The Migratory Catalyst: α-Olefin Polymerization by 2,ω-Linkage

Abstract

With the homogeneous catalyst system nickel(0)compound bis(trimethylsilyl)aminobis(trimethylsilylimino)phosphorane linear α-olefins and singly branched α-olefins can be polymerized. The structure of the poly-α-olefins is unusual. When a linear α-olefin is polymerized, the polymer contains only methyl branches, regularly spaced along the chain with a separation corresponding to the chain length of the monomer. Thus, in the polymer of a linear α-olefin with n CH2 groups the distance between two methyl branches is (n+1) CH2 groups. Analysis of the C-NMR spectra of polymers of deuterated α-olefins revealed that the growing chain is bound to the next α-olefin via Cω➝C2 linking; C1 forms the latter methyl branch in the polymer. Taking all results into account, a reaction scheme was developed with which the origin of the special structure of the poly-α-olefin can be explained. The main points of the scheme are: The monomer can only insert into a CH2-Ni bond at the end of the growing chain; the insertion is regioselective; only Cω ➝ C2 coupling of the growing chain with the next monomer takes place; the nickel-catalyst complex ’migrates’ along the polymer chain between two insertions. During this ’migration’ transfer reactions to the monomer can occur, but not insertions.