Syndiotactic CO/Styrene copolymerization catalyzed by α-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 been 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 characterizing the reaction intermediates. The reactivity of the catalyst toward carbon monoxide was studied, and the corresponding methyl carbonyl Pd initiator 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 intermediate, a five-membered palladacycle, in which the acetylic oxygen is coordinated to the palladium. Moreover, the intermediate resulting after the second sequence of CO and styrene insertion unexpectedly showed a doubling of the signals, both in the proton and carbon-13 NMR spectra. This finding, attributed to the presence of two diastereoisomeric complexes, showed that the chain-end stereochemical control is not fully effective during the initial steps of the copolymerization. Last, the fluxional character of some examined complexes was studied and the relative activation energies were determined.