Chiral crystalline, helical chain conformations

Peptoids based on a-chiral aliphatic side chains can form stable helices as well [43]. A crystal of a pentameric peptoid homooligomer composed of homochiral N-(1-cyclohexylethyl)glycine residues was grown by slow evaporation from methanol solution, and its structure determined by X-ray crystallographic methods. In the crystalline state, this pentamer adopts a helical conformation with repeating cis-... 

Circularly polarised PL and EL emission has been obtained from polymers, e.g., 22 and 23 (Scheme 8), bearing chiral side-chains [42-45]. The copolymer 24 also produced circularly polarised emission, but the degree of dissymmetry was much reduced [42 - 44]. This was attributed to the dioctylfluorene units preferring a planar conformation of the backbone while the chiral-substituted units adopt a helical structure. A combination of a helical backbone conformation and liquid crystallinity is thought essential for obtaining a high degree of circular polarisation. 

The chain conformation for the ordered a [23] and y [172] forms of nylon 6, as reported in the literature, correspond to chiral 2/1 helices with all coplanar amide groups. In both crystalline forms the chains are in nearly... 

Crystallization of polymers in chiral crystals, even in the case of achiral polymers, is quite frequent and strictly related to the occurrence of helical conformations of the chains. The crystallizable polymer consists of a regular sequence of a chemical repeating unit which can be chiral if it presents an asymmetric center or achiral. On the contrary, helical conformations assumed by the polymer chains in the crystalline state are intrinsically chiral, even though the chemical repeat is achiral. Three possible cases can be distinguished ... 

The above reasoning regarding helical hand in the crystal rests on the assumption that the polymer melt is either made of random coils, or that, if helical stretches exist in the melt, both right- and left-handed helices exist for chiral but racemic polymers such as isotactic (or syndiotactic) polyolefins. For random coils, the conformation of the incoming chain would simply have to adapt to the crystalline substrate structure. When helical stretches do exist, the sorting-out process described above would have to be fully operative. 

Isotactic poly(x-olcfin)s crystallise in a helical conformation, and, in the case of polypropylene, with three units per turn [4,5], Isotactic polypropylene has a melting point of 175°C and does not dissolve in boiling n-heptane [6,7], Note that, depending upon the configuration of the tertiary carbon atom of the polymer main chains, the poly(x-olefin) helices will be characterised by right-handedness or left-handedness. It should be mentioned that the helical structure of the poly(x-olcfin) chain per se is sufficient for the appearance of chirality of such a macromolecule [8], Figure 3.3 presents the helical conformation of chains of isotactic poly(a-olefin)s in the crystalline state (with three units per turn - the case of polypropylene) [5],... 

Most isotactic vinyl polymers assume helical conformations in the crystalline state [4], but ovnng to the substantial achirality of the macromolecules both screw senses are found in the lattice ceils in equal amounts. This is even more true in a melt or in solution, where left-handed and right-handed helical sections alternate even within the same macromolecular chain. Certainly, an appreciable optical rotation would be observed in the crystalline state provided that crystallization occurred under a chiral field inducing a single screw sense helidty in all chains. Such an optical rotation would promptly be lost on melting or dissolution, as an immediate equilibration between the two opposite helical senses would occur. 

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