Electron spin resonance studies on radiation‐induced polymerization of ethylene and isobutene on silica gel

Edlund, O.; Kinell, Per‐Olof; Lund, Anders; Shimizu, Akira

doi:10.1002/pol.1968.110060208

Cited by 7 publications

(2 citation statements)

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“…In fact, the linkage to silica, while preventing radical decay by inhibiting their translational diffusion, does not hinder fast intramolecular rotational motions, which lead to the averaging of hf and g tensor anisotropies. 19,20,22,24,[26][27][28]94 In agreement with this conclusion, no stable radicals are obtained from neat polybutadiene after irradiation near room temperature. For the assignment of the spectra in Figure 7, the three species A 1 , A 2 , and A 3 , all bearing the same number and type of interacting protons and arising from the reaction of silica species with polybutadiene double bonds, can be considered (Figure 8).…”

Section: The Lack Of Detection Of Siloxane Radical-cations [(−O) 3 Si−h]supporting

confidence: 76%

Reaction Mechanisms in Irradiated, Precipitated, and Mesoporous Silica

et al. 2013

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A matrix EPR spectroscopy study of the low temperature γ radiolysis of precipitated (Zeosil) and mesoporous high surface silica has afforded evidence of the formation of trapped H-atoms, H-atom centers, siloxy radicals ≡Si-O(•), anomalous silyl peroxy radicals ≡Si-OO(•) with reduced g tensor anisotropy, siloxy radical-cations (≡Si-O-Si≡)(+•), E' centers, and two species from Ge impurity. Coordination of peroxyl radicals with diamagnetic ≡Si(+) centers is proposed and tested by DFT computations in order to justify the observed g tensor. Coordination of H-atoms to ≡Si(+) centers is also proposed for the structure of the H-atom centers as an alternative model not requiring the intervention of Ge, Sn, or CO impurities. The DFT method has been employed to assess the electronic structure of siloxy radical-cations and its similarity with that of the carbon radical-cation analogues; the results have prompted a revision of the structures proposed in the literature for ST1 and ST2 centers. The comparison between the two types of silica has afforded evidence of different radiolysis mechanisms leading to a greater yield of trapped H-atoms and H-atom centers in zeosil silica, which is reckoned with the 4-fold greater concentration of silanol groups. Parallel radiolysis experiments carried out by using both types of silica with polybutadiene oligomers as adsorbate have afforded evidence of free valence and energy migration phenomena leading to irreversible linking of polybutadiene chains onto silica. Reaction mechanisms are proposed based on the detection of SiO2-bonded free radicals whose structure has been defined by EPR.

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