Poly energy calculations

For syndiotactic polydienes, experimental data and energy calculations have been reported for ds-l,4-poly(l,3-pentadiene)83,99,100 and ds-l,4-poly(3-methyl-1,3-pentadiene).87 In both cases energy minima are obtained for the tc conformation (A cwA TA+cwA+T) 47,83,87 according to the observed chain axes of 8.5 and 8.6 A.87,99... 

Isotactic poly(methyl methacrylate), also, is an intricate case, resolved only after a 20-year debate. The repetition period along the chain axis is 10.40 A corresponding to S monomer units the entire cell contains 20 monomer units (four chains). At first, the stmcture was resolved as a 5/1 helix (183) with = 180° and 62 — 108° but no reasonable packing was found using this assumption. Further conformational calculations showed that helices like 10/1 or 12/1 should be more stable than the 5/1 helix. The structure was solved by Tadokoro and co-workers (153b) who proposed the presence of a double helix. Two chains, with the same helical sense and the same direction but displaced by 10.40 A one from the other are wound on each other, each chain having 10 monomer units per turn [i(10/l)] and a 20.80-A repeat period. As a result, the double helix has a 10.40-A translational identity period, identical to that found in the fiber spectmm. The conformational parameters are Of = 179° and 2 = -148°. Energy calculations indicate that the double helix is more stable by 4.4 kcal per-mole of monomer units than two isolated 10/1 helices, a result that is in line with the well-known capacity of this polymer to form complexes in solution (184). 

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 calculations are carried out on poly(di-n-hexylsilanes). The most significant finding from the energy calculations is that the a -trans conformation is not the lowest energy structure for the symmetrically alkyl-substituted silane polymers. A helical structure is preferred for the isolated molecule. 

Intramolecular energy calculations have been made on polygermane chains, so additional information on their conformational preferences is available 43 More specifically, molecular orbital calculations have been carried out as a function of rotations about the backbone bonds. For poly(dimethylgermane), the results predict a broad energy minimum located at the trans conformation, with two symmetrical, steeper and somewhat shallower minima near the usual gauche locations. The results are very similar to those for poly(dimethylsilane), except that the barriers are considerably lower.43 This is apparently due to the fact that Ge-Ge bonds are approximately 0.30 A longer than Si-Si bonds.44 6 Unfortunately, no relevant experimental data might be used to test the validity of the calculations are available at the present time. 

The intramolecular interaction energy was calculated for five isotactic polymers, namely, isotactic polypropylene, poly(U-methyl-l-pentene), poly(3-methyl-1-butene), polyacetaldehyde, and poly(methyl methacrylate) (23). The molecular structures of the first four polymers have already been determined by x-ray analyses as (3/1) (2k), (7/2) (18,25.,26), (U/l) (21), and (U/l) helices (28), respectively. Here (7/2) means seven monomeric units turn twice in the fiber identity period. For isotactic poly(methyl methacrylate) (29), a (5/l) helix was considered reasonable at the time of the energy calculation in 1970, before the discovering of... 

These energy calculations can provide suitable and stable molecular models, and have been successfully utilized for the structure analyses of many other polymers, such as poly(tert-butylethylene oxide) (3 ) and polyisobutylene (35). 

The evolution of the frequency of the maxima show a linear dependency with the inverse temperature according to the expectations as shown in Fig. 2.41. The activation energies calculated from the Arrhenius plot for each process are 68.2 0.6kJ mol-1 for p relaxation and48.0 0.3 kJ mol-1 for y relaxation. These values are very close to those found for structurally related to P4THPMA such a PCHM [29,30], PMCHMA [59], poly(2-chlorocyclohexyl methacrylate) [33] and PDMA [104] (70-82 and 41-50 kJ mor1 [38],... 

Baker, K.N., Fratini, A.V., Resch, T., Knachel, H.C., Adams, W.W., Socd, E. P. and Farmer, B.L (1993) Crystal structures, phase transition and energy calculations of poly-p-phenylene oligomers. Polymer, 34, 1571-87. 

A polymer related to poly-L-proline, in the sense that the amide nitrogen is substituted and, therefore, cannot take part in hydrogen bonding, is poly-N-methyl-L-alanine. Conformational energy calculations for this... 

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. 

F. T. Hesselink, T. Ooi, and H. A. Scheraga, Macromolecules, 6, 541 (1973). Conformational Energy Calculations. Thermodynamic Parameters of the Helix—Coil Transition for Poly(L-lysine) in Aqueous Salt Solution. 

Figure 4. Theoretical CD spectra of poly[Ala -1-napAla] in a-helical conformation with the least-energy side-chain conformation predicted from the ECEPP energy calculation. Numbers in the figure indicate m.

Baker, K.N. et al.. Crystal-structures, phase-transitions and energy calculations of poly(p-phenylene) oligomers. Polymer 34, 1571-1587, 1993. 

The crystal structure of PHB is a orthorhombic form with unit cell parameters fl = 0.576 nm, = 1.320 nm, and c(fiber axis)=0.596 nm, and space group P2,2,2, (Alper et al. 1963 Okamura and Marchessault 1967). The conformational analysis by intermolecular energy calculation has indicated that the PHB molecule has a left-handed 2j helix conformation (Comibert and Marchessault 1972 Yokouchi et al. 1973 Bruckner et al. 1988). The crystal structure of random copolymers of 3HB and 3HV has been investigated extensively (Bloembergen et al. 1986 Kamiya et al. 1991 VanderHart et al. 1995). A structural characteristic of poly(3HB-co-3HV) is isodimorphism, i.e., cocrystallization, of the two monomer units. In contrast, the cocrystallization of 3HB with 3HH or (7 )-6-hydroxyhexanoate (6HH) does not occur. 

Patnaik, S.S. and B.L. Farmer. 1992. Energy calculations of the crystal-structure of poly(di-methyl silane). Polymer 33 5121. 

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