Branching Regulation in Olefin Polymerization via Lewis Acid Triggered Isomerization of Monomers

Deciphering the Olefin Isomerization-Polymerization Paradox of Palladium(II) Diimine Catalysts: Discovery of Simultaneous and Independent Pathways of Olefin Isomerization and Living Polymerization

This work elucidates a long-standing unexplained paradox commonly observed within the polymerization of α-olefin using palladium (Pd)(II)-diimine catalysts, in which isomerization and living polymerization of α-olefins are both observed. With a classical mechanistic understanding of these complexes, this behavior is often dismissed and interpreted as experimental error. Herein, we present a comprehensive mechanistic investigation into this phenomenon that supports the existence of a novel mechanistic pathway for Pd(II)-diimine complexes. Part one of the mechanistic study lays the foundation of the proposed mechanism, in which neutral Pd(II)-diimine complexes were found to exhibit a moderate to good catalytic activity for olefin isomerization of α-olefins despite the established notion that catalyst activation is required. Extensive experimental and computational studies reveal the possibility of a partial dissociation of the diimine ligand, which frees up one coordination site and enables coordination-insertion. This finding is significant as the coexistence of two reactive coordination sites at the palladium center becomes a valid proposal for the activated cationic Pd(II)-diimine complexes. In part two, we examined and validated the simultaneously observed α-olefin isomerization and living polymerization using the cationic Pd(II)-diimine catalyst, which supports the presence of two independent reaction pathways of isomerization and polymerization, respectively. Moreover, the addition of a strong Lewis acid, such as AlCl₃, accelerates the ligand dissociation and the consequential isomerization as it weakens the palladium-nitrogen bond through competitive binding. In part three, Lewis acid-triggered olefin isomerization-polymerization is employed to prepare living olefinic block copolymers and further synthesize novel polyolefin-polar block copolymers with unique architectures, distinct levels of branching, crystallinity, and polar functionality in a one-pot manner.

Linear Block Copolymer Synthesis

Block copolymers form the basis of the most ubiquitous materials such as thermoplastic elastomers, bridge interphases in polymer blends, and are fundamental for the development of high-performance materials. The driving force to further advance these materials is the accessibility of block copolymers, which have a wide variety in composition, functional group content, and precision of their structure. To advance and broaden the application of block copolymers will depend on the nature of combined segmented blocks, guided through the combination of polymerization techniques to reach a high versatility in block copolymer architecture and function. This review provides the most comprehensive overview of techniques to prepare linear block copolymers and is intended to serve as a guideline on how polymerization techniques can work together to result in desired block combinations. As the review will give an account of the relevant procedures and access areas, the sections will include orthogonal approaches or sequentially combined polymerization techniques, which increases the synthetic options for these materials.

Parts-per-million of ruthenium catalyze the selective chain-walking reaction of terminal alkenes

The chain–walking of terminal alkenes (also called migration or isomerization reaction) is currently carried out in industry with unselective and relatively costly processes, to give mixtures of alkenes with significant amounts of oligomerized, branched and reduced by–products. Here, it is shown that part–per–million amounts of a variety of commercially available and in–house made ruthenium compounds, supported or not, transform into an extremely active catalyst for the regioselective migration of terminal alkenes to internal positions, with yields and selectivity up to >99% and without any solvent, ligand, additive or protecting atmosphere required, but only heating at temperatures >150 °C. The resulting internal alkene can be prepared in kilogram quantities, ready to be used in nine different organic reactions without any further treatment.

The chain-walking of terminal alkenes is an industrially relevant reaction. Here, the authors show that part-per-million amounts of a variety of ruthenium compounds catalyze the reaction in yields and selectivity up to >99%, without any solvent or additive.

Selective branch formation in ethylene polymerization to access precise ethylene-propylene copolymers

Polyolefins with branches produced by ethylene alone via chain walking are highly desired in industry. Selective branch formation from uncontrolled chain walking is a long-standing challenge to generate exclusively branched polyolefins, however. Here we report such desirable microstructures in ethylene polymerization by using sterically constrained α-diimine nickel(II)/palladium(II) catalysts at 30 °C–90 °C that fall into industrial conditions. Branched polyethylenes with exclusive branch pattern of methyl branches (99%) and notably selective branch distribution of 1,4-Me₂ unit (86%) can be generated. The ultrahigh degree of branching (>200 Me/1000 C) enables the well-defined product to mimic ethylene-propylene copolymers. More interestingly, branch distribution is predictable and computable by using a simple statistical model of p(1-p)ⁿ (p: the probability of branch formation). Mechanistic insights into the branch formation including branch pattern and branch distribution by an in-depth density functional theory (DFT) calculation are elucidated.

Selective branch formation from uncontrolled chain walking is a longstanding challenge to generate exclusively branched polyolefins. Here the authors report such desirable microstructures in ethylene polymerization enabled by a nickel catalyst at 30 °C–90 °C that fall into industrial conditions and mimic ethylene-propylene copolymers.

Aliphatic Anion Exchange Ionomers with Long Spacers and No Ether Links by Ziegler-Natta Polymerization: Properties and Alkaline Stability

In this work we report the synthesis of poly(vinylbenzylchloride-co-hexene) copolymer grafted with N,N-dimethylhexylammonium groups to study the effect of an aliphatic backbone without ether linkage on the ionomer properties. The copolymerization was achieved by the Ziegler-Natta method, employing the complex ZrCl4 (THF)2 as a catalyst. A certain degree of crosslinking with N,N,N',N'-tetramethylethylenediamine (TEMED) was introduced with the aim of avoiding excessive swelling in water. The resulting anion exchange polymers were characterized by 1H-NMR, FTIR, TGA, and ion exchange capacity (IEC) measurements. The ionomers showed good alkaline stability; after 72 h of treatment in 2 M KOH at 80 °C the remaining IEC of 76% confirms that ionomers without ether bonds are less sensitive to a SN2 attack and suggests the possibility of their use as a binder in a fuel cell electrode formulation. The ionomers were also blended with polyvinyl alcohol (PVA) and crosslinked with glutaraldehyde. The water uptake of the blend membranes was around 110% at 25 °C. The ionic conductivity at 25 °C in the OH- form was 29.5 mS/cm.

Polar additive triggered chain walking copolymerization of ethylene and fundamental polar monomers

The use of a polar additive efficiently triggers chain walking copolymerization of ethylene with a broad scope of fundamental polar monomers, which is long-sought in an α-diimine Pd(ii) system.

Transition metal-catalyzed alkene isomerization as an enabling technology in tandem, sequential and domino processes

One-pot reactions based on catalytic isomerization of alkenes not only offer the inherent advantages of atom-, step- and redox-economy but also enable the preparation of value-added products that would be difficult to access by conventional methods.

Custom-made polar monomers utilized in nickel and palladium promoted olefin copolymerization

In this review, the functions of custom-made polar monomers are insightfully emphasized in the preparation of functional polyolefins.