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Title: Quantitative Comparison of the Mesitoyl vs the Benzoyl Fragment in Photoinitiation: A Question of Origin
Authors: Voll, Dominik
Barner-Kowollik, Christopher
Issue Date: 2011
Source: MACROMOLECULES, 44(8). p. 2542-2551
Abstract: Photolytically generated radicals (at a wavelength of 351 nm) derived from the acetophenone-type photoinitiators benzoin (2-hydroxy- 1,2-diphenylethanone) and 2,4,6-trimethylbenzoin (2-hydroxy-1-mesityl-2-phenylethanone, TMB) (specifically the benzoyl and mesitoyl radical) are quantified in their ability to serve as initiating species in methyl methacrylate (MMA), ethyl methacrylate (EMA), and butyl methacrylate (BMA) bulk free radical polymerizations under optimized conditions. Herein, 2,4,6-trimethylbenzoin is employed for the first time as photoinitiator in pulsed laser polymerizations (PLP) employing a high-frequency excimer laser, constituting a new source for mesitoyl radicals. The current work presents an improved method for quantifying radical efficiency of photoinitiation processes using coupled online size exclusion chromatography-electrospray ionization mass spectrometry (SEC/ESI-MS) to analyze the obtained polymers. Because of the occurrence of side reactions during the benzoin-initiated MMA polymerization, reduced laser energies (similar to 0.35 mJ/pulse) as well as low polymerization temperatures (similar to-5 degrees C) were employed, which avoids side product formation. A plot of the ratio of benzoyl to mesitoyl (derived from 2,4,6-trimethylbenzoin) end groups vs the ratio of both initiators in the reaction mixture indicates that the benzoin-derived benzoyl radical is 3.0 (2.6, 2.4) times more likely to initiate the polymerization process of MMA (EMA, BMA) than the TMB-derived mesitoyl fragment. This observation is in sharp contrast to the case when mesitil is employed as a source of mesitoyl radicals (8.6 times higher likelihood of benzoyl incorporation). These results clearly support the notion that the origin of a radical species significantly determines its propensity to be incorporated at a polymer chain's terminus. The cause of such an origin dependence is tentatively assigned-at least in part-to different triplet lifetimes or intersystem crossing efficiencies (Phi(ISC)) or both of TMB and mesitil.
Notes: [Voll, Dominik; Barner-Kowollik, Christopher] Karlsruhe Inst Technol KIT, Inst Tech Chem & Polymerchem, D-76128 Karlsruhe, Germany. [Junkers, Thomas] Univ Hasselt, Inst Mat Res IMO, Polymer React Design Grp, BE-3590 Diepenbeek, Belgium.
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ISSN: 0024-9297
e-ISSN: 1520-5835
DOI: 10.1021/ma2001977
ISI #: 000289593200021
Category: A1
Type: Journal Contribution
Validations: ecoom 2012
Appears in Collections:Research publications

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