Preferential screw sense, optically active

Another significant cooperativity effect in preferential helical screw sense optically active copolymers is the majority rule phenomenon.18bl8q In this case, the screw sense of a helical main chain with unequal proportions of opposite chirality enantiopure chiral side groups is controlled by the enantiomeric excess only. Since this phenomenon was first reported from poly-a-olefins made of vinyl co-monomers bearing nonenantiopure chiral moieties by Green et al.8b and Pino et al.,16b this majority rule has been established in... 

In this section, we comprehensively focused on the controlled synthesis, chiroptical characterization, and manipulation of optically active poly(dialkyl-silane)s. Although many artificial polymers adopting preferential screw sense... 

Polymerization of optically active isonitiiles, 72, also leads to the formation of helical polymers with a preferential screw sense (219-222). Various factors distinguish this system from the preceding ones In the isonitrile case no new stereogenic atoms are formed during polymerization the helices are rigid and there is no indication of conformational equilibrium in the system the formation of a preferential screw sense is very probably a kinetically controlled process. 

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. 

Optically Active Homopolymers with a Preferential Screw Sense. 131... 

For example, the sergeants and soldiers experiment features a preferential screw-sense helical amplification in optical active copolymers with racemic helical structures named by Green et al. [17,18], This effect means that a small portion of enantiopure chiral side groups determines the overall screw sense (P or M) of the helical main chain bearing a majority of achiral side groups, and a population of helicity with one preferential screw-sense is nonlinearly amplified as a function of the chiral impurity. Since the first report of this phenomenon in poly-a-olefin copolymers by Pino et al. [21],... 

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. 

Moller and co-workers co-polymerized dichlorodi- -pentylsilane with either dichloro-bis-(d )-2-methylbutylsilane or dichloro-(d )-2-methylbutyl- -pentylsilane in various ratios and found a linear dependence of optical activity on mole fraction of chiral co-monomer.313 On the other hand, studies by Fujiki on co-polymers 109 formed by the copolymerization of achiral (racemic) dichlorohexyl-2-methylbutylsilane and chiral dichlorohexyl-(d )-2-methylbutylsi-lane or dichlorohexyl-(l )-2-methylbutylsilane have shown that a preferential helical screw sense can be induced by even as little as 0.6 mol% of chiral co-monomer, and that at 5 mol%, the helicity, as gauged by the gabs value, is essentially the same as that of the chiral homopolymer, as shown in Figure 40. This indicates a positive non-... 

Helix-sense-selective polymerization is the reaction that yields polymers with a helical conformation with excess screw sense. Right- and left-handed helices are mirror images (atropisom-ers), and if one of the two is preferentially synthesized for a polymer chain, the polymer can be optically active (1-9]. 

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