Palladium complexes for Co/Styrene copolymerization. Study of the influence of the ligand

by Bastero Rezola, Amaya

Abstract Summary and evaluation The development and study of new palladium catalysts for the copolymerization of 4-tert-butylstyrene with carbon monoxide was the main objective in the present thesis. The role played by the chelating ligands and in particular by their donating atoms is known to be fundamental in this catalytic process and therefore sulfur and nitrogen-donating ligands were studied. As a result from this study, the following conclusions can be drawn: The new palladium(II) neutral and cationic complexes containing bis(thio)ethers as chelating ligands, synthesized in Chapter 3, show different behavior in solution depending on the rigidity of the ligand. The cationic complexes are precursors for the copolymerization of CO/4-tertbutylstyrene although they are quite unstable under carbon monoxide pressure and get reduced to inactive palladium metal. By choosing chiral bis-(thio)ethers with the appropriate backbone rigidity, good stereocontrol of the styrene insertion in the polyketone chain may be obtained. In Chapter 4 the coordination of a family of Cs-symmetrical pyrazolcontaining bisnitrogen ligands to palladium-methyl cationic complexes leads to different stereoisomers depending on the steric hindrance of the bisnitrogen ligand. They behave as active precursors for the copolymerization reaction and allow the synthesis of syndiotactic polyketones, irrespectively of the stereochemistry of the precatalyst. Under mild copolymerization conditions the pyrazol-containing catalysts are an example of living copolymerization catalysts. Chapter 5 deals with the synthesis of new chiral N1-substituted imidazolines which allow tuning the electronic properties of the metal to which they are coordinated. The differently substituted C1-symmetrical pyridine-imidazolines may lead to the selective synthesis of palladium cationic stereoisomers depending on the basicity of the ligand. Similarly under carbon monoxide atmosphere, palladium-acyl stereoisomers may be selectively obtained. When the different stereoisomers are used as precatalysts for the copolymerization of CO and 4-tert-butylstyrene, polyketones with different degrees of stereoregularity may be obtained. The combination of a pyridine or pyrimidine ring with a 5-membered nitrogen containing heterocycle (pyrazol or imidazoline) leads to effective achiral or chiral ligands, respectively, for the CO/4-tertbutylstyrene copolymerization and also CO/ethene/4-tert-butylstyrene terpolymerization reactions, as shown in Chapter 6. The palladium precatalysts with pyrazol-containing ligands are more stable under co- and terpolymerization conditions than the imidazoline-derived ones. The steric hindrance caused by the chiral imidazoline ligands may be responsible for the low reactivity of the palladium species towards 4-tert-butylstyrene,while the insertion of ethylene is more favored. This reactivity is reversed when less steric hindered nitrogen ligands are used.