Intermediate radical termination in reversible addition-fragmentation chain transfer-mediated polymerization : identification of termination products

Rate retardation in reversible addition-fragmentation chain transfer (RAFT)-mediated polymerizations is the topic of an ongoing debate. One of the main explanations for this rate retardation is termination of the intermediate radical. The use of a RAFT agent with a UV label in the leaving group acts as a means to identify products of intermediate radical termination (IRT). In the analysis of the polymeric samples with size exclusion chromatography (SEC), two different UV absorbance traces were compared (i.e., the UV absorbance of the label used and that of the thiocarbonyl thio moiety of the RAFT agent). Deviations in the ratio of the C=S of the RAFT moiety to the UV label indicate that IRT products could be formed. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry was used to identify these IRT products formed during a "normal" RAFT-mediated polymerization (not using forced conditions). These experimental findings support the postulated IRT model and therefore help to elucidate the fate of the intermediate radical during a RAFT mediated polymerization.