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

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 polymer. The nickel α-diimine-catalyzed copolymerizations produce high molecular weight copolymers with structures that vary from nearly linear to moderately branched. Both four- and five-membered chelates are catalyst resting states but are rapidly opened by ethylene, and thus turnover frequencies are only slightly reduced relative to ethylene homopolymerizations. Finally, a convenient and practical nickel-based system has been developed for the efficient synthesis of this copolymer which can be cross-linked to form PEX- b, a commercial PE plastic used for hot water plumbing pipes and power cable coatings.