DETERMINATION OF THE STRUCTURAL CONFIGURATION AND THE CROSS-LINKING REACTION BETWEEN 2-P-(FORMYLSTYRYL)-3,4-DIMETHYLTHIAZOL-3-IUM MOIETIES IN PHOTOSENSITIVE POLYMERS

Poly(vinyl alcohol) modified with 2-p-(formylstyryl)-3,4-dimethylthiaz ol-3-ium shows a very high photosensitivity even when substitution lev els are only 1-2 mol%. The trans structure of the model chromophore ha s been confirmed by FT NMR. Upon exposure to light, in the solid state or as a polymeric film, a cross-linking reaction occurs which insolub ilises the compound. The nature of this photoproduct has been characte rised by FTIR and FT NMR as a cyclobutane structure. Upon exposure eac h spectra shows loss of the alkenic bands and the appearance of alkani c bands, specifically, a proton multiplet at 5.5 ppm typical of a prot onated cyclobutane ring. Investigation of the cross-link mechanism ind icates that aggregation occurs as the first step in the reaction. Mole cular modelling of the chromophore indicates that electrostatic attrac tion is the driving force in the aggregation and that head-to-tail typ e cyclobutane molecules will predominate. HPLC separation shows two ba nds attributed to dimeric species, in a ratio of ca. 10 : 1. Compariso n of the multiplet at 5.5 ppm with simulations of those for head-to-he ad and head-to-tail dimer suggests that the HPLC bands relate to the t wo possible head-to-tail-dimers and that the alpha-isomer predominates .