Palladium(II)-Catalyzed Copolymerization of Carbon Monoxide with Ethylene. Direct Evidence for a Single Mode of Chain Growth

The series of cationic Pd(II) compounds [Pd(PPh₃)_n(CH₃CN)_4-n](BF₄)₂ (n= 1-3), which may be synthesized in situ or separately, were found to catalyze the copolymerization of CO and C₂H₄ at 25 °C and at a combined pressure of as low as 300 psi, in noncoordinating solvents such as CHC1₃ and CH₂C1₂. Higher reaction temperatures were required in coordinating solvents, in the presence of excess PPh₃, or when more basic tertiary phosphines were used instead of PPh3. The ethylene-carbon monoxide copolymer (E-CO copolymer) was a high melting solid having a regular structure with alternating carbon monoxide and ethylene units. The mechanism of the copolymerization is thought to involve a single mode of stepwise chain growth with alternate insertions of carbon monoxide and ethylene into a preformed Pd-alkyl bond. The intermediacy of Pd(II)-alkyl and -acyl species was supported by the observed catalytic activity of Pd(PPh₃)₂(Me)(solv)⁺ and Pd(PPh₃)₂(COMe)(solv)⁺. The use of alcohols as solvents for the copolymerization reaction resulted in the synthesis of polyketo esters of the type RO(-COCH₂CH₂-)_nH. Schultz-Flory plots of the oligomeric polyketo esters formed resulted in straight lines, supporting a stepwise chain growth mechanism. The rate of termination was found to decrease with increasing steric size and decreasing nucleophilicity of the alcohol used. The ν_C=o for the E-CO copolymer was abnormally low, and there was a monotonic decrease in ν_C=o with increasing n for the solid oligomeric polyketo esters. This was tentatively ascribed to the presence of intra- and intermolecular dipolar bonding between carbonyl groups.