Addition-Fragmentation Kinetics of Fluorodithioformates (F-RAFT) in Styrene, Vinyl Acetate, and Ethylene Polymerization: An Ab Initio Investigation

Abstract

The kinetics and thermodynamics of the addition-fragmentation equilibrium in fluorodithioformate (SC(F)SR; F-RAFT) mediated polymerization of styrene and vinyl acetate were investigated via high-level ab initio molecular orbital calculations. The fragmentation efficiencies of a wide range of leaving groups (R = C(CH3)2CN, CH2CN, C(CH3)2Ph, CH(Ph)CH3, CH2Ph, CH(COOCH3)CH3, CH2COOCH3, CH(OCOCH3), CH2OCOCH3, C(CH3)3, CH2CH3, CH3) were also investigated. The calculations confirm earlier predictions, on the basis of thermodynamic considerations alone, that these agents are likely to function as genuine multipurpose RAFT agents. Thus, stable propagating radicals (as in styrene polymerization) are capable of adding to the RAFT agent with high rate coefficients (1.8 × 106 L mol-1 s-1 at 333.15 K), comparable to those observed with normal dithioesters such as SC(CH3)SR (3.8 × 106 L mol-1 s-1). Concurrently, unstable propagating radicals (as in vinyl acetate polymerization) are capable of undergoing fragmentation with significantly higher rate coefficients (1.7 × 104 s-1) than that for SC(CH3)SR (8.4 s-1) and are not expected to be rate retarded. On the basis of an examination of leaving group abilities and known reinitiation rate coefficients, the agents SC(F)SC(CH3)2CN or SC(F)SC(CH3)2Ph are identified as optimal F-RAFT agents for styrene polymerization, while SC(F)SCH2CN or SC(F)SC(CH3)3 are identified as optimal F-RAFT agents for vinyl acetate and ethylene polymerization. The potential suitability of employing F-RAFT to invoke living free radical polymerization of ethylene has been tested by a general kinetic screening exercise as well as specific simulations that employ quantum chemically predicted F-RAFT rate coefficients. These results indicate that F-RAFT is expected to control ethylene free radical polymerization.