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)]PF6 are effective catalyst precursors for the alternating copolymerization of carbon monoxide with ethene in CH2Cl2 (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)(C2H4)] as well as opening of â-chelates [Pd- (CH2CH2C(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 CH2Cl2. High-pressure NMR studies under catalytic conditions show the occurrence of chain transfer by protonolysis with adventitious water to give í-hydroxo compounds [Pd(í-OH)(P-P)]2 2+. With time, the bischelates [Pd(P-P)2]2+ are the only species visible on the NMR time scale. Independent copolymerization reactions in CH2Cl2 with either í-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(í-OH)(rac-bdpp)]2 2+ < [Pd(í-OH)(meso-bdpp)]2 2+ < [Pd(í-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 í-hydroxo and bis-chelate compounds contribute appreciably to determine the overall productivity of the methyl precursors [Pd(Me)(MeCN)(P-P)]+ in CH2Cl2. 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.