Main chain free radicals produced from 248 nm photolysis of poly(alkyl acrylate)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, leading to an oxo-acyl radical from the ester side chain and a main chain polymeric radical, the existence of which has been previously postulated but never confirmed by direct spectroscopic observation. There is a strong stereochemical influence on the methacrylate spectra, which manifests itself through changes in the TREPR spectra as a function of polymer tacticity and temperature. There is also a strong solvent dependence. Computer simulation provides unambiguous assignment of the signal carriers for the acrylate polymers at room temperature and above, and for PMMA main chain radicals at high temperature, where the fast motion limit for the P-methylene hyperfine coupling constants is achieved. The methacrylate spectra give remarkably similar coupling constants for all tacticities at high temperature, a phenomenon explained by an unexpected land fortuitous) pseudosymmetry relationship between the diastereotopic protons in the radicals.