‐b‐PLA, in the presence of branched alcohols suggests nature of catalyst governs the formation of macromolecular architecture (Figure 4.8) [119].