Trans-1,4-poly , disordered

The high-temperature polymorphic form of trans-1,4-poly(1,3-butadicne) (form II) provides another example of conformational disorder.111 At... 

Figure 2.17 Model of disorder for conformation of chains of cis-1,4-poly(isoprenc). Thick lines indicate double bonds. The symbols D and U indicate structural units having double bond down or up, respectively, with respect to two adjacent single bonds. Subscripts A+ and A- indicate torsion angles assumed by two single bonds adjacent to double bonds. Central single bond, -CH2-CH2-, is always in trans conformation.

Another example of a conformationally disordered mesomorphic form is the high-temperature phase of trans-1,4-poly(1,3-butadiene).111112 As discussed in Section 2.6, in this phase the disorder corresponds to a statistical succession... 

The same type of addition—as shown by X-ray analysis—occurs in the cationic polymerization of alkenyl ethers R—CH=CH—OR and of 8-chlorovinyl ethers (395). However, NMR analysis showed the presence of some configurational disorder (396). The stereochemistry of acrylate polymerization, determined by the use of deuterated monomers, was found to be strongly dependent on the reaction environment and, in particular, on the solvation of the growing-chain-catalyst system at both the a and jS carbon atoms (390, 397-399). Non-solvated contact ion pairs such as those existing in the presence of lithium catalysts in toluene at low temperature, are responsible for the formation of threo isotactic sequences from cis monomers and, therefore, involve a trans addition in contrast, solvent separated ion pairs (fluorenyllithium in THF) give rise to a predominantly syndiotactic polymer. Finally, in mixed ether-hydrocarbon solvents where there are probably peripherally solvated ion pairs, a predominantly isotactic polymer with nonconstant stereochemistry in the jS position is obtained. It seems evident fiom this complexity of situations that the micro-tacticity of anionic poly(methyl methacrylate) cannot be interpreted by a simple Bernoulli distribution, as has already been discussed in Sect. III-A. 

In this section information on possible condis states of the following macromolecules are reviewed polyethylene, polytetrafluoroethylene, poly(vinylidene fluoride), poly-chlorotrifluoroethylene, polypropylene, trans-1,4-polybutadiene, cis-l,4-poly(2-me-thylbutadiene), polyoxybenzoate, polyethylene terephthalate), nylon, poly(diethyl siloxane), and polyphosphazene. There is no reason to assume that this selection is complete. Station ni) has shown, for example, already in 1959 on a list of 29 macromolecules that longitudinal and lateral disorder may exist. Similarly, textbooks18> u2)... 

Williams, G. Dielectric information on chain mobility, chain configuration, and an order-disorder transition in amorphous poly(acetaldehyde), Trans. Farad. Soc., 59, 1397 (1962)... 

A recent study of poly(di-n-hexylsilane) films by V.R. McCrary et al. (17) measured the polarization dependent Cls and Si2p NEXAFS spectra, EXAFS, and UPS as a function of temperature. Above 42°C it was found that the hexyl side chains were disordered and that the Si backbone was partially disordered (no longer an all-trans configuration). Below 42°C there was coexistence of the disordered phase with the well-ordered (all-trans backbone and side chains) phase. 

In addition to the order-disorder transition, observed for a helices, helical structures can also be induced to undergo transitions from one ordered form to another. For example, a crystalline form of poly[p-(p-chlorobenzyl)-L-aspartate] can be made to undergo a phase transition from an a-helical to an co-helical form by heating rotational entropy is computed to play a role in this process.68 Another order-order transition is the solvent-induced interconversion between polyproline 1 (with cis peptide bonds) and polyproline 11 (with trans peptide bonds), a process that has also been subjected to conformational energy calculations.85 The transition has been accounted for in terms of differences in the binding of solvent components to the peptide 0=0 groups. 

These results support current interpretations of the bathochromic shifts observed in dialkyl-substituted poly silane. Experimental results for poly(di-n-hexylsilane) indicate that as the temperature is cooled below a transition temperature of roughly -35 °C, the major absorption peak shifts from a broad peak at about 310-320 nm (3.9-4.0 eV) to a narrower peak at about 350-370 nm (3.3-3.5 eV), with the red shift being attributed to a transition from a disordered system with a large population of gauche bond twists in the silicon backbone and in the alkyl substituent to a planar dll-trans backbone conformation (5-8, 15). Results from polarized absorption spectra of stretch-oriented samples for the cooled samples exhibit absorbance only for polarizations parallel to the stretch (and presumably the chain axis) direction (22). 

The closely related trans-I,4-poly(2 ethylbutadiene) (gutta percha) has also two crystalline polymorphs. The stable, monoclinic a-crystal form (P2jC) grows from the melt above 318 K (T = 353 K) and has an entropy of fusion of 36.4 J/(K mol), indicative of full conformational order. The s nd polymorph, the orthorhombic P-crystal form (Pnam) grows at lower crystallization temperatures, but has only a somewhat lower entropy of fusion [29.7 J/(K mol)]. Conversion from p to a is not possiUe, or at least extremely slow. Both polymorphs have been asigned distinct chain conformations , i.e. neither seems to show large stale dynamic conformational disorder. 

Photoswitchable supramolecular dendrimer-like structures were constructed starting from azobenzene-based and CD-based building blocks. For example a bis-azobenzene with a dipyridyl linker trans-Azo dimer) and a p-CD trimer formed an hyperbranched structure whose shape could be controlled by light (Fig. 11). AFM evidenced branched structures of several microns turning to disordered particles upon UV irradiation. Moreover a supramolecular dendrimer was recently built up in water starting from an hydrophilic hyperbranched polyglycerol with a-CD apex (CD-g-HPG) and an hydrophobic hyperbranched poly(3-ethyl-3-oxetane-methanol) with azobenzene apex (AZO-g-HPBO) (Fig. 12). The two components in 1 1 molar ratio selfassembled into a Janus-like dendrimer (JHBP, HPBO-6-HPG) of diameter ca. 5.1 nm, due to inclusion complexation of the azobenzene apex into the CD cavity of the partner... 

Crystalline polymers characterized by disordered conformations of the chains are, for instance, polytetrafluoroethylene [10-12,49-54], cis-l,4-poly (isoprene) [100-102] and trans-1,4-poly( 1,3-butadiene) [8,9,55-58]. In these cases, disorder does not destroy the crystallinity because of the similar shape of the various conformational units. The occmrence of cases of conformational isomorphism of the first kind demonstrates that a polymer chain can remain straight, as if it was constrained to run inside the walls of a tight cylinder, while its conformational freedom remains of the same order of magnitude as that in the melt. 

Molecular Structure.—Improved Af-ray diffraction data have been reported for highly purified poly(dichlorophosphazene). The results are consistent with a cis-trans planar chain conformation and with a chain-packing arrangement allowing appreciable rotational disorder between adjacent chains. Measurements of crystalline transitions and related physical properties of poly(dichloropho phazene) have also been described. ... 

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