Resonance polyisoprenes

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. 

Fig. 1.24 Two examples of frequency-depen-dent relaxation times - 7"i is plotted as a function of the proton resonance frequency V = ou/2 JI, which was obtained from measurements at different magnetic fields strengths. Left polyisoprene (PI) melts and solutions of the same samples at 19wt-% concentration in cyclohexane. Numbers indicate the average molecular weight. The difference between the melt and solution increases towards lower magnetic fields strengths, the frequency dependence is more pronounced for melts.

The determination of the various types of geometric isomers associated with unsaturation in Polymer chains is of great importance, for example, in the study of the structure of modern synthetic rubbers. In table below are listed some of the important infrared absorption bands which arise from olefinic groups. In synthetic "natural" rubber, cis-1, 4-polyisoprene, relatively small amounts of 1, 2 and 3, 4-addition can easily be detected, though it is more difficult to distinguish between the cis and trans-configurations. Nuclear magnetic resonance spectroscopy is also useful for this analysis. 

In contrast to the spin-lattice relaxation parameters, which remain invariant, a sijbstantial broadening of the resonant lines occurs upon crystallization. The effect is relatively modest for cis polyisoprene at 0°C and 57.9 MHz, where comparison can be made at the same temperature. Here there is about a 50% increase in the linewidths upon the development of 30% crystallinity. Schaefer (13) reports approximately 3- to 5-fold broader lines (but they are still relatively narrow) for the crystalline trans polyisoprene relative to the completely amorphous cis polyisoprene at 40°C and 22.5 MHz. It is interesting to note in this connection that for carbon black filled cis polyisoprene the line-widths are greater by factors of 5-10 relative to the unfilled polymer. 

On the other hand, Schaefer ( ) has shown from selective saturation experiments of amorphous cis polyisoprene, crystalline trans polyisoprene, as well as carbon black filled cis polyisoprene, that the resonant lines are homogeneous. The linewidths in these cases are thus not caused by inhomogeneous broadening resulting from equivalent nuclei being subject to differing local magnetic fields. The results for these systems are thus contrary in part to what has been found here. 

On the other hand, the fact that the methyl resonances appear as sharp singlets at 1.53 ppm for the hydrochlorinated product and at 1.69 ppm for the hydrobrominated product indicates the exclusiveness of Markownikoff s rule for the addition of either hydrogen halide to the repeating units of 1,4-polyisoprene. 

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. 

The NMR spectra of - and Z-II are complicated. Eight vinyl and eight allylic carbon resonances were observed. This result is consistent with the NMR of ( )- and (Z)-l,4-polyisoprene (14) (equation 2). 

Most of the data referred to above were obtained in earlier work, and were based on infrared spectroscopy. In recent years, more reliable data were obtained by means of NMR spectroscopy, using both and resonances (9-14). Some of these investigations suggested that, aside from the dramatic effects of polar solvents on the chain structure in organolithium systems, there were some subtle effects even in non-polar media, e.g., caused by initiator concentration and type of non-polar solvent. Sinn and coworkers ( ), for example, used infrared spectroscopy to show an effect of butyl lithium concentration on the chain structure of polyisoprene and polybutadiene. Hence an extensive study was carried out recently (, M) on the influence of reaction parameters on the chain structure of polybutadiene and polyisoprene prepared in non-polar media. 

NMR peak assignments were as follows. For polyisoprene, the 3,4-unit content was determined from the olefinic methylene protons at 4.61 and 4.67 ppm, while the trans-1,4 units were determined from the methyl proton resonance at 1.54 ppm. The cis-1,4 content was then calculated by difference. No 1,2 olefinic methylene protons could be seen at 5,4 ppm, where they would be expected. For polybutadiene, the 1,2-unit content was calculated from a comparison of the 1,2-olefinic methylene protons at 4.B ppm with the 1,4-methine protons at 5.4 ppm. The cis/trans ratio was then computed from the 1,4-methylene protons at 1.9B and 2.03 ppm. These methods made it possible to estimate the chain unit structures within about 1%>,... 

Spec. Pap. 261, 17—35. Boulder Geol. Soc. of America. SchaefferJ. (1972) Comparison of the carbon-13 nuclear magnetic resonance of some solid cis- and trans-polyisoprenes. 

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—. 

Reactions of lithium alkyls are generally considered to be carbanionic in nature, but in reactions with alkyl halides free radicals have been detected by electron spin resonance.32 Lithium alkyls are widely employed as stereospecific catalysts for the polymerization of alkenes, notably isoprene, which gives up to 90% of 1,4-cA-polyisoprene numerous other reactions with alkenes have been studied.33 The TMED complexes again are especially active not only will they polymerize ethylene but they will even metallate benzene and aromatic compounds, as well as reacting with hydrogen at 1 atm to give LiH and alkane. 

Miller, J. B., K. J. McGrath, C. M. Roland, C. A. Trask, and A. N. Garroway. 1990. Nuclear-magnetic-resonance study of polyisoprene poly(vinylethylene) miscible blends. Macromolecules 23 4543 547. 

Resonance Raman spectroscopy was used to investigate the products formed upon iodine doping of cis-polyisoprene. Evidence for the production of polyenes was found and that these are in part responsible for the increased conductivity of iodine-doped cis-polyisoprene (374). 

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