Structure and Configuration of Polymers

We have seen that NOESY provides information on internuclear (principally interproton) distances. For many organic molecules (as distinguished from macromolecules such as proteins and nucleic acids) structure elucidation often involves only the establishment of the structural formula and bonding scheme. However, where ambiguities in configuration or preferred conformation remain to be settled, NOESY is often crucial for establishing stereochemistry. 

Even when the structures of the individual monomer units in a polymer are known, the determination of their sequence and of the geometric arrangement, configuration, and conformation of the entire polymer presents challenging problems. We comment on only a few aspects here. 

The NMR spectrum of a synthetic homopolymer may be very simple if the monomeric unit repeats regularly. On the other hand, irregularities, such as head-to-head junctions mixed with head-to-tail junctions, in such cases as vinyl polymers, for example, introduce additional lines that can often be valuable in structure elucidation. 

A synthetic copolymer provides additional degrees of freedom in the arrangement of the repeating units. For example, the spectrum of a copolymer of vinylidine chloride and isobutylene, shown in Fig. 13.5, indicates that various tetrad sequences (sequences of four monomer units) display significantly different spectra. Copolymers composed of more than two monomer types, including biopolymers, have much more complex spectra, as we discuss later. 

When the repeating unit possesses a center of asymmetry, further complexity is introduced into the spectrum. For example, in a vinyl polymer 

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