Determination of the structural configuration and the cross-linking reaction between 2-p-(formylstyryl)-3,4-dimethylthiazol-3-ium moieties in photosensitive polymers

Abstract

Poly(vinyl alcohol) modified with 2-p-(formylstyryl)-3,4-dimethylthiazol-3-ium shows a very high photosensitivity even when substitution levels are only 1–2 mol%. The trans structure of the model chromophore has been confirmed by FT NMR. Upon exposure to light, in the solid state or as a polymeric film, a cross-linking reaction occurs which insolubilises the compound. The nature of this photoproduct has been characterised by FTIR and FT NMR as a cyclobutane structure. Upon exposure each spectra shows loss of the alkenic bands and the appearance of alkanic bands, specifically, a proton multiplet at 5.5 ppm typical of a protonated cyclobutane ring.

Investigation of the cross-link mechanism indicates that aggregation occurs as the first step in the reaction. Molecular modelling of the chromophore indicates that electrostatic attraction is the driving force in the aggregation and that head-to-tail type cyclobutane molecules will predominate. HPLC separation shows two bands attributed to dimeric species, in a ratio of ca. 10 : 1. Comparison of the multiplet at 5.5 ppm with simulations of those for head-to-head and head-to-tail dimer suggests that the HPLC bands relate to the two possible head-to-tail-dimers and that the α-isomer predominates.