Application Of Late Transition Metal Catalysts In Ethylene Polymerization

With the advancement and development of science and technology,the demand for polyolefin materials is increasing,and polymers with different topological structures are closely associated with the structure of catalysts.From the initial Ziegler-Natta catalysts and metallocene catalysts to the later non-metallocene catalysts and late transition metal catalysts,the breakthrough of the polyolefin industrial revolution is inseparable from the design of new catalyst structures.The core of coordination polymerization is still the structure of the catalyst.This dissertation mainly focused on two classic late transition metal catalyst systems(α-diimide nickel palladium system and salicylaldimine nickel system),and explored the effect on olefin polymerization by changing the ligand structure of the catalysts.The details are as follows:(1)A class of unsymmetrical α-diimine nickel catalysts was designed and synthesized by combining large sterically hindered(CHPh2)and small sterically hindered(H)substituents.This type of catalyst has high polymerization activity but low molecular weight,and can realize the transformation from highly linear semi-crystalline polyethylene plastic to highly branched polyethylene oil through temperature control.These catalysts enable "tandem" reactions through the terminal Friedel-Crafts alkylation of polyethylene to achieve in situ functionalization.In addition,these catalysts can also polymerize ethylene in different polar solvents(hexane,chlorobenzene,anisole)and the Friedel-Crafts alkylation was more likely to occur in electron-donating aromatic solvents.(2)A series of α-diimide nickel catalysts with polyethylene glycol groups(PEG)were designed and synthesized.Through the interaction of ligand structures with aluminum alkyls,these catalysts can effectively control the polymer properties.With the growth of the PEG chain,the polymerization activities increased by about 7 times,and the branching degrees could be reduced from 106/1000C to 19/1000C.This result suggests that the transition from amorphous polymers to highly linear polymers can be achieved by controlling the PEG chain length.It is supposed that the interaction between the PEG chain and the alkyl aluminum effectively inhibits the chain walking process.In addition,the Nickel catalyst also exhibits the characteristics of living polymerization.This work provides a new secondary coordination approach for olefin polymerization to prepare polymers with different topologies.(3)A new class of large sterically hindered asymmetric a-diimide nickel-palladium catalysts Ipty/iPr-Ni and Ipty/iPr-Pd were designed and synthesized,by the combination the pentiptycenyl/2,6-diisopropylaniline with bulky dibenzobarrelene group.Under the activation of MMAO,the nickel catalyst exhibited high polymerization activity(1.4 × 107 g mol-1 h-1)and ultra-high molecular weight(166.3× 104 g mol-1).Compared to the symmetrical Ipty-Ni and ’Pr-Ni catalysts,the unsymmetric catalyst had a larger steric hindrance,an 8.5-fold and 3.2-fold improvement in polymerisation activity respectively,and the molecular weight is increased to millions.In the copolymerization of ethylene with methyl 10-undecanoate,the insertion rate of polar monomers can reach 0.41%.In terms of palladium catalyst,the polymerization activities approach 1.24 × 106 g mol-1 h-1,the branching degrees show a special downward trend.Moreover,it can still maintain moderate activities at high temperature of 120℃.In copolymerisation ethylene with methyl acrylate,polar monomer insertion rates reach to 1.98%.and polymers with two polar monomer insertion modes(main chain insertion and terminal insertion)can be accessed,which can be regulated by temperature.(4)A series of novel trinuclear symmetric salicylaldimine neutral nickel catalysts Ni-iPr,Ni-Ph and Ni-Cs were designed,synthesized and characterized accordingly.Under the activation of the cocatalyst Ni(COD)2,three catalysts can be polymerized under low pressure ethylene,and their polymerization activities can reach to 1.21×106 g mol-1 h-1.The Ni-iPr and Ni-Ph exhibited multiple active centers during the polymerization process,indicated a wide molecular weight distribution(PDI=3.5-7.1),and the resulting polymers had low molecular weights and internal double bond structures.In contrast,Ni-Cs catalyst exhibits double sandwich structure that effectively inhibits chain transfer reactions,which has a single active centre,narrow molecular weight distribution(PDI=1.9-2.6),high thermal stability,and molecular weights up to 136 kg mol-1.In addition,ethylene polymerization can carry out in the presence of polar additives.