A low symmetry structure of isotactic poly(4-methyl-pentene-1), Form II. An illustration of the impact of chain folding on polymer crystal structure and unit-cell symmetry

The crystal structure of the Form II modification of isotactic poly(4-methyl-pentene-1) (P4MP1) is derived by molecular mechanics modeling based on electron diffraction patterns of single crystals and on earlier X-ray powder diffraction patterns. This turns out to be a second structure derivation, since we became aware, after its completion, of the earlier work of De Rosa [De Rosa C. Macromolecules; 2003, 36, 6087] based on X-ray powder patterns and NMR data. In essential agreement with his analysis, the unit-cell is monoclinic with parameters a=18.50 Å, b=10.43 Å, c=7.22 Å, γ=113°. The chain has four propylene units per turn but two fold symmetry. Different structures are considered. The most straightforward ones have a space group P21/a (c unique axis) and the cell contains two anticline enantiomorphous helices (i.e. Right up-Left down, or Right down-Left up), with possible statistical presence of up- and down-pointing helices at each helix site. However, detailed single crystal electron diffraction patterns display weak equatorial reflections that should be extinct for P21/a, indicating a lower cell symmetry. A model with P1121 symmetry made of antichiral but isoclined (e.g. Right up-Left up) helices accounts better for the experimental pattern. It is suggested that this lower symmetry results from, or at least is favored by, conformational restrictions set by chain folding on the stem chirality and clinicity of helical polyolefins, as analyzed by Sadler et al. [Sadler DM, Spells SJ, Keller A, Guenet JM. Polym Commun, 25, 290, 1984] and Petraccone et al. [Petraccone V, Pirozzi B, Meille SV. Polymer, 27, 1665, 1986]: two stems linked by a fold must be either antichiral or anticline, which rules out the P21/a symmetry. The structure taking into account the impact of folds is consistent with the single crystal habit (preferred ac growth planes). It corresponds to the 'chain folded lamellar' variant of the P4MP1 Form II crystal structure whereas the cells with P21/a symmetry would be suitable models for the 'fiber' (non-folded) variant.