Solid-state polymerization of diacetylenes

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Abstract
Differences in the solid‐state reactivity of two substituted diacetylenes with identical lattice symmetry (P21/c) are described in terms of the molecular rotational and translational motion necessary for 1,4‐addition polymerization. The structural relationship between monomer and polymer has been investigated in detail for the solid‐state polymerization of 2,4‐hexadiynediol. For the thermal, uv, or γ‐ray‐induced polymerization of this monomer the same crystallographic relationship was found between the polymer chain direction and the monomer lattice. Mutually reacting monomer molecules are related by a unit‐cell translation along the a/c diagonal, which is perpendicular to the unique axis of this monoclinic crystal. X‐ray diffraction determination of unit‐cell parameters for γ‐ray‐irradiated samples of differing conversion have confirmed that the polymer enters the monomer lattice as a solid solution and verify the mentioned polymerization mechanism. When these irradiated samples were thermally annealed, phase separation occurred, producing discrete monomer and amorphous polymer phases. Despite the instability of the partially polymerized 2,4‐hexadiynediol lattice, ir measurements reveal no free hydroxyl hydrogens either before or after phase separation. Neighboring monomer molecules are indirectly hydrogen bonded together in the c‐axis direction, but no hydrogen bonding exists between mutually reacting monomer molecules in disagreement with a previously suggested model for lattice‐controlled diacetylene polymerizations. During the polymerization of 2,4‐hexadiynediol the center‐to‐center separation of mutually reacting molecules must change by − 0.44 Å and the conjugated molecular rods must rotate by an angle of at least 28°. This large required molecular rotation suggests that the activation energy for rotation contributes significantly to the activation energy for polymerization. The high entropy of fusion (17.0±1.2 eu) measured for this compound is consistent with low solid‐state rotational mobility. No solid‐state phase transitions were observed calorimetrically for 2,4‐hexadiynediol from − 100°C to the melting point (114.5±0.3°C). The low solid‐state reactivity observed for diphenyldiacetylene is reasonable in light of the much larger translation and rotation necessary for 1,4‐addition polymerization, respectively, − 1.14 Å and 37.5°. No ethanol insoluble polymer was obtained for γ‐ray radiation dosages which resulted in high monomer‐to‐polymer conversions for 2,4‐hexadiynediol. No measurable difference was observed in the melting behavior of γ‐ray‐irradiated and nonirradiated samples of this high entropy of fusion compound (14.1±0.7 eu).