INDEX single-site catalysts

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. 

These metallocene/aluminoxane catalysts are considered "single site" catalysts and therefore produce PE with veiy narrow polydispersily index (close to Stockmayer s distribution where the polydispeisity index is 2) and narrow chemical composition distribution, CCD. They ate usefiil in producing HDPE, and LLDPE. However, one drawback to the metallocene/aluminoxane catalysts is that significant quantities of aluminoxane by-products must be removed after production, due to the high Al/Zr ratios required for optimum performance. The primary role of the aluminoxane is in formation of the active site species. (The initial stracture in the catalytic cycle shown in Figure 5.) The series of complexation and fast alkylation... 

LyondellBasell (2009) New single-site catalyst boosts process performance of LyondellBasell s Metocene PP technology. http //lyondellbasell.mediaroom.com/index.php s=43 item=678. Accessed 27 March 2013... 

Melt Flow Ratio of the FI and MI values is a relative measure of the polymer MWD where an MFR value of 16 indicates that the MWD is very narrow due to the single-site catalyst that produces PE with a single Flory MWD with a Mw/Mn value of 2.0. Melt Index, Flow Index, MFR and density are only approximate values for each apphcation. 

The density (corrected to a 1.0 melt index value) vs comonomer content (mol%) relationship is dependent on variables such as branching distribution and branch length. For example, a single-site catalyst that provides a homogeneous branching distribution requires significantly less comonomer... 

Examination of this catalyst system under solution polymerization conditions carried out at 160°C in 216 ml of cyclohexane as solvent containing 10 or 20 ml 1-octene as comonomer, 140 psig total reactor pressure, the catalyst exhibited a relatively high activity of 2430 g PE/mmol catalyst/hr. Characterization of the polyethylene showed a molecular weight (Mw) of 114,000 with a polydispersity index value of 2.6, indicating near single-site MWD. 

However, since the 1990s, Dow scientists discovered a single-site designated as a constrained geometry catalyst system which has been used to significantly expand the product mix available from the Dow solution process. Dow presently produces copolymers based on 1 -butene, 1 -hexene and 1-octene over a wide density and Melt Index range. Note the reader is referred to Chapter 4 of this book, which discusses the Dow catalyst in more detail. 

In some applications, narrow molecular weight distributions are desirable. Polydispersity indexes in the range of 3-4 have been reported for systems in which the catalyst is supported on MgCl2 (selective promotion of certain types of sites ) or when additives are used (selective poisoning of certain types of sites ). Single-site (EXXPOL) catalysts have recently been announced, They are reportedly capable of producing polyolefins with a minimum polydispersity index of 2. As this is written, the tot is being commercialized. ... 

In recent years metallocene complexes have also been successfully used as polymerization catalysts for methyl methacrylate. Collins et al. and Soga et al. reported that cationic zirconocene complexes catalyse the polymerization of MMA [217-220]. But these metallocene complexes consisted of more components than the only metallocene complex. Hocker et al. investigated some novel single-component zirconocene complexes as catalysts for the stereospecific polymerization of MMA [221]. MMA was polymerized by the cationic bridged zirconocene complex [rPr(Cp)(Ind)Zr(Me)(THF)][BPh4] at temperatures between -20 and 20 °C. The polymerization led to mainly isotactic PMMA due to an enantiomorphic site mechanism and a low polydispersity index (1.12-2.33). Also it has been assumed that the polymerization mechanism is of living character. 

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