Trans-1,4-poly , disordered conformation

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. ... 

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... 

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. 

These chromisms were first recognized for a series of polydiacetylene [39] and polysilane [40] compounds. In these systems, extent of the disorder can be mea-smed by deviation from the fully-stretched aM-trans backbone conformation [41]. In the case of the poly thiophene and its derivatives, the conformational disorder is caused by the distortion around the cr-bonding interconnecting the thiophene rings [42]. The TT-delocalization along the polythiophene backbone will be maximized when the polymer chains assume the fully-stretched S-anti form. This delocalization will be hampered by the ring distortion of any amount. 

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