Electron Spin Resonance (ESR) Studies of Mid-Chain Radicals Formed During Radical Polymerizations of Acrylates

Abstract

ESR spectroscopic studies directed at clarifying the fundamentals of radical polymerizations have been conducted. Optimization of measurement conditions allows direct detection of radicals in polymerization reactions and well-resolved ESR spectra can be obtained. The spectra provide information not only on the structure, properties, and concentration of radicals but also information on the initiating and propagating (oligomeric and polymeric) radicals during radical polymerization. A combination of ESR and atom transfer radical polymerization (ATRP) provided significant new information on the properties of radicals in radical polymerizations, e.g. dependency of chain length, dynamics, and reactivity (hydrogen transfer) of propagating radicals. For acrylates, each ESR spectrum of dimeric, trimeric, tetrameric, and pentameric tert-butyl acrylate (tBA) model radicals observed at various temperatures provided clear experimental evidence of the 1,5-hydrogen shift. In this research work, ESR spectra observed during radical polymerization of tBA above 30°C, could be completely explained as being due to mid-chain radicals, using several experiments and a special simulation program.