Methyl pentads

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

Figure 24 Methyl pentad region of the 13C NMR spectrum (100 MHz, 120 °C, C2D2CI4) of aPP with pentad assignments. Catalyst rac-CH2(1 -Me3C-2-lnd)2HfMe2/MA0.669...

While aPP is obviously amorphous, an aPP is not necessarily atactic. The methyl pentad region of a fully regioregular, nearly perfect aPP is shown in Figure 24 with pentad assignments. 

This material, often incorrectly referred to as atactic, is actually neither atactic nor fully amorphous, but is a mixture of chains of different stereoregularity and molecular masses, as proved by polymer fractionation.315,374,671,672 The methyl pentad region of the 13C NMR spectrum of such a ZN PP clearly shows both isotactic and syndiotactic stereoblocks. Although the molecular masses of this material are low, amPPs have been used as bitumen additives and in hot-melt adhesives. With the improvement of the stereospecificity of the ZN catalysts for iPP, amPP was no longer available as a byproduct. Fully or largely amorphous PP from ZN has since been produced on purpose, also by adding co-monomers to further reduce crystallinity.673-680... 

Figure 4. The 100 MHz NMR spectra of isotactic (top), atactic (middle), and syndiotactic (bottom) polypropenes (a) methyl pentad region, (b) methine pentad region, and (c) methylene pentad region.

All other sPP and the catalyst samples were recorded with a Varian VXR-5000 spectrometer. The polymers were 5-10% (w/w) solutions in TCB with dg-benzene added as a lock solvent. C-13 NHR spectra were obtained at 75.43 HHz and 120 C using SCpulse width and a 15 second delay time. Inverse gated decoupling with Waltz modulation was used to suppress NOE s for purposes of quantitation. The LAB ONE curve fitting program from New Methods Research Inc. (NHRi) in New York was used to quantify the SPP rr centered methyl pentads and hexads in the SPP spectra. 

Background. The statistical models derived in this section account for the methyl pentad distributions in the C-13 NMR spectra and the sPP microstructures. 

Reaction stereoregulation mechanisms have been reduced to mathematical, kinetic models in several cases. A one-parameter Bernoullian statistical model accounts for the NMR methyl pentad intensities of both syndiotactic and isotactic polypropylenes obtained at subambient temperatures with homogeneous, achiral catalyst precursors. The polypropylenes have. ..rrrrrmrrrrr... and. ..mmmmmrmmmmm... microstructures respectively. The m and r deffects are consistent with the chain-end configurations being responsible for stereoregulation. Two additional models of the chain-end control type are the first- and the... 

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. 

By contrast, the synthesis of syndiotactic PP, s-PP, is generally catalyzed by Cs-symmetry ansa- metallocenes. For example, (16)/MAO affords PP with a pentad (rrrr) content of 86% at 25 °C.77 The stereoselectivity is highly sensitive to ligand variation. For example, substitution at the 3-position of the Cp ring with a methyl group affords heterotactic PP,78 whilst the Bu analog favors i-PP production.50,75,79... 

Of course, the stereoselectivity (and hence the isospecificity) of the catalytic models strongly depends on the encumbrance of the jt-ligand, increasing along the following series 3-methyl-cyclopentadienyl (e.g., 3.7 kcal/mol for 10), indenyl (e.g., 4.9 kcal/mol for 13), 4,7-dimethyl-indenyl (e.g., 5.3 kcal/mol for 17), and tetrahydroindenyl (e.g., 5.9 kcal/mol for 18). This is in good qualitative agreement, for instance, with the percent of mmmm pentads evaluated for polypropylene samples obtained for different catalytic systems in strictly similar conditions by Resconi and co-workers.58... 

Statistical analysis is important and relatively easy. Suppose we have a fully atactic polymer which we analyse for the triad content for isotactic polymer. Only three methyl resonances due to triads are observed, and the statistical ratio of mm, rr, and mr is 1 1 2. Thus even in the atactic polymer our isotactic content is 25% Pentad analysis, however, would give only 8% mmmm isotactic content Especially for catalysts with low enantiospecificity it is worthwhile keeping this in mind. 

In a recent study on poly(a-methyl-a-ethyl-piopiolactone) and poly(a-methyl-a-propyl-propiolactone) (112) Spassky et al. extended the earlier examination carried out on oxiranes and thiiranes (113) to the tetrad and pentad level. Because of the chirality of these polymers, Spassky avoids the use of the terms m and r as being inapplicable (21). To indicate the dyads RR (or SS) and RS (or SR) 64 he used the symbols ij and Sj where the indices stand for dyad. In their turn the triads RRR (or SSS) are indicated with i, RSR (or SRS) with s, RRS (or SRR) with hi (heterotactic triad beginning with an dyad) and RSS (or SRR) with (heterotactic triad beginning with a dyad), 65 (Scheme 14) (114). 

The first point of the stereochemical analysis is in the recognition of the sequence to which a given nucleus is sensitive the problem seems rather obvious for vinyl or vinylidene polymers where the sequence must extend equally from the two sides of the nucleus in question but for diene polymers or those containing heteroatoms, the problem is not so simple. In the present case, the methylene protons are sensitive to the structure of the even sequences, dyads and tetrads, whereas the methyl protons are sensitive to the odd sequences, triads, and pentads. 

Zambelli, Lxrcatelli, Bajo, and Bovey (143) proposed assignment of the pentads by comparison with the spectra of two diastereomeric mixtures of the 3,5,7,9,11,13,15-heptamethylheptadecane enriched in C on the 9-methyl. In the synthesis, which started from optically active precursors of known configuration, a selection of the stereoisomers occurred, which permitted the assignments to be made by means of intensity criteria. These authors reached the same conclusions as Stehling and Knox. A parallel analysis was effected with the methylene subspectrum by using 2,4,6,8,10,12-hexamethyltridecane enriched in C at position seven (144, 145). 

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... 

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. 

Seven other possible pentad sequences (mmmr, rrmr, mmrm, mmrr, mrmr, mrrm, and rmmr) are atactic. Residues R may be CH3 for polypropylene, C6H5 for polystyrene, Cl for polyvinyl chloride, CN for polyacrylonitrile, and C02CH3 for polyacrylic acid methyl ester. 

Melhyl-l-(pemafluoro-ethyl)-E10b,. 406 (SiR, - Ar) 4-MetIiyl-1-(pentad uoro-isopropen-yl)- E10b2, 144 (— Cl2 - En) 4-Methyl-l -(2-phenylsulfonyl-l, 2,2-trifluoro-ethyl)- ElOb, 92 (2-0H - 2-F)... 

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