Polyolefins single site catalysts

Plastomers represent a major advancement for polyolefins. Their success allows polyolefins to have a continuum of products from amorphous EPR to thermoplastic PE and iPP. This development coincides with the advent of single-site catalysts these are necessary for copolymers of components of widely different reactivity such as ethylene and octene. Their rapid introduction into the mainstream polymer use indicates that this spectrum of properties and the inherent economy, stability and processibility of polyolefins are finding new applications to enter. 

The theoretical lower limit of the molecular weight distribution for the diblock OBC is 1.58. The observed MJMn of 1.67 indicates that the sample contains a very large fraction of polymer chains with the anticipated diblock architecture. The estimated number of chains per zinc and hafnium are also indicative of a high level of CCTP. The Mn of the diblock product corresponds to just over two chains per zinc but 380 chains per hafnium. This copolymer also provides a highly unusual example of a polyolefin produced in a continuous process with a molecular weight distribution less than that expected for a polymer prepared with a single-site catalyst (in absence of chain shuttling). 

This is a major achievement, mainly due to Basset and his group, in surface organometallic chemistry because it has been thus possible to prepare single site catalysts for various known or new catalytic reactions [53] such as metathesis of olefins [54], polymerization of olefins [55], alkane metathesis [56], coupHng of methane to ethane and hydrogen [57], cleavage of alkanes by methane [58], hydrogenolysis of polyolefins [59] and alkanes [60], direct transformation of ethylene into propylene [61], etc. These topics are considered in detail in subsequent chapters. 

Figure 6.6 Structure of a single-site catalyst described by Goodall. Catalyst is capable of copolymerizing ethylene with polar comonomers without cocatalysts (BL Goodall, NT Allen, DM Conner, TC Kirk, LH McIntosh III and H Shen, International Conference on Polyolefins, Society of Plastics Engineers, Houston, TX, February 25-28, 2007).

These developments and non-metallocene single site catalysts in general represent the next wave of innovation in polyolefin catalysis which should permit production of polyethylenes with unique properties at lower cost. They will complement, and perhaps even supplant, many of the metallocene single site catalysts commercialized in the 1990s. 

Progress in (heterogeneous) Ziegler-Natta catalysis has continued unabated over the last 50 years, while the last 20 years have seen the advent of homogeneous (metallocene and other single-site) catalysts. However, despite the enormous research effort and many advances made in the field of homogeneous catalysis, polyolefins manufacture is still dominated by Ziegler-Natta systems. 

In contrast to Group IV-based polymerization catalysts, late transition metal complexes can carry out a number of useful transformations above and beyond the polyinsertion reaction. These include isomerization reactions and the incorporation of polar monomers, which have allowed the synthesis of branched polymer chains from ethylene alone, and of functional polyolefins via direct copolymerization. The rational design of metallocene catalysts allowed, for the first time, a precise correlation between the structure of the single site catalyst and the mi-crostructure of the olefin homo- or copolymer chain. A similar relationship does not yet exist for late transition metal complexes. This goal, however, and the enormous opportunities that may result from new monomer combinations, provide the direction and the vision for future developments. 

S. P. Chum, C. I. Kao, and G. W. Knight, Structure, properties and preparation of polyolefins produced by single-site catalyst technology, in Metallocene-Based Polyolefins Preparation, Properties and Technology, Wiley Series in Polymer Science, Vol. 1, J. Schiers and W. Kaminsky (eds.), Wiley, New York, 2000, p. 261. 

Their ability as so-called single site catalysts to produce polyolefins with narrow molecular weight distributions MJMn 2). 

One of the remarkable advantages of metallocene catalysts is their ability to make polyolefins with much more uniform microstructure than the Ziegler-Natta or the Phillips catalysts. Metallocene catalysts are considered to have only one type of active site (single-site catalysts) making polymer chains with the same average properties, while heterogeneous Ziegler-Natta and Phillips catalysts are multiple-site catalyst that makes polyolefins with broad, and sometimes multimodal, microstructural distributions [9]. 

Equation 5.1 is applicable to polyolefins made with single-site catalysts, such as metallocenes, and predicts a polydispersity index of 2.0. It is discussed later how this equation can also be used to model the CLD of polyolefins made with multiple-site catalysts, such as heterogeneous Ziegler-Natta and Phillips catalysts. Despite its simplicity, this equation can be used to predict the complete CLD of single-site polyolefins instantaneously using an easy-to-estimate parameter, t. 

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