Determination of tacticity

Tacticities have been determined for polymers of various derivatives of norbomene, norbomadiene and 7-oxanorbomene. Three methods have been used (i) by polymerizing single enantiomers of 5-, or 5,5-, or e rfo,exo-5,6-disubstituted derivatives, and determining whether the polymers have an HT (m) or HH/TT (r) stmcture, or both (m and r) (ii) by polymerizing 5,6-disubstituted derivatives in which the substituents contain a chiral centre of a single handedness, and 

The substitution parameters, along with those of the singly substituted methyl compounds, are summarized in Table 11.6. The cis olefinic lines are generally in a different position from the trans olefinic lines but the line order remains the same. 

X Y HH line positions/ ppm Splitting/ ppm Relative intensities/ (downfield/upfield)  

The substitution parameters, though different from those for trans dyads, remain within the expected ranges. 

Turning now to the spectrum of the polymer made from racemic monomer (Fig. 11.9b), we see that the HH trans resonance is split into two peaks, one in the same position as in Fig. 11.9a, already assigned to HH,r the other, 0.29 ppm upheld, can therefore be assigned to HH,m. The relative intensities of these two peaks provide confirmation of the value of = 0.57. The other three peaks, TH, TT, and HI, do not show detectable m r splitting. 

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Nuclear magnetic resonance has proved to be a valuable tool in determination of configurational sequences in poly(MMA) (14). In Figure 3 is shown the NMR of poly(MMA) synthesized with an anionic polymerization catalyst known to produce predominantly isotactic sequences. In these polymers, the NMR spectrum of the methylene units In the polymer backbone gives an unequivocal determination of tacticity. The methylene signal, occurring about 1.8... 

Chapman, C. B. Determination of Tacticity Constants from Hydrolytic Measurements. J. Polymer Sci. 45, 237 (1960). 

The 5,6-dicarboxylic chiral esters (71, 72) have already been mentioned in connection with the determination of tacticity see Section VIIIA.5. Other polysubstituted norboma-dienes, not containing fused aromatic rings, which undergo ROMP are 21510,126,325,550, 217551, 218551, 21910 325 552 - 555, 220539, 221551, 222121,550,551 but not 21610. 

Tacticity is one of the most valuable parameters obtained from NMR spectra, and the stereochemical assignments of H and 13C NMR spectra of various vinyl polymers have been established.156 As discussed in Section 2.1, reliability of signal intensity is of prime importance in the determination of tacticity by NMR spectroscopy, since no other analytical means provide quantitative data on tacticity. 

In addition to identihcation of unknowns, NMR can be used for conformational and stereochemical analyses. This includes the determination of tacticity in polymers, that is, whether the side chains are arranged regularly (isotactic and syndiotactic) or randomly (atatic) along the polymer backbone. Fundamental studies of bond distances from dipolar coupling and molecular motion from relaxation time measurements are used by physical chemists and physical organic chemists. In biology and biochemistry, the use... 

Table 2. Determination of tacticity for dibutylamino-substituted polysilanes.

An absolute determination of tacticity requires the use of proton spectra of the CH2 group. However, nuclei are usually sensitive to the tacticity over a longer range than protons. 

O DeU, R. McConviUe, D. H. Hofmeister, G. E. Schrock, R. R. Polymerization of enantiomeric-aUy pure 2,3-dicarboalkoxynorbornadienes and 5,6-disubstituted norbornenes by well-characterized molybdenum ring-opening metathesis polymerization initiators. Direct determination of tacticity in cis, highly tactic and trans, highly tactic polymers. /. Am. Chem. Soc. 1994, 116, 3414-3423. 

A major breakthrough in PP characterisation was made possible with the application of high-resolution and C-NMR, which allows absolute determination of tacticity. These methods, which have been comprehensively reviewed [8, 9, 25], are of paramount importance because they permit not only quantification of tacticity, but also elucidation of stereo-sequences. Although NMR characterisation provides the most fundamental information about stereostructure, secondary methods based on IR spectroscopy or solvent extraction are very widely employed. The continuing use of these secondary methods is a direct consequence of simplicity, speed, and low equipment cost which are particularly important for routine or screening analysis. 

However, the structural characterization of polymers by NMR does not lend itself to an easy quantitative determination of the polymers being studied. Indeed, it is well known that the carbon atoms exist in different hybridization states with different relaxation times. Moreover, due to the Overhauser effect, decoupling by irradiation of the protons modifies the population of the excited atoms. Consequently, it is difficult to consider that the intensity of the signals is directly proportional to the population of carbon atoms which they represent. Nevertheless, for the determination of tacticity, one generally considers that all stereosensitive carbon atoms are in the same hybridization state and that the intensities of their signals are an accurate measure of the content in different x-ads. Figure 3.6 shows... 

In terms of polymer structure, the ROMP of norborn-2-enes and norborna-dienes using ruthenium-based systems generally results in the formation of polymers that predominantly contain trons-vinylene units (vide infra). For a detailed discussion on the stereochemistry of ROMP-derived polymers and the determination of tacticity, the reader should refer to the chapter by J.G. Hamilton in Ref. [1], and the references cited therein. Polymerizations initiated by Grubbs-lype initiators are best terminated by the use of ethyl vinyl ether, yielding methylidene-terminated polymers according to the equation [25] ... 

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