Synthesis Of Aliphatic Polyesters For Biomaterials Via Controllable Polycondensation

In the past few decades, biodegradable polymers have been drawn much attention due to their potential applications in biomedical materials. Aliphatic polyesters as a class of biodegradable polymers have been developed rapidly, and some of them have been commercially available. Polycondensation is one of traditional methods to synthesize polyesters using a hydroxyl acid. The method suffers from some major shortcomings, such as high temperature, high vacuum, long reaction time and removal of byproducts. The ring-opening polymerization (ROP) of cyclic diesters (lactides and glycolides) is an alternative method, which could be employed to yield polymers with a predictable polydispersity. However, due to the high cost of cyclic monomers like lactides and glycolides, some applications of the ROP are limited.Therefore, it would be significant to improve the synthesis of aliphatic polyesters. Chain-growth polycondensation in solid-liquid phase was studied by Yokozawa. The monomers in solid phase which were unable to react with each other, were dispersed in an organic solvent and transferred by a phase transfer catalyst to the solution phase for participating into the polymerization. The condensation proceeded in a chain-growth polymerization manner, so the polymerization degree could be controlled by the feed ratio of monomer to initiator. Encouraged by these studies, chain-growth polycondensation in solid-liquid phase would be used to obtain aliphatic polyesters. A significant effort was made in this paper to explore the possibility of developing polyesters with exact structure, including a well-defined molecular weight and definite end groups. The further crosslinking could be carried out with the functional end groups. The degradation period of crosslinked products could be controlled by the length of biodegradable moieties.First, well-defined aliphatic polyesters, polyglycolide (PGA), polylactide (PLA) and their copolymers (PLGA) with allyl end-groups were prepared by chain-growth polycondensation of potassium2-bromocarboxylates in solid-liquid phase. The polymerization proceeded in a living fashion. The polymerization degree of polyesters was in agreement with the feed ratio of monomer to initiator. The synthesized polyesters with allyl end groups were successfully crosslinked with crosslinkers under thermal condition. The crosslinked products were transparent and uniform, and possessed biodegradability in phosphate buffer solution. For yielding more aliphatic polyesters with different topological structure, a series of three-armed star-shaped polyesters were synthesized via chain-growth polycondensation in solid-liquid phase. The chemical structure and molecular weight of star-shaped polyesters could be controlled by means of changing the feed ratio of monomer to initiator. Due to having more functional end-groups, the gel time of star-shaped polyesters was shorter than that of linear polyesters. The crosslinked products possessed better thermal stabilities and degradabilities in buffer solution. The allyl end-groups in linear and star-shaped aliphatic polyesters could undergo an oxidation reaction to obtain a,ω-diepoxylpolyesters. The epoxyl end-groups would provide an effective approach to further modify the polymers by reacting with many nucleophilic reagents, such as alcohols, amines and carboxyl acids.To increase the thermal properties of the aliphatic polyesters, a series of linear and star-shaped polyesters with biomesogenic units as end-groups, like cinnamic acid (CA) and ferulic acid (FA), were synthesized via the chain-growth polycondensation in solid-liquid phase. The transition from anisotropic to isotropic was observed in the DSC measurement of poly(glycolic acid)-based products, and the X-ray diffraction pattern was typical of the nematic state of liquid-crystalline polymers. There was a needle form which is widely seen in the crystals under polarizing-light microscopy. The polyesters with vinyl and cinnamoyl end groups were easily crosslinked under thermal condition. The crosslinked products possessed good thermal stabilities and degradability in buffer solution, which would be a potential application as biomaterials.Cyclic aliphatic polyesters were obtained via phase transfer catalysis in solid-liquid phase. According to the kinetic studies, chain-growth polycondensation happened first to synthesis linear polyesters, and then intrachain condensation occurred to form cyclic products. Due to the ring-chain equilibrium, the cyclization only occurred in a certain degree. Cyclic poly(lactic acid)s formed via a kinetically controlled polymerization with end-to-end cyclization. Cyclic poly(lactic acid)s possessed a kind of crystalline structure like liquid-crystalline polymers. After thermal process, this crystalline structure could be destroyed, and the cyclic polyesters would become amorphous. According to molecular simulation, the radius of gyration of cyclic heptamer and dodecamer was close to the linear polyesters with the same polymerization degree, which would expain that cyclic heptamer and dodecamer were obtained with the largest percentage in the polymerization.