Deprotonation of coordinated ethylene may start Phillips catalysis.

The metal-catalyzed polymerization of simple olefins, such as ethylene and propylene, has been one of the transformational discoveries in chemistry of the 20th century. It is difficult to imagine today’s civilization without ubiquitous plastics, such as polyethylene and related polymers. Although often attributed to Karl Ziegler and Giulio Natta, the winners of the 1963 Nobel Prize in Chemistry for the discovery of titanium-based catalysts and their use in propylene polymerization, much (∼40–50%) of the polyethylene produced worldwide is actually made with catalysts containing chromium as the catalytic metal. Discovered in 1951 by Paul Hogan and Robert Banks at the Phillips Petroleum Co. in Bartlesville, Oklahoma, the so-called “Phillips catalyst” remains a major workhorse of the polymerization industry to this very day (1). Despite its significant commercial impact, this apparently “simple” heterogeneous catalyst has been the focus of longstanding scientific arguments regarding its mechanism of operation. Most enigmatic among the questions posed is the issue of its initiation. In other words, because the formation of the polymer chains presumably proceeds by repeated insertion of olefin monomer into the metal–carbon bond of an ever-growing chromium alkyl (2), what sequence of chemical events transforms the inorganic catalyst precursor into an organometallic compound— i.e., a material containing a covalent chromium–carbon bond—that catalyzes the polymerization?Delley et al. address this important question, among others, in their elegant study published in PNAS (3). To appreciate the conundrum posed by the Phillips catalyst, it is necessary to review its preparation and the conditions of its use. At the most basic level, the catalyst consists of chromium atoms bound to the surface of an oxide support (typically silica; i.e., SiO2). It is prepared by contacting the support particles with an aqueous solution of a chromium compound, followed by calcination in oxygen at high temperatures (600–900 °C). Although hexavalent chromium compounds (e.g., CrO3) were originally used as chromium source, concerns about the carcinogenicity of the former eventually led to the substitution of a variety of simple Cr(III) salts as precursors. Either way, reaction with oxygen at high temperature oxidizes the surface-bound chromium to Cr(VI). Another important effect of the heat treatment is the condensation of silanol groups [(-O)3Si-OH], which terminate the surface of silica under ambient conditions. Extrusion of water leaves behind a surface populated by hexavalent chromium, likely in the form of chromate esters, Si–O–Si linkages, and the occasional isolated silanol group. A schematic representation of this transformation is shown in Fig. 1A. The hexavalent catalyst precursor can be directly introduced into the polymerization reactor. However, this process results in an induction period, with full catalytic activity developing over the course of an hour or so. During this time, the chromium is likely reduced and transformed into an organometallic derivative (i.e., it acquires a ligand— such as alkyl group or a hydride—that can start a polymer chain). In this context it is worth noting that the Phillips catalyst does not require any cocatalyst, which might serve as the source of the organic ligand. Typical additives for Ziegler/Natta catalysts are aluminum alkyls (AlR3), which are thought to transfer their alkyl groups to the titanium. Because no such compounds are needed for the development and maintenance of the catalytic activity of the chromium catalyst, it must somehow acquire its alkyl group from the substrate (ethylene, C2H4). The chemistry of chromium derives much of its interest from the multitude of possible oxidation states exhibited by this metal, ranging from 0 to +VI. The decades since