QUANTUM-CHEMICAL INVESTIGATION OF THE MECHANISM OF DIRECT INITIATION OF ISOBUTYLENE POLYMERIZATION BY BORON-TRICHLORIDE

Quantum chemical calculations by the MNDO and PM-3 methods, taking int o account nonspecific solvation effects, were carried out in order to elucidate the mechanism of direct initiation of living carbocationic p olymerization of isobutylene (IB) by BCl3. Enthalpies and activation e nergies obtained by these investigations are consistent with direct ad dition of BCl3 to IB (chloroboration) in polar solvents like CH3Cl and CH2Cl2. These calculations also suggest that the ion generation react ion proceeds between excess of BCl3 and the product of the chloroborat ion reaction (Cl2BCH2C(CH3)(2)Cl, III), leading to an ion pair, Cl2BCH 2C(CH3)(2)(BCl4-)-B-- (IV). Addition of IB (initiation) to such an ion pair was shown to be more probable than that to a zwitterionic interm ediate (Cl3B-CH2C(CH3)2(+), I), which may be formed in the first stage of the chloroboration reaction. It was found that the MNDO method giv es three local minima of IV having different orientations of the BCl4- counterion in polar solvent and interionic distances (R) in the range of 4.6-6.1 Angstrom. This indicates that a real value for R can be fo und in this range. The propagation enthalpy values (Delta H-p) of IB p olymerization obtained by the MNDO calculations are higher by 20-22 kc al/mol than the experimental value of Delta H-p = -17.2 kcal/mol. This difference is attributed to the tendency of overestimating the repuls ion between nonbonded atoms by the MNDO method. PM-3 calculations, fre e from this problem, gave Delta H-p in good agreement with the experim ental data.