Optically Active Polymers

30 Jan 2018

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  1. Introduction

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. Most of the naturally occurring molecules/macromolecules, such as nucleic acids, proteins, and polysaccharides are chiral and optically active. Chirality is essential for life. This situation can be very obviously seen if we look at the chirality of nearly 800 drugs (about 97%) derived from natural sources. Only 2% are racemates and only 1% is achiral. Synthetic optically active polymers are of great interests, since they might mimic the fascinating functions of naturally occurring polymers, leading extensive studies being conducted on their synthesis, conformations and functions. In fact, most naturally occurring macromolecules possess the ability to organize to more complex high structure rather than single one and manifest their functions. Optical activity is a physical spectral property of chiral matter caused by asymmetric configuration, confirmations and structures which have no plane and no centre of symmetry and consequently have two mirror image enantiomeric forms of inverse optical rotation. The recemic mixture of chiral enantiomers is optically inactive.

The great majority of natural molecules contain chiral centres and are optically active. This is the case because living systems and their extracts as enzymes are able to produce completely stereoselective asymmetrical synthesis or transformations. This led Pasteur to say that ‘life is asymmetrical’ at the molecular level. The majority of food and drug molecules of physiological activity are chiral [1]. Xi et al. [2-8] investigated about chirality of optically active compounds. Optically active polymers today have also become of great interest owing to their chiral structure which may play an important role in molecular arrangement and assembly, which is critical for optoelectronics super molecular structure [9-12]. Chiral polymers with helical chain backbone have received increasing attention due to their helicity generating from secondary interactions such as hydrogen bonds and van der Waals forces. These chiral helical polymers undergo conformational change as well as helical reversal easily. The concept of the optically active aromatic chromophore as ‘conformational probe’ in isotactic polymers can be further extended by the use of optically active monomers [13]. Optically active polymers have exhibited a number of interesting properties in several highly specialized areas such as chromatographic resolution of steroregular [14], chiral [15-16], asymmetric catalysis and phase of the separation of racemic mixtures [17], thermosensitivity [18], synthesis molecular receptors and chiral liquid crystals for ferroelectric and nonlinear optical applications [20].

In the last year [52], Angiolini and co-workers have synthesized and investigated methacrylic polymers bearing in the side chain the chiral cyclic (S)-3- hydroxypyrrolidine moiety interposed between the main chain and the trans azoaromatic chromophore, substituted or not in the 4’ position by an electron withdrawing group. In these materials, the presence of a rigid chiral moiety of oneprevailing absolute configuration favours the establishment of a chiral conformation of one prevailing helical handedness, at least within chain segments of the macromolecules, which can be observed by circular dichroism (CD). The simultaneous presence of the azoaromatic and chiral functionalities allows the polymers to display both the properties typical of dissymmetric systems (optical activity, exciton splitting of dichroic absorptions), as well as the features typical of photochromic materials (photorefractivity, photoresponsiveness, NLO properties).

Recently, highly efficient methodologies and catalysts have been developed to synthesize various kinds of optically active compounds. Some of them can be applied to chiral polymer synthesis and in a few syntheses for optically active polymers; chiral monomer polymerization has essential advantages in applicability of monomer, apart from both asymmetric polymerization of achiral or prochiral monomers and enantioselective polymerization of a recemic monomer mixture. Optically active chiral polymers are not only fundamentally interesting, due to the rich and complex architecture of macromolecular chirality as compared to that of small molecules, but also technologically important because their unique chiral arrays give rise to a number of potential, and in some cases commercially implemented.

Classification of Optically active polymers:

Optically active polymers are divided into three types:

  1. Biopolymers: Biopolymers are the main type of biomaterials. According to their degradation properties, biopolymers can be further classified into biodegradable and non-biodegradable biopolymers. Many implants, such as bone substitution materials, some bone fixing materials, and dental materials, should possess long term stable performance in the body. Recently biopolymers acts as developments in bone tissue engineering, vascular tissue engineering, nerve tissue engineering, genitourinary tissue engineering, regenerative medicine, gene therapy, and controlled drug delivery have promoted the need of new properties of biomaterials with biodegradability. Biologically derived and synthetic biodegradable biopolymers have attracted considerable attention [21].
  2. Polymers prepared by almost completely isotactic polymerization by modification of naturally occurring polymer backbones such as polysaccharides.
  3. Synthetic polymers: Polymers synthesized from low molecular weight compounds are called synthetic polymers, e.g., polyethylene, PVC, nylon and terylene [7]. This polymer is also divided into three types:

(a) Addition polymers: Addition polymers are including vinyl, aldehyde, isocyanide and acetylene polymers that were prepared via addition polymerization reaction such as poly(acryl amide)s, polyolephynes, polystyrene derivatives, polyazulenes, poly(vinyl ether)s, polymethacrylate, polymethacryloylamine, polychloral, polyisocyanides, polyisocyanates, polyacethylene and polyethers [22–32].

(b) Condensation polymers: Condensation polymerization continues to receive intense academic and industrial attention for the preparation of polymeric materials used in a vast array of applications [28]. One of application is synthesis of chiral polymers. For this purpose, monomer must be optically active.

(c) Cross-linked gels: One of application is synthesis of chiral polymers. For this purpose, monomer must be optically active. One of application is synthesis of chiral polymers. For this purpose, monomer must be optically active.

Why optically active polymers are important?


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

Polymerization is the process of combining many small molecules known as monomers into a covalently bonded chain or network. During the polymerization process, some chemical groups may be lost from each monomer. This is the case, for example, in the polymerization of PET polyester. The monomers are terephthalic acid (HOOC-C6H4-COOH) and ethylene glycol (HO-CH2-CH2-OH) but the repeating unit is -OC-C6H4-COO-CH2-CH2-O-, which corresponds to the combination of the two monomers with the loss of two water molecules. The distinct piece of each monomer that is incorporated into the polymer is known as a repeat unit or monomer residue.

The identity of the monomer residues (repeat units) comprising a polymer is its first and most important attribute. Polymer nomenclature is generally based upon the type of monomer residues comprising the polymer. Polymers that contain only a single type of repeat unit are known as homopolymers, while polymers containing a mixture of repeat units are known as copolymers. Poly(styrene) is composed only of styrene monomer residues, and is therefore classified as a homopolymer. Ethylene-vinyl acetate, on the other hand, contains more than one variety of repeat units and is thus a copolymer. 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 [33].

  1. Synthesis of optically active polymers

Much of the attention in chiral polymers results from the potential of these materials for several specialized utilizations that are chiral matrices for asymmetric synthesis, chiral stationary phases for the separation of racemic mixtures, synthetic molecular receptors and chiral liquid crystals for ferroelectric and nonlinear optical applications. Presently optically active compounds are synthesized by highly efficient methodologies and catalysts. In a few synthetic approaches for optically active polymers, chiral monomer polymerization has essential advantages in applicability of monomer, apart from both asymmetric polymerization of achiral or prochiral monomers and enantioselective polymerization of a racemic monomer mixture [17].

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