Copolymerization Of CO₂ And Epoxides Under Aluminum Porphyrin Catalyst

Fixation of carbon dioxide into polymers is becoming increasingly attractive in polymer chemistry,among which ring-opening copolymerization(ROCOP)of propylene oxide(PO)and CO2 is an industrially viable reaction.Though heterogeneous catalyst like rare earth ternary catalyst has been industrialized,the catalytic activity as well as polymer selectivity need fiurther improvement.The past 2 decades witnessed the great progress in homogeneous catalyst;especially,(Salen)Co(?)X displayed high catalytic activity and excellent polymer selectivity.However,the toxicity nature of cobalt keeps it away from industry application,since the cobalt residue in biodegradable plastics is not acceptable,especially for the soil compostable requirement in which the toxic cobalt is strictly limited.In the past decade,aluminum(?)porphyrin complex has shown potentiality as environmental friendly catalyst for copolymerization of CO2 and PO,there are great space to raise the activity and polymer selectivity.In this thesis,we developed a series of porphyrin aluminum complexes for copolymerization of CO2 and PO,focusing on highly active and selective catalyst system.The main results are summarized as follows:1.To improve the chemical selectivity of porphyrin aluminum,we introduced a dimethanoanthracen substituent to increase steric hindrance in porphyrin aluminum complexes 4 and 5,while bis(triphenylphosphoryl)ammonium chloride([PPN]Cl)was used as cocatalyst.Both catalyst systems afforded PPC with 99%carbonate linkage,where complex 5 completely inhibited the formation of polyether,indicating that bulky steric ligands can significantly improve the PPC selectivity in PO/CO2 copolymerization.The head-to-tail segment(HT%)of PPC was significantly improved,e.g.,PPC from complex 5 showed regioselectivity of 92%,which was the highest record among the aluminum porphyrin catalyst systems.2.To regulate the product selectivity,the porphyrin aluminum catalyst with temperature response selectivity was designed.For complex 5,a critical temperature from transition of complete coupling reaction to copolymerization reaction was observed at 75,it promoted cyclic carbonate formation above 75? and turn-over frequency(TOF)reached 5352 h-at 120?.Copolymerization reaction became dominant below 50?,especially,the PPC selectivity reached 99%at 25?.In addition,porphyrin aluminum complexes were synthesized possessing a gradient of electron donating ability by introducing small volume substituents such as methoxy and halogen for eliminating the influence of steric hindrance.Among which complex 6 exhibited the same selectivity as complex 5,indicating that the temperature-responsive selectivity highly relied on strong electron donating effect of substituent in porphyrin ligand,not the steric hindrance.3.In pursuit of metal catalysts with multiple cooperativity,polynorbornene via ring-opening metathesis polymerization was selected as a platform for constructing oligomeric aluminum catalysts,where complex 14b afforded TOF value of 4574 h-1 at 110? with selectivity of 90%,producing PPC with 40.4%carbonate unit.Halogen substituted oligomeric porphyrin aluminum complexes were designed,among which bromo-substituted complex 15 exhibited the best catalytic performance even at very low catalyst loading,achieving TOF value up to 8110 h'1 with>99%polymer selectivity,it should be noted that thus obtained PPC was ether-rich polycarbonate.The high activity for catalyzing PO homopolymerization resulted in the high ether content of the copolymerization product.4.To identify the synergistic effect of the enrichment of active center,three oligomeric porphyrin aluminum complexes 17-19 with different link lengths were synthesized,the catalytic activity decreased with the increasing length of spacer segment,indicating that weakening active center synergistic effect resulted lower catalytic performance.