Biodegradable Polycarbonates With High Molecular Weight; Enzymatic Synthesis And Properties Improvement

Aliphatic polycarbonate is one kind of surface erosion biodegradable polymers, which has been used in the biomedical fields such as suture, bone fixation plats and drug delivery systems due to its good biocompatibility and favorable mechanical properties. Biodegradable aliphatic polycarbonate is generally prepared by polycondensation of diols with phosgene or dialkyl carbonate, copolymerization of carbon dioxide with epoxide, and ring-opening polymerization of cyclic carbonates. The properties of biodegradable aliphatic polycarbonate can be improved by introduction of functional pendant groups, copolymerization with other monomers and blend with other materials. In this paper, the development of biodegradable polycarbonates is reviewed in the aspects of synthesis, properties improvement and application.Recently, enzymatic ring-opening polymerization has been known as new methodology of polymer syntheses, which can easily ensure the good biocompatibility and reliability of resulting polymer materials and can provide an environmentally friend process. Nevertheless, this method generally leads to rather lower molecular weight polymers with high polydispersities. This main problem exist in enzymatic polymerization method should limit its actual applications in the synthesis of biodegradable polymer materials for biomedical uses. Enzyme immobilization has been proven an effective method to improve catalytic activity and thermal stability of enzyme. It was reported that cyclic carbonates (6- and 7-membered) polymerize with volume expansion. In this paper, pressure-associated enzymatic ring-opening polymerization was first adopted to synthesize high molecular weight poly(5,5-dimethyl-trimethylene carbonate)(PDTC) and poly(5-benzyloxy-trimethylene carbonate)(PBTMC) using immobilized enzyme on silica particles as catalyst. The influences of pressure and immobilized enzyme concentration on the molecular weight and yield of the polymers have been investigated. This novel method was expected to be favorable and feasible for the enzymatic synthesis of polycarbonates with controlled high molecular weight.Although aliphatic polycarbonate has many good properties and has been used in the biomedical fields, the rather lower hydrophilicity and hydrodegradability introduced by the presence of the carbonate groups in the polymer chain decreases their compatibility with soft tissues and lowers their biodegradability. Poly(ethylene glycol) (PEG) has been reported to be hydrophily, good solubility in water and organic solvents, lack of toxicity, as well as absence of antigenicity and immunogenicity. Polycarbonate can be copolymerized with PEG to improve its lower hydrophilicity and hydrodegradability, so as to fill various applications requiring different hydrophility-hydrophobility, degradation rates and mechanical properties. In this paper, using immobilized enzyme as catalyst, a series of PDTC-PEG-PDTC triblock copolymers of DTC and PEG were successfully synthesized. The copolymers were characterized by 'H NMR, GPC, and DSC. The influences of immobilized enzyme concentration and monomer feed ratios on the compositions, molecular weight and yield were investigated. The thermal properties of triblock copolymers with different compositions were also studied. In addition, enzyme- and chemical-catalyzed copolymerizations of DTC with PEG were compared. It was found that the molar ratios in the copolymers were close to that in feed. Immobilized PPL on silica particles is one kind of powerful candidate compared with organometallic initiators, such as Sn(Oct)2 and Al(O'Pr)3.The hydrophilicity, mechanical property, hydrolytic and enzymatic degradation, and drug-release of PDTC and PDTC-PEG-PDTC triblock copolymers were studied. The PEG content influence significantly on the hydrophilicity, mechanical property and drug-release behavior of PDTC-PEG-PDTC triblock copolymers, but little on their degradation.