| Because of their biocompatibility and environmental degradability, polymers derived from lactic acid have long been important materials for medical applications. Once used primarily as sutures and implants, polylactide materials are now entering high volume areas such as packaging. To be successful in these new applications, polylactides must be available that exhibit a broad spectrum of physical properties while retaining the degradability of the parent polymers. This objective can be achieved by altering the substituents on the polylactide chains.; A variety of substituted lactides were synthesized and polymerized to high molecular weight polymers using solution and bulk polymerization. The polymers show a wide range of physical properties: glass transition temperatures range from –37°C to 85°C, hydrolytic degradation rates are 2–3 times slower than polylactide's, and degradation temperatures are around 350°C. We established the relationship between the pendant group and the physical properties of the polymers.; The kinetics and mechanism of lactide polymerization was studied. Lactide polymerizations are first-order reactions in both solution and bulk polymerization. In bulk polymerization, polymerization rates follow the order of size of substituted group, the bigger the substituted group, the slower the polymerization rate. In solution polymerizations, the polymerization rates do not follow the order of size of substituted group, which is probably caused by different rate-determining steps for the polymerizations. We proposed that lactide polymerization mechanism is similar to “atom-transfer polymerization” when initiated by Sn(OCt)2. Using it as guide, we achieved control of the molecular weights and molecular weight distributions of the polymers.; We studied the influence of stereochemistry on the kinetics of polymerizations and the physical properties of the polymers. We found that racemic monomers polymerize faster than either of the optically pure monomers. The racemic polymers are usually amorphous polymers and optically pure polymers are usually crystalline polymers.; Copolymerization of lactide with substituted lactides were investigated. The architectures of polymers depend on the size difference of substituted groups between the lactide and substituted lactide. By controlling the size of substituted group, random, block and alternating copolymers can be synthesized. |
