Chemical Modification of Oxidized Polyethylene Enables Access to Functional Polyethylenes with Greater Reuse

Abstract

Polyethylene is a commodity material that is widely used because of its low cost and valuable properties. However, the lack of functional groups in polyethylene limits its use in applications that include adhesives, gas barriers, and plastic blends. The inertness of polyethylene makes it difficult to install groups that would enhance its properties and enable programmed chemical decomposition. To overcome these deficiencies, the installation of pendent functional groups that imbue polyethylene with enhanced properties is an attractive strategy to overcome its inherent limitations. Here, we describe strategies to derivatize oxidized polyethylene that contains both ketones and alcohols to monofunctional variants with bulk properties superior to those of unmodified polyethylene. Iridium-catalyzed transfer dehydrogenation with acetone furnished polyethylenes with only ketones, and ruthenium-catalyzed hydrogenation with hydrogen furnished polyethylenes with only alcohols. We demonstrate that the ratio of these functional groups can be controlled by reduction with stoichiometric hydride-containing reagents. The ketones and alcohols serve as sites to introduce esters and oximes onto the polymer, thereby improving surface and bulk properties over those of polyethylene. These esters and oximes were removed by hydrolysis to regenerate the original oxygenated polyethylenes, showing how functionalization can lead to materials with circularity. Waste polyethylenes were equally amenable to oxidative functionalization and derivatization of the oxidized material, showing that this low- or negative-value feedstock can be used to prepare materials of higher value. Finally, the derivatized polymers with distinct solubilities were separated from mechanically mixed plastic blends by selective dissolution, demonstrating that functionalization can lead to novel approaches for distinguishing and separating polymers from a mixture.

A versatile modification strategy to enhance polyethylene properties through solution-state peroxide modifications

“…Yet even using just 0.5% peroxide loading in air showed oxygenated moieties ( M19 ), indicating that these functional groups arise from the presence of air. Some studies have shown that oxygenated functional groups can enhance the polymer properties, 68 yet here, the presence of oxygenated moieties in M19 clearly decreases the viscosity profiles, providing further support that the enhanced viscosity profiles are due to the presence of long chain branching.…”

Synthesis and Deconstruction of Polyethylene-type Materials

“…These postpolymerization reactions, however, can suffer from drawbacks such as uncontrolled chain cleavage and selectivity, if promoted in a free-radical fashion. , Substantial efforts led to the development of more controlled, transition-metal catalyzed C–H functionalization. − The introduction of keto and hydroxyl groups via this method can enhance materials properties, for example, enabling better adhesion to polar surfaces . Despite significant advances, the functional group selectivity remains a main challenge for postpolymerization modification of polyethylenes, and usually mixtures of ketones and hydroxyl groups are obtained from Ni- or Ru-catalyzed C–H functionalization. , The synthesis of exclusively keto-containing polymers via this pathway was also reported but requires an additional oxidation step of such mixed “oxo-polyethylenes” by Cp*Ir-catalyzed transfer dehydrogenation using acetone as oxidant . Even though a high selectivity for keto groups (>99%) can be achieved initially with Cu catalysts and benzaldehyde as a reagent, the C–H oxidation results in a significant molecular weight decrease vs the original material .…”

Review on Recycling of Cross-Linked Polyethylene

“…This advancement facilitates programmed chemical decomposition. 6 Depending on the desired qualities of the final product, polyethylene is manufactured at high pressures and temperatures in the presence of any one of a number of catalysts. Depending on the synthesis technique, other structures (leading to long and short branches) may be present.…”

“…This process produces monofunctional variants with bulk properties surpassing those of unmodified polyethylene. This advancement facilitates programmed chemical decomposition . Depending on the desired qualities of the final product, polyethylene is manufactured at high pressures and temperatures in the presence of any one of a number of catalysts.…”