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

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) x 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 ft 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 networks are both glassy at the degradation temperature of 37 degrees 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 degrees C. The 20:80 and 0:100 DLL:TMC networks were the slowest to degrade owing to their hydrophobicity. (C) 1997 Elsevier Science Ltd.