Alternating Copolymerization and Theoretical Study on CO₂ and Propylene Oxide over Ternary Rare Earth Metal/TMAF Complexes

The copolymerization behavior of CO₂ and propylene oxide(PO) over ternary rare earth metal complex(ZnEt₂-Y(CCl₃COO)₃-glycerine/tetramethylammonium fluoride(TMAF)) were investigated systematically.The copolymerization process and chemical properties were characterized by ¹H-Nuclear Magnetic Resonance(¹H-NMR),Gel Permeation Chromatography(GPC),Differential Scanning Calorimetry(DSC),Thermal Gravity(TG) and in-situ Fourier Transform Infrared Spectrometer(in situ FT-IR).The initial step of CO₂ and PO insertion was calculated by Density Functional Theory(DFT) method.Results showed that the application of TMAF in ZnEt₂-Y(CCl₃COO)₃-glycerine catalyst dramatically increased the catalytic activity up to 4 740.6 g/mol(mass of polymer for 1 mol Zn) and decreased the catalytic induction period from 100 min to 20 min.The theoretical results demonstrated that the rate-determining step of CO₂ and PO copolymerization was the ring-opening/insertion period of PO.The Gibbs free energy of the rate-determining step decreased with the increase of natural bonding orbital of the zinc metal center.In addition,the Gibbs free energy of CO₂ insertion to form "back-bite" cyclic chain growth structure was high.Thus,the copolymerization of CO₂ and PO might follow the linear chain growth mechanism to obtain poly(propylene carbonate).