Exploring Ethylene/Polar Vinyl Monomer Copolymerizations Using Ni and Pd α-Diimine Catalysts

Abstract

The most ubiquitous polymer, polyethylene (PE), is produced either through a radical-initiated process or, more commonly, through a coordination/insertion process employing early transition metal catalysts, particularly titanium- and chromium-based systems. These oxophilic early metal catalysts are not functional-group-tolerant and thus cannot be used to synthesize copolymers of ethylene and polar vinyl monomers such as alkyl acrylates and vinyl acetate. Such PE copolymers have enhanced properties relative to PE and are made through radical polymerization processes, requiring exceptionally high pressures and temperatures. Copolymerizations of polar vinyl monomers with ethylene using more functional group-tolerant late metal catalysts potentially offer an attractive alternative for generating such value-added copolymers since ligand variations may provide more control of polymer microstructures and milder reaction conditions would apply. This Account describes our efforts, particularly through detailed mechanistic studies, to probe and develop this potential using Pd(II) and Ni(II) α-diimine catalysts. To inform discussions of the copolymerizations, we briefly review key aspects of ethylene homopolymerizations using these diimine catalysts. These include ligand designs that incorporate axial blocking groups that retard chain transfer and promote production of a high polymer rather than an oligomer. These ligand designs also lead to unique branched polyethylenes via migration of the metal along the chain ("chain-walking") prior to insertion. Mechanistic investigations of copolymerizations of ethylene with polar vinyl monomers using the diimine complexes have revealed several impediments to developing practical catalysts: (1) The polar group of the comonomer can coordinate strongly to the metal center, blocking coordination of ethylene. (2) Weak binding affinity of the polar monomer relative to ethylene can result in very low levels of comonomer incorporation. (3) A metal alkyl chain bearing a heteroatom, X, on the β-carbon atom can undergo β-X elimination leading to deactivation of the catalyst. (4) Stable chelate formation following insertion of a polar comonomer can greatly retard the rate of chain growth. (5) A metal alkyl chain bearing an electron-withdrawing heteroatom at the □-carbon atom can result in a high insertion barrier. A patent disclosure by the DuPont Versipol group and our extensive mechanistic studies reveal that, remarkably, vinyl trialkoxysilanes are ideal comonomers and circumvent all of the impediments noted above. The Pd-catalyzed copolymerization of vinyl trialkoxysilanes with ethylene produces highly branched, low molecular weight copolymers with activities comparable to those of analgous ethylene homopolymerizations. A 1,2- insertion of the vinyl silane results in the formation of a five-membered Pd-O(R)Si chelate which is readily opened by ethylene and thus does not reduce the rate of chain growth. β-Silyl elimination results in chain transfer and accounts for the lower molecular weight p...

C−H Activation and Olefin Insertion in d8 and d0 Complexes: Same Elementary Steps, Different Electronics

“…In both cases, olefin coordination leads to a lowering of the π *(M−C) orbital, leading to a deshielding for d 0 systems, but to a shielding for d 8 systems. Notably, the inverse polarization of the methyl group in d 8 and d 0 systems entails a fundamentally different reactivity, as evidenced in the opposing regioselectivities in propylene insertion, or the reactivity and regioselectivity observed in the reaction of acrylates with d 8 systems …”

“…[21][22][23][24][25][26] The functional group tolerance of these d 8 complexes has allowed for the copolymerization of unfunctionalized α-olefins with polar vinyl monomers such as methyl acrylate. [27][28][29][30][31] Furthermore, it is also noteworthy that insertion of propylene and methyl acrylate preferentially occurs in a 2,1-fashion, [21,28,32] that is opposite to the observed regioselectivity for d 0 systems and is often followed by isomerization through chain walking. [33] In analogy to d 0 catalysts, CÀ H bond activation has also been observed for d 8 systems: Pd(II) polymerization catalysts are prone to deactivation through intramolecular ligand CÀ H activation.…”

“…In particular, cationic α ‐diimine Ni(II) and Pd(II) methyl complexes and related complexes have been developed as alternative polymerization catalysts to traditional d 0 metallocene or Ziegler ‐type systems, with the advantage of being tolerant towards olefins bearing polar functional groups . The functional group tolerance of these d 8 complexes has allowed for the copolymerization of unfunctionalized α ‐olefins with polar vinyl monomers such as methyl acrylate . Furthermore, it is also noteworthy that insertion of propylene and methyl acrylate preferentially occurs in a 2,1‐fashion, that is opposite to the observed regioselectivity for d 0 systems and is often followed by isomerization through chain walking .…”

Synthesis of Poly(Arylene Alkenylene)s by Pd‐Catalyzed Three‐Component Coupling Polycondensation of Diiodoarenes, Non‐Conjugated Dienes, and Nucleophiles that Involves Chain Walking Isomerization

“…Pd‐diimine complexes were reported to catalyze polymerization of ethylene, α‐olefins, and cycloolefins to give the corresponding polymers . The polymer growth is accompanied by rapid olefin migration, named as chain walking, which is caused by repetitive β‐hydrogen elimination and reinsertion of the vinyl group‐terminated polymer molecules.…”

Direct and Tandem Routes for the Copolymerization of Ethylene with Polar Functionalized Internal Olefins

“…In comparison, fundamental polar monomers with polar functional groups directly attached to the double bond are industrially more relevant and are more difficult to handle during copolymerization reactions.T he seminal works by Brookhart et al in 1996 demonstrated the copolymerization of ethylene with some acrylates and vinyl ketone comonomers. [17] After two decades of extensive research, [18][19][20][21] the Brookhart-type catalysts were only suitable for af ew other fundamental polar monomers. [22][23][24][25] In 2002, Drent and co-workers reported another major breakthrough in this field.…”