Determination of chemical composition and molecular microstructure

The previous sections of this chapter have been concerned with methods for determination of average molar masses, molar mass distributions and molecular dimensions. In many instances this information is all that is necessary to characterize a homopolymer when its method of preparation is known. However, for certain homopolymers (e.g. polypropylene, polyisoprene) knowledge of molecular microstructure is of crucial importance. Additionally, for a copolymer it is necessary to determine the chemical composition in terms of the mole or weight fractions of the different repeat units present. It is also desirable to determine the distribution of chemical composition amongst the different copolymer molecules which constitute the copolymer (Section 3.17.6), and to determine the sequence distribution of the different repeat units in these molecules. Furthermore, when characterizing a sample of an unknown polymer the first requirement is to identify the repeat unit(s) present. Thus methods for determination of chemical composition and molecular microstructure are of great intportance. 

Simple methods of chemical analysis are useful in certain cases. Qualitative chemical tests such as the Beilstein and Lassaigne tests for the presence of particular elements (e.g. halogens, nitrogen, sulphur) are of use in the identification of an unknown polymer. Combustion analysis for quantitative determination of elemental composition can be used to confirm the purity of a homopolymer and to determine the average 

The use of spectroscopic methods to analyse polymers was a natural extension of their initial use for studying the structures of low molar mass compounds. There now are available an enormous number of fully-interpreted spectra of low molar mass compounds and of polymers, and these provide a firm foundation for structural characterization. In the following sections, spectroscopic characterization of polymers is reviewed with particular emphasis upon infrared spectroscopy and nuclear magnetic resonance spectroscopy, since they are most widely used. Fundamental aspects, and descriptions of the instrumentation and experimental procedures used, will be treated only briefly because a full account would require a disproportionate amount of space and there already exist many excellent texts on spectroscopy which deal with these topics. Also due to the limitations of space, only a few spectra and a brief survey of the vast potential of these and other spectroscopic methods for characterization of polymers are given. The reader is referred to specialist texts on spectroscopy of polymers for a more complete appreciation of their uses. 

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