Studies in polymerization - XXI. On free-radical formation by oxidation of molybdenum carbonyl with carbon tetrahalides: carbon tetrabromide

Determination of the conversion as a function of reaction time in the polymerization of methyl methacrylate initiated by Mo(CO) 6 + CBr 4 a t 80 °C indicates that the rate constant for carbonyl consumption is the same as in the CCI 4 system and corresponds to displacement of a CO ligand by monomer; further, each Mo(CO) 6 consumed gives rise to three free radicals. The limiting initial rate of polymerization at high [CBr 4 ] is consistent with the rapid formation of three radicals per Mo(CO) 6 . It is concluded that primary and secondary oxidation steps occur as with CCI 4 , but that on account of the relatively high reactivity of CBr 4 the two stages occur effectively simultaneously and cannot readily be separated. Tracer investigations with 14 CBr 4 reveal that both stages yield CBr 3 radicals. The electron spin resonance (e. s. r.) spectra of the paramagnetic products formed in ethyl acetate solution at 80 °C are shown to arise from Mo v species. The signal intensity grows at a remarkably high rate, corresponding to a half-life of 0.25 h; the rate is markedly reduced by the presence of free radical scavengers such as methyl methacrylate or cyclohexane. These observations are thought to indicate the occurrence of a chain process involving CBr 3 and Mo I , which react with Mo(CO) 6 and CBr 4 , respectively. Experiments in which CBr 3 radicals were produced photochemically from Mn 2 (CO) 10 + CBr 4 in the presence of Mo(CO) 6 support the proposed mechanism. Although the carbonyls of chromium and tungsten, in association with halides, initiate polymerization, the products do not give observable e. s. r. spectra. It is suggested that these reactions lead to oxidation states III and VI respectively, rather than V, which is particularly stable in the case of molybdenum.