Catalytic polymerization of acrylates and in supercritical carbon dioxide

Abstract

Since the seminal reports by Ziegler and Natta, catalytic olefin polymerization has gained a tremendous impact. While the initial discoveries resulted from the nickel effect , today s industrial catalysts are based on early transition metals. Later on, late transition metals were commercialized for ethylene oligomerization. More recently, olefin polymerization by late transition metals has received growing interest. This interest is mostly based on two major differences between late transition metal complexes and their early transition metals counterparts. First, due to their lower oxophilicity, late transition metal complexes are more tolerant to polar groups. And second, due to their propensity for beta-hydride elimination, polymers with unique microstructure can be prepared.A major advance in this area was the finding by Brookhart and co-workers that cationic diimine complexes of Ni(II) or Pd(II) can polymerize ethylene to a highly branched polymer, due to chain walking of the catalyst. This discovery motivates the interest of the chemical community to develop new more active late transition metal catalysts for olefin polymerization (chapter 2). A general issue is the suppression of chain transfer in order to form high molecular weight polymer Recent progress in catalyst design allows for the preparation of a broad scope of polyolefin microstructures, offering new opportunities in polymerization processes. In this context, carbon dioxide is an interesting reaction medium, due to its unique solvent properties. Obviously, the high oxophilicity of Ziegler catalysts and metallocenes (the catalysts used industrially) prevent their use in any oxygenated solvent. Thus, late transition metal catalysts appear to be particularly suited for this purpose. However this perspective requires the finding of catalysts well soluble in scCO2 and with increased polymerization productivity and a control of polymer microstructure and properties (chapters 3 and 4).In addition, late transition metal catalysts have attracted attention not only for their tolerance towards polar molecules as ideally unreactive reaction media but more importantly for the copolymerization of hydrocarbon monomer with readily available polar monomers as substrates. Up to now, from the large numbers of late transition metal catalysts reported, only two families of complexes have been found to incorporate such polar monomers. Brookhart demonstrated the insertion copolymerization of methyl acrylate with ethylene by Pd(II) diimine catalysts. However, other common comonomers such as vinyl acetate, acrylonitrile and vinyl chloride could not be copolymerized by these cationic complexes. By contrast, neutral Pd(II) complexes with chelating phosphinosulfonate ligands copolymerize ethylene with a relatively broad scope of functional vinyl monomers. However, hindered dissociation of the catalyst precursor complexes renders a large part of the metal sites inactive. There is a need for more reactive complexes for utilization as catalyst precursors (chapter 5) and for stoichiometric studies directed towards a mechanistic understanding (chapter 6). This enables unprecedented reactions and polymer microstructures.