Hosts, Inclusion polymerization

Keywords Cyclodextrin, Cyclodextrin complex, Hydrophilic hosts, Inclusion, Polymerization... 

PHTP is a chiral host which can be resolved into enantiomers DCA and ACA are (or derive from) naturally occurring optically active compounds. Using these hosts inclusion polymerization can be performed in a chiral environment and can be used for the synthesis of optically active polymers. This line of research has been very fruitful, both on the synthetic and on the theoretical plane. It has been ascertained that asymmetric inclusion polymerization occurs by a "through space" and not by a "through bond" induction only steric host-guest interactions (physical in nature) and not conventional chemical bonds are responsible for the transmission of chirality (W). 

CONTENTS Preface. George W. Gokel. Cryptophanes Receptors for Tetrahedral Molecules, Andre Collett, Jean-Pierre Dutasta and Benedict Lozach. Inclusion Polymerization in Steroidal Canal Complexes, Kiichi Takemoto, Mikiji Miyata. Functionalized Tetraazamacrocycles Ligands with Many Aspects, Thomas A. Kaden. Calixarenes as the Third Supramolecular Host, Seiji Shinkai, Kyushu University, Japan. Fluorescent Chemosensors for Metal and Non-Metal Ions in Aqueous Solutions Based on the Chief Paradigm, Anthony W. Czamik. Index. 

As Tonelli et al. [44,45] have pointed out, the study of crystalline inclusion complexes provides an approach to investigate the behaviors of single polymer chains in isolated and well - defined environments. Then, it is helpful in understanding the mechanism of molecular recognition between hosts and polymeric guests. 

In inclusion polymerization a monomeric clathrate is transformed into a polymeric one (where monomeric and polymeric refer to the nature of the guest molecules)(Fig. 1) or into a mixture of polymer and host when the polymer does not possess the steric requirements for inclusion. 

A discussion of inclusion polymerization within the frame of solid state polymerization requires the specification of the points which distinguish the two processes. In inclusion polymerization the solid phase consists of two components, host and guest. The former is a crystalline substance which possesses a strong tendency to polymorphism. Generally hosts are able to crystallize in... 

The most common hosts for inclusion polymerization are urea, thiourea, perhydrotriphenylene (PHTP), deoxycholic acid (DCA), apocholic acid (ACA) and tris(o-phenylenedioxy)cyclotriphosphazene (TPP)(Fig. 2). They have the common feature of forming channel-like clathrates, but differ in many specific properties. For instance, urea and thiourea have a rigid structure in which the host molecules are connected by hydrogen bonds and possess a high selectivity towards the guests. In urea channels are rather narrow whereas in thiourea they are wider as a consequence, linear molecules include only in urea and branched or cyclic molecules in thiourea. On the contrary, chainnels existing in PHTP clathrates are very flexible and can accomodate linear, branched and cyclic molecules. 

Apocholic acid ACA Figure 2. Hosts used In Inclusion polymerization. 

Finally, we wish to comment briefly on a recent development in inclusion polymerization. As already discussed, this reaction can be carried out on the pure clathrate or in the presence of an excess monomer. Consequently, the vapor pressure of a volatile monomer during polymerization ranges from the decomposition pressure of the clathrate to the vapor pressure of the saturated solution of the host in the guest, which is generally very close to that of the pure liquid monomer. For example, the vapor pressure... 

The characteristic of crystal lattices is a strict periodical succession of structurally identical molecular units, in the sense of an inclusion lattice also of holes, channels, layers etc. which may include guest molecules in an oriented fashion. This organizing principle makes topochemistry possible. One of the early studies in this area was the inclusion polymerization of dienes in the channels of urea, respectively thiourea, leading to stereoregular polymers (Eq. 1) Although stereodifferentiating inclusion polymerization/co-polymerization has been performed in other host lattices, too, e.g. in the channels of the perhydrotriphenylene host (6) it is still a problem of actual interest... 

In this chapter, we will describe how the host frameworks based on PCPs can be designed for the fields of polymerizations in Sect. 2. Later in the following section, details of inclusion polymerization in PCPs such as radical and catalytic polymerization processes are considered. Finally, in Sect. 4, the significant effects of host framework structures on polymer primary structure are discussed. 

Polymer inclusion compounds, which can be prepared not only by inclusion polymerization but also by direct cocrystallization of polymers with small molecule hosts, are interesting composite materials awaiting practical applications, e.g., incorporation of conjugated polymers into channels should lead to new composite materials with potential use as molecular wires.New polymer-polymer molecular composites with interesting properties can be produced by embedding polymer inclusion compound into a carrier polymer. 

APA J has long been known to form the inclusion compounds similar to DCA. The crystal structures and inclusion abilities are nearly the same as shown in Fig. 4. They are served as hosts for inclusion polymerization. The former has larger channels than the latter. 

Goonewardena, W. Miyata, M. Takemoto, K. Onedimensional inclusion polymerization of diene and vinyl monomers by using methyl cholate as a host. Polym. J. 1993. 25. 731. 

Inclusion polymerization usually results in host-polymer composites (Fig. Id). The addition of suitable solvents induces separation of the resulting polymers from the hosts. The separated polymers sometimes involve the host component even after thorough solvent treatment. This may be explained by the fact that the initiating radicals come from the hosts in some cases. The separated host components assemble together by recrystallization to give original inclusion compounds with solvents (Fig, la). 

In Fig. 1, a cyclic process for inclusion polymerization is shown. The process consists of four steps 1) hybridization of a host assembly with a solvent 2) complexation of the host with a guest monomer 3) polymerization of the monomer and 4) separation of the resulting polymer from the host. This cycle is based on spontaneous and non-covalent phenomena, such as intermolecular association and dissociation among host-host, host-guest, and guest-guest components. 

A chiral host could readily be available from a naturally occurring compound. The use of steroidal acid, deoxycholic acid (Fig. 3d), yielded coinprehensive polymers, particularly, optically active polymers from pro-chiral monomers. Many derivatives of deoxy cholic acid have the corresponding characteristic inclusion abilities. For example, use of apocholic acid (Fig. 3e), cholic acid (Fig. 3f), and chenodeoxycholic acid (Fig. 3g) enabled us to perform one-dimensional inclusion polymerization of various diene and vinyl monomers. 

Fig. 3 Organic hosts used for one-dimensional inclusion polymerization (a) urea (b) thiourea (c) perhydrotriphenylene (d) deoxycholic acid (e) apocholic acid (f) cholic acid (g) chenodeoxycholic acid (h) m5 (6>-phenylenedioxy)cyclotriphosphazene and (i) n5(2,3-naphthalenedioxy)cyclotriphosphazene.

Radical species during inclusion polymerization can readily be detected by ESR spectroscopy, indicating that the radicals are thermally stable in the channels. The reason is that the radicals in the channels do not meet with each other due to the host walls. y-Irradiation produces radicals of the host component as well as the monomers. Monomeric and propagating radicals were observed in the case of urea, while only the propagating radicals were observed in the case of perhydrotriphenylene, deoxycholic acid, and apocholic acid. Simulation of the spectra clarified that the propagating radicals do not rotate freely, indicating that mobilities of the radicals are constrained in the channels. 

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