Catalytic Hydroxylation of Polyethylenes

Abstract

Polyolefins account for 60% of global plastic consumption, but many potential applications of polyolefins require that their properties, such as compatibility with polar polymers, adhesion, gas permeability, and surface wetting, be improved. A strategy to overcome these deficiencies would involve the introduction of polar functionalities onto the polymer chain. Here, we describe the Ni-catalyzed hydroxylation of polyethylenes (LDPE, HDPE, and LLDPE) in the presence of mCPBA as an oxidant. Studies with cycloalkanes and pure, long-chain alkanes were conducted to assess precisely the selectivity of the reaction and the degree to which potential C–C bond cleavage of a radical intermediate occurs. Among the nickel catalysts we tested, [Ni(Me4Phen)3](BPh4)2 (Me4Phen = 3,4,7,8,-tetramethyl-1,10-phenanthroline) reacted with the highest turnover number (TON) for hydroxylation of cyclohexane and the highest selectivity for the formation of cyclohexanol over cyclohexanone (TON, 5560; cyclohexanol/(cyclohexanone + ε-caprolactone) ratio, 10.5). The oxidation of n-octadecane occurred at the secondary C–H bonds with 15.5:1 selectivity for formation of an alcohol over a ketone and 660 TON. Consistent with these data, the hydroxylation of various polyethylene materials by the combination of [Ni(Me4Phen)3](BPh4)2 and mCPBA led to the introduction of 2.0 to 5.5 functional groups (alcohol, ketone, alkyl chloride) per 100 monomer units with up to 88% selectivity for formation of alcohols over ketones or chloride. In contrast to more classical radical functionalizations of polyethylene, this catalytic process occurred without significant modification of the molecular weight of the polymer that would result from chain cleavage or cross-linking. Thus, the resulting materials are new compositions in which hydroxyl groups are located along the main chain of commercial, high molecular weight LDPE, HDPE, and LLDPE materials. These hydroxylated polyethylenes have improved wetting properties and serve as macroinitiators to synthesize graft polycaprolactones that compatibilize polyethylene–polycaprolactone blends.

Regioselective C−H Xanthylation as a Platform for Polyolefin Functionalization

“…[11] Significantly,t he regioselectivity of borylation is catalyst-controlled and selective for terminal functionalization of the polymer,w ith oxidation providing hydroxylated polyolefins.M ost recently,H artwig and coworkers reported anickel-catalyzed CÀHoxidation to install amixture of hydroxy,ketone,and chloride functional groups in polyethylene. [12] Despite these advances,m inimization of the amount of metal catalyst in the final polymer is imperative owing to cost considerations and oxidative degradation processes that can be catalyzed by these residual metals. [13] Metal-free methods for polyolefin functionalization are less studied, and none of those reported to date occur on branched materials without coincident chain scission.…”

C−H Functionalization of Commodity Polymers

“…This method was able to install up to 5mol %o ft he desired hydroxy functionality,b ut unintentional chlorination, formylation, and esterification occurred coincidently. [90] Metal-catalyzed C À Hi nsertion has proven to be as uccessful approach for the functionalization of polyolefins. PØrez and co-workers reported the copper-catalyzed generation of carbenes from ethyl diazoacetate (EDA) to achieve the addition of up to 13 mol %e ster groups onto various polyolefins ( Figure 7C).…”

“…The mild conditions and use of an abundant transition‐metal catalyst are notable compared to previous work. This method was able to install up to 5 mol % of the desired hydroxy functionality, but unintentional chlorination, formylation, and esterification occurred coincidently …”

C‐H‐Funktionalisierung von Standardpolymeren

“…Gegenüber bisherigen Arbeiten sind die milden Bedingungen und die Verwendung eines reichlich vorhandenen Übergangsmetalls als Katalysator bemerkenswert. Mit dieser Methode konnten bis zu 5 Mol‐% der gewünschten Hydroxygruppe eingebaut werden, zugleich fanden jedoch eine unbeabsichtigte Chlorierung, Formylierung und Veresterung statt …”