| Popis: |
Relation between catalyst structure and selectivity of the oxidative coupling of 2,6-dimethylphenol, I , into poly[oxy-(2,6-dimethyl)-1,4-phenylene], II , an important industrial polymer, in the presence of copper( II ) complexes with acetate or formate and pyridine or piperidine in chloroform and chloroform + toluene solutions has been studied. Variations in the structure of the catalyst during various steps of the catalytic process have been elucidated using EPR and NMR. Six different types of Cu( II ) complexes have been observed and their roles in the catalytic process have been identified. Intermediates that control reaction selectivity towards the desired polymeric product II seem to be mixed Cu( II ) complexes with monomeric and oligomeric phenolate ions, acetic or formic ion and amines. The rate-determining step of oxidative coupling in steady-state conditions seems to be electron transfer from phenolate ions to the Cu( II ) atom in these complexes. The selectivity is enhanced at high concentrations of these complexes, since in these conditions the monomeric phenoxyl radicals formed in the reaction mixture react mainly with coordinated phenolate and polyphenolate ligands to yield finally II , rather than recombining to form 3,3',5,5'-tetramethyldiphenoquinone, III , which is the main side product of the reaction. Complexes with oligomeric phenolate ligands seem to provide better selectivity than complexes with monomeric ones, since for the former complexes intramolecular electron transfer does not produce the monomeric phenol radicals necessary to make compound III . |
