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 {κ2-1-PiPr2-2-P­(O)­(OEt)2-5-Me-Ph}­PdMe­(2,6-lutidine)+ (3c), 125 kg mol–1 h–1 for {κ2-1-PPh2-2-P­(O)­(OEt)2-5-Me-Ph}­PdMe­(2,6-lutidine)+ (3a), and 1470 kg mol–1 h–1 for {κ2-1-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
{κ2-1-P(4-tBu-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 %.