Synthesis of low-molecular-weight copoly( l-lactic acid/ɛ-caprolactone) by direct copolycondensation in the absence of catalysts, and enzymatic degradation of the polymers

Abstract

Low-molecular weight copolymers ofl-lactic acid (LA) and ɛ-caprolactone (CL) were synthesized by direct copolycondensation without catalysts at 200°C under a nitrogen atmosphere. The reaction proceeds by direct condensation between linear LA and linear 6-hydroxycaproic acid produced by hydrolysis of the cyclic CL. These copolymers were characterized with respect to molecular weight by gel permeation chromatography, composition by1H nuclear magnetic resonance (n.m.r.) spectroscopy, sequence by13C n.m.r. spectroscopy, and crystallinities by both differential scanning calorimetry and X-ray measurement. The morphology of copoly (LA/CL) can be subdivided into three states as a function of monomer composition according to the differences inTg and crystallinity, e.g. solid (0–15 mol% CL composition ranges), paste (30–70 mol% CL composition ranges) and wax (85–100 mol% CL composition ranges). Thein vitro degradation of the copolymer was examined by treatment in buffer solutions with and without enzymes and, as a result,Rhizopus delemer lipase showed the strongest degradation activity. In this case, the degree of enzymatic degradation is strongly dependent on the morphology of copoly(LA/CL), in which the pasty copolymer is much more subject to hydrolysis than the solid and waxy copolymers. For comparison thein vivo degradation of the copolymer was investigated by implanting it subcutaneously in the back of rats.