Mechanism of the Chain Termination of the Allylnickel(II)-Catalyzed Polymerization of 1,3-Butadiene. A Density Functional Investigation for the Cationic [NiII(RC3H4)(cis-C4H6)L]+ Active Catalyst

Abstract

Alternative mechanisms of chain termination of the allylnickel(II)-catalyzed polymerization of 1,3-dienes have been investigated by employing a gradient-corrected DFT method: namely, (I) the hydrogen transfer mechanism and (II) the hydrogen elimination mechanism. Several conceivable pathways for each of these mechanisms have been scrutinized for the well-characterized trans-1,4 regulating cationic allylnickel(II) [NiII(η3-RC3H4)(cis-C4H6)L]+ active catalyst. The transfer of a hydrogen atom from the C4(butenyl) carbon of the growing polymer chain to the coordinated monomer, which starts with the butadiene π-complex, is suggested to be the favorable mechanism of chain termination. This process is found to preferably proceed in a concerted fashion outside of the immediate proximity of the nickel atom. On the other hand, the hydrogen elimination occurring from the monomer insertion product by hydrogen abstraction from the polymer chain (C4(butenyl) carbon) yielding initially an diene hydride species, is seen to be kinetically disfavored. Accordingly, hydrogen elimination represents a less likely channel for chain termination. The monomer π-complex is the crucial species of the entire polymerization reaction course, as it serves as the precursor for the critical monomer insertion and allylic anti−syn elementary steps of the chain propagation cycle, as well as for the favorable hydrogen transfer chain termination channel. The present study provides, for the first time, a theoretically well-founded insight into the mechanism of molecular weight regulation for the allylnickel(II)-catalyzed polymerization of 1,3-dienes.