D. La Camera et al., Crystal structure of the clathrate form of syndiotactic poly(p-methylstyrene) containing benzene, MACROMOLEC, 34(22), 2001, pp. 7762-7766
The crystal structure of the beta class clathrate of syndiotactic poly(p-me
thylstyrene) containing benzene is presented and compared to that of the cl
athrate containing tetrahydrofuran, also belonging to the beta class. The s
tructure is characterized by polymer chains, in s(2/1)2 helical conformatio
n, and benzene molecules packed in an orthorhombic unit cell with axes alph
a = 19.5 A, b = 13.3 Angstrom, and c = 7.7 Angstrom, according to the space
group C222(1). The calculated crystalline density is 1.05 g/cm(3) for two
polymer chains (eight monomer units) and four benzene molecules included in
the unit cell. Our analysis affords an insight into the structural feature
s typical of the beta class clathrates. As in the packing model proposed fo
r 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, comple
tely different in the two structures. bc layers of isochiral chains alterna
te with bc layers of chains having opposity chirality in the model proposed
for the tetrahydrofuran clathrate. A new and unusual example of chiral cry
stalline phase, in which all the polymer helices assume the same chirality
in the lattice, is proposed for the crystal structure of the benzene clathr
ate.