| Abstrakt: |
Summary: The vast majority of head-to-tail stereoregular crystallizable polymers with an isotactic structure generally assume a helical symmetry in the crystalline state, compatible with the regular repetition along the chain axis of isomorphous isoclined units. There are at least two cases, in which an isotactic polymer chain does not adopt a helical conformation in the crystalline state. One is the crystalline alternating copolymer ethylene-cis-butene-2, (E2B) adopting a (T3G+T3G−)n conformation in the crystalline state, and the second case is isotactic polystyrene (i-PS) in crystalline gels. In this contribution, the case of E2B and i-PS is discussed in detail. In the first case, the theoretical provision that an isotactic polymer may show a chain conformation in the crystal state is verified, which corresponds to a regular succession of enantiomorphous, anticlined structural units, instead of a helical repetition of isomorphous isoclined structural units. A new conformational model is presented for the second case, i-PS chains in the crystalline gels, which could account for all the experimental data concerning this system collected so far in the literature. The limit ordered conformational model of i-PS chains in crystalline gels proposed is built up of regular helical blocks of chains of opposite handedness alternating along the chain and explains the 0.51 nm meridional reflection in the x-ray diffraction patterns of stretched i-PS gels. For the most ordered conformation, it includes 16 monomeric units in a period of 3.06 nm. Minimum energy conformations for i-PS chains including (TG)n helical stems of opposite chirality (R, L indicate the right- and the left-handed helical portions, respectively) A: The right-handed spiral sequence (G−T)n is followed by a left handed one, (TG+)m. B: The left-handed helical sequence (TG+) is followed by a right-handed one trough the junction (TT'). C: Possible conformational model for i-PS chains, corresponding to the sequence of dihedral angles
(G−T)3n(TG+)3j(TT')(G−T)3m
These types of junctions shown in A and B must alternate along the chain. |
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