Study On Linear-comb/Star-comb Highly Branched Aliphatic Polyesters

Aliphatic polyesters, a kind of environmentally friendly green polymer material, have a wide range of applications in biomedical field owing to their excellent biodegradation, biocompatibility and bioabsorbability. So far, most of the biodegradable aliphatic polyesters are linear, star, dendrimer and hyperbranched polymer with the shortcomings of low molecular weight, lose control of branched structure and the complicated synthetic process. Therefore, it has an important significance in research and applications to develop a simple method for the synthesis of well-defined polyesters with high molecular weight and highly branched structure. Furthermore, the requirements are very strict for aliphatic polyesters as biomedical materials. In general, strong acids/bases are used to catalyze the ring-opening polymerization (ROP) of cyclic esters which have a negative impact on the reaction and products. Thus, it is of great importance in biomedical applications to further explore new biocompatible catalysts those are suitable for ROP of cyclic esters.In this thesis, linear-comb/star-comb poly(?-caprolactone) (PCL), poly(<?-valerolactone) (PVL), poly(trimethylene carbonate) (PTMC) and poly(?-caprolactone)-block-polylactide (PCL-b-PLA) were synthesized by ring-opening polymerization using linear/star hydroxylated polybutadiene as macroinitiator. The influence of highly branched structure on the properties of polymers was investigated by a systematic study of physicochemical properties of the above aliphatic polyesters. A new biocompatible boric acid catalyst was used in the ROP of cyclic esters, and then the studies of polymerization kinetics and polymerization mechanism were carried out. The main contents and results are as follows:(1) Linear-comb and star-comb PCL (LC-PCL and SC-PCL) with highly branched structure, high molecular weight, narrow dispersity and controlled branching level were synthesized simply using linear/star hydroxylated polybutadiene as macroinitiator. The weight-average molecular weight Mw of PCL reached up to 276 KDa with narrow dispersities (<1.15)and the branching factor g'= 0.24. The influences of side chain length on the thermal properties and crystal structure of PCL were studied:with the increase of side chain length, the crystallization temperature (Tc) and melting point (Tm) of PCL increased which indicated that the crystallization capacity of PCL was improved; the degree of crystallinity (Xc) increased and the crystallite dimension (D) decreased. Then the influences of different branched structures (linear, star, linear-comb and star-comb) on the intrinsic viscosity, rheological behavior and crystallization property of PCL were investigated comprehensively, it showed that:the intrinsic viscosity of PCL decreased with the degree of branching increasing; the order of complex viscosity(?*) for PCL with different structures was L-PCL> S-PCL> SC-PCL> LC-PCL; with the degree of branching increasing, the degree of crystallinity (Xc) increased greatly (from 58.0% of L-PCL to 87.8% of SC-PCL) and the crystallite dimension (D) decreased for PCL.(2) Linear-comb and star-comb PVL (LC-PVL and SC-PVL) with highly branched structure, high molecular weight, narrow dispersity and controlled branching level were synthesized simply using linear/star hydroxylated polybutadiene as macroinitiator. The weight-average molecular weight Mw of PVL reached up to 264 KDa with narrow dispersities (<1.15) and the branching factor g'= 0.37. The influences of side chain length on the thermal properties and crystal structure of PVL were studied:with the increase of side chain length, the crystallization temperature (Tc) and melting point (Tm) of PVL increased which indicated that the crystallization capacity of PVL was improved; the degree of crystallinity (Xc) increased and the crystallite dimension (D) decreased. Then the influences of different branched structures (linear, star, linear-comb and star-comb) on the intrinsic viscosity, rheological behavior and crystallization property of PVL were investigated comprehensively, it showed that:the intrinsic viscosity of PVL decreased with the degree of branching increasing; the order of complex viscosity (?*) for PVL with different structures was L-PVL> S-PVL> SC-PVL> LC-PVL; with the degree of branching increasing, the degree of crystallinity (Xc) increased greatly (from 52.3% of L-PVL to 82.7% of SC-PVL) and the crystallite dimension (D) decreased for PVL.(3) Linear-comb and star-comb PTMC (LC-PTMC and SC-PTMC) with highly branched structure, high molecular weight and controlled branching level were synthesized simply using linear/star hydroxylated polybutadiene as macroinitiator. The weight-average molecular weight Mw of PTMC reached up to 364 KDa and the branching factor g'=0.46. The influences of different branched structures (linear, star, linear-comb and star-comb) on the intrinsic viscosity, thermal property and creep behavior of PTMC were investigated comprehensively, it showed that:the intrinsic viscosity of PTMC decreased with the degree of branching increasing; the branched structures had no significant impact on the glass transition and thermal degradation of PTMC; with the degree of branching increasing, the creep strain and creep rate of PTMC increased which indicated that the processability of PTMC was improved.(4) Linear-comb and star-comb PCL-b-PLA (LCPCL-b-PLA and SCPCL-b-PLA) with highly branched structure and high molecular weight were synthesized using linear/star hydroxylated polybutadiene as macroinitiator. The influence of PCL block on the crystallization property of PLA block was studied, it can be seen that:the introduction of PCL block was favorable for PLA crystallization due to the dilution effect of PCL; the crystallization capacity of PCL-b-PLA was the strongest when the molar contents of PCL and PLA were comparable. Then the influences of different branched structures (linear, star, linear-comb and star-comb) on the crystallization property of PLA block were researched, it showed that:star and linear-comb branched structures improved the crystallization ability of PLA block significantly.(5) Anew kind of catalysts, boric acid (1), phenylboronic acid (2),2-thienylboronic acid (3), 2-furanylboronic acid (4) and 5-pyrimidinylboronic acid (5) were used to synthesize PCL, PVL, PTMC and PCL-b-PVL. The outstanding advantage of boric acid catalyst is completely biocompatibility. The studies on polymerization kinetics of ROP of CL catalyzed by boric acid and its derivates confirmed that the reaction rates are first-order with respect to monomer and their catalytic activities follow the order 1>3>2>4>5. "Activated monomer" mechanism was proposed based on comprehensive research.