Carbon monoxide-ethylene copolymerization catalyzed by a Pd(AcO)(2)/dppp/TsOH system: The promoting effect of water and of the acid

For the title copolymerization the catalyst productivity (g-polymer/g-Pd . h) is significantly influenced by the presence of water and of the acid as it passes through a maximum upon increasing concentration of H2O and of TsOH. In the presence of 450 ppm of H2O, the maximum productivity is ca. 3.7 times higher than when the copolymerization is carried out in the presence of 1% of trimethylorthoformate, used as H2O scavenger in MeOH as solvent, at 90 degrees C, under 45 atm of total pressure, employing the catalyst precursor in the molar ratio Pd/dppp/TsOH 1/1/2 ([Pd] 5.6 x 10(-5) mol . l(-1)). Under similar conditions, but under 60 atm of the two monomers, in the presence of 900 ppm of H2O and when employing an excess of the acid (TsOH/Pd 6.4) the productivity reaches a maximum of ca. 11500 g-polymer/g-Pd . h, which is 1.4 times higher than that obtained when the TsOH/Pd ratio is 2/1. The promoting effect of H2O is ascribed to the possibility that a higher concentration of active Pd-H species, which are proposed to initiate the catalytic process through the insertion of the olefin into a Pd-H bond, is achieved through the interaction_of carbon monoxide with water on the metal center, via a reaction closely related to the water gas shift reaction. It is also proposed that the promoting effect of the acid is due to the reactivation of inactive Pd(O) species, which inevitably form under the reducing reaction conditions, with formation of active Pd-H species. When the copolymerization is carried out in the presence of benzoquinone (BQ), either under the reaction conditions in which the productivity reaches a maximum or under unfavorable conditions, that is, in the presence of low or relatively high concentrations of water, the productivity has an average value of ca. 7000 g-polymer/g-Pd . h. Since it was found by other research groups that in the presence of BQ the polymer takes origin mainly through the insertion of CO into a Pd-OCH3 species whose formation is favored in the presence of BQ, the findings presented above give further support to the suggestion that the promoting effect of H2O and of TsOH are due to the possibility that, when present in appropriate amounts, they favour the formation of Pd-II species which start the catalytic cycle.