Olefin polymerization, metallocene catalysts

In the early stages of the metallocene-catalyzed olefin polymerizations the focus of research lay on C2- and CY-symmetric complexes [3, 12, 13], Since the beginning of the 1990s C,-symmetric catalysts have had more and more impact. The reason is... 

Insite Not a process, but a range of constrained-geometry metallocene catalysts for polymerizing olefins. Developed by Dow Chemical. 

Finally, as already mentioned, metallocene catalysts can polymerize a variety of olefins. In certain cases the structural features of the monomer lead to the formation of novel polymers. Two such examples are shown by reactions 6.6 and 6.7. It is clear that the polymerization processes involve considerable rearrangements of the bonds. Reactions 6.8 and 6.9 show the formal mechanisms of such rearrangements for 1,5-hexadiene and methylenecyclobutane, respectively. 

Insite Not a process, but a range of constrained-geometry metallocene catalysts for polymerizing olefins. Olefin block copolymers made using these catalysts have the trade name Infuse. Developed by J.C. Stevens at the Dow Chemical Company, for which he received several medals. 

Solid-state hydroxyisobutylaluminoxane co-catalysts prepared by Wu [4] were as effective in activating metallocenes in olefin polymerization as the corresponding alkyl aluminoxanes but at a lower aluminum/metal ratio. 

That is, in terms of reaction rates, the molecular weight of polyolefins is given by the ratio between the overall rate of propagation (Rp) and the sum of all rates of chain release (Rr) reactions this means that the molecular weight is dependent on the type of catalyst and the kinetics of the process, that is, the polymerization conditions (polymerization temperature, monomer concentration, catalyst/cocatalyst ratio). Hence, understanding the details of the mechanisms of chain release reactions is the key to molecular weight control in metallocene-catalyzed olefin polymerization. Here, chain release reactions (usually referred to as termination or transfer reactions) are all those steps that cause release of the polymer chain from the active catalyst, with the formation of a new initiating species (see section... 

Mulhaupt and coworkers have reported the details of several studies related to the preparation of block copolymers from thiol, maleic acid and hydroxy-functional polypropylene prepared by a metallocene catalyst [157, 158]. The same group also reported the transformation of metallocene-mediated olefin polymerization to anionic polymerization by a novel consecutive chain-transfer reaction for the preparation of polypropylene-based block copolymers [159]. The latter were also... 

Keywords metallocene catalyst, Ziegler-Natta catalyst, olefin polymerization, polyolefins, homogeneous catalysts, supported catalysts, stereoregularity, molecular weight distribution (MWD), chemical composition distribution, Unipol , Novolen , stereoselectivity, single site catalyst, multiple site catalyst, gas phase process, slurry process, homopolymerization, copolymerization. 

Since the mid-1980s bilhons of dollars have been invested worldwide in research focusing on modifying and improving the metallocene-based olefin polymerization catalysts. Key structures are summarized in Figure 2. 

Kawai, K. Yamashita, M. Tohi, Y Kawahara, N. Michiue, K. Kaneyoshi, H. Mori, R. Metallocene compound, process for producing metallocene compound, olefin polymerization catalyst, process for producing polyolefin, and polyolefin. PCX Int. Pat. Appl. WO 2001/027124A1 (Mitsui Chemicals, Inc.), April 19, 2001. 

Rather glaring deficits in our understanding of metallocene-catalyzed olefin polymerization concern the catalyst resting state(s), i.e. the identity of those species that make up the majority of the total metallocene concentration of a catalyst system at work. At first glance, it would appear quite feasible to identify at... 

The history of the breakthrough in metallocene catalysts from 1955 till now is given (Table 12). Natta pioneered the use of metallocene for olefin polymerization, but because their activity was low they were put aside. Twenty years later, much interest and attention was focussed on metallocenes with the discovery of a new activator, alumoxane. Since then innovations in this catalyst [67] have spurred the growth of business activity in this area. 

Cocatalysts for metallocene-based olefin polymerization catalyst systems Metallocenes supported on ion exchange resins Olefin polymerization catalysts Process and a catalyst for preventing reactor fouling Supported metallocene catalysts for the production of polyolefins... 

Here, we review (1) research activities in metallocene catalysts, (2) polymerization performances of metallocene catalysts and other single-site catalyst technologies, with examples for polyethylene (PE), cyclo-olefin copolymer (COC), polypropylene (PP), syndiotactic polystyrene (SPS), and cyclo-olefin polymers, and (3) the computational design of metallocene catalysts. 

Eric D. Schwerdtfeger received BS degrees in mathematics and in chemistry from the Massachusetts Institute of Technology in 2003. He then studied metallocene-mediated olefin polymerization under the dir lion of Professor Stephen A. Miller at Texas A M University where he obtained a PhD in chemistry in 2007. Since that time, he has been a research chemist at Chevron Phillips Chemical Company in Bartlesville, Oklahoma, where his research primarily involves polymerization of ethylene by heterogeneous chromium on silica catalysts. 

While group 4 metallocene-based olefin polymerization catalysts have dominated the field of homogenous olefin polymerization catalysis since the late 1950s, the development of complexes bearing non-Cp ligands as potential olefin polymerization catalysts has become a rapidly expanding area over... 

In metallocene-catalyzed olefin polymerization, the propagation reaction is terminated usually by chain transfer. It is generally believed that three major chain-transfer reactions exist in homogeneous Ziegler-Natta catalysts (Scheme 3) [8] ... 

The second type of solution polymerization concept uses mixtures of supercritical ethylene and molten PE as the medium for ethylene polymerization. Some reactors previously used for free-radical ethylene polymerization in supercritical ethylene at high pressure (see Olefin POLYMERS,LOW DENSITY polyethylene) were converted for the catalytic synthesis of LLDPE. Both stirred and tubular autoclaves operating at 30—200 MPa (4,500—30,000 psig) and 170—350°C can also be used for this purpose. Residence times in these reactors are short, from 1 to 5 minutes. Three types of catalysts are used in these processes. The first type includes pseudo-homogeneous Ziegler catalysts. In this case, all catalyst components are introduced into a reactor as hquids or solutions but form soHd catalysts when combined in the reactor. Examples of such catalysts include titanium tetrachloride as well as its mixtures with vanadium oxytrichloride and a trialkyl aluminum compound (53,54). The second type of catalysts are soHd Ziegler catalysts (55). Both of these catalysts produce compositionaHy nonuniform LLDPE resins. Exxon Chemical Company uses a third type of catalysts, metallocene catalysts, in a similar solution process to produce uniformly branched ethylene copolymers with 1-butene and 1-hexene called Exact resins (56). 

Polymerization Reactions. Polymerization addition reactions are commercially the most important class of reactions for the propylene molecule and are covered in detail elsewhere (see Olefin polymers, polypropylene). Many types of gas- or liquid-phase catalysts are used for this purpose. Most recently, metallocene catalysts have been commercially employed. These latter catalysts requite higher levels of propylene purity. 

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