Mapping free radical reactivity: A high-resolution electrospray ionization-mass spectrometry study of photoinitiation processes in methyl methacrylate free radical polymerization

High-resolution electrospray ionization-mass spectrometry (ESI-MS) was applied to study the polymeric product spectrum generated by the pulsed laser polymerization (PLP) of methyl methacrylate (MMA) at temperatures <= 0 degrees C in the presence of the photoinitiators 2,2-dimethoxy-2-phenylacetophenone (DMPA), benzoin, benzil, benzoin ethyl ether (BEE), and 2,2-azobisisobutylnitrile (AIBN). Termination products, both combination and disproportionation, were identified with high accuracy. Both the benzoyl and acetal fragments generated as a result of DMPA photocleavage were found to initiate and highly likely terminate polymerization. Under the conditions studied, the acetal radical produced upon DMPA photolysis fragments further to yield methyl radicals which seem to act predominantly as terminating moieties. Both the benzoyl and ether fragments produced as a result of benzoin photocleavage were found to act as initiating and probable terminating species, indicating that the ether radical fragment does not act exclusively as a terminating species. Additionally, increasing laser intensity and/or irradiation repetition rate (i.e., energy deposition into the system) results in more complex product distributions of the MMA polymers produced via photoinitiation (with the exception of AIBN). Temperature was determined to have a minor influence on the resulting product distribution under the conditions examined.