Mechanistic aspects of isotactic CO/styrene copolymerization catalyzed by oxazoline palladium(II) complexes

Catalytic systems of the type [Pd(CH3)(NCCH3)(N-N)](+)[B{3,5-(CF3)(2)C6H3}(4)](-), where N-N = (4S,4'S)-(-)-4,4',5,5'-tetrahydro-4,4'-bis(1-methylethyl)-2,2'-bioxazole (BIOX) or N-N = (4S,4'S)-(-)-2,2'-(1-methylethyhdene)bis[4,5-dihydro-4-(phenylmethyl)oxazole]) (BISOX), afford highly isotactic CO/styrene or p-methylstyrene copolymers. The reactivity of the catalyst with the BIOX ligand toward carbon monoxide was studied and the corresponding methyl carbonyl Pd complex was isolated and identified as the real catalytic species. Starting from this compound the first steps of the copolymerization process were investigated and particular attention was paid to the stereochemistry of the intermediates. Specifically, NOE experiments carried out on the five-membered palladacycle, obtained from the first insertion of p-methylstyrene, indicated that regiochemistry is of the 2,1 type and that one diastereo-isomeric species is exclusively generated, with an R configuration of the new -CHAr-stereogenic center. Moreover, the investigation of the intermediate resulting after the second sequence of p-methylstyrene and CO insertion showed the presence of only one diastereoisomer. This evidence indicates that the stereocontrol of the isospecific catalyst is already very efficient at the first stages of copolymerization.