Study On The Synthesis Of Biodegradable Poly Lactic Acid And Polyester By The Chain-Extending Reaction

Chain-extending reaction is one of the important methods to increase the molecular weight of the polylactic acid(PLA) and the polyesters prepared from melt polycondensation. Chain extenders are usually low molecular weight chemicals with bifunctional groups. They can increase the molecular weight of the PLA or the polyesters in a fast reaction.In this study, original hydroxyl-terminated poly lactic acid(PLA) with the molecular of1000-1500was synthesized by melt polycondensation.2,2’-(1,4-phenylene)-bis(2-oxazoline)(PBOX) were prepared and used as chain-extenders for PLA. The chain-extending reaction of PLA was researched and the result showed that the optimal molar ratio of PBOX/PLA is1.1; chain extension temperature is180℃, the Mr of original PLA is968-1200, and the optimal reaction time is about2hours. PLA with [η] up to0.46dl/g was prepared at these optimal conditions.In this study, polybutylene adipate (PBA) and polybutylene succinate (PBS) were synthesized by melt polycondensation.2,2’-(1,4-phenylene)- bis(2-oxazoline)(PBOX) and adipoyl biscaprolactamate(ABC) were synthesized and used as chain-extenders for PBA and PBS. PBA and PBS with high molecular weight were obtained by the chain-extending reaction. The influence factors such as the chain-extenders, the structure of the PBA prepolymer and reaction temperature etc, on the chain-extension were studied. The results show that chain extension temperature is200℃, the optimal reaction time is3hours, and both PBOX and ABC were used. PBA with Mn up to39110and Mw up to94525, as well as PBS with [η] up to1.0was prepared at these optimal conditions.The chain-extended polyesters were characterized by IR and NMR spectra and the mechanism of the chain-extending reaction was proposed. The chain-extending reaction proceeds through a ring-opening reaction of the oxazoline rings, polyester prepolymers were connected by amide groups resulted from the chain-extenders, making the molecular weight of the polyesters increased. The chain-extending reaction of hydroxyl-terminated polyester with ABC proceeds through the elimination of caprolactam ring in the chain-extender, the groups such as adipoyl and terephthaloyl couple the hydroxyl-terminated polyester together and achieve the aliphatic polyester with high molecular weight.The prepolymers and the chain-extended PBA and PBS were also characterized by differential scanning calorimeter (DSC). The DSC spectrum shows that the melting point of the chain-extended polyester prepared with ABC increases duo to the fact that the adipoyl group in the ABC was identical to the adipoyl structure in the original hydroxyl-terminated polyester, the increase of molecular weight of chain-extended polyester leads to the enhancement of regularity of the molecular chain of polyester. While PBOX was used to extend the original carboxyl-terminated polyester, the polyester would be imported different structure which destroyed the regularity of the structure.So melting point of the chain-extended polyester would be reduced. When two kind of chain-extenders were used together, the higher amout chain-extender would determine the melting point of the polyester.