Synthesis And Properties Of Biodegradable Main-chain Liquid Crystalline Polycarbonates

Over the past decades,biodegradable polymer materials have received more and more attention for their wide application in biomedical fields such as drug carriers,tissue engineering,surgical medical,etc.Among them,aliphatic polycarbonates are an important class of biodegradable materials with good biocompatibility,favorable biodegradability and low toxicity.Many biodegradable materials with more excellent performance can be obtained by the further study of such compounds and modified products of aliphatic polycarbonates.The previous research showed that major classes of biological compounds including lipids,proteins and nucleic acids have been found to exist in various liquid-crystalline(LC)phases in vivo.As known,cholesterol in the living cell membrane has high thermodynamic affinity and ability to change the membrane's permeability and fluidity.These characteristics make cholesterol to be a promissing candidate for preparing bioactive LC material.Therefore,the main ideal of study in this article is to introduce cholesteryl into biodegradable main-chain LC polycarbonates to obtain biodegradable material with excellent performance and widen applications.In the second chapter,the synthesis of a novel cholesterol derivatives 2-cholesterylethanol is described.The cholesteryl end-capped polymers/oligomers were obtained by the ring-opening polymerization of trimethylene carbonate(TMC)using 2-cholesterylethanol as an initiator.By varying the feed ratio of the initiator to the TMC,four kinds of cholesteryl end-capped polymers/oligomers were prepared.The chemical structures and properties of all the cholesteryl end-capped polymers/oligomers were characterized by FT-IR,1H-NMR,DSC,POM,etc.The results showed that all the cholesteryl end-capped polymers/oligomers were in LC state at room temperature,and the fan-shaped texture could be observed.X-ray diffraction analysis showed the mesophase of the cholesteryl end-capped polymers/oligomers was a smectic A phase.And the thermal stability of four kinds of resulted copolymers/oligomers was similar.What's more,with an decrease in initiator content,the Tg and the clear points decreased,but the molecular weight increased.In the third chapter,biodegradable main-chain LC polycarbonates were synthesized by the the ring-opening polymerization of TMC and caprolactone(CL)using cholesterol with a hydroxyl group as an initiator,in the presence of stannous octoate as a catalyst.To investigate the effect of monomer molar ratio on copolymer's performance,five copolymers Chol-(TMCL)x+y with different composition were prepared,in which the molar ratio of cholesterol to the monomers was given at 1:4,and the moalr ratios of TMC to CL was variable.In another experiment,to investigate the influnce of the cholesterol amount on the copolymer's performance,four copolymers were also prepared,in which the moalr ratio of TMC to CL was given at 1:1,while the moalr ratio of cholesterol to the monomers was variable.The chemical structures and properties of all the copolymers were characterized by FT-IR,1H-NMR,DSC,POM,etc.The results showed that the content of CL had a great influence on the Tg,LC behavior,and molecular weight as well as the thermal stability of copolymer.The more CL content,the lower Tg,and the weaker LC behavior of the resulted copolymer.As well,with an increase in CL content,the molecular weight decrease and thermal stability of copolymer increase.Meanwhile,the cholesterol content also had a great influnce on the Tg,and LC behavior as well as molecular weight of copolymers.The LC behavior of copolymer became weak with the decrease of the molar ratio of cholesterol to the monomers.Moverover,with the decrease of cholesterol content,the Tg decreased and molecular weight of copolymer increased.X-ray diffraction analysis showed the mesophase of resulted polymers was a smectic A phase.In the forth chapter,the enzymatic degradation of Chol-(TMCL)x+y was investigated.The results showed that all copolymers underwent surface errosion during the enzymatic degradation,and porous structure was observed on the sample surfaces.The content of CL had a great influence on the enzymatic degradation rate,the higher degradation rates,the higher mass and thickness loss as well as the faster decreases in molecular weight.During the enzymatic degradation,continuous release of acid degradation products into the surrounding media caused by the cleavage of carboxylic groups was observed.The copolymers with higher CL content had higher water uptake and loss thermal stability more significantly.