Optically active polymers naturally occurring

Both synthetic and naturally occurring polymers have been used as CSPs. Figure 3.2 shows typical CSPs prepared from optically active polymers (1-18) 1-15 are totally synthetic polymers, including vinyl polymers (1-7), polyamides (8-12), polyurethanes (13), polyacetylene (14), and polysaccharide analogue (15). The CSPs 16-18 are based on natural polymers, proteins (16), and polysaccharides (17, 18). 

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). 

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. 

Many naturally occurring polymers, such as proteins, DNA and cellulose, are optically active, and some of them show characteristic functionalities such as molecular recognition ability and catalytic activity, owing to their specific chiral structure as represented by genes and proteins. These considerations have motivated considerable interest in the synthesis and application of optically active polymers [127-129]. Introducing chirality into polymers has distinctive advantages over the use of non-chiral or atactic polymers since it adds a higher level of... 

Optically active polymers are important functional materials for several industrial and bio-m ical applications and are extensively used as chiral catalysts for asymmetric synthesis, packing materials of chromatographic columns and chiral materials for the preparation of liquid crystal polymers (7). Polymers such as poly hydroxy alkanoates (PHAs), naturally occurring microbial optically active polyesters, are important materials in biomedical applications owing to their biodegradability (2). In synthetic polymer chemistry, synthesis of optically active polymers has been one of the most challenging tasks. Most synthetic chiral polymers are prepared from optically pure starting materials which are, except when isolated from nature, in limited supply and difficult to prepare (7, 3). 

Optically active polymers play a very important role in our modem society. The specialities of optically active polymers are known with their various characteristics as occurred naturally in mimicry. The present review describes the monomers and synthesis of optically active polymers from its helicity, internal compounds nature, dendronization, copolymerization, side chromophoric groups, chiral, metal complex and stereo-specific behaviour. The various properties like nonlinear optical properties of azo-polymers, thermal analysis, chiroptical properties, vapochromic behaviour, absorption and emission properties, thermosensitivity, chiral separation, fabrication and photochromic property are explained in detail. This review is expected to be interesting and useful to the researchers and industry personnel who are actively engaged in research on optically active polymers for versatile applications. 

From the examples discussed in this chapter it results clearly that chiroptical techniques supply a powerfull method for investigating the molecular conformation of macromolecules in solution. The evident limitation of the method is that optically active polymers only display chiroptical properties. While this occurs in the most naturally occurring polymers, it is not true for the most common stereoregular synthetic polymers. In these last the asymmetric carbon atomsliave either one or the other absolute configuration with the same probability and inter-and intra-molecular compensations cancel any optical rotation. 

The stereochemistry of step polymerization is considered now. Bond formation during step polymerization almost never results in the formation of a stereocenter. For example, neither the ester nor the amide groups in polyesters and polyamides, respectively, possess stereocenters. Stereoregular polymers are possible when there is a chiral stereocenter in the monomer(s) [Oishi and Kawakami, 2000 Orgueira and Varela, 2001 Vanhaecht et al., 2001], An example would be the polymerization of (R) or (S)-H2NCHRCOOH. Naturally occurring polypeptides are stereoregular polymers formed from optically active a-amino acids. 

With the notable exceptions of natural rubber and gutta-percha, almost all naturally occurring polymers are optically active. Historically, interest in optically active synthetic polymers has focused on modeling natural polymers, interpreting the conformational properties of macromolecules in solution, and investi-... 

Esters of acrylic and methacrylic acids may, of course, be polymerized by the conventional free-radical techniques that have been described at length in this series of Polymer Syntheses. If the carbinol portions of these esters are optically active and these groups are not involved in the free radical process, the products will be optically active. The distance of the asymmetric carbon atom from the poly(vinyl) backbone may be expected to influence the amount of rotation [100]. Many naturally occurring carbinols, already being optically resolved, have been converted to the appropriate asymmetric esters. Among the starting materials described for this were menthol, bomeol, various carbohydrates, and resolved synthetic carbinols such as 1-a-methylbenzyl alcohol [101,102]. 

Lactide is the cycUc dimer of lactic acid, which exists as two optical active isomers d and l. L-Lactide is the naturally occurring isomer, while OL-lactide is the synthetic blend of D-lactide and L-lactide. The polymerization of these monomers leads to either a semicrystalline polymer or an amorphous polymer. Poly(L-lactide) (PLLA), for example, is a semicrystaUine polymer with a degree of crystallinity around 37%. It has a glass transition temperature of 60-65 C and a melting temperature of approximately 175 °C. Conversely, poly(DL-lactide) (DLPLA) is an amorphous polymer with random distribution of both isomeric forms of... 

Mucopolysaccharides are naturally occurring carbohydrate polymers containing glucosamine and usually a uronic acid share to some degree the properties listed for heparin and heparinoids less sulfur, much weaker metachromatic and anticoagulant activities, but show negative optical rotation, differ in solubility characteristics, aqueous solutions show considerable viscosity. 

In a first step the monomer reacts with the initiator to form a full spectrum of sites having different R and S character. Some of these formed species have a complete selectivity and produce crystalline isotactic polymers. The proportion of such selective species for a given initiator is depending on the nature of the monomer. We have seen that for monomers with bulky substituents like t-butyl thiirane almost all the sites are purely selective, while for other monomers like methyl oxirane only 20 % of the active species are selective. If the initiator is optically active there is an unbalanced amount of R type and S type species and therefore stereoelection will occur when polymerizing a racemic monomer mixture. 

Naturally occurring polymers, such as proteins, DNA, and polysaccharides, are optically active. Consequently, the design, characterization, and preparation of chirad pol)oners are of particular interest [89]. The methods of preparing OAPs involve the pol)anerization of optically active monomers and as)anmetric pol)nnerization, which produces OAPs starting from optically inactive monomers. Concerning the aromatic PA synthesis, the simplest approach, starting with chiral monomers, is commonly used. 

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