Parallel chains, pairing

The softening behaviour of a thermoplastic material depends to a large extent on the flexibility of the chain and the ability to crystallise. Significant cross-linking of a reasonably stiff-chained polymer will lead to material that is unlikely to soften below its decomposition temperature. Intermediate to the linear and cross-linked polymers are various ladder polymers in which the polymer molecule consists of a pair of more-or-less parallel chains bridged in a manner analogous to the rungs of a ladder. 

Chain Pair Modeling. In the following analysis, we assume that the chains are regular helices, i.e. that they have screw symmetry, with a repeat distance, t. In a perfect crystal, such chains must either be parallel or antiparallel. Four interhelical parameters are required to define the geometric orientation of chain A relative to chain B (see Figure 2). The parameters and their ranges are ... 

Combined X-ray and electron diffraction analysis led to an orthorhombic unit-cell, with a = 2.468 mn, 1) = 1.152 nm, and c = 1.054 nm. The space group is P2,2,21. Two parallel chains are related, pairwise, by a two-fold screw-axis parallel to the chain axis, and pairs of chains pack in an antiparallel array. The (110) growth planes ol the crystal are parallel to the direction of highest atomic densities. The transformation CTA II cellulose II was discussed. The R factor is 30% with the X-ray diffraction data, and 26% with the electron diffraction data. 

Ti18028(0Bu )16( Bu0H)], where the parallel chains of pairs or triades ofoctahedra are connected with each other via [14-, p3-0 and p-OR-groups. The [Ti16016(0Et)32] molecule contains 2 perpendicularly placed blocks of 8 octa-hedra, where each of them contains a layer of 6 octaheclra with NiAs-type structure and 1 octahedron below and 1 above the layer [1177] (see also Section 12.11). 

Fig. 4. The structure of human vasodilator-stimulated phosphoprotein (VASP) is that of a parallel chain, four-stranded coiled-coil (Kuhnel et al., 2004). The sequence contains a pair of 15-residue repeats, resulting in the formation of a right-handed coiled coil with a pitch length of 18.5 nm, a value directly comparable to that seen in left-handed two-stranded coiled coils. Figure courtesy of Sergei Strelkov.

In these and in later equations the symbols placed over the arrows define the rate constants for the various reactions. Reactions 4 and 5 represent a pair of chain-carrying steps for two parallel chain reactions, each of which is initiated by Reactions 1 to 3. As the second chain-carrying step, the two chain reactions have a common link, since the OH radical generated in Reactions 4 and 5 can disappear only in Reaction 6 (or in chain-terminating steps which contribute negligibly to the over-all stoichiometry). 

The double-stranded DNA consists of two parallel chains folded into an a-helix. The chains are joined by hydrogen bonds that follow the rules of complementarity. In accordance with these rules, adenine is always bound with thymine of the other chain and guanine is always bound to cytosine (see the following scheme). The principles of molecular complementarity do not apply only to nucleic acids. Rather, they represent general rules for the formation of various supramolecular structures. If nucleotides are labeled by their first letters, the complementary pairs would be AT and GC, respectively. The structural key for complementarity is the number of hydrogen bonds. There are two such bonds in the AT pair and three in... 

From the above discussion, it is clear that there are two kinds of mutual chain orientations of the overgrowing polymers with respect to the oriented substrates for polymer epitaxies, that is, the parallel chain alignment of the epitaxial pairs and the intersect chain alignment of polymers at fixed angles. Tables 12.1 and 12.2 listed the known parallel and nonparallel epitaxial polymer systems reported in the literature to date. 

The optimal electron count for a square net can be determined in a straight forward manner [23], as follows. We have shown that the hypervalent linear chain is one which has seven electrons per A atom. Bringing an infinite number of parallel chains together generates 14.27. Recall that there is one electron in a p AO that runs along each chain. Therefore, removing one electron from the in-plane lone pair on... 

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