Cocrystals have been attracting much attention in the field of organic solid-state chemistry, including CT complexes in superconductivity and inclusion complexes in molecular recognition.] We have recently reported on the formation of a mixed crystal from a couple of diolefinic compounds, which gave a random copolymer by repeating topochemical [2 + 21 photocycloaddition.2 In contrast, a regulated polymer is expected to be formed by the topochemical[2 + 21 photopolymerization of a molecular complex of a couple of diolefinic compounds with an ordered molecular arrangement in the crystal. Here we report on the first example of the topochemical photoreaction of a molecular complex to afford a perfectly ordered polymer composite.Each of 2,5-distyrylpyrazine (l)3 and ethyl 4-[2-(2-pyrazinyl)-ethenyllcinnamate (2)4 has been found to give a crystalline polymer upon UV irradiation in the crystalline state, respectively (Scheme I). On the other hand, when an equimolar mixture of 1 and 2 was recrystallized from benzene, very thin, platelike yellow crystals deposited. The 1H-NMR spectrum of the crystal showed that it comprised 1 and 2 in a molar ratio of 1 :2. This composition ratio in the crystal was not affected upon changing the molar ratio of 1 and 2 in solution in a range from 1:l to 1 5 . In the differential scanning calorimetric curve of this crystal, a single endothermic peak was observed at 166 "C, which is different from the melting point of either 1 (223 "C) or 2 (156 "C). Furthermore, the X-ray powder diffraction pattern of the crystal was quite different from those of the homocrystals 1 and 2. From these results, it is suggested that the crystal, obtained by recrystallization of a mixture of 1 and 2 from benzene, is not a simple blend but a cocrystal of 1 and 2 with a molar ratio of 1:2.Upon irradiation with a 500-W super-high-pressure mercury lamp, cocrystal 1.2 afforded a crystalline polymer (vinh = 1.0 dL/g: trifluoroacetic acid). The 'H-NMR spectrum of the polymer revealed that all of the olefin bonds were converted into cyclobutane rings through [2 + 21 cycloaddition and perfectly coincided with that of a 1:2 mixture of poly-l3 and poly-2.4 Partially dimerized photoproduct crystal was obtained when cocrystal 1.2 was irradiated with wavelengths longer than 410 nm, which excite only the monomers. In the dimer crystal, only 1-dimer and 2-dimer were detected by high-performance liquid chromatography (HPLC), but the corresponding cross dimer consisting of 1 and 2 was not detected at all. These observations + Toin University of Yokohama. 1 The University of Tokyo. (1) (a) Desiraju, G. R. Crystal Engineering. The Designof OrganicSolids; Elsevier: Amsterdam, 1989; Chapter 9. (b) Kitaigorodskii, A. I. Molecular Crystals and Molecules; Academic Press: New York, 1973; pp 94-130. (2) Maekawa, Y.; Lim, P.-J.; Saigo, K.; Hasegawa, M. Macromolecules 1991, 24, 5752. Part A-1 1969, 7, 743. (3) Hasegawa, M.; Suzuki, Y.; Suzuki, F.; Nakanishi, H. J. Polym. Sci. (4) Haseaawa. M.; Aoyama, M.; Maekawa, Y.; Ohashi, Y.Macromole...