Crystal structures and solid-state reactivities of 1,4- and 1,2-bis(5-hydroxypenta-1,3-diynyl)benzenes and 1-(5-hydroxypenta-1,3-diynyl)-4-ethynylbenzene

Abstract

Crystal structures and the solid-state polymerization of 1,4-bis(5-hydroxypenta-1,3-diynyl)benzene (1), 1,2-bis(5-hydroxypenta-1,3-diynyl)benzene (2) and 1-(5-hydroxypenta-1,3-diynyl)-4-ethynylbenzene (3) were investigated. The crystal structure of 1 depends on the recrystallization solvent. Crystals obtained from acetone–benzene mixtures polymerize when irradiated with γ-rays or heated. The colourless crystals become black and are insoluble in common organic solvents. Only one of the butadiynyl groups participates in the radiation-induced polymerization, whereas both do, in the thermal polymerization. Polymerization does not occur for crystals obtained from pure acetone. The polymerization reactivities are correlated to the crystal structures. Polymerizable crystals were not obtained from 2. The arrangement of the butadiynyl groups in crystals of 2 is unfavorable for the polymerization. Crystals of 3, obtained from hexane–acetone mixtures, polymerize upon γ-irradiation but not upon heating. In the radiation-induced polymerization of 3, the propagation by the ethynyl and butadiynyl groups is considered to proceed independently along different crystal axes. The ethynyl groups are more reactive than the butadiynyl groups because they have a shorter intermolecular carbon–carbon distance through which the polymerization occurs. The difference in reactivity toward heating between 1 and 3 may also be interpreted in terms of intermolecular carbon–carbon distances of the butadiynyl groups.