NMR pentads

Metallocenes are far more versatile in controlling polymer stereochemistry compared to Ziegler-Natta catalysts, as extensively demonstrated in the case of PP. Also in 1-butene polymerization, all kinds of chain microstructures can be obtained with different metallocenes. The 13C NMR pentad analysis of polybutene is somewhat less immediate than that of PP, and has been reported for both ZN 886,887 and metallocenes.180 The 13C NMR spectrum of atactic polybutene, with pentad assignments of the C(3) methylene region, is shown in Figure 37. 

Figure 22 deals with the MAO-activated propylene polymerization of a series of metallocene catalysts (see Figs. 18 and 19) having a substituent of increasing steric demand at the cyclopentadienyl ring. To each catalyst is appointed the syndio-tactic (rrrr), hemiisotactic, or isotactic (mmmm) NMR pentads (%). These pentad distributions are measured from samples polymerized at 10°C and 70°C. 

FIGURE 2.21 A NMR pentad analysis of nine different isotactic-hemiisotactic polypropylenes reveals an increase in catalyst isoselectivity with increasing 3-substituent (R) size, but a consistently low level of stereoerror pentads, mmrm, rrmr, and mrmr. 

The nmr spectmm of PVAc iu carbon tetrachloride solution at 110°C shows absorptions at 4.86 5 (pentad) of the methine proton 1.78 5 (triad) of the methylene group and 1.98 5, 1.96 5, and 1.94 5, which are the resonances of the acetate methyls iu isotactic, heterotactic, and syndiotactic triads, respectively. Poly(vinyl acetate) produced by normal free-radical polymerization is completely atactic and noncrystalline. The nmr spectra of ethylene vinyl acetate copolymers have also been obtained (33). The ir spectra of the copolymers of vinyl acetate differ from that of the homopolymer depending on the identity of the comonomers and their proportion. 

Silicone oil OV 61, a linear polymer, contains exclusively D- and Dph2-groups with a resolved pentade pattern in the 29Si NMR spectrum [3]. The relaxation times were found to be between 17 and 52 s for D, and 60 to 93 s for Dph2. NOE values lie between -1.24 and -1.60, showing no structural relevance. The silicon molecules seem to be convoluted with no differences in segmental motion. 

Above we mentioned the results reported by Ewen [13] who found that Cp2TiPh2/alumoxane gives a polypropene with isotactic stereoblocks. Naturally, this achiral catalyst can only give chain-end control as it lacks the necessary chiral centre for site control. In the 13C NMR the stereoblocks can be clearly observed as they lead to the typical 1 1 ratio of mmmr and mmrm absorptions in addition to the main peak of mmmm pentads. These are two simple examples showing how the analysis of the 13C NMR spectra can be used for the determination of the most likely mechanism of control of the stereochemistry. Obviously, further details can be obtained from the statistical analysis of the spectra and very neat examples are known [18],... 

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]. 

Table 8.1 Experimental c NMR chemical shifts for the methyl pentads of regioregular polypropylene (in 1,2,4-trichlorobenzene, 137°C).

The advent of high-resolution 13 C NMR allows the determination of tetrad, pentad, and even higher sequence distributions in many polymers [Bovey, 1972 Bovey and Mirau, 1996 Farina, 1987]. The tetrad distribution consists of the isotactic sequence mmm, the syndiotactic sequence rrr, and the heterotactic sequences mmr, rmr, mrm, rrm. The sum of the tetrad fractions is unity, and the following relationships exist ... 

The study of the stereoregularity of the polymers prepared, provides also Information about the stereoregulating mechanism. The probability of formation of the different types of sequences, was determined on the basis of the resonance of the quaternary carbon of pVP (12). The NMR spectrum performed at 15 MHz allows one to determine the concentration of triads. The values summarized In Table 4 do not agree with those expected for bernoullllan statistics. Hence, more than the last unit of the living chain Is Involved In the process. In order to obtain more precise Information about the process, It is necessary to measure the probability of formation of pentads. Such measurements are possible with spectra performed at 63 MHz (Figure 18). In spite... 

C NMR spectra are recorded for a low molecular weight atactic PP dissolved in a variety of solvents over a broad temperature range [293 - 393 K). Comparison of chemical shifts calculated via the y effect method with the observed resonances, whose relative chemical shifts are solvent independent, permits their assignment to most of the methyl heptad, methylene hexad, and methine pentad stereosequences. Agreement between observed and calculated chemical shifts requires y effects, he., upfield chemical shifts produced by a gauche arrangement of carbon atoms separated by three bonds, of ca. - 5 ppm for the methyl and methine carbons and ca. - 4 ppm for the methylene carbons. 

C NMR chemical shifts are calculated for the carbon nuclei in PVA to the pentad and hexad levels of stereosequence for the methine and methylene carbons, respectively. The RIS model developed by Wolf and Surer (A 069) is employed to calculate the frequencies. The relative orders of the observed methine pentad and methylene hexad resonances agree with the calculated chemical shifts, in addition to the agreement between the overall chemical shift dispersions measured and predicted for the methylene carbons. 

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