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

The reaction of L′2PdR2 (L′ = pyridine (py), pyridazine; L′2 = cyclooctadiene, TMEDA) with 2-{(2-OMe-Ph)2P}-4-Me-benzenesulfonic acid ([PO-OMe]H, [1a]H) or 2-{(2-Et-Ph)2P}-4-Me-benzenesulfonic acid ([PO-Et]H, [1b]H) yields [PO-OMe]Pd(R)(L) (L = py, R = CH2SiMe3 (2a), CH2tBu (3a), CH2Ph (4a); R = Me, L = pyridazine (5a), py (6a), PPh3 (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(C6F5)3 yields {[PO-OMe]Pd(R)}2 (R = CH2SiMe3 (8a), CH2tBu (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 Mn 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(CH2CH2R) species (III) formed by insertion of II.