Synthesis, characteristics and in vitro degradation of star-block copolymers consisting of L-lactide, glycolide and branched multi-arm poly(ethylene oxide)

Biodegradable star-block copolymers containing a hydrophilic 4- or 8-arm branched poly(ethylene oxide) (PEO) central unit and hydrophobic poly(L-lactide) (PLLA) or poly(L-lactide-co-glycolide) (PLLG) were synthesized by solution polymerization in toluene catalysed by aluminium triethylene. Using g.p.c. and laser light scattering analysis, a significant increase in the molecular weights could be demonstrated, corresponding to the ratio of the monomers relative to the multi-arm poly(ethylene oxide) in the feed. Polydispersities were of same order as those of the parent PEOs. N.m.r. spectra show that all hydroxy end groups of multi-arm PEO are esterified after copolymerization, which supports the star architecture of the products. The star-block copolymers have physico-chemical properties differing from their starting materials: branched PEO and PLLG or PLLA. The glass transition temperature, T-g, and the crystallinity of star-block PLLA-PEO or PLLG-PEO were significantly reduced in comparison to the respective linear polymers, demonstrating the influence of the steric architecture of the star-block copolymers on the thermal properties. The in vitro degradation properties of star-block PLLG-PEO copolymers differ from those of linear ABA triblock copolymers of PLLG and PEO due to the steric architecture and shorter PLLG block. While mass loss and molecular weight decay are of comparable order in the initial phase, loss of branched PEO seems to be reduced. These novel biodegradable polyether-polyesters may have potential for parenteral protein delivery systems. (C) 1998 Elsevier Science Ltd. All rights reserved.