Possible side reactions due to water in emulsion polymerization by late transition metal complexes II:: Deactivation of the catalyst by a Wacker-type reaction

The transition metal catalyzed ethylene polymerization in water suspensions has been increasingly successful in the last couple of years. Water, however, adversely affects the polymerization process by deactivating the catalyst in a Wacker-type reaction. The cationic Pd(II)-diimine Brookhart catalyst 1 is studied here by a combination of quantum mechanics and molecular mechanics to determine the nature of the decomposition reaction. The study considers the decomposition process to take place in two stages. In the first stage, a coordinated olefin is attacked by a hydroxide group to ultimately produce a beta-hydroxy-ethyl complex 2. The second step represents a decomposition to acetaldehyde and Pd(O), the latter in the form of palladium black. For the attack of OH on the coordinated ethylene in 1, both an internal path involving transfer from a palladium-coordinated OH group produced from hydrolysis of a Pd-CH3 bond and an external path based on attack of H2O on coordinated ethylene were considered. Both paths are found to be feasible. For the second stage the most promising decomposition mode involves isomerization of 2 to the a-hydroxy-ethyl complex 5 followed by abstraction of a proton from the C-OH link to produce the Pd(O)-eta(2)-acetaldehyde complex 12. Finally, 12 releases acetaldehyde under the deposition of Pd(O) as palladium black.