Catalytic synthesis of functionalized (polar and non-polar) polyolefin block copolymers

Catalytic Chemical Recycling of Post-Consumer Polyethylene

Among commercial plastics, polyolefins are the most widely produced worldwide but have limited recyclability. Here, we report a chemical recycling route for the conversion of post-consumer high-density polyethylene (HDPE) into telechelic macromonomers suitable for circular reprocessing. Unsaturation was introduced into HDPE by catalytic dehydrogenation using an Ir-POCOP catalyst without an alkene acceptor. Cross-metathesis with 2-hydroxyethyl acrylate followed by hydrogenation transformed the partially unsaturated HDPE into telechelic macromonomers. The direct repolymerization of the macromonomers gave a brittle material due to the low overall weight-average molecular weight. Aminolysis of telechelic macromonomers with a small amount of diethanolamine increased the overall functionality. The resulting macromonomers were repolymerized through transesterification to generate a polymer with comparable mechanical properties to the starting post-consumer HDPE waste. Depolymerization of the repolymerized material catalyzed by an organic base regenerated the telechelic macromonomers, thereby allowing waste polyethylene materials to enter a chemical recycling pathway.

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 Pd^II -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.

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.

Chemically Recyclable Ester-Linked Polypropylene

Polyolefins represent the largest class of commodity materials due to their excellent material properties; however, they have limited pathways to chemical recycling and are often difficult to mechanically recycle. Here we demonstrate a new catalyst for the isoselective copolymerization of propylene and butadiene capable of favoring 1,4-insertion over 1,2-insertion while maintaining good molecular weights and turnover frequencies. This isotactic propylene copolymer with main-chain unsaturation was depolymerized to a telechelic macromonomer using an olefin metathesis catalyst and 2-hydroxyethyl acrylate. After hydrogenation, the telechelic macromonomer was repolymerized to form an ester-linked polypropylene material. This polymer shows thermal and mechanical properties comparable to linear low-density polyethylene. Finally, the telechelic macromonomer could be regenerated through the depolymerization of the ester-linked polypropylene material, which allows for the chemical recycling to macromonomer. This process provides a route to transform partially unsaturated polyolefins to chemically recyclable materials with similar properties to their parent polymers.

Tandem ROMP/Hydrogenation Approach to Hydroxy-Telechelic Linear Polyethylene

Hydroxy-telechelic polyalkenamers have long been synthesized using ring-opening metathesis polymerization (ROMP) in the presence of an acyclic olefin chain-transfer agent (CTA); however, this route typically requires protected diols in the CTA due to the challenge of alcohol-mediated degradation of ruthenium metathesis catalysts that can not only deactivate the catalysts, but also compromise the CTA. We demonstrate the synthesis and implementation of a new hydroxyl-containing CTA in which extended methylene spacers isolate the olefin and alcohol moieties to mitigate decomposition pathways. This CTA enabled the direct ROMP synthesis of hydroxy-telechelic polycyclooctene with controlled chain lengths dictated by the initial ratio of monomer to CTA. The elimination of protection/deprotection steps resulted in improved atom economy. Subsequent hydrogenation of the backbone olefins was performed by a one-pot, catalytic approach employing the ruthenium complex used for the initial ROMP. The resultant approach is a streamlined, atom-economic, and low-waste route to hydroxy-telechelic linear polyethylene that uses a green solvent, succeeds with miniscule quantities of catalyst (0.005 mol %), and requires no additional purification steps.

Tandem Living Insertion and Controlled Radical Polymerization for Polyolefin-Polyvinyl Block Copolymers

Practical synthesis of polyolefin-polyvinyl block copolymers remains a challenge for transition-metal catalyzed polymerizations. Common approaches functionalize polyolefins for post-radical polymerization via insertion methods, yet sacrifice the livingness of the olefin polymerization. This work identifies an orthogonal radical/spin coupling technique which affords tandem living insertion and controlled radical polymerization. The broad tolerance of this coupling technique has been demonstrated for diverse radical/spin traps such as 2,2,5-trimethyl-4-phenyl-3-azahexane-3-nitroxide (TIPNO), 1-oxyl- (2,2,6,6-tetramethylpiperidine) -4-yl-α-bromoisobutyrate (TEMPO-Br), and N-tert -butyl-α-phenylnitrone (PBN). Subsequent controlled radical polymerization is demonstrated with NMP and atom transfer radical polymerization (ATRP), yielding polyolefin-polyvinyl di- and triblock copolymers ( Đ <1.3) with acrylic, vinylic and styrenic segments. These findings highlight radical trapping as an approach to expand the scope of polyolefin functionalization techniques to access polyolefin macroinitiators.

Distorted Sandwich a-Diimine Pd(II) Catalyst: Linear Polyethylene and Synthesis of Ethylene/Acrylate Elastomers

The introduction of m-xylyl substituents to α-diimine Pd(II) catalyst promotes living ethylene polymerization at room temperature and low pressure to yield high molecular weight polyethylene (PE) with low branching (<17/1000C). 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.

The II-I Phase Transition Behavior of Butene-1 Copolymers with Hydroxyl Groups

The crystallization and II–I phase transition of functionalized polybutene-1 with hydroxyl groups were investigated by differential scanning calorimetry. The results show that the incorporated hydroxyl groups increase the nucleation density but decrease the growth rate in melt crystallization. Interestingly, for the generated tetragonal form II, the presence of polar hydroxyl groups can effectively accelerate the phase transition into the thermodynamically stable modification of trigonal form I, especially with stepwise annealing and high incorporation. Using stepwise annealing, II–I phase transition was enhanced by an additional nucleation step performed at a relatively low temperature, and the optimal nucleation temperature to obtain the maximum transition degree was ‒10 °C, which is independent from the content of hydroxyl groups. Furthermore, the accelerating effect of hydroxyl groups on the II–I transition kinetics can be increased by reducing the crystallization temperature when preparing form II crystallites. These results provide a potential molecular design approach for developing polybutene-1 materials.

Synthesis of telechelic polyolefins

A comprehensive review of all the methodologies developed for the synthesis of telechelic polyolefins is reported.

Polystyrene Chain Growth Initiated from Dialkylzinc for Synthesis of Polyolefin-Polystyrene Block Copolymers

Polyolefins (POs) are the most abundant polymers. However, synthesis of PO-based block copolymers has only rarely been achieved. We aimed to synthesize various PO-based block copolymers by coordinative chain transfer polymerization (CCTP) followed by anionic polymerization in one-pot via conversion of the CCTP product (polyolefinyl)2Zn to polyolefinyl-Li. The addition of 2 equiv t-BuLi to (1-octyl)2Zn (a model compound of (polyolefinyl)2Zn) and selective removal or decomposition of (tBu)2Zn by evacuation or heating at 130 °C afforded 1-octyl-Li. Attempts to convert (polyolefinyl)2Zn to polyolefinyl-Li were unsuccessful. However, polystyrene (PS) chains were efficiently grown from (polyolefinyl)2Zn; the addition of styrene monomers after treatment with t-BuLi and pentamethyldiethylenetriamine (PMDTA) in the presence of residual olefin monomers afforded PO-block-PSs. Organolithium species that might be generated in the pot of t-BuLi, PMDTA, and olefin monomers, i.e., [Me2NCH2CH2N(Me)CH2CH2N(Me)CH2Li, Me2NCH2CH2N(Me)Li·(PMDTA), pentylallyl-Li⋅(PMDTA)], as well as PhLi⋅(PMDTA), were screened as initiators to grow PS chains from (1-hexyl)2Zn, as well as from (polyolefinyl)2Zn. Pentylallyl-Li⋅(PMDTA) was the best initiator. The Mn values increased substantially after the styrene polymerization with some generation of homo-PSs (27-29%). The Mn values of the extracted homo-PS suggested that PS chains were grown mainly from polyolefinyl groups in [(polyolefinyl)2(pentylallyl)Zn]-[Li⋅(PMDTA)]+ formed by pentylallyl-Li⋅(PMDTA) acting onto (polyolefinyl)2Zn.

End-functional polyolefins for block copolymer synthesis

Polyolefins that contain polar functionalities are highly desired because they could extend the range of applications of these low production cost materials by modifying surface and other interfacial properties. Block copolymers containing polyolefin and polar segments are among the most sought-after architectures because of their ability to span the phase boundaries. This review focusses on the end-functionalisation of polyolefins by catalytic olefin polymerisation processes, almost invariably by metal-catalysed routes, followed by the growth polar blocks by various polymerisation techniques.