Asymmetric Regio- And Stereo-selective Alternating Copolymerization Of CO₂ And Epoxides

Carbon dioxide (CO₂) is the only carbon source of all the biomass and can transform to the carbohydrate through photosynthesis, but excessive CO₂ from industrial emission leads to the greenhouse effect.Therefore, recycling CO₂ and chemical fixation of CO₂ are receiving increased attention. It is very attractive to establish the synthetic methodology of using CO₂ as starting material for the synthesis of small organic molecular and especially polymer. The coupling of epoxide and CO₂ to produce the biodegradable polycarbonate has been regarded as the most promising method, which was first reported by Inoue and his coworkers in 1969. Although there is growing effort to develop the effective catalyst for the copolymerization of CO₂ and epoxide, some problems still plague these catalyst systems, such as low catalyst activity, inferior polymer selectivity, irregular regio- and stereo- chemistry and so on. In this dissertation, it is found that the binary catalyst systems composed of the chiral tetradentate Schiff base cobalt complex [SalcyCo(â…¢)â…©] as the electrophlie and ionic ammonium salt or sterically hindered strong base as the nucleophile can effectively promote the asymmetric copolymerization of CO₂ and racemic propylene oxide(PO) to produce poly(propylene carbonate) (PPC) with excellent activity, relative high enantioselectivity(ee%≈70%),＞95% head-to-tail connectivity, and＞99% carbonate linkages.The systematical investigation of the effects of steric and electronic factor of catalyst is carried out. Indeed, the characters of both electropile and nucleopile dramatically influence the copolymerization reaction. Generally, a bulky chiral SalcyCo(â…¢)â…©complex with an axial group of poor leaving ability as the electrophile, in conjunction with a bulky ionic ammonium salt consisting of a bulky cation and a nucleophilic anion with poor leaving ability as the nucleophile, composes the ideal binary catalyst system for the copolymerization of CO₂ and rac-PO. The system of SalcyCo(â…¢)â…©/PPNCl exhibits very high activity of TOF＞530h^-1 at 25℃and 1.5MPa CO₂. A sterically hindered strong organic base with low coordinating ability and a bulky electrophile of chiral SalcyCo(â…¢)â…©compose the ideal binary catalyst system for asymmetric living copolymerization of CO₂ and rac-PO with improved enantioselectivity.The binary catalyst system of SalcyCo(â…¢)â…©with MTBD or PPNCl can operate effectively even at 0℃and with CO₂ at atmospheric pressure. Up to now, it is the first catalyst system that can keep excellent activity and polymer seclectivity under such condition. Of surprise, the elevated temperature does not result in much lost in polymer selectivity, regio- and enantio-selectivity. In the presence of (R,R)SalcyCo(â…¢)â…©and PPNCl, various terminal aliphatic epoxide with longer chain and CO₂ form the polycarbonate with unprecedented regio-regularity of head-to-tail content of＞98%.Electrospray ionization mass spectrometry (ESI-MS) in combination with kinetic and in situ FT-IR spectra study can provide a straightforward approach to insight into the details of copolymerization of CO₂ and epoxide. It is clearly suggested that polymer chain growth predominately occurs in the nucleophile and the coordination activation of epoxide by the electrophile plays a very important role in the copolymerization process. Additionally, we tentatively assume that both the enantiomorphic site effect resulting from chiral electrophile and the polymer chain end effect coorperatively control the stereochemistry of CO₂/epoxide copolymerization process, but the former effect should be predominant.The binary catalyst of (R,R)SalcyCo(â…¢)â…©and PPNCl is also very effective in terpolymerization of PO/CHO/CO₂ in mild condition. Interestingly, the result from mass spactra study shows that the two different carbonate unites predominately arrange with an alternative nature in the terpolymer main chain. Of importance, the resulting terpolymer exhibit only one glass transition temperature, which can be easily adjusted in a wide temperature range(50-100℃) by changing the composition of terpolymer.