AN INVESTIGATION OF HEMIBENZOPINACOLATE RADICALS USED IN THE SYNTHESIS OF BLOCK-COPOLYMERS - ESR MEASUREMENTS AND SEMIEMPIRICAL CALCULATIONS

Hemibenzopinacolate radicals may be incorporated in a polymer chain an d the latter used as a macromolecular initiator. Kinetics of polymeriz ation are strongly dependent on the substitution of the pinacolate. A large family of substituted (CH3)(3)SiOC(C6H5)(2) radicals has been sy nthesized and studied by electron spin resonance (ESR) and semiempiric al molecular orbital calculations. ESR spectra are well resolved and, except for one derivative, allow an unambiguous determination of the h yperfine coupling splitting (HFS) constants. The ground state properti es have been calculated within different approximations, namely MNDO, AM1 and PM3 for the geometry optimization and INDO for spin density. G eometries obtained within AM1 and PM3 approximations are satisfactory. The HFS constants evaluated using McConnell type relations and spin d ensities are in a quite good agreement with the experimental ones. It turns out that the effect of substitution on kinetics of polymerizatio n cannot be attributed to any drastic change in the charge distributio n.