Recent Advances in Controlled Production of Long-Chain Branched Polyolefins.

Polyolefins, composed of carbon and hydrogen atoms, dominate global polymer production. This stems from the wide range of physical and mechanical properties that various polyolefins can cover. Their versatile properties are largely tuned by chain microstructure, including molar mass distribution, comonomer content, and long-chain branching (LCB). Specifically, LCB imparts unique characteristics, notably enhances processability crucial for downstream applications. Tailoring LCB structural features has encouraged academic and industrial efforts, chronicle in this review from a chemistry standpoint. While encompassing post-reaction modification based traditional methods like peroxide grafting, ionizing beam irradiation, and coupling reactions, the main focus is given to catalyst-centric strategies and innovative polymerization schemes. The advent of single-site catalysts-metallocenes and late transition metals catalysts-amplifies interest in tailored chemical methods, but the progress in LCB formation flourishes via tandem catalytic systems and bimetallic catalysts under controlled reaction conditions. Specifically, the breakthrough in coordinative chain transfer polymerization unveils a novel avenue for controlled LCB synthesis by sequential chain propagation, transfer, liberation, and enchainment. This short review highlights recent approaches for the production of LCB polyolefins that can provide a roadmap crucial for researchers in academia and industry, steering their efforts toward further advancements in the production of tailored polyolefin.

Block Copolymers of Polyolefins with Polyacrylates: Analyzing and Improving the Blocking Efficiencies Using MILRad/ATRP Approach.

Despite their industrial ubiquity, polyolefin-polyacrylate block copolymers are challenging to synthesize due to the distinct polymerization pathways necessary for respective blocks. This study utilizes MILRad, metal-organic insertion light-initiated radical polymerization, to synthesize polyolefin-b-poly(methyl acrylate) copolymer by combining palladium-catalyzed insertion-coordination polymerization and atom transfer radical polymerization (ATRP). Brookhart-type Pd complexes used for the living polymerization of olefins are homolytically cleaved by blue-light irradiation, generating polyolefin-based macroradicals, which are trapped with functional nitroxide derivatives forming ATRP macroinitiators. ATRP in the presence of Cu(0), that is, supplemental activators and reducing agents , is used to polymerize methyl acrylate. An increase in the functionalization efficiency of up to 71% is demonstrated in this study by modifying the light source and optimizing the radical trapping condition. Regardless of the radical trapping efficiency, essentially quantitative chain extension of polyolefin-Br macroinitiator with acrylates is consistently demonstrated, indicating successful second block formation.

Temperature Dependence of Kinetic Friction: A Handle for Plastics Sortation?

Sortation is a crucial step in mechanical recycling of post-consumer plastics (PCR) whereby properties such as density or spectral signature are used to separate plastics. However it is difficult to sort polyolefin flakes at high throughput by these properties. We ask whether the frictional properties of plastics as a function of temperature may be used as an alternate sorting property. However, fundamental studies of friction at temperatures near their melting points are limited. Here we measure the temperature dependence of kinetic friction for three common polyolefins (high and low den- sity polyethylene and polypropylene) as well as polyethylene terephthalate (PET), focusing on the softening/melting regime. The results are augmented by differential scanning calorimetry and temperature dependence measurements of both dynamic modulus and and probe tack. For the polyolefins, we find strong increases in the coefficients of kinetic friction during temperature ramps in the melting/softening regime. For the PET, we report a notable peak in the kinetic friction which we associate with the glass transition and cold-crystallization. We discuss the enhanced friction in the context of rub- ber friction, which exhibits comparable coefficients of kinetic frictions.

Synthesis of Photoresponsive Fast Self-healing Polyolefin Composites by Nickel-Catalyzed Copolymerization of Ethylene and Lignin Cluster Monomers.

Polymers may suffer from sudden mechanical damages during long-term use under various harsh operating environments. Rapid and real-time self-healing will extend their service life, which is particularly attractive in the context of circular economy. In this work, a lignin cluster polymerization strategy (LCPS) was designed to prepare a series of lignin functionalized polyolefin composites with excellent mechanical properties through nickel catalyzed copolymerization of ethylene and lignin cluster monomers. These composites can achieve rapid self-healing within 30 seconds under a variety of extreme usage environments (underwater, seawater, extremely low temperatures as low as -60 °C, organic solvents, acid/alkali solvents, etc.), which is of great significance for real-time self-healing of sudden mechanical damage. More importantly, the dynamic cross-linking network within these composites enable great re-processability and amazing sealing performances.

A Vitrimer Acts as a Compatibilizer for Polyethylene and Polypropylene Blends.

Polymer compatibilization plays a critical role in achieving polymer blends with favorable mechanical properties and enabling efficient recycling of mixed plastic wastes. Nonetheless, traditional compatibilization methods often require tailored designs based on the specific chemical compositions of the blends. In this study, we propose a new approach for compatibilizing polymer blends using a dynamically crosslinked polymer network, known as vitrimers. By adding a relatively small amount (1-5 w/w%) of a vitrimer made of siloxane-crosslinked high-density polyethylene (HDPE), we successfully compatibilized unmodified HDPE and isotactic polypropylene (iPP). The vitrimer-compatibilized blend exhibited enhanced elongation at break (120 %) and smaller iPP domain sizes (0.4 µm) compared to the control blend (22 % elongation at break, 0.9 µm iPP droplet size). Moreover, the vitrimer-compatibilized blend showed significantly improved microphase stability during annealing at 180 °C. This straightforward method shows promise for applications across various polymer blend systems.

Covalent Organic Framework-Supported Metallocene for Ethylene Polymerization.

The loading of homogeneous catalysts with support can dramatically improve their performance in olefin polymerization. However, the challenge lies in the development of supported catalysts with well-defined pore structures and good compatibility to achieve high catalytic activity and product performance. Herein, we report the use of an emergent class of porous material-covalent organic framework material (COF) as a carrier to support metallocene catalyst-Cp2 ZrCl2 for ethylene polymerization. The COF-supported catalyst demonstrates a higher catalytic activity of 31.1×106  g mol-1 h-1 at 140 °C, compared with 11.2×106  g mol-1 h-1 for the homogenous one. The resulting polyethylene (PE) products possess higher weight-average molecular weight (Mw ) and narrower molecular weight distribution (Ð) after COF supporting, that is, Mw increases from 160 to 308 kDa and Ð drops from 3.3 to 2.2. The melting point (Tm ) is also increased by up to 5.2 °C. Moreover, the PE product possesses a characteristic filamentous microstructure and demonstrates an increased tensile strength from 19.0 to 30.7 MPa and elongation at break from 350 to 1400 % after catalyst loading. We believe that the use of COF carriers will facilitate the future development of supported catalysts for highly efficient olefin polymerization and high-performance polyolefins.

Outer-Shell Self-Supported Nickel Catalysts for the Synthesis of Polyolefin Composites.

In situ heterogeneous olefin polymerization has attracted much attention for the synthesis of polyolefin composites. However, the complicated syntheses of specially designed catalysts or the detrimental effects of interactions between catalyst and solid supports pose great challenges. In this contribution, an outer-shell self-supporting strategy was designed to heterogenize nickel catalysts on different fillers via precipitation homopolymerization of ionic cluster type polar monomer. These catalysts demonstrated high activity, good product morphology control, and stable performances in ethylene polymerization and copolymerization. Moreover, various polyolefin composites with great mechanical and customized properties can be efficiently synthesized.

Rapid Polyolefin Hydrogenolysis by a Single-Site Organo-Tantalum Catalyst on a Super-Acidic Support: Structure and Mechanism.

The novel electrophilic organo-tantalum catalyst AlS/TaNpx (1) (Np=neopentyl) is prepared by chemisorption of the alkylidene Np3 Ta=CHt Bu onto highly Brønsted acidic sulfated alumina (AlS). The proposed catalyst structure is supported by EXAFS, XANES, ICP, DRIFTS, elemental analysis, and SSNMR measurements and is in good agreement with DFT analysis. Catalyst 1 is highly effective for the hydrogenolysis of diverse linear and branched hydrocarbons, ranging from C2 to polyolefins. To the best of our knowledge, 1 exhibits one of the highest polyolefin hydrogenolysis activities (9,800 (CH2 units) ⋅ mol(Ta)-1 ⋅ h-1 at 200 °C/17 atm H2 ) reported to date in the peer-reviewed literature. Unlike the AlS/ZrNp2 analog, the Ta catalyst is more thermally stable and offers multiple potential C-C bond activation pathways. For hydrogenolysis, AlS/TaNpx is effective for a wide variety of pre- and post-consumer polyolefin plastics and is not significantly deactivated by standard polyolefin additives at typical industrial concentrations.

Polyolefin Innovations toward Circularity and Sustainable Alternatives.

The unprecedented growth and socioeconomic impacts of polyolefins clearly outline a major success story in the world of polymer science. Polyolefins revolutionizes industries such as health care, construction, and food packaging. Despite the benefits of polyolefins, there is a rising concern for the environment due to high production volume (i.e., fossil fuel consumption), often short usage time, and problems related to waste management and accumulation in the natural environment. Creating a circular economy for polyolefins through effective recycling technologies has the potential to decrease the environmental impact of these materials. This perspective discusses polyolefins and their impact, existing and emerging recycling/upcycling solutions, and recycle-by-design alternatives that are challenging the status quo.

Transition Metal-(µ-Cl)-Aluminum Bonding in α-Olefin and Diene Chemistry.

Olefin and diene transformations, catalyzed by organoaluminum-activated metal complexes, are widely used in synthetic organic chemistry and form the basis of major petrochemical processes. However, the role of M−(µ-Cl)−Al bonding, being proven for certain >C=C< functionalization reactions, remains unclear and debated for essentially more important industrial processes such as oligomerization and polymerization of α-olefins and conjugated dienes. Numerous publications indirectly point at the significance of M−(µ-Cl)−Al bonding in Ziegler−Natta and related transformations, but only a few studies contain experimental or at least theoretical evidence of the involvement of M−(µ-Cl)−Al species into catalytic cycles. In the present review, we have compiled data on the formation of M−(µ-Cl)−Al complexes (M = Ti, Zr, V, Cr, Ni), their molecular structure, and reactivity towards olefins and dienes. The possible role of similar complexes in the functionalization, oligomerization and polymerization of α-olefins and dienes is discussed in the present review through the prism of the further development of Ziegler−Natta processes and beyond.

One-For-All Polyolefin Functionalization: Active Ester as Gateway to Combine Insertion Polymerization with ROP, NMP, and RAFT.

This work develops the Polyolefin Active-Ester Exchange (PACE) process to afford well-defined polyolefin-polyvinyl block copolymers. α-Diimine PdII -catalyzed olefin polymerizations were investigated through in-depth kinetic studies in comparison to an analog to establish the critical design that facilitates catalyst activation. Simple transformations lead to a diversity of functional groups forming polyolefin macroinitiators or macro-mediators for various subsequent controlled polymerization techniques. Preparation of block copolymers with different architectures, molecular weights, and compositions was demonstrated with ring-opening polymerization (ROP), nitroxide-mediated polymerization (NMP), and photoiniferter reversible addition-fragmentation chain transfer (PI-RAFT). The significant difference in the properties of polyolefin-polyacrylamide block copolymers was harnessed to carry out polymerization-induced self-assembly (PISA) and study the nanostructure behaviors.

Switch from Anionic Polymerization to Coordinative Chain Transfer Polymerization: A Valuable Strategy to Make Olefin Block Copolymers.

Anionic polymerization of butadiene or/and styrene is performed with lithium initiators, functional or not. The polymer chains are subsequently transferred to magnesium. The resulting polymeryl-magnesium compounds were combined with {(Me2 Si(C13 H8 )2 )Nd(µ-BH4 )[(µ-BH4 )Li(THF)]}2 metallocene complex to act as macromolecular chain transfer agents (macroCTAs) in coordinative chain transfer polymerization (CCTP) of ethylene (E) or its copolymerization (CCTcoP) with butadiene (B). Block copolymers were produced for the first time by this switch from anionic polymerization to CCTP. Hard and soft blocks such as PB, polystyrene (PS), poly(styrene-co-butadiene) (SBR) obtained by anionic polymerization and PE or poly(ethylene-co-butadiene) (EBR) produced by CCT(co)P were combined and the corresponding structures were characterized.

Stereomodulation of Poly(4-methyl-1-pentene): Adoption of a Neglected and Misunderstood Commercial Polyolefin.

A toolbox of living coordination polymerization methods based on CPAM group 4 metal initiators has been applied for the stereomodulation of poly(4-methyl-1-pentene) (PMP) tacticity to provide new fundamental forms in which thermal phase transitions, Tg and Tm , can be adjusted over a wide range. Also reported for the first time are the synthesis and properties of authenticated 1,2-regioregular atactic PMP, which serves to correct past errors regarding this fundamental polyolefin.

First-principles based theoretical investigation of impact of polyolefin structure on photooxidation behavior.

Despite their mass production and large applications, polyolefins' stability and durability toward the air, moisture, and weather resistance is a challenge for the ecosystem. After long-term exposure to ultraviolet (UV) radiation or high-temperature or erosion, polyolefins undergo degradation generating microplastics (MPs). The MPs generated after the degradation of these polyolefins are hazardous for the ecosystem. In the present work, we have carried out density functional theory (DFT) studies to investigate the photodegradation of six different polyolefins ranging from polyethylene to polydecene, differing in side-chain. Herein, we have investigated photooxidized derivatives of different polyolefins and analyzed their relative stability, conformations, UV-visible spectral behavior, and carbonyl index. The photooxidized derivatives of various polyolefins formed during degradation are examined. The time-dependent density functional theory analysis confirms that the carbonyl groups of photooxidized products show absorption peak in Infrared (IR) and visible region, acting as light-absorbing species. The relative stabilities of hydroperoxide formed during photo/thermal oxidation of different polyolefins have been evaluated to explain the degradation behavior. The oligomerization and stabilization energies of their corresponding hydroperoxide's were computed and analyzed to explain the degradation behavior of the polyolefins. The computed results suggest that polyolefins in their pristine state are stable toward photooxidation, but chemical impurities like carbonyl, unsaturated carbonyl, carboxylic acid, and hydroperoxide derivatives make them prone to undergo degradation, a fundamental process leading to generation of MPs. The comparative results confirmed that the side-chain length affects the stability and degradation of different polyolefins toward photooxidation.

{Cyclopentadienyl/Fluorenyl}-Group 4 ansa-Metallocene Catalysts for Production of Tailor-Made Polyolefins.

The development of new metallocene-based polymerization catalysts and innovative processes derived thereof still constitutes a challenge for the manufacturing of polyolefinic materials with tailored properties (e. g. particular microstructure or topology, ultra-high molecular weight, high melting transition, and their combinations) for contemporary commercial applications. This personal account summarizes our continuing endeavors to advance the family of industry-relevant stereoselective propylene polymerization catalysts based on C1 -symmetric group 4 ansa-metallocenes incorporating multi-substituted fluorenyl-cyclopentadienyl {Cp/Flu} ligands. Within the framework of this project, valuable structural and catalytic data, harvested both for neutral metallocenes and for metallocenium ion-pairs, have been used for rational design of more efficient catalytic systems, reluctant towards side reactions, and for providing new stereoregular value-added polymer materials.

Development of an LC-MS method for the semiquantitative determination of polyamide 6 contaminations in polyolefin recyclates.

Recycling will be of increasing importance in the future, especially for plastic packaging waste mainly consisting of polyolefins. One major problem of recyclates comprises impurities which can have a significant negative impact on future product properties. Polyamide 6 can be found widely as contaminant in recycled polyolefins, leading to a need of quantification methods thereof. In this paper, a method development for the quantitative analysis of polyamide 6 is presented based on analysing ε-caprolactam and related cyclic oligomers as marker compounds in model recyclates of high- and low-density polyethylene and polypropylene compounded with low amounts of polyamide 6. For the method development and tentative identification of the different cyclic compounds, a HPLC-QTOF-MS was used and it was possible to detect six different compounds, ε-caprolactam and the corresponding cyclic di- to hexamer. The quantification was performed with a HPLC-QQQ-MS, equipped with a HILIC column, after sample preparation via microwave-assisted extraction. It could be shown that a good linearity from 0.2 up to 5 wt% polyamide 6 in the different polyolefins can be achieved. The cyclic trimer and tetramer show a low limit of quantification and are therefore well-suited for the quantification, whereas the other cyclic compounds can be then used as qualifiers to avoid false positives. To guarantee the applicability of the method, six real recyclate materials were analysed, whereby in three of them low amounts of polyamide 6 could be detected. Graphical abstract.

Extraction of stabilizers from polymers: Separation of oligomeric hindered amine light stabilizers and phenolic antioxidants from polyolefins using liquid chromatography and high-temperature solid-phase extraction.

The extraction of different stabilizers from a polymer matrix and the subsequent separation of said stabilizers is one of the most important as well as challenging undertakings in polymer chemistry. A multitude of stabilizers exists, each of which may be hard to extract, be difficult if not impossible to separate from other stabilizers or necessitate very selected and time-consuming intermediate stages for separation. Certain polymer matrices even pose additional challenges, such as polyolefins being only soluble at elevated temperatures. One of the most well-established approaches for the extraction of stabilizers is Soxhlet extraction. However, even this highly successful approach shows only limited success with regard to the extraction of the ever more relevant oligomeric stabilizers or the extraction of multiple stabilizers in a one-shot approach. Moreover, performing Soxhlet extractions often necessitates ≥24 h. For these reasons, alternative approaches for the extraction of stabilizers from polymers are highly sought after. An approach with enormous potential is solid-phase extraction, which allows the selective retention and enrichment of stabilizers. Herein, the very first application of high-temperature solid-phase extraction for the extraction of stabilizers from polyolefin matrices is described; as with other extraction techniques, the identification and quantification of the stabilizers is then allowed. At temperatures of 140-160°C, it was possible to adsorb common polyolefin stabilizers selectively on a silica solid phase from their polyolefin matrix. To predict high-temperature solid-phase extraction test conditions, first LC tests are necessary, offering an elegant approach for the separation of polyolefins from oligomeric stabilizers, which was not achievable until now.

Distorted Sandwich α-Diimine PdII Catalyst: Linear Polyethylene and Synthesis of Ethylene/Acrylate Elastomers.

The introduction of m-xylyl substituents to α-diimine PdII catalyst promotes living ethylene polymerization at room temperature and low pressure to yield high molecular weight polyethylene (PE) with low branching (<17/1000 C). m-Xylyl groups provide a highly effective blockage to the axial sites of the catalytic center and form a distorted sandwich geometry. The shielding prevents chain-transfer and easy accessibility of polar monomers, leading to a living polymerization. Conducting a light irradiation as part of the one-step metal-organic insertion light initiated radical (MILRad) process leads to diblock copolymers of ethylene and acrylates. Incorporation of different acrylate block sequences can significantly modify the mechanical and chemical properties of block copolymers which can be modulated to be a hard plastic, elastomer, or semi-amorphous polymer.

Hydrogen-Bonding-Induced Heterogenization of Nickel and Palladium Catalysts for Copolymerization of Ethylene with Polar Monomers.

The practical synthesis of polar-functionalized polyolefins using transition-metal-catalyzed copolymerization of olefins with polar monomers is a challenge; the use of heterogeneous catalysts is little explored. Herein, we report the synthesis of heterogeneous naphthoquinone-based nickel (Ni/SiO2 ) and palladium (Pd/SiO2 ) catalysts through hydrogen bonding interactions of the ligands with the silica surface. Ni/SiO2 exhibits high activities (up to 2.65×106  g mol-1 h-1 ) during the copolymerization of ethylene with 5-hexene-1-yl-acetate, affording high-molecular-weight (Mn up to 630 000) polar-functionalized semicrystalline polyethylene (comonomer incorporation up to 2.8 mol %), along with great morphology control. The resulting copolymers possess improved surface properties and great mechanical properties. Pd/SiO2 can mediate ethylene copolymerization with polar monomers with moderate activity to produce high-molecular-weight copolymers with tunable comonomer incorporation.

2,4,6-Triphenylpyridinium: A Bulky, Highly Electron-Withdrawing Substituent That Enhances Properties of Nickel(II) Ethylene Polymerization Catalysts.

The reactivity of NiII and PdII olefin polymerization catalysts can be enhanced by introduction of electron-withdrawing substituents on the supporting ligands rendering the metal centers more electrophilic. Reported here is a comparison of ethylene polymerization activity of a classical salicyliminato nickel catalyst substituted with the powerful electron-withdrawing 2,4,6-triphenylpyridinium (trippy) group to the -CF3 analogue. The trippy substituent is substantially more electron-withdrawing (σmeta =0.63) than the trifluoromethyl group (σmeta =0.43) which results in a ca. 8-fold increase in catalytic turnover frequency. An additional advantage of trippy is the high steric bulk relative to the trifluoromethyl group. This feature results in a four-fold increase in polymer molecular weight owing to enhanced retardation of chain transfer. A significant increase in catalyst lifetime is observed as well.