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Environmental fate & pathways

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The rates of the reactions of OH radicals with styrene, alpha-methylstyrene, and beta-methylstyrene were measured by irradiating mixtures of these aromatic olefins and NO in an environmental chamber at 298K. Based on the experimentally determined reaction rate constant k(OH)=5.3 +/- 0.6 x 10E-11 cm³/molecule x sec (25°C) and assuming a tropospheric OH radical concentration of 5 x 10E5 radicals/cm³ (24 h day) a half-life of 7.27 h can be estimated for alpha-methylstyrene. Based on these results, for alpha-methylstyrene present in air rapid photodegradation by reaction with OH radicals is to be expected. Acetophenone and formaldehyde were observed as main breakdown products which underwent further decomposition. Similar results were obtained using the generally accepted calculation model EPIWIN v3.11, AOPWIN v1.91 (t1/2=7.2 h). The reaction of alpha-methylstyrene with ozone is a further elimination path (t1/2=2 h, calculated via EPIWIN). Due to the spectral properties of alpha-methylstyrene (absorption of light in the environmental UV spectrum) direct photolysis is to be expected. In a study conducted by CITI (Japan) according to OECD Guideline 111 (Hydrolysis as a Function of pH), alpha-methylstyrene proved to be stable at pH 4, 7, and 9 at 25°C.