DFT CALCULATIONS OF ALTERNATIVE STRUCTURES IN THE ALLYL-NICKEL CATALYZED POLYMERIZATION OF BUTADIENE

The pi-allyl insertion mechanism of the 1,4-cis polymerization of buta diene by means of allyl-nickel catalysts has been studied theoreticall y by density functional theory (DFT) for the ligand free cationic bute nyl nickel(II) complexes [Ni(C3H5)(C4H6)](+), I, [Ni(C3H5)(C4H6)(C2H4) ](+), II, and [Ni(C7H11)(C4H6,)](+), III DFT energy profiles have been determined for the insertion of s-cis-butadiene into the anti-eta(3)- butenyl nickel(II) bond in the supine and prone orientation of the rea cting ligands. One of the objectives of the study was to support the p roposed pi-allyl insertion mechanism. With increasing size of the mode l compound there is better agreement between the calculated reaction a nd activation energies in relation to the known experimental behavior. Among the different arrangements (anti- or syn-butenyl) of the allyli c end of the growing polymer chain and the relative orientation (supin e or prone) of cisoid butadiene at the catalytic site, the anti-buteny l/prone reacts in compound III with a small activation barrier (8.2 kc al/mol) to the product (-5.7 kcal/mol). The stabilizing effect of the coordination of an additional double bond of the growing polymer chain for the correct description of the geometrical and energetical aspect s of the insertion reaction was demonstrated by II. The results suppor t the s-cis-butadiene insertion into the anti-eta(3)-butenyl nickel(II ) bond according to the proposed pi-allyl mechanism. (C) 1996 John Wil ey & Sons, Inc.