Experimental Variables in Quantitative NMR Studies of Polymers

Choice of Solvent. The most appropriate solvent for NMR studies of polymers would allow a range of polymer concentrations to be investigated, be free of overlap problems and hopefully provide a signal for internal lock. Not all of these conditions can usually be met as many high molecular weight polymers pose solubility problems and can be examined in only a limited number of solvents. Deuterium resonance is the typical choice for an internal lock signal on most modern NMR spectrometers. Unfortunately, the majority of available deuterated solvents are poor solvents for many addition polymers such as the polyolefins while it is generally possible to find a number of appropriate deuterated solvents for many of the condensation polymers. The 

From a comparison of peak heights, the long chain branching is approximately 1 per 10,000 carbons. 

Relaxation Time and Nuclear Overhauser Effects. It is well known that for 90 pulse angles, a pulse spacing of 5 x Ti will ensure that 99% of rf excited nuclei will be fully relaxed between pulses (14). Such pulse spacings will ensure reliable quantitative results and are recommended although it is possible to obtain quantitatively reliable results with lower pulse angles. The following relaxation time (Ti) data was obtained for a 97/3 ethylene-l-hexene copolymer  

Nuclear Overhauser effects (14) can arise from energy transfer from the proton nuclear spin reservoir to the 13c nuclear spin reservoir 

Relative Peak Heights and Integrated Areas from 50.3 MHz NMR Spectra of a 97/3 Ethylene-l-Hexene Copolymer Utilizing Diffe Pulse Angles and Pulse Spacings. 

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