SITE-SELECTIVE PHOTOCHEMISTRY IN AN ALTERNATING 2-NORBORNYL-CO COPOLYMER - IMPORTANCE OF STEREOELECTRONIC EFFECTS

The electronic structure of a highly stereoregular, alternating 2-norb ornyl-CO copolymer has been investigated using ab initio calculations on model compounds, and the photochemical consequences of these calcul ations are discussed in regard to recent laser flash photolysis/time r esolved electron paramagnetic resonance (TREPR) experiments on this po lymer. The polymer is believed to be synthesized in the exo-syn geomet ry, While photolysis is expected to lead to exo-substituted 2-norborny l radicals, only endo-substituted radicals could be detected in the TR EPR experiment. Evidence is presented that the endo radicals arise fro m 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 a long the polymer backbone. The ab initio calculations support the expe ctation of different photochemical reaction rates of the defects compa red to the exo-syn linkages, due to the smaller electronic coupling ma trix elements for triplet energy transfer at anti defect sites than at exo-syn linkages.