Bimetallic Effects for Enhanced Polar Comonomer Enchainment Selectivity in Catalytic Ethylene Polymerization

摘要

The synthesis and characterization of the bimetallic 2,7-di-[(2,6-diisopropylphenyl)imino]-1,8-naphthalenediolato group 10 metal polymerization catalysts {[Ni(CH_3)]_2[1,8-(O)_2C_(10)H_4-2,7-[CH=N(2,6-~iPr_2C_6H_3)](PMe_3)_2} and {[Ni(1-naphthyl)]_2[1,8-(O)_2C_(10)H_4-2,7-[CH=N(2,6-~iPr_2C_6H_3)](PPh_3)_2} [FI~2-Ni_2(PR_3)_2] are presented, along with the synthesis and characterization of the mononuclear analogues {Ni(CH_3)[3-~tBu-2-(O)C_6H_3CH=N(2,6-~iPr_2C_6H_3)](PMe)_3} and {Ni(1 -naphthyl)[3-~tBu-2-(O)C_6H_3CH=N(2,6-~iPr_2C_6H_3)](PPh)_3} [FI-Ni (PR_3)]. Monometallic Ni catalysts were also prepared by functionalizing one ligation center of the bimetallic ligand with a trimethylsilyl group (TMS), yielding {Ni(CH_3)[1,8-(O)(TMSO)C_(10)H_4-2,7-[CH=N(2,6-~iPr_2C_6H_3)](PMe_3)} [TMS-FI~2-Ni(PMe_3)]. The FI~2-Ni_2 catalysts exhibit significant increases in ethylene homopolymerization activity versus the monometallic analogues, as well as increased branching and methyl branch selectivity, even in the absence of a Ni(cod)_2 cocatalyst. Increasing ethylene concentrations significantly suppress branching and alter branch morphology. FI~2-Ni_2-mediated copolymerizations with ethylene + polar-functionalized norbornenes exhibit a 4-fold increase in comonomer incorporation versus FI-Ni, yielding copolymers with up to 10% norbornene copolymer incorporation. FI~2-Ni_2-catalyzed copolymerizations with ethylene + methylacrylate or methyl methacrylate incorporate up to 11% acrylate comonomer, while the corresponding mononuclear Fl-Ni catalysts incorporate negligible amounts. Furthermore, the FI~2-Ni_2-mediated polymerizations exhibit appreciable polar solvent tolerance, turning over in the presence of ethyl ether, acetone, and even water. The mechanism by which the present cooperative effects take place is investigated, as is the nature of the copolymer microstructures produced.