Study On Copolymerization Of CO₂ With Epoxides Over Organo-zinc/cobalt Complex Catalyst System

With the societal and industrial development, there is a dramatical increase in the globle emission of CO2. As a main green house gas, the increasing CO2 concentration exacerbates climate change, leading to the environmental issues of glaciers melting, climate warming and land draining. In consequence, people began to realize the importance of CO2 conversion and utilization. In 1969, Inoue and coworkers used ZnEt2/H2O successfully catalyzed CO2 and propylene oxide copolymerization. Since then, a growing number of scientists started to work on this field and develop catalysts with both high activity and selectivity. In this paper, copolymerization of CO2 and epoxides with different organo-zinc/cobalt catalyst systems and reaction conditions are systemlly investigated by experimental and theoretical studies, the main contents are as follows:First, the effect of ZnEt2-glycerine-Y(CCl3COO)3 rare earth ternary complex on the copolymerization of CO2 and propylene oxide was investigated by experimental and DFT calculation methods. The results showed that the introduction of TMAF in the ZnEt2-glycerine-Y(CCl3COO)3 rare earth ternary complex could dramatically increase the activity of CO2 and propylene oxide copolymerization. TMAF facilitated the mutual solubility of CO2/propylene oxide and heterogeneous ZnEt2-glycerine-Y(CCl3COO)3 catalyst system, making more active centers expose to the copolymerization monomer and resulting in the increasing rate of CO2/propylene oxide coordinative insertion. The use of TMAF facilitated the insertion of CO2, but showed no significant effect on the selectivity of ZnEt2-glycerine-Y(CCl3COO)3 catalyst system. Instead, this facilitated the polymer chain growth process and the molecular weight appeared to rise sharply. In addition, the introduction of TMAF in ZnEt2-glycerine-Y(CCl3COO)3 catalyst system decreased the catalyst induction period for about 80%, the reason was that TMAF interacted with zinc metal center and reduced the electron density (increase of electron deficiency), facilitating the the ability of the coordination and ring-opening process in zinc active center and propylene oxide. The DFT calculation results of the initial step of CO2/propylene oxide copolymerization demonstrated that the insertion of propylene oxide was proposed to overcome high Gibbs free energy barrier and this process is the rate-determining step of CO2/propylene copolymerization. However, the Gibbs free energy barrier of CO2 insertion was low, leading to possibility of reverse reaction. Hence the chain growth reaction of CO2/propylene oxide copolymerization was more inclined to proceed on the ZnEt-OR active center. For cyclic chain growth model, the steric hindrance of cyclic alkyl zinc oxide increased with the insertion of CO2 and propylene oxide, hindered the further insertion of CO2 and propylene oxide to afford polymer. But the cyclic chain growth mechanism was also an evidence for the obtained cyclic propylene carbonate. The DFT calculation results further proved the experimental data.Second, the effect of different synthesis methods on the particle size and crystallinity of ZnGA, the effect of particle size/crystallinity and the ZnGA/DMC composite catalyst system on CO2 and propylene oxide copolymerization were systematically studied. The results demonstrated that the use of non-ionic surfactant NP-40 dramatically decreased the mean volume size and mean surface size, and increased the specific surface area of ZnGA particle. In addition, the crystallinity of ZnGA was also improved. Compared to the traditional ZnGA catalyst, the one with bigger specific area and smaller particle size obtained by NP-40 showed much higher activity, but the carbonate linkage content stayed the same. Poly(propylene oxide) with rather high molecular weight and good thermostability was successfully obtained. The ZnGA/DMC composite catalyst was obtained by cacination, which further facilitated the activity of CO2 and propylene oxide copolymerization with carbonate linkage content more than 97.7% and good thermostability. Compared to the ZnGA/DMC mixing system, ZnGA/DMC composite catalyst system presented not only high activity characteristic of DMC, but also high carbonate linkage and molecular weight characteristic of ZnGA. By characterization, the carbonate linakge content of final product was about 99.9%which further proved that the polymer was the perfect alternating copolymerization product poly(propylene carbonate).Finally, the copolymerization of CO2 and cyclohexene oxide over DMC/salen-Co(III) acetate dual complex catalyst system was investigated under supercritical CO2 conditons. The results indicated that supercritical CO2 effectively facilitated the mass and heat transfer of CCVcyclohexene oxide copolymerization system. Hence, the activity was significantly improved. Moreover, the use of supercritical CO2 increased the selectivity and the carbonate linkage content of final product. The thermostability increasing trend of poly(cyclohexene carbonate) were all consistent with the carbonate linkage content. Supercritical CO2 increased the molecular weight of poly(cyclohexene carbonate) as well, which could not be affected by different reaction temperature and time. However, there are isotactic and syndiotactic stuctures of poly(cyclohexene carbonate), demonstrating that supercritical CO2 had poor stereo-control to alternating copolymer poly(cyclohexene carbonate). In addition, supercritical CO2 significantly decreased the catalyst induction period. In the traditional poly(cyclohexene carbonate) purification process, organic solvents such as methanol and ethanol were environmentally hazardous. Therefore, using supercritical CO2 for final product purification had potential application value. The supercritical CO2 flushing experiment indicated that superciritcal CO2 could effectively purge the final product in situ in an environmentally friendly way. The DMC/salen-Co(III) acetate dual catalyst could be recycled by a simple decompression process.