Evaluation of the Mode of Termination for a Thermally Initiated Free-Radical Polymerization via Matrix-Assisted Laser Desorption Ionization Time-of-Flight Mass Spectrometry

Abstract

Styrene and methyl methacrylate have been polymerized at 90 °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 °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.

Characterization and relative ionization efficiencies of end-functionalized polystyrenes by matrix-assisted laser desorption/ionization mass spectrometry

“…Generally, it has been assumed that the end groups have little effect on the ionization efficiency of PS, such as when MALDI has been used to quantify products with differing end-functionality following the polymerization of styrene. This assumption was used by both Quirk et al, who quantified the ethylene oxide oligomerization during end-functionalization of poly(styryl)lithium showing good agreement with data obtained by 1 H NMR [18], and by Zammit et al, who quantified the mode of termination of a free radical polymerization of styrene and found good agreement with multiple previous studies using other methods [19]. The assumption was valid in both cases, i.e., quantification of oligomers with different end groups by MALDI showed no obvious bias towards a particular end group, and as such no detailed investigation of an end group bias was done.…”

Synthesis of semitelechelic poly[N-(2-hydroxypropyl)methacryl-amide] by radical polymerization in the presence of alkyl mercaptans

“…The method is better for samples with a narrow distribution, because the higher molecular weight masses may be underestimated in the spectra 27) . Nevertheless, the new technique has been used to quantitatively investigate the polymerization reaction, e. g., the determination of chain transfer constants 29) and the evaluation of termination mode 30) of free radical polymerization. The evaluation of the termination mode of free radical polymerization by MALDI-TOF MS end group analysis gave a good agreement with the data reported in the literature 30) .…”

Photocopying Living Chains. 1. Steady-State

“…This latter value should be taken with some caution, however, as "there remains considerable uncertainty in existing termination mode measurements and there is wide variation in results for common monomers such as styrene and methyl methacrylate" 57 as reviewed and discussed by Zammit et al 57 and by Solomon and Moad. 16 We remind the reader that the calculation of N h o is sensitive to the integration limits set on the FL chromatograms.…”