Chain configuration spectroscopy

Since about 1960 nuclear magnetic resonance (NMR) spectroscopy has become an important tool for the study of chain configuration, sequence distribution and microstructure of polymers. Its use started from early broad-line studies of the one-set of molecular motion in solid polymers and passed through the solution studies of proton NMR, to the application of the more difficult but more powerful carbon-13 NMR methods to both liquids and solids. 

Proton NMR spectroscopy has been used fruitfully for many years in the analysis of polymer chain configuration (i). The proton solution spectra (2) for poly(methyl-n-propylsilylene) (PMPS), poly(methyl-n-hexylsilylene) (PMHS), poly(methyl-n-dodecylsilylene) (PMDS), and poly(di-n-hexylsil-ylene) (PDHS) are shown in Figure I, At this high field (11.75 T), the res-... 

Figure 8 (Left) Force spectroscopy Shear forces between poly(ethylene propylene) end functionalized with a zwitterionic group (PEP-X) brush-bearing surfaces at separation D=7.0 0.4nm. Upper trace, applied lateral motion (AXo) of top mica surface lower trace, shear force fs transmitted to the lower mica surface. The horizontal broken line represents the midpoint between the shear forces on the back and forth cycles, and is therefore the position of zero shearforce when the shear springs are unbent. Inset (i) illustrates the chain configuration in the initial force rise regime a- b. Inset (ii) illustrates the relaxation of a chain following cessation of the applied motion at point d (lower trace) the thick part is as yet unrelaxed, while the thin part has relaxed by arm retraction. (Right) The variation of the plateau normalized shearforce (kinetic normalized friction force, fnin IR, in units of pN m" ) with sliding velocity. Vs, taken from traces as on the left. The cartoons indicate schematically the effect of the self-regulation mechanism on the interpenetration zone S (see also Section 2.24.3.5). Adapted from Tadmor, R. Janik, J. Fetters, L. J. Klein, J. Phys. Rev. Lett. 2003, 91,115503. ...

Measurement of Unsaturation. The presence of double bonds in a fatty acid side chain can be detected chemically or through use of instmmentation. Iodine value (IV) (74) is a measure of extent of the reaction of iodine with double bonds the higher the IV, the more unsaturated the oil. IV may also be calculated from fatty acid composition. The cis—trans configuration of double bonds may be deterrnined by infrared (59) or nmr spectroscopy. Naturally occurring oils have methylene-intermpted double bonds that do not absorb in the uv however, conjugated dienes maybe deterrnined in an appropriate solvent at 233 nm. 

The application of NMR spectroscopy to tacticity determination of synthetic polymers was pioneered by Bovey and Tiers.9 NMR spectroscopy is the most used method and often the only technique available for directly assessing tacticity of polymer chains. "2 7 8 0JI The chemical shift of a given nucleus in or attached to the chain may be sensitive to the configuration of centers three or more monomer units removed. Other forms of spectroscopy (e.g. TR spectroscopy l2 lJ) are useful with some polymers and various physical properties (e.g. the Kerr effect14) may also be correlated with tacticity. 

The determination of the various types of geometric isomers associated with unsaturation in Polymer chains is of great importance, for example, in the study of the structure of modern synthetic rubbers. In table below are listed some of the important infrared absorption bands which arise from olefinic groups. In synthetic "natural" rubber, cis-1, 4-polyisoprene, relatively small amounts of 1, 2 and 3, 4-addition can easily be detected, though it is more difficult to distinguish between the cis and trans-configurations. Nuclear magnetic resonance spectroscopy is also useful for this analysis. 

Four diastereomers of 2-ethynyl-7a-hydroxy-3a,7-dimethyl-4,6-diphe-nylperhydropyrrolo[3,2-c]pyridine (58B) were recently isolated (92KGS-903). It was established by means of H-NOESY and C-NMR spectroscopy that the stereochemical differences in the structures of the diastereomers involve cis and transfusion of the rings and different configurations at C(2). In solution and in the gas phase, the corresponding ring-chain equilibrium 58A 58B was detected, with two open-chain epimers (different configurations at C—NH2). 

Chemical constitution, steric configuration and, in some cases, details about chain conformation, aggregation, association, and supramolecular self-organization behavior of macromolecular substances can be determined using high-resolution nuclear magnetic resonance (NMR) spectroscopy. This spectroscopic technique is sensitive towards nuclei with a nuclear spin different from zero. 

NOED spectroscopy was used to determine the configurations of the four conjugated side-chain double bonds in some retinoids and their methoxy derivatives, for example 4. Arrows indicate NOE perturbances directed from the irradiated proton(s) towards those which show an NOE response280. Analogously, the configuration of C-5 in carboprostacyclin 5 was determined281. 

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