Topochemical control of solid‐state polymerization

A survey of compounds that undergo polymerization in the solid state supports the topochemical postulate that such a reaction is possible only if the monomer crystal structure affords close contact between the reactive centers of neighboring molecules. Even when this condition is fulfilled, chain propagation may be diffusion‐limited because of the contraction of the chain on polymerization. When the crystal structure makes such contraction unnecessary, polymerization can proceed without diffusion and the crystal can exert a direct control over the geometric course of the reaction. Several kinds of structures are known in which such behavior has been demonstrated or can be expected.