Preparation Of High Molecular Weight Poly(p-dioxanone) By Chain Extending Of Hydroxyl-terminated Prepolymers

As one of aliphatic polyether-ester, poly (p-dioxanone) (PPDO) shows not only excellent biodegradability, biocompatibility and bio-absorbability, but also good flexility, tensile strength and knot strength. The PPDO has already been used in some biomedical materials such as suture, bone repair device and may have potential application in peroral or parenteral drug delivery systems. However, its application was restricted because the rigorous synthetic condition and longer reaction time were needed in order to obtain high molecular weight products by common ring opening polymerization. In fact, the molecular weight of aliphatic polyesters is an important factor that determines their properties.Chain extending has been considered as an effective and convenient method to enhance the molecular weight of aliphatic polyesters, which has been used frequently in synthesis of polyurethanes and biodegradable polymers. Diisocyanates are the most important chain extenders for polymers with hydroxyl-terminated, which can connect two chains together through urethane bonds to form a longer chain. Toluene-2,4-diisocyanate (TDI) has been chosen as chain extender for PPDO in our previous work, although high molecular weight of PPDO has been achieved by this method while the molecular weight of prepolymer reach a quite high level. In fact, the molecular weight of chain extended products was limitedly increased, owing to the PPDO prepolymer contained one terminal hydroxyl group and one terminal carboxyl group when it is prepared by ring-opening polymerization of PDO initiated by traces of water and catalyzed by SnOct2.In order to prepare high molecular weight PPDO, in the present paper, 1,4-butanediol (BD) as the dihydroxyl compound was employed to produce hydroxyl-terminated PPDO prepolymers, then TDI and hexamethylenediisocyanate (HDI) were used as chain extender to synthesize high molecular weight PPDO in shorter time. Several instruments such as NMR, DSC, WXRD, POM, TG and Capillary Rheometer have been employed to investigate the structure-properties relationship of chain-extended HPPDO.The reaction conditions were optimized as conducting at 150°C within 60 minutes after systematically investigation. The results approved that the chain extending occurred between dihydroxyl-terminated prepolymers and TDI (or HDI) in high effeciency with good controllability, and high molecular PPDO can be obtained very easily. Comparing with its prepolymer, HPPDO10, the viscosity-average molecular weight (Mv) of HPPDO10-T has increased 42 times when TDI was used as chain extender, and the Mv of HPPDO10-H has increased 52 times while HDI was employed. The highest viscosity-average molecular weight of resulting polymer could reach 257000g/mol. 1H NMR have been used to characterize the structure of HPPDO prepolymer and chain-extended products.POM, DSC and WXRD had been utilized to investigate the crystallization and morphology of chain-extended HPPDO. The products could also develop well-defined spherulites with a clear Maltese cross and bands while cooling from the melt. The spherulitic morphology changed as a function of crystallization temperature. The structure of polymers is also one of the key factors, which not only governed the ability of crystallization and the crystalline morphology, but also influenced melt point (Tm) and glass transition temperature (Tg) of polymers during the dynamic crystallization process. From the DSC data, it showed that the Tm was decreased and the Tg was increased after chain extending, and this variation appeared more obviously with the increase of chain extender in feed. From the WXRD curves it could see that the chain-extended HPPDO had the same peaks at 2θas that of HPPDO prepolymer.Thermal property of PPDO homopolyner and chain-extended HPPDO had been compared by TG analysis. The results showed that the chain structure and molecular weight of polymers do have effect on their thermal stability. with similar intrinsic viscosity(or molecular weight) of PPDO and chain-extended HPPDO, the thermal stability of chain-extended product is higher than that of pure PPDO, which is inflected by the higher value of initial decomposition temperature and maximum decomposition temperature. Furthermore, the thermal stability of HPPDO-T is better than that of HPPDO-H.The research on rheology properties of chain extened products was carried out by using a Capillary Rheometer. The results showed that chain extended HPPDO exhibits a shear-thinning phenomenon, and the apparent viscosity of melt increased after chain extending. And the apparent viscosities of melt of products are especially sensitive to temperature and decreased sharply with the increasing of temperature.The in-vitro degradation tests of PPDO homopolyner and chain-extended HPPDO were carried out in a buffer solution based on hydrogenated sodium phosphate and di-hydrogenated potassium phosphate with an initial pH of 7.4 at 37oC. The results showed that the degradation rates of chain extended products were still similar to PPDO homopolymer.