Apocholic 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. 

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

The crystals of apocholic acid canal complexes kept their original shapes and appearance during the polymerization just like the case of deoxycholic acid. It was assured that all monomers used polymerize in the canals and not outside of them, because the polymer yield was less than 1.0 mg per 0.50 cm of monomer in the absence of apocholic acid under the condition studied. The methanol-insoluble solid or rubbery polymers after the extraction of apocholic acid with boiling methanol were characterized. 

Table I summarizes the result of the inclusion polymerization of various diene monomers in apocholic acid canals.

INCLUSION POLYMERIZATION OF DIENE MONOMERS IN APOCHOLIC ACID CANALS... 

This microstructure is similar to that of a polymer obtained by free radical polymerization in solution [19], which demonstrates that inclusion polymerization proceeded without the steric control in the apocholic acid canals. So it is assumed that the monomer molecules are loosely trapped into the canals and not oriented in them regularly enough to yield a highly 1,4-trans polymer. 

Fig. 3. %-NMR (left) and IR (above) spectra of the polymers obtained by inclusion polymerization in apocholic acid canals.

The irj,elusion polymerization of various diene monatiers in apDcholic acid canals was studied in the same way as the case of deoxycholic acid canal complexes [6,7]. The apocholic acid-monomer canal ccxtplexes were at first prepared in sealed tubes by absorbing the monomer molecules into vacant canals of solvent-free apocholic acid crystals. Then the moncxners present in the canals were post-polymerized by heating after y-ray irradiation. 

Most of canal corplexes obtained after polymerization melted at higher temperatures than that of apocholic acid itself (176-8°C). The most remarkable increase of the melting point was observed in the case of 2,3-dimethylbutadiene. This is probably because the polymer chains fit in the canals best among the polymers studied due to its suitable size and symnetric shape. On the other hand, polybutadiene does not seem to stabilize the canal complexes due to its small cross section as compared with that of the canal. 

Apocholic acid is a flexible host v ich can include a variety of monomers with different sizes and shapes. And the stereochemical control for the polymerization in the canals work effectively in case of two methyl-substituted butadienes, such as 2,3-dimethylbutadiene, 2-methylpentadi-ene and so on, v ile not in case of small monomers, such as butadiene and isoprene. 

In contrast to apocholic acid, urea and thiourea hosts are highly selective [ 25], v ile perhydrotriphenylene host is highly flexible with respect to the sizes and shapes of their guest monomers [23]. The inclusion polymerization in their canals affords the corresponding diene polymers with almost exclusively 1,4-trans structure. 

It can be concluded from the comparison of these results that apocholic acid has an intermediate capability between urea, thiourea and perhydrotriphenylene as regards both the selectivity of the guest molecules and the stereochemical control for inclusion polymerization in the canals. 

We found earlier that deoxycholic acid (3a, 12a-dihydroxy-5B-cho-lan-24-oic acid DCA) and apocholic acid (3a, 12a-dihydroxy-53-chol-8(14)-en-24-oic acid apoCA) can provide tunnel-like spaces, called canals or channels, which are suitable for one-dimensional polymerization [3-8]. 

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