Poly isotactic/syndiotactic structures, conformational

Rather recently, we have studied the solid-state structure of various polymers, such as polyethylene crystallized under different conditions [17-21], poly (tetramethylene oxide) [22], polyvinyl alcohol [23], isotactic and syndiotactic polypropylene [24,25],cellulose [26-30],and amylose [31] with solid-state high-resolution X3C NMR with supplementary use of other methods, such as X-ray diffraction and IR spectroscopy. Through these studies, the high resolution solid-state X3C NMR has proved very powerful for elucidating the solid-state structure of polymers in order of molecules, that is, in terms of molecular chain conformation and dynamics, not only on the crystalline component but also on the noncrystalline components via the chemical shift and magnetic relaxation. In this chapter we will review briefly these studies, focusing particular attention on the molecular chain conformation and dynamics in the crystalline-amorphous interfacial region. 

Being acquainted with the structure of poly(a-olefin)s, one may reasonably explain some of the differences in their physicochemical properties. For example, isotactic polypropylene, the chains of which in the helical conformation can be closely packed, has rather a high density (0.92-0.94 g/cm3) and melting point (175°C) and is insoluble in low-boiling aliphatic hydrocarbons at boiling point. Syndiotactic polypropylene, consisting of chains in the form of binary helices, which cannot be packed so closely as in the previous case, has a density of 0.89-0.91 g/cm3 and a melting point of 135°C, which is 40 k lower from that of isotactic polypropylene syndiotactic polypropylene is also moderately soluble in... 

Because atactic polymer has no ordered structure and shows only slight intramolecular interactions, the interactions between atactic polymers is the strongest (Fig. 10 a). The isotactic polymers may be stabilized by assuming the helix conformation reported for isotactic poly(methyl methacrylate)401. Nucleic add bases are situated outside the polymer chain so that they can form the complex, although the interaction is not so strong. On the other hand, the syndiotactic polymer may have a rod-like conformation that is supported by the low solubility of the polymer and by NMR spectra321. Tlierefoie, it is well understood that the complex formation ability of the syndiotactic polymers is very low. 

MM calculations with full geometry optimization have been carried out to calculate the molecular structure and conformational energies od, ..modeJ -.cpmpounds for poly (phenylmethylsilylene) [-SiPhMe-] and poly(silastyrene) [-SiPhH-SiH2-]. In each case the calculations considered both the isotactic and syndiotactic stereochemical isomeric forms of the model compounds. The structures considered are illustrated in Figures 6 and 7. 

The protective effects of syndiotactic and isotactic poly-2-vinylpyridine-l-oxide remarkably differ in the spatial arrangement of the structural units (Holt et al. 1970). The isotactic polymer has probably a helical conformation. When the cultures... 

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