Design And Synthesis Of Binary Catalyst Systems For The Asymmetric Copolymerization Of CO₂ And Propylene Oxide

The tetradentate Schiff-base compounds (designated as Salen ligand), formed by thecondensation of salicylhydride or its derivatives and various diamino compounds, can easilycomplex with transition metal ions. Many efforts has been denoted to the use of binarycatalyst system consisted of a SalenM(â…¢) complex and an ionic ammonium salt or organicbase in the copolymerization of CO₂ and propylene oxide. The improved catalytic activity aswell as high polymer selectivity and narrow molecular weight distribution were usuallyobtained in these systems. Prominent among these binary catalysts systems being the mostefficient for CO₂/epoxides copolymerization and in some cases approach to the regio- and/orstereo-selective polymerization. But there still remain some problems to be further solved,such as the low enantioselectivity (ee value) of poly(propoylene carbonate) with regard to thelow kinetic resolution coefficient (K_rel).In this thesis, on the basis of "double chiral centers induction", a chiral bicyclic guanidinewas first synthesized and used as chiral nucleophile of binary catalyst system based on chiralSalenCo(â…¢)NO₃ as electrophile for the asymmetry copolymerization of CO₂ and racemicpropylene oxide. Unfortunately, compared with the result of the previous work in our group,the enantioselectivity of the resulted polycarbonates did not show observable improvement,probably because the chiral induction effect of chiral bicyclic guanidine to propylene oxideonly affects at the very beginning of the copolymerization due to the "polymer chain endcontrol" mechanism. Also, the synthetic process of the chiral bicyclic guanidine was furtheroptimized.Furthermore, a new chiral SalenCo(â…¢)complex was designed and synthesized. By theformation of Co-O bond of the phenyloxyl in the diamine backbone to the central metal ion,the twist of the diamine backbone happened, and the other substituent group of the diaminebackbone could be forced to axial position thus to increase the steric hindrance. The resulting(S,S)-SalenCo(â…¢) complex in conjunction with an ionic ammonium salt was applied inasymmetry copolymerization of CO₂ and racemic propylene oxide, but all the catalyticactivity, K_rel and polymer selectivity are low. These results are reasonable assumpation thathigh steric hindrance of the (S,S)-SalenCo(â…¢) complex and lack of leaving axial group affect the coordination of propylene oxide. In addition, the detailed investigations on the syntheticprocesses of some key intermediates offer an economical route for synthesizing the chiralSalenCo(â…¢)complex.