Solution-state NMR determination of polymer end-groups, substituents and minor structures

Identification of the abnormal groups in a polymer may not be sufficient it is often necessary also to determine the numbers of such groups for, say, 1000 normal units. NMR techniques can give the required information but precautions are needed if reliable results are to be obtained. The problems can be illustrated by considering the use of NMR to compare the number 

Consider, as an example, the case of poly(methyl methacrylate) prepared in such a way that there is one Me2C(CN)- end-group, with enriched methyl carbons, for n monomeric units of formula -CH2 C(Me)(COOMe)- in a typical case, n might lie in the range 100-5000. Suppose that a is the abundance of in the enriched methyl groups and that b is the abundance at each of the sites in the monomeric units. If A is the total area under the NMR peaks for the two methyl carbons of the end-groups and B is the corresponding area for all five of the carbons of the monomeric units and, very importantly, if the spectral response of a nucleus is independent of its position in the polymer molecule (see section 3.1.2), then 

It is usually supposed that the enrichment factor for the special carbons in the end-groups is the same as that for the corresponding carbons in the original initiator from which they were derived. The abundance of at the enriched sites in the initiator may be appreciably less than 100% and an isotope effect in the initiation process would mean that a different enrichment factor would be applicable to the end-groups. Any isotope effect is likely to be small for C-labelled initiators although not for those enriched with deuterium. 

Determination of the ratio of end-groups to monomeric units in systems of another type must be mentioned. Suppose that the end-group contains an element readily detected by NMR, such as fluorine or phosphorus, and that the bodies of the macromolecules are free from that element. Clearly, determination of, say, the fluorine content of the whole polymer could lead to a value of the required ratio. It is necessary to use as a standard a properly 

In this chapter, there are frequent mentions of certain monomers and the polymers derived from them. Abbreviations are used for styrene (STY), methyl methacrylate (MMA), vinyl acetate (VAC), acrylonitrile (AN) and methacrylo-nitrile (MAN). Abbreviations for other materials are specified at the points at which they first appear. 

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Solution-state NMR determination of polymer end-groups, substituents and minor structures... 

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