The electronic structure of a highly stereoregular, alternating 2-norbornyl-CO copolymer has been investigated using ab initio calculations on model compounds, and the photochemical consequences of these calculations are discussed in regard to recent laser flash photolysis/time resolved electron paramagnetic resonance (TREPR) experiments on this polymer. The polymer is believed to be synthesized in the exo-syn geometry, While photolysis is expected to lead to exo-substituted 2-norbornyl radicals, only endo-substituted radicals could be detected in the TREPR experiment. Evidence is presented that the endo radicals arise from selective photochemistry at anti defect sites in the polymer chain. NMR results from a (CO)-C-13-enriched sample show that enol formation can be responsible for the disruption of the exo-syn stereochemistry along the polymer backbone. The ab initio calculations support the expectation of different photochemical reaction rates of the defects compared to the exo-syn linkages, due to the smaller electronic coupling matrix elements for triplet energy transfer at anti defect sites than at exo-syn linkages.