Controlled Open-loop Polymerization Of Block And Star-shaped Aliphatic Polyester

Dihydroxyl capped biodegradable poly(DTC-b-PEG-b-DTC) triblock copolymer and poly(TMC-b-DTC-b-PEG-b-DTC-b-TMC) pentablock copolymer have been synthesized by PEG and the triblock copolymer as macroinitiator in the presence of yttrium tris(2,6-di-tert-butyl-4-methylphenolate) (Y(OAr)3). The copolymers without random segments have been thoroughly characterized by 1~H, 13~C NMR, SEC and DSC. Molecular weights of obtained copolymers are dependent on the amount of PEGs and coincide with the theoretical values. The exchange reaction of yttrium alkoxide and hydroxyl end group is essential for controlling product's molecular weight. Their thermal behaviors are relevant to the chain lengths of PEG and PDTC segments. Monte Carlo method has been developed to estimate the chain propagation constant and exchange reaction constant. In average, one exchange reaction will occur after approximate six monomer molecules insert into the growing chain.The ring opening polymerization of e-caprolactone (CL) was initiated by glycol and Y(OAr)3, preparing dihydroxy-capped poly(e-caprolactone) (PCL) with controllable molecular weight. !H NMR and SEC analyses indicate two kinds of active species and corresponding PCL with different structures are existed in the system. Increasing the ratio of glycol to Y(OAr)3 benefits the formation of monofunctional active species. PCL obtained from butyl glycol (BG) system and diethylene glycol (DEG) system were compared with that from ethylene glycol (EG) system, which indicates that increasing chain length of glycol benefits the formation of bifunctional active species in the polymerizaton. PEG/Y(OAr)3 system only contains sole bifunctional active species to synthesize copolymer of CL with PEG (poly(CL-b-PEG-b-CL)). Dihydroxy-capped PCL as macroinitiator can further initiate the polymerization of 2,2-dimethyltrimethylene carbonate (DTC). Thus, triblock copolymer of CL with DTC (poly(DTC-b-CL-b-DTC)) has been prepared.Triol/lanthanum tris(2,6-di-terf-butyl-4-methylphenolate) (La(OAr)3) can initiate CL polymerization, preparing three armed star-shaped PCL. The structure of resultant PCL is influenced by the amount and structure of triol added. Mn of the PCL increases with the increasing ratio of CL to triol, which can control Mn of three armed star-shaped PCL. DSC curves shows Tg and Tm of star-shaped PCL increasewith the increasing Mn and are lower than those of linear PCL with similar Mn.Tetraol system can also initiated CL polymerization for the synthesis of four armed star-shaped PCL. The tetraol and triol system are both efficient catalysts to prepare star-shaped PTMC.