Spectroscopy of Poly l-alkenes Polypropylene

A particularly important application of NMR spectroscopy is the structural characterisation of polymers. While initial attempts were made using NMR, the wide 

The determination of the microstructure of vinyl polymers is not merely a characterisation tool. Each polymer molecule is unique, and each polymer chain is a record of the history of its formation, including mis-insertions, rearrangements, the incorporation of co-monomers, and the mode of its termination. NMR analysis of polymers can therefore be used to provide detailed mechanistic and kinetic information. This approach has been applied particularly successfully to the microstructure, i. e. the sequence distribution of monomer insertions, of polypropylene, giving rise to a wealth of studies far too numerous to cover here. Progress in this area has recently been summarised in two excellent and very comprehensive review articles [122, 123[. Here we will cover only the most fundamental aspects of stereoselective polymerisations. 

In principle, therefore, the microstructure of such a polymer could be described by a sequence of m and r diads. However, the limits of NMR resolution is such that the microstructure can only be determined over a limited section of the polymer, usually no longer than 5-7 monomer units. NMR spectra on comparatively insoluble polymers such as polyethylene and polypropylene are usually recorded in 1,2,4-trichlorobenzene or tetrachloroethane-d2 at elevated temperatures (110-150 °C). 

The NMR chemical shift of a given CH3 group is influenced by its neighbors. Most modem NMR instruments will allow resolution at the pentad level, i. e. the resonance of a methyl group is determined by the orientation of the two monomer 

A significant improvement in resolution and information content of NMR spectra was achieved by high-field (150 MHz) NMR which allows resolution at hep-tad or even higher levels. At present, this level of resolution is still the preserve of the specialist NMR practitioner. In any case, for most purposes of structural assignments resolution at pentad level is satisfactory. For chemical shifts of high-resolution spectra see Ref [123]. 

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