Evaluation of the mode of termination for a thermally initiated free-radical polymerization via matrix-assisted laser desorption ionization time-of-flight mass spectrometry

Styrene and methyl methacrylate have been polymerized at 90 degrees C with azobis(isobutyronitrile) (AIBN) as a thermal initiator. The molecular weight distributions were analyzed using matrix-assisted laser desorption ionization (MALDI) mass spectrometry and size exclusion chromatography (SEC). By synthesizing polymer of low molecular weight (M(p) = 1000), we were able to achieve excellent agreement between molecular weight data from MALDI and SEC for the same polymer. Evaluation of the termination mode in free-radical polymerization was performed by integrating the peaks on the mass spectra which corresponded with either one or two initiator fragments per polymer chain. Evidence for some primary radical termination was noted at high initiator concentrations. An additional peak in the spectra for polystyrene was ascribed to the copolymerization of styrene with a single methacrylonitrile unit originating from the initiation process with AIBN. Two other anomalous peaks were noted in the polystyrene mass spectrum; the first relates to chain scission under MALDI conditions and the second to a Diels-Alder rearrangement thermal initiation product. The ratios of the termination modes (disproportionation to recombination, K-td/k(tc)) were evaluated for methyl methacrylate and styrene as 4.37 +/- 1.1 and 0.057 +/- 0.032, respectively, at 90 degrees C. These values are in excellent agreement with data reported previously in the literature. Potential errors arising from the use of MALDI for quantitative chain end analyses are discussed