On predicting free radical polymerizability of allyl monomers. MINDO/3 and 13C NMR results

Abstract

Two new methods of predicting free radical polymerizability of allyl monomers have been developed. MINDO/3 calculations of eigenvalues for the C—H bond a to the atlyl substituent allow comparison of relative bond strengths. Monomers known to undergo degradative chain transfer display more positive eigenvalues correlating with weaker α-C—H bonds. Polymerizable monomers have more negative eigenvalues and stronger α-C—H bonds. The latter possess strongly polarized or protonated substituents not capable of resonance stabilization of a chain-terminating allyl radical. The inductive effect of such substituents on the ¹³C NMR peaks of the vinyl carbons is the basis for the empirical spectroscopic method. Allyl compounds display β-carbon peaks farther downfield than the γ-carbon peaks. Polymerizable monomers generally have β peaks shifted upfield and γ peaks shifted downfield from those of poor monomers. This effect brings the two peaks closer together in the spectrum. Thus, the smaller the Δδ value (δ_β − δ_γ), the more likely the monomer is to polymerize well. Monomers with intermediate eigenvalues or Δδ values may polymerize with difficulty or only under special conditions. Combined use of the theoretical and empirical techniques can allow prediction of inherent polymerizability and can facilitate evaluation of reaction media most favorable to polymer formation.