ESR studies on chain transfer reactions: Rate constants for the reactions of model radicals of polystyrene growing chain ends with tetrahalomethanes and thiophenols.

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The absolute rate constants for reactions of the model radicals of the growing chain ends of styrene and methacrylonitrile with tetrahalomethanes and thiophenols were determined by spin-trapping methods using ESR spectroscopy. The model radicals were produced by the photodecomposition of 1,1′-diphenylazoethane and 2,2′-azobisisobutyronitrile. In the case of the styrene model radical, the rate constants for the halogen abstraction reaction from CBr4, CBrCl3, and CCl4 were 1.1×107, 2.0×106, and 1.1×105 M−1s−1, respectively, and increased with increasing electron affinity of tetrahalomethanes. The rate constants for the hydrogen abstraction reaction from thiophenols were of the order 106 M−1s−1, independent of the electron-donating and -accepting properties of the para substituents. In the case of the methacrylonitrile model radical, the rate constants for the halogen abstraction reaction from CBr4 and CBrCl3 were 1.1×105 and 4.5×103 M−1s−1, respectively. The rate constants for the hydrogen abstraction reaction from thiophenols with –CH3, –H, and –Cl substituent in the para position were 4.5×104, 2.7×104, and 1.7×104 M−1s−1, respectively. The rate constants decreased with increasing the electron-withdrawing power of the substituent.
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