Copolymer main chain carbon chemical shifts

Main Chain Carbon Chemical Shifts in Styrene-SO>> Copolymers... 

An important first step in interpreting the C-13 spectra is to distinguish a-carbons from 3-carbons, i.e. methine from methylene. Observation of multiplicity when the proton decoupler is off is one way, but this is not always easy if the lines are broadened by chemical shift multiplicity. Measurement of has been used for this purpose since the 3-carbon with two bonded protons relaxes about twice as fast as the a-carbon with only one. A very positive way is by deuterium labelling. In Fig. 3 is shown the main-chain 25 MHz carbon spectrum of two styrene-S02 copolymers containing 58 mol% styrene, or a ratio of styrene to SO2 of 1.38 (7 ). In the bottom one, 3,3-d2-styrene has been used, cind all the 3-carbon resonances are distinguishable from the a-carbon resonances since the presence of deuterium has eliminated their nuclear Overhauser effect because of this eind the deuterium J coupling ( 20 Hz), they are markedly smaller eind broader than the a-carbon resonances. 

Chemical structure and 75.5 MHz C NMR chemical shift assignments for the main- and side-chain carbon atoms in a P(3HB-co-20 mol% 3HV) sample produced by B. cepacia from xylose and levulinic acid. Expanded inset displays three distinct carbonyl chemical shifts and integrated peak areas, corresponding to the relative intensities of the different diad sequences (3HB-3HB, 3HB-3HV, and 3HV-3HV, from right to left) in this statistically random copolymer (Keenan eta/., 2004). 

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