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 alpha,omega-disilyl-polydimethylsiloxane and alpha,omega-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 alpha,omega-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 alpha,omega-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 alpha,omega-diallyl-poly(L-lactide) ((M) over bar(n) = 2000 g.mol(-1)) and various alpha,omega-disilyl-polydimethylsiloxanes ((M) over bar(n) from 1750 to 9000 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 (DPn > 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.