Solid-state polymerization of pentoxane (─CH2─O─)5

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Abstract
In the course of systematic study of the solid-state polymerization of the cyclic oligomers of formaldehyde (─CH2─O─)m, it was found that pentoxane (─CH2─O─)5 undergoes polymerization in the solid state to form polyoxymethylene (─CH2─O─)n by X-irradiation. The present article contains (a) the molecular and crystal structure determination of pentoxane, (b) the determination of the relative crystal orientation of pentoxane and the resultant polyoxymethylene, and (c) some discussions on the role played by the crystal structure of pentoxane in the solid state polymerization comparing with the cases of trioxane and tetraoxane. Pentoxane forms an orthorhombic crystal. The crystallographic data are a = 8.194 Å, b = 10.691 Å, c = 7.682 Å, D14 2h - Pbcn, four molecules in the unit cell. The relative crystal orientation of pentoxane and polyoxymethylene was determined using X-ray diffraction from single crystals of pentoxane which were partially converted into polyoxymethylene by X-irradiation. The polyoxymethylene obtained is in the form of “twinned main crystals”: the polymer chain axis is parallel to either of the two equivalent [110] and [110] axes of the pentoxane crystal, indicating that the solid-state polymerization is classified as a “topotactic reaction.” In addition to the main crystals, a small portion of the polyoxy-methylene is in the form of “subcrystals”: the polymer chain axis of which is parallel to either of the two equivalent [101] and [101] axes of the original pentoxane crystal. These characteristic orientations are quite different from the cases of trioxane and tetraoxane, and the relative crystal orientation of these oligomers and the resultant polyoxymethvlene were interpreted in terms of the molecular arrangements of the original crystals. The polymer yield from the solid-state polymerization of pentoxane induced by X-rays was found to increase with temperature to near the melting point of pentoxane. However, above the melting point polymerization by X-irradiation is abruptly suppressed.

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