Pentad spectra

JC Hilico, JP Champion, S Toumi, VG Tyuterev, SA Tashkun. New analysis of the pentade system of methane and prediction of the (pentad-pentad) spectrum. J Mol Spectrosc 168 455-476, 1994. 

The spectrum shown in Fig. 7.5 shows the appropriate portion of the spectrum for a copolymer prepared from a feedstock for which fj = 0.153 It turns out that each polyene produces a set of three bands The dyad is identified with the peaks at X = 298, 312, and 327 nm the triad, with X = 347 367, and 388 nm and the tetrad with X = 412 and 437 nm. Apparently one of the tetrad bands overlaps that of the triad and is not resolved. Likewise only one band (at 473 nm) is observed for the pentad. The identification ol these features can be confirmed with model compounds and the location and relative intensities of the peaks has been shown to be independent of copolymer composition. 

Example 2. Vinviidene Chloride Isobutylene Copolymer. The next example is for the carbon-13 spectrum of copolymer vinylidene chloride isobutylene. Figure 5 shows the full spectrum and the peak assignment listing for the non-protonated vinylidene chloride carbon in the 84-92 ppm range. Triad assignments were made (Crowther, M. W., 1987, Syracuse University, unpublished data) using the two-dimensional COLOC (20) experiment. There are ten v-centered pentads representing different environments for the vinylidene chloride carbon. The i represents the non-protonated carbon in the isobutylene polymer unit. 

A portion of the database for this polymer is shown in Figure 6. Literature reports that this polymer follows second-order Markov statistics ( 21 ). And, in fact, probabilities that produced simulated spectra comparable to the experimental spectrum could not be obtained with Bernoullian or first-order Markov models. Figure 7 shows the experimental and simulated spectra for these ten pentads using the second-order Markov probabilities Pil/i=0.60, Piv/i=0.35, Pvi/i=0.40, and Pvv/i=0.55 and a linewidth of 14.8 Hz. 

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. 

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... 

The 13C-NMR spectrum of the polymer (Fig. 11) exhibits in the methyl region a large signal of the mmmm pentad, ruling out the possibility of for-... 

In a separate paper Harris and Kimber (30) examined mixed polymeric siloxanes of the type MDj.D. M where M = Me3SiOo.s, D = Me2Si(Oo.5)2, and D = HMeSi(Oo 5)2- The Si spectrum of these polymers shows three distinct areas of resonance, M at +9-71 to + 7 07 ppm, D from —18-81 to —22 03 ppm, and D from —35-19 to -37-35 ppm. The resolution at 2-35 T is sufficient to show triad and pentad fine structure at the D and D resonances, respectively, and chemical shift sensitivity at the M resonance to structural changes occurring up to six bonds away (Table X). 

This interpretation was confirmed by time-resolved fluorescence spectroscopy using the single photon timing technique. The pentad as a 1 X 10 M solution in chloroform was excited at 590 nm, and emission decay curves were recorded at 14 wavelengths. All 14 decays were then fit simultaneously to four exponential functions (x = 1.12) using a global analysis technique. The results were used to construct the decay associated spectrum shown in Figure 16. The two major components of the decay had lifetimes of 0.039 and 1.2 ns. (The two minor components represent impur-... 

Figure 16. Decay-associated spectrum resulting from a global analysis of the decay of the porphyrin fluorescence of pentad 22 following excitation of a — lxlO M chloroform solution with a 590-nm laser pulse. Data were obtained using the single photon counting technique, and the instrument response time was 0.035 ns. Decays at 14 different wavelengths were analyzed simultaneously, and the goodness of fit parameter was 1.12.

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...

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... 

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. 

Stereochemical assignments have also been made at the pentad level for poly (vinyl fluoride) from the 19F COSY spectrum, in which the cross-peaks arise from four-bond scalar coupling (about 7 Hz) between the central pair of fluorines in the pentad sequences that share a common hexad.200... 

The 1H COSY spectrum of isotatic PMMA enables unambiguous assignment of erythro and threo methylene protons based on the fact that only the erythro proton is capable of forming with a-methyl protons a W -shaped four-bond path of long-range coupling.203 Tetrad-pentad correlation was also observed in the 1H COSY spectrum of radically prepared PMMA.203... 

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