X-ray crystallographic study of solid-state polymerization of trioxane and tetraoxymethylene

Abstract

Abstract An X-ray study has been applied to clarify the relationship between crystal structures and crystal orientations of tetraoxymethylene and polyoxymethylene obtained by solid- state polymerization of tetraoxymethylene. The polyoxymethylene crystal obtained by the solidstate polymerization of a tetraoxymethylene single crystal was found to be definitely oriented with respect to the original tetraoxymethylene single crystal without any aftertreatment; i.e., the polymer chain (the c-axis) is parallel to the b-axis of tetraoxymethylene and the a-axis of polyoxymethylene coincides to the c-axis of tetraoxymethylene. In addition to the main orientation, other preferred crystalline orientations (“twin structure”) of polyoxymethylene were observed in polymers polymerized at lower temperatures (60-80°C); i.e., the polymer chains are parallel to either the [100], [001], or [101] direction of the original tetraoxymethylene crystal, though the amount of such oriented crystallites is small. The twin structure is different from that of polyoxymethylene obtained from trioxane. This difference is due to the crystal structures of these original materials; tetraoxymethylene occurs in the monoclinic system, while trioxane belongs to the trigonal system, as does polyoxymethylene. These characteristic orientations are explained in terms of the molecular arrangements in the crystals of the original materials.