Crystal Structures and Topochemical Polymerizations of 7,7,8,8-Tetrakis(alkoxycarbonyl)quinodimethanes

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 (θ₁), and the shorter axis of the monomer molecule (θ₂), and then the polymerization reactivity of these quinodimethanes in the solid state was discussed on the basis of these parameters.