Ni(II) α-Diimine-Catalyzed α-Olefins Polymerization: Thermoplastic Elastomers of Block Copolymers

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.

π-Carbazolyl supported bis(alkyl) complexes of Sc, Y and La for α-olefin polymerization and hydrogenation

A series of new half-sandwich bis(alkyl) rare-earth metal complexes coordinated by a sterically demanding 1,3,6,8-tetra-tert-butyl-carbazol-9-yl ligand [tBu₄Carb]La(CH₂C₆H₅)₂(THF) (1-La), [tBu₄Carb]Ln(o-NMe₂C₆H₄CH₂)₂ (Ln = Sc (2-Sc), Y (2-Y), La (2-La), [tBu₄Carb]Ln(CH₂SiMe₃)₂(THF) (Ln = Sc (3-Sc), Y (3-Y)), were synthesized. 1-La, 2-La, and 2-Y were prepared by an alkane elimination protocol, while 2-Sc, 3-Sc, and 3-Y became accessible only when salt metathesis reactions of tBu₄CarbK with R₂Ln(THF)_n⁺[BPh₄]^- were employed. X-ray analysis revealed that in all complexes the carbazolyl ligand exhibits π-coordination with metal ions. 2-Sc and 3-Sc when activated with [Ph₃C][B(C₆F₅)₄] demonstrate excellent activity in α-olefin (octene-1, nonene-1, decene-1 and 1,1-diphenyl-but-1-ene) polymerization. When H₂ was used as a chain transfer agent (1 bar, rt) in the presence of 3-Sc/[Ph₃C][B(C₆F₅)₄] or 2-Y, 2-La olefin hydrogenation occurred with quantitative conversion.

Soft, Formstable (Co)Polyester Blend Elastomers

High crystallization rate and thermomechanical stability make polylactide stereocomplexes effective nanosized physical netpoints. Here, we address the need for soft, form-stable degradable elastomers for medical applications by designing such blends from (co)polyesters, whose mechanical properties are ruled by their nanodimensional architecture and which are applied as single components in implants. By careful controlling of the copolymer composition and sequence structure of poly[(L-lactide)-co-(ε-caprolactone)], it is possible to prepare hyperelastic polymer blends formed through stereocomplexation by adding poly(D-lactide) (PDLA). Low glass transition temperature T_g ≤ 0 °C of the mixed amorphous phase contributes to the low Young’s modulus E. The formation of stereocomplexes is shown in DSC by melting transitions T_m > 190 °C and in WAXS by distinct scattering maxima at 2θ = 12° and 21°. Tensile testing demonstrated that the blends are soft (E = 12–80 MPa) and show an excellent hyperelastic recovery R_rec = 66–85% while having high elongation at break ε_b up to >1000%. These properties of the blends are attained only when the copolymer has 56–62 wt% lactide content, a weight average molar mass >140 kg·mol⁻¹, and number average lactide sequence length ≥4.8, while the blend is formed with a content of 5–10 wt% of PDLA. The devised strategy to identify a suitable copolymer for stereocomplexation and blend formation is transferable to further polymer systems and will support the development of thermoplastic elastomers suitable for medical applications.

Propylene homopolymerization and copolymerization with ethylene by acenaphthene-based α-diimine nickel complexes to access EPR-like elastomers

A series of acenaphthene-based α-diimine nickel complexes were synthesized and subsequently used for accessing branched EPR-like elastomers with different compositions and chain structures.

Concerted steric and electronic effects on α-diimine nickel- and palladium-catalyzed ethylene polymerization and copolymerization

For transition metal-based olefin polymerization catalysts, ligand steric and electronic effects can strongly influence important catalytic properties. However, the simultaneous tuning of both steric and electronic effects has not been explored in most of the previous studies. In this contribution, a strategy to tune the ligand electronic and steric effects in a concerted fashion is reported. In such a system, both dibenzhydryl groups and multiple methoxy/fluoro groups were installed in α-diimine ligands. In addition to strongly influencing ligand electronics, the methoxy/fluoro groups can interact with the dibenzhydryl groups and efficiently increase ligand sterics. In ethylene polymerization, this concurrent tuning of electronic and steric effects can lead to simultaneous enhancement of several parameters (activity, stability, polymer molecular weight, melting point, branching density) for both the nickel and palladium catalysts. The effectiveness of this strategy is highly attractive for future studies in other catalytic systems.

Living coordination–insertion copolymerization of 1-hexene and ligated α-olefins using an α-diimine nickel catalyst and preparation of metal–ligand coordination crosslinked polymers

Metal–ligand coordination crosslinked polymers were prepared by coordination–insertion copolymerization of 1-hexene and ligated α-olefins.

Comprehensive studies of the ligand electronic effect on unsymmetrical α-diimine nickel(ii) promoted ethylene (co)polymerizations

The ligand electronic effects on the horizontal axis and vertical axis are systematically disclosed in unsymmetrical α-diimine Ni(ii)-promoted ethylene polymerization and copolymerization reactions.

Studies on chain shuttling polymerization reaction of nonbridged half-titanocene and bis(phenoxy-imine) Zr binary catalyst system

In this contribution, olefin block copolymers were produced via chain shuttling polymerization (CSP), using a new combination of catalysts and a chain shuttling agent (CSA) diethylzinc (ZnEt₂). The binary catalyst system included nonbridged half-titanocene catalyst, Cp*TiCl₂(O-2,6-ⁱPr₂C₆H₃) (Cat A) and bis(phenoxy-imine) zirconium, {η ²-1-[C(H)=NC₆H₁₁]-2-O-3-^tBu-C₆H₃}₂ZrCl₂ (Cat B), as well as co-catalyst methylaluminoxane (MAO). In contrast to dual-catalyst system in the absence of CSA, the blocky structure was obtained in the presence of CSA and rationalized from rheological studies. The binary catalyst system could cause the CSP reaction to occur in the presence of CSA ZnEt₂, which yielded broad distribution ethylene/1-octene copolymers (M _w/M _n: 35.86) containing block polymer chains with high M _w. The presented dual-catalytic system was applied for the first time in CSP and has a potential to be extended to produce a library of olefin block copolymers that can be used as advanced additives for thermoplastics.

Bulky yet flexible substituents in insertion polymerization with α-diimine nickel and palladium systems

The bulky yet flexible substituent on the N-aryl moieties of α-diimine ligands may adopt different conformations and provide dynamic steric hindrance in the process of ethylene (co)polymerization.

Switchable living nickel(ii) α-diimine catalyst for ethylene polymerisation

Design and synthesis of a Ni(ii) “sandwich” α-diimine complex (1) resulted in a switchable catalyst for the living polymerisation of ethylene over a range of temperatures and pressures.

N,N-chelated nickel catalysts for highly branched polyolefin elastomers: a survey

The physical properties and end applications of polyolefin materials are defined by their chain architectures and topologies. These properties can, in part, be controlled by a judicious choice of the steric and electronic properties of the catalyst and, in particular, the ligand framework. One major achievement in this field is the discovery of thermoplastic polyolefin elastomers that combine the processing and recyclable characteristics of thermoplastics with the flexibility and ductility of elastomers. These polymers are highly sought after as alternative materials to thermoset elastomers. In this perspective, works in the literature related to the development of nickel catalysts as well as their implementations for the synthesis of polyolefin elastomers are summarized in detail. Throughout the perspective, attention has been focused on developing the relationship between catalyst structure and performance, on strategies for the synthesis of polyolefin elastomer using nickel catalysts, on properties of the resultant polyolefin, such as degree of branching and crystallinity, as well as on their effects on mechanical properties. The future perspective regarding the most recent developments in single-step production of polyethylene elastomers will also be presented.