Ethylene polymerization catalyzed by bridging Ni/Zn heterobimetallics

The effect of proximal Zn halides on Ni-catalyzed ethylene polymerization is reported in this work. A series of (NON)NiLX (NON = 2,6-bis-((2,6-diisopropylphenyl)imino)methyl phenoxide; LX = methallyl or L = py, X = tolyl, 2-4) ethylene polymerization precatalysts have been synthesized, as well as a heterobimetallic Ni/Zn complex, (NON)Ni(C₄H₇)ZnBr₂ (5). Each precatalyst could be activated (or promoted) by ZnX₂ (X = Cl, Br, Et) to polymerize ethylene. In situ recruitment of ZnX₂ by the free imine binding pocket of the NON complexes results in the generation of heterobimetallic active species that produce lower M_n polyethylene than monometallic controls. Room temperature ZnX₂-promoted polymerizations with these catalysts resulted in bimodal M_n distributions that result from different catalyst speciation: "dangling" imine-ligated ZnX₂ species yield higher M_n polymer while N,O-chelated ZnX₂ species yield lower M_n polymer. Running polymerizations at higher temperature yields in only lower M_n polymer resulting from exclusive formation of the thermodynamically favored N,O chelated Ni/Zn heterobimetallic. DFT calculations indicate that this bridging bimetallic complex undergoes β-H elimination more facilely than monometallic Ni analogues, resulting in lower molecular weight polymers.