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Mechanistic Studies of the 1,4-Polymerization of Butadiene According to the π-Allyl-Insertion Mechanism. 2. Density Functional Study of the C−C Bond Formation Reaction in Cationic and Neutral (η 3 -Crotyl)(η 2 -/η 4 -butadiene)nickel(II) Complexes [Ni(C 4 H 7 )(C 4 H 6 )] + , [Ni(C 4 H 7 )(C 4 H 6 )L] + (L = C 2 H 4 , PH 3 ), and [Ni(C 4 H<sub…

1998; American Chemical Society; Volume: 17; Issue: 6 Linguagem: Inglês

10.1021/om9705923

1520-6041

Sven Tobisch, Horst Bögel, Rudolf Taube,

biodegradable polymer synthesis and properties

The entire catalytic cycle of the 1,4-polymerization of butadiene has been theoretically studied according to the π-allyl-insertion mechanism. 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 and thermodynamic control of the catalytic activity and cis−trans selectivity as well as into the elucidation of the stereoregulation mechanism. The supposed π-allyl-insertion mechanism was supported in all essential features by this research. The stability and reactivity of different isomers of η4-butadiene π-complexes was calculated to be very similar, regardless of the donor−acceptor ability of the neutral or anionic ligand. The thermodynamically more stable syn-butenyl forms are also more reactive than the anti counterparts. The intrinsic reactivity diminishes while the ligand's donating ability increases. The favored pathway proceeds in an exothermic process as follows: starting from stable syn-butenyl η4-cis-butadiene complexes, followed by a required ligand conversion via prone butadiene transition states, and subsequently anti−syn isomerization of the actual anti insertion product to a new transoid C4 unit in the polymer chain. The polymer chain should not have any stereoselectivity in the methylene groups. Alternative 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 higher kinetic barriers. The rate-determining step is the cis-butadiene insertion for the neutral complexes and the anti−syn isomerization for the cationic complexes. To achieve a well-balanced description of both thermodynamic and kinetic control of trans-1,4-polymerization of butadiene, a careful modeling of the organophosphorus ligand's basicity was necessary.