Comments on experimental procedures

The principal experimental techniques involved in the examination of polymers by solution state NMR are considered in chapters 1 and 4 but certain points should be made here. This chapter contains examples of the use of substances highly enriched with Most of them are not at present available 

Frequently, it is possible to make effective use of initiators with about 50% at the selected site. Accurate information on the enrichment factor can be obtained by mass spectrometry and also, in some cases, by examination of the NMR spectrum of the substance. The signals due to protons attached to the atom are flanked by pairs of satellite lines arising from spin-spin coupling of the protons to the nucleus measurements of their intensities can yield the required information [1]. 

Some use has been made in polymer systems of materials enriched with deuterium or with N, followed by recording of or N NMR spectra as appropriate. Many deuterated substances and a few enriched with N are available and can be used in syntheses. 

It should be mentioned that the ratio between the intensities of signals from abnormal groups derived from a reagent, such as an initiator, and 

Quite commonly, the signals from isotopically enriched abnormal units in a polymer are obscured by responses from the many normal units. This problem can be overcome by the use of difference spectroscopy. A polymer is prepared with a reactant, say initiator, enriched with a second polymer is prepared under identical conditions but with the reactant having at only its natural level. The two polymers are examined spectroscopically under precisely the same conditions. The only difference between the two spectra should be the considerable reinforcement of signals from the enriched sites in any of the C-labelled reactant incorporated in the polymer. Subtraction of the unenriched spectrum from the enriched should yield a difference spectrum containing only the responses from the incorporated enriched reactant or C-enriched fragments derived from it. 

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