Nonsymmetric alpha-diimines in Pd-catalyzed ehtene/polar monomer copolymerization

One of the major unsolved problems in polymer chemistry is represented by the synthesis of functionalized polyolefines, that are currently produced through radical processes, a technology that suffers from high energy consumption, low cost efficiency and poor control over the macromolecule structure.1 The direct, controlled, homogeneously catalyzed copolymerization of ethylene with polar vinyl monomers represents a sustainable technology to overcome these limits. The main catalytic systems reported in the literature are based on Pd(II) compounds with either -diimines or phosphino-sulfonate ligands.2 Their productivity is thus far low from the values required for an industrial application, and better performing catalysts are strongly needed. With this aim we are studying Pd(II) complexes with new nonsymmetric -diimines (Ar,Ar'-BIAN) featured by an acenaphthene skeleton and one aryl ring substituted in meta positions with electron-withdrawing groups, while the other ring bears electron-releasing substituents on the ortho positions (Figure up). The related monocationic complexes, [Pd(CH3)(Ar,Ar'-BIAN)(L)][PF6], that include both acetonitrile and dimethyl sulfoxide derivatives, have been applied as precatalysts in the ethylene/methyl acrylate copolymerization under mild conditions of temperature and pressure showing a remarkably different catalytic behaviour depending on the nature of the L ligand (Figure below).3 While the catalyst originated from the acetonitrile precursor was found to be less active than the dmso counterpart and to deactivate within 16 h of reaction, that obtained from the dmso derivative was still active after 48 h achieving a productivity of 350 g P/g Pd. The catalysts with the nonsymmetric Ar,Ar'-BIAN showed a productivity twice as high as that of the catalysts with the corresponding symmetric ligands together with a higher incorporation of the polar monomer into the catalytic product. The latter is a mixture of ethylene oligomers and ethylene/methyl acrylate cooligomers, having the polar monomer at the end of the branches. Detailed investigation on the studied catalytic system will be presented. 1A. Nakamura, S. Ito and K. Nozaki, Chem. Rev., 2009, 109, 5215. 2 a) L.K. Johnson, S. Mecking and M. Brookhart, J. Am. Chem. Soc., 1996, 118, 267; b) E. Drent, R. van Dijk, R. van Ginkel B. van Oort and R.I. Pugh, Chem. Commun., 2002, 744. 3A. Meduri, T. Montini, F. Ragaini, P. Fornasiero, E. Zangrando and B. Milani, ChemCatChem published on the web, DOI: 10.1002/cctc.201200520.