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(CH(3))(CH(3)CN)](+)[BArF](-) (1a) and [(2,2'-bipyridine)Pd(CH(3))(CH(3)CN)](+)[BArF](-) (1b) (BArF = 3,5-(CF(3))(2)-C(6)H(3)) catalyzed perfectly alternating copolymerization of styrenes with CO is reported. The copolymerization in CH(2)Cl(2) 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(CHArCH(2)C(O)CH(3)](+)[BArF](-) (5b sigma) and [(2,2'-bipyridine)Pd(eta(3)-CH(CH(2)C(O)CH(3))Ar)](+)[BArF](-) (5b pi), existing in equilibrium, were prepared. Treatment of 5b sigma,pi with (13)CO followed by 4-tert-butylstyrene at -78 degrees C allowed for (13)C 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, K(4), measured between [(phen)Pd(CO)(C(O)CH(3)](+)[BArF](-) (3a) and [(phen)Pd(C(O)CH(3))-(C(6)H(5)C=CH(2))](+)[BArF](-) (8a), was determined to be 2.84 +/- 2.8 x 10(-7) at -66 degrees C. The barrier to migratory insertion in 8a was determined (DeltaG(double dagger) (-66 degrees 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-XC(6)H(4)CH=CH(2) (X = -OCH(3), -CH(3), -H, -Cl), were copolymerized with CO. A Hammett treatment of TOF for the series showed that electron-donating groups increase the rate of copolymerization (rho 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 alpha, beta-enone end groups. The reactivity of model systems, coupled with a study of the effect of added acetonitrile, support a chain transfer mechanism involving beta-hydrogen transfer to monomer from a palladium alkyl styrene intermediate.

Palladium complexes with simple iminopyridines as catalysts for polyketone synthesis

“…The effect of the ligand nature on the productivity is reflected on the molecular weight values of the polyketones, that increase in the order 1b < 2b < 3b < 4b regardless of the vinyl arene, thus indicating that the ratio between the propagation and the termination rates increases in the same order. The highest values of Mw obtained with catalyst having ligand 4 are in agreement with the fact that the termination step occurs with an associative mechanism assisted by an incoming vinyl arene molecule (Scheme 4b), 45 that is disfavored by the steric hindrance created by the methyl groups on ligand 4.…”

“…The termination reaction is the β-hydrogen elimination leading to the formation of the Pd-H intermediate (v), on which either catalyst decomposition, through the dissociation of the Ndonor ligand, or the growth of a new polymeric chain can occur (Scheme 4b). 26,44,45 Scheme 4. a) The propagation and b) the termination steps of the copolymerization catalytic cycle.…”

α-Substituted Benzylic Complexes of Palladium(II) as Precursors of Palladium Hydrides

“…The fact that insertion of carbon monoxide into Pd-benzyl bonds is faster than β-H elimination makes carbonylation reactions of substituted benzylic derivatives possible, including copolymerization of Scheme 5. Carbonylation reaction of complex 1. styrene and CO. 12 The preferred insertion of CO vs. palladium migration has also been observed before for η 2 -σ alkenyl complexes of palladium. 13 Trap of the Pd-H generated from the benzylic complexes with dienes.…”

(Iminopyridine)Pd (II) complexes as versatile catalysts for copolymerization and terpolymerization of vinyl arene, ethylene, and carbon monoxide

“…The mechanism of polyketone formation using vinyl arenes favors the coordination of electron‐rich alkenes, as opposed to electron‐poor alkenes, with palladium ions, which results in higher catalytic activities. [ 39–41 ] By comparison, polymerization reactions conducted with p ‐methoxystyrene also exhibited high catalytic activity and vinyl arene insertion (entries 3 and 6 of Table 3). Other groups have reported that the copolymerization of p ‐methoxystyrene and CO is unfavorable and that homopolymerization of p ‐methoxystyrene occurs in the presence of CO. [ 38,41 ] Based on 1 H NMR analyses of the polymers of entry 3 (Table 3), polymers synthesized from p ‐methoxystyrene and CO are estimated to be a mixture of polyketone (minor component) and homopolymers of p ‐methoxystyrene (major component).…”