Stereochemistry and pseudosymmetries in main chain polymeric free radicals

Main chain free radicals produced from 248 nm photolysis of poly(alkyl acry late)s and poly(alkyl methacrylate)s have: been unambiguously characterized for the first time by time-resolved electron paramagnetic resonance (TREPR ) spectroscopy. Side chain cleavage via the Norrish I process dominates, le ading to an oxo-acyl radical from the ester side chain and a main chain pol ymeric radical, the existence of which has been previously postulated but n ever confirmed by direct spectroscopic observation. There is a strong stere ochemical influence on the methacrylate spectra, which manifests itself thr ough changes in the TREPR spectra as a function of polymer tacticity and te mperature. There is also a strong solvent dependence. Computer simulation p rovides unambiguous assignment of the signal carriers for the acrylate poly mers at room temperature and above, and for PMMA main chain radicals at hig h temperature, where the fast motion limit for the P-methylene hyperfine co upling constants is achieved. The methacrylate spectra give remarkably simi lar coupling constants for all tacticities at high temperature, a phenomeno n explained by an unexpected land fortuitous) pseudosymmetry relationship b etween the diastereotopic protons in the radicals.