Conformational analysis of polymers

P. G. Khalatur and Yu. G. Papulov, "Computer-Aided Experiment in the Conformational Analysis of Polymers, Kalinin. Gos. Univ., Kalinin, USSR, 1982. 

During the past five decades, spectroscopy has moved out from the laboratories of physicists and theoretical chemists into every area of analysis and chemical research. Applications vary from routine, single data point measurements for control of plant streams to structural analysis of complex molecules and conformational analysis of polymers. Enhancement of the sensitivity of all spectroscopic methods has been achieved through computer-assisted data handling. It is possible to find structural differences in polymers under different degrees of stress and to analyze for very low levels of chemical structures at surfaces and interfaces. For difficult structure determinations, data from several spectroscopic disciplines may be combined and require months or years of research. 

There is even a separate branch of research called conformational analysis of polymers. It deals with the geometry of atoms positions in reasonably short chain segments, for much more complex structures than polyethylene, of course. An example is depicted in Figure C2.4 a strand of a DN A double helix. (We shall talk about DNA structure in more detail in Sections 5.5... 

Finally, it should be emphasized that, in the refined RIS scheme, due consideration can be given to effects arising from variation in the shape of the potential energy minima. With this device, the RIS scheme has gained a wider applicability in the conformational analysis of polymer chains. In fact, as demonstrated by Suter et al. in the treatment of polypropylene (PP), this method allows a reasonable interpretation of the experimental results even when the shape of the potential energy well is highly distorted due to intramolecular steric conflicts. 

On the other hand, the method examines the rotations of contiguous residues about the glycosidic bonds, for example, C-l-O and O-C-4 in a (1 —> 4)-linked polysaccharide. This method scans the entire conformational space available to a polymer. The analysis can thus proceed either with a regular, helical structure, or a random conformation. The method has been used quite extensively in the conformational analysis of polysaccharides.9 26 The steric... 

These same considerations were expressed in a manner only seemingly different by Corradini (47, 48), who introduced a representation widely used in conformational analysis of vinyl polymers. He defined the chain bonds adjacent to the tertiary carbon atom as or — according to the clockwise or counterclockwise sequence of the two substituents in the Cahn-Ingold-Prelog sense, as shown in 8 and 9, as one proceeds along the chain. 

In recent years new NMR techniques offering broad applications in stereochemical analysis have come into use. A prominent example is 2D-NMR (both 2D-resolved and 2D-correlated spectroscopy), which has been extensively applied to biopolymers (149-151). Its use with synthetic polymers has, until now, been limited to but a few cases (152, 153). A further technique, cross-polarization magic-angle spinning spectroscopy (CP-MAS NMR) will be discussed in the section on conformational analysis of solid polymers. 

An analysis of polymer end groups provided insight into the mechanism of stereo-control in such catalysts. The first polymerisation step, where propene inserts into a Zr-Me bond, is in fact not stereoselective, while the insertion into a Zr-iso-butyl bond proceeds with high enantioselectivity. Ligand stereo-control operates therefore by an indirect mechanism the ligand determines the conformation of the polymery] chain, and this in turn influences the preferred orientation of the incoming alkene [127], as illustrated in structure 89 for a syndiospecific case. 

Several work are concerned with the synthesis, characterization, dielectric behavior and conformational analysis of dendronized Polymers. Poly(methacrylates) containing phtalimidoalkyl moieties in the side chain have been recently studied i.e. poly(3,5-diphtalimido alkylphenyl methacrylate)s with ethyl (P-EthylGi), propyl (P-PropylGi) and butyl (P-ButylGi) chains as spacer groups. Where Gi indicates first generation [112],... 

The foregoing typical results clearly illustrate another very important application of the polyethyleneglycol support method for the synthesis of peptides and protein sequences. The unique suitability of this linear, soluble macromolecular support with optimum hydrophilicity-hydrophobicity balance for the conformational analysis of the bound peptides originates from the peculiar conformational properties of the polymer chain. 

The SANS technique has already contributed to the analysis of polymer conformation in systems at equilibrium (i.e. dilute and concentrated solutions, bulk) [5-7] and to the molecular response of polymer systems under external constraints (i.e. polymer networks, polymer melts under uniaxial or shear deformation) [8-14]. 

The helical structure of polychloral was proposed by Vogl in 198028 and was demonstrated by Ute, Hatada, and Vogl via a detailed conformational analysis of chloral oligomers.29 As an example of a helical polymer with an inorganic backbone, polysilanes bearing a chiral side chain were synthesized and their conformational aspects were studied. A helical conformation with an excess screw sense for this class of polymers in solution was found in 1994 independently by Fujiki30a and by Moller.300 Matyjaszewski had pointed out such a conformation for chiral polysilanes in the solid state in 1992.30c... 

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