Asymmetric center polymerization

All of these functions are made possible by the characteristic chemical features of carbohydrates (1) the existence of at least one and often two or more asymmetric centers, (2) the ability to exist either in linear or ring structures, (3) the capacity to form polymeric structures via glyeosidie bonds, and (4) the potential to form multiple hydrogen bonds with water or other molecules in their environment. 

On the other hand if the monomer contains an optically asymmetric center, then four independent species participate in the polymerization although the reaction is still determined by only two rate constants. The relevant reactions are ... 

A wide variety of homogeneous and hetrogeneous systems are effective for polymerizing various monomers to isotactic structures. Therefore the question must be raised as to whether macro-surface theories have any required validity for steric control but whether some other factor is important. Fig. 10 summarizes the monomers polymerized to isotactic polymers and shows that all propagate with an asymmetric center at the end of the chain. 

The same concept is presented by Tsuruta, Inoue, Ishimori and Yoshida (95) in their study of propylene oxide polymerization. They concluded that the stereochemistry of the monomeric units in the isotactic structure is regulated by the asymmetric center of the proceeding monomer unit. 

Diene Polymers Polymerization of a 1,3-diene yields a polymer having true asymmetric centers in the main chain and ozonolysis of the polymer gives a chiral diacid compound (12) whose analysis of optical purity discloses the extent of chiral induction in the polymerization (Scheme 11.2) [12,35-39], The polymerization of methyl and butyl sorbates methyl and butyl styrylacrylates and methyl, ethyl, butyl, and /-butyl 1,3-butadiene-1-carboxylates using (+)-2-methylbutyllithium, butyllithium/(-)-menthyl ethyl ether, butyllithium/menthoxy-Na, butyllithium/bomeoxy-Na, butyllithium/Ti((-)-menthoxy)4, and butyllithium/bomyl ethyl ether initiators [35-37] and that of 1,3-pentadiene in the presence of... 

Optically active polyaldehydes possessing optically active side chains, such as poly-(R)(+)-citronellal, poly-(R)(+)-6-methoxy-4-methylhexanal, and poly-(S)(+)-2-methylbutanal, have been prepared by Goodman (1, 22). The optical activity of the polymers was enhanced as compared with their model compounds. It was concluded that the enhancements of the optical activity arose from a conformational rigidity around the asymmetric center in the side chain of the polymer. From degradation studies of the polymers it was concluded that the optical activity of the monomer was unchanged, and no racemization had occurred during polymerization and degradation. 

Vinylidene chloride doesn t polymerize in isotactic, syndiotactic or atactic forms because no asymmetric centers are formed during polymerization. 

The most self-evident method for formation of a stereoregular polymer is the polymerization of enantiomerically pure monomers with structural properties supporting the assembly of a confor-mationally uniform and stable secondary structure. Alternatives such as asymmetric polymerization generating stereogenic centers ( asymmetric synthesis polymerization ) or P- or M-he-... 

In 1978 Lahav et al. (Weizmann Institute, Israel) succeeded in an absolute asymmetric syntheses of chiral oligomeric crystals by the four-center polymerization of achiral monomer. 

Biopolymers are polymers formed in nature during the growth cycles of all organisms hence, they are also referred to as natural polymers. The biopolymers of interest in this review are those that serve in nature as either structural or reserve cellular materials. Their syntheses always involve enzyme-catalyzed, chain-growth polymerization reactions of activated monomers, which are generally formed within the cells by complex metabolic processes. The most prevalent structural and reserve biopolymers are the polysaccharides, of which many different types exist, but several other more limited types of polymers exist in nature which serve these roles and are of particular interest for materials applications. The latter include the polyesters and proteins produced by bacteria and the hydrocarbon elastomers produced by plants (e.g. natural rubber). In almost all cases (natural rubber is an exception), all of the repeating units of these biopolymers contain one or more chiral centers and the repeating units are always present in optically pure form that is, biopolymers with asymmetric centers are always 100% isotactic. 

A compound such as benzofuran is a monomer with no asymmetric center. In a formal sense, its oxygen-containing five-membered ring may be considered as a vinyl ether. Upon polymerization two asymmetric centers form from the two vinyl carbon atoms. The product is optically active although its exact structure and its optical purity is not known. 

FIGURE 3.3 Construction of asymmetric reaction field for acetylene polymerization by dissolving Ziegler-Natta catalyst, Ti(0-n-Bu)4-AlEt3, into the chiral nematic LC. The chiral nematic LC includes an axially chiral binaphthyl derivative or an asymmetric center containing chiral compound. 

In the polymerization of the above racemic a-olefins with TiCl4/Zn[(S)-2-CH2CH(CH3)C2H5]2 catalyst, enantiomer selectivity has been observed, although it is rather low. The selectivity decreased as the distance between double bond and the asymmetric carbon increased, and no selectivity was observed for 5-methy-l-heptene (283), whose asymmetric center is at the y-position with respect to the double bond. Enantiomer selectivity has also been observed in a-olefin polymerization with MgCl2-supported catalysts modified with optically active Lewis bases and in the copolymerization... 

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... 

One purpose of the present investigation was to obtain some additional information on structure-crystallinity relationships in this family of polymers by the preparation of stereoregular isotactic polyesters from a single asymmetric isomer of the chiral monomer that is, from an optically-active a,a-disubstituted-s-propiolactone. Because the polymerization reaction mechanism operates through scission of the alkyl-oxygen bond and does not involve bond reorganizations at the asymmetric center, it was fully expected that polymerization of the optically-active monomer... 

In addition to this structural problem, there is also a stereochemical problem. Unlike vinyl monomers or a-olefins, propylene oxide has an asymmetric carbon atom before polymerization. Thus, it is possible to obtain optically active polymers if the polymerization proceeds with either complete retention or complete inversion at the asymmetric center. Four different dimer units in the main chain are therefore possible [eq.(l)]. 

He concluded that this remarkable difference in physical properties resulted from identical configuration of all the asymmetric centers in the crystalline polymer. The liquid polymer from racemic monomer, on the other hand, was evidently a stereorandom atactic polymer (4). Both polymers were later shown to be formed almost exclusively by head-to-tail polymerization. Vandenberg, Price, and others later showed that addition of each epoxide unit to the polymer chain occurs with inversion of configuration at the carbon atom where ring opening occurs. The asymmetry in this case is not disturbed, however, because the bond between the oxygen and the asymmetric carbon is never broken. 

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