Palladium(ii) salts containing [PdCl4]2− and [Pd2Cl6]2− ions as pre-catalysts for the vinyl-polymerization of norbornene—evidence for the in situ formation of PdCl2 as the active species

Abstract

The polymerization behaviour of a new series of palladium(II) pre-catalysts in the vinyl-polymerization of norbornene has been investigated. Compounds containing [PdCl₄]²⁻ or [Pd₂Cl₆]²⁻ anions and the organic cations [K(18-crown-6)]⁺, [H₃O(18-crown-6)]⁺, [Ph₃PCH₂C(O)CH₃]⁺ or [Him]⁺ (im = imidazole) can be activated with the Lewis acids methylalumoxane (MAO), tris(pentafluorophenyl)borane, B(C₆F₅)₃ or a combination of B(C₆F₅)₃/triethylaluminum (TEA). The catalytic systems show very high norbornene polymerization activities of up to 10⁷ g_polymer mol_Pd⁻¹ h⁻¹. In the case of an activation with B(C₆F₅)₃ alone it is possible to obtain soluble palladium-catalyzed poly(norbornene)s which exhibit a fully saturated polymer backbone and a molar mass (M_n) between 8 × 10³ and 3 × 10⁵ g mol⁻¹. The activation process of the pre-catalyst [Ph₃PCH₂C(O)CH₃]₂[Pd₂Cl₆] (2C) in combination with B(C₆F₅)₃ can be followed by multinuclear (¹H, ¹³C, ¹⁹F and ³¹P) NMR investigations and points to the in situ formation of PdCl₂ which then represents the active species for the polymerization process. The X-ray structure of compound 2C is reported.