Structure-reactivity investigations of ethylene polymerization and copolymerization using zwitterionic nickel and palladium complexes, and copolymerization of carbon dioxede and epoxides using salen chromium chloride complexes

Historically, late transition metals were not considered good candidates for producing high molecular weight polymers due to their rapid β-hydride elimination. Brookhart and coworkers discovered the use of late transition metal Ni(II) and Pd(II) diimine catalysts systems in olefin polymerization and copolymerization. The key to the polymerization activity of Brookhart's late transition metal catalysts are the bulky ortho substituents (such as 2,6 diisopropyl on the aryl group) in the catalyst system. A series of new nickel precatalysts have been synthesized to study the relationship between ligand design and reactivity. Investigations demonstrate that tert-Bu is an effective bulky group for blocking the axial site of the nickel center due to a shortened nickel-bulky group bond distance. The tert-Bu is a ligand that also increases catalyst activity and shortens the activation time. When the cocatalyst attaches to the carbonyl group, the shorter the C-N bond, the higher the activities and the molecular weights. Substituting electron donating groups on the ligand tend to increase activity and the lifetime of the precatalysts.; Jacobsen's chiral Cr(III) chloride salen type complex [salen = N,N ′-bis (3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediimine] is an excellent catalyst for asymmetric ring opening of epoxides and for copolymerization with CO₂ and cyclohexene oxide. In order to optimize polymerization activity, a series of new salenCrCl catalysts have been synthesized to study the relationship between ligand design and copolymerization activity of CO ₂ and cyclohexene oxides. Replacing 1,2-cyclohexanediimine in Jacobsen's Cr(III) chloride salen type complex with 1,2-ethylenediamine yielded compound [N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-ethylenediamine Cr(III)Cl salen] 16, which has more than double the copolymerization activity of Jacobson's catalyst and a much shorter activation time (12 min. vs. >90 min.) under the same reaction conditions. The presence of the neutral Lewis base (N-methyl imidazole) enhances 16 copolymerization activity, but excess N-MeIm loading decreases activity and increases activation time. Compound 16 is also an effective precatalyst for copolymerization of CO₂ and different epoxides such as 2-(3,4-epoxycyclohexyl)ethyl-trimethoxysilane, 1,2,3,4-tetrahydro-2,3-epoxynaphthalene, α-pinene oxide and limonene oxide. The diversity of these epoxides' physical properties, suggests a promising new avenue in catalyst design for producing novel copolymers.