Vinyl polymers helical conformation, optical activity

Synthetic polymers with conformational chirality have become a research field of widespread interest in recent years, and a wide range of polymers with conformational chirality have been synthesized from various types of monomers including vinyl monomers [9, 61-63, 128-136]. The existing examples of optically active vinyl polymers with conformational chirality include isotactic, helical polyolefins bearing asymmetric side chains [133-135] and isotactic, hehcal polymethacrylates bearing bulky, achiral side chains [61-63,136]. These polymers have stereocenters in the main and/or side chains. Optically active poly(PDBS) is the first vinyl polymer with conformational chirality bearing no stereocenters in the main and side chains whose chiroptical properties arise only from a chiral conformation. 

Figure 13 shows several optically active vinyl polymers and copolymers with azobenzene or stilbene residues in the side groups. All these polymers were prepared by the conventional radical polymerization, so that the main chains should be atactic or syndiotactic and may not be able to take a helical conformation even in the presence of chiral pendants. The homopolymers (28-31) [83-85]... 

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... 

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. 

Isotactic and syndiotactic vinyl polymers cannot be chiral unless they adopt a single-handed helical conformation because they possess a mirror plane, and they do not show optical activity except for that due to the asymmetric centers in the vicinity of the chain ends. Several negative examples have been shown such as those for the polymers of styrene,propylene, methacrylates,and acrylonitrile. ... 

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. 

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