Syndiotactic CO/Styrene copolymerization catalyzed by alpha-diimine Pd(II)complexes: Regio- and stereochemical control

Using the catalytic system [(Pr(i)DAB)Pd(Me)(NCMe)](+)[(3,5-(CF3)(2)C6H3)(4 )B](-), where Pr(i)DAB = 1,4-diisopropyl-1,4-diaza-1,3-butadiene, it has be en possible to obtain copolymers of styrene or p-methylstyrene with carbon monoxide, which have alternating, head-to-tail, syndiotactic structure and a stereochemical purity of 92%. To investigate the reaction mechanism, the first steps of the copolymerization were studied by isolating and character izing the reaction intermediates. The reactivity of the catalyst toward car bon monoxide was studied, and the corresponding methyl carbonyl Pd initiato r complex was isolated. The regiospecific 2,1-insertion of p-methylstyrene in the Pd-acyl bond was proved by NOE measurements performed on the first i ntermediate, a five-membered palladacycle, in which the acetylic oxygen is coordinated to the palladium. Moreover, the intermediate resulting after th e second sequence of CO and styrene insertion unexpectedly showed a doublin g of the signals, both in the proton and carbon-13 NMR spectra. This findin g, attributed to the presence of two diastereoisomeric complexes, showed th at the chain-end stereochemical control is not fully effective during the i nitial steps of the copolymerization. Last, the fluxional character of some examined complexes was studied and the relative activation energies were d etermined.