Laser-induced decomposition of 2,2-dimethoxy-2-phenylacetophenone and benzoin in methyl methacrylate homopolymerization studied via matrix-assisted laser desorption/ionization time-of-flight mass Spectrometry

Pulsed laser polymerization (PLP) experiments were performed on the bulk polymerization of methyl methacrylate (MMA) at -34 degreesC. The aim of this study was to investigate the polymer end groups formed during the photoinitiation process of MMA monomer using 2,2-dimethoxy-2-phenylacetophenone (DMPA) and benzoin as initiators via matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. Analysis of the MALDI-TOF spectra indicated that the two radical fragments generated upon pulsed laser: irradiation show markedly different reactivity toward MMA: whereas the benzoyl fragment-common to both DMPA and benzoin-clearly participates in the initiation process, the acetal and benzyl alcohol fragments cannot be identified as end groups in the polymer. The complexity of the MALDI-TOF spectrum strongly increased with increasing laser intensity, this effect being more pronounced in the case of benzoin. This indicates that a cleaner initiation process is at work when DMPA is used as the photoinitiator. In addition, the MALDI-TOF spectra were analyzed to extract the propagation-rate coefficient, k(p), of MMA at -34 degreesC. The obtained value of k(p) = 43.8 L mol(-1) s(-1) agrees well with corresponding numbers obtained via size exclusion chromatography (k(p) = 40.5, L mol(-1) s(-1)). (C) 2002 John Wiley Sons, Inc.