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

The synthesis and characterization of the bimetallic 2,7-di-[(2,6-diisopropylphenyl)imino]-1,8-naphthalenediolato
group 10 metal polymerization catalysts {[Ni(CH3)]2[1,8-(O)2C10H4-2,7-[CH=N(2,6-iPr2C6H3)](PMe3)2} and {[Ni(1-naphthyl)]2[1,8-(O)2C10H4-2,7-[CH=N(2,6-iPr2C6H3)](PPh3)2} [FI2-Ni2(PR3)2] are presented, along with the synthesis and
characterization of the mononuclear analogues {Ni(CH3)[3-tBu-2-(O)C6H3CH=N(2,6-iPr2C6H3)](PMe)3} and {Ni(1-naphthyl)[3-tBu-2-(O)C6H3CH=N(2,6-iPr2C6H3)](PPh)3} [FI-Ni
(PR3)]. Monometallic Ni
catalysts were also prepared by functionalizing one ligation center
of the bimetallic ligand with a trimethylsilyl group (TMS), yielding
{Ni(CH3)[1,8-(O)(TMSO)C10H4-2,7-[CH=N(2,6-iPr2C6H3)](PMe3)} [TMS-FI2-Ni(PMe3)]. The FI2-Ni2 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. FI2-Ni2-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. FI2-Ni2-catalyzed copolymerizations with ethylene
+ methylacrylate or methyl methacrylate incorporate up to 11% acrylate
comonomer, while the corresponding mononuclear FI−Ni catalysts incorporate negligible amounts. Furthermore, the FI2-Ni2-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.