Alternating backbone bonds

The extended chains, that is, the backbones, may further organize into assemblies that have characteristic properties. The two most important of these are the a- helix and the /3-sheet. The latter is illustrated in Figure 1.23. Panel (a) shows the extensive hydrogen bond organization of peptide chains that are oriented in opposite directions. The arrows indicate the nitrogen to carbonyl (arrowhead) direction. The lower panel (b) shows an alternate FI-bond organization when the two peptide chains are parallel rather than antiparallel. 

Dudek and Scheraga32 developed an alternative formulation of these equations, and Palmer and Scheraga33-34 modified the original formulation24 to take into account the different equilibrium values for the backbone bond... 

Crystalline and amorphous silicons, which are currently investigated in the field of solid-state physics, are still considered as unrelated to polysilanes and related macromolecules, which are studied in the field of organosilicon chemistry. A new idea proposed in this chapter is that these materials are related and can be understood in terms of the dimensional hierarchy of silicon-backbone materials. The electronic structures of one-dimensional polymers (polysilanes) are discussed. The effects of side groups and conformations were calculated theoretically and are discussed in the light of such experimental data as UV absorption, photoluminescence, and UV photospectroscopy (UPS) measurements. Finally, future directions in the development of silicon-based polymers are indicated on the basis of some novel efforts to extend silicon-based polymers to high-dimensional polymers, one-dimensional superlattices, and metallic polymers with alternating double bonds. 

The true conformation in the crystalline structure of a polymer is more complicated, the backbone chain often taking a helical shape in which alternate chain bonds take trans and gauche positions, such that the carbon chain is not in a plane. For example. 

The electrical properties of a number of semiconductor polyoximes was studied.209,210 These structures offer whole-chain resonance, where the polymer backbone consists entirely of alternating double bonds with the exception of the metal atom. Values for D (dissipation factor) ranged from 3 to 10 (at 103 hertz), bulk resistivity from 0.05 to 0.6 (at 103 hertz), and bulk capacitance from 0.2 K to 0.9 K. 

Note that polymers with a simple silicone-type backbone, i.e., with a backbone consisting solely of alternating silicon and oxygen atoms, such as poly(dimethyl siloxane) (Figure 5.2) and poly[oxy(methylphenylsilylene)] (Figure 6.11), have two Si-O backbone bonds per repeat unit. 

Fig. 9.6 Some polymers with alternation (conjugation) of single and double backbone bonds. (nms-Polyacetylene (/-PA) N. Polythiophene (PT) ( W...

The bond alternation means that the repeat unit is twice the length that it would be if the bonds were equivalent, so that there are two nearly free electrons per repeat unit and the band gap therefore appears at the Fermi level. The magnitude of this energy gap for /-PA turns out be about 1.5 eV. There can thus be no metallic-type conductivity and intrinsic /-PA is a semiconductor. In the other polymers of the type shown in fig. 9.6 the backbone bonds are intrinsically non-equivalent and a band gap therefore exists for them too, so that they also cannot exhibit metallie-type conduetivity. 

A common use of the rotational isomeric state model is to learn how speciflc structural properties (at the local level) affect conformational properties at the level of a long chain. Polyisobutylene provides a good example. The bond angles in the backbone of pol5dsobutylene alternate between 110° (for CH2—C—CH2) and 124° (for C—CH2—C). How strongly, and in what direction, does this alternation in bond angles affect the mean square dimensions of the chains in a polyisobutylene... 

Alternatively, if we were to attempt to achieve rotation about one single backbone bond, the motion would require a large element of the polymer chain to move through space. This is very unlikely to occur. So we next look at the coupled nature of the motions in polymers. 

The agreement betw the predicted and the observed pattern of NOESY cross peaks for the co-hetero triad of 1 1 alternating S-MM copolymer confirms the vdidity of the Koinuma et al. [67] conformational model It is pardculariy noteworthy that this agreement requires the assumption of 20° displacements from the perfectly staggered rotational states as predicted for the backbone bonds in polystyrene by Yoon et al. [63] (see the Newman projections below). As an example, in the t, t conformation (see Figure 2.15), 4>z = —20% 20° because this produces relief from steric interactions of the phenyl ring and the methyl methacrylate C as seen in the following Newman projections ... 

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