Influence of racemization on stereocomplex-induced gelation of water-soluble polylactide-poly(ethylene glycol) block copolymers

Ring-opening polymerization of L- or D-Iactide was realized at 140 degrees C for a period of 7 days in the presence of dihydroxyl poly(ethylene glycol) (PEG), with (M) over bar (n) = 40009 mol(-1), using zinc lactate as initiator. The resulting poly(L-lactide)-PEG- poly(L-lactide) and poly(D-lactide)-PEG- poly(D-lactide) triblock copolymers are water soluble with polylactide (PLA) block length ranging from 11 to 17 units. Both the tube inverting method and rheological measurements were used to evaluate the gelation properties of aqueous solutions containing single copolymers or L/D copolymer pairs. Stereocomplexation between poly(L-lactide) and poly(o-lactide) blocks is observed for mixed solutions. Hydrogel formation is detected in the case of relatively long PLA blocks ((DP) over bar (PLA) = 17), but not for copolymers with shorter PLA blocks ((DP) over bar (PLA) = 11-13) due to partial racemization of L-lactyl units. Racemization is largely reduced when the reaction time is shortened to 1 day. Under these conditions, (DP) over bar (PLA) of 8 is sufficient for the stereocomplexation of PLA-PEG block copolymers, and (DP) over bar (PLA) above 10 leads to the formation of hydrogels of PLA - PEG block copolymers. On the other hand, racemization appears as a general phenomenon in the (co)polymerization of L-lactide with Zn(Lac)(2) as initiator, although it is negligible or undetectable in the case of high molar mass polymers. Therefore, racemization is the limiting factor for the stereocomplexation-induced gelation of water-soluble PLA - PEG block copolymers where the PLA block length generally ranges from 10 to 30. Reaction conditions including initiator, time and temperature should be strictly controlled to minimize racemization. (C) 2010 Society of Chemical Industry