Polyisoprenes spectra

Successive 1,4 units in the synthetic polyisoprene chain evidently are preponderantly arranged in head-to-tail sequence, although an appreciable proportion of head-to-head and tail-to-tail junctions appears to be present as well. Apparently the growing radical adds preferentially to one of the two ends of the monomer. Which of the reactions (6) or (7) is the preferred process cannot be decided from these results alone, however. Positive identification of both 1,2 and 3,4 units in the infrared spectrum shows that both addition reactions take place during the polymerization of isoprene. The relative contributions of the alternative addition processes cannot be ascertained from the proportions of these two units, however, inasmuch as the product radicals formed in reactions (6) and (7), may differ markedly in their preference for addition in one or the other of the two resonance forms available to each. We may conclude merely that structural evidence indicates a preference for oriented (i.e., head-to-tail) additions but that the 1,4 units of synthetic polyisoprene are by no means as consistently arranged in head-to-tail sequence as in the naturally occurring poly-isoprenes. 

Caution should be used in assuming an inherent advantage exists in obtaining a magic angle spectrum of a rubber gum stock. We found the line widths obtained on uncured polyisoprene with... 

Figure 6. Magic angle spinning, high-power proton decoupling, FT C-13 NMR spectrum of cured, carbon-black-loaded polyisoprene at ambient temperature, FT of normal FID without proton enhancement.

Figure 10 shows a spectrum of butyl rubber gum stock obtained on the solid at 80°C using normal pulsed FT techniques. Clearly it could be identified as a component in fabricated materials by direct nmr spectral analysis. Figure 11 shows spectra obtained from various portions of typical rubber products. These samples were cut from the rubber product, placed in an nmr tube without solvent, and spectra obtained at an elevated temperature. The data show how polyisoprene, a polyisoprene/polybutadiene blend and a polyisobutylene/polyisoprene/polybutadiene rubber blend are quickly identified in the materials. Figure 11a shows processing oil was present, and which was confirmed by solvent extraction. 

Hydrogenated 1,4-po1yisoprene. Figure 5 shows the 22.6 MHz NMR spectrum of the hydrogenated 1,4-polyisoprene sample. The... 

Figure 5. Proton noise-decoupled 22.6-MHz C-13 NMR spectrum of the hydrogenated 1,4-polyisoprene sample. Perdeuteriohenzene solution at 25°C with TMS as internal reference. Approximately 5000 pulses with an acquisition time of 0.7 sec and a flip angle of 30°.

The empirical shift parameters calculated from the NMR data of the 1,4-polyisoprene derivatives will provide, hereafter, a basis for assigning the carbon resonances observed in the spectrum of hydrochlorinated 1,4-polydimethyl butadiene, which like hydro-chlorinated 1,4-polyisoprene has quaternary carbons substituted by one chlorine atom. 

Spectra have been recorded of the following polymers Poly isobutylene. Schaufele has reported a spectrum of a non-turbid specimen of this material complete with depolarization data (18). Bands at Av - 2919 and 720 cm 1 are clearly polarized and must result from symmetrical modes. Schaufele was also able to show that the short chain hydrocarbon CH3(CH2)12CH3 could be examined satisfactorily but the results were very much sharpened by cooling to liquid air temperatures. Polyisoprene and Polybutadiene. Both of these in the cis form have also been examined at Southampton very recently. The trans isomers gave no spectra due to fluorescence but no interpretation has been attempted as yet. Spectra are shown in Fig. 7 and 8. 

Fig. 49. Standard 13C NMR spectrum (top) and DEPT spectra of sulfur cured natural rubber (6h at 138 °C). The label T indicates peaks from trans-polyisoprene units, X marks residual peaks from other subspectra and arrows indicate peaks due to crosslink sites (adapted from Ref. 194>)...

The system Cl-buty 1-natural rubber (or cw-polyisoprene) could not be resolved by differential solvent techniques because the polymeric solubility parameters were too similar. At one end of the spectrum—i.e., with styrene at — 25 °C—natural rubber could be highly swollen while restricting the chlorobutyl swell, but the reverse was not possible, as indicated by the swelling volumes in the trimethylpentane. As displayed in Table II, attempts to use a highly symmetrically branched hydrocarbon with a very low solubility parameter, served only to reduce both the swelling of natural rubber and chlorobutyl. (Neopentane is a gas above 10°C and a solid below — 20°C). Therefore, for this report the use of differential solvents in the study of interfacial bonding in blends was limited to systems of Cl-butyl and cw-polybutadiene or SBR. 

Comparison of the dielectric and viscoelastic relaxation times, which, according to the above speculations, obey a simple relation rn = 3r, has attracted special attention of scholars (Watanabe et al. 1996 Ren et al. 2003). According to Watanabe et al. (1996), the ratio of the two longest relaxation times from alternative measurements is 2-3 for dilute solutions of polyisobu-tilene, while it is close to unity for undiluted (M 10Me) solutions. For undiluted polyisoprene and poly(d,/-lactic acid), it was found (Ren et al. 2003) that the relaxation time for the dielectric normal mode coincides approximately with the terminal viscoelastic relaxation time. This evidence is consistent with the above speculations and confirms that both dielectric and stress relaxation are closely related to motion of separate Kuhn s segments. However, there is a need in a more detailed theory experiment shows the existence of many relaxation times for both dielectric and viscoelastic relaxation, while the relaxation spectrum for the latter is much broader that for the former. 

A typical first derivative spectrum from rro-polyisoprene tensile tested at 77 K in a nitrogen atmosphere, is shown in Fig. 32 Identification of the radical species... 

Fig. 32. First derivatwe ESR spectrum at 123 A from peroxide cross linked cis-polyisoprene, tensile tested at 77 K in an oxygen free nitrogen atmosphere...

The spectrum from cis-polyisoprene tensile tested in (nominally) oxygen free nitrogen, shown in Fig. 32, is a six line spectrum with approximately 12 G hyper-fine splitting. Polymer radicals are centred around the free spin -value (g-= 2) and structure in the spectrum is invariably due to nuclear hyperfine structure. The peroxy... 

Fig. 34. First derivative spectrum from tensile tested cis-polyisoprene, after spectral subtraction of the peroxy radical content...

A minor part of the spectra had a singlet contour which was assigned to polyenyl radicals, —(—CR CR2 ) CH—, where R2 and R2=H or CH3, and n is about 3, judging from the linewidth. Free radicals were found to be stable at —196° C but decayed sharply near the glass-transition temperature. Trans-l,4-polyisoprene irradiated with UV light gave a similar spectrum to that of the ds-form. 

A minor part was assigned to radicals—CH2—CH=CH—CH—CH2— formed by hydrogen abstraction from one of the a methylene groups. These radicals were not resonance-stabilized at -196° C due to steric hindrance. On heating, a singlet spectrum similar to the one observed for 1,4-polyisoprene was observed and interpreted as due to polyenyl radicals -fCH=Cf% H—. 

TOF-SIMS spectrum of polyisoprene (MW = 2300). A. Oligomer distribution. B. Region showing overlap between oligomer and fragment-ion peaks. - Oligomer peaks, o - Fragment-ion peaks. 

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