E clathrates

Weber, E. Clathrate Chemistry Today — Some Problems and Reflections, 140, 1-20 (1987). 

Our recent studies [74-76,85-93] show that almost all prepared polycrystalline adducts of zinc(II)-dithiocarbamates (see previous section) may form solvates, i.e. incorporating small guesf molecules such as benzene, various chlorohy-drocarbons and N-donor bases, which are held by van-der-Waals forces between molecules of the host compound. X-ray diffraction studies [85-91,93] have revealed an ordered system of molecular channels occupied by outer-sphere guest molecules in the crystal lattice of solvated forms of the adducts (see Fig. 23), i.e., clathrate type structures. 

Weber. E. Clathrate chemistry today—Some problems and reflections. In Molecular Inclusion and Molecular Recog-nition-Clathrates I Weber. E., Ed. Topics in Current 39. Chemistry. Springer-Verlag Berlin. Germany. 1987 Vol. 

More recently, the occurrence of a third class of SPS co-crystals has been established [53-56]. In this third class of SPS co-crystals, guest molecules are imprisoned into channels formed between enantiomorphous s (2/1)2 polymer helices, typical of the nanoporous e phase [55]. As a consequence, longer guest molecules can be hosted by placing their molecular plane nearly parallel to the to the axis of the polymer helices [53,54]. The two classes of SPS co-crystals with guest molecules imprisoned in cavities and channels will be thereafter dehned as 5 clathrates and e clathrates, respectively. 

Figure 10.1 Presentations of along-the-chain-projections of SPS co-crystalline phases (a) 5 clathrate phase with DCE (b) intercalate phase with norbornadiene (c) schematic e clathrate phase with DCE.

As for the e clathrates, they can be only obtained by guest sorption in into SPS samples presenting the e phase, which in turn can be obtained by chloroform sorption and desorption in y form samples [53-56],... 

Conformational Equilibria of the Guest Molecules FTIR studies relative to chlorinated hydrocarbons sorption into SPS films have shown that the trans conformation of DCE and i,2-dichloropropane (DCP) is largely prevailing in the clathrate phase, while the trans and gauche conformations are nearly equally populated when both chlorinated compounds are sorbed in the amorphous phase [88,137-139]. Because essentially only their trans conform-ers are included into the clathrate phases (both 8- and e clathrates), while both trans and gauche conformers are included in the amorphous phase, quantitative evaluations of vibrational peaks associated with these conformers allow the evaluation of the amounts of DCE and DCP confined as guest in the clathrate phase or simply absorbed in the amorphous phase [88,111,137-139]. 

Figure 10.7 X-ray diffraction patterns (CuKa) of SPS semicrystallme powder samples presenting different helical crystalline and co-crystalline phases (A) 5 form (B) 5 clathrate with DCE (C) e form (D) e clathrate with DCE (E) y form. Bragg distances (d in nm) of relevant reflections are indicated.

Particularly suitable are guest-extraction procedures by solvents that are temporary volatile guests of these nanoporous phases, like acetone, acetonitrile [43], or supercritical CO2 [47,48], The same extraction procedures are suitable to obtain the nanoporous e phase only when used on e clathrate samples. 

Several exciting new material based on co-crystalline and nanoporous crystalline phases of syndiotactic polystyrene have been achieved. In particular, several kinds of polymer co-crystalline phases have been prepared, belonging to three different classes 8- and e-clathrates and intercalates. Polymer cocrystals with active guest molecules show unusual physical properties, hence are promising for several kinds of advanced materials. Moreover, the unprecedented achievement of polymeric nanoporous crystalline phases (8 and e) has given very interesting results in the fields of molecular separations, water/ air purification and sensorics. 

As for perspectives of applications of SPS-based co-crystaUine phases, studies will be mainly devoted to films, also trying to exploit the unique availability of three different kinds of uniplanar orientations, which allow macroscopic control of the guest orientation. Particular attention will be devoted to possible applications of the recently discovered e clathrates, mainly due the possibility of controlling the orientation of very long guest molecules, which could give relevant nonlinear optical properties. 

Relevant new materials could also be obtained by chemical reactions (e.g., polymerization) between guest molecules, for polymer co-crystals exhibiting guest-guest proximity (e clathrates and intercalates). 

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