Mechanisms for pyrolysis of poly(α-methylstyrene) must rationalize high selectivity for monomer formation, negligible formation of volatile oligomers, and notably slow decrease in molecular weight compared with the rate of weight loss, i.e., unzipping dominates both back-biting and transfer. Backbone homolysis should form both a tert-benzylic radical Rtb and a prim radical Rp, with formation of the latter potentially supplemented in chain propagation steps emanating from the former. Hence product-forming pathways characteristic of each are expected to compete. Simulations of initial product distributions based on assigned rate constants for chain propagation steps indicate that Rtb is indeed predicted to efficiently unzip with minimal transfer or back-biting. However, Rp is predicted to give comparable amounts of transfer and back-biting with minimal unzipping, behavior inconsistent with experimental data. The proposed escape from this impasse is a previously unrecognized pathway, 1,2-phenyl shift in Rp to form a tert radical. If it undergoes β-scission, the net result is an inter-conversion of Rp to Rtb. Quantitative simulations suggest that this sequence is indeed highly competitive with other reactions of Rp and thus efficiently subverts the otherwise expected propagation of chains emanating from Rp. 相似文献
The metal salts mediated oxidative free radical reaction of 2-amino-1,4-benzoquinones is described. Imine radicals can be generated by the oxidation of 2-amino-1,4-benzoquinones with Mn(III) and Ag(II). The dimeric products 4 and 14 were formed via the intermolecular radical coupling reaction of the corresponding radical intermediates 5 and 15. In the presence of styrene, twistane 17 was afforded from 2-phenylamino-1,4-benzoquinone 1 via a radical annulation reaction of imine radical 5. 相似文献
The reactivities of mono- and dihalocarbene anions (CHCl−, CHBr−, CF2−, CCl2−, and CBrCl−) were studied using a tandem flowing afterglow-selected ion flow tube instrument. Reaction rate constants and product branching ratios are reported for the reactions of these carbene anions with six neutral reagents (CS2, COS, CO2, O2, CO, and N2O). These anions were found to demonstrate diverse chemistry as illustrated by formation of multiple product ions and by the observed reaction trends. The reactions of CHCl− and CHBr− occur with similar efficiencies and reactivity patterns. Substitution of a Cl atom for an H atom to form CCl2− and CBrCl− decreases the rate constants; these two anions react with similar efficiencies and reactivity trends. The CF2− anion displays remarkably different reactivity; these differences are discussed in terms of its lower electron binding energy and the effect of the electronegative fluorine substituents. The results presented here are compared to the reactivity of the CH2− anion, which has previously been reported. 相似文献
Using 266 nm laser flash photolysis it has been demonstrated that Berberine (BBR) in aqueous solution is ionized via a mono-photonic process giving a hydrated electron, anion radical that formed by hydrated electron react with steady state of BBR, and neutral radical that formed from rapid deprotonation of the radical cation of BBR. The quantum yield of photoionization is determined to be 0.03 at room temperature with KI solution used as a reference. Furthermore utilizing pH changing method and the SO4− radical oxidation method, the assignment of radical cation of BBR was further confirmed, the pKa value of it was calculated, and the related set up rate constant was also determined. 相似文献
Solid-state optical sensor membranes based on immobilised chromogenic radicals for the assessment of antioxidant activity have been studied. Two stable lipophilic chromogenic radicals, DPPH (2,2-diphenyl-1-picrylhydrazyl radical) and galvinoxyl radical, GV, (2,6-di-tert-butyl-α-(3,5-di-tert-butyl-oxo-2,5-cyclohexadien-1-ylidene)-p-tolyloxy radical), were immobilised in plasticised PVC films and screened for suitability as indicators of antioxidative activity. The spectrophotometric characterisation of the polymer films containing immobilised free radicals was performed, and the response of the immobilised free radicals toward standard antioxidants was studied. It has been demonstrated that the immobilised radicals retain their reactivity towards antioxidants and the results suggest that the reactivity of immobilised radicals is comparable to standard solution-based DPPH assays. Polymer films containing immobilised DPPH radical respond to standard antioxidants in aqueous solutions by changing colour irreversibly from purple (absorption maximum at 520 nm) to yellow. The initial slopes of the response curves to the phenolic antioxidant gallic acid, obtained in the 1-50 mM concentration range, gave a linear calibration plot in a 1 min exposure cuvette test. The polymer films were used to screen antioxidative activity of beverage and food samples known to contain antioxidants, such as black and green tea, coffee, red wine, fruit juice, olive oil and sunflower oil. It has been demonstrated that a rapid and simple qualitative screening test of untreated samples is possible using a test strip based on immobilised DPPH radical. 相似文献
Summary: A kinetic model for the thermal crosslinking of rubbery polymers is presented. The reaction mechanism used to develop the model includes thermal radical generation producing a polymeric radical and a primary radical, crosslinking from attack of a polymer radical to any inactive polymer molecule, bimolecular radical termination among chains of any degree of branching, and radical termination between a polymer radical and a primary radical from the thermal radical generation. The overall polymer population is divided into “generations”, which are defined in terms of the number of primary (linear) chains of original rubbery polymer attached to the polymer molecule. Mass balances for each generation are written down, and expressions to calculate fraction of branched molecules, gel fraction, and number and weight average chain lengths are derived. Model simulations, parameter sensitivity analyses and preliminary parameter estimation studies are presented, taking the thermal crosslinking of linear polybutadiene as a case study.
Development of a kinetic model for thermal crosslinking of rubbery polymers: reaction mechanism, model equations and parameter sensitivity analyses. 相似文献
