Study On Chain Extending Of Poly(P-dioxanone)

Poly(p-dioxanone) (PPDO) is one of the most prospective biodegradable aliphatic polyesters not only because of its outstanding biodegradability, bioabsorbability and biocompatibility, but also because of its good mechanical properties. It has been used in medical, such as bone or tissue fixation device and drug delivery system. Furthermore, PPDO also has great potentiality for general uses such as films, molded products, laminates, foams, non-woven materials, adhesives, and coatings.Until now, PPDO was synthesized by ring-opening polymerization of p-dioxanone (PDO) with effective catalysts such as organic-tin, organic-aluminum, organic-zinc, organic rare earth compounds, and enzyme etc. Due to the critical reaction conditions such as high purity of monomer PDO and high activity of catalysts, etc., it is difficult to obtain high-molecular-weight PPDO that is suitable for preparing a series of biodegradable polymeric materials with good mechanical properties. How to obtain higher molecular weight PPDO has attracted great interest of researchers in recent years.Using chain extenders to achieve high molecular polymers is an effective way. Nowadays, the chain extending reaction in the synthesis and modification of the polymeric materials has been widely used. However, the synthesis of high- molecular-weight PPDO polymers by chain-extending reaction hasn't been reported in literature, as far as we know.In this investigation, PPDO polymer, with higher molecular weight, was synthesized by a two-step process: a ring-opening polymerization of PDO using stannous octoate (SnOct2) as a catalyst, followed by an increase in molecular weight through reaction with toluene-2, 4-diisocyanate (TDI). Several instruments such as infrared (IR), nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), wide-angle-X-ray diffractometry (WAXD), hot-stage polarizing optical microscope (PM) and thermogravimetry analysis (TG) have been employed to investigate the structure-properties relationship of PPDO prepolymer and chain-extended PPDO.When intrinsic viscosities of PPDO prepolymers are low ( [η]=0.17dL/g,0.45dL/g), PPDO with higher intrinsic viscosities can be obtained easily after chain extension at 150(C within twenty minutes. However, it is difficult for PPDO prepolymers with high intrinsic viscosities ( [η]=1.50dL/g,2.08dL/g) to react in such way because the increment of intrinsic viscosities reduces the molecular activities and the numbers of free chain ends. However, PPDO with higher intrinsic viscosities can still be obtained through increasing reaction temperature and reaction time. PPDO with an intrinsic viscosity of 3.72dL/g has been achieved by chain-extending of PPDO prepolymer with an intrinsic viscosity of 2.08dL/g using TDI as a chain extender at 180(C for 4 hours, which reflects that this is an effective method to obtaining high-molecular-weight PPDO. IR and NMR have been used to characterize the structure of PPDO prepolymer and chain-extended PPDO.PM, DSC and WAXD have been utilized to investigated the crystallization and morphology of PPDO and chain-extended PPDO. Chain-extended PPDO can also develop well-defined spherulites with a clear Maltese cross and bands while cooling from the melt. From the DSC data, we can see that the melting temperatures (Tm ) of chain-extended PPDO are not obviously changed. From the wide X-ray diffraction curves it can see that the chain-extended PPDO has the same peaks at 2( as that of PPDO prepolymer.Thermal property of PPDO and chain-extended PPDO has been investigated by TG. The results showed that the thermal stability of PPDO after chain extension is higher than that of pure PPDO, which is inflected by the higher value of initial decomposition temperature and maximum decomposition temperature. The results of thermal-oxidative degradation experiment also showed that thermal stability of PPDO could be improved by chain extending with TDI.