Polymer crystal structures determination

Chain Packing and Crystal Structures. The chain packing and the suhmolecular arrangement of repeat units and pendant side groups of macromolecules in crystalline domains of polymers can be visualized using contact mode SFM. The resolution is in most cases not true resolution, since the area of the contact area (1 — few nm ) exceeds the molecular scale and must be considered lattice resolution instead. The first example of molecularly resolved structures of a polymer dates back to 1988, when Marti and co-workers reported on an SFM study on a polydiacetylene film (128). Examples for resolved chain packing and polymer crystal structure determination at the surface of semicrystalline polymers include poly(tetrafiuoroethylene) (PTFE) (129,130), polyethylene (PE) (131-133), polypropylene (PP) (134,135), poly(ethylene oxide) (PEO) (136), aramids (137,138), and poly(oxy methylene) (POM) (139). 

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The mixed metal alkyl-amido base [BuNa(TMEDA)][TMP2Mg] (TMP = 2,2,6,6-tetramethylpiperidine) is capable of deprotonating furan selectively at its 2-position in THF as a solvent. The product obtained is a complex tris-furylmagnesiate, with the empirical formula [Na2(THF)3][2-furyl6Mg2(TMEDA)] (15) . An X-ray crystal-structure determination showed that in the solid state this compound exists as a coordination polymer of [Na2(THF)3][2-furyl6Mg2] units linked by bridging TMEDA molecules (Figure 16). 

Bis[(dimethylisopropoxysilyl)methyl]zinc (74) has proven to be a valuable reagent in the nickel-catalysed cross-coupling reactions with alkenylsulfoximines, giving allylic silanes with complete retention of configuration. An X-ray crystal structure determination of 74 showed this compound to exist in the solid state as a coordination polymer (Figure 41). In 74, in addition to the two covalent Zn—C bonds [Zn—C(l) 1.953(7)... 

Diethylzinc forms a 1 1 complex with DABCO, Et2Zn(DABCO) (94). An X-ray crystal structure determination revealed that Et2Zn(DABCO) exists in the solid state as a coordination polymer in which Et2Zn units are linked by the two coordinating nitrogen atoms... 

A next step in the assignment of the excited states responsible for emissions in gold-dithiophosphate dimers is the contribution of Eisemberg et al. [37]. They analyzed the optical properties of the complexes [Au2 S2P(OR)2 2] (R = Me, Et, n-Pr, n-Bu), for which the crystal structure determinations of the complexes with R = Me and R = Et revealed that these are extended linear chain polymers formed by gold interactions between dinuclear units of about 3 A, of the same type as those described previously. 

Nonstandard amino acids can be either incorporated globally at multiple sites within a protein or inserted at specific locations (1, 15). Global misincorporation of nonstandard amino acids can produce protein polymers with altered physical properties that confer, for example, varied tensile strengths and elasticities (16). These unique biomaterials can be used in many medical applications, such as altering properties associated with cell adhesion. In other applications, routine replacement of methionine by selenomethionine aids in X-ray crystal structure determination. 

While the balance of evidence is therefore in favor of a cyclic formulation for all compounds (PNCU) except the elastomer, a direct crystal structure determination of the pentamer or higher polymer would be of great value. 

Oligomers have been studied to test the assumptions about the chain conformation of the polymer. The major difficulty is the low oxidative stability of these compounds. Crystal structure determinations for the dimer and the trimer by means of x-ray studies are briefly mentioned in a paper by Street et al. [254]. In both cases, the molecules are planar and have the alternating orientation of the rings. While the dimer molecules are stacked in parallel fashion (monoclinic cell), the trimer molecules are inclined with respect to their neighbours (orthorhombic cell). 

A second important focus of our work is the development of suitable analytical methods for the solid state and in solution. The physical characterization of metallo-supramolecular systems has mainly relied on crystal structure determination. Studies have also been performed on surface layers 40-42). The classical analytical methods (like FAB mass spectrometry) or most polymer methods (like light scattering, vapor pressure osmometry or membrane osmometry) can not be used. In solution, ESI mass spectrometry (43-45) and NMR (27,46) have been succesfully applied. We have explored whether MALDI-TOF mass spectrometry in the solid state (Schubert, U. S. Lehn, J.-M. Weidl, C. H. Spickermann, J. Goix, L. Rader, J. Mullen, K., unpublished data.) and sedimentation equilibrium analysis in the analytical ultracentrifuge for solutions may be employed. Grid-like cobalt coordination arrays ([2 X 2] Co(n)-Grid) were used as model systems in the analytical ultracentrifuge (47). 

Program Description. In order to determine polymer crystal structures, each of the participating Institutions uses computer programs that reflect different resources and abilities as well as philosophies. Models to be used for calculation of Intensities are constructed In various ways, and each has a different strategy for finding the best result. Each of the methods has some unique abilities and some disadvantages. 

Although polymer crystal structures are known, and some slip mechanisms (slip plane and slip direction) determined, these are less important than for metals. Firstly, the amorphous phase plays an important part in the mechanical properties. Secondly, polymer yield strengths are not determined by obstacles to dislocation movement. However, it is possible to fabricate highly anisotropic forms of semi-crystalline polymers, so crystal characterization and orientation are important. 

The values of o and I were determined from crystallographic data for various polymers provided by Miller and Nielsen (1960) and made use also of much accurate pictorial representation of polymer crystal structures given by Geil (1963). 

Very recently, several five and six-coordinate ruthenium monocarbene complexes have also been structurally characterized by the groups of Che [141] and Miyamoto [142-144], X-ray crystal structure determinations revealed Ru = C distances of 1.806(3)-1.876(3) A, comparable to those in the previously reported analogues (Table 3). In one case, (TPFPP)Ru[C (p-C6H40Me)2], the complex is a one-dimensional coordination polymer [141]. In this particular complex, the carbene groups function as bridges through the carbene carbon atom and one of the two methoxy oxygen atoms. Many of these complexes were also characterized by H and C NMR [141]. 

In the polymers tin becomes live-coordinated, trigonal bipyramidal, with the oxygen atoms in axial positions. Intra-unit Sn- -O distances are ca. 3.0 A, which suggests very weak interaction, and in many crystal structure determinations they were simply ignored. They usually distort the trigonal bipyranaidal geometry, however. 

In summary, it seems clear that particulate fillers can have significant nucleating effects in semi-crystalline polymers and that this may lead to effects on mechanical properties. Much work remains to be done to clarify this and provide a clear, coherent description of the effects involved, however. This would be greatly helped if simple techniques for determining polymer crystal structures in filled systems were available. It would also be highly desirable to have a better understanding of how the structure of polymer crystallinity affects composite properties. 

The polymorphism in poly(l, 4-trans-isoprene) (gutta percha) has been studied in detail. Based on a detailed analysis of chain stereochemistry Bunn (261) predicted the possibility of four different crystalline modifications of this polymer, each with a different chain structure. Two of these, crystallized solely by cooling the polymer to an appropriate temperature, have been identified and their crystal structures determined.(261-263) A third form, that crystallizes upon stretching, has also been identified.(264) However, its structure has been questioned.(264)... 

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