Studies of Ligand and Solvent Effects in the Alternating Copolymerization of Carbon Monoxide and Ethene by Palladium-Diphosphine Catalysis

The Pd(II) methyl complexes [Pd(Me)(MeCN)(P−P)]PF 6 are effective catalyst precursors for the alternating copolymerization of carbon monoxide with ethene in CH 2 Cl 2 (P−P = 1,3-bis(diphenylphosphino)propane (dppp), meso -2,4-bis(diphenylphosphino)pentane ( meso -bdpp), rac -2,4-bis(diphenylphosphino)pentane ( rac -bdpp)). The productivity in high molecular weight polyketones within 30 min follows the ligand order dppp > meso -bdpp > rac -bdpp. All the methyl precursors exhibit comparable values of both intrinsic activity and energy barriers to migratory insertions [Pd(Me)(CO), Pd(COMe)(C 2 H 4 )] as well as opening of β-chelates [Pd(CH 2 CH 2 C(O)Me)(P−P)] + by CO. It is concluded that the presence and/or stereochemistry of methyl groups in the 1,3-positions of dppp do not exert a significant influence on the propagation rate of the copolymerization reaction in CH 2 Cl 2 . High-pressure NMR studies under catalytic conditions show the occurrence of chain transfer by protonolysis with adventitious water to give μ-hydroxo compounds [Pd(μ-OH)(P−P)] 2 2+ . With time, the bis-chelates [Pd(P−P) 2 ] 2+ are the only species visible on the NMR time scale. Independent copolymerization reactions in CH 2 Cl 2 with either μ-OH or bis-chelate precursors show that both resting states can reenter the catalysis cycle to give alternating polyketones with productivities increasing in the orders [Pd(μ-OH)( rac -bdpp)] 2 2+ < [Pd(μ-OH)( meso -bdpp)] 2 2+ < [Pd(μ-OH)(dppp)] 2 2+, and [Pd( rac -bdpp) 2 ] 2+ < [Pd(dppp) 2 ] 2+ < [Pd( meso -bdpp) 2 ] 2+ . On the basis of the batch reactions and in situ NMR experiments it is suggested that both μ-hydroxo and bis-chelate compounds contribute appreciably to determine the overall productivity of the methyl precursors [Pd(Me)(MeCN)(P−P)] + in CH 2 Cl 2 . The bis-chelates are also active catalysts for the CO/ethene copolymerization in MeOH with productivities that increase in the ligand order dppp ≪ rac -bdpp < meso -bdpp.