Possible conformation poly

In contrast to polypeptides that have many possible conformations, poly(hexyl isocynate) is known to have a stiff rodlike helical conformation in the solid state and in a wide range of solvents, which is responsible for the formation of a nematic liquid crystalline phase.45-47 The inherent chain stiffness of this polymer is primarily determined by chemical structure rather than by intramolecular hydrogen bonding. This results in a greater stability in the stiff rodlike characteristics in the solution as compared to polypeptides. The lyotropic liquid crystalline behavior in a number of different solvents was extensively studied by Aharoni et al.48-50 In contrast to homopolymers, interesting new supramolecular structures can be expected if a flexible block is connected to the rigid polyisocyanate block (rod—coil copolymers) because the molecule imparts both microphase separation characteristics of the blocks and a tendency of rod segments to form anisotropic order. 

All the possible line repetition groups for cis and trans poly dienes compatible with the isotactic or syndiotactic configurations are reported in Figure 2.15,47,68 In order to consider only the possible conformations assumed in the crystalline state, the torsion angle of the central single bond is assumed to be 180° trans) in both the cis and trans polydienes. This condition produces conformations sufficiently extended to be packed in a crystalline lattice for each value of the torsion angles 0i and 02 (Figure 2.15). 

Fig. 4. Different possible conformations of cw-l,4-polyisoprene (poly[(Z)-2-methylbut-2-ene-l,4-diyl]) in the crystalline state, as viewed sideways along two orthogonal axes [17].

Fig. 7 Possible conformations of poly(cyclopent-1 -enylene-1 -vinylene)s and poly(cyclohex-l-ene-3-methylidene)s...

Fig. 11-4. Possible conformations of poly inerchains al the surfaces of chain-folded single crystals, (a) Adjacent reentry model with smooth, regular chain folds, (b) adjacent reentry model with rough fold surface, and (c) random reentry (switchboard) model.

Our procedure in predicting poly(MDI-PDO and poly(MDI-EDO) conformations has been to take the extended minimum energy conformation for the diphenylmethane diurethane section (Xi = -90°, l=-60°, 2 = -60°, X2 = +90°) and to look at possible conformations of the diol sections, comparing their relative energies and fiber repeats. (12) These conformations are defined by the torsion angles (i)3 and [for poly(MDI-PDO) only] as shown... 

Figure 10-9 A possible conformation of the cis isomer of an orrAo-substituted poly(phenylacetylene). The steric requirements of the substituents enforce a planar conformation.

In order to examine possible conformational differences between the different classes of poly(RCOT)s, both force-field (MM2) [132] and semiempirical quantum mechanical (AMI) [133] calculations on model polyene oligomers were employed (Fig. 10-23) [34]. Using either routine, it is observed that the single bonds adjacent to the trisubstituted double bond (0] and 2) both strongly deviate from planarity in the models of the soluble polymers. In contrast, in the model of trans- poly (terr-butoxyCOT), an insoluble polymer, j is large, but 2 is not. The good correlation between polymer solubility and calculated chain twist is indicated... 

The possible conformations of PFS chains in the solid state have attracted significant interest from several research groups and much work has also focused on the prototypical material, poly(ferrocenyldimethylsilane) 73 (R = R =Me). One contribution toward the understanding of conformations adopted by poly(ferrocenyldimethyl-silane) chains involved X-ray structural studies of well-defined oligomers. In particular, a model pentamer 97 was successfully characterized by single crystal X-ray diffraction. The pentamer molecules possess a trans-... 

Chart 3, Diagrammatic illustrations of (A and B) single- and double-stranded helical chains of leucine-containing polyphenylacetylene le stabilized by intra-and inter-chain hydrogen bonds and (C) theoretically possible conformations of chain segments of a substituted poly(phenylacetylene). 

For example, in the solid state one structure predominates for poly(ethyl-ene oxide), the tgt conformation. The O—C bond is trans, the C—C bond is gauche, and the C—O bond is trans. As illustrated in Table 2.4 (18), all possible conformations exist in the molten state—tgt, tgg, ggg, ttt, ttg, and... 

The calculations for poly-a-butene are reported, as an example, in fig. 1. The E(0, 02) map represented is that one relevant to the possible conformations of left-handed helices. Di ferently from the case of PP (above cited lecture) the energetic minimum is split into two. Correspondigly, chain polymorphism is experimentally observed for PB(2). The different crystalline modifications have s(3/l)l, s(ll/3)l, s(4/l)l chain conformations the loci of points corresponding to such symmetries are also represented in the figure. 

Figure 12 Possible conformations of tightly (top) and loosely (bottom) twisted helical c/s-stereoregular poly(/i/-propargylamide) [-CH=C(CH2NHC0H)-] , which accompany helically arranged intramolecular hydrogen-bonding strands (dotted lines) formed between the amide groups at the rth and ( +3)th units (top) and the th and ( +2)th units (bottom). Methine and methylene hydrogen atoms are omitted for clarity.

In most cases a mixing of cis and ttans conformatitms is considered, for example in poly-Chtho-MethylPA (10). Obviously a mixture of the two possible conformers increases the number of available configurations, rendering the description of the statistical conformation much more difficult. Preliminary SANS studies have been performed on PPA synthesized widi M0CI5 and WQ. The scattering curves for the two samples in solution are almost identical. They correspond to a coil behavior without any indication on the existence of a persistence of curvature or a partial rod confcumatitm due to an helical local order (6). 

The most possible reason may be in the higher free energy of the protein adsorption on PolyPROPYL A materials. Chemisorbed neutral poly(succinimide) of molecular weight 13000 apparently forms a diffuse interface as predicted by theory (see Sect. 2.2). Controversially, a short polyethyleneimine exists on a surface in a more flat conformation exhibiting almost no excluded volume and producing... 

The recent interest in substituted silane polymers has resulted in a number of theoretical (15-19) and spectroscopic (19-21) studies. Most of the theoretical studies have assumed an all-trans planar zig-zag backbone conformation for computational simplicity. However, early PES studies of a number of short chain silicon catenates strongly suggested that the electronic properties may also depend on the conformation of the silicon backbone (22). This was recently confirmed by spectroscopic studies of poly(di-n-hexylsilane) in the solid state (23-26). Complementary studies in solution have suggested that conformational changes in the polysilane backbone may also be responsible for the unusual thermochromic behavior of many derivatives (27,28). In order to avoid the additional complexities associated with this thermochromism and possible aggregation effects at low temperatures, we have limited this report to polymer solutions at room temperature. 

In addition, data on the size, shape and solvation of the polymer particles in aqueous solutions at temperatures below and above the transition phenomena registered by HS-DSC have been obtained [42]. Table 2 shows the results of capillary viscometry and light scattering experiments for the fractions p and s of poly(NVCl-co-NVIAz) synthesized at 65 °C from the feed with the initial molar comonomer ratio equal to 85 15. Since fraction p precipitates from the aqueous solution at temperatures > 34 °C, its intrinsic viscosity can be determined only at 20 °C, whereas for the fraction s such measurements were possible above and below the temperatures of the HS-DSC-registered conformational transition. 

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