A series of three-dimensional (3D) nuclear magnetic resonance ment is used, signals originating from the chain ends often (NMR) pulse sequences, utilizing pulsed-field gradient (PFG) have intensities which are comparable to those originating techniques, were developed or adapted from biological experifrom the polymer backbone. One drawback of this technique ments for applications in the characterization of polymer strucinvolves the interference from overlap with extremely inture. These experiments form the foundation of a suite of experitense signals of the polymer backbone. These signals often ments which can be used to characterize the structure of polymers interfere with detection of the weak chain end signals of and other heteroatom-containing organic materials, in much the interest. same way that the data from multiple 3D NMR experiments have To resolve this problem, utilization of a third NMR-active been used in biological structure determination. Several variations nucleus can often be useful. This can be accomplished by of an 1 H/X/Y chemical shift correlation (HXY) experiment are exhibited, and the HCX sequence (where Y Γ
13 C) is combined artificial enrichment or by utilizing an X nucleus such as 31 P with 13 C homonuclear isotropic mixing to generate new pulse se-or 19 F (100% natural abundance) which is already present quences which provide additional structural information. In this in the structure. In favorable cases, direct observation of this paper, the spectra of poly(a,b-13 C 2 -styrene) (PS) prepared by third nucleus provides a spectrum with very large chemical diphenylphosphinyl radical (DPPR) initiated polymerization of shift dispersion and only a few resonances (when there are a,b-13 C 2 -styrene are used to illustrate the application of these a limited number of X atoms in the molecule). The idea of techniques for characterization of polymer chain end structures. using isotopic labeling together with triple resonance NMR Comments on the relative advantages of the pulse sequence are techniques to selectively observe resonances from the label provided. Although polymers are used to illustrate the applications site, while filtering the remainder of the signals from the of these pulse sequences, the sequences can just as easily be used to study other organic structures containing an NMR-active X spectrum, was described in 1982 (4, 5). nucleus. Organometallic chemistry is especially suited for applica-Recently, triple resonance experiments have taken the tions of these NMR experiments. α§ 1998 Academic Press form of three-dimensional (3D) NMR and have been used Key Words: 3D NMR; triple resonance NMR; polystyrene; 13 Cin conjunction with isotopic labeling to study the structures labeling.
of biological macromolecules. The HNCO experiment was originally developed by Bax et al. (6, 7) to help determine the structures of 13 C-and 15 N-labeled proteins. Later, Berger