Optically Active Polymers

Optically active polymers play very important role in our modern society. The speciality of optically active polymers are known with its 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 with details. This review is expected to be of interesting and useful to the researchers and industry personnel who are actively engaged in research on optically active polymers for versatile applications.

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Optically active materials are those which can able to rotate the plane of polarization of a beam of transmitted plane-polarized light containing unequal amounts of corresponding enantiomers. The optical activity originates from the presence of chiral elements in a polymer such as chiral centres or chiral axes due to long-range conformational order in a macromolecule. In fact, most naturally occurring macromolecules possess the ability to organize to more complex high structure rather than single one and manifest their functions.
Optically active polymers are related to problems of the charged and reactive polymers, since optical activity is an inherent property of both natural macromolecules as well as a great variety of polymers synthesized. Chiral compounds are optically active and essential for life such as proteins, polysaccharides, nucleic acids, etc. and chirality is most important for existence. About 97% drugs are formed from natural sources, 2% are recemates and only 1% is achiral, in looking of chirality of nearly 800 drugs. Optically active polymers today have also become of great interest and thus play an important role in molecular arrangement and assembly, which is critical for optoelectronics super molecular structure [1-4]. The synthetic optically active polymers may also play important role like mimicry of naturally occurring polymers and that’s why the extensive studies are required on their synthesis, conformations and properties. Various kinds of optically active polymers e.g., from its helicity, internal compounds nature, dendronization, copolymerization, side chromophoric groups, chiral, metal complex and stereo-specific behaviour are reported, however, those are not placed in a systematic manner. In the present review an effort has been made to collect most of those works in one place for better understanding in the subject with detailed explanation of properties like nonlinear optical properties of azo-polymers, thermal analysis, chiroptical properties, vapochromic behaviour, absorption and emission properties, thermosensitivity, chiral separation, fabrication and photochromism.
-Classification of optically active polymers
Optically active polymers are divided into three types:

Biopolymers as obtained from nature.
Polymers prepared by almost completely isotactic polymerization by modification of naturally occurring polymer backbones such as polysaccharides.
Synthetic polymers as per the requirement with proper tailoring of functional groups.

-Speciality of optically active polymer
Optical properties of polymers are not so different of other substances, excepting those characteristics related to the chain dimension and structure or conformational changes. Optically active polymers have found interesting applications because of their specific properties. The optical properties of these materials lie at the basis of many applications, for example in chromatographic methods for enantiomeric separations or creating complex optical devices. The dispersion of the specific rotation offers information regarding the conformational changes or Cotton effect. Optically active polymers characteristics as follows:

Optically active polymers with configurational chirality: the optical activity is given by the presence of an asymmetric carbon atom in the backbone or in the side chain of the monomer;
Optically active polymers with conformational chirality: the optical activity is related to the conformational changes;
Optically active polymers with both configurational and conformational chirality: the optical activity is given by macromolecular asymmetry and by the presence of the asymmetrical centers.

-Monomers of optically active polymers
Some biological polymers are composed of a variety of different but structurally related monomer residues; for example, polynucleotides such as DNA are composed of a variety of nucleotide subunits. The solid-state structures of polystyrene – poly(Z-L-lysine) block copolymers were examined with respect to the polymer architecture and the secondary structure of the polypeptide using circular dichroism, quantitative small and wide-angle X-ray scattering, and electron microscopy [5].

Synthesis of optically active polymers

The optically active compounds are synthesized by highly efficient methodologies and catalysts. The various synthetic approaches for optically active polymers are described below:

Helical polymer: Helicity is one of the subtlest aspects of polymer chain where the polymer chain spiral structure along the chain axis acts like a spring. Helical polymers are frequently occurring in nature in single, double or triple helices form in genes, proteins, DNA, collagen, enzymes, and polypeptides. The helical conformations increase the stability of the natural polypeptides.

Preparation of artificial helical polymers is a great challenge to the researchers. So far, only limited success has been achieved in constructing microscale particles using helical polymers, despite the great number of analogous microparticles created from vinyl polymers and even from other conjugated polymers like poly(thiophene), poly(phenylene ethynylene), and poly(fluorene) and polyacetylenes. Mecking’s et al has performed extensive investigations on preparing nanoparticles from polyacetylenes, which have shown interesting potential in inkjet printing. Later on, various group of researchers have successfully prepared both nano and microparticles consisting of optically active helical substituted polyacetylenes [6]. Such nano- and microarchitectures demonstrated remarkable optical activity and significant potential applications ranging from asymmetric catalysis, chiral recognition/resolution, and enantiomer-selective crystallization to enantio-selective release [7-9].
Synthetic helical polymers may be classified as either static or dynamic helical polymers, depending on the inversion barrier of the helical conformation [10-11]. Static helical polymers have a relatively high energy barrier for helix inversion and are stable in solution, while dynamic helical polymers have a relatively low energy barrier for helix inversion and exist as a mixture of right- and left handed helical domains that are separated by rarely occurring helix reversals. Even a slight incorporation of optically active repeat units can shift the equilibrium to excess one-handed helicity.
The chiral recognition properties of biopolymers with skilled emulating of synthetic helical polymers are currently a focus of much interest. Enantioseparation, catalysis, and sensing are among the more promising applications of molecular recognition based on responsive three-dimensional intramolecular or intermolecular superchiral structures. Optically active conjugated polymers represent an attractive class of chiral macromolecules adaptable to this purpose because their chiral behaviour can be augmented by nonlinear electrically conductive or optical properties arising from conjugation along the backbone. The first example of optically active polycarbazoles, poly[N-(R)- or (S)-3,7-dimethyloctyl-3,6-carbazole]s (R- or S-PDOC) were synthesized in 60-70% yield using modified nickel coupling method [12].
Helical polymers are easily denaturalized by certain physical factors e.g. heat, ultraviolet irradiation, and high pressure and by other chemical factors such as organic solvents. Various helical polymers have been synthesized, which include polyisocyanates, polyisocyanides, polychloral, polymethacrylates, polysilanes, polythiophenes, poly(p-phenylene)s, poly(1-methylpropargyl-ester)s, poly(phenylacetylene)s and poly(-unsaturated ketone) [13-19] (Fig. 1). Other polymers are whose optical activity is main chain or side chain chirality dependent e.g. amino-acid-based polymers are nontoxic, biocompatible and biodegradable.