Unsaturation, vinylene

Foremost among the classes of the nonaromatic unsaturated systems is that of compounds in which the substituent is bonded to a carbon-carbon double bond. It is convenient to divide the data sets for carbon-carbon double bonds into five categories vinyl, vinylidene, frans-vinylene, cis-vinylene, and reactions of carbon-carbon double-bond sets. 

The effect of antioxidants such as hindered phenohcs, secondary amine, and thioester on the radiation cross-linking efficiency of LDPE has been reported [260]. Amount of cross-linking at a given dose decreases with aU the antioxidants, the thioester being the most effective. IR absorption spectroscopy has been used to demonstrate dose-rate dependence of trani -vinylene unsaturation in irradiated Marlex 50 PE [261]. When the irradiated polymer is stored in vacuum a decrease is observed in trani-vinylene absorbance over a period of several weeks. After high dose-rate irradiation the decay is preceded by an initial increase. These phenomena have been ascribed to the reaction of trapped radicals. 

On 7-irradiation, a decay of unsaturation is observed in the microstmcture of EPDM mbber containing 2-4 mol% of 5-ethyhdene-2-norbomene [368]. The formation of trani -vinylenes and conjugated dienes has been noted in ethylene-rich EPR rubbers [369]. Gas production in EPR under the influence of 7-radiation has also been reported [370]. 

The data of Table II indicate that the etch rates for CB and its "homologues"—TP, CO (or TO), and EPM—tend to increase monotonically with a decrease in vinylene (-CH=CH-) unsaturation. The elastomeric EPM was chosen instead of crystalline polyethylene as a model for the fully saturated CB to avoid a morphology factor in etch rates, as was observed with crystalline TB. The difference in etch rates for the partially crystalline TO and the elastomeric CO (ratio of about 1.2 1.0) is attributable more to a morphology difference between these polyoctenamers than to the difference in their cis/trans content. Cis/trans content had likewise no perceptible effect on etch rates in the vinyl-containing polybutadienes (see Table I) if there was a small effect, it was certainly masked by the dominant effect of the vinyl groups. 

Alternatively, l,3-dithiol-2-ones have been obtained by reacting di-isopropylxanthogen disulfide with unsaturated substrates, such as disubstituted alkynes R2C2, in the presence of AIBN (azoisobutyrylnitrile), a radical initiator.78 Finally, vinylene dithiocarbonates can be obtained by reacting mer-cury(II) acetate with the corresponding trithiocarbonates, generated by reaction of electrophilic alkynes with ethylene trithiocarbonate.79 Analogously,... 

Irradiation of the cycloalkenone (218) with vinylene carbonate in THF resulted in the separation of (219), which yielded the p,y-unsaturated aldehyde (220) upon treatment with KOH in MeOH at room temperature (Table 12) 73 . 

The IR-spectrum of the polymer was measured with a Nippon Bunko Model DS-403G infrared spectrometer. The concentration of the unsaturated groups in the polymer (number of unsaturated group per 1000 carbons) was determined from the absorbances at 966 cm l (trans-vinylene), 910 (vinyl) and 890 (vinylidene) in the IR-spectrum by the method reported by Cernia et al. (7). 

As discussed kinetically in detail (9), the reactivity of each unsaturated group in the cross-linking reactions (7-l)-(7-3) was in the order, vinylidene> vinyl> trans-vinylene. 

Apart from this behavior, the concentration of frarw-vinylene increases with dose, at first linearly but then tends to a limiting value which depends on the type of radiation (7). This limit is possibly caused by the capture by frarw-vinylene of the thermalized hydrogen atoms released elsewhere by radiation. From the limiting concentration of unsaturation the number of collisions made by each thermalized hydrogen atom before it abstracts can be deduced this lies in the range 103 to 104. 

Inasmuch as the production of tram-vinylene unsaturation is one of the important processes occurring during the y-ray irradiation of polyethylene, it is interesting to compare G(t—Vl) in polyethylene with G(Vl) values in other substances. Data are presented in Table II where G(Vl) represents a monoene group, whether cis or trans. 

Infrared analysis (6) of irradiated film showed an increase in the yield of trans-vinylene unsaturation (Figure 2). The calculation from the... 

To determine the concentration of cis-vinylene groups in irradiated specimens, the original cw-polybutadiene itself was used as a standard, and a value for A (assumed independent of concentration) was determined from Av and logr (I0/I)vo of unirradiated specimens. The total concentration of unsaturation was accepted (15) as 100% of theoretical and the density as 0.90 gram per cc. The cis concentration was obtained by subtracting from the total unsaturation the trans-vinylene and vinyl contents determined from the calibration curves developed as described below. The value of A so determined was 2.67 X 103 liters/mole-sq. cm. 

The linear portions of the curves for c/.v-vinylene concentration (Figure 3) give G values of —15 for cis destruction by gamma radiation and —12 by the reactor field. The value for gamma radiation is in reasonable agreement with that observed by Golub (12) if his yield for loss of unsaturation is combined with his isomerization yield (G = —7.9... 

Table II. Extinction Coefficients for trans-Vinylene Unsaturation in Various Ethylene Homo- and Copolymers...

Figure 7. Growth of tmns-vinylene unsaturation at low doses of ionizing radiation...

Polymer Density gem-3 MI g/10 min MFR 1-octane (from 13C NMR) W% Unsaturation Vinyl mol L 1 t-Vinylene mol L 1 Vinylidene mol L 1 Total double bond... 

Recycled HDPE items (blow-moulding grade) produced from HDPE made by the Phillips process were analysed by IR spectroscopy. The absorption bands at 888 and 965 cm-1, corresponding to unsaturations of vinylidene and vinylene type, respectively, are common in PE produced by either the Ziegler process or metallocenes [104]. The absence of these bands in the IR spectrum of the different samples confirmed that the resins had been produced using a Cr-type catalyst [118]. 

The 13C NMR parameters for vinylene carbonate (30) show significant shielding of the carbonyl carbon atom due most probably to the proximity of the lone pair electrons on the adjacent heteroatoms (77JOC2237). The carbonyl shift is very similar in ethylene carbonate, the saturated analog (B-72MI43000). The alkenic carbons in (30) appear at 5132.2, a typical resonance for sp2 carbons of unsaturated alkenes and aromatics. On protonation the chemical shifts for the 1,3-dioxolium ion (31) are deshielded by 6.1 p.p.m. for the alkenic carbon atoms and 9.5 p.p.m. for the carbonyl carbon atom. 

Another type of unsaturated mesogenic structure, which is based on a structure in between an azo- and vinylene-type, is that in which a Schiff base unit links two phenylene groups. This unit is similar in geometry to the three previous mesogenic units, the azo, azoxy and stilbene groups The only reported example of this structure, however, was Polymer 8 with a decamethylene spacer, and it was quite similar in properties to the polymers just discussed. It also possessed a smectic phase, and the mesogenic unit had terminal carboxy groups. 

In polyethylene, the tertiary carbon atom, which dominated the chemistry of the oxidative degradation of PP, is present only at branch points. This suggests that there may be a difference among LDPE, LLDPE and HDPE in terms of the expected rates of oxidation. This is complicated further by the presence of catalyst residues from the Ziegler-Natta polymerization of HDPE that may be potential free-radical initiators. The polymers also have differences in degree of crystallinity, but these should not impinge on the melt properties at other than low temperatures at which residual structure may prevail in the melt. Also of significance is residual unsaturation such as in-chain tra s-vinylene and vinylidene as well as terminal vinyl, which are defects in the idealized PE strucmre. 

The rate of formation of trans-vinylene (—CH2—CH=CH—CH2—) unsaturation and the rate of decay of vinyl (—CH=CH2), vinylidene... 

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