Synthesis and characterization of multiblock copolymers poly[poly( L ‐lactide)‐ block ‐polydimethylsiloxane]

The preparation of multiblock copolymers poly[poly( L ‐lactide)‐ block ‐polydimethylsiloxane] by polycondensation of bifunctional oligomers via hydrosilylation is described. The procedure consists first to synthesize the two bifunctional oligomers α,ω‐disilyl‐polydimethylsiloxane and α,ω‐diallyl‐poly( L ‐lactide). The former is prepared in one step by cationic polymerization of octamethylcyclotetrasiloxane in the presence of 1,1,3,3‐tetramethyldisiloxane as end‐blocker. Two steps are necessary to prepare α,ω‐diallyl‐poly( L ‐lactide). The first one is the polymerization of L‐lactide initiated by the system ethylene glycol/tin 2‐ethylhexanoate. In a second step, the hydroxyl end‐groups of the resulting α,ω‐dihydroxy‐poly( L ‐lactide) are transformed, by reaction with allyl isocyanate, into terminal allylic functions. Different multiblock copolymers were prepared by hydrosilylation (catalyzed by hexachloroplatinic acid) using the same α,ω‐diallyl‐poly( L ‐lactide) (M̄ n = 2000 g · mol −1 ) and various α,ω‐disilyl‐polydimethylsiloxanes (M̄ n from 1 750 to 9 000 g · mol −1 ). The influence of parameters such as temperature, stoichiometry of reactive end‐groups and catalyst concentration on the molecular weight of the copolymers was studied. High molecular weight copolymers were obtained (DP n > 12 by SEC). In addition to the biodegradability of the lactic acid units, the immiscibility of the polydimethylsiloxane and poly( L ‐lactide) blocks imparts thermoplastic elastomer properties to these copolymers. The crystallinity of the poly( L ‐lactide) phase is dependent on the molecular weight of the polydimethylsiloxane blocks.