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 νC=o for the E-CO copolymer was abnormally low, and there was a monotonic decrease in νC=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.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry