Synthesis And Characterization Of Biodegradable Polymers By Novel Rare Earth Catalysts

Ring-opening homopolymerization of e-caprolactone (ε-CL), D,L-lactide (DL-LA), L-lactide (L-LA), 2,2-dimethyltrimethylene carbonate (DTC) and copolymerization of 2,2-dimethyltrimethylene carbonate and e-caprolactone have been carried out by using single component rare earth tris(4-tert-butylphenolate)s (Ln(OTBP)3) initiators for the first time. It have been found that rare earth tris(4-tert-butylphenolate)s are effective initiators for these polymerizations. Characteristics and kinetics of these homopolymerizations and the copolymerization initiated by Ln(OTBP)3 have been examined in detail. The structures and properties of the polymers such as poly(s-caprolactone) (PCL), poly(D,L-lactide) (PDLLA), poly(L-lactide) (PLLA), poly(2,2-dimethyltrimethylene carbonate) (PDTC) and poly(2,2-dimethyltrimethylene carbonate-co-s-caprolactone) [P(DTC-co-CL)j obtained by Ln(OTBP)3 were characterized and analyzed thoroughly by GPC, DSC, 'HNMRand13CNMR.Various Ln(OTBP)3 were applied to the homopolymerization of CL, the influence of the rare earth elements and solvents on the polymerization were investigated. It was found that La(OTBP)3 shows the highest activity for CL polymerization in toluene. The optimum conditions are as follows: [CL] = 2.70 mol/1, [CL]/[La] =1000 molar ratio, 60℃, 1.5 h, toluene. Under these conditions, PCL with Mn of 4.17 x 104 is obtained and the catalytic efficiency of lanthanum is 1.16 ℃ 105g PCL/mol.La. The PCL obtained with La(OTBP)3 has a melting temperature of 70癈 and 65% crystallinity. GPC measurement showed that cyclic oligomers were not presented in the polymer. The kinetic study indicated that the CL polymerization is first order with respect to the initiator concentration and monomer concentration. That is: Rp = kp[CL][ La(OTBP)3], at 60℃, kp = 33.3 1 mol"1 min"1. The overall activation energy of CL polymerization with La(OTBP)3 amounts to 51.9 kJ/mol. End-group analysis showed that CL inserted into the growing chains with the acyl-oxygen bond scission.IVThe polymerization of D,L-LA with Ln(OTBP)3 were conducted. La(OTBP)3, Gd(OTBP)3 and Nd(OTBP)3 are effective initiators for the D,L-LA polymerization. La(OTBP)3 exhibits the highest activity with the catalytic efficiency about 1.44 x l O5 g PDLLA/mol.La. Influences of different reaction conditions on the polymerization were studied, and the optimum conditions for the D,L-LA polymerization by La(OTBP)3 can be described as: [D,L-LA] = 1.0 mol L'1, [D,L-LA]/[La] - 1000 molar ratio, 90 癈, 50mm, hi toluene, under which PDLLA with Mn = 5.3 x 104 can be obtained. It can be seen that in the !H NMR the resonance peaks due to iss, sss and ssi tetrads are observed with very low intensity, indicating that transesterification is very slow at temperatures 90 . From DSC curves only one glass transition can be observed, suggesting that PDLLA prepared with La(OTBP)3 are completely amorphous and that La(OTBP)3 shows no selectivity toward the two isomers. The kinetics study indicates that the polymerization rate is first order with respect to D,L-LA concentration and initiator concentration, that is: Rp = p[D,L-LA][La(OTBP)3]. At 90癈 the polymerization rate constant Jfcp is 40.4L mol'1 min'1. The overall activation energy of the polymerization is 81.7kJ mol"1.Ring-opening polymerization of L-LA initiated by Ln(OTBP)3 has been conducted. It has be found that La, Gd, Nd and Y compound are effective initiators for the ring-opening polymerization of L-LA and can prepare PLLA with 100 % optical purity. The catalytic activity of various rare earth initiators for the polymerization has following order: La > Gd > Nd > Y > Er. Investigations on the effect of solvent on the polymerization of L-LA with La(OTBP)3 showed that L-LA can be polymerized effectively to give PLLA with full optical activity in all the solvents used. The characteristics of L-LA polymerization catalyzed by La(OTBP)3 were intensively studied and the optimum conditions for the polymerization are: [L-LA] = 1.5 mol L"1, molar ratio of [L-LA]/[La] = 1000, 70 癈, 110 min, in toluene. The resul...