ABSTRACT
The study of the transition metal-catalyzed alternating copolymerization of ethylene with CO dates back to the 1950s.1 While the polymer ethylene/CO ratio is higher than one when the copolymer is produced via a radical process, a completely alternating copolymer is produced when transition metal complexes, particularly palladium species, are employed as precatalysts.2 Because the melting point of ethylene/CO alternating copolymer is almost as high as its decomposition temperature, decreasing its melting point is essential to obtain a melt-processable material. Thus, for practical usage, a termonomer like propylene or a higher 1-alkene is typically added. The catalytic systems most frequently used for ethylene/propylene/CO terpolymerization and propylene/CO copolymerization are palladium complexes of the type [Pd(LˆL′)(S)2][X]2, where LˆL′ (L = or = L′) is a cis-chelating bis(phosphine) ligand such as 1,3-propanediyl-bis(diphenylphosphine) (dppp), S is a solvent molecule, and X is an anion with low coordination capability. In addition, an oxidant is often added to the dicationic palladium complexes in order to minimize the formation of inactive reduced palladium species.3,4 On the other hand, for styrene/CO copolymerization or for ethylene/styrene/CO terpolymerization, bis-sp2-nitrogen ligands, such as bipyridine or diimines, are suitable as the LˆL′ in [Pd(LˆL′)(S)2][X]2.
