Crystal Structure of the Clathrate Form of Syndiotactic Poly(p-methylstyrene) Containing Benzene

The crystal structure of the β class clathrate of syndiotactic poly(p-methylstyrene) containing benzene is presented and compared to that of the clathrate containing tetrahydrofuran, also belonging to the β class. The structure is characterized by polymer chains, in s(2/1)2 helical conformation, and benzene molecules packed in an orthorhombic unit cell with axes a = 19.5 A, b = 13.3 A, and c = 7.7 A, according to the space group C2221. The calculated crystalline density is 1.05 g/cm3 for two polymer chains (eight monomer units) and four benzene molecules included in the unit cell. Our analysis affords an insight into the structural features typical of the β class clathrates. As in the packing model proposed for the crystal structure of the s-PPMS clathrate containing tetrahydrofuran, the guest molecules occupy cavities delimited by isomorphic helices along the b axis and are related by the crystallographic 2-fold screw axis of the unit cell. The packing of the helices along the a axis is, instead, complet...