Photopolymerization Reaction Intermediates in Diacetylene Crystals

Abstract

Recent spectroscopic investigations and model calculations concerning the reaction intermediates of the solid state polymerization in diacetylene crystals are reviewed. It will be shown, that diacetylenes are a unique model system for the study of reaction mechanisms. The reaction intermediates and final reaction products are best described by diradical (DR), asymmetric carbene (AC), dicarbene (DC) and stable oligomer (SO) molecules of different chain lengths. The respective singlet (S=0), triplet (S=1) or quintet (S=2) states and their roles in the polymerization process are investigated in detail by ESR and optical absorption spectroscopy. The optical and thermal reaction kinetics are characterized by the photoinitiation, the chain propagation and chain termination reactions. A one-dimensional electron gas model and an exciton model is applied to the different absorption series of the intermediates. The structural transition from diradicals to dicarbenes observed at a critical chain length of six repeat units is quantitatively explained by the generation of “up” and “down” radical electrons at the chain ends of the reaction intermediates.