Termination of Acetoxyethyl Radicals Generated by Br-Atom Abstraction from 1-Bromoethyl Acetate: Implications for the Spontaneous and Catalyzed Termination of Poly(vinyl acetate) Radicals

Abstract

The present contribution explores the termination modes of acetoxyethyl radicals – unimer models of poly(vinyl acetate) radicals – generated by Br-atom abstraction from 1-bromoethyl acetate (BEA). Using [(Me6TREN)Cu(BF4)] alone, with or without UV light irradiation, results in a ca. 50:50 distribution of combination (Comb) and disproportionation (Disp). Using Sn2Me6 or Pb2Ph6 alone with UV light irradiation yields greater Comb fractions (67% and 88%, respectively). This trend is due to the simultaneous presence of conventional Comb-dominated bimolecular termination and a Disp-selective in-cage hydrogen-atom transfer (HAT) to the Me3Sn•/Ph3Pb• radical, which decreases in the order Me3Sn• > Ph3Pb•. By extrapolation, poly(vinyl acetate) growing radical chains are suggested to terminate predominantly by Comb (>88%). On the other hand, the simultaneous use of Sn2Me6 or Pb2Ph6 and [(Me6TREN)Cu(BF4)], with UV light irradiation, leads to 100% Disp. These results reveal that Catalyzed Radical Termination (CRT) completely overrides the intrinsic termination behavior of acetoxyethyl radicals, switching the product distribution from predominant combination to exclusive disproportionation. More generally, they suggest that CRT may represent an overlooked limitation in the Atom Transfer Radical Polymerization (ATRP) of less-activated monomers by highly active copper catalysts.