Helix sense

Interestingly, Reggelin et al. [147] prepared helical chiral polymers by helix-sense selective anionic polymerization of methacrylates, using an asymmetric base mixture as initiator (Scheme 61). 

The problem is further complicated for vinyl polymers with their problems of stereoisomerism. The first descriptions of the conformational state of isotactic polypropylene in solution go back 25 years (178, 179, 192, 193). Corradini, Allegra, and Ganis proposed a model, still essentially valid today, according to which macromolecules possess a local helical structure analogous to that observed in the crystalline state. The helix segments are rather short, only a few monomer units, after which an inversion of the helix sense occurs, with simultaneous alteration of its direction (Figure 15). As a whole this disordered con-... 

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]. 

Anionic Catalysis Several bulky methacrylates afford highly isotactic, optically active polymers having a single-handed helical structure by asymmetric polymerization. The effective polymerization mechanism is mainly anionic but free-radical catalysis can also lead to helix-sense-selective polymerization. The anionic initiator systems can also be applied for the polymerization of bulky acrylates and acrylamides. The one-handed helical polymethacrylates show an excellent chiral recognition ability when used as a chiral stationary phase for high-performance liquid chromatography (HPLC) [97,98]. 

TABLE 11.2. Helix-Sense-Selective Polymerization of IYY1A with (-)-Sp-, (+)-DDB, and (+)-PMP-FILi Complexes in Toluene at -78°Ca... 

Helix-sense-selective polymerization of methyl, benzyl, and f-butyl methacrylates was attempted by using the complexes of chiral crown ethers, 91 and 92, and that of a chiral diamine 93 with n-BuLi however, these esters seem to be too small to form and maintain helical conformation [138,139], The complexes of BuLi with 58a and 59 failed in producing an optically active, helical polymer in the polymerization of methyl and benzyl methacrylates [104b]. 

Isocyanide Polymers Optically active polyisocyanides with excess helix sense are obtained from optically active monomers by polymerization with NiCI-,. The polymers obtained from (R)-(CH3)2CHCH(CH,)NC, (fl)-(CH3)2CHCH2CH(CH3)NC, and (R)-n-C6H13CH(CH3)NC have M helical sense with screw-sense excesses of 62%. 56%, and 20%, respectively [189]. The copolymerization of achiral phenyl isocyanide with optically active... 

Various optically active polymers have also been used as CSPs (Figure 3). We can produce the one-handed helical polymer 14 by helix-sense-selective polymerization... 

Photoinduced Helix-sense Reversal in Azobenzene-containing Poly(L-aspartate)s... 

Large photoresponse effects could be observed in solvent mixtures, provided that the irradiation was carried out at appropriate solvent compositions. A copolypeptide composed of 33 mol% P-benzyl-L-aspartate and 67 mol% para-phenylazo-L-aspartate (X) was found to give different kinds of photoresponse, depending on the composition of the solvent in which irradiation was carried out. In dichloroethane (DCE)/ hexafluoropropanol (HFP) = 95/5, irradiation at 320-390 nm produced an increase in right-handed helix content in DCE/HFP = 54/26, a light-induced conformational change from left-handed helix to random coil was observed while, finally, reversal of the helix sense occurred in DCE/HFP = 65/35.150,511... 

More recently, Ueno et al. have prepared and investigated a new series of copolymers containing p-phenylazobenzyl-L-aspartate and n-octadecyl-L-aspartate residues (Scheme 7, Structure XII). 55 561 In the case of copolymers containing less than 50 mol% azo residues, the CD spectra at 25 °C were consistent with the presence of right-handed helical conformations, which were not affected by irradiation at 320 nm. In contrast, in the case of copolymers containing 68 and 89 mol% azobenzene groups, irradiation caused the reversal of helix sense from the left-handed to the right-handed form. 

While the double-helices formed from A and B may formally be regarded as fibrils because they are constructed from two ribbons, it is important to note that the formation mechanism would be entirely different from the one described for pure oligopeptides [122]. In the latter case, the stacking of P-sheets is the driving force for fibril formation, which implies that the helix sense of ribbons and fibrils must be identical. Whereas such P-sheet stacking was assumed to be suppressed... 

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