Quantum Chemical Investigation of the Mechanism of Direct Initiation of Isobutylene Polymerization by Boron Trichloride

Abstract

Quantum chemical calculations by the MNDO and PM-3 methods, taking into account nonspecific solvation effects, were carried out in order to elucidate the mechanism of direct initiation of living carbocationic polymerization of isobutylene (IB) by BCl 3 . Enthalpies and activation energies obtained by these investigations are consistent with direct addition of BCl 3 to IB (chloroboration) in polar solvents like CH 3 Cl and CH 2 Cl 2 . These calculations also suggest that the ion generation reaction proceeds between excess of BCl 3 and the product of the chloroboration reaction (Cl 2 BCH 2 C(CH 3 ) 2 Cl, III), leading to an ion pair, Cl 2 BCH 2 C(CH 3 ) 2 +Bcl 4 - (IV). Addition of IB (initiation) to such an ion pair was shown to be more probable than that to a zwitterionic intermediate (Cl 3 B-CH 2 C(CH 3 ) 2 + , I), which may be formed in the first stage of the chloroboration reaction. It was found that the MNDO method gives three local minima of IV having different orientations of the BCl 4 - counterion in polar solvent and interionic distances (R) in the range of 4.6-6.1 A. This indicates that a real value for R can be found in this range. The propagation enthalpy values (ΔH p ) of IB polymerization obtained by the MNDO calculations are higher by 20-22 kcal/mol than the experimental value of ΔH p = -17.2 kcal/mol. This difference is attributed to the tendency of overestimating the repulsion between nonbonded atoms by the MNDO method. PM-3 calculations, free from this problem, gave ΔH p in good agreement with the experimental data.