Polymers, helical

Moffitt W. Optical rotatory dispersion of helical polymers. / Chem Phys 1956 25 467-78. 

Extensive studies have been performed on the (1- 6)-)8-D-glucan (pustulan) and the (l- 4)-a-D-glucan (amylose). These are linear polysaccharides that may exist as helical polymers in aqueous solution, as demonstrated by c.d. spectroscopy. Characteristic of the helical structure of these glucans is a negative band at 182 nm, a crossover at 177 nm, and a more intensely positive band at shorter wavelengths (see Figs. 8 and 9). 

Stevens and coworkers used their c.d. data on the various D-glucans to assign, tentatively, the bands to specific chromophores. They found that derivatives of these polysaccharides that have all of their hydroxyl groups acetylated still exhibit the 177-nm band. They assigned this band (which occurs at somewhat shorter wavelengths for the helical polymers) to the ether of the acetal chromophore. This assignment is essentially consistent with the results obtained by Johnson and coworkers on unsubstituted monosaccharides. 

The preparation of helically well-ordered polymers with stable screw-sense, which is able to be transmitted to newly formed polymer main-chains effectively, is highly desired for the development of new methodology for the synthesis of optically active helical polymers. An aromatizing polymerization of 1,2-diisocyanobenzenes is promoted by methylpalladium(II) complexes, producing poly(quinoxaline-2,3-diyl)s.146-148 The polymerization proceeds with successive insertion of the two isocyano groups of the diisocyanobenzene to the carbon palladium bond of... 

The zinc complex of 1,1,1,5,5,5-hexafluoroacetylacetonate forms coordination polymers in reaction with either 2,5-bis(4-ethynylpyridyl)furan or l,2-bis(4-ethynylpyridyl)benzene. The X-ray crystal structures demonstrate an isotactic helical structure for the former and a syndio-tactic structure for the latter in the solid state. Low-temperature 1H and 19F NMR studies gave information on the solution structures of oligomers. Chiral polymers were prepared from L2Zn where L = 3-((trifluoromethyl)hydroxymethylene)-(+)-camphorate. Reaction with 2,5-bis(4-ethy-nylpyridyl)furan gave a linear zigzag structure and reaction with tris(4-pyridyl)methanol a homo-chiral helical polymer.479... 

Multimolecular helical inclusion networks formed by rigid alicyciic diols, urea, deoxycholic acid, and tri-o-thymotide are described and contrasted, followed by discussion of DNA intercalates, amylose compounds, and other inclusion systems formed by helical polymers. 

Figure 14 The left hand side shows the band structures of poly(pyridine) calculated using a DFT-LMTO method for helical polymers. The right hand side shows its calculated density of states spectrum (solid line) and the experimental UPS spectrum (dashed line). The UPS spectrum was taken from Miyamae et al. [104]. Reproduced with permission from Vaschetto et al. [103], Figure 6. Copyright 1997 the American Chemical Society.

The mechanism we propose for chiral crystallization of helical polymers appears to be also supported by the analysis of the circumstances under which some chiral polymorphs are obtained [13]. 

Table 2 Selected data of helical polymers (or of enantiomeric polymer pairs) displaying both chiral and achiral modifications (adapted from [13])... 

We can conclude this section noting that understanding chiral crystallization of helical polymers is still hardly satisfactory, but quite certainly different mechanisms are likely to apply for different polymers and different crystallization conditions. A lot of experimental work still needs to be carried out if we want to progress beyond speculation and generalization of results that may in fact apply only to very specific instances. 

Downs et al. reported the synthesis of methylzinc tetrahydridoborate 164 by the two routes shown in Scheme 99.230 The compound is an extremely moisture and oxygen sensitive, colorless, polycrystalline solid, whose solid-state structure was determined by X-ray analysis. Figure 76 shows that 164 consists of helical polymers of alternating methylzinc and tetrahydridoborate ions. The zinc atom is formally pentacoordinate, making this one of the few organozinc compounds with five-coordinate zinc atom. 

Additionally, copolymers of 30-37 containing 20% enantiopure chiral silane units (see Chart 4.6) are optically active helical polymers which obey the sergeants-and-soldiers principle, as shown in Figure 4.18.29g Interestingly, from the observation of the CD sign in the phenyl region, the arrangement of phenyl... 

A few synthetic helical polymers are known to act as chiral selectors.7a,918d l8k i9d i9h ancj are widely used as chiral stationary phases (CSP) in gas or liquid chromatography.73,53 Recently, it has been reported that the preference of one helical sense in isotropic solution can be induced by some interaction between optically inactive polymers and chiral solvents/additives. Examples of this include poly(n-hexyl isocyanate)18d l8k and poly(phenylacetylene)s bearing functional group.19d 19h The polysilane derivatives also show chiral recognition ability in solution at room temperature. Poly(methyl-ft-pinanylsilane) includes two chiral centers per bulky hydrophobic pinanyl side group28 and... 

It is considered that, if ideal, optically active poly(alkyl(aryl)silane) homopolymer and copolymer systems could be obtained which had stiffer main-chain structures with longer persistence lengths, it should be possible to clarify the relationship between the gabs value and the chiral molar composition. The magnitude of the chirality of the polyisocyanates allowed precise correlations with the cooperativity models.18q In the theory of the cooperative helical order in polyisocyanates, the polymers are characterized by the chiral order parameter M, which is the fraction of the main chain twisting in one helical sense minus the fraction of the main chain twisting in the opposing sense. This order parameter is equal to the optical activity normalized by the value for an entirely one-handed helical polymer. The theory predicts... 

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