Highly conjugated monomers, 7,7,8,8-tetrakis(alkoxycarbonyl)quinodimethanes (methoxy (1a), ethoxy (1b), isopropoxy (1c), benzyloxy (1d), chloroethoxy (1e), and bromoethoxy (1f)), were synthesized. Recrystallizations of 1a, 1c, 1e, and 1f yielded two crystal forms (prisms (1a-A) and needles (1a-B), needles (1c-A) and plates (1c-B), prisms (1e-A) and plates (1e-B), and prisms (1f-A) and needles (1f-B)), which have different molecular packing modes by X-ray crystal structure analysis, indicating that the crystals are polymorphic. In the photopolymerizations of these monomer crystals in the solid state, 1a-A, 1e-A, and 1f-A polymerized topochemically to give crystalline polymers. For their thermal polymerizations in the solid state, in addition to 1a-A, 1e-A, and 1f-A, 1e-B and 1f-B polymerized, but polymers formed from the 1e-B and 1f-B were amorphous. The packing of quinodimethane molecules in the crystals was defined by four kinds of parameters, stacking distance (d(s)), the distance between the reacting exomethylene carbon atoms (d(cc)), the angles formed between the stacking axis and longer axis of the monomer molecule (theta(1)), and the shorter axis of the monomer molecule (theta(2)), and then the polymerization reactivity of these quinodimethanes in the solid state was discussed on the basis of these parameters.

S Supporting Information ■ INTRODUCTIONTopochemical polymerization is counted as a specific case in the solid-state polymerizations, and it proceeds with no movement of a center of gravity of the monomer molecule and only slight rotation of the monomer molecule around the center of gravity, indicating that the crystallographic position and symmetry of the monomer crystals are retained in the resulting polymer crystals. X-ray single-crystal structure determination of monomer and especially polymer crystals can provide directly an experimental evidence to prove the topochemical polymerization. Such direct proofs are available for the polymerizations of derivatives of diacetylenes, 1 distyrylpyrazines and phenylene diacrylates, 2 trienes and triacetylenes, 3 muconic and sorbic acids, 4 and [2,2′-bi-1H-indene]-1,1′-dione-3,5-diyldialkyl carboxylate. 5 Previously, we investigated photochemical and thermal polymerizations of 7,7,8,8-tetrakis(alkoxycarbonyl)quinodimethanes with various alkoxy groups in the solid state 6 and found that 7,7,8,8-tetrakis(alkoxycarbonyl)quinodimethane monomers with methoxy, chloroethoxy, and bromoethoxy groups afford their highly crystalline polymers (Scheme 1).Unfortunately, photochemical and thermal polymerizations of these quinodimethanes in the solid state did not afford the polymer crystals suitable for single-crystal structure determination. Therefore, on the basis of the indirect experimental evidence such as infrared, elemental analysis, and powder X-ray analysis for the monomers and the corresponding polymers, we concluded that solid-state polymerizations of these monomers might proceed via a topochemical reaction mechanism. On photochemical polymerization in the solid state, UV light is absorbed mainly by the molecules present at the surface of the crystals, and the strength of the UV light is weakened rapidly inside of the crystals. That is, polymerization rate is significantly different between the surface and inside of the crystals. It is therefore considered that the difference in the polymerization rate induces defects or cracks in the crystals, resulting in polymer single crystals with poor quality. Moreover, when the crystals of these quinodimethanes were kept at room temperature in dark for 6 months, their solid-state polymerizations took place slowly, and white polymer crystals, indicating the presence of cracks, were formed. It is considered that it is difficult to obtain their polymer crystals suitable for Xray single-crystal structure analysis on the thermal polymerization. To obtain the polymer crystals suitable for X-ray singlecrystal determination, polymerization reaction is required to proceed rapidly and homogeneously in the crystal. As X-ray, γ-ray, and electron beam have generally excellent penetration ability, solid-state polymerization reactions under their irradiation are expected to take place rapidly and homogeneously and to provide polymer crystals with fewer defects. It is reported that the solid-state polymerizations of diacetylene and diene derivatives ...