Researches On PLLA And Its Copolymers

Polylactide (PLA) is a biodegradable and biocompatible crystalline polymer which has resultant environmental advantages among all synthetic polymers. Its metabolic end products in our bodies are water and carbon dioxide whilst its metabolic intermediate product -lactic acid is also one of the metabolic products of sugar which can not be accumulated in our main organs. These desirable attributes make it an attractive material in both biomedical and environmental fields. However, its application is limited by its lower holding water property due to high hydrophobicity. which in turn results in a slow hydrolytic rate, and its fragile nature. Other potential disadvantages come from the accumulation of lactic acid produced in the process of PLA degradation may cause an acidic micro-circumstance. The improvement on the crystallization behaviour of PLA and the synthesis of new copolymers of PLA are strategies that can provide PLLA with new properties.In this dissertation, the poly(L-lactide)(PLLA) synthesis and its crystallization behaviour were studied in detail, and then the multi-arm PLLA synthesis and its degradation properties, as well as the synthesis of the PLLA/Chitosan graft copolymer were also researched successfully. All of these have efficiently improved the properties of the PLLA. The main conclusions and contributions of this dissertation on PLLA and its copolymers are summarized as follows:(1) PLLA was synthesized by ring-opening polymerization of L-lactide. The chemical structure, crystallization and thermal behavior of PLLA were characterized. The influence of the processing conditions on the molecular weight was also discussed. The results showed that in the process of ring-opening polymerization of L-lactide the ratio of initiator to monomer, the vacuum and the purity of L-lactide were critical to the molecular weight of PLLA. The yield of L-lactide was 88.37% and its optical purity was 99.3%. PLLA with a weight-average molecular weight of 140,000 was obtained. In vitro degradation experiments showed that PLLA in...