Poly conformational energy

Figure 2.10 Maps of conformational energy of various isotactic polymers as function of backbone torsion angles 0i and 02 (a) Isotactic polystyrene, (b) polypropylene, (c) poly(l-butene), and (d) poly(4-methyl-l-pentene). Succession of torsion angles. .. 0i020i02 [s(M/N) symmetry] has been assumed. Isoenergetic curves are reported every 10 (a,c,d) or 5 (b) kJ/mol of monomeric units with respect to absolute minimum of each map assumed as zero.

Recently, a similar analysis of the conformational energy has been performed also for various new syndiotactic polymers.27,47 The conformational energy maps of syndiotactic polypropylene (sPP),48 polystyrene (sPS),49 poly butene (sPB),25 and poly(4-methyl-l-pentene) (sP4MP)26 are reported in Figure 2.12. A line repetition group s(M/N)2 for the polymer chain, and, hence, a succession of the torsion angles. .. 0i, 0i, 02, 02,..., has been... 

Figure 2.14 Maps of conformational energy as function of backbone torsion angles 9i and 02 of a chain of isotactic poly((S)-3-methyl-l-pentene) for (a,b) left-handed helix and (c) right-handed helix.29 For each pair of Oi and 02, reported energy corresponds to minimum obtained by varying torsion angles of lateral group 03 and 04. Curves are reported at intervals of 0.5 kcal/mol of monomeric unit. Values of energies corresponding to minima are also indicated. (Reprinted with permission from Ref. 29. Copyright 1976 by Elsevier Science.)...

Figure 2.16 reports the conformational energy maps as a function of the torsion angles 0i and 02 of the two single bonds adjacent to the double bonds for 03 = T = 180° for cis-1,4-poly (1,3-butadicnc) (cisPBD),69 tranx-l,4-poly(l,3-butadiene) (transPBD),70 ds-l,4-poly (isoprene) (cisPI),68 trans-1,4-poly(isoprene) (transPI),71 ds-l,4-poly(2,3-dimethyl-l,3-butadiene) (cisPMBD),68 and lrans-, 4-poly(2,3-dimethy 1-1,3-butadicnc) (transPMBD).68 These polymers are representative examples of polydienes with A = A = H... 

According to the conformational energy minima, isotactic trans- 1,4-poly (1,3-pentadiene)73,74,90 - 94 and trans-, 4-poly(2-methyl-1,3-pcntadicnc)95 are characterized by chains in the conformation (A trans A+T)n (tl symmetry) and chain axes c values of 4.85 and 4.82 A, respectively. The conformation (A cisA 1 T) with s(2/l) symmetry characterizes the chains in the structures of isotactic m-l,4-poly(l,3-peiiladiene)96 98 and cis-1,4-poly (2-methyl-1,3-pentadiene).85... 

Fossey, S. A., Nemethy, G., Gibson, K. D., and Scheraga, H. A. (1991). Conformational energy studies of beta-sheets of model silk fibroin peptides. 1. Sheets of poly(Ala-Gly) chains. Biopolymers 31, 1529-1541. 

Calculations of conformational energy made by means of molecular mechanics fully confirm these conclusions. Such calculations were first introduced into the examination of synthetic crystalline polymers by Liquori and co-workers (175, 176) and were extensively used by Natta, Corradini, Allegra, Ganis, and co-workers (168, 177-179). The conformational energy map of isotactic poly-... 

Conformational energies as function of rotational angles over two consecutive skeletal bonds for both meso and racemic diads of poly(Af-vinyl-2-pyrrolidone) are computed. The results of these calculations are used to formulate a statistical model that was then employed to calculate the unperturbed dimensions of this polymer. The conformational energies are sensitive to the Coutombic interactions, which are governed by the dielectric constant of the solvent, and to the size of the solvent molecules. Consequently, the calculated values of the polymeric chain dimensions are strongly dependent on the nature of the solvent, as it was experimentally found before. 

Contrary to the usual procedure which considers only the local conformations of minimum energy, the mean-square end-to-end distance of crs-1,4-poly butadiene is recalculated taking into account the whole continuum of conformational states elastic strain on the C—C-C bond angles is also allowed, but only the values which minimize the conformational energy are retained, for every set of rotation angles. 

Geometry-optimized CNDO/2 molecular orbital calculations are carried out on poly(5,5 -bibenzoxazole-2,2 -diyl-1,4-phenylene)- and poly(2,5-benzoxazole)-model compounds to determine conformational energies as a function of rotation about each type of rotationable bond within the repeat units. 

Empirical conformational energy calculations are performed on helical poly(2,3-quinoxaline)s to predict stable conformations. Two energy minimum conformations are found by varying the dihedral angle, y, between two adjacent quinoxaline units from 5 to 180°. Circular dichroism spectra are calculated for the two stable conformations (v - 45 and 135°) on the basis of exciton theory. 

Conformational energies are calculated for chain segments in poly(vlnyl bromide) (PVB) homopolymer and the copolymers of vinyl bromide (VBS and ethylene (E), PEVB. Semlempirical potential functions are used to account for the nonbonded van der Waals and electrostatic Interactions. RIS models are developed for PVB and PEVB from the calculated conformational energies. Dimensions and dipole moments are calculated for PVB and PEVB using their RIS models, where the effects of stereosequence and comonomer sequence are explicitly considered. It is concluded from the calculated dimensions and dipole moments that the dipole moments are most sensitive to the microstructure of PVB homopolymers and PEVB copolymers and may provide an experimental means for their structural characterization. 

Allowing for rotation about the Ca—C bond (/.e., variation of ijr) and for some degree of freedom about the peptide bond [i.e., small variation of ro), the characteristic ratios of the form / (crs) and form II [trans) poly(L-proline) chain are calculated by a Monte Carlo method in which the conformational energies are used as weighting factors. The Monte Carlo method enabled short-range interactions (beyond those involved in a single residue) to be taken into account. 

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