Metallocene as Olefin Polymerization Catalysts: An Introduction

Metallocene based catalyst technology is expected to revolutionize the immense polyolefin industry, particularly in polyethylene and polypropylene markets. Some have called metallocenes the single most important development in catalyst technology since the discovery of Ziegler-Natta catalysts. This optimism is reflected in the R&D efforts of the major polyolefin producers who have spent an estimated one billion dollars on metallocene research in the past year. This amounts to approximately 75% of the total polyolefin research effort, with the remaining 25% being spent toward the incremental improvement of conventional technologies. A further one billion dollars has been committed toward the construction of new plants.

Metallocene polyolefins are projected to penetrate a broad array of polymer markets. First with the higher priced specialty markets, followed by the high volume and commodity markets. New markets are also expected to be created with the development of new classes of polymer that were not possible with conventional Ziegler-Natta technologies. It is estimated that by the turn of the century, over 20 million tons per year of metallocene based polymer will be produced accounting for over 10% of the global thermoplastics and elastomer market . The primary reason for the frenzy of activity in this area is that compared to conventional Ziegler-Natta technology, metallocenes offer some significant process advantages and produce polymers with very favourable properties.

Metallocenes are a relatively old class of organometallic complexes, with ferrocene being the first discovered in 1951 . At the time the term metallocene was used to described a complex with a metal sandwiched between two eta5-cyclopentadienyl (Cp) ligands. Since the discovery of ferrocene, a large number of metallocenes have been prepared and the term has evolved to include a wide variety of organometallic structures including those with substituted Cp rings, those with bent sandwich structures, and even the half-sandwich or mono-Cp complexes (see Figure 1)
 
 

Metallocenes as olefin polymerization catalysts also have a long history. As early as 1957 Natta reported the polymerization of ethylene with the titanocene catalyst Cp2TiCl2 and the cocatalyst triethyl aluminum, a cocatalyst traditionally used in Ziegler-Natta olefin polymerization systems. The activity of the metallocene with the Ziegler-Natta cocatalyst was very low and therefore showed little commercial promise. The current interest in metallocenes originated with a discovery by Walter Kaminsky's laboratory at the University of Hamburg in the mid 1970's . While studying a homogenous Cp2ZrCl2/Al(CH3)3 polymerization system, water was accidentally introduced into the reactor leading to an extremely active ethylene polymerization system. Subsequent studies revealed that the high activity was due to the formation of the cocatalyst methylaluminoxane (MAO) as a result of the hydrolysis of the trimethyl aluminum, Al(CH3)3. It is because of the discovery of this new cocatalyst in Kaminsky's lab that metallocenes are also commonly called "Kaminsky" catalysts.

Olefin polymerization catalyzed by metallocenes is believed to occur via the Cossee-Arlman mechanism as stolen from traditional Ziegler-Natta catalysis and shown in Figure 2. A significant amount of experimental and theoretical evidence suggests that a cationic alkyl-metallocene complex is the active species in the polymerization . The MAO cocatalyst is believed to i) alkylate the metallocene, forming the active complex, ii) scavenge for impurities, iii) stabilize the cationic center in an ion-pair interaction, and iv) possibly prevent bimetallic deactivation processes from occurring. The exact details of the reaction mechanism including the role of the cocatalyst are currently the topic of intense research and debate.
 
 

Since Kaminsky's discovery of the high activity Cp2ZrCl2/MAO ethylene polymerization system, metallocene catalysts have slowly evolved as producers push to commercialize the technology. The innovations have primarily been directed at solving the deficiencies associated with the early metallocene systems. The most serious shortcoming being that, in order to achieve the high activities, extremely high molar Al to transition metal ratios (Al:M) of between 1000-15000:1 were required. Such ratios are commercially unacceptable in terms of the cost and the amount of residues left in the polymer. (Commercial Ziegler-Natta systems typically require Al to M ratios of between 50-200:1.) A significant effort has been put into reducing the amount of MAO required and this has led to the development many systems with non-aluminum cocatalysts, such as [B(C6F5)4]- and [CH3(BC6F5)3]- . Other developments include the mono-Cp constrained geometry catalysts which have been primarily developed by Dow and Exxon. These are typically titanium based and have one Cp ligand replaced by a heteroatom that is "constrained" by a bridging group. (Nitrogen is the preferred heteroatom and silicon the most preferred bridging center as shown in Figure 2.) Significant effort has also gone into heterogenizing the catalyst system by supporting the metallocene and cocatalyst onto an inorganic support such as silica (This will be discussed in more detail in Section V). Despite all the innovations Zr still remains the most common metal center, followed by Ti and Hf. Work has also been done with Sc, Cr, and lanthanides.