Synthesis, Characterization and Enzyme Degradation Study of a New Biodegradable Copolymer of Poly(L-lactide) and Poly(L-glutamic acid)

Abstract

Polylactide is a widely used biodegradable polymer which finds many uses in biomedical applications such as surgical sutures, implant materials and drug carriers. However, the rather low hydrophilicity of polylactides, due to the presence of apolar methyl substituents, decreases their compatibility with soft tissues and lowers their biodegradability. This problem may be overcome by introduction of hydrophilic segment, e.g. poly(amino acid)s, into the polylactide chain. This thesis describes the synthesis, characterization and enzyme degradation of a novel biodegradable copolymer comprising poly(L-glutamic acid) and polylactide segments. First, the preparation of protected poly(y-benzyl-L-glutamate) (PBLG) bearing NH2 group at both ends with narrow molecular weight distribution was conducted. Under appropriate conditions, the PBLG then initiated the ring-opening polymerization of L-lactide to produce block copolymer, using low toxicity stannous octoate as catalyst. The kinetics of the polymerization was studied, and the characterization of the copolymer was performed by the use of 1H and 13C NMR FT-IR and GPC (gel permeation chromatography). A tri-block structure of the copolymer was deduced. The thermal behavior of the copolymers was investigated by DSC, and the morphology of crystallized polymer films was examined by polarized light microscopy. The composition of the copolymer affected not only the degree of crystallinity but also the morphology of the crystals. Since both the PBLG and polylactide segments are hydrophobic, PBLG was deprotected by catalytic hydrogenation using Pd/C (10%) as catalyst. Films of poly(L-glutamic acid) and polylactide copolymers were found to degrade in enzyme (Proteinase K) solution. Amorphous films lost weight even on the first day of degradation, and weight loss reached 73% on the 8th day for one sample. In comparison, the solvent cast films only lost weight 22% by the 10th day. Crystallization led to slower degradation. Degradation rate was also controlled by the hydrophilic/hydrophobic composition of the copolymer. Finally, the biodegraded samples were characterized by FT-IR, NMR DSC and SEM (scanning electronic microscopy).