The influence of the structure of various dialkylmagnesium additives on the retardation of styrene polymerization, initiated by alkyllithium in cyclohexane, was investigated. The effect of n,sec-dibutylmagnesium, di-sec-butylmagnesium, butyloctylmagnesium and dibenzylmagnesium, in ratios ranging from 0–20 with respect to alkyllithium, was studied in detail. The presence of the magnesium derivatives leads to a drastic reduction of the overall reactivity depending on the type, concentration and proportion of dialkylmagnesium used. This was related to the formation of mixed complexes as shown by UV-Visible spectroscopy. The spectral changes were interpreted by the formation of several types of “ate” complexes of different stoichiometry and structure. Experimental polystyrene molar masses and MALDI-TOF measurements show a different degree of participation of magnesium alkyl groups as chain initiator (or reversible transfer agent) of styrene polymerization: secondary alkyls are the most efficient ones, whereas primary alkyls (or aryl) yield only a slow initiation (or transfer) and can be considered as almost ineffective.
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Ménoret et al. (2001) studied this question.