Ethylene Polymerization by Palladium Alkyl Complexes Containing Bis(aryl)phosphino-toluenesulfonate Ligands

Abstract

The reaction of L′ 2 PdR 2 (L′ = pyridine (py), pyridazine; L′ 2 = cyclooctadiene, TMEDA) with 2-{(2-OMe-Ph) 2 P}-4-Me-benzenesulfonic acid ([PO-OMe]H, [ 1a ]H) or 2-{(2-Et-Ph) 2 P}-4-Me-benzenesulfonic acid ([PO-Et]H, [ 1b ]H) yields [PO-OMe]Pd(R)(L) (L = py, R = CH 2 SiMe 3 ( 2a ), CH 2 t Bu ( 3a ), CH 2 Ph ( 4a ); R = Me, L = pyridazine ( 5a ), py ( 6a ), PPh 3 ( 7a )) or [PO-Et]Pd(Me)(py) ( 6b ). 2a and 6b have square-planar structures in which the alkyl group is cis to the phosphine and the [PO]Pd chelate rings are puckered. The reaction of 2a and 3a with B(C 6 F 5 ) 3 yields {[PO-OMe]Pd(R)} 2 (R = CH 2 SiMe 3 ( 8a ), CH 2 t Bu ( 9a )). 8a is a sulfonate-bridged dimer in the solid state. 2a, 6a, and 6b polymerize ethylene to linear polyethylene that contains low levels of Me branches, one C═C unit per chain (mostly 1- or 2-olefins), and M n in the range 6000 to 19 000. 6a is slightly more active but produces polymers with similar molecular weight and structure compared to 6b . 6a copolymerizes ethylene and hexene at low ethylene pressure (5 atm), but no α-olefin incorporation is observed at high pressure (30 atm). An ethylene polymerization mechanism is proposed, which involves insertion and chain transfer of [PO]Pd(R)(ethylene) species ( II ) and ethylene trapping and much slower chain-walking of the [PO]Pd(CH 2 CH 2 R) species ( III ) formed by insertion of II .