Introduction

One of the subtlest aspects of polymer chain structure is that of helicity where the polymer chain structure spirals along the chain axis like in a spring. The most common examples of this phenomena are found in nature in the alpha helix of some polypeptides, the triple helix of collagen, and, of course, the double helix of DNA. Nature seems to favor helicity. Though there are both right and left hand versions, for some reason nature chose to use only the left hand version of the amino acids that are synthesized by plants and animals. The fact that only one of the two isomers is used leads to some interesting stereochemical consequences. The helical conformations increase the stability of the natural polypeptides. Did you know that some bacteria can survive in boiling water? This is because their natural polymers have been stabilized by such helical structures. Small segments of such helical structures are what nature uses to mold enzymes into certain shapes so that they can do their catalytic magic. For example, a flexible randomly coiled segment may be joined by two alpha-helix segments so that they can react together on some substrate. Thus, this special structure relates to a molecule’s special functions of molecular recognition ability and catalytic activity.

Synthetic routes to helical polymers

The existence of helical folding in polymers like proteins and nucleic acids is very important in biological systems, but biological polymers are not the only polymers to exhibit such special arrangements, Synthetic polymers also show helicity. Isotactic polypropylene [1] and the halogenated polyolefins also exhibit this stereo property for drawn solid-state fibers though in solution a random configuration is thermodynamically favored. However stereochemistry in man-made polymeric materials remained an unexplored area until the development of the Ziegler catalysts and that of Natta on stereo regularity and optical activity in polyolefins.

Today it is known that the intramolecular self-organization necessary to induce a molecule to fold into a specific conformation is driven by the interplay of multiple molecular recognition processes such as hydrogen-bonding, Van der Waals, electrostatic and solvophobic interactions. Many researchers today are developing polymers that adopt a chiral or helical conformation as a consequence of the cooperative action of intramolecular hydrogen-bonding and nonbonded packing interactions [2] that develop at higher generations and in poor solvents. Under chiral synthetic conditions it is possible to synthesize a polymer with predominantly one screw sense and thus to obtain a solid-state polymer with optical activity. In principle, the chirality will be lost upon melting and dissolving because of the fast conformational equilibrium. In order to synthesize helical chiral polymers, the technique of helix-sense-selective polymerization has been used. The key point in this kind of polymerization is to prevent the random conformation change by creating either rigidity in the main chain or steric repulsion of the side groups. The successful examples include synthesis of polymethacrylates, polyisocyanides, polyisocyanates, etc.

Helix-Sense-Selective Polymerization of Methacrylates: A Successful example

The main technique to form excess one-handed sense polymer is to take advantage of the different reaction rate of right- or left-handed helical propagating polymer chain with either

	a chiral initiator, or
	a chiral chain-transfer agent.

Use of Chiral Initiators

In helix-sense-selective polymerization of methacrylates, chirality is introduced to the polymer by using chiral initiators. The one screw helical conformation of the polymer is maintained by steric repulsion between the bulky ester groups. Because of the electron-withdrawing effect of the ester group in methacrylate, anionic polymerization has been widely used. The first example of an optically active vinyl polymer with one-handed helical conformation is poly(trityl methacrylate) (Poly(TrMA)) [3] polymerized from trityl methacrylate(TrMA) shown below.

i. It was found here that optical activity is lost when the side-chain of the polymer is a methyl ester group. This indicates that the helical conformation is maintained by steric hindrance of the bulky triphenylmethyl group.

ii. It was also found here that polymerization ability depends not only on types of the initiator, but also on the position and the number of methyl groups substituted at the phenyl rings of TrMA[5]. The ability to form a helical structure is in following order:

o-MeTrMA	o-Me2TrMA	o-Me3TrMA.

This trend is due to the change of stereoscopic structure of the triarylmethyl group. More methyl groups on the ortho position results in more steric hindrance between the adjacent monomer units. In fact, o-Me2TrMA only gives low polymerization yield and the polymerization of o-Me3TrMA is completely inhibited.

iii. When three methyl groups are at different positions on the phenyl rings, the size of the group and the steric hindrance is changed. m-Me3TrMA shows little change to polymerization yields compared to TrMA but p-Me3TrMA gives a lower polymerization yield.

This suggests that substitutions of the meta position have about the same size and steric hindrance of the bulky group as in TrMA and that para substituted TrMA has a larger group which results in the increased hindrance between the bulky side groups of the adjacent monomer units.

Polymerization in the presence of a chiral radical chain transfer agent has also been able to introduce helix-sense-selectivity to the polymer main chain. Different chiral chain transfer agents are used, including (+)- and (-)-neomenthanethiol (NMT, 20a and 20b) [6] .

Basic chain transfer reaction happens as shown below (This is for the free radical polymerization of 1-phenyldibenzosuberyl methacrylate (PDBSMA) using a chiral thiol like (+)- or (-)-NMT and
(-)-MT)

Step A: Chain transfer from a propagating polymer chain to a thiol

Step B: Reinitiation step

Step C: Termination by combination of a polymer propagating radical with a thiol radical

The reaction rates of Step A and C might be different for right- or left-handed sense helical chain. This results in excess one-handed helicity of the main chain.

Similar success with other polymers

	Polyisocyanides have a rather well defined helical conformation and are accessible in an optically active form by a nickel catalyzed polymerization reaction. The macromolecular helix is stable in solution when bulky side groups are present, but slowly unfolds in the case of less sterically demanding side groups. Because of the regular helical structure, it has been proposed that hydrogen bonds could be formed between the amide groups present in the side chains[7] which are more or less above each other. This would result in an increased regularity, stability, and rigidity of the macromolecular helix.
	Recently Jeffrey S. Moore et al [8] have reported that synthetic oligomers with an all-carbon backbone, linear phenyl-acetylenes with ester-substituted benzene rings linked to one another by acetylene groups, spontaneously fold into a stable helical configuration in acetonitrile, and that this apparently involves a "solvophobic" mechanism similar to the hydrophobic collapse model of protein folding in water. In both systems, the phenylacetylene oligomers and biological proteins, hydrophobic groups associate to form a compact structure that excludes the solvent. The phenylacetylene oligomers have longitudinal cavities that might be used for binding metals and other reactive species. It has also been suggested that such systems could be used in the design and construction of synthetic enzymes.
	The Intelligent Polymer Research Institute has recently reported the first synthesis of optically active polyaniline, a polymer that possesses the unusual feature of being both electrically conducting and chiral. The remarkably facile synthesis involved the enantioselective electropolymerisation of aniline [9,10] in the presence of either (+)- or (-)-camphorsulfonic acid (HCSA). The polyaniline chain preferentially adopts a right- or left-handed helical screw depending on which hand of the CSA- anion it incorporates. They have also recently discovered routes to optically active substituted polyanilines in which the aniline rings in the polymer chains contain methoxide or sulphonate sustituents. These latter polymers, which like the parent polyaniline can be made in either helical hand, have the advantage that they are readily soluble in either organic solvents or water, respectively.

Applications

Today researchers are doing work pointing to the possibility that chiral polymers might be developed with enantiomeric excess for varied uses such as chiral chromatography, asymmetric catalysis and many others.

Chiral chromatography

Chiral materials in which enantiomeric excess and enantiomeric content is nonlinearly related [11] can be applied to uses such as chiral chromatography Some helical polymers such as cellulose esters whose chirality comes from helicity also can be used for enantiomeric separations. It is thought that the mechanism of separation on these columns involves a combination of attractive interactions and inclusion of the analyte in a chiral cavity. One-handed helical poly(TrMA) has been widely used as a chiral stationary phase in HPLC to resolve many classes of racemates including drugs and their precursors. Poly(TrMA) with high molecular weight is crystalline and insoluble in solvents and thus it is used as a chiral stationary phase when it is ground and sieved to small homogeneous particles.

Use of Chiral Electrically Conducting Polymers

It has been recently found that chiral polyanilines have considerable potential as:

	chiral electrodes for the electrochemical asymmetric synthesis of drugss
	chiral membranes for the separation of chiral species.

Particularly attractive would be their use as chiral electrodes in electrochemical asymmetric synthesis. This little explored area could have major advantages over conventional chemical asymmetric syntheses, such as:

	no requirement for expensive chiral auxiliaries and
	the reduced number of by-products.

Asymmetric Synthesis

The chiral conformational preference of these polymeric materials is also being exploited in enantioselective catalysis. It has been have discovered that the Lewis acid complexes of the optically active binaphthyl molecules [12] and polymers can carry out highly enantioselective organic reactions such as organozinc additions to aldehydes, hetero-Diels-Alder reactions [13], 1,3-dipolar cycloadditions, reductions of ketones, Michael additions, epoxidations and others. Using the chiral polymers has the advantage of easy recovery of the catalysts and simplified product purification.

> Optical Applications

Chirality may be introduced into polyelectrolytes in water solution. This allows new understanding of the glass-like restrictions to motions associated with the disorderly cooperative hydrophobic capsules formed in various pH conditions and opens the possibility of developing new kinds of chiral-based switches [14]. Also new kinds of sensors and switches based on chirality, involving circularly polarized light or temperature have now become possible. The behavior of liquid crystal forming optically active helical polymers [15] offers insight into the forces exerted on the accessible conformational states. One particular use is based on the synthesis of chiral-photochromic (Ch-Ph) acrylic copolymers containing chiral and photochromic groups in a joint monomer unit. The Ch-Ph monomer units of such copolymers are characterized by the appropriate value of the helical twisting power, predetermining the pitch of the helical supramolecular structure and the selective light reflection wavelength. Irradiation of the films with UV light causes the photoisomerization of the Ch-Ph groups and their helical twisting power can be drastically varied changing the color and other optical properties of the polymer films.

Recently, researchers have also been investigating some synthetic helical polymers that mimic the properties of proteins. These polymers respond in interesting ways to different solvent environments. These are "living" polymers that seem to reconstitute their structures and will prove to be "smart" materials that mimic DNA in some ways. Thus, slowly but surely we are beginning to understand how nature puts large molecules together and we have figured out how to do it ourselves. However, we are not very good at it and cannot make very large molecules very efficiently. Therefore, it is a long way to go and it seems that the road is far from straight. It is a helical convoluted one.

Written by: Jain Prashant * and Shah Pratik *
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References

* B.Tech (Polymer Engineering & Technology), UICT, Mumbai, India

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