Methyl resonance atactic

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. 

Figure 50 The 25-MHz 1D-NMR spectra of polypropylene with different tacticity (a) full spectrum, (b) model spectrum showing assignment of methyl resonances to various pentad stereosequences and expansions of the methyl regions (c and d) syndiotactic polymer, (e and f) atactic polymer, and (g and h) isotactic polymer. Reprinted with permission adaptation from Zambelli, A. Dorman, D. E. Richard Brewster, A. I. Bovey, F. A. Macromoiecuies 1973, 6,925. Copyright 1973 American Chemical Society.

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. 

Poly(PO) formed with 17 as initiator under irradiation showed virtually the same NMR pattern as that for the polymer formed in the dark at 70 C, where the resonance due to the methyl group was very simple, indicating that the polymer consists of regular head-to-tail linkages. The diad and triad tactic-ities of the polymer, as determined by NMR [82], indicated the atactic struc-... 

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. 

The broad resonance of the a-methyl carbon observed in the CP/MAS/DD 13C NMR spectrum of atactic PMMA can be decomposed to three peaks corresponding to different tacticities by taking advantage of the differences in the rotational motions of the a-methyl carbons in different... 

This can be seen from the data in Table 10.1, where isotactic, syndiotactic, and atactic poly(methyl methacrylate) (PMMA) have different a values. If the size of a polymer chain can be affected by its configuration, the microstructure must be well characterized before an accurate assessment of experimental data can be made. This can be achieved using nuclear magnetic resonance (NMR) and infrared techniques. 

Figure 3-9. Section from the proton resonance spectra of isotactic (it), syndiotactic (st), and atactic (at) poly(methyl methacrylates). The signals of the methyl ester protons are not shown. TMS = reference signal of tetramethyl silane. (According to P. Goeldi and H.-G. Elias.)...

Hgure 6 The 90 MHz methyl spectrum of atactic polypropylene in 1,2,4-trichlorobenzene at 100°. Empirical shift predictions for different stereoisomers reflecting the meso (m) or racemic (r) relative orientation of neighboring methyl groups. (Reprinted with permissbn from Schilling FC and Tonelli AE (1980) Carbon-13 nuclear magnetic resonance of atactic polypropylene. Macromolecules 13 270 American Chemical Society.)... 

As mentioned before (Section II,E,3), the determination of tac-ticity by X-ray analysis is limited by the requirement that the polymer be crystalline. For the study of poly (methyl methacrylate), which may or may not be crystalline, nuclear magnetic resonance spectroscopy has been more useful. In order to interpret the spectra, it has been found necessary to describe the stereochemistry of a unit by the configurations on both sides. Therefore, an isotactic configuration, or isotactic triad, is one where the central unit is fianked by units of the same asymmetry, that is ddd or III. Similarly, for a syndiotactic triad, the stereochemistry is did or Idl. To overcome the disadvantages of the term atactic, a new term heterotactic was introduced. The stereochemistry for heterotactic configurations is, therefore, Idd, dll, lid, and ddl. 

The NMR spectra presented in Fig. 20.9 [11] for the same PP samples whose H NMR spectra appear in Fig. 20.8 make the superior microstructural sensitivity of NMR plainly evident. While resonances are spread over an 30 ppm range, aU H resonances observed for PPs are within < 1 ppm of each other. In addition, the absence of homonuclear ( C- C) and the easy removal of hetero-nuclear ( C- H) scalar couplings further simplify the spectra. Both of these advantages result in the kind of microstructural sensitivity seen in the methyl carbon region of the PP spectra note that in atactic PP sample all ten possible pentad stereosequences mmmm, rrrr, mmrm, etc.) are distinctly observed. (Also see in Fig. 20.10 an expansion of the methyl region of the atactic PP spectrum observed at a higher magnetic field, which we will subsequently discuss.)... 

NMR spectrum of atactic PP were sensitive to pentad stereo-sequences. At 90.5 MHz (see Fig. 20.10), the methyl carbon resonances show sensitivity to heptad stereosequences (rmnmmmm, rrrrrr, mrmmrr, etc.) [13]. The NMR spectra of PPs are sensitive to stereosequences extending over 4 (pentads) and 6 (heptads) bonds in both directions along the PP backbone. This long-range sensitivity to microstructural detail makes NMR a valuable tool in the determination of polymer structures. 

Differential scanning calorimetry (DSC) experiments indicated that atactic polystyrene and polyvinyl methyl ether (PVME) form miscible blends [8,9]. Syndiotactic and isotactic polystyrene when blended with PVME, phase separate at aU temperatures above the glass transition temperature of PVME. Only weak van der Waals interactions between the phenyl rings in polystyrene with the methoxy group of PVME were detected using 2-dimensional nuclear magnetic resonance (NMR) spectroscopy. 

There are four possible types of diads for this polymer depending on the asymmetric silicon centers S-S, S-R, R-S and R R (Figure 8). Two methyl carbons in S-S (or R-R) diad are located in quite different environments and therefore have two chemical shifts. In the case of S-R (or R-S) diad, two methyl groups are in very similar environments, and their carbon resonances will appear at the similar position, i.e., methyl groups of S-R and R-S diads will appear as one inseparable peak. Si(CH3)2 in atactic polymer, therefore, would be split into three peaks (the centrd peak represents the S-R and R-S diads, two side peaks represent S-S and R-R diads) in the NMR spectrum, with an intensity of 1 2 1. While pure isotactic polymer, containing only S-S (or R-R) diad, would show two peaks of methyl carbons, and pure syndiotactic polymer only one (S-R and R-S diads). The present polymer showed two distinct peaks of methyl carbons, indicating that the polymer is highly isotactic. 

Judging from the resonances in the methyl regions, these polymers are not isotactic but atactic. However, the peaks at 36.7 ppm (lay), 29.2 ppm (Tg ) and 29.9-29.0 ppm (S g) assignable to the carbons in the sequences with inverted propene cannot be observed. From these results, the following scheme may be applied to the present polymerization. 

However, it is clear from the NMR spectra of three PP samples presented in Figure 2.2 [13] that the multiple resonances appearing in the spectrum of atactic-PP (b), which are well known to be produced by different stereosequences [11], cannot be explained by the usual fi-, and y-substituent effects. Each methyl carbon in PP has one a-, two / -, and two y-substituents, each methine carbon has two a-, two / -, and four y-substituents, and each methylene carbon has two a-, four and two y-substituents independent of stereosequence. Some factor other than numbers and types of a-, P-, and y-substituents, and which depends on PP stereosequence, must be responsible for the multiplicity of... 

Having established the assignment of resonances observed in the methyl region of the NMR spectrum of atactic PP to the appropriate heptad stereosequen-... 

The comparison presented in Figure 2.6 makes it apparent that we have been able successfully to simulate the methyl region of the NMR spectrum of atactic PP based on our ability to calculate and assign all of the heptad stereosequence resonances. Thus, from this successful simulation we know how much of each heptad stereosequence is present in our atactic PP sample. When we compare these heptad stereosequence populations with those predicted by simple statistical models, we are able to conclude that our atactic PP sample cannot be described by any simple statistical polymerization model, such as Bernoullian or first-order Markovian. 

Sometimes more than two empirical rules must be developed to make proper assignments, but it is seldom necessary to use more than three or four rules. This general approach can also be used for assigning resonances due to stereosequences in homopolymers, as has been mentioned in the discussion on stereoregular homopolymers. Klesper employed four rules to completely assign the a-methyl proton resonances of atactic meth-acrylic acid-methyl methacrylate copolymers [27]. Since he was the first to develop and apply rules of the sort considered here, it seems appropriate to refer to them in a general sense as "Klesper s rules."... 

In the free radical polymerization proeess, conducted usually at elevated temperatures, these effects are insignificant and the reaction usually leads to the formation of atactic polymer only. However, in some cases, like, for example, in free radical polymerization of methyl methacrylate at temperature below 0°C one obtains a crystalline polymer with syndiotactic structure, as it was proven by the high resolution nuclear magnetic resonance spectroscopy. These results confirm the rule that according to which the degree of stereoregularity decreases with increasing temperature. 

Fig. 3.26 N.m.r. spectra for proton resonance in atactic, syndiotactic and isotactic poly (methyl methacrylate) in CDCl at 331 K. (McCall and Slichter, in Newer Methods of Polymer Characterization ed. B. Ke, Wiley-Interscience, New York, 1964, reproduced with permission).

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