Carbon backbone methylene

Aromatic carbons of styrene appeared as three different peaks viz., peak at 145 ppm due to aromatic-C carbon and peaks at 127 and 129 ppm due to o and m, and p carbons respectively, whereas carbonyl carbons, backbone methylene and quaternary carbons showed resonance in the region 174 ppm, 40 ppm and 78 ppm respectively. 

The spectrum of 6, 20 and 40 wt. % PEMA solutions at 34 C are shown in Figure 1. All of the resonances are easily discernible except for the backbone methylene at 40%. At low concentration the poljimer a-CH3, quaternary carbon, and backbone methylene carbon exhibit resolved or partially resolved chemical shifts due to the various stereochemical sequences since the pol3rmer was not stereoregular. A rough estimate indicates the polymer is essentially atactic. 

The Tetrad Chemical Shift Sequence with Respect to an Increasing Field Strength for Some Representative Vinyl Polymer Backbone Methylene Carbon Resonances... 

In a similar manner. It can be shown that the backbone methylene carbons are sensitive to an even number of structural units with the simplest configurational sensitivity being dyad. 

Figures 13.7 and 13.8 are two examples of two-dimensional NMR spectroscopy applied to polymers. Figure 13.7 is the proton homonuclear correlated spectroscopy (COSY) contour plot of Allied 8207A poly(amide) 6 [29]. In this experiment, the normal NMR spectrum is along the diagonal. Whenever a cross peak occurs, it is indicative of protons that are three bonds apart. Consequently, the backbone methylenes of this particular polymer can be traced through their J-coupling. Figure 13.8 is the proton-carbon correlated (HETCOR) contour plot of Nylon 6 [29]. This experiment permits the mapping of the proton resonances into the carbon-13 resonances.

Syndiotactic polystyrene is produced by well stereo-controlled coordination polymerization by titanium compounds. The polymer has a high percentage of mr pentad structure. The 13C NMR chemical shift for the phenyl-1 carbon and backbone methylene carbon are approximately 145.3 and 44.9 ppm, respectively [1]. In general, these polymers are found to be >99% pure in syndiotactic structure as defined by NMR. The most stereoselective catalysts produce a >99.6% pure syndiotactic structure (Figure 18.2)... 

The two-carbon precursor, acetyl coenzyme A (acetyl-CoA), is the initial substrate for synthesis of the carbon backbone of plant polyketides. As the name implies, polyketides are naturally occurring polymers of ketene (CH2CO) and contain alternating carbonyl and methylene groups derived from the acetate pathway. Polyketides and their derivatives are ubiquitous and are found in all organisms known to produce secondary metabolites. Because of their immense stmctural diversity, a... 

Comparison of the these makes the peak assignment unambiguous. The a-methyl group of phenyl methacrylate appears as a broad peak at 16.4 ppm, the backbone methylene group shows a sharp intense line at 44.0 ppm and the small broad quaternary carbon peak is at 54.0 ppm. The O—C aromatic... 

Backbone methylene carbons, poly(1-hexene), 141 Benzoyl peroxide catalyst, 13C NMR of PS, 190... 

Reduced PVCs were observed as 33% (w/v) solutions in 1,2,4-trichlorobenzene at 110°C. At this temperature, Ti for the normal backbone carbons is approximately 1.3 sec. Methyl branch frequencies were measured quantitatively by comparing the intensities of the M-a carbon resonances (see Figure 1) with those of the carbon resonances arising from the normal backbone methylenes. The absence of tri-n-butyltin groups from reduced polymers prepared with n-Bu3SnH was inferred from the absence of a resonance at ca. 9.1 ppm arising from n-butyl methylene carbons a to tin (30). 

Tetrad Assignment using Backbone Methylene Carbon ... 

The a-methyl carbon showed peaks around 19 ppm which were assigned as partial pentad splitting. Whereas, quaternary carbon and backbone methylene carbons showed triad and tetrad tacticity respectively. P and P values calculated were in agreement with those previously r ort-... 

Figure U C NMR spectra of the backbone methylene carbons of (a) a tactic poiy(but-l-ene sulphide), (b) of the tactic poly(but-l-ene sulphone) made from it by oxidation, and (c) of an atactic polysulphone. The dispersion of shifts of the sulphone polymer is greater because of the gamma- rauc/ie effect of the oxygens. The small peak at 5 = 49.2 ppm is from H — H sequence.

The maximum of the complexity was observed in poly-BF3a (Ri=H), which showed two broad peaks attributable to aliphatic quaternary carbon, while the signals attributable to backbone methylene were dispersed in a wide range of chanical shifts. 

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