Monomers, inclusion polymerization

The trick used in asyrmnetric inclusion polymerization is to perform the reaction in a rigid and chiral environment. With more specific reference to chirality transmission, the choice between the two extreme hypotheses, influence of the starting radical (which is chiral because it comes from a PHTP molecule), or influence of the chirality of the channel (in which the monomers and the growing chain are included), was made in favor of the second by means of an experiment of block copolymerization. This reaction was conducted so as to interpose between the starting chiral radical and the chiral polypentadiene block a long nonchiral polymer block (formed of isoprene units) (352), 93. The iso-prene-pentadiene block copolymer so obtained is still optically active and the... 

Color sites can be introduced either through inclusion of monomers possessing color sites as in the case of titanocene polyfluorocene dyes, shown in Figure 4.3, or by the generation of color sites through polymerization, as in the case of polyphenylenes. 

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... 

A topochemical condition for polymerization is the proper approach of successive monomers at the growing chain-end within the channels. In this respect, conjugated dienes like butadiene, isoprene, etc. possessing reactive atoms in terminal positions, are very suited to inclusion polymerization. However, even bulkier monomers such as substituted styrenes or methyl methacrylate can polymerize if the space available inside the channels permits a favorable orientation and/or conformation of the monomer. The most studied examples are butadiene, vinyl chloride, bromide and fluoride, and acrylonitrile in urea 2,3-dimethylbutadiene and 2,3-dichlorobutadiene in thiourea butadiene, isoprene, cis- and trans-pentadiene, trans-2-methylpentadiene, ethylene and propylene in PHTP butadiene, cis- and trans-pentadiene, cis- and trans-2-methylpentadiene in DCA and ACA butadiene, vinyl chloride, 4-bro-mostyrene, divinylbenzene, acrylonitrile and methyl methacrylate in TPP. 

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... 

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. 

A variety of monomers can be trapped in the inclusion spaces at the molecular level and polymerized under suitable conditions. Such a reaction is called inclusion polymerization. " The study of inclusion polymerization started soon after the discovery of a honeycomb structure of urea inclusion compoxmds. The early study aimed to obtain highly stereoregular and asymmetric polymers in the spaces. Further studies brought about a profound understanding of the space effects from various viewpoints. Now. inclusion polymerization is classified between bulk or solution polymerization and solid state polymerization. In other words, it may be situated as low-dimensional and space-dependent polymerizations. Such a polymerization closely relates to supramolecular chemistry from a viewpoint of molecular information and expression. 

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. 4 Various monomers used for one-dimensional inclusion polymerization...

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. 

Molecular motions are much smaller in channels than in solution at the same temperature. This indicates that the polymerization may proceed slowly. But the neighboring monomers are located very near, suggesting that the reaction may proceed rapidly. This is how the reversed effects work during the propagation reaction. It is known that inclusion polymerization smoothly occurs at low temperatures in the chaimels of urea, thiourea, and perhydrotriphenylene. However, in the case of steroidal hosts, we observed a deerease in polymerization rates. For example. in one case, the polymerization reached a saturated state after 1 month. 

There are many possible schemes for addition reactions of diene monomers from electronical and steric viewpoints. Because the monomer molecules arrange along the direction of the channels, a,co-addition may selectively take place in one-dimensional inclusion polymerization. Therefore, conjugated polyenes, such as dienes and trienes, may selectively polymerize by 1,4- and 1,6-addi-tion, respectively. 1,3-Butadiene polymerized via 1,4-addition exclusively in the chaimels of urea and perhydrotriphenylene. while the same monomer polymerized via both 1,2- and 1,4-additions in the channels of deoxycholic acid and apocholic acid. Moreover, we have to evaluate head-to-tail or head-to-head (tail-to-tail) additions in the case of dissymmetric conjugated diene monomers such as isoprene and 1.3-pentadiene. 

Only a definite scope of monomers can be polymerized in a given host. This indicates that inclusion polymerization displays a sterically different boundary condition from other polymerizations. That is, an increase of one methylene unit of one monomer can induce an inhibition of the polymerization in a given channel (Fig. 5a,b). Moreover, the relative sizes of the channels change the polymerizabilities of the identical monomers. Even though a monomer does not polymerize in a small channel, the same monomer polymerizes in a larger channel, (Fig. 5b,c). For example, 4-methyl-1,3-pentadiene polymerized in the smaller channels of deoxycholic acid. 

Stereoregularities of the resulting polymers depend on the sizes of the host channels. Moreover, the space effect in chirality was observed in asymmetric inclusion polymerization of trans- or cis-2-methy 1-1,3-pentadiene by using a pair of hosts, deoxycholic acid and apocholic acid. We obtained optically active polymers with predominant absolute configurations (R). Optical yields varied with the polymerization conditions and the hosts. A maximum optical yield of the trans monomer was 36% m the channel of apocholic acid. 

Nylon-11 was prepared via the inclusion polymerization of pseudo-rotaxanes [52, 53]. Inclusion compounds between CDs and nylon-6 have been formed from their solutions [54-56]. Polyurethane-CD psewdo-polyrotaxanes have been prepared by mixing isocyanate and a dihydroxyl monomer in the presence of permethylated a-CD or permethylated /3-CD in dimethyl formamide (DMF) [57]. 

Inclusion Polymerization of Various Diene Monomers in ocholic Acid Canals... 

ABSTRACT. The inclusion polymerization of diene monomers with different sizes and shapes in apocholic acid canals was studied under y-ray irradiation. It was found that the sizes and shapes of monomers profoundly influenced the microstructures of the corresponding polymers obtained. Thus, polybutadiene contained a significant amount of 1,2-units like usual radical polymerization in solution. Polyisoprene consisted of a mixture of head-to-tail and head-to-head (tail-to-tail) addition. The introduction of two methyl groups into butadiene led to the synthesis of polymers with almost exclusively head-to-tail, 1,4-trans structure. 

By the way, the sizes and shapes of both canals and guest molecules v ich can be accommodated depend on the mutual positions and on the separation of two adjacent bilayers formed by hydrogen bonds among host molecules [8]. This suggests that monomers with different sizes and shapes can be included into the canals in different ways, which may bring different microstructures of the polymers obtained by inclusion polymerization in the canals. 

The inclusion polymerization of these diene monomers in apocholic acid canals was carried out as previously reported [4,7]. 

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