Formation and thermal‐induced decay of radical ions in polymer matrices

Abstract

The formation of radical ions in γ‐irradiated polymethyl‐methacrylate (PMMA) matrices at 77°K and thermal‐induced reaction of these radical ions were of studied by optical absorption spectroscopic measurements. The radical ions of stilbene and pyrene were investigated. These radical ions decay according to second‐order kinetics, which means that the neutralization reaction of the cationic species and anionic species participates in the decay process. The kinetic plots consist of two straight lines; that is, fast and slow decay processes are concerned. The activation energies were estimated to be Efast = 2.4 kcal/mol and Eslow = 6.4 kcal/mol, respectively. The probability of recombination reaction depends on the distance between cationic and anionic species.

Evidence for rapid chain growth in the Fischer-Tropsch synthesis over iron and cobalt catalysts

“…We have published a brief communication of a portion of the results herein which show that chain growth is very rapid on an iron catalyst. 7 Prior to these studies, estimates of chain growth (made from site turnover frequencies13,9,10 and non-steady-state experi-ments11"13) were 2-3 orders of magnitude slower. By studying the isotopic transient in detail, including the position dependence of isotope incorporation within the molecules, we can extend our knowledge about the chain growth process in the Fischer-Tropsch reaction.…”

“…This requirement will tend to "freeze" the rotation of the bond to the benzene ring. It is possible that this symmetrical conformation is disfavored in the unsymmetrical radicals from 7 and 8 and that a higher than expected activation energy might result.…”

Ionic and excited species in irradiated poly(dimethylsiloxane) doped with pyrene

“…5). Such radicals can be generated in the reaction of Py •− with PDMS(H + ) at low temperatures, as found for poly(methyl methacrylate)–solute systems23, 26: If such a process takes place, PDMS(H + ) formation via proton transfer has to be considered, in addition to reaction 3: This type of proton‐transfer process is very common in other polymer systems27 as an additional source of matrix macroradicals detected by the electron spin resonance technique.…”

“…Tentatively, we may propose the following explanation of these observations. The absorption peak at approximately 404 nm, characteristic of PyH • , cyclohexadienyl‐type radicals,23, 24 was found even at 77 K (Fig. 5).…”

“…The optical absorption spectrum of PDMS film irradiated and analyzed at 77 K and doped with Py (∼5 × 10 −3 mol dm −3 ) is presented in Figure 5. Two strong absorption bands at 450 and 493 nm can be ascribed to Py radical cations and anions, respectively22 whereas the very narrow peak at 404 nm corresponds to Py radicals (PyH • ) 23, 24. The temperature dependences of the absorption bands with maxima at 404, 409, 450, and 493 nm are shown in Figure 6(a).…”