Photochemical Reactions in Polymeric

Photochemical reactions have been examined in relation with the physical properties of the solid polymeric matrix in which they are carried out. Three main reaction types are considered: (i) photocrosslinking reactions resulting from cyclodimerization of unsaturated groups and from the decomposition of photolabile groups into very reactive species, e.g. nitrenes from azides; (ii) photoisomerization reactions in the case of aromatic azoderivatives, and photochromic spirobenzopyran compounds; (iii) photodissociation reactions in the case of bisanthraceneand benzpinacol derivatives. Each class of reaction is illustrated by recent data from the literature, as well as by new results obtained in our laboratory in photochemical isomerization and dissociation experiments. The reversibility of these reactions is discussed in terms of chain segment mobility and free volume distribution, and correlated diffusion effects in the case of dissociation reactions. The importance of the chemical incorporation of the photosensitive groups into the polymer is stressed by comparison with the photoresponse of corresponding model compounds dissolved in a polymeric matrix. INTRODUCTION Photochemical reactions in high polymers are based on a primary light energy absorption step followed by a reversible or irreversible chemical transformation. The initial photophysical act is evidently the absorption of a quantum of light, which is directly related with the electronic structure of the compound. By this absorption the molecule is brought into a singlet excited state of which the lifetime is determined by several competing phenomena, such as fluorescence back to the ground state, non-radiative decay, intersystem crossing to the triplet state, and eventually chemical reaction. Similarly the deactivation of the triplet state can proceed by phosphorescence, by non-radiative (energy transfer) processes and by chemical reactions. Photochemically induced reactions are therefore one of the possible deactivation processes of the singlet and triplet excited states; these highly reactive intermediates are responsible for the course of the reaction, which very often differs fundamentally from thermal reactions in the ground state.