Helical conformation optically active polymers

It is possible that the helicity is a result of the chiral substitution itself and that the polymers with achiral substituents have, in fact, all-anti conformations. While this possibility cannot be directly ruled out, comparison of the spectroscopic data for the polymers with chiral substituents and achiral substituents, for example, 47 and 48, respectively, indicates similar main-chain dihedral angles, since the UV absorption maxima are so similar. Both polymers should therefore be latent helical, that is, contain segments of opposite screw sense separated by strong kinks (helix reversal points), with the difference being that in the case of 47 the overall numbers of P and M turns are equal, whereas for 48, one of the screw senses predominates, resulting in net helicity and optical activity. 

Incorporation of the (.S )-2-mcthyloctoxy group afforded optically active polymers with preferential helical screw sense (see Section 3.11.6.1). The observed helicity was corroborated by force field calculations, which indicated similar helical conformations for both dialkoxy- and dialkyl-substituted polymers. Based on their similar conformational properties, it was suggested that the origin of the spectral red shift was electronic, due to a a-n mixing interaction, as for polymers 76 above, rather than conformational. 

Isocyanide Polymers Bulky isocyanides give polymers having a 4 1 helical conformation (115) [154]. An optically active polyisocyanide was first obtained by chromatographic resolution of poly(f-butyl isocyanide) (poly-116) using optically active poly((S)-sec-butyl isocyanide) as a stationary phase and the polymer showing positive rotation was found to possess an M-helical conformation on the basis of CD spectral analysis [155,156]. Polymerization of bulky isocyanides with chiral catalysts also leads to optically active polymers. 

Olefin Polymers Isotactic polymers of propylene and 1-butene obtained by optically active metallocene catalyst (145) have been reported to show large specific rotation in suspension ([a]D-123°, -250° for polypropylene [a]D+130° for polybutene), which was lost when the polymers were completely dissolved or heated [176,177]. The optical activity was ascribed to a helical conformation of the polymer chain with preferential screw sense. 

Several biopolymers and synthetic optically active polymers are known to exhibit an inversion of helicity (helix-helix transition) between right- and left-handed helical conformations when changing the external conditions, such as solvent, temperature, or by light irradiation. However, switching of the macromolecular helicity by chiral stimuli is rare, and can be used to sense the chirality of specific chiral guests. The helicity of optically active helical poly(phenylacetylene)s 67-69 can be switched by external chiral and achiral stimuli [123-126]. The first example of such a helix inversion induced by... 

Asymmetric Polymerization. Polymerization of methacrylate derived from (1) affords optically active polymer of helical conformation of single screw sense. ... 

V-1 -Naphthylmaleimide (NMI, 36) affords an optically active polymer ([a]435 +152° to 296°) by polymerization using an EtzZn—Bnbox complex.110 The obtained polymer resolves l,T-bi-2-naphthol when used as an HPLC packing material. Although the tacticity of the polymer is not clear, the polymer may have a helical conformation with an excess screw sense in this case. 

Isotactic vinyl polymers often possess a helical conformation in the solid state however, without bulky substituents present (vide infra) in solution at room temperature, helix—helix reversal takes place fast and no optical activity is observed. Ortiz and Kahn reported a borderline case in which a non-bonded interaction between the monomers leads to the formation of isotactic 39 (Chart 7) by anionic polymerzation at —78 °C. Optically active polymers can be isolated, but in solution the proposed one-handed helicity is lost in less than 1 h.148 An intriguing class of polymers formed by polycondensation of diboronic acid and chiral tetraalcohols has been studied by Mikami and Shinkai and is exemplified by polymer 40 (Chart 7). In this D-mannitol-based polymer, the noncovalent intramolecular interaction between the amines and the boron atoms imposed a sp3-hybridization on boron, which, according to calculations, results in a helical conformation of the macromolecule.149... 

Sterically Restricted Poly (methacrylate ester)s. It was recognized by Okamoto and coworkers150 that the anionic polymerization of tri-phenylmethyl methacrylate (TrMA, 41) (Chart 8) at low temperature in the presence of an optically active initiator results in the formation of an isotactic, optically active polymer. The helical conformation of the backbone in these macromolecules is the result of steric interactions between the bulky trityl groups, as was shown by the loss of optical activity upon their conversion to methyl ester groups. This class of bulky... 

Some isotactic polymers such as polychloral and poly(triphenylmethyl methacrylate)289 are known to exist only in purely helical conformation. The helical structure of the polymers is rigid even in solution, owing to the bulkiness of the side-groups. This has been demonstrated by the measurement of high optical activity of the polymers prepared by asymmetric polymerizations the optical activity is based on a one-handed helical conformation of the polymer chain. 

In our laboratories, we have used a related approach to covalently attach chiral camphorsulfonate groups to N centers of PAn, by the reaction of EB with (lS)-(+)-10-camphorsulfonyl chloride in NMP/pyridine.153 The optically active product 13, isolated as the HC1 salt, is believed to preferentially adopt a one-handed helical conformation for its polymer chains. This provides the first example of chiral induction in a PAn species through a covalently attached group. A significant advantage for the product 13 compared to the chiral PAn/HCSA salts described earlier is that it consequently retains its optical activity upon alkaline dedoping in solution to its EB form. 

ORD and CD measurements of optically active thiolacetates (39), poly(thiolactides) (XV) (II), and poly(e-thiolactones) (21) show two Cotton effects, one centered around 230-240 nm and the other at about 280 nm. The ORD spectra of the polymers are nearly identical with those of low-molecular-weight model compounds, thus indicating the absence of a helical conformation of the polymers in solution. 

Optically active polymers can be prepared by free-radical additions that give polymers whose chirality is the result of an excess of one single-screw sense. Most polymers will not maintain a helix screw conformation in solution unless the chain backbone is rigid or the polymer side-chains are very large and prevent conformational relaxation. Polymers derived from trityl and related methacrylates have this apparent capacity, i.e. they display excess helical content in solution. Comprehensive reviews of helix-sense-selective anionic polymerizations have appeared [12], and in this section, we highlight some of the recent developments in this field related to radical polymerizations of these highly hindered methacrylates. 

Compounds 54 and 56 also afford optically active polymers by asymmetric coordination polymerization using optically active zirconocene catalyst, 65. Racemic 65 is known to afford a highly isotactic (mm > 99%) polymer in the polymerization of 54. In the polymerization of 56, the optical activity of the products increased with the [56]/[65] ratio in feed. The specific rotation of the product at [56]/[65] = 50 was [a] D+180°. This strongly suggests that poly-56 synthesized using 65 possesses a single-handed helical conformation. The advantage of this method over the asymmetric anionic polymerization is that the polymerization of 65 can be conducted at room temperature. 

The anionic polymerization of optically active (+)- or (-)-m-tolyl vinyl sulfoxide ([a]o+486°, -486°) using BuLi or BuLi-(-)-Sp leads to an optically active polymer 79([a]o +274 ° to+311° (from (+)-monomer) [aj -272° to -310° (from (-)-monomer)). Oxidation of 79 afforded polymer 80 with an achiral side group that was still optically active ([a]o+19° to+42° starting from the (+)-monomer, -16° to -41° starting from the (-)-monomer). Polymer 80 may have a helical conformation with a prevailing helicity of the main chain. 

By anionic polymerization using t-BuOK, an optically active, binaphthyl-based carbonate monomer (228) gives a polymer (poly-228) that has a single-handed 41-helical conformation. An analogous polymer has been synthesized from a biphenyl-based monomer... 

Also, in studies with optically active vinyl ethers it was observed [104] that trimethyl vinyl silane, which is bulky and non-chiral forms highly syndiotactic polymers. Equally bulky, but chiral (—)-menthyl vinyl ether, however, produces isotactic polymers in polar solvents. This suggests that isotactic propagation is preferred in a polar medium because of helical conformation of the polymer... 

Finally, it may be noted that some polymers have been obtained in which optical activity is ascribed mainly to conformational asymmetry. In these cases there is a predominance of either right-handed or left-handed enantiomorphs of helical polymer molecules, in contrast to the more usual situation wherein equal amounts of the two enantiomorphs are produced and there is no resultant optical activity. Optically active polymers of this type have been obtained from a-olefins possessing optically active side chains, e.g., 3-methylpent-l-ene, 4-methylhex-l-ene and 5-methylhept-l-ene. Isotactic polymers from these monomers have greatly enhanced optical activity compared to the monomer. Since these polymers are vinyl polymers this optical activity cannot be associated with the asymmetry of the carbon atom in the polymer backbone (for the reasons given above). Thus it is supposed that the presence of optically active side groups favours a particular screw sense of the helix so that the resultant polymer shows a large optical rotation. Optical activity of this type has not been observed when the side groups are not asymmetric. 

Synthetic optically active polymers and their chiroptical properties during neutralization was thoroughly studied by Selegny, Vert et al. [78—80]. For further details see the lecture of M. Vert, this Symposium. Though beyond the scope of this chapter a special case of reaction between two optically active polyelectrolytes should be described. As models for connective tissue systems the interaction between mucopolysaccharides and cationic polypeptides were studied by Blackwell et al. [44]. The results of CD measurements indicate that the mucopolysaccharides have a conformation directing effect on polypeptides such that the polypeptides adopt the a-helical conformation. The strength of the interaction and the stoichiometry depend on position, number and type of anionic groups attached to the polysaccharide backbone. 

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