Enhancement of Chain Growth and Chain Transfer Rates in Ethylene Polymerization by (Phosphine-sulfonate)PdMe Catalysts by Binding of B(C6F5)3 to the Sulfonate Group

Binding of B­(C6F5)3 to a sulfonate oxygen of (ortho-phosphino-arenesulfonate)­PdR catalysts results in a 3–4 fold increase in the rate of chain growth and a larger increase in the rate of chain transfer. The reaction of (PO-Et)­PdMe­(py) (1a, [PO-Et]− = ortho-{(2-Et-Ph)2P}-para-toluenesulfonate) with 1 equiv of B­(C6F5)3 yields the base-free dimer {(PO-Et)­PdMe}2 (2a), in which the (PO-Et)­PdMe units are linked through an eight-membered [PdSO2]2 ring. The reaction of {(PO-3,5-tBu2)­PdMe}2(TMEDA) (4b; [PO-3,5-tBu2]− = ortho-{(3,5-tBu2-Ph)2P}-para-toluenesulfonate, TMEDA = N,N,N′,N′-tetramethylethylenediamine) with BF3·Et2O yields the soluble base-free dimer {(PO-3,5-tBu2)­PdMe}2 (2b), in which the (PO-3,5-tBu2)­PdMe units are linked through a four-membered Pd2O2 ring. 2b reacts with 2 equiv of B­(C6F5)3 to yield {[PO·B­(C6F5)3-3,5-tBu2]­PdMe}2 (5b, [PO·B­(C6F5)3-3,5-tBu2]− = [2-{(3,5-tBu2-Ph)2P}-4-Me-C6H3SO2OB­(C6F5)3]−), which crystallizes from Et2O as the monomeric complex [PO·B­(C6F5)3-3,5-tBu2]­PdMe­(Et2O) (6b). In both 5b and 6b, the B­(C6F5)3 binds to a sulfonate oxygen. In toluene solution at 60 °C, 2b polymerizes ethylene (80 psi) to linear polyethylene with Mn = 3,000, while the B­(C6F5)3 adducts 5b and 6b yield ethylene oligomers (Mn = 160–170). 5b and 6b are 3–4 times more active than 2b. Similarly, 1a polymerizes ethylene to linear polyethylene with Mn = 29,300 (toluene, 80 °C, 435 psi), while 1a-4 B­(C6F5)3 yields polymer with Mn = 2,520 with a 4 fold increase in activity. 2b reacts with ethylene at 7 °C to form the ethylene adduct (PO-3,5-tBu2)­PdMe­(CH2CH2) (7b) followed by multiple insertions to generate (PO-3,5-tBu2)­Pd­(CH2CH2)nCH3 species. In contrast, 5b reacts with ethylene to form [PO·B­(C6F5)3-3,5-tBu2]­PdMe­(CH2CH2) (8b) followed by insertion and β-H transfer to yield propene with subsequent catalytic formation of 1-butene and higher olefins. The rate of ethylene insertion of 8b is 3 times greater than that of 7b, consistent with the batch polymerization results. The polymer yield and molecular weight data show that binding of B­(C6F5)3 to 2b and 1a increases the chain transfer rates by a factor of 80 and 42, respectively.