Hyperbranched−Linear Polyethylene Block Polymers Constructed with Chain Blocks of Hybrid Chain Topologies via One-Pot Stagewise Chain Walking Ethylene “Living” Polymerization

Hyperbranched Polyethylene Ionomers Containing Quaternary Ammonium Ions and Their Functionalization of Nanomaterials

Ionomers containing a small number of ionic side groups are a unique class of polymers with some valuable properties and distinct applications. To date, commercially important ionomers are exclusively anionomers that contain covalently bonded anions and are synthesized commonly by radical polymerization. The catalytic synthesis of polyethylene-based cationomers is challenging, while it is attractive due to the low cost of ethylene stocks and less stringent polymerization conditions, along with their desirable properties and broadened scope of commercial applications. Advances in catalyst technology-specifically, Pd-diimine catalysts-have recently enabled the synthesis of a class of hyperbranched polyethylene cationomers that are designed to contain quaternary ammonium cations. With their unique hydrophobic hyperbranched polyethylene backbone, this class of ionomers enables the successful functionalization of negatively surface-charged nanomaterials, thus improving the processing and application of the latter. This review summarizes the developments of this class of ionomers, including their synthesis, properties, and functionalization of various nanomaterials.

Effect of architectural asymmetry of hyperbranched block copolymers on their phase boundaries

Asymmetric architecture of AB-type block copolymers can induce additional spontaneous curvature to the A/B interface, accordingly deflecting the phase boundaries. However, it is often difficult to determine or compare the asymmetric effects of different asymmetric architectures. In this work, we proposed to use the equivalent arm number nequ, which was originally defined as nequ = n/iĐ for ABn with unequal B-arms and iĐ being the intramolecular polydispersity of these B-arms, to quantify the asymmetric effect of various linear-hyperbranched copolymers. For each linear-hyperbranched copolymer, nequ is estimated by matching its phase boundaries on the side with expanded spherical phase region with those of ABn with unequal B-arms but tunable iĐ. Our results suggest that the addition of B-blocks at the further location from the A-B joint point has less influence on nequ, i.e. the asymmetric effect, because these B-blocks can access more space. For the linear-dendrimer copolymers, nequ changes from 2 to about 3.8 when the overall generation number of the copolymer increases from 2 to 5. In other words, the asymmetric effect of these linear-dendrimer copolymers is intermediate between those of AB2 and AB4 miktoarm star copolymers. In brief, nequ can effectively describe the asymmetric effect on the interfacial curvature of complex asymmetric architectures.

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.

Light-mediated olefin coordination polymerization and photoswitches

This review outlines photoswitchable, transition metal-based olefin coordination polymerization catalysts ranging from homogeneous to heterogeneous, and monometallic to bimetallic regimes.

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.

Photochemical regulation of a redox-active olefin polymerization catalyst: controlling polyethylene microstructure with visible light

The utility of photoredox chemistry is expanded to include microstructural control of polyolefins.

Toward Maximizing the Mechanical Property of Interconnected Macroporous Polystyrenes Made from High Internal Phase Emulsions

Macroporous materials polymerized from high internal phase emulsions (PolyHIPEs) possess well-defined interconnected porous structures and tunable device shapes. This provides interesting property characteristics well-suited for a variety of applications. However, such materials also demonstrate poor mechanical performances, which limit their potential use. As will be demonstrated, this results from the high surfactant content required by PolyHIPEs. Herein, a new approach is introduced for designing a highly efficient polymeric surfactant, which generates interconnected pores in PolyHIPEs through designing an incompatible surfactant and skeleton material. The surfactant also possesses a hyperbranched topology, which combines the strong amphipathy of small molecular surfactants and the nanosphere structure of Pickering emulsifiers to provide an excellent colloidal stability to HIPEs. A hyperbranched polyethylene having pendant sodium sulfonate groups (HBPE-SO₃Na) was thus designed and synthesized via chain walking copolymerization of ethylene and 2-trimethylsilyloxyethyl acrylate followed by sulfonation. Stable HIPEs of styrene/divinylbenzene and water at a weight ratio of 1 to 5 were obtained with using HBPE-SO₃Na. The polymerization of HIPEs produced interconnected macroporous polystyrenes (PSs) at a substantially lower surfactant content, for example, 0.5 wt % HBPE-SO₃Na. The compressive Young's moduli of PolyHIPEs reached 104-111 MPa with 0.5-2 wt % HBPE-SO₃Na, which is the first reported case of a PS-based PolyHIPE achieving its theoretical modulus. The PolyHIPE was used to support Au nanoparticles and embed in a column for oxidation of dimethylphenylsilane. A complete conversion of dimethylphenylsilanol was achieved with low column back pressure in a 50 h continuous reaction with no degradation of PolyHIPE integrity and mechanical property.

"Living" Polymerization of Ethylene and 1-Hexene Using Novel Binuclear Pd⁻Diimine Catalysts

We report the synthesis of two novel binuclear Pd–diimine catalysts and their unique behaviors in initiating “living” polymerization of ethylene and 1-hexene. These two binuclear catalysts, [(N^N)Pd(CH₂)₃C(O)O(CH₂)_mO(O)C(CH₂)₃Pd(N^N)](SbF₆)₂ (3a: m = 4, 3b: m = 6) (N^N≡ArN=C(Me)–(Me)C=NAr, Ar≡2,6–(iPr)₂C₆H₃), were synthesized by simply reacting [(N^N)Pd(CH₃)(N≡CMe)]SbF₆ (1) with diacrylates, 1,4-butanediol diacrylate and 1,6-hexanediol diacrylate, respectively. Their unique binuclear structure with two identical Pd–diimine acrylate chelates covalently linked together through an ester linkage was confirmed by NMR and single crystal XRD measurements. Ethylene “living” polymerizations were carried out at 5 °C and under ethylene pressure of 400 and 100 psi, respectively, with the binuclear catalysts, along with a mononuclear chelate catalyst, [(N^N)Pd(CH₂)₃C(O)OMe]SbF₆ (2), for comparison. All the polyethylenes produced with both binuclear catalysts show bimodal molecular weight distribution with the number-average molecular weight of the higher molecular weight portion being approximately twice that of the lower molecular weight portion. The results demonstrate the presence of monofunctional chain growing species resembling catalyst 2, in addition to the expected bifunctional species leading to bifunctional “living” polymerization, in the polymerization systems. Both types of chain growing species exhibit “living” characteristics under the studied conditions, leading to the simultaneous linear increase of molecular weight in both portions. However, when applied for the “living” polymerization of 1-hexene, the binuclear catalyst 3a leads to polymers with only monomodal molecular weight distribution, indicating the sole presence of monofunctional chain growing species. These two binuclear catalysts are the first Pd–diimine catalysts capable of initiating bifunctional ethylene “living” polymerization.

Modulating polyolefin properties through the incorporation of nitrogen-containing polar monomers

In this work, ethylene copolymerization and terpolymerization reactions with some nitrogen-containing monomers were investigated using α-diimine palladium and phosphine-sulfonate palladium catalysts.

Modulating Polyolefin Copolymer Composition via Redox-Active Olefin Polymerization Catalysts

The ability to precisely modulate polymer architecture and composition is a long-standing goal within the field of polymer synthesis. Herein, we demonstrate that redox-active olefin polymerization catalysts may be used to predictably tailor polyolefin comonomer incorporation levels for the copolymerization of ethylene and higher α-olefins. This ability is facilitated via the utilization of a redox-active olefin polymerization catalyst that once reduced via in situ addition of a chemical reductant results in a notable drop in α-olefin incorporation. We attribute this behavior to the reduced catalyst's increased electron density and its concomitant decreased rate of α-olefin consumption. These results are supported by investigations into propylene and 1-hexene homopolymerizations as well as detailed GPC, DSC, GC, and NMR analyses.

Olefin Polymerization Catalyzed by Double-Decker Dipalladium Complexes: Low Branched Poly(α-Olefin)s by Selective Insertion of the Monomer Molecule

Dipalladium complexes of a cyclic bis(diimine) ligand with a double-decker structure catalyze polymerization of ethylene and α-olefins and copolymerization of ethylene with 1-hexene. The polymerization of 1-hexene yields a polymer that is mainly composed of the hexamethylene unit formed by 2,1-insertion of the monomer into the palladium-carbon bond, followed by chain-walking (6,1-insertion). The polymerization of 4-methyl-1-pentene proceeds by 2,1-insertion with a selectivity of 92-97 %, and affords the polymer with methyl and 2-methylhexyl branches. 2,1-Insertion occurs selectively in all of the polymerization reactions of α-olefins catalyzed by the dipalladium complexes. Ethylene polymerization with the catalyst at 100 °C lasts over 24 h, whereas the monopalladium-diimine catalyst loses its activity within 8 h at 60 °C. Polyethylene obtained by the dipalladium catalyst is less-branched and has a higher molecular weight compared to that of the monopalladium catalyst under the same conditions. Copolymerization of ethylene with 1-hexene affords solid products with melting points and molecular weights that vary depending on the polymerization time, suggesting formation of a block and/or gradient copolymer.

Synthesis of polyethylene and polystyrene miktoarm star copolymers using an “in–out” strategy

Miktoarm star copolymers having multiple polyethylene and polystyrene arms joined at the crosslinked polydivinylbenzene core were synthesized using an “in–out” strategy with the combined Pd-catalyzed ethylene “living” polymerization and atom transfer radical polymerization.