Potential of Functionalized Polyolefins in a Sustainable Polymer Economy: Synthetic Strategies and Applications

A π-π interaction strategy for targeting highly thermostable bis(arylimino)pyridyliron precatalysts in ethylene polymerization

The suboptimal catalytic performance of bis(arylimino)pyridyliron precatalysts in ethylene polymerization at elevated temperatures remains a significant challenge. In this study, a series of six bis(arylimino)pyridyliron chloride precatalysts with the incorporation of the benzosuberyl steric bulk were prepared and investigated for ethylene polymerization. Capping the axial sites of the iron center with the benzosuberyl steric bulk resulted in π-π interactions between the phenyl group and the chelate backbone, with a distance of ca. 3.293 Å for Fe3iPr, shorter than the sum of the van der Waals radii of carbon atoms (3.4 Å), as confirmed by single-crystal X-ray diffraction. These non-covalent interactions enhanced the thermal stability, catalytic activity, and polymer molecular weights. On activation with MAO or MMAO cocatalysts, these precatalysts exhibited the maximum activity of 2.53 × 107 gPE mol-1Fe h-1 at 80 °C and 1.88 × 107 gPE mol-1Fe h-1 at 100 °C, demonstrating their unprecedented thermal stability. The molecular weight of the produced polyethylene remained high, even at elevated temperatures. Moreover, ligand modifications had a pronounced impact on polymerization outcomes: the least sterically hindered complex, while producing comparatively lower molecular weight polyethylene, displayed higher activity and the most sterically hindered one showed the opposite tendency. The molecular weight dispersity of polyethylene showed a strong correlation with the precatalyst structure and reaction conditions. High melting points confirmed the presence of strictly linear structures with high vinyl end groups (up to 74%), as verified using 1H/13C NMR spectra. A comparison of structurally related iron precatalysts revealed significant improvements in both thermal stability and catalytic activity, attributed to the π-π interactions present in the current iron precatalysts.

Boryloxy Titanium Complex-Enabled High Polar Monomer Contents in Catalytic Copolymerization of Olefins

The preparation of polyolefins with high polar monomer contents (above 20 mol %) has long been a challenge. Half-titanocenes (Cp')[HCN(Ar)]2BOTiCl2 bearing bulky electron-donating N-heterocyclic boryloxy ligands have been designed and synthesized. The complexes (Cp*)[HCN(Ar)]2BOTiCl2 (2, Ar=2,6-iPr2C6H3; 5, Ar=2,4,6-Me3C6H2) supported by Cp* and the boryloxy ligands have been shown to efficiently catalyze the copolymerization of ethylene and long chain α-olefins. In particular, precatalyst 5 enabled the controlled synthesis of poly(ethylene-co-9-decen-1-ol) with unprecedented high polar monomer contents up to 32.1 mol % while maintaining the high catalytic activity. The structural analysis and DFT calculations disclosed that the bulky and strong electron-donating boryloxy ligands could effectively stabilize cationic active species. The mechanical studies on the hydroxyl-functionalized copolymers disclosed that they exhibited high strength and toughness because of the existence of hydrogen bonds in the polymer network.

Recent Advancements in the Synthesis of Functional Polyolefins by Non-Bridged Half-Titanocenes

Polyolefins are used widely due to their benefits such as being lightweight, chemical inertness, low cost, tunable properties, and easy processability. However, their nonpolar nature significantly limits their high-end applications. The non-bridged half-titanocenes exhibit remarkable catalytic activities with good comonomer incorporations in the olefin polymerization. The synthesis of functional polyolefins has attracted more and more attention recently. The non-bridged half-titanocenes have been used in the preparation of functional polyolefins, in particular the functional olefin copolymers. Herein, the recent advancements in the synthesis of functional polyolefins by non-bridged half-titanocenes were reviewed. The functional polyolefins have been synthesized by direct copolymerization of olefin with functional comonomers using half-titanocenes as precatalysts. In addition, polyolefins containing reactive groups could be synthesized by the olefin (co)polymerization using half-titanocenes as precatalysts. The functional polyolefins were synthesized successfully by the post-functionalization of polyolefin containing reactive groups.

Boosting the Strength and Toughness of Polymer Blends via Ligand-Modulated MOFs

Mechanically robust and tough polymeric materials are in high demand for applications ranging from flexible electronics to aerospace. However, achieving both high toughness and strength in polymers remains a significant challenge due to their inherently contradictory nature. Here, a universal strategy for enhancing the toughness and strength of polymer blends using ligand-modulated metal-organic framework (MOF) nanoparticles is presented, which are engineered to have adjustable hydrophilicity and lipophilicity by varying the types and ratios of ligands. Molecular dynamics (MD) simulations demonstrate that these nanoparticles can effectively regulate the interfaces between chemically distinct polymers based on their amphiphilicity. Remarkably, a mere 0.1 wt.% of MOF nanoparticles with optimized amphiphilicity (ML-MOF(5:5)) delivered ≈1.1- and ≈34.1-fold increase in strength and toughness of poly (lactic acid) (PLA)/poly (butylene succinate) (PBS) blend, respectively. Moreover, these amphiphilicity-tailorable MOF nanoparticles universally enhance the mechanical properties of various polymer blends, such as polypropylene (PP)/polyethylene (PE), PP/polystyrene (PS), PLA/poly (butylene adipate-co-terephthalate) (PBAT), and PLA/polycaprolactone (PCL)/PBS. This simple universal method offers significant potential for strengthening and toughening various polymer blends.

A Review on the Application of Deep Eutectic Solvents in Polymer-Based Membrane Preparation for Environmental Separation Technologies

The use of deep eutectic solvents (DESs) for the preparation of polymer membranes for environmental separation technologies is comprehensively reviewed. DESs have been divided into five categories based on the hydrogen bond donor (HBD) and acceptor (HBA) that are involved in the production of the DESs, and a wide range of DESs' physicochemical characteristics, such as density, surface tension, viscosity, and melting temperature, are initially gathered. Furthermore, the most popular techniques for creating membranes have been demonstrated and discussed, with a focus on the non-solvent induced phase separation (NIPS) method. Additionally, a number of studies have been reported in which DESs were employed as pore formers, solvents, additives, or co-solvents, among other applications. The addition of DESs to the manufacturing process increased the presence of finger-like structures and macrovoids in the cross-section and, on numerous occasions, had a substantial impact on the overall porosity and pore size. Performance data were also gathered for membranes made for various separation technologies, such as ultrafiltration (UF) and nanofiltration (NF). Lastly, DESs provide various options for the functionalization of membranes, such as the creation of various liquid membrane types, with special focus on supported liquid membranes (SLMs) for decarbonization technologies, discussed in terms of permeability and selectivity of several gases, including CO2, N2, and CH4.

Preparation of High-Performance Polyethylene Nanocomposites with Oleic Acid-Siloxene-Supported Ziegler-Natta Catalysts

The addition of two-dimensional inorganic nanomaterials can effectively enhance the properties of polyethylene (PE). In the present study, a series of high-performance PE/oleic acid (OA)-siloxene nanocomposites were prepared by in situ polymerization using OA-siloxene-supported Ziegler-Natta catalysts. Compared with the conventional Ziegler-Natta catalyst, the polymerization activity of the OA-siloxene-supported Ziegler-Natta catalyst was enhanced to 100 kg/mol-Ti•h, an increase of 56%. The OA-siloxene fillers exhibited excellent dispersion within the PE matrix through the in situ polymerization technique. Compared to pure PE, PE/OA-siloxene nanocomposites containing 1.13 wt% content of OA-siloxene showed 68.3 °C, 126%, 37%, and 46% enhancements in Tdmax, breaking strength, modulus, and elongation at break, respectively.

Nitriles as Functionalization and Coupling Agents for Polyolefins Obtained by Coordinative Chain Transfer Polymerization

Coordinative chain transfer polymerization (CCTP) of ethylene and its copolymerization with 1,3-butadiene is conducted in toluene at 80 °C using a combination of {(Me2Si(C13H8)2)Nd(μ-BH4)[(μ-BH4)Li(THF)]}2 (1) metal complex and various organomagnesium compounds used as chain transfer agents including n-butyl-n-octyl-magnesium (BOMAG), n-butyl-mesityl-magnesium (n-BuMgMes), n-butyl-magnesium chloride (n-BuMgCl), n-pentyl-magnesium bromide (n-C5H11MgBr), pentanediyl-1,5-di(magnesium bromide) (PDMB) and isobutyl-magnesium chloride (i-BuMgCl). Kinetics and performance in terms of control of the (co)polymerization are comparatively discussed particularly considering the presence of ether and the nature of the organomagnesium compounds employed. Taking advantage of the well-known reactivity between nitrile and molecular organomagnesium compounds, the functionalization of the chains is further carried out by deactivation of the polymerization medium with benzonitrile or methoxybenzonitrile compounds leading to ketone ω-functionalized chains. The success of the functionalizations is extended to coupling strategies using dinitrile reagents and to the functionalization of high molar mass ethylene butadiene rubber (EBR).

Light, Heat, and Force-Responsive Polyolefins

Stimuli-responsive polymers have found applications as shape-memory materials, optical switches, and sensors, but the installation of these responsive properties in non-polar and inert polyolefins is challenging. In this contribution, a series of spiropyran (SP)-based comonomers are synthesized and copolymerized with ethylene or ethylene/cyclic monomers. In addition to great mechanical and surface properties, these functionalized polyolefins responded to light, heat, and force, which induced changes in the polymer structure to transmit color or mechanical signals. These interesting responsive properties are also installed in a series of commercial polyolefin materials through reactive extrusion, making the scalable production of these materials possible.

Modulating Thermal Conductivity and Flame Retardancy of Polyolefin Composites via Distributed Structures of Magnesium Hydroxide and Hexagonal Boron Nitride

Thermally conductive and flame-retardant polyolefin composites are facing great challenges in meeting the increasing demands for fire safety and thermal management. Aiming at simultaneously enhancing thermal conductivity and flame retardancy, hexagonal boron nitride (hBN) and magnesium hydroxide (MH) were adopted in ethylene-vinyl acetate copolymer/polyolefin elastomer (EVA/POE) blends to design composites with selective filler distributions and co-continuous networks via different processing schemes. The thermal conductivity and flame retardancy show strong dependence on the distributed structure of hBN and MH. The composites with hBN-rich centers and MH-rich edges in the filled POE phase show a thermal conductivity of 0.70 W/(m·K) and an LOI of 27.7%, which are very close to the thermal conductivity of EVA/POE/hBN and the LOI of EVA/POE/MH at the same total filler content. The composites with MH-rich centers and hBN-rich edges show pHRR, THR and TSP values of 169 kW/m2, 49.8 MJ/m2 and 1.8 m2, which are decreased by 40%, 33% and 62% in comparison with EVA/POE/MH, respectively. Modulating the filler structure distribution provides a strategy to co-enhance thermal conductivity and flame retardancy.

Advances of polyolefins from fiber to nanofiber: fabrication and recent applications

Polyolefins are a widely accepted commodity polymer made from olefinic monomer consisting of carbon and hydrogen. This thermoplastic polymeric material is formed through reactive double bonds of olefins by the addition polymerization technique and it possesses a diverse range of unique features for a large variety of applications. Among the various types, polyethylene and polypropylene are the prominent classes of polyolefins that can be crafted and manipulated into diversified products for numerous applications. Research on polyolefins has boomed tremendously in recent times owing to the abundance of raw materials, low cost, lightweight, high chemical resistance, diverse functionalities, and outstanding physical characteristics. Polyolefins have also evidenced their potentiality as a fiber in micro to nanoscale and emerged as a fascinating material for widespread high-performance use. This review aims to provide an elucidation of the breakthroughs in polyolefins, namely as fibers, filaments, and yarns, and their applications in many domains such as medicine, body armor, and load-bearing industries. Moreover, the development of electrospun polyolefin nanofibers employing cutting-edge techniques and their prospective utilization in filtration, biomedical engineering, protective textiles, and lithium-ion batteries has been illustrated meticulously. Besides, this review delineates the challenges associated with the formation of polyolefin nanofiber using different techniques and critically analyzes overcoming the difficulties in forming functional nanofibers for the innovative field of applications.

Density Functional Theory Study of the Regioselectivity in Copolymerization of bis-Styrenic Molecules with Propylene Using Zirconocene Catalyst

Density functional theory (DFT) was used to study the regioselectivity of the copolymerization of propylene and the bis-styrenic molecules (DVB and BVPE) using a zirconocene catalyst. This study reveals the following: when hydrogen is introduced to reactivate the catalyst on the vinyl bonds containing DVB or BVPE, the second vinyl bond is inserted into the polymer in a regio-irregular 1,2-way. (I) The 1,2-insertion mode forms more thermodynamically stable products. (II) The 2,1 insertion, DVB-PP1, or BVPE-PP1 needs to rotate 180° along the Zr-C1 bond to complete the process; thus, it is easier to accomplish the 1,2 insertion. (III) The analysis of the local electrophilicity/nucleophilicity index and the Fukui functions also indicate that the 1,2-insertion mode is the optimal insertion mode. Investigating the mechanism of this experimental phenomenon is important in the development of a functionalization strategy for polypropylene (PP) polymers.