Conformational and Packing Modeling of Optically Active Polyesters. 2. Helical Structure of an Isotactic Polylactone

Abstract

Optically active poly(α-methyl-α-n-propyl-β-propiolactone) (PMPPL) crystallizes in a 21 helical conformation, but its conformational structure and packing symmetry have not been solved and refined yet. On the basis of the rotational isomeric state approximation and the conformational algorithm for polymer helices, optimum conformational models derived from single helices were used as a starting point in building crystal structures. Both intra- and intermolecular interactions were simultaneously optimized. Semiempirical molecular mechanics calculations of an isotactic single chain having fixed experimental helical parameters revealed that the preferred conformation for PMPPL entails the ttg-g- backbone chain conformation with a g-t side-chain orientation. Crystal models compatible with the observed orthorhombic lattice dimensions (a = 10.6, b = 11.1 Å) were built and refined against electron diffraction data, X-ray powder diffraction spectra, and structure factor calculations. The favored structure contains two 2-fold screw helices packed antiparallel in an orthorhombic lattice with space group P212121−D2 4. With the ttg-g- main chain conformation, both the g-t and tt conformations are possible for the side chain according to the refinement of X-ray structure factors. However, the g-t side-chain conformation shows a better fit than that of the tt conformation with the electron diffraction patterns. The flexible procedure of energy minimization yields distinct values of fiber period for the g-t and tt conformations, e.g., 6.31 and 6.14 Å, respectively. The final discrepancy R factors are 0.20 for the g-t and 0.17 for the tt side-chain models when compared to X-ray data and 0.17 for the g-t and 0.27 for the tt side-chain models when compared to electron diffraction data.

Structural and Spectroscopic Investigations of Bulk Poly[bis(2-ethyl)hexylfluorene]

“…As a supplement to the SF calculations, a large number of single chain decamers were modeled by systematically varying both the backbone and side chain conformation. As already noted, a combinatorial approach has proven useful for identifying low-energy conformations in other alkyl-substituted polymers. ,, For PFs the alkyl chain spacing along the backbone is much sparser, and so interchain interactions could be expected to play a more dominant role. In view of the X-ray evidence for cold crystallization and, thereafter, the relatively few overt changes in the average PF2/6 backbone conformation on thermal cycling, this suggests that the chain conformational structure is already established while in solution, and the main impact of interchain interactions is to influence side chain order on local length scales.…”

“…Universal efficacy of this approach is not proven, but it has been shown to be useful for identifying viable low-energy conformers in selected σ-conjugated polysilanes 21,32 and conventional polyesters. 33 At present only a single RR enantiomer was modeled (for comparisons with the RR structure adopted in the structure factor calculations), and over 4000 starting conformers were examined.…”

A New Crystal Form, Polymorphism, and Multi‐Morphology in Biodegradable Poly(3‐hydroxypropionate)

“…A previous conformational analysis10 predicted that PHP and the α ‐ or β ‐substituted PHPs would all adopt either the 2 1 helix or the trans conformation in the crystalline state. The prediction has been well validated by research on PHP,5–8 PHB,4,12 polypivalolactone (PPL),13 poly( α ‐methyl‐ α ‐ n ‐propyl‐ β ‐propiolacone) (PMPPL),14,15 and poly( α ‐methyl‐ α ‐ethyl‐ β ‐propiolactone) (PMEPL),15,16 where the crystalline chains take either the 2 1 helix or the trans conformation.…”