Topochemically controlled solid-state polymerization

Abstract

Certain kinds of crystalline organic monomers, when exposed to u.v. light, are smoothly converted into extended-chain polymers (or oligomers) in a diffusionless manner, which is rendered possible because of the favourable packing of the monomeric units within the parent crystal. The principles that govern such topochemical reactions are critically assessed, starting with the ideas of Hirshfeld & Schmidt (1964), who first drew attention to the advantage (as far as facile solid-state polymerization is concerned) of using monomers with two unsaturated centres such as the diacetylenes and diolefins. We have examined eleven substituted diolefins and related their ease of polymerization to various crystallographic properties such as symmetry, space group, packing mode and separation distance between reactive centres on adjacent molecules. The two principal techniques used were X-ray diffraction and low-temperature transmission electron microscopy. We show that, for molecules such as distyrylpyrazine (in its photosensitive α polymorphic form) and divinyl benzene, nucleation begins and propagation proceeds within the defect-free structure - imperfections in the crystal appear not to play a direct role in the course of the polymerization. This fact offers considerable scope for the design of other monomeric entities which may be ‘engineered’ to pack in certain photolabile forms. We also briefly discuss the factors that permit achiral molecules to crystallize in such a way as to enable solid-state photochemical reactions to generate crystalline optically active products.