Abstract
Certain small molecules, such as urea (U), perhydrotriphenylene (PHTP) and cyclodextrins (CDs), can be co-crystallized with polymers to form inclusion compounds (ICs). The guest polymer chains are confined to narrow, cylindrical channels created by the host, small-molecule lattice. The number and conformation of included polymer chains depend on the relative cross-sectional dimensions of polymer chains and the host channel diameter. For the hosts U, PHTP and α-CD ( D≈5 Å), only highly extended single chains can be squeezed inside the channel and are separated from neighboring polymer chains by the IC channel walls composed exclusively of the small-molecule lattice. However, for the host γ-CD ( D≈8 Å), two side-by-side, parallel extended polymer chains can be incorporated inside the channel, and thus, are also decoupled from all other neighboring chains by the channel walls. Therefore, the unique solid-state environment for polymers residing in IC channels can be utilized as model systems for ordered, bulk polymer phases. Comparison of the behavior of isolated, extended polymer chains in different host environments with the behavior observed for ordered, bulk phases of polymers permits an assessment of contributions made by the inherent, single chain, interactions between adjacent side-by-side pairs of chains and the overall co-operative, interchain interactions to the properties of ordered, bulk polymers. Solid-state NMR spectroscopy is an efficient technique to study the conformations and molecular motions of polymer ICs. This review paper mainly discusses the solid-state NMR study of the conformations and dynamics of a series of crystalline polymers observed in their ICs. In order to facilitate interpretation of the NMR observations, at the beginning of this review, we also discuss the related modeling results obtained by rotational isomeric state modeling and molecular dynamics simulations.
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