Main chain polymeric radicals from several acrylic polymers, produced by laser flash photolysis at 248 nm in liquid solution, have been studied using direct detection time-resolved electron paramagnetic resonance (TREPR) spectroscopy at 9.5 GHz. Highly isotactic poly(methyl methacrylate) (i-PMMA) shows a sharp, well-resolved spectrum at about 95 degrees C. Using synthetic methodology to disrupt the tacticity of i-PMMA, we observed different fast-motion hyperfine coupling constants for the main chain radicals. By raising the temperature of observation, we returned the coupling constants to the same value as those in the highly isotactic sample. This result is related qualitatively to the degree of stiffness of the polymer chains as a function of tacticity. The concept is tested further by comparison to two other acrylic polymers with bulky side chains: poly(fluorooctyl methacrylate) (PFOMA) and poly(adamantyl methacrylate) (PAMA), whose main chain radicals show significant line broadening even at 110 degrees C. Solvent effects on both spectral appearance (the alternating line-width effect) and kinetic decays (attributed to T1 relaxation) are also presented and discussed in terms of main chain conformational motion.
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