Topochemical photopolymerization of diolefin crystals

Topochemical [2+2] photocyclo-dimerization and -polymerization of olefinic derivative crystals are reviewed from the points of view of synthetic and structural chemistries. In 1964 Schmidt and co-workers established the topochemical concept concerning to the photoreactive olefinic crystals: the topochemical reaction tends to occur with a minimum of atomic and molecular motion. The first example of topochemical polymerization (four-center type photopolymerization) was found in 1967, where 2,5-distyrylpyrazine crystal affords highly crystalline linear polymer, repeating intermolecular [2+2] photocyclo-dimerization of two olef in bonds in a molecule with retention of crystal symmetry. Almost all of the reaction behaviors and configurations of the product are interpreted in terms of topochemical principle (reactive double0bonds are arranged in parallel with a distance within 4.2 A) while several exceptional examples have been recently reported. A great variety of reactivities and photoproducts has been revealed in the four-center type photopolymerization of unsymmetric diolefin crystals, depending on a slight modification of chemical structure or crystallization process of the monomer. A single type of dimer is formed exclusively and accumulated at the intermediate stage, and only the reactive species having even-numbered degree of polymerization participates in chain growing process. Optically active polymers and polymers with different substituents in alternative order can be prepared from the unsymmetric diolef in crystals.