An in-depth analytical approach to the mechanism of the RAFT process in acrylate free radical polymerizations via coupled size exclusion chromatography-electrospray ionization mass spectrometry (SEC-ESI-MS)

Coupled size exclusion chromatography (SEC)-electrospray ionization mass spectrometry (ESI-MS) was applied to carefully map the product spectrum of a series of acrylate free radical polymerizations mediated via the reversible addition fragmentation chain transfer (RAFT) process. The product stream of a significantly rate retarded RAFT system (i.e. n-butyl acrylate (BA)/cumyl dithiobenzoate (CDB)) was compared with the less rate retarded RAFT polymerizations of BA mediated by cumyl phenyl dithioacetate (CPDA) and methyl acrylate (MA)/CPDA. In each case excellent agreement between the theoretical and experimental masses, as well as the simulated isotopic peak distributions, of polymeric species in the product stream was observed. Although conventional disproportionation and combination bimolecular termination products were clearly identified within the product spectra, the presence of irreversibly terminated RAFT intermediates, i.e. 3-armed star polymers, was not observed. The mass spectroscopic results are compared to modeling estimations (carried out via the PREDICI^® program package) of the concentration ratios of 3-armed stars vs. conventional termination products. It is demonstrated that the occurrence of conventional termination products should be accompanied by a significant product stream associated with 3-armed star polymer material if cross termination was operational—at least under the current reaction conditions. The absence of three armed star polymer products in the polymers stream suggests that irreversible cross termination reactions may be of minor importance in the present systems.