Structures of three crystal modifications of poly(3,3-dimethyloxacyclobutane)

Three crystal modifications of poly(3,3-dimethyloxacyclobutane) [? CH₂C(CH₃)₂CH₂O? ]_n were found and their structures were analyzed by x-ray diffraction. Modification I is obtained only under tension and disappears on relaxing the tension. From the fiber period of 4.83 Å, the molecular structure seems to be planar zigzag. In modification II, two chains in T₃GT₃? conformation pass through a monoclinic cell with parameters a = 8.93 Å, b = 7.48 Å, c (fiber axis) = 8.35 Å, β = 97.9°, and the space group P2₁/c-C. In modification III, two (T₂G₂)₂ chains pass through an orthorhombic cell with parameters a = 15.60 Å, b = 5.74 Å, c (fiber axis) = 6.51 Å, and the space group, C222₁–D. Molecular conformations of the three crystal modifications correspond to those of polyoxacyclobutane.     

Structural studies of polyethers. IV. Structure analysis of the polyoxymethylene molecule by three-dimensional fourier syntheses

Detailed structure analysis of the polyoxymethylene molecule was carried out by the method of three-dimensional Fourier synthesis. The diffraction data were measured on a highly ordered polyoxymethylene sample prepared from a single crystal of tetraoxane by solid-state radiation polymerization. By assuming the anisotropic thermal factors for the oxygen and carbon atoms and taking into account the contribution of the hydrogen atoms, good agreement between the observed and calculated structure factors was obtained; the R factor is 8.8%. The distance between the oxygen atom and the helix axis r(O) = 0.671 A. is slightly shorter than r(C) = 0.691 A. The bond distance C? O = 1.42₁ A., the angle ∠COC = 112°24′, and ∠OCO = 110°49′. All the internal rotation angles of the skeletal bonds are 78°13′. It is deduced from the experimental results that each crystallite of polyoxymethylene is composed of only one type of helix, right-handed or left-handed.     

Crystal structure of α-gutta percha: Modification of the constrained least-squares method

The crystal structure of α-gutta percha has been determined by x-ray diffraction. The unit cell parameters are a = 7.98 Å, b = 6.29 Å, c (fiber period) = 8.77 Å, and β = 102.0° (monoclinic). The space group is P2₁/c–C_2h⁵. Two molecular chains of nearly trans-CTS-trans-CTS? conformation pass through a unit cell; C, T, S, and S? being the cis, trans, and two types of skew forms, respectively. The constrainedle astsquares method was modified so that the order of the least squares matrix could be reduced and was applied to the refinement of the crystal structure.     

Structural studies on polyformals. III. Poly-1,3-dioxocane and poly-1,3-dioxonane

Molecular and crystal structures of poly-1,3-dioxocane and poly-1,3-dioxonane, the polyformals [? OCH₂O? (CH₂)_m? ]_m with m = 5 and m = 6, respectively, are analyzed. Poly-l13-dioxocane is triclinic, space group with m = 5 and m = 6, respectively, are analyzed. Poly-1,3-dioxocane is triclinic, space group (fiber axis) and N (number of chains per unit cell) = 2. Poly-1,3-dioxonane is orthorhombic, (fiber axis) = 18.8 Å, and N = 2. The molecular conformations are roughly where T, G, and G are the trans, and the two gauche forms, respectively.     

Polymerisation im festen Zustand und Polymerkristallisation

Ohne Zusammenfassung     

Structure of poly(ethylene oxide) complexes. II. Poly(ethylene oxide)–mercuric chloride complex

The structure of the crystalline complex of poly(ethylene oxide) with mercuric chloride, whose composition is (CH₂CH₂O)₄ · HgCl₂, has been determined by x-ray diffraction. The unit cell is orthorhombic with the dimensions of a = 13.55 Å, b = 8.58 Å, and c (fiber axis) = 11.75 Å, and the unit cell contains 4 HgCl₂ molecules and 16 CH₂CH₂O units. Four chains pass through the lattice and four monomeric units are contained in the fiber identity period. The space group is Ccmm–D_2h¹⁷, Ccm2¹–C_2v¹², Cc2m–C_2v¹⁶ or C222₁–D₂⁵. The positions of Hg and Cl atoms have been determined by the Patterson function synthesized by the use of intensity data of the fiber sample, and the molecular conformation of poly(ethylene oxide) has been determined by examining the space not occupied by mercuric chloride molecules in the crystal lattice. The conformation of polyethylene oxide in the complex has been found to be the form of T₅GT₅G ; that is, where G and G mean the right- and left-handed gauche forms, respectively. This molecular structure has been confirmed further by the results of the Fourier syntheses by using the more accurate data refined by the intensity measurements with a diffractometer on the powder sample. The bond length between Hg and Cl in the complex (2.30 Å) is a little longer than that of HgCl₂ in the crystal (2.25 Å). This is consistent with the fact that the infrared absorption band associated with the antisymmetric stretching vibration of HgCl₂ shifts to 353 cm^?1 in the complex from 367 cm^?1 in the crystal. It was also found that another type of complex, giving a different infrared spectrum and x-ray diffraction pattern, was obtained when the original complex was soaked much longer in a saturated ether solution of HgCl₂.     

Construction of higher-ordered monolayer membranes derived from archaeal membrane lipid-inspired cyclic lipids with longer alkyl chains

A series of artificial cyclic lipids that mimic archaeal membrane ones has been synthesized. The structural features of these molecules include a longer cyclic framework, in which the alkyl chain length ranges from 24 to 32 in carbon number, which is longer than our first analogous molecule with 20-carbon long alkyl chains [K. Miyawaki, T. Takagi, M. Shibakami, Synlett 8 (2002) 1326]. Microscopic observation reveals that these molecules have a self-assembling ability: hydration of the lipids yields multilamellar vesicles in aqueous solution and monolayer sheets on solid supports. High-sensitivity differential scanning calorimetry (24- and 28-carbon alkyl chain lipids) indicates that (i) the alkyl chain length affects their phase behavior and (ii) the enthalpies of endothermic peaks accompanied by phase transition were considerably lower than those of their monomeric phospholipid analogs. Fluorescence polarization measurements suggest that the membranes made from the 24-carbon alkyl chain lipid have a higher polarization factor than membranes composed of DMPC and DMPC plus cholesterol. These findings imply that the cyclic lipids containing 24- and 28-carbon alkyl chain construct well-organized monolayer membranes and, in particular, that the molecular order of the 24-carbon alkyl chain lipid is higher than that of bilayer membranes in the liquid-ordered phase.