Ethylene polymerization iron catalysts

We have discussed the structure and synthesis of the library of molecular catalysts for polymerization in Section 11.5.1. In the present section we want to take a closer look at the performance of the catalyst library and discuss the results obtained [87], The entire catalyst library was screened in a parallel autoclave bench with exchangeable autoclave cups and stirrers so as to remove the bottleneck of the entire workflow. Ethylene was the polymerizable monomer that was introduced as a gas, the molecular catalyst was dissolved in toluene and activated by methylalumoxane (MAO), the metal to MAO ratio was 5000. All reactions were carried out at 50°C at a total pressure of 10 bar. The activity of the catalysts was determined by measuring the gas uptake during the reaction and the weight of the obtained polymer. Figure 11.40 gives an overview of the catalytic performance of the entire library of catalysts prepared. It can clearly be seen that different metals display different activities. The following order can be observed for the activity of the different metals Fe(III) > Fe(II) > Cr(II) > Co(II) > Ni(II) > Cr(III). Apparently iron catalysts are far more active than any of the other central metal... 

Examples of supports modifying the properties of transition metal oxides have also appeared in the literature. Recent work points to iron oxide phases as important species in Fischer-Tropsch synthesis (3 ). Iron oxide supported on SiO2 (4 ) and TiO ( ) resist reduction under conditions in which bulk iron oxide easily reduces. Thus supported iron oxide catalysts are potentially interesting Fischer-Tropsch catalysts. The extensive studies on ethylene polymerization catalysts suggests that chromium (VI) species exist on a SiOp surface at temperatures above which bulk chromic anhydride (CrOg) decomposes ( ). 

In addition to the FBI complexes, several additional classes of ethylene polymerization catalysts based upon tridentate ligand complexes of iron have been developed. These include the furan, 5, and pyrrole, 6, derivatives. The oxidation state of the... 

Griffiths, Britovsek, Gibson, and Gould report an ethylene insertion barrier of 3 kcal/mol for the recently reported iron ethylene polymerization catalyst, 5. In contrast Deng, Margl, and Ziegler find insertion to not be a facile process for a related small model complex. For 5 they find that the alkyl complex must isomerize prior to insertion, insertion then proceeding with a 0.3 kcal/mol barrier. 

The initiation of the polymerization of alkylene oxides with iron has been known for some time. A recent patent extends this concept to the alkoxylation of many active hydrogen compounds. The catalyst for this process is a polycrystalline iron oxide (a-iron(lll)oxide). For example, in an autoclave, under nitrogen, n-decanol and 2 wt% of the iron catalyst are treated with ethylene oxide at a pressure of less than 6 bars for 4 hr. The resulting liquid had a degree of polymerization of 5 [42]. 

As with ethylene polymerization, the metal-mediated conversion of ethylene to short chain Hnear a-olefins (range C4-C20) represents an important industrial process. Such oHgomers find considerable use in the manufacture of detergents, and plasticizers and in the production of linear low density polyethylene (LLDPE). An additional benefit of bis(imino)pyridine iron and cobalt catalysts is the ability to tune the Hgand environment to allow the formation of exclusively Unear a-olefins with activities and selectivities comparable with other weU-known late transition metal catalysts (e.g., the SHOP catalyst) [115]. 

Despite the discovery of iron and cobalt catalysts capable of ethylene polymerization occurring only ten years ago, the impact on polymerization catalysis has been... 

LTM complexes have attracted increasing attention [22, 23], especially after the reports of Brookhart and Gibson about new Fe (Il)-based complexes containing 2,6-bis(imino) pyridyl ligands as efficient catalyst precursors for ethylene polymerization (Scheme 3.12). After MAO activation, the complexes show high activity and produce strictly linear PE [24]. The parent catalysts that were based on iron(II)-and cobalt(ll)-based precursors showed that there is a clear relationship between the molar mass of the polymer produced and the bulky groups on the ortho position of the aromatic auxiliary ligands. 

Abu-Surrah et al. [29] also reported on highly active 2,6-bis(arylimino) pyridine iron(II)- and cobalt(II)-based ethylene polymerization catalysts which lack the ortho alkyl substituents on the aryl groups. Modifications of the steric bulkiness of the aromatic groups in the tridentate ligands influenced not only the catalytic activity, but also the molecular weight, and for the first time the microstructure of the resulted material (Scheme 3.14) [28]. 

McTavish et al. [30] reported another form of catalysts containing BIP-based ligands. The resultant iron dichloride complexes were highly active ethylene polymerization catalysts after the activation with methylaluminoxane (MAO). 

S. McTavish, G. Britovsek, T. Smit, V. Gibson, A. White, D. Williams, Iron-based ethylene polymerization catalysts supported by Bis(imino)pyridine ligands derivatization via depiotonation/alkylation at the ketimine methyl position. J. Mol. Cat. A (Them. 261, 293-300 (2007)... 

G. Britovsek, S. Baugh, O. Hoarau, V. Gibson, D. Wass, A. White, D. Williams, The role of bulky substituents in the polymerization of ethylene using late transition metal catalysts a comparative study of nickel and iron catalyst systems. Inorg. Chimica Acta 345, 279-291 (2003)... 

Baugh et al. [22] synthesized and characterized a series of nickel(II) and iron(II) complexes of the general formula [LMX2] containing bidentate (for M = Ni) and tridentate (for M = Fe) heterocycle-imine ligands. Activation of these pre-catalysts with methyl aluminoxane yields active catalyst systems for the oligomerization/polymerization of ethylene. Compared to a-diimine nickel and bis (immo)pyridine iron catalysts, both metal systems provide only half of the steric protection and consequently the catalytic activities are significantly lower. 

Supported bis(imino)pyridyl metal catalysts on SiO2/Si(100) wafers were used for ethylene polymerization in solvent. Since the catalysts are covalently anchored to the flat surface, polymers can only grow perpendicularly to the flat surface and form films of almost constant height. As the polymerization reaction occured weU below the dissolution temperature of polyethylene, all polymer remained on the catalyst surface [27]. A typical morphology is shown in the SEM images in Fig. 1 for a PE film polymerized from the SiO2/Si(100) wafer-supported bis(imino)pyridyl iron(II) catalyst in a toluene solution. Islands of polymer are observed on the top of the PE films. Between these islands, a fiber texture is found. This morphology is more apparent from the side view. 

It was reported [20, 96] that the counter-ion type on mica influences the polymerization behavior of the supported catalyst Even though micas do not have exactly the same structure of montmoriUonites, they are considered as clay materials and some of their behavior may be translated to montmoriUonites. Hiyama et al. [20] compared ethylene polymerization with an iron catalyst supported on micas with different counterions. They observed that when the polymerization catalyst was supported on M" mica (where M" = Mg, Zn, and Fe " ), polymerization activities were approximately 10-fold higher than those obtained when Na mica was used as a support. Kurokawa et al. [96] supported Cp2ZrCl2 on fluorotetrasUicic micas with different counter ions. During ethylene polymerization, itwas observed that the polymerization activity of the catalyst supported on Na mica (Cp2ZrCl2/... 

During 1997 and 1998, highly active iron(II)- and cobalt)II)-based ethylene polymerization catalysts bearing 2,6-bis(imino)pyridyl ligands (Fig. 5) were described independently by Brookhart (183,184), and Gibson (187,188), as well as by DuPont (185,186). Oligomerization of ethylene to linear a-olefins (189) as well... 

Griffiths, E. A. H. Britovsek, G J. R Gibson, V. C. Gould I. R. Highly active ethylene polymerization catalysts based on iron An ab initio study. Chem. Commun. 1999, 1333-1334. 

Deng, L. Margl, R Ziegler, T. Mechanistic aspects of ethylene polymerization by iron(II)-bisimine pyridine catalysts A combined density functional theory and molecular mechanics study. J. Am. Chem. Soc. 1999,121, 6479-6487. 

Ray et al. [93] treated organically modified MMT (OMMT) with a MAO solution after vacuum-drying at 100°C. The resulting MAO-treated clay was subsequently used for ethylene polymerization in the presence of 2,6-bis [l-(2,6-diisopropylphenylimino)ethyl]pyridine iron(ll) dichloride with additional MAO in a glass reactor. In addition, they compared the methods of nanocomposite preparation and observed that the nanocomposite produced by catalyst supported on MAO-pretreated OMMT was more efficiently exfoliated than the nanocomposite produced when only a mixture of catalyst and clay was used. This result led them to conclude that at least some of the active centers resided within the clay galleries. Similarly, Guo et al. [100] in a separate studies successfully used pyridine diimine-based iron(ll) catalysts for preparation of exfoliated PE/clay nanocomposites. 

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