Clathrates, tunnel

Miscellaneous Applications.—Polyesters can be prepared from their monomers by interaction with (NPCls)8/LiCl in pyridine. Alkenes have been polymerized in the clathrate tunnel system of (16), resulting in polymers with enhanced stereoregularity. ... 

Recently, Allcock et al. reported the inclusion polymerization of vinylic, acrylic [89], and diene monomers [90] within the clathrate tunnels formed by tris(o-phenylenedioxy)cyclotriphosphazene. Varying degrees of stereoregularity were obtained from the inclusion polymerization. For example, poly(methacrylonitrile) synthesized within the host adduct showed an enhanced isotactic microstructure. No cross-linked material was obtained from the polymers derived from the acrylates and methacrylonitrile. The copolymer poly(vinylacetate-co-methylacrylate) was also prepared by the inclusion polymerization method. 

Fig. 32. Packing relations and steric fit of the 26 acetic acid (1 1) clathrate (isomorphous with the corresponding propionic acid clathrate of 26)1U- (a) Stereoscopic packing illustration acetic acid (shown in stick style) forms dimers in the tunnel running along the c crystal axis of the 26 host matrix (space filling representation, O atoms shaded), (b) Electron density contours in the plane of the acetic acid dimer sa First contour (solid line) is at 0.4 eA" while subsequent ones are with arbitrary spacings of either 0.5 and 1 eA 3. Density of the enclosing walls comes from C and H atoms of host molecules.

A further example of the steric fit and thus the conditions of the second rank interactions between host and guest is illustrated by the channel structure of the acid inclusions of 26 (see inclusion compound with acetic acid, Fig. 32a). The tunnel has a mostly hydrophobic character being made up mainly from the aromatic portions of the roof-shaped host molecule. We must note that this arrangement applies possibly for the acetic acid clathrate of 1 as well. 

Not all clathrates are hydrates. Other well-known examples have host lattices formed from hydrogen bonded aggregates of hydroquinone, phenol, and similar organic compounds. Non-hydrogen bonded host structures are also known. One example is a cyclotriphosphazene. (C6H402PN). that traps molecules such as benzene in tunnels in the crystal.2 In addition, coordination polymers are formed by ambidentate ligands, such as CN and SCN, which coordinate to metal ions at both ends (Chapter 12). Perhaps the best known of this type of compound is the series of Ni(CN)2NHj M compounds, where M may be benzene, thiophene, furon. pyrrole, aniline, or phenol. 

When Cd(CN)2 is crystallized in the presence of other molecules that can stuff cavities or tunnels, many different structures are formed depending on the size and shape of the guests that stuff the cavities. Similar behavior is, of course, found elsewhere, e.g., in gas hydrates and hydrothermal synthesis of zeolites. These cadmium cyanide structures may be considered as a new class of clathrates. 

Alcock and coworkers studied the polymerization of butadiene (as well as of monoolefins, acetylene and aromatic olefins) trapped within the tunnel clathrate system of tris((9-phenylenedioxy)cyclotriphosphazene, induced by Co-y-radiation. The host was used in order to find if the concatenation and orientation of the monomer molecules under the steric forces generated within the host crystal lattice will lead to stereospecific polymerization. The clathrate was prepared by addition of liquid butadiene to the pure host at low temperature. The irradiation was conducted at low temperatures. Irradiation of pure butadiene (unclathrated bulk monomer) leads to formation of a mixture of three addition products f,2-adduct, cis- and trons-f,4-adducts. In contrast, the radiation-induced polymerization within the tunnel system of the host yielded almost pure trans-1,4-polybutadiene. A small percentage of f, 2-addition product was observed, but no evidence for the formation of c/s-f,4-adduct was found, confirming the earlier observation by Fin ter and Wegner. The average molecular weight was about 5000,... 

Inclusion compound A host compound that forms a crystal lattice that has spaces large enough for guest molecules. The only bonding between the host and the guest compounds is van der Waals forces. The spaces in the lattice are in the form of long tunnels or channels, unlike clathrate compounds in which the spaces are completely enclosed. 

Crystals of tris(o-phenylenedioxyde)cyclotriphosphazene (97) can act as hosts for the inclusion of a number of organic polymers, e.g. cis-1,4-poly butadiene, 1,4-polyisoprene, polyethylene (PE), poly(ethylene oxide) (PEO) and polytetrahydrofuran. X-ray studies of the PE and PEO inclusion compounds show that the polymer chains are extended along the tunnel-like voids of the host lattice. The formation of clathrates appears to be limited by the tunnel dimension of the host crystal lattice. The melting points of the inclusion adducts appear to be higher than those of either the pure host or the pure... 

Crystalline inclusion compounds containing unsaturated monomers are effective reactive systems for the production of linear polymers (1-6). This process belongs to the wider class of solid state polymerization, but possesses some specific features which make it worthy of a separate description. Throughout this article, the polymerization in inclusion compounds will be referred to as "inclusion polymerization" (other names currently used in the scientific literature are channel, canal or tunnel polymerization), and the terms "clathrate" will be used as synonymous with "inclusion compound". When there is no risk of confusion, the more general term of "adduct" will be used for clathrate in principle a... 

This chapter describes the formation of intercalation compounds as clathrates, as tunnel structures and as sheets. 

A clathrate is thus an inclusion compound which is a solid solution of the guest atoms or molecules in a metastable crystalline form characterized by isolated large spherical voids of the host species. These structural criteria differentiate clathrates from the other intercalation compounds subsequently described (see 16.3 and 16.4), in which the available voids exhibit a uni- or two-dimensional arrangement (tunnel and sheet structures). 

Methyl methaerylate does not appear to polymerize in the solid state upon simple UV radiation [63,64]. However, under pressure sufheiently high to solidify the monomer at a relatively high temperature or in a solid solution in paraffin wax, polymerization was found to be possible. It is remarkable that the y-radiation-induced solid-state polymerization is influenced significantly when the polymerization proceeds in tunnel clathrates [1]. 

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