Olefin Polymerization by Late Transition Metal Complexes : A Root of Ziegler Catalysts Gains New Ground

Abstract

Polyolefins are of vast economic importance, which is reflected by an annual production of more than 70 million tons of polyethylene and polypropylene. While the major portion of these materials is produced with Ziegler- and chromium-based catalysts, the older free-radical process that affords low-density polyethylene (LDPE) has maintained its significance. Despite the necessity of working at over 1500 bar, 16 million tons of LDPE are currently consumed annually and new large plants continue to be built. One attractive feature of the high-pressure process is the possibility of incorporating functionalized olefins, such as vinyl acetate or acrylates. Incorporation of even small amounts of polar moieties can increase adhesion properties and compatibility of polyolefins with other materials. Another attractive feature is the different property profile of LDPE compared to the linear ethylene homo- and copolymers produced by Ziegler catalysts. In the free-radical polymerization of ethylene, short- as well as long-chain branches are formed without any added co-monomer. Short-chain branches affect polymer properties, such as crystallinity and melting temperature, and are important in controlling polyolefin application properties. Long-chain branches (typically containing 100 or more carbon atoms) particularly influence the rheology of polyolefin melts, and result in good processing properties of LDPE.These considerations exemplify existing challenges for transition metal catalyzed coordination polymerization in low-pressure processes. In regard to the desirable incorporation of polar monomers, early transition metal basedZiegler catalysts and metallocenes are, unfortunately, highly sensitive to polar reagents. By comparison, late transition metal complexes are generally much more functional-group tolerant as a result of their less oxophilic nature. In addition, they can provide access to unique polyolefin branching structures. Recent discoveries of novel olefin-polymerization catalysts based on late transition metals represent major advances. These findings are highlighted and put into perspective with previous developments, by using ethylene polymerization as a guideline.