Topochemical photopolymerization of 2,5-distyrylpyrazine (2 in Scheme 1) was first reported in 1967.(1) The space group of crystal 2 (Pbca) was found to be retained during the [2 + 2] cycloaddition polymerization from the crystallographic study.(2) The result was the first evidence of a polymerization reaction in the crystal lattice. After this report, the [2 + 2] topochemical reaction was investigated on a large number of diolefin compounds in detail, to afford a variety of products such as ...
