Electron Spin Resonance (ESR) Studies of Radicals in the Initial Stage of Radical Polymerizations of tert-Butyl Methacrylate Using Model Radicals

Abstract

Electron Spin Resonance (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 in radical polymerizations. A combination of ESR and atom transfer radical polymerization (ATRP) provided significant new information on the properties of radicals in radical polymerization, e.g. dependency of chain length, dynamics, and reactivity (hydrogen transfer) of propagating radicals. Up to now, it has been extremely difficult, even impossible, to obtain such information from ESR spectra during conventional radical polymerization. To overcome this difficulty radical precursors of oligo- and poly(meth)acrylates were prepared by ATRP and purified. Model radicals, with given chain lengths were generated by the reaction of well defined radical precursors with an organotin compound and were observed by ESR spectroscopy. tert-Butyl methacrylate (tBMA) radicals with various chain lengths showed clear chain length dependent ESR spectra. Especially, the ESR spectra of the dimeric model propagating radical were examined in detail at various temperatures.