Palladium(ii)-catalyzed copolymerization of styrenes with carbon monoxide: mechanism of chain propagation and chain transfer

Abstract

A mechanistic interpretation of the [(1,10-phenanthroline)Pd(CH₃)(CH₃CN)]⁺[BArF]⁻ (1a) and [(2,2′-bipyridine)Pd(CH₃)(CH₃CN)]⁺[BArF]⁻ (1b) (BArF = 3,5-(CF₃)₂-C₆H₃) catalyzed perfectly alternating copolymerization of styrenes with CO is reported. The copolymerization in CH₂Cl₂ or chlorobenzene has been found to be first order in styrene and inverse first order in CO concentrations. The microscopic steps involved in the catalytic cycle have been studied via low temperature NMR techniques. Palladium alkyl chelate complex [(2,2′-bipyridine)Pd(CHArCH₂C(O)CH₃]⁺[BArF]⁻ (5bσ) and [(2,2′-bipyridine)Pd(η³-CH(CH₂C(O)CH₃)Ar)]⁺[BArF]⁻ (5bπ), existing in equilibrium, were prepared. Treatment of 5σ,π with ¹³CO followed by 4-tert-butylstyrene at −78 ^oC allowed for ¹³C NMR monitoring of the alternating chain growth of a series of palladium acyl carbonyl complexes. The acyl carbonyl species, representing the catalyst resting state, is in equilibrium with a palladium acyl styrene complex. The equilibrium constant, K₄, measured between [(phen)Pd(CO)(C(O)CH₃]⁺[BArF]⁻ (3a) and [(phen)Pd(C(O)CH₃)-(C₆H₅CCH₂)]⁺[BArF]⁻ (8a), was determined to be 2.84 ± 2.8 × 10⁻⁷ at −66 °C. The barrier to migratory insertion in 8a was determined (ΔG^‡ (−66 °C) = 15.6 ± 0.1 kcal mol⁻¹). From the experimentally determined kinetic and thermodynamic data for the copolymerization of styrene with CO a mechanistic model has been constructed. The ability of this model to predict catalyst turnover frequency (TOF) was used as a test of its validity. A series of para-substituted styrenes, p-XC₆H₄CHCH₂ (X = –OCH₃, –CH₃, –H, –Cl), were copolymerized with CO. A Hammett treatment of TOF for the series showed that electron-donating groups increase the rate of copolymerization (ρ^p = −0.8). The ratio of chain transfer to chain propagation was found to increase with styrene concentration and decrease with CO concentration. Polymer end group analysis showed the presence of α, β-enone end groups. The reactivity of model systems, coupled with a study of the effect of added acetonitrile, support a chain transfer mechanism involving β-hydrogen transfer to monomer from a palladium alkyl styrene intermediate.