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 (ds), the distance between the
reacting exomethylene carbon atoms (dcc), 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.