Mo(CO)5Py as an Initiator of Polymerization

In a series of publications we have described the kinetics of free-radical formation from molybdenum carbonyl in association with organic halides in a number of electron-donating solvents. Radical formation involves the displacement of carbon monoxide from the carbonyl by the solvent or vinyl monomer in a rate-determining process giving rise to complexes of the type Mo (CO)5L (where L represents the solvent or monomer) which then react directly with the halide. This paper describes a study of the behaviour of Mo (CO)5Py in association with organic halides in methyl methacrylate solution. The results show that Mo (CO)5Py does not react directly with the halide but first undergoes activation by reaction with monomer, mainly with displacement of pyridine. The relative reactivities of complexes of the type Mo (CO)5L are discussed.The mechanism previously proposed to account for the inhibition processes observed in Mo (CO) 6-organic halide systems satisfactorily explains the extensive inhibition observed in the present work.

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

Proceedings of the Royal Society of London Series A - Mathematical and Physical Sciences ◽

10.1098/rspa.1972.0019 ◽

1972 ◽

Vol 326 (1567) ◽

pp. 431-451 ◽

Cited By ~ 9

Keyword(s):

Free Radical ◽

Carbon Tetrachloride ◽

Methyl Methacrylate ◽

Ethyl Acetate ◽

Reaction Times ◽

Radical Formation ◽

Secondary Oxidation ◽

Time Curve ◽

Molybdenum Carbonyl ◽

The Individual

The interaction of molybdenum carbonyl and carbon tetrachloride in a suitable medium gives rise to paramagnetic molybdenum products which can be studied by electron spin resonance (e. s. r.) spectroscopy. Most of the observations described in this paper have been made with ethyl acetate at 80 °C as solvent; in many respects methyl methacrylate gives similar results, but the rapid increase in viscosity accompanying polymerization limits investigation to short reaction times. E. s. r. spectra are consistent with the formation of Mo v species with one unpaired spin per Mo atom as the final oxidation state. Mo I and Mo III derivatives are not stable in the presence of carbon tetrachloride, but are rapidly oxidized to Mo v compounds. Kinetic investigations show that the signal-intensity versus reaction-time curve is sigmoid, the initial rate of growth of the signal being much lower than the rate of free-radical generation. It is concluded that there are at least two rate-determining processes in the reaction, the first being displacement of a CO ligand by solvent and the second the oxidation of an intermediate Mo species to a Mo v derivative. The first rate-determining reaction is followed rapidly by primary oxidation, producing one radical per Mo(CO) 6 consumed; this is essentially the only radical-generating step observed at short reaction times. The secondary oxidation, leading to Mo v , yields two radicals. Clearly some of the individual oxidation steps occur without radical formation; suggestions about the nature of the secondary oxidations are advanced. Measurements of carbon monoxide evolution indicate that all the CO ligands are ultimately released, and lead to the tentative conclusion that, in ethyl acetate, evolution of 5 molecules of CO per Mo(CO) 6 consumed accompanies formation of the primary radical. In methyl methacrylate CO evolution is slower, with a greater contribution from processes involved in secondary oxidation. The paramagnetic products of the reaction decompose to a diamagnetic black solid containing Mo IV on removal of volatile materials.

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