Symmetrical Chains

The suppliers of nylon 46 have laid particular emphasis on the fact that this polymer, with its highly symmetrical chain structure, leads to both a high level of crystallinity and a high rate of nucleation. In turn the high nucleation rate leads to a fine crystalline structure which in this case is claimed to lead to a higher impact strength (dry as moulded) than with nylons 6 and 66. 

To see this, consider the mixing free energy expression for a polymer blend with symmetrical chain lengths and with only one crystallizable component (i.e., p = 0 for one component and Ep 0 for the other). In that case the (mean-field) partition function for the liquid mixture is... 

To illustrate this point it is particularly instructive to examine the configurational defects existing in isotactic polypropylene. The steric control during polymerization can in principle be attributed to two different factors the influence of the chirality of the last entered unit (this case falls within the discussion of symmetric chains reported previously) or that of the catalytic site. The chirality of the latter may be preexisting or may arise at the moment of polymer-... 

S simple symmetric chains maximum 1 relevant torsion angle in side chains... 

For symmetric chains, furthermore For asymmetric chains, furthermore ... 

Since the solidity or fluidity of the bilayer membrane is likely to depend on the alkyl chain interactions and consequently their length, an understanding of the relationship between chain order and chain length for tightly packed monolayers of phospholipids are important. As an example of how VSFS can be employed to study phospholipids at a liquid surface, a series of saturated symmetric chain phosphatidylcholines (PCs) were examined at the air/water and CCfr/water interfaces [49]. At the air/D20 interface, chain order within the monolayer was found to increase as the length of the chains increased (Figure 2.9a) under conditions of constant phospholipid head group area. 

These reagents have been used for the conversion of Q symmetrical chains into chiral non-racemic products. Condensation of mcio-dialdehydes with Cp(/ ,/ )-Ti[All] or Cp(5,5)-Ti[All] reagents led (after reduction with NaBH4) to optically active polyketides (eq 12). ... 

The chemical structure of a polymer determines whether it will be crystalline or amorphous in the solid state. Both tacticity (i.e., syndio-tactic or isotactic) and geometric isomerism (i.e., trans configuration) favor crystallinity. In general, tactic polymers with their more stereoregular chain structure are more likely to be crystalline than their atactic counterparts. For example, isotactic polypropylene is crystalline, whereas commercial-grade atactic polypropylene is amorphous. Also, cis-pol3nsoprene is amorphous, whereas the more easily packed rans-poly-isoprene is crystalline. In addition to symmetrical chain structures that allow close packing of polymer molecules into crystalline lamellae, specific interactions between chains that favor molecular orientation, favor crystallinity. For example, crystallinity in nylon is enhanced because of... 

Figure 12.14 Molecular models illustrating secondary conformational transitions for symmetrical chains. (From Ref. 16.)...

Polymers that have fairly symmetrical chains and strong interchain forces can be drawn into fibres. Plastics are polymers with lower degrees of crystallinity which can be moulded. Further down the rigidity scale are rubbers and elastomers, whose properties are well known. 

Fig. 3.5. A self-avoiding chain joining the opposite edges A and B of a rectangle and contained in this rectangle, is associated with symmetric chains, in order to construct a self-avoiding cycle.

Some of the chloro-bridged palladium 105a (R = C H2 +1, R = OC H2m+i), a mixed-bridged /r-acetato /r-thiolato 106b ( = ot = 6), and a mononuclear complex 111 (R R, R, R = OCioH2i) discussed above were also found to exhibit additional lyotropic mesophases in contact with apolar organic solvents such as linear alkanes (octane, decane, dodecane, and pentadecane), cycloocta-1,5-diene, and the chiral limonene. A lyotropic lamellar phase was induced for the dichloro complexes with symmetrical chain length (n = m = 6, 10) in linear alkanes the transition temperatures were found to decrease from pentadecane to octane. 

Cell Symmetry. For many years. It was accepted that the cellulose molecule embodied 2-fold screw symmetry (space group F2 ). Later, Honjo and Watanabe (12) showed electron diffraction diagrams that Indicate that Valonla cellulose crystallizes In the FI space group that does not contain symmetric chains. The unit cell also contained 8 chains. Other diffraction work showed the presence of faint meridional reflections on the odd layer lines that also Indicate that the chains do not have exact 2-fold screw symmetry. While the validity of this evidence has been accepted for Valonla, It has not been used In structural studies because of the low Intensities of the spots that Indicate the large cell and that break the symmetry. It has been argued that the deviations from symmetry must be relatively small (11) assumptions regarding the cell size have been necessary In order for the needed computations to be manageable. 

Symmetrical chains, which allow the regular close packing required for crystallite formation. 

Valence Forces, Deformation Forces, Valence Angle and NuclearDistances OF Triatomic Symmetrical Chains... 

Theoretical Calculations.- A molecular interpretation of the chain-length dependent thermotropic behaviour of saturated symmetrical-chain phosphatidylcholine bilavers has been put forward. Thermodynamic parameters of the thermal phase-transition were found to be linearly related to a perturbation parameter and could be used to predict the minimum number of carbon atoms in the acvl chain needed for a bilayer phase-transition to occur. A model has been developed, consistent with NMR data, for hydrocarbon-chain dynamics in lipid bilayers. Involving concerted rotations around at least two C-C bonds at a... 

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