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 C_ω➝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.

The ’migration’ can be explained in terms of an addition-elimination mechanism via alkylnickel/nickel hydride species with 1,2-hydride shift.