Crystal structure tacticity

The zinc complex of 1,1,1,5,5,5-hexafluoroacetylacetonate forms coordination polymers in reaction with either 2,5-bis(4-ethynylpyridyl)furan or l,2-bis(4-ethynylpyridyl)benzene. The X-ray crystal structures demonstrate an isotactic helical structure for the former and a syndio-tactic structure for the latter in the solid state. Low-temperature 1H and 19F NMR studies gave information on the solution structures of oligomers. Chiral polymers were prepared from L2Zn where L = 3-((trifluoromethyl)hydroxymethylene)-(+)-camphorate. Reaction with 2,5-bis(4-ethy-nylpyridyl)furan gave a linear zigzag structure and reaction with tris(4-pyridyl)methanol a homo-chiral helical polymer.479... 

Heat capacities of polymer are complicated by physical and chemical structural changes, namely by changes in crystallinity, crystal structure, morphology, tacticity, molecular weight and copolymer unit content. Furthermore the picture can be complicated by changes in mechanical, thermal and possibly electrical history of the polymer. 

In the present paper, differences between isotactic PMEPL and the stereocomplex are examined by solid state nuclear magnetic resonance (NMR) spectroscopy. These studies reveal new polymorphic behaviour of the isotactic polymer and differences in crystal structure which depend on tacticity. Crystal structures of the various polymorphs were also determined by electron and x-ray diffraction studies. 

In the conformational analysis, we assumed that the main chain conformation of sample polymers in the glassy state maintains locally their crystalline structure. Since the crystal structure is 3/1 helix for both isotactic MA (24) and isotactic tBA (25), the local conformation was taken to be 3/1 helix for the isotactic sequence in both polymers, though the present samples are both atactic. Furthermore, we assumed to be planer zig-zag for the syndio-tactic sequence. 

Stereospecific behavior of the catalyst site is related to the chirality of the surface sites of the solid TiCls. Models by Corradini can explain a number of observations—the type of tacticity errors along a predominantly isotactic chain, stereospecificity of the initiation reaction, and the maintenance of isotacticity after the insertion of ethylene monomer in the chain (241). Furthermore, since violet TiCls and MgCl2 solid-state crystal structures are similar, Corradini s model relates well to both TiCls and MgCl2-supported catalysts. 

When Rj is different from R2 in Formula (1.1) the carbon atom is asymmetric and may have d or 7 forms. If all the asymmetric carbon atoms have either d or 7 forms, the polymer chain is said to be isotactic. If these carbon atoms are instead alternating d and 7 , the polymer chain is said to be syndiotactic. If the d and 7 assignments are random along the chain, it is said to be atactic [8,18-20] (see Fig. 1.1). Isotactic polypropylene, poly(butene-l) and poly(4-methylpentene-l) are commercially available. Both isotactic and syndiotactic polypropylene and polystyrene have been synthesized, subjected to extensive investigation. The two isomeric polymers have different crystal structures and their atactic forms do not crystallize. Isotactic and syndiotactic polymers were originally developed by Natta and his coworkers [18, 19] at Milan Polytechnic and Montecatini. In recent years, there has been interest in producing polyolefins with controlled intermediate tacticities [20]. 

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