An Unusual Chain Conformation:  s(6*4/3) Helices in Form II of Isotactic Poly(vinylcyclopentane)

Abstract

The crystalline form II of isotactic poly(vinylcyclopentane) was studied on the basis of fiber X-ray diffraction and of potential energy calculations of both isolated and packed chains. The hypothesis of a s(6*4/3) conformation of the chain has been investigated in detail. According to our interpretation, s(6*4/3) helices, which represent an anomalous case among isotactic poly(α-olefins), result from the superposition of a basically s(2*4/1) conformation of the backbone chain and of side groups following one another with a regular sequence of three conformations, mainly different for their orientation. Slight deformations of the main chain torsion angles from s(2*4/1) symmetry are found to be relevant to improve the data fitting. The resulting structure is indicative of a quite singular polymorphism, mainly ascribable to the different conformations accessible to the side groups.

Crystal Structure of Alternating Isotactic Ethylene−Cyclopentene Copolymer

“…44 In the case of poly-(vinylcyclopentane), instead, the conformational freedom of cyclopentane rings is restricted in a narrow range of the pseudorotational path, dictated by severe ring-to-chain steric interactions. 45 The crystal structure of alternating ECC presents structural features also similar to the crystal structure of another important class of cyclic olefin copolymers, the alternating ethylene-norbornene copolymers (ENCs), in particular as far as the statistical occupancy of the lattice sites from chains having S-and Z-shapes is concerned. 8 However, other kinds of structural disorder are also present in the crystal structure of ENCs, depending on the degree of stereoregularity.…”

“…It is worth noting that disorder in the conformation of cyclopentene rings for ECC is analogous to the disorder described also for other crystalline polymers containing cyclopentane rings along the main chain, for instance poly(methylene-1,3-cyclopentane) (PMCP), 43,44 or as side groups, for instance isotactic poly(vinylcyclopentane). 45 In particular, independent of the microstructure of PMCP chains, similar crystalline structures are formed, characterized by hexagonal packing of the chain axes and a high degree of disorder as far as the rotation of the chains around the chain axes and the relative translation of the chains parallel to the axes. This disorder is linked either to disorder in the cis or trans configuration of cyclopentane rings or to the disorder in the relative chirality of consecutive stereoisomeric centers determining a different tacticity of PMCP chains.…”

“…This disorder is linked either to disorder in the cis or trans configuration of cyclopentane rings or to the disorder in the relative chirality of consecutive stereoisomeric centers determining a different tacticity of PMCP chains. , In all cases, a high degree of dynamic disorder in the conformation of cyclopentane rings is present as in the case of ECC . In the case of poly(vinylcyclopentane), instead, the conformational freedom of cyclopentane rings is restricted in a narrow range of the pseudorotational path, dictated by severe ring-to-chain steric interactions …”

Crystal Structure of Form I of Syndiotactic 1,2-Poly(4-methyl-1,3-pentadiene)

“…Furthermore, although we obtained in the past good results using just surface integrals, [10][11][12] the frequent reflection overlap suggested to prefer the method by Cella et al 13 who assert that, in the case of fiber spectra, the correct structure factors must be computed by linear integration along increasing ϑ lines and applying appropriate correction factors to these integrals.…”

TRY, a new computer program for crystal structure analysis from diffraction data based on internal coordinates and on a molecular modelling procedure free of redundant coordinates

“…The most significant structures studied using TRY (all polymeric) are cited in the references (Antinucci et al, 2001;Immirzi et al, 2000Immirzi et al, , 2003Immirzi et al, , 2005Immirzi et al, , 2007. The program has been employed both for setting up models and for refinement.…”