Crystal structure of the clathrate form of syndiotactic poly(p-methylstyrene) containing o-dichlorobenzene

The crystal structure of the clathrate form of syndiotactic poly(p-methylst yrene) (s-PPMS) containing o-dichlorobenzene (o-DCB) is presented. The stru cture is characterized by polymer chains in s(2/1)2 helical conformation an d o-dichlorobenzene molecules packed in a monoclinic unit cell with axes a = 23.4 Angstrom, b = 11.8 Angstrom, c = 7.7 Angstrom, and gamma = 115 degre es, according to the space group P2(1)/alpha. The calculated crystalline de nsity is 1.07 g/cm(3) for two polymer chains (eight monomer units) and two o-DCB molecules included in the unit cell. The o-DCB molecules occupy cavit ies delimited by the phenyl rings of two enantiomorphic polymer chains. A d isorder in the positioning of the o-DCB molecules inside the cavity is pres ent. A comparison with the crystal structure of the clathrate form of s-PPM S containing tetrahydrofuran is presented. Remarkable differences were foun d, confirming the previous hypothesis that two different types of crystal s tructures (alpha and beta class) are presented by the clathrate forms of s- PPMS depending on the guest molecule.