CONFORMATIONS AND MOTIONS OF POLYPROPYLENES CONFINED TO CHANNELS

It is possible to study the solid-state conformations and motions of individual polymer chains, well-separated from neighboring chains, by observing their behavior in the narrow channels formed in their inclusion compounds with host clathrate molecules. The host clathrate channels provide a well-defined, constraining geometry for each included guest polymer chain. As a consequence, the experimental observations of the conformations and motions of constrained and isolated polymer chains are much easier to interpret than those of a bulk polymer sample where cooperative interchain interactions often predominate. In this paper we model the conformations and motions of polypropylene (PP) chains isolated in the narrow channels (D = 5.5 angstrom) of their inclusion compounds with perhydrotriphenylene (PHTP). We explicitly treat PP's with various stereosequences and find the following behavior: (i) neither stereoregular PP [isotatic (i) or syndiotactic (s)] can adopt its crystalline conformation in the PHTP channels; (ii) interconversion between channel conformers is not possible; and (iii) the channel conformers of s-PP are much more probable (have a much lower intramolecular energy) than the channel conformers of i-PP. These results are compared to the observations of PP-PHTP inclusion compounds reported by Sozzani et al.