Iron(III)-Catalyzed C─H Hydroxylation of Low-Density Polyethylene Coupled with Short Chain Branching Growth

Low-density polyethylene (LDPE) is widely used in packaging applications, but after being discarded its environmental impact is a pressing concern due to a lack of effective chemical recycling strategies, especially owing to its chemical inertness and nonpolar nature. To address these challenges, we present a mild iron(III)-catalyzed oxidative upcycling of LDPE in which C─H hydroxylation of LDPE occurs coupled with the growth of methyl short chain branching (Me-SCB) and ethyl shortchain branching (Et-SCB). As a result, the resulting products achieve significant improvements in surface wettability, crystallinity, and mechanical properties despite a concomitant reduction in molecular weight. CH…F interactions and σ-π interactions are found between LDPE and the catalyst. Density functional theory (DFT) calculations elucidate the catalytic mechanism that fluorine on the ligand facilitates hydrogen peroxide activation and subsequent deprotonation, leading to the formation of high-valent ironoxo species. The growth of short-chain branching (SCB) involves the β-scission of CC bonds and a radical-mediated chain-walking mechanism. This work represents a transformative advancement in deep understanding of polyolefin upcycling and opens a new approach of polyolefin functionalization and architecture modulation.

Zirconium-Catalyzed C-H Alumination of Polyolefins, Paraffins, and Methane

C-H/Et-Al exchange in zirconium-catalyzed reactions of saturated hydrocarbons and AlEt₃ affords versatile organoaluminum compounds and ethane. The grafting of commercially available Zr(OtBu)₄ on silica/alumina gives monopodal ≡SiO-Zr(OtBu)₃ surface pre-catalyst sites that are activated in situ by ligand exchange with AlEt₃. The catalytic C-H alumination of dodecane at 150 °C followed by quenching in air affords n-dodecanol as the major product, revealing selectivity for methyl group activation. Shorter hydrocarbon or alcohol products were not detected under these conditions. Catalytic reactions of cyclooctane and AlEt₃, however, afford ring-opened products, indicating that C-C bond cleavage occurs readily in methyl group-free reactants. This selectivity for methyl group alumination enables the C-H alumination of polyethylenes, polypropylene, polystyrene, and poly-α-olefin oils without significant chain deconstruction. In addition, the smallest hydrocarbon, methane, undergoes selective mono-alumination under solvent-free catalytic conditions, providing a direct route to Al-Me species.

Diversification of aliphatic C-H bonds in small molecules and polyolefins through radical chain transfer

The ability to selectively introduce diverse functionality onto hydrocarbons is of substantial value in the synthesis of both small molecules and polymers. Herein, we report an approach to aliphatic carbon-hydrogen bond diversification using radical chain transfer featuring an easily prepared O-alkenylhydroxamate reagent, which upon mild heating facilitates a range of challenging or previously undeveloped aliphatic carbon-hydrogen bond functionalizations of small molecules and polyolefins. This broad reaction platform enabled the functionalization of postconsumer polyolefins in infrastructure used to process plastic waste. Furthermore, the chemoselective placement of ionic functionality onto a branched polyolefin using carbon-hydrogen bond functionalization upcycled the material from a thermoplastic into a tough elastomer with the tensile properties of high-value polyolefin ionomers.

The post-modification of polyolefins with emerging synthetic methods

This Perspective highlights the present state of polyolefin post-modification research, especially concerning recently developed C–H functionalization chemistry. Remaining challenges and emerging strategies within the field have also been discussed.

Chemo- and Regioselective Functionalization of Isotactic Polypropylene: A Mechanistic and Structure-Property Study

Polyolefins represent a high-volume class of polymers prized for their attractive thermomechanical properties, but the lack of chemical functionality on polyolefins makes them inadequate for many high-performance engineering applications. We report a metal-free postpolymerization modification approach to impart functionality onto branched polyolefins without the deleterious chain-coupling or chain-scission side reactions inherent to previous methods. The identification of conditions for thermally initiated polyolefin C-H functionalization combined with the development of new reagents enabled the addition of xanthates, trithiocarbonates, and dithiocarbamates to a variety of commercially available branched polyolefins. Systematic experimental and kinetic studies led to a mechanistic hypothesis that facilitated the rational design of reagents and reaction conditions for the thermally initiated C-H xanthylation of isotactic polypropylene (iPP) within a twin-screw extruder. A structure-property study showed that the functionalized iPP adheres to polar surfaces twice as strongly as commercial iPP while demonstrating similar tensile properties. The fundamental understanding of the elementary steps in amidyl radical-mediated polyolefin functionalization provided herein reveals key structure-reactivity relationships for the design of improved reagents, while the demonstration of chemoselective and scalable iPP functionalization to realize a material with improved adhesion properties indicates the translational potential of this method.

Synthesis of Brominated Polyethylene by Copolymerization of Ethylene with ω–bromoalkene Catalyzed by a Metallocene and Methylaluminoxane System

The metallocene-catalyzed copolymerization of ethylene and 11–bromo–1–undecene was carried out to synthesize brominated polyethylene (PE). A modified methylaluminoxane (MMAO) solution was used as a cocatalyst and rac–Et(H4Ind)2ZrCl2 as a catalyst. The copolymerization showed a high activity and afforded the copolymer with a 11-bromo-1-undecene incorporation ranging from 1.0 to 4.3 mol%. When using a dried methylaluminoxane (dMAO) as a cocatalyst, the incorporation ratio of 11-bromo-1-undecene increased remarkably to 25.2 mol%. It was demonstrated that the type of MAO used as a cocatalyst affects the composition of the ethylene/11-bromo-1-undecene copolymer without changing the structure of the catalytic complex.

A direct functionalization of polyolefins for blend compatibilization by an insertion of 1,1-bis(phenylsulfonyl)ethylene (BPSE)

A mild post-modification of polyethylene (PE) & polypropylene (PP) to insert phenylsulfonyl groups is reported, the modified samples demonstrated to act as blend compatibilizers.

Acyloxyimide derivatives as efficient promoters of polyolefin C–H functionalization: application in the melt grafting of maleic anhydride onto polyethylene

Acyloxyimides as a new H-abstracting agent have been developed for the radical grafting of maleic anhydride (MA) onto polyethylene in the melt state.