Multi-Block Copolymers of PLA

The syntheses of PLA copolymers have been widely studied, but most efforts have been focused on random, diblock, and triblock copolymers by the ring-open polymerization of lactide that not only affects the properties of copolymers by phase separation but also causes a rather higher cost of production for the lactide as a monomer. However, growing interests have been recently given to a new class of PLA copolymers multi-block copolymers. Compared to the diblock or triblock copolymers, multiblock copolymer has more and shorter blocks and PLA segments, which alternate in the polymer chain. Consequently, it is possible to get some special properties such as better miscibility between the two components and lower crstallinity. Thus the degradability of the copolymer is expected to be enhanced. Multi-block copolymers can be synthesized by direct polycondensation, coupling reaction or chain-extension reaction of prepolymers. 

To improve the properties of PLA copolymers with high-molecular-weights and reduce the cost of production, direct polycondensation of prepolymers has been developed to synthesize multiblock copolymers. For example, Luo et al. [117] synthesized PLLA-PEG multiblock copolymers with predetermined block lengths by polycondensation of PLA diols and PEG diacids. Chen et al. [118] 

In addition, Kricheldorf et al. [126] demonstrated the synthesis of Z-lactide-PEG multiblock copolymers by means of spirocyclic Ge-PEG initiators. Spirocyclic initiators were prepared by condensation of Ge(OEt)4 and tetra(ethylene glycol), 

or PEG-2000. These spirocycles were used as initiators for the ring-expansion polymerization of Z-lactide in chlorobenzene at 120 °C. The resulting cyclic block co-polymers were reacted in situ with aliphatic dicarboxylic acid dichlorides (ADADs) or with hexamethylene diisocyanate (HMDI) to achieve chain extension. 

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