Chromium allyl and alkyl catalysts for the vinyl polymerization of norbornene and ethylene–norbornene copolymerizations

International audienceHomoleptic allyl complexes of divalent metals, MII[1,3-C3H3(SiMe3)2]2 (1, M= Cr; 2, M= Fe; 3, M= Ni; 4, M= Co) activated withmethylaluminoxane (MAO) have been tested as catalysts for the polymerization of norbornene. Whereas the iron and cobalt systems werepoorly active, both the nickel and chromium complexes were very productive and gave high molecular weight poly(norbornene)s, althoughthe polymers generated by Cr and Ni differed significantly in the degree of stereoregularity. CrII[1,3-C3H3(SiMe3)2]2 polymerized ethylenein the absence of any cocatalyst but, surprisingly, was deactivated by MAO, whereas 1/B(C6F5)3 showed moderate activity at 20 ◦C under1 bar of ethylene. NMR experiments suggest that B(C6F5)3 acts as a one-electron oxidant to generate the [CrIII(allyl)2]+ cation. Althoughethylene/norbornene copolymerizations were possible with 1/B(C6F5)3, the activity was much lower than in ethylene homopolymerizations,and only limited amounts of norbornene could be incorporated. The catalytic behavior of this Cr(II) precursor contrasts sharply with thatof the Cr(IV) alkyl complex Cr(CH2SiMe3)4 (5)/MAO which polymerizes ethylene but reacts with norbornene to give oligomers, mostlytrimers–pentamers. On the other hand, (5)/MAO is highly active for ethylene/norbornene copolymerizations and gives high molecular weightcopolymers. Significantly, catalyst activity increases with increasing norbornene concentration. The copolymers show block-structure, andincorporated norbornene is present at the least as di-norbornene units, even at incorporation levels as low as 10 mol%. At higher norborneneconcentrations, NNN sequences prevail