Nuclear magnetic resonance model compound analysis

With the development of polymer structural characterizations using spectroscopy, there has been a considerable effort directed to measurements of tacticity, sequence distributions and number average sequence lengths (59 65). Two methods have been traditionally used for microstructure analysis from polymer solutions. Vibrational spectroscopy (infrared) and Nuclear Magnetic Resonance (NMR). Neither of these techniques is absolute. The assignment of absorption bands requires the use of model compounds or standards of known structure. 

While nuclear magnetic resonance spectroscopy has become an invaluable tool in lignin analysis, relatively little effort has been directed toward the calculation of such spectra for lignin model compounds. Liptaj et al. [68] compared conformational data for cinnamaldehyde models derived from NMR to calculation, and similar work on quinone methides at both semiempirical and ab initio levels has been reported by Konschin et al. [69]. 

The elimination reaction of this chloride was carried out with potassium 2-methyl-2-butoxide to give the desired monomer. Monomer 2 was prepared in the same way using 4-methyl catechol. The structures were confirmed by infrared (IR), proton magnetic resonance (PMR), 13-C nuclear magnetic resonance (CMR) and elemental analysis. Model compounds 3 - 6 were synthesized as aids in establishing the structure of polymers derived from 1. 

Comparison of the nuclear magnetic-resonance spectrum of methyl 4-0-acetylmycaroside with the spectra of some model compounds, and the failure to obtain an isopropylidene derivative of methyl a-L-mycaroside, led Foster and coworkers to propose for mycarose the ij-xylo configuration. However, the more detailed analysis made of the nuclear magnetic-resonance spectrum of di-O-acetylmycarose by Hofheinz and coworkers as well as the stereospecific syntheses of mycarose by Korte and coworkers and by Woodward and coworkers, leave little doubt that mycarose is 2,6-dideoxy-S-C-methyl-L-nho-hexose (61) and that cladinose is its 3-methyl ether (62). 

Verbruggen and co-workers [13] prepared a model NR gumstock (i.e., cure system only, no fillers or other additives) compound and subjected a thin film (300 pm thick) of it to DPDS at 200 "C. Solvent extraction (with acetone and tetrahydrofuran), swelling experiments (in toluene) and chemical analysis by GPC and nuclear magnetic resonance (NMR) spectroscopy were used to assess the degree of devulcanisation and to characterise the samples produced. It was found that complete network breakdown was obtained with 2.4% w/w of DPDS after 2 h of heating, but that both crosslink scission and main-chain scission had occurred. 

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