In MeOH, [Pd(AcO)(2)(dppp)] becomes an efficient catalyst for the perfectly alternated CO-C2H4 copolymerization when used in combination of a relatively large amount of CH3COOH (CH3COOH/Pd, ca. 2.0 x 10(4)). Under 4.56 MPa (CO/C2H4 = 1/1), at 90 degreesC, 7.5 kg polyketone/g Pd*h are obtained. A significantly higher productivity is obtained in H2O-CH3COOH, in place of MeOH (27.5 kg polymer/g Pd*h under 4.56 MPa (CO/C2H4 = 1/1), 90 degreesC, H2O = 37% (mol/mol)). Under these conditions the catalyst undergoes only a minor deactivation with time as after 3 h the productivity decreases only by ca. 10%. The C-13 NMR analysis shows that [Pd(AcO)(2)(dppp)] in CH3COOH-H2O yields a perfectly alternated polyketone of ca. 8000 g/mol bearing only ketonic end groups. During the course of the copolymerization reaction CO2 forms in significant amount. These findings suggest that the insertion Of C2H4 into Pd-H bond starts the catalytic cycle, and that alternating insertions of the monomers followed by protonolysis of a Pd-C bond of a Pd-CH2-CH2-(CO-C2H4)(n)-H growing chain yields the polymer with only keto ending groups and a Pd-OH species; a reaction closely related to the WGS reproduces the starting Pd-hydride; deprotonation of this species to inactive Pd(0) is prevented by the acid. (C) 2003 Elsevier Science B.V. All rights reserved.