Synthesis of copoly(d,l-lactic acid) with relatively low molecular weight and in vitro degradation

Copoly(d-lactic acid/l-lactic acid; d-LA/l-LA) with relatively low molecular weight (Mn) of 300–3800, was synthesized by direct copolycondensation without catalyst in an atmosphere of nitrogen at 200°C, to evaluate the difference in degradation between crystalline and amorphous copolymers. The non-enzymatic degradation of crystalline copolymer, e.g. a copoly(d-LA/l-LA, 2/98 mol%), showed a parabolic-type degradation pattern in the Mn 900 to 2700 range; degradation preferentially occurs in the amorphous regions because the matrix remains highly crystalline in the initial stages. In contrast, the non-enzymatic degradation of amorphous copolymer, e.g. a copoly(d-LA/l-LA, 50/50 mol %), can be divided into three types according to Mn, viz. parabolic-type for Mn = 1500, linear-type for Mn = 2200, S-type for Mn = 3500. For comparison, the copolymers were degraded in the presence of enzymes; the process was markedly accelerated by the action of esterase-type enzymes, of which Rhizopus delemer lipase gave the highest degradation activity.