Methacrylate-endcapped poly(d,l-lactide- co-trimethylene carbonate) oligomers. Network formation by thermal free-radical curing

Abstract

A series of 3-arm, methacrylate-endcapped poly( d,l-lactide- co-trimethylene carbonate) prepolymers was synthesized using d,l-lactide:trimethylene carbonate (DLL:TMC) molar feed ratios of 100:0, 80:20, 60:40, 40:60, 20:80, and 0:100. Number average molecular weights were in the range (2.3–2.6) × 10 3 g mol −1. The prepolymers were free-radically crosslinked in the absence of reactive diluents to give amorphous, bioabsorbable networks with a broad range of thermal, mechanical, and degradative properties. Extraction studies indicated that sol-contents ranged from 2.89%–6.17%. Tensile modulus, ultimate strength, and T g increased with increasing d,l-lactide content. Networks containing higher contents of d,l-lactide, 100:0, 80:20, and 60:40 (DLL:TMC), were strong and fairly rigid, but failed catastrophically at the yield point; networks containing lower contents of d,l-lactide, 20:80 and 0:100, showed a higher elongation to break, failing catastrophically at the yield point. A 40:60 DLL:TMC network fit perfectly within the series of compositions with regard to modulus and tensile strengh; however, it showed a yield point, followed by a regime of plastic flow prior to break. Hydrolytic degradation experiments revealed that the network based on poly( d,l-lactide) homopolymer degraded fastest owing to its hydrophilicity. Hydrolytic degradation in the copolymer networks was controlled by two opposing effects which occurred as the trimethylene carbonate was increased: T g depression, which increased water uptake, and increased hydrophobicity, which decreased water uptake. Increasing trimethylene carbonate in the 80:20 and 60:40 DLL:TMC copolymer networks caused a decrease in the water uptake and the degradation rate since these network are both glassy at the degradation temperature of 37°C. The observed increase in degradation rate in the 40:60 copolymer network was due to increased water uptake caused by depression of the T g to a value below the test temperature of 37°C. The 20:80 and 0:100 DLL:TMC networks were the slowest to degrade owing to their hydrophobicity. © 1997 Elsevier Science Ltd.