Ethene/norbornene copolymerization with palladium(II) α-diimine catalysts:: From ligand screening to discrete catalyst species

Sixteen palladium(II) alpha-diimine catalysts were investigated in a screening-like procedure for the copolymerization of ethene with norbornene. The resulting copolymers were characterized by C-13 NMR spectroscopy, differential scanning calorimetry, gel permeation chromatography, and viscosimetry. The degree of incorporation of norbornene in the polymer chain is very high for most of the catalysts. To validate the results achieved in the screening, two catalysts, [{ArN=CHCH=NAr}Pd(Me)-(CH3CN)]BAr4f (1b'; Ar = 2,6-Me2C6H3, BAr4f = B[3,5-C6H3(CF3)(2)](4)) and [{ArN=C(CH3)C(CH3)=NAr}Pd(Me)-(CH3CN)]BAr (f)(4) (2c'; Ar = 2,6-iPr(2)C(6)H(3)), were synthesized as discrete catalytically active species, and their copolymerization behaviour was investigated in detail. In agreement with the screening results, 1b' incorporates norbornene much better in the polymer chain than ethene, a property that has no analogue in metal-locene catalysts.