Synthesis of Optically Active Polymers

B. Synthesis of Optically Active Polymers from Chiral Monomers. 72... 

Interest in optically active polymers arose from analogy with macromolecules of biological origin. In addition, there was the hope to obtain new information to clarify the stereochemical features of synthetic polymers this, in fact, did come about. Attempts to direct the course of polymerization using chiral reagents had been made already prior to the discovery of stereospecific polymerization. It was only after the 1950s, however, that the problem of polymer chirality was tackled in a rational way. The topic has been reviewed by several authors (251-257). In this section I shall try to illustrate three distinct aspects the prediction of chirality in macromolecular systems, the problems regarding the synthesis of optically active polymers, and polymer behavior in solution. 

The synthesis of optically active polymers was tackled with the purpose not only of clarifying the mechanism of polymerization and the conformational state of polymers in solution, but also to explore the potential of these products in many fields as chiral catalysts, as stationary phases for chromatographic resolution of optical antipodes, for the preparation of liquid crystals, and so on. 

Until now I have discussed the methods of synthesis of optically active polymers from chiral monomers. As is well known in organic chemistry, it is also possible to produce chiral molecules with one preferred configuration by reaction of achiral molecules in the presence of some chiral influence. These reactions are known as asymmetric syntheses (36, 323-325) when an unsatuiated compound is involved, the term enantioface-differenriating reaction is often used (281). 

Inoue,S., Tsukuma.I., Kawaguchi,M., Tsuruta,T. Synthesis of optically active polymers by asymmetric catalysts. VI. Behavior of organozinc catalyst systems in the stereoselective polymerization of propylene oxide. Makromol. Chem. 103,151 (1967). 

The synthesis of optically active polymers from racemic monomers could in principle be ascribed i) to asymmetric induction by optically active terminal groups of the macromolecules originally present in the catalyst, ii) to a steric control of the propagation exerted by the intrinsically asymmetric active centers of the catalyst, or iii) to both these factors. 

Further investigations are necessary to better evaluate whether the monomers modified by the presence of easily cleavable asymmetric groups, may be used for the synthesis of optically active polymers. 

The synthesis of optically active polymers is an important area in macromolecular science, as they have a wide variety of potential applications, including the preparation of CSPs [31-37]. Many of the optically active polymers with or without binding to silica gel were used as CSPs and commercialized [38]. These synthetic polymers are classified into three groups according to the methods of polymerization (1) addition polymers, including vinyl, aldehyde, isocyanide, and acetylene polymers, (2) condensation polymers consisting of polyamides and polyurethanes, and (3) cross-linked gels (template polymerization). The art of the chiral resolution on these polymer-based CSPs is described herein. 

During polymerization of (R3 )-2-(phenoxymethyl)thiirane by diethylzinc/L-amino acid, the S-enantiomer of the thiirane was consumed preferentially <2002PSA3443>. Synthesis of optically active polymers prepared from thiiranes was described <2005TA2149>. 

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). 

There are several alternative methods for the synthesis of optically active polymers from achiral or racemic monomers that do not involve polymerization catalysts. Optically active polymers have been formed from achiral dienes immobilized in a chiral host lattices [ 106]. In these reactions, the chiral matrix serves as a catalyst and can be recovered following the reaction. For example, 1,3-penta-dienes have been polymerized in perhydrotriphenylene and apochoUc acid hosts, where asymmetric induction occurs via through-space interactions between the chiral host and the monomer [107,108]. The resultant polymers are optically active, and the optical purities of the ozonolysis products are as high as 36%. In addition, achiral monomers have been found to pack in chiral crystals with the orientations necessary for topochemical soHd-state polymerization [109]. In these reactions, the scientist is the enantioselective catalyst who separates the enantiomeric crystals. The oligomers, formed by a [27H-27i] asymmetric photopolymerization, can be obtained in the enantiomeric pure form [110]. 

S l gny E, Merle-Aubry L (1979) General methods of synthesis of optically active polymers. In Selegny E (ed) Optically Active Polymers. D. Reidel, Dordrecht, p 15... 

Coates, G W. Waymouth, R. M. Enantioselective cyclopolymerization of 1,5-hexadiene catalyzed by chiral zirconocenes a novel strategy for the synthesis of optically active polymers with chirality in the main chain. J. Am. Chem. Soc. 1993,115, 91-98. 

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). 

The brief presents a systematic study of synthesis of optically active polymers. It discusses in detail about the syntheses of three different types of optically active polymers from helical polymers, dendronized polymers and other types of polymeric compounds. The brief also explains the syntheses of optically active azoaromatic and carbazole containing azoaromatic polymers and copolymers optically active benzodithiophene and optically active porphyrin derivatives. The final chapter of the brief discusses different properties of optically active polymers such as nonlinear optical properties, chiroptical properties, vapochromic behavior, absorption and emission properties, fabrication and photochromic properties. The intrinsic details of different properties of optically active polymers will be useful for researchers and industry personnel, who are actively engaged in application oriented investigations. 

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. 

GENERAL METHODS OF SYNTHESIS OF OPTICALLY ACTIVE POLYMERS... 

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