The mechanism of vinyl polymerization initiated by metal alkyls: Copolymerization studies

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Abstract
Monomer reactivity ratios have been determined for the copolymerization of methyl methacrylate (M1) and acrylonitrile (M2) catalyzed by alkyl derivatives of lithium, sodium, beryllium, magnesium, boron, aluminum, zinc, and cadmium. Lithium, sodium, beryllium, and magnesium alkyls gave reactivity ratios averagingr1= 0.34,r2= 6.7, while alkyls of boron, aluminum, zinc, and cadmium gaver1= 1.24,r2= 0.11. These values compare favorably with published reactivity ratios for anionic copolymerization (r1= 0.25,r2= 7.9) and free‐radical copolymerization (r1= 1.35,r2= 0.18). Therefore, alkyl derivatives of groups Ia and IIa metals propagate by an anionic mechanism, while those of groups IIb and IIIa do so by a free‐radical mechanism. Confirmatory experiments were run using styrene (M1)–acrylonitrile (M2) solutions. For groups Ia and IIa alkyls the reactivity ratios averaged,r1= 0.20,r2= 12.5; and for Groups IIb and IIIar1= 0.33,r2= 0.03. The published values for free‐radical catalysts arer1= 0.41,r2= 0.04. The formation of free radicals from the groups IIb and IIIa metal alkyls probably arises from oxidation of the metal alkyls by chance traces of oxygen.

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