Asymmetric 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). 

Chiral hosts provide chiral molecular-level spaces, where asymmetric inclusion polymerization may occur. A... 

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. 

DCA and apoCA can serve as effective host components for asymmetric inclusion polymerization of prochiral monomers such as 1-substituted butadienes. We reported previously the preparation of optically active polymers with extremely high specific optical rotation of arbitrary sign from (E)- or (Z)-2-methyl-l,3-pentadiene by inclusion polymerization in the canals [7,12-14]. Moreover we have found that butadiene derivatives with polar groups such as cyano or carbomethoxy group can be polymerized to yield optically active polymers. The [ajp values of the resulting polymers were much higher than those of polymers obtained by other known polymerization method. 

Chiral solid matrices are used for asymmetric synthesis polymerization of 1,3-dienes (inclusion polymerization), although the matrix reaction is not exactly a catalytic synthesis [40,41]. (R)-trans-anti-trans-anti-trans-Perhydrotriphenylene (13) [42,43], deoxyapocholic acid (14) [44,45], and apocholic acid (15) [46,47] are known as effective matrices for the... 

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. 

One of the most important results of inclusion polymerization is the synthesis of optically active polymers from nonchiral compounds. Asymmetric polymerization of /ra 5-pentadiene in PHTP has been reported. The optical purity of the polypentadiene is about 7%. DCA and ACA, as natural hosts, induce a greater asymmetric polymerization. The cis and rra/i5-2-methyl-pentadiene gave the highest asymmetric polymerization values [88]. The optical rotatory power disappeared with temperatures higher than 70°C, indicating that this process is reversible and results from a conformational transformation. 

The polymerization of trans-1,3-pentadiene, 149, in a chiral channel inclusion complex with enantiomerically pure perhydrotriphenylene affords an optically active polymer, 150 (236). Asymmetric polymerization of this monomer guest occurs also in deoxycholic acid inclusion complexes (237). 

Type II sorbents are based on an inclusion mechanism. Chiral recognition by optically active polymers is based solely on the helicity of that polymer. Optically active polymers can be prepared by the asymmetric polymerization of triphenylmethyl methacrylate using a chiral anionic initiator [264]. Helical polymers are unique from the previously discussed chromatographic approaches because polar functional groups are not required for resolution [265]. These commercially available sorbents have been used to resolve enantiomers of a-tocopherol [266]. The distinction between this group (lib) and the sorbents containing cavities is vague (Ila). 

Through-Space Asymmetric Polymerization in Inclusion Complexes and Liquid Crystals... 

Optical activity in biopolymers has been known and studied well before this phenomenon was observed in synthetic polymers. Homopolymerization of vinyl monomers does not result in structures with asymmetric centers (The role of the end groups is generally negligible). Polymers can be formed and will exhibit optical activity, however, that will contain centers of asymmetry in the backbones [73]. This can be a result of optical activity in the monomers. This activity becomes incorporated into the polymer backbone in the process of chain growth. It can also be a result of polymerization that involves asymmetric induction [74, 75]. These processes in polymer formation are explained in subsequent chapters. An example of inclusion of an optically active monomer into the polymer chain is the polymerization of optically active propylene oxide. (See Chap. 5 for additional discussion). The process of chain growth is such that the monomer addition is sterically controlled by the asymmetric portion of the monomer. Several factors appear important in order to produce measurable optical activity in copolymers [76]. These are (1) Selection of comonomer must be such that the induced asymmetric center in the polymer backbone remains a center of asymmetry. (2) The four substituents on the originally inducing center on the center portion must differ considerably in size. (3) The location... 

Farina etal. also reported a breakthrough result in 1967 which was the first example of an asymmetric polymerization reaction in the solid state, in which optically active isotactic tra i-l,4-polypentadiene was formed by irradiation of the inclusion compound containing trans-... 

However, membranes can be symmetric or asymmetric. Many porous or dense membranes are asymmetric and have one or several more porous supporting layers and a thin skin layer which, in fact, gives selectivity. If these two layers are made of different materials, the membrane is a composite one. On some occasions, dense membranes have inclusions of other materials these are, of course, also composite membranes. In the case of gas separation membranes it has became usual to include inorganic charges in a polymeric membrane to get what is called a mixed matrix composite membrane. 

There is no doubt that these principles are applicable too to other systems which undergo topochemically controlled polymerizations, such as the diacetylenes [33] and, to a lesser extent perhaps, to mixed crystals (to give copolymers) and to inclusion complexes, in urea for example. Trans, fraws-pentadiene in urea has been reported to give a stereoregular polymer [42]. Further, it is known that crystals of the urea channel complexes have chiral structures. Thus we expect that this polymerization in a single crystal would give rise to an asymmetric polypentadiene, and therefore would provide a further example of absolute synthesis. 

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