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Effect of Electron Donors on the Radical Polymerization of Vinyl Acetate Mediated by [Co(acac) 2 ]: Degenerative Transfer versus Reversible Homolytic Cleavage of an Organocobalt(III) Complex
Author(s) -
Maria Sébastien,
Kaneyoshi Hiromu,
Matyjaszewski Krzysztof,
Poli Rinaldo
Publication year - 2007
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200601457
Subject(s) - chemistry , polymerization , homolysis , electron transfer , vinyl acetate , polymer chemistry , ligand (biochemistry) , radical polymerization , pyridine , photochemistry , medicinal chemistry , radical , polymer , organic chemistry , copolymer , receptor , biochemistry
The molecular structure of bis(acetylacetonate)cobalt(II) ([Co(acac) 2 ]) in solution and in the presence of the electron donors (ED) pyridine (py), NEt 3 , and vinyl acetate (VOAc) was investigated using 1 H NMR spectroscopy in C 6 D 6 . The extent of formation of ligand adducts, [Co(acac) 2 (ED) x ], varies in the order py>NEt 3 >VOAc (no interaction). Density functional theory (DFT) calculations on a model system agree with CoED bond strengths decreasing in the same order. The effect of electron donors on the [Co(acac) 2 ]‐mediated radical polymerization of VOAc was examined at 30 °C by the addition of excess py or NEt 3 to the complex in the molar ratio [VOAc] 0 /[Co] 0 /[V‐70] 0 /[py or NEt 3 ] 0 =500:1:1:30 (V‐70=2,2′‐azobis(4‐methoxy‐2,4‐dimethylvaleronitrile)). As previously reported by R. Jerome et al., the polymerization showed long induction periods in the absence of ED. However, a controlled polymerization without an induction period took place in the presence of ED, though the level of control was poorer. The effective polymerization rate decreased in the order py>NEt 3 . A similar behavior was found when these electron donors were added to an ongoing [Co(acac) 2 ]‐mediated radical polymerization of VOAc. On the basis of the NMR and DFT studies, it is proposed that the polymerization is controlled by the reversible homolytic cleavage of an organocobalt(III) dormant species in the presence of ED. Conversely, the faster polymerization after the induction period in the absence of ED is due to a degenerative transfer process with the radicals produced by the continuous decomposition of the excess initiator. Complementary experiments provide additional results in agreement with this interpretation.