Configurational disorder

J- K mole added to account for electronic contribution to configurational disorder. 

Figure 3.9 Conformation of Gibbs free energy curve in various types of binary mixtures. (A) Ideal mixture of components A and B. Standard state adopted is that of pure component at T and P of interest. (B) Regular mixture with complete configurational disorder kJ/mole for 500 < r(K) < 1500. (C) Simple mixture IF = 10 - 0.01 X r(K) (kJ/ mole). (D) Subregular mixture Aq = 10 — 0.01 X T (kJ/mole) = 5 — 0.01 X F (kJ/ mole). Adopting corresponding Margules notation, an equivalent interaction is obtained with IFba = 15 - 0.02 X r(kJ/mole) Bab = 5 (kJ/mole).

Regular Mixtures with Complete Configurational Disorder (Zeroth Approximation)... 

The tendency toward configurational disorder is expressed by parameter... 

If we imagine that both defects A and B and associate defect (AB) are distributed in conditions of complete configurational disorder, and if we substitute thermodynamic activity for the molar concentration, we have... 

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 his early papers Natta referred to the state of the art of that time along the lines discussed by Floty (28). Until the early 1950s, the lack of crystallinity in vinyl polymers was attributed to the configurational disorder present in such compounds, considered as copolymers of d and I stmctural units. These units were defined with reference to a macromolecule having the chain in a zigzag planar disposition, the substituents of one type being above the plane... 

The Influence of Configurational Disorder on the M Phase of Pure Materials. 77... 

Several chiral substituted a-phenyl-alkyl alcohols, which were partially enantio-merically enriched, were successfully purified by this method after esterification with 9-anthroic acid optical purities higher than 95% were obtained.Fluorescence studies of racemic compounds show a characteristic a-type excimer emission, whereas the chiral enantiomer exhibits a typical y-type monomer emission. Figure 7 shows the spectra of l-(2-chlorophenyl)ethyl 9-anthroate for a crystalline sample of enantiomeric (1) and racemic (2) at 77 K. It has been demonstrated that in the absence of configurational disorder chiral impurities in the range of 1 % can be detected by this method. ... 

The pattern of disorder and the relationship between racemic and enantiomeric crystal structures for phthalamide (11) is similar to that of the phthalate (10). The A -ray diffraction data were measured at —170 °C. The racemate crystallizes in space group PT (Z = 4) and the enantiomer in FI (Z = 4) (Table 2). Thus there are two independent molecules in the racemate and four in the chiral crystal. The s-butyl moeities in the racemate adopt the trans conformation and exhibit configurational disorder of different measures (44 56 and 28 72) at the two independent sites, as shown in Figure 11. The chiral crystal structure is shown in Figure 12. 

Thus both configurational disorder of the s-butyl group and the isomorphism between enantiomeric crystals and their racemic counterparts occur by virtue of the following molecular property a s-butyl group is able to occupy almost the same space occupied by its enantiomer by a change in conformation and position. We may define this property as conformational isomorphism. ... 

Increases in configurational disorder on melting are particularly prominent with flexible molecules, such as the homologues of long-chain aliphatic compounds with the general formula C H2 +xX, where X is an end-group such as H, I, COOH, etc. The crystal lattice normally selects only one out of a number of configurations, usually the one which is fully stretched, and packs... 

These faetors lead to broadening of the absorption or seattering peaks and to the presenee of extra peaks. The configurational disorder also leads to conformational disorder, which introduces further new vibrational modes and takes intensity away from others. The problem then beeomes very eomplieated, so that essentially empirieal methods are often used. For instance the ratio of the absorbances or intensities at two points in the spectrum that do not necessarily coincide with obvious peaks may be found to correlate with the tacticity. 

The presence of configurational disorder in the crystalline state of atactic PAN necessarily implies the presence of conformational disorder to alleviate intramolecular strains. The conformational disorder, in turn, yields small deviations from the perfect straightness of crystalline portions of the chains, and thus some waviness, which comply more easily with the local packing of - CN groups. 

Additionally, the solid-state structure of (/ )-10a was determined by X-ray crystallography. Although a configurational disorder was observed for the methyl as well as the OH groups and the positions of these groups could not be refined exactly, the X-ray analysis revealed that the OH groups are fully enclosed by the bulky (MeaSilaSi substituents and that no intra- or intermolecular hydrogen bonds exist in the solid state. 

This assumes that there is no contribution from configurational disorder, which must be corrected for separately, if relevant. For convenience, the sum of the vibrational energy and entropy term can expressed together, due to the cancellation of a common term, as ... 

Most polyphenylquinoxalines show no crystallinity in wide-angle X-ray diffraction studies. Their amorphous nature is primarily due to the configurational disorder carried out in the backbone of the polymer chains by the presence of... 

The cases treated in the previous sections were restricted to the study of the few cases of mass defects and of the very many cases of conformational defects. Most of the relevant polymers, however, may contain configurational disorder which carries as a consequence also some confonnational disorder. This case has been considered within the theoretical approach discussed in this chapter. 

---