1,3,5-Triazacyclohexane Complexes of Chromium as Homogeneous

Chapter 7

1,3,5-Triazacyclohexane Complexes of Chromium as Homogeneous Model Systems for the Phillips Catalyst 1

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Randolf D. Köhn , D. Smith , D. Lilge , S. Mihan , F. Molnar , and M. Prinz Downloaded by FUDAN UNIV on January 17, 2017 | http://pubs.acs.org Publication Date: July 31, 2003 | doi: 10.1021/bk-2003-0857.ch007

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Department of Chemistry, University of Bath, Bath BA2 7AY, United Kingdom Basell Polyolefins GmbH, Carl-Bosch-Strasse M505, D-67056 Ludwisshofer, Germany BASF Aktiengesellschaft GKT/P B1, 87056 Ludwigshafen, Germany 2

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Complexes of N-substituted 1,3,5-triazacyclohexanes with CrCl can be activated by MAO or PhNMe H(B(C F ) )/AlR to give solutions that can polymerize and/or trimerize ethylene depending on the N-substituents R with activities and polymer products similar to those of the heterogeneous Phillips catalysts, α-olefins are selectively trimerized or copolymerized with ethylene. Variation of these substituents R showed a large dependence of the trimerization/polymerization ratio on branching in the N-substituent. Spectroscopic studies show that the triazacyclohexane stays co-ordinated during the catalysis and that mono-nuclear metallacyclic complexes with a weakly coordinating anion in one coordination site are likely involved. 3

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© 2003 American Chemical Society

Patil and Hlatky; Beyond Metallocenes ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Downloaded by FUDAN UNIV on January 17, 2017 | http://pubs.acs.org Publication Date: July 31, 2003 | doi: 10.1021/bk-2003-0857.ch007

Introduction Several transition metal systems have been found over the past 50 years that can catalyze the polymerization and oligomerization of olefins. Heterogeneous Ziegler-Natta systems based on early transition metals are the most successful and produce a large variety of polyolefms. Homogeneous single site model systems, foremost the metallocenes, have been developed which are able to produce highly stereo regular polymers and have become useful industrial catalysts. The study of these homogeneous systems has tremendously improved the detailed understanding of the mechanism. Chain propagation is largely based on olefin insertion into metal-C or H bonds followed by chain transfer via ß-H elimination. This general mechanism is often termed the hydride mechanism and seems to be common among other transition metal systems as well. Late transition metal systems often have a higher tendency for chain transfer versus propagation that leads to good dimerization or oligomerization catalysts. One highly successful system is the nickel based SHOP catalyst for the ethylene oligomerization. Blocking the chain transfer pathway with sterically demanding groups in similar systems has lead to an exciting development of highly active homogeneous polymerization catalysts based on late transition metals such as Fe, Co, N i , and Pd. Again, a hydride mechanism appears to be involved.

Chromium based Catalysts

Polymerization The heterogeneous Phillips catalysts (1,2,3,4,5) based on C r 0 / S i 0 for the polymerization of ethylene have been known as long as the Ziegler-Natta systems and still produce a large fraction of the world production of H D P E (>7 million t/a)(6,7). This system has many unusual features compared to the other transition metal catalysts. First of all, it is a purely inorganic system that does not require any metal alkyl co-catalyst and can be activated by ethylene itself although activation can also be achieved with other reducing reagents such as C O or aluminum alky Is. Contrary to most catalysts based on the hydride mechanism, the molecular weight of the polymer is quite insensitive to hydrogen but can be regulated by the reaction temperature. End group analysis of the polymer shows not only the expected single methyl and vinyl end groups but additional methyl groups, vinylidene and some internal olefin end groups. Such end groups have been found in systems based on the hydride mechanism when additional isomerisation, mis-insertion or chain-walking steps are involved. 3


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However, these groups are found in Phillips systems in consistently similar ratios. The Phillips catalysts show characteristically broad molecular weight distributions. Previous studies have shown that several different chromium sites are formed during the calcinations and reduction steps and that only a small fraction of these are actually active. When activated with metal alkyls the Phillips catalyst can also oligomerize ethylene to α-olefins that can be in-situ co-polymerized giving polymer with side chains. However, these oligomers do not follow the Flory-Schultz distribution typical for systems based on the hydride mechanism. There is a high selectivity for the trimer of ethylene, 1-hexene, and subsequent co-polymers with butyl side chains. Co-polymers can also be obtained directly by adding α-olefins. Contrary to the early and late transition metal systems, the nature of the active species and the mechanism in the chromium systems is still a matter of debate. This is largely due to the fact that the Phillips catalyst is very difficult to study. Highly active catalysts are obtained only at high dilution of chromium on the silica surface (< 1 w%) and the chemistry of the surface chromium is very rich with many different species at various oxidation states and nuclearities. However, only a small fraction of the total chromium is known to be active. Thus, surface analytical methods mostly detect inactive compounds. Generally, a mononuclear chromium compound in the oxidation state +III directly bound to the silica via two or three oxygen atoms is believed to be the active species (Figure 1). How additional Cr-C bonds are formed during catalysis is still unclear.

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Figure 1. Possible co-ordination environment of the active site in the Phillips catalyst

Many homogeneous model systems (8) show only limited activity (9,10) and most of them fail to reproduce the properties of the Phillips catalyst and a true model has yet to be found. As an alternative chromium system, the Union Carbide catalysts based on chromocene on silica has been introduced 30 years ago which is more accessible to studies. One cyclopentadienyl appears to stay attached to chromium and various mono-cyclopentadienyl chromium complexes show polymerization activity. However, these cyclopentadienyl systems are quite


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different from the Phillips catalyst. They generally do not co-polymerize ethylene and α-olefins, the molecular weight of the polymer shows high hydrogen response, the polymers do not have the same end group distribution and are mostly linear and there is no selectivity for trimerization.


Ethylene Trimerization In an interesting development after the initial report by Briggs in 1987 (77), homogeneous chromium systems have been found that can trimerize ethylene with >90% selectivity to 1-hexene. These systems generally consist of some soluble chromium complex, aluminum alkyls and some amine, mostly pyrroles (72) or more recently PNP ligands (75). The increasing demand in 1-hexene has sparked a growing interest in these selective trimerization catalysts as can be seen in the number of publications in the past few years (Figure 2).

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Figure 2. Chemical abstract entries on trimerization of ethylene However, little is known about the active species in these complex mixtures largely due to the difficulty of obtaining useful N M R spectra of these highly paramagnetic compounds. Briggs (77) and others have proposed a mechanism via metallacycles analogous to Figure 8 and Jolly (14) was able to support this by showing that complex shown in Figure 3 can react with ethylene under reducing conditions (activated Mg) to a metallacyclopentane complex and that the larger metallacycloheptane complex readily decomposes under reductive elimination to the trimer 1-hexene. However, insertions into the Cr-C bonds are prevented by the amine donor and addition of M A O leads to an active ethylene polymerization catalyst without trimerization probably by opening the the metallacycle.


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1,3,5-Triazacyclohexane Complexes of Chromium as Homogeneous

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