Nuclear magnetic resonance spectroscopy branching structures

With the advent of advanced characterization techniques such as multiple detector liquid exclusion chromatography and - C Fourier transform nuclear magnetic resonance spectroscopy, the study of structure/property relationships in polymers has become technically feasible (l -(5). Understanding the relationship between structure and properties alone does not always allow for the solution of problems encountered in commercial polymer synthesis. Certain processes, of which emulsion polymerization is one, are controlled by variables which exert a large influence on polymer infrastructure (sequence distribution, tacticity, branching, enchainment) and hence properties. In addition, because the emulsion polymerization takes place in an heterophase system and because the product is an aqueous dispersion, it is important to understand which performance characteristics are influended by the colloidal state, (i.e., particle size and size distribution) and which by the polymer infrastructure. 

Nuclear Magnetic Resonance Spectroscopy. Like IR spectroscopy, NMR spectroscopy requires little sample preparation, and provides extremely detailed information on the composition of many resins. The only limitation is that the sample must be soluble in a deuterated solvent (e.g., deuterated chloroform, tetrahydro-furan, dimethylformamide). Commercial pulse Fourier transform NMR spectrometers with superconducting magnets (field strength 4-14 Tesla) allow routine measurement of high-resolution H- and C-NMR spectra. Two-dimensional NMR techniques and other multipulse techniques (e.g., distortionless enhancement of polarization transfer, DEPT) can also be used [10.16]. These methods are employed to analyze complicated structures. C-NMR spectroscopy is particularly suitable for the qualitative analysis of individual resins in binders, quantiative evaluations are more readily obtained by H-NMR spectroscopy. Comprehensive information on NMR measurements and the assignment of the resonance lines are given in the literature, e.g., for branched polyesters [10.17], alkyd resins [10.18], polyacrylates [10.19], polyurethane elastomers [10.20], fatty acids [10.21], cycloaliphatic diisocyanates [10.22], and epoxy resins [10.23]. 

Another definition, taking into account polymerization conversion, has been more recently proposed.192 Perfect dendrimers present only terminal- and dendritic-type units and therefore have DB = 1, while linear polymers have DB = 0. Linear units do not contribute to branching and can be considered as structural defects present in hyperbranched polymers but not in dendrimers. For most hyperbranched polymers, nuclear magnetic resonance (NMR) spectroscopy determinations lead to DB values close to 0.5, that is, close to the theoretical value for randomly branched polymers. Slow monomer addition193 194 or polycondensations with nonequal reactivity of functional groups195 have been reported to yield polymers with higher DBs (0.6-0.66 range). 

The direct detection of radiation induced crosslinks in polyethylene has been a major goal of radiation chemists for many years. It was recognized as early as 1967 that solution 13c nuclear magnetic resonance (NMR) spectroscopy could be used to detect structures produced in polymers from ionizing radiation. Fischer and Langbein(l) reported the first direct detection of radiation induced crosslinks (H-links) in polyoxymethylene using 13c NMR. Bennett et al.(2) used 13c NMR to detect radiation induced crosslinks in n-alkanes irradiated in vacuum in the molten state. Bovey et al.(3) used this technique to identify both radiation induced H-links and long chain branches (Y-links) in n-alkanes... 

Py-GC Fourier-transform (FT) nuclear magnetic resonance (NMR) spectroscopy is another complementary technique that provides more detailed information on the structure of polymers than does Py-GC alone. As an example of the application of this technique Leibman and co-workers [2] applied it to a study of short-chain branching in polyethylene (PE) and polyvinylchloride (PVC). The nature and relative quantities of the short branches along the polymer chains were determined, providing detailed microstructural information. Down to 0.1 methyl, ethyl, or w-butyl branches per 1000 methylene groups can be determined by this procedure. Other structural defects can also be determined, thus providing significant information on polymer microstructure that is not otherwise readily obtained. 

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