Ortho-Phosphinobenzenesulfonate: A Superb Ligand for Palladium-Catalyzed Coordination–Insertion Copolymerization of Polar Vinyl Monomers

Abstract

Ligands, Lewis bases that coordinate to the metal center in a complex, can completely change the catalytic behavior of the metal center. In this Account, we summarize new reactions enabled by a single class of ligands, phosphine-sulfonates (ortho-phosphinobenzenesulfonates). Using their palladium complexes, we have developed four unusual reactions, and three of these have produced novel types of polymers. In one case, we have produced linear high-molecular weight polyethylene, a type of polymer that group 10 metal catalysts do not typically produce. Secondly, complexes using these ligands catalyzed the formation of linear poly(ethylene-co-polar vinyl monomers). Before the use of phosphine-sulfonate catalysts, researchers could only produce ethylene/polar monomer copolymers that have different branched structures rather than linear ones, depending on whether the polymers were produced by a radical polymerization or a group 10 metal catalyzed coordination polymerization. Thirdly, these phosphine-sulfonate catalysts produced nonalternating linear poly(ethylene-co-carbon monoxide). Radical polymerization gives ethylene-rich branched ethylene/CO copolymers copolymers. Prior to the use of phosphine-sulfonates, all of the metal catalyzed processes gave completely alternating ethylene/carbon monoxide copolymers. Finally, we produced poly(polar vinyl monomer-alt-carbon monoxide), a copolymerization of common polar monomers with carbon monoxide that had not been previously reported. Although researchers have often used symmetrical bidentate ligands such as diimines for the polymerization catalysis, phosphine-sulfonates are unsymmetrical, containing two nonequivalent donor units, a neutral phosphine, and an anionic sulfonate. We discuss the features that make this ligand unique. In order to understand all of the new reactions facilitated by this special ligand, we discuss both the steric effect of the bulky phosphines and electronic effects. We provide a unified interpretation of the unique reactivity by considering of the net charge and the enhanced back donation in the phosphine-sulfonate complexes.

Palladium-Catalyzed Homo- and Copolymerization of Polar Monomers: Synthesis of Aliphatic and Aromatic Polymers

“…The use of palladium catalysts bearing a bidentate phosphinesulfonate ligand, with a typical structure of o-(Ar 2 P)C 6 H 4 SO 3 ¹ , 11 led to this breakthrough. 2,9 One of the key characteristics of the catalysts is the ability to produce highly linear polyethylene main chains, in sharp contrast to conventional palladium/α-diimine catalysts, which generally produce branched polyethylenes. 17,18 Theoretical mechanistic studies of ethylene polymerization catalyzed by the palladium/phosphinesulfonate system 19,20 revealed that the unsymmetric ligand structure bearing a strong σ-donor and a weak σ-donor plays an indispensable role, suppressing chaintransfer reactions caused by β-hydride elimination.…”

Supported Single-Site Organometallic Catalysts for the Synthesis of High-Performance Polyolefins

“…In addition, in early 2000, Drent and co-workers [54] at Shell reported that neutral Pd(II) catalysts generated in situ from phosphonium-sulfonate ligands copolymerize ethylene and methyl acrylate to produce linear copolymers (Fig. 5) [55]. Especially in the case of group 4, these new catalysts and cocatalyst/activators have achieved great success in the production of advanced polyolefin materials [4].…”

“…Especially in the case of group 4, these new catalysts and cocatalyst/activators have achieved great success in the production of advanced polyolefin materials [4]. The new systems are able to control product molecular weight, polydispersity, comonomer enchainment level and pattern, the tacticity of poly(a-olefins) [17,56,57], copolymerization of olefins with polar comonomers (group 10) [54,55,[58][59][60][61][62], and the catalytic synthesis of block copolymers by processes such as chain shuttling polymerization [63][64][65][66][67].…”

A Greener Approach for Synthesis of Functionalized Polyolefins by Introducing Reactive Functionality into Ethylene Copolymers

“…Preparation of the copolymer with high olefin (ethylene, propylene) content by adopting the living radical copolymerization is difficult, because the equilibrium shift to the left (dormant) upon increasing olefin contents. The direct copolymerization using Pd catalysts containing so called "posphino-sulfonate" ligands afforded low molecular weights, and more efficient catalysts are thus highly desired at this stage [25] [26]. The conventional post-modification approach (introduced as the second approach, Scheme 2), still faces difficulties, because only limited chemistry like a free radical grafting reaction is available to activate the completely saturated aliphatic molecular structure (Scheme 2(a)) [27] [28].…”