Experimental Determination Of The Geometrical Relation Between Monomer And Polymer Species Of 2,5-Distyrylpyrazine Single Crystal In The Topotactic Photoinduced Polymerization Reaction

The topotactic polymerization reaction is a special case of more general topochemical reactions, and it is defined as the photoinduced solid-state polymerization reaction, in which the space group symmetry is kept unchanged between the initial monomer and the final polymer-crystals and the morphology or crystal shape is also mostly unchanged through this solid-state polymerization reaction. The experimental check of the topotactic reaction is made by analyzing the structure relation between the original monomer single crystal and the resultant polymer single crystal. A half century ago, the 2,5-distyrylpyrazine (DSP) monomer single crystal was discovered as the first example of the [2 + 2]-type cycloaddition topotactic polymerization reaction. However, while the crystal structure of DSP monomer single crystal was analyzed, the crystal structure analysis of the corresponding polymer species had been left pending over these several tens of years, although the unit cell parameters and space group were reported in the literature. Because of such a situation, some argument was given to the proposed reaction system. Using synchrotron X-ray diffraction data, we have succeeded to analyze the crystal structure of the DSP polymer single crystal, by which clear and straightforward evidence of the topotactic polymerization reaction was extracted through the geometrical comparison between the monomer and polymer species.