Study On Carbon Dioxide/Epoxides Copolymerization With Rare-earth Ternary Catalyst And Its Mechanism

Polymerization involving carbon dioxide is one of the most important utilizations of carbon dioxide. In this work, we explored two kinds of rare-earth catalytic systems for CO2/epoxide copolymerization producing aliphatic polycarbonates with high molecular weight and carbonate content. The polymerization features were examined in detail and preliminary investigations on the copolymerization mechanism have been carried out through the structural analysis of resulted polymerization products.A new ternary catalyst composed of lanthanide borohydrides, diethylzinc, and glycerin was found to be effective for the copolymerization of propylene oxide with carbon dioxide. Among them, Y(BH4)3-3THF/ZnEt2/glycerin was most effectively for the copolymerization to afford poly(propylene carbonate)s (PPC) having a carbonate content more than85%in most cases. The weight-average molecular weights were observed in a range between6.6×104and49.6×104with Mw/Mn ranging from2.0to9.1. The orthogonal tests were used to optimize catalyst composition and polymerization conditions. At an appropriate combination of the variables, the highest yield value of23190g-polymer/mol Y and60260g-polymer/mol Y could be obtained in the solution and bulk polymerizations, respectively. The influences of solvent on catalyst structure and copolymer region-structure were investigated detailedly. NMR technique was employed to analyze the interaction between Y(BH4)3and solvent. It was found that DME could replace THF to coordinate Y and Y(BH4)3reacted with PO to form Y(O’Pr)3regardless of the preceding solvent added. Thus, we postulated that solvent could not change catalyst structure and its regionselectivity.Combination of rare-earth phosphonates (Ln(P204)3) with ZnEt2and glycerin formed another effective catalyst for CO2/epoxide copolymerization. Ln(P204)3-ZnEt2-Gly showed an improved stability to moisture compared to Ln(BH4)3-based system. It was found that Y(P204)3-based catalytic system exhibits high activity in the CO2/PO polymerization at60-100℃under0.5-3.5MPa CO2pressure to produce PPC with high yield (120845g-PPC/mol-Y) and carbonate linkage (49.5-82.8%) and cyclic propylene carbonate (6-50.8%). On the basis of ESI-MS spectra analysis of polymer degraded by LiO Bu and low molecular weight polycarbonates obtained with different catalytic systems, a possible polymerization mechanism was proposed, in which in-situ formed Y-Zn bimetallic active species would contain hydroxyl and ethyl initiating groups.Y(P204)3-ZnEt2-Gly has been successefully used for CO2/PO/CHO copolymerization to afford the desired ter-polymer with a high glass-transition temperature. DSC data demonstrated that PPC easily degraded at127℃to produce cyclic propylene carbonate. Using the catalytic system, a ter-polymer (PCm) was synthesized by CO2/PO copolymerization in the presence of glycidyl methacylate as third monomer. Then, an amphiphilic network was prepared by radical polymerization of N-isopropylacrylamide (NIPAM) with PCm as macro-monomer. The resulted amphiphilic material has proven to be hydrogel with an equilibrium swelling ratio734%for water.Also, a scale-up experimental setup was founded for the evaluation of CO2/PO copolymerization with rare-earth catalytic systems.