Olefinic resonance

The exact enthalpy of polymerization for a particular monomer will depend on the steric and electronic effects imposed by the substituents attached to the E=E double bond. For olefins, resonance stabihzation of the double bond and increased strain in the polymer due to substituent interactions are the most important factors governing AHp For example, propylene has a calculated AH of -94.0 kJ moT, whereas the polymerization of the bulkier 2-methylpropene is less exothermic (-78.2 kJ moT ) [63]. Due to resonance effects, the experimentally determined AH of styrene (-72.8 kJ mol ) is less exothermic than that for propylene, while that for bulkier a-methylstyrene is even less favorable (-33.5 kJ moT ) [63]. In general, bulky 1,2-disubstituted olefins (i.e., PhHC= CHPh) are either very difficult or impossible to polymerize. 

A further example is the ready assignment of an olefinic resonance which appears in the spectrum of ruberoside G in addition to the H-ll signal at 5.3 ppm, common to asterosaponins. Its low-field chemical shift of 6.2 ppm (solid frame in Figure 5.1.4, and Figure 5.1.5) indicates the presence of a double bond in the steroidal side-chain in proximity to a deshielding functional group such as a carbonyl. 

Solid frame olefinic resonance at 6.2 ppm indicates a double bond in the side-chain in the vicinity of a carbonyl function... 

To compensate for this contribution the olefinic resonances are used as an internal standard. Cloprostenol possesses four olefinic protons, two from the 5-c/ s -olefin and two from the 3-trans-olefin which give rise to an overlapping multiplet between 5 5.2 and 8 5.8 ppm. The integrated intensity of this multiplet is equivalent to four protons and so by subtracting 0.75 of this value from the integrated intensity of the 5 A.12 ppm peak the three hydroxyl groups are accounted for and the remainder is due to water. 

Measurement of the rate at which the olefinic resonances disappear and the peaks due to the saturated addition products appear permits a rough... 

Transfer to A1 was reported to be operative with several non-metallocene catalysts. It is the only chain-release mechanism operative with the diamido complexes MCl2 ArN(CH2) NAr catalysts, as well as with the mono-and tris(benzamidinate) catalysts, since no olefinic resonances were observed in the H or 13C NMR spectra of these polymers.275 276 This chain-release reaction is also dominant with bis(phenoxy-imine)zirconium cat-... 

Polymerization experiments carried out using Pd(dpm)2 also showed that norbornene was polymerized quite effectively in the absence of AlEt3 at a 4000 1 9 monomer Pd B(C6F5)3 ratio (run 6, Tab. 4.2). The polymer was soluble enough at 50°C in ortho-dichlorobenzene to obtain a H NMR spectrum. No olefinic resonances were observed, confirming the formation of addition poly(norbornene). However, the solubility difficulties encountered prevented further characterization. 

In the presence of alkyl groups, the vinylic proton in open chain and cyclic olefins resonates in the range of 5 = 4.7-5.7 ppm with the exception of cyclopropene (59) and cyclobutene (8) (59 <5 = 7.01 8 d = 5.97 ppm) (Table 9). Even the oop distortions in the bridgehead olefins 69, 176, 177 affect the chemical shift of the vinylic proton only to a small extent. Noteworthy is the small shift difference between trans- 1-methylcyclooctene (37c) and 1-methylcyclohexene (42c) (Table 10). 

Going one step further, we might ask about the influence that a phosphorus multiple bond may have on the chemical shift. From C-NMR, we know that an olefin resonates downfield from an alkane, and an acetylene is found in between, but closer to the alkane. This is explained by diamagnetic anisotropy and the behaviour of P—C, P=C, and I C bonds should be analogous. 

IRRADIATION OF THE 5 6.52 OLEFINIC RESONANCE. In a second NOE experiment, the olefinic resonance at 8 6.52 was irradiated, and an NOE was seen for two resonances that for the other olefinic proton at 8 6.25 and that for the one aromatic proton at 8 7.12. In the C-l/C-q isomer (Scheme II), the single aromatic proton is H-4 in the C-3/C-q isomer (Scheme I), the aromatic proton is H-12. This observation is further evidence for the C-l/ C-q isomer because the 8 6.52 resonance corresponds to H-3, which should not show an NOE on either methyl group. In the C-3/C-q isomer, either olefinic proton (H-l or H-2) should show an NOE on a methyl group. 

Aris et al. (9) examined cyclopentadienyl (propene) irondicarbonyl tri-fluoroacetate and dipropenerhodiumacetylacetonate. In the iron complex the olefinic resonances are shifted upheld by approximately 60 ppm (methylene carbon) and approximately 50 ppm (methine carbon). These shifts indicated, in the authors view, that there was extensive backbonding. For the rhodium complex, the methylene and methine resonances are also shifted upheld upon coordination, but the difference between the upheld shifts (J C(l)-d C(2)) is smaller. 

Another type of emulsion (co)polymer that has been analysed in toms of compositional sequence distributicHi is, for example, polybutadioie. By means of carbon-13 NMR, it is possible to determine the vinyl-1,2, cfr-1,4 and the trans-1,4 units from the olefinic resonances [142], Alternatively, the alifAiatic carbon-13 resonances of polybutadiene have been analysed in the same microsctructural sequences by means of low MM model compounds [143]. The fractions of the microstructural units of a polybutadiene pr rared in emulsirm are typically 18% vinyl-1,2,20% cfr-1,4 and 62% mons-1,4 and depend only on the polymerization temperature [144]. 

By integration of the NMR olefinic resonances Bimodal distribution from GPC analyses Less than 0.05 g... 

Chlorination of the analogous ISB-isoprene copol3nner is known to be highly regioselective (Figure 2). More than 90% of the product consists of the H2C=C-6h-C1 moiety, characterized by two olefinic resonances in the C NMR spectrum H2C=, 116 ppm =C-, 147 ppm (Figure 3 shows the C spectrum of a partially chlorinated ISB-isoprene copolymer). H NMR confirms the presence of H2C= and -tn-Cl protons, with minor amounts of =tH at 5.3 ppm.2... 

The interesting feature of the H NMR spectra of these organometallic polymers is that while some of the olefinic resonances were overlapped with the complexed aromatic resonances in monomer 19, all of these peaks can be observed in the spectra of the polymers. Figure 14 shows the H NMR spectrum of polymer 22a. The olefinic proton at 5.23 now overlaps slightly with the cyclopentadienyl... 

Cyclooctadiene and dicyclopentadiene terpolymers have olefinic protons with the same chemical shift (4.55 t), so these cannot be differentiated by this technique, but they may be distinguished by iodine monochloride. The hexadiene type of terpolymers may be identified by its olefinic resonance at 4.7 x. 

These three monomers have what appears as a single olefinic resonance in the terpolymers, but, the two norbornadiene types of monomer each show two characteristic resonances. In the methylene noroborene terpolymer, the olefinic resonances arise from two protons, each giving a separate signal whereas in the ethylidene norbonene terpolymers this is only one proton, the signal of which appears as a doublet. In view of these considerations, it is more difficult to detect the olefinic resonance in the latter instance. 

Chain end analysis of the polypentene by H NMR shows the presence of geminally disubstituted olefin resonances, indicating chain termination by /6-H elimination. NMR analysis of the polymer at the pentad level shows a remarkably high degree of isotacticity. This catalyst also initiates the polymerization of 1-hexene and afford polymers (M = 24,000) with M jM =1.75. A similar C NMR analysis showed the tacticity of more than 95%. 

Parallel results were found when bryostatins 4 and 10 were allowed to react with m-chloroperbenzoic acid in methylene chloride at room temperature. Introduction of one oxygen atom required two days. The SP-SIMS spectra were consistent with an additional 16 mass units, but while epoxidation of bryostatin 4 occurred at the A -olefin, epoxidation of bryostatin 10 occurred at the A -olefin, a conclusion reached by considering the H-NMR spectra. The spectrum of bryostatin 10 epoxide retained the olefinic resonances of the A double bond but the H-30 olefinic resonance at 85.658 was shifted to higher field at 53.358, a shift characteristic of hydrogen on an epoxide. In addition, the C-36 methyl... 

NMR spectra can give detailed information on structure and stereochemistry, especially with the availability of very hi field instruments, even on quantities as small as 10 Mg or less. The dienoid alkaloids are readily distinguished by the presence of an ABX system corresponding to the three olefinic protons, whereas the alkenoid alkaloids only have a single olefinic resonance and the lactonic alkaloids lack the two aromatic singlet resonances. 

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