MECHANISTIC STUDIES OF THE 1,4-POLYMERIZATION OF BUTADIENE ACCORDING TO THE PI-ALLYL-INSERTION MECHANISM - 2 - DENSITY-FUNCTIONAL STUDY OF THE C-C BOND FORMATION REACTION IN GATIONIC AND NEUTRAL (ETA(3)-CROTYL)(ETA(2)- ETA(4)-BUTADIENE)NICKEL(II) COMPLEXES [NI(C4H7)(C4H6)](+), [NI(C4H7)(C4H6)L](+) (L = C2H4, PH3), AND [NI(C4H7)(C4H6)X] (X- = I-)/

The entire catalytic cycle of the 1,4-polymerization of butadiene has been theoretically studied according to the pi-allyl-insertion mechani sm. This has been performed using density functional theory (DFT) with cationic butenylbis(ligand) and neutral dimeric butenyl complexes as the catalyst. The calculations give a clear insight into the kinetic a nd thermodynamic control of the catalytic activity and cis-trans selec tivity as well as into the elucidation of the stereoregulation mechani sm. The supposed pi-allyl-insertion mechanism was supported in all ess ential features by this research. The stability and reactivity of diff erent isomers of eta(4)-butadiene pi-complexes was calculated to be ve ry similar, regardless of the donor-acceptor ability of the neutral or anionic ligand. The thermodynamically more stable syn-butenyl forms a re also more reactive than the anti counterparts. The intrinsic reacti vity diminishes while the ligand's donating ability increases. The fav ored pathway proceeds in an exothermic process as follows: starting fr om stable syn-butenyl eta(4)-cis-butadiene complexes, followed by a re quired ligand conversion via prone butadiene transition states, and su bsequently anti-syn isomerization of the actual anti insertion product to a new transoid C-4 unit in the polymer chain. The polymer chain sh ould not have any stereoselectivity in the methylene groups. Alternati ve pathways (e.g., via anti-butenyl prone butadiene transition states, thus forming a cis-1,4 polymer, or the direct generation of trans-1,4 -products by inserting trans-butadiene) are strongly unfavored by high er kinetic barriers. The rate-determining step is the cis-butadiene in sertion for the neutral complexes and the anti-syn isomerization for t he cationic complexes. To achieve a well-balanced description of both thermodynamic and kinetic control of trans-1,4-polymerization of butad iene, a careful modeling of the organophosphorus ligand's basicity was necessary.