Clathrate
Inclusion adducts, also called clathrates, consist of a host molecule whose crystal structure has voids in which guest molecules may be trapped. Spirocyclotriphosphazenes, such as tris(o-phenylenedioxy)cyclotriphosphazene (Structure 1), are examples of such hosts.
1
Through X-ray crystallography, it has been shown that these cyclotriphosphazene host molecules can stack in such a manner that allows the formation of tunnels 5A in diameter through its crystal lattice.
Crystal Structure of 1 with 5A Diameter Tunnels
Research in this group has focused on the development of other, unusual cyclotriphosphazene clathrates. The versatility of phosphazene chemistry has allowed for the synthesis of clathrates with variable tunnel diameters variable by the choice of different side groups. Tris(2,3-naphthalenedioxy)cyclotriphosphazene (2) has crystal structure tunnels 10A in diameter. Tris(1,8-naphthalenedioxy)cylclotriphosphazene (3) has 5.2-7A diameter tunnels. A clathrate based on tris(o-phenylenediamino)cyclotriphosphazene (4) was also prepared.
2 3
4
Inclusion adducts are formed by the direct exposure of the host molecule to a liquid or vapor guest. It has been found that the host can selectively include a particular guest based on its size and shape, and this process can be fine-tuned by varying the structure of the side group.
Because the clathrate voids are in the shape of tunnels, the polymerization of various organic monomers included as guest with these tunnels has been possible. Many different monomers have been polymerized inside the tunnels of various cyclotriphosphazene clathrates and, in some cases, it was demonstrated that the macromolecules produced were stereoregular. Butadiene polymerizes, after exposure to gamma-irradiation to yield 1,4-trans polymers when included within the tris(o-phenylenedioxy)cyclotriphosphazene host.
This is a significant difference from the bulk polymerization, which yields a mixture of 1,4-cis, 1,4-trans and 1,2- addition polymers.
Polymer Inclusion and Separation
Recently it has been demonstrated that host 1 can include a wide variety of preformed polymers within its tunnels. This host has also been shown to selectively include one polymer over another, such as the preferential inclusion of linear 1,4-polybutadiene over the branched 1,2-polybutadiene. Ongoing work in this area involves investigating polymer separations based on molecular weight and endgroup functionality.