The alternating copolymerization of carbon monoxide with ethene is efficiently accomplished using [PdCl2(dppf)] as catalyst precursor in H2O-HCOOH as a solvent. At 90 degrees C, under 4.5 MPa of total pressure with the monomers in the ratio 1/1, the productivity passes through a maximum of 18,100 gPK(gPd h)(-1), when the concentration of H2O is 50 mol.%, in which case the limiting viscosity number of the polyketone is 0.51 dLg(-1). A much higher LVN is obtainable at a lower temperature. A satisfactory balance between productivity and LVN is achievable under higher pressure (10,100 gPK(gPd h)(-1), LVN = 6.5 dL g(-1) under 9.0 MPa, CO/ethene = 1/1, at 45 degrees C, H2O 40 mol.%). The influence of the operative conditions on productivity and LVN is discussed. The kinetics under different pressure of CO, keeping constant that of ethene, and the kinetics under constant total pressure at different CO/ethene ratios show that the copolymerization rates are virtually constant with time within 1 h reaction. The PK presents only keto-end groups (-COCH2CH3) and catalysis occurs with the concomitant evolution of CO2. NMR evidences show that the chloride-precursor is transformed, through the interaction of H2O with CO on the metal center, into a Pd-hydride species that starts the catalysis. A catalytic cycle is proposed. (C) 2012 Elsevier B.V. All rights reserved.