Nuclear Magnetic Resonance tacticity from

Figure 7.10 Nuclear magnetic resonance spectra of three poly(methyl methacrylate samples. Curves are labeled according to the preominant tacticity of samples. [From D. W. McCall and W. P. Slichter, in Newer Methods of Polymer Characterization, B. Ke (Ed.), Interscience, New York, 1964, used with permission.]...

With the development of polymer structural characterizations using spectroscopy, there has been a considerable effort directed to measurements of tacticity, sequence distributions and number average sequence lengths (59 65). Two methods have been traditionally used for microstructure analysis from polymer solutions. Vibrational spectroscopy (infrared) and Nuclear Magnetic Resonance (NMR). Neither of these techniques is absolute. The assignment of absorption bands requires the use of model compounds or standards of known structure. 

Nuclear Magnetic Resonance. The successful study of polymers in solution by high resolution NMR spectroscopy started with the pioneering work on the sequence structure of poly methyl methacrylate in 1960. Since then, an ever-increasing number of investigations have been carried out ranging from the elucidation of the statistics of homopolymer and copolymer structure to the study of conformation, relaxation and adsorption properties of polymers. The aspects of sequence length determination and tacticity have received considerable attention (Klesper 84, for example, reports more than 500 entries). Therefore, a detailed review will not be attempted. (For a detailed description of the NMR Theory and statistics of polymer structure, see Bovey 59, Randall 23, and Klesper 84). 

Many radical polymerizations have been examined from the point of view of establishing the stereosequence distribution. For most systems, it is claimed that the tacticity is predictable within experimental error by Bemoullian statistics (i.e., by the single parameter P m] - see Section 3.04.3.1.1). It should be noted that, in some studies, deviations of 5-10% in expected and measured nuclear magnetic resonance (NMR) peak intensities have been ascribed to experimental error. Such error is sufficient to hide significant departures from Bemoullian statistics. ... 

IR spectroscopy is the most widely-used method for characterizing the molecular structures of polymers, principally because it provides a lot of information and is relatively inexpensive and easy to perform. However, it is not simple to interpret absolutely the more subtle features of IR spectra, such as those due to differences in tacticity. Such interpretations are usually made on the basis of information obtained from other techniques, in particular nuclear magnetic resonance spectroscopy which is by far the most powerful method for determining the detailed molecular microstructures of polymers. 

Spectroscopic methods are finding increasing use in the characterization and analysis of polymers. All of the methods that are employed were developed initially for use with low-molar-mass materials and they have been extended for the analysis of polymers. The spectrum obtained for a particular polymer is often characteristic of that polymer and can therefore be used for identification purposes. Polymer spectra can be surprisingly simple given the complex nature of polymer molecules and they are often similar to the spectra obtained from their low-molar-mass counterparts. This can make the analysis of the spectra a relatively simple task allowing important spectral details to be revealed. In fact, certain information can only be obtained using spectroscopic methods. For example, nuclear magnetic resonance (n.m.r.) is the only technique that can be used to measure directly the tacticity of a polymer molecule. 

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