Exploring the effects of phenolic compounds on bis(imino)pyridine iron-catalyzed ethylene oligomerization

Polymer formation could be efficiently retarded with the activation of phenol-modified MAOs in the iron-catalyzed oligomerization of ethylene.

Post-metallocenes in the industrial production of polyolefins

Research on "post-metallocene" polymerization catalysis ranges methodologically from fundamental mechanistic studies of polymerization reactions over catalyst design to material properties of the polyolefins prepared. A common goal of these studies is the creation of practically useful new polyolefin materials or polymerization processes. This Review gives a comprehensive overview of post-metallocene polymerization catalysts that have been put into practice. The decisive properties for this success of a given catalyst structure are delineated.

Frustrated Lewis Pairs: from dihydrogen activation to asymmetric catalysis

The non-self-quenched property of Frustrated Lewis Pairs (FLPs) contradicts the classical Lewis acid-base theory, but this peculiarity offers unprecedented possibilities for the activation of small molecules. Among all of their fascinating applications, FLP mediated hydrogen activation and the associated catalytic hydrogenations are currently considered as the most intriguing illustration of their reactivity. The FLPs enabled the catalytic reduction of a wide range of substrates with molecular hydrogen and tuning of the structural properties of the FLP partners allowed broadening of the substrate scope. Based on detailed mechanistic knowledge, FLP based asymmetric hydrogenation of various substrates could be achieved with high enantioselectivities. More importantly, FLP based enantioselective catalysis is not limited to the field of asymmetric hydrogenation, and other exciting catalytic applications have already appeared.

Polymerization by classical and frustrated Lewis pairs

Main-group classical and frustrated Lewis pairs (CLPs and FLPs) comprising strong Lewis acids (LAs) and strong Lewis bases (LBs) are highly active for polymerization of conjugated polar alkenes, affording typically high molecular weight polymers with relatively narrow molecular weight distributions. Especially effective systems are the Lewis pairs (LPs) consisting of the strong LA Al(C6F5)3 and strong LBs, such as achiral phosphines and chiral chelating diphosphines, N-heterocyclic carbenes, and phosphazene superbases, for polymerization of methacrylates and acrylamides as well as renewable α-methylene-γ-butyrolactones. Chain initiation involves cooperative addition of LPs to the monomer to generate zwitterionic active species, and chain propagation proceeds via a bimetallic, activated-monomer addition mechanism. Transition metal nucleophile/electrophile pairs comprising neutral metallocene bis(ester enolate)s and strong LAs E(C6F5)3 (E = Al, B) generate two drastically different polymerization systems, depending on the LA. With E = Al, catalyst activation and chain initiating events lead to dually active ion-pairs, thereby effecting ion-pairing polymerization that affords polymers with unique stereo-multiblock microstructures. With E = B, on the other hand, the FLP-induced catalyst activation generates metallacyclic cations paired with the hydridoborate anion [HB(C6F5)3](-); uniquely, such ion-pairs effect catalytic polymerization of conjugated polar alkenes by an H-shuttling mechanism, with the cation catalyzing chain growth and the anion promoting chain transfer by shuttling the hydride between the cation and anion centers through the neutral borane.