Polymer stereochemical configuration

With the discovery of crystalline polypropylene in the early 1950 s, polymer stereochemical configuration was established as a property fundamental to formulating both polymer physical characteristics and mechanical behavior. Although molecular asymmetry was well understood, polymer asymmetry presented a new type of problem. Both a description and measurement of polymer asymmetry were essential for an understanding of the polymer structure. 

The measurement of polymer configuration was difficult and sometimes speculative until the early 1960 s when it was shown that proton NMR could be used, in several instances, to define clearly polymer stereochemical configuration. Bovey was able to identify the configurational structure of poly(methylmethacrylate) in terms of the configurational triads, mm, mr and rr, in a classic example (3). In the case of polypropylene, configurational information appeared available but was not unambiguously accessible because severe overlap complicated the identification of resonances from the mm, mr and rr triads (4). Several papers appeared on the subject of polypropylene tacticity but none totally resolved the problem (5). 

With the advent of C-13 NMR in the early 1970 s, the measurement of polymer stereochemical configuration became routine and reasonably unambiguous. 

The advantages of C-13 NMR in measurements of polymer stereochemical configuration arise primarily from a useful chemical shift range which is approximately 20 times that of proton NMR. 

The advantages of C-NMR in measurements of polymer stereochemical configuration arise primarily from a useful chemical shift range which is approximately 20 times that obtained by proton NMR. Structural sensitivity is enhanced through well-separated resonances for different types of carbon atoms. Overlap is generally not a limiting problem. The low natural abundance (-1%) of C nuclei is another favourable... 

Structures [VIII] and [IX] are not equivalent they would not superimpose if the extended chains were overlaid. The difference has to do with the stereochemical configuration at the asymmetric carbon atom. Note that the asymmetry is more accurately described as pseudoasymmetry, since two sections of chain are bonded to these centers. Except near chain ends, which we ignore for high polymers, these chains provide local symmetry in the neighborhood of the carbon under consideration. The designations D and L or R and S are used to distinguish these structures, even though true asymmetry is absent. 

It is well known that the mechanical and physical properties of vinyl polymers are dependent upon their stereochemical configuration. It is critical, therefore, that the stereoregularity of poly(TBTM/MMA) be determined accurately and conveniently if the field performance of the material is to be predicted with any certainty. The effectiveness of organometallic polymers as an anti-... 

High polymers which show the stereochemical configurations known as isotactic and... 

Another important application of NMR to polymer systems is the elucidation of the stereochemical configurations of Polymer chains. Poly (methyl methacrylate) was first studied by Bovey in 1960. It is now possible to analyse for the statistical frequency of occurrence of all possible combinations of up to four successive pairs of units (dyads) capable of occurring with either the same (meso) or opposite (racemic) configurations. 

Table 7. Stereochemical Configuration of a.-Methyl styrene Polymers (21)...

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