Influence of the Synthesis Conditions on the Structural and Thermal Properties of Poly( L- lactide)- b- Poly( ethylene glycol)- b- Poly( L- lactide)

The poly(l-lactide)-b-poly(ethylene glycol)-b-poly(l-lactide) block copolymers (PLLA-b-PEG-b-PLLA) were synthesized in a toluene solution by the ring-opening polymerization of 3,6-dimethyl-1,4-dioxan-2,5-dione (LLA) with PEG as a macroinitiator or by transterification from the homopolymers {[}polylactide and PEG]. Two polymerization conditions were adopted: method A, which used an equimolar catalyst/initiator molar ratio (1-5 wt %), and method B, which used a catalyst content commonly reported in the literature (<0.05 wt %). Method A was more efficient in producing copolymers with a higher yield and monomer conversion, whereas method B resulted in a mixture of the copolymer and homopolymers. The copolymers achieved high molar masses and even presenting similar global compositions, the molar mass distribution and thermal properties depends on the polymerization method. For instance, the suppression of the PEG block crystallization was more noticeable for copolymer A. An experimental design was used to qualify the influence of the catalyst and homopolymer amounts on the transreactions. The catalyst concentration was shown to be the most important factor. Therefore, the effectiveness of method A to produce copolymers was partly due to the transreactions. (c) 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40419. (AU)