Direct Access to Dithiobenzoate RAFT Agent Fragmentation Rate Coefficients by ESR Spin-Trapping

摘要

The beta-scission rate coefficient of tert-butyl radicals fragmenting off the intermediate resulting from their addition to tert-butyl dithiobenzoate-a reversible addition-fragmentation chain transfer (RAFT) agent-is estimated via the recently introduced electron spin resonance (ESR)-trapping methodology as a function of temperature. The newly introduced ESR-trapping methodology is critically evaluated and found to be reliable. At 20 degrees C, a fragmentation rate coefficient of close to 0.042 s(-1) is observed, whereas the activation parameters for the fragmentation reaction-determined for the first time-read E-A = 82 +/- 13.3 kJ mol(-1) and A = (1.4 +/- 0.25) x 10(13) s(-1). The ESR spin-trapping methodology thus efficiently probes the stability of the RAFT adduct radical under conditions relevant for the pre-equilibrium of the RAFT process. It particularly indicates that stable RAFT adduct radicals are indeed formed in early stages of the RAFT polymerization, at least when dithiobenzoates are employed as controlling agents as stipulated by the so-called slow fragmentation theory. By design of the methodology, the obtained fragmentation rate coefficients represent an upper limit. The ESR spin-trapping methodology is thus seen as a suitable tool for evaluating the fragmentation rate coefficients of a wide range of RAFT adduct radicals.