Single site catalyst silica

Future challenges for polymerization model catalysts are to study the structure of polymers below their melting point in what is called the nascent morphology. Such work can be undertaken on silica-supported chromium catalysts as discussed above, or on so-called single-site catalysts, such as metallocenes, applied on flat silica supports. 

A new strategy has been used by Tilley et al. to prepare a series of single-site catalysts that consist of iron [13] and cobalt [14] centers supported on mesoporous SBA-15 silica. The iron centers were introduced via grafting reactions of the tris(tert-butoxy)siloxy-iron(III) complex [Fe(OSi Bu3)3(THF)] with SBA-15 in dry hexane to form, finally, an immobilized iron(III) complex of the type [(=SiO) Fe 0Si(0 Bu)3 2(THF)j and to eliminate H0Si(0 Bu)3. Calcination of these species... 

Tris-allyl-neodymium Nd(//3-C3I Ishdioxane which performs as a single site catalyst in solution polymerization was heterogenized on various silica supports which differed in specific surface area and pore volume. The catalyst was activated by MAO. In the solution polymerization the best of the supported catalysts was 100 times more active (determined by the rate constant) than the respective unsupported catalyst [408]. 

High-Density Polyethylene (HDPE). Polymerization of ethylene to polyethylenes is most often carried out at low temperature and pressure, using either the Ziegler aluminum triethyl plus titanium tetrachloride catalyst system, the Phillips chromic oxide plus silica plus alumina system, or more recently the newer metallocene single-site catalyst systems. 

FIGURE 81 Statistical distribution of LCB within the MW distribution as determined from model calculations based on random incorporation. A Single-site catalyst having Mw/Mn— 2.0, B Cr/silica catalyst having MW/MN 83, assuming that sites exhibit the same relative preference for macromer incorporation as for 1-hexene. 

In a tour-de-force with respect to the arcane patent literature, Hlatky reviews heterogeneous single-site catalysts. This is complemented by an article by Fink, Steinmetz, Zechlin, Przybyla, and Tesche on the specific subject of propylene polymerization with silica-supported metallocene/MAO catalysts. The mechanistic role of MAO and other activators is then analyzed by Chen and Marks. They define structure-activity relationships that are certain to promote future research and advances. 

Zeolites have begun to attract more interest as supports for single-site catalysts. Unlike silica, with its amorphous structure and wide distribution of pore sizes, zeolites have more regular structures, pore sizes, and supercages. Because the zeolite countercations (Na+, H+, NH4+) can be ion exchanged, the possibility exists of electronically tuning the support. 

Another worthwhile area of endeavor is new support materials, ones which are as single-sited as the catalysts they carry. Eor example, the zeolite carriers discussed above have more uniform pore sizes and volumes than the amorphous silicas hitherto widely used. The importance of single-site catalysts lies in the ability to characterize them comprehensively and alter them in a rational fashion in order to enhance desired polymer properties extending this degree of understanding and control to their interaction with supports and their performance in numerous polymerization processes could only be beneficial. 

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