Design Of Trinuclear Catalysts And Their Application In Epoxide Copolymerization

As an important organic chemical raw material,epoxide plays a crucial role in modern chemical industry with low price and abundant varieties.Because of its green chemical characteristics,the preparation of degradable polyesters and polycarbonates by ring-opening copolymerization of epoxides with anhydrides or CO₂ has been the focus of scientific attention.However,there are still some problems in this field,such as insufficient catalyst activity and poor material properties,which seriously restrict the practical application of the above processes.Compared with mononuclear and dinuclear catalysts,trinuclear catalysts have more advantages in catalytic activity and sequence control with more metal active centers.So,aiming at above problems,in this contribution,various trinuclear complexes are designed to increase the reactivity of the copolymerization of epoxides with cyclic anhydrides,and to realize the regulation of polyester or polycarbonate properties.（1）Herein,trinuclear Salph Cr^Ⅲcomplexes based on different ligands,in conjunction with a nucleophilic cocatalyst,are demonstrated as excellent catalysts for the copolymerization of cyclohexene oxide（CHO）and phthalic anhydride（PA）,selectively affording perfectly alternating polyesters.Trinuclear chromium complex S3 is found to be the most efficient catalyst for the copolymerization among the above catalysts.In particular,a high activity of10620 h^-1 is obtained for the copolymerization under a low catalyst loading（<0.01 mol%）at100°C.Kinetic experiments are performed using in situ FTIR spectroscopy.And a reaction order of 1.06 is obtained from S3,which is consistent with the first-order reaction.The kinetic experiments also suggest that the activation energy of the trinuclear chromium complex S3 for CHO/PA copolymerization is 15 k J/mol lower than that of the corresponding mononuclear catalyst owing to an intramolecular synergistic effect among the metal atoms.The structures of trinuclear complexes are calculated by density functional theory（DFT）.And the Cr-Cr distances of trinuclear chromium complex S3 are approximately 7.3（?）,which proves that the distances between the chromium species are closely related to catalytic activities.（2）In this study,trinuclear Salph Co^III complexes based on different ligands are designed,among which complexes T5 with flexible linking groups prefer consecutive epoxide insertion for the formation of AB_n（n=2,3）ether sequences in the ring-opening copolymerization of phthalic anhydride（PA,A）with propylene oxide（PO,B）.This new class of polymers featuring controlled AB_n sequences cannot be obtained using traditional catalysts,which tend to favor alternating AB structures.Research shows that the formation of AB_n segments during copolymerization is proposed to occur through a bimetallic mechanism,while alternating AB sequences mainly proceed via a monometallic process.The sequence-controlled polyesters have evenly distributed backbone ether groups,resulting in a single-glass transition and narrow thermal decomposition range.The glass transition temperature and thermal stability can be regulated by changing the proportion of AB_n sequence.（3）The sequence-controlled polycarbonates with AB_n sequence are successfully prepared by the copolymerization of CO₂（A）with propylene oxide（PO,B）using trinuclear complexes T5.The polymers have evenly distributed backbone ether groups,and the proportion of AB_nsequence remains unchanged in the reaction process.The structure and property of sequence-controlled polycarbonates can be regulated by changing the reaction temperature,due to the proportion of AB_n sequence decreasing with reaction temperature increment.