Copolymerization of Ethylene and Methyl Acrylate by Cationic Palladium Catalysts That Contain Phosphine-Diethyl Phosphonate Ancillary Ligands

A series of benzo-linked phosphine-diethyl phosphonate (P-PO) and phosphine-bis(diethyl phosphonate) (P-(PO)2) ligands and the corresponding (P-PO)PdMe(2,6-lutidine)(+) and (P(PO)(2))PdMe(2,6-lutidine)+ complexes were synthesized. Cationic (P-PO)PdMe(2,6-lutidine)+ complexes are active for ethylene oligomerization/polymerization, with activities of 2 kg mol(-1) h(-1) for {k(2)-1-PiPr(2)-2-P(O)(OEt)(2)-5-Me-Ph}PdMe(2,6-lutidine)(+) (3c), 125 kg mol(-1) h(-1) for {k(2)-1-PPh2-2-P(O)(OEt)(2)-5-Me-Ph}PdMe(2,6-lutidine) (3a), and 1470 kg mol(-1) h(-1) for {k(2)-1-P(2-OMe-Ph)(2)-2-P(O)(OEt)(2)-Ph}PdMe(2,6-lutidine) (3b). The polyethylene is highly linear, with over 80% terminal unsaturation and low (230-1890 Da) molecular weight in all cases. 3b copolymerizes ethylene with methyl acrylate, exhibiting highly selective (95%) in-chain (rather than chain-end) acrylate incorporation. The P-(PO)2 catalyst {k(2)-1-P(4-Bu-t-Ph)(2-P(O)(OEt)(2)-5-Me-Ph)-2-P(O)(OEt)(2)-5-Me-Ph}PdMe(2,6-lutidine) (3d) is more active for ethylene homopolymerization (2640 kg mol(-1) h(-1)), yielding linear, low-molecular-weight polymer (1280-1430 Da) with predominantly internal olefin placement. In ethylene/methyl acrylate copolymerization, 3d incorporates 2.6 mol % methyl acrylate, 60% of which is in-chain. Both 3b and 3d catalyze ethylene/acrylic acid copolymerization, albeit with low (<10 kg mol(-1) h(-1)) activities and acrylic acid incorporation up to 1.1 mol %.