13C NMR analyses

Useful information on the nature of the active sites can also be obtained by 13C NMR analyses of the polymers produced. These analyses indicate that some active sites can interconvert in a time shorter than the average time of chain growth118 120 and that at least one of the active sites has an environment of Ci symmetry.119... 

Careful 1H and 13C NMR analyses were carried out for both monomers and polymers in order to prove the chemical structures of the polymers. The H NMR spectra of 50 and 52 are shown in Figure 8. As polymerization proceeded, an acetylenic proton peak at 2.0-2.2 ppm disappeared, while a new vinylic proton peak appeared broadly in the 6.8-7.2 ppm range. Since the new peak is weaker than those for the aromatic biphenyl rings and the two peaks are superimposed, it is hard to separate them clearly. The broad peaks at 2.6 and 3.4 ppm are assignable to the methylene protons and methine proton in the ring, respectively. 

Based on 13C-NMR analyses, it has been assumed [62] that 6-methoxymellein, a polyketide compound, is synthesized by head-to-tail condensation of one acetyl-CoA and four malonyl-CoA to form 3,4-dehydro-6-hydroxymellein as an intermediate. This isocoumarin derivative is then reduced to its dihydro-form, 6-hydroxymellein, which then accepts a methyl-unit via Omethyltransferase. We have shown [63] that cell extracts prepared from elicitor-treated carrot root disks catalyzes... 

No characterization of the catalysts after reaction was reported. Detailed 13C NMR analyses were used to identify structures in the PPO products. A mechanism involving ring opening of PO after coordination of a PO monomer to the surface of the LDH leading to discrete sterically hindered catalytic sites was invoked to explain the high stereodirecting influence of the mixed oxide catalyst. 

The macromolecules were obtained as brittle, non-film forming materials with molecular weight ranges of 5000-35000 and Mw/Mn < 1.5. They were soluble in many organic solvents and thermally stable up to 550 °C. Branching efficiencies were estimated to be approximately 70% by 13C-NMR analyses. For this reason, no evidence for Malthusian-like, dendrimer dense-packing was observed as a function of molecular weight enhancement. 

This method is based on that of Fahim,4 who isolated 4-alkylbenzoic acids in 40-60% yields by hydrolysis of the corresponding acid chlorides. The present improved procedure includes those conditions believed to be optimum for a one-step synthesis of 4-substituted benzoyl chlorides in good yields and apparently free of positional isomers, as indicated by gas chromatography/mass spectroscopy as well as H and 13C NMR analyses. The procedure has been used successfully for the synthesis of 4-halobenzoyl chlorides and several other aryl acid chlorides,5 6 as well as for 4-alkylbenzoyl chlorides up through the decyl derivative. Some of these results are summarized in Table 1. The reaction has been run on a 1-mol scale by the submitters with no difficulty. 

The reaction of formaldehyde with a mixture was investigated by 13C-NMR analyses of samples taken from a synthesis with P/C/U/F molar ratios of 0.75 1 0.5 2.5. The carbohydrate was glucose. 

Values illustrating product yields and gas compositions from isothermal experiments are shown in Table II. The yield data shown were obtained after a reaction period of two hours at a reaction temperature of 450°C. The gas and liquid yields are higher and the residue yield is lower when a catalyst is present. The gas compositions with and without catalyst are fairly similar. However 13C NMR analyses showed that the liquid products obtained in the catalyzed experiment have a higher aromatic content than those formed in the absence of the catalyst. 

Cobalt compounds are used as a mixture with organoaluminum in the catalytic 1,3-butadiene polymerization. CoCl2/MAO initiates living polymerization of 1,3-butadiene to produce cis- 1,4-polymer with 98-99% selectivity based on the 13C NMR analyses (Eq. 13) [71]. The molecular weight of the polymer increases... 

Table 3 shows the results of compositional analysis for radically prepared copolymer of MMA and styrene.14 H and 13C NMR analyses and elemental analysis gave consistent results as seen in the table, indicating good accuracy for all these analyses. 

With the detailed structural information provided through 13c NMR analyses of polymers, the polymer scientist is in a position to determine physical property - structure relationships. Many excellent studies will be forthcoming during this decade, largely because of the development of NMR spectrometers with high sensitivity and sophisticated pulsing techniques which greatly simplify quantitative studies. 

In this chapter, we will briefly summarize some of the results reported in the literature concerning the mechanisms of stereospecific polymerizations of ot-olefins and discuss some of the latest results obtained by our research groups via 13c NMR analyses. 

The result of 13C-NMR analyses for the asphaltenes and hard resins in Bitumen 200 Elf is presented in table 8.3. From table 8.3, it is evident that asphaltenes (heavier fraction) have a higher aromaticity in comparison to hard resins and at the same time a lesser amount of paraffin side chains. This shows that asphaltenes have a higher tendency to coke formation in comparison with hard resins (see section 8.2). 

Johnson RA, Hamilton JA, Worgall TS, Deckelbaum RJ. Free fatty acids modulate intermembrane trafficking of cholesterol by increasing lipid mobilities novel 13C NMR analyses of free cholesterol partitioning. Biochemistry 42(2003) 1637-1645. 

In order to test the reliability of compositional results obtained by the 13c NMR, analyses of several copolymers were carried out by both 13c NMR and titration. Titrations were carried out in 50% aqueous ethanol medium to the phenol phthalein endpoint using 0.15 N aqueous potassium hydroxide as the titrant. As evidenced in Table 7.49 there is an excellent correlation between the compositional analysis by NMR and titration (correlation coefficient = 0.998) with most of the comparative analyses differing by 2% or less. 

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