Cooperativity preferential screw sense, copolymers

On the other hand, noticeable cooperative preferential screw sense induction was reported in rodlike polysilane copolymers bearing (.S )-2-mcthylbutyl and 2-methylpropyl groups (10) and (R)-2-melhylbutyl and 2-methylpropyl groups (11) in isooctane (Chart 4.3). Since the UV-CD spectral features of the polysilane copolymers are almost identical with those of 4-6 and the Xmax varies sensitively with changes in the molar fraction of chiral substituents and temperature,28 the copolymer systems are suitable for testing the sergeants-and-soldiers phenomenon by their CD-UV spectra. 

In this part, two series of 44 copolymers with coiled main-chain structures and 45 copolymers with stiff main-chain conformations were described. It was concluded that both optically inactive 42 and 43 adopt helical conformations with an equal proportion of P and M screw senses by means of UV and CD spectra as well as molecular mechanics calculations. A marked positive cooperative induction effect of the preferential screw sense in 44 and 45 copolymers was found. However, there is a marked difference in the helical cooperativity between 44 and 45, probably because of the differences in their global and local conformations. This difference can be related to the persistence of the helical conformation against defects allowing change of... 

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... 

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