Olefin Polymerisation Catalysts

The isolobal analogy between cyclopentadienyl and imido ligands held promise for the development of new a-olefin polymerisation catalysts based on imido ligation. However, our early endeavours were to prove disappointing. Cationic 

The relatively short journey from the Group 4 metallocenes on the left hand side of the transition series to Group 6 bis(imido) metal systems in the middle of the series has led to a continuing journey towards the late transition metals in a quest for new olefin polymerisation catalysts. This journey is still in progress and so an account of the outcome of these studies will have to wait until another occasion. 

---

A number of highly active ethylene polymerisation catalysts have resulted from the combination of functionalised NHC ligands with Ti, the first of these was the bis(phenolate)carbene ligated complex 3 [8], Upon activation with modified MAO (MMAO), this species gave an activity of 290 kg-mol bar h in the one test reported, making it one of the most active carbene-based olefin polymerisation catalysts known. In later work the same complex was evaluated with straight MAO activation, and activities of up to ca. 100 kg mol -bar" -h" were reported for linear polyethylene production [9],... 

Iron-Based and Cobalt-Based Olefin Polymerisation Catalysts... 

For more general overviews of post-metallocene a-olefin polymerisation catalysts, the reader is referred to a series of reviews [8, 9, 10, 11, 12], while recent reviews pertaining to the importance of 2,6-bis(imino)pyridines and to iron and cobalt systems per se have also been documented [13, 14],... 

Extensive efforts have also been made to develop olefin polymerisation catalysts based on metallocenes with only one ligand of the cyclopentadienyl type. Ethylene-,dimethylsilylene- or tetramethyldisilylene-bridged mono(l-tetra -methylcyclopentadienyl), mono(l-indenyl) or mono(9-fluorenyl)-amidotita-nium complexes, such as dimethylsilylene(l-tetramethylcyclopentadienyl)(t-butyl)amidotitanium dichloride [Me2Si(Me4Cp)N(/-Bu)TiCl2] (Figure 3.10), have recently attracted both industrial and scientific interest as precursors for methylaluminoxane-activated catalysts, which polymerise ethylene and copolymerise ethylene with 1-butene, 1-hexene and 1-octene [30,105,148-152]. 

Homogeneous alkylaluminium (alkylaluminoxane)-free olefin polymerisation catalysts are commonly referred to as catalysts containing group 4 metallocene cationic species. These catalysts resemble the structure and properties of the respective metallocene-based Ziegler-Natta catalysts but, by definition, are not included among the latter catalysts. The development of alkylaluminium-free metallocene-based olefin polymerisation catalysts is connected with discoveries... 

Homogeneous alkylaluminium-free olefin polymerisation catalysts also comprise non-metallocene cationic group 4 metal complexes such as those with benzyl ligands [162]. A distinct group of alkylaluminium-free homogeneous olefin polymerisation catalysts consists of nickel complexes [181-183],... 

Similarly to the case of heterogeneous Ziegler Natta olefin polymerisation catalysts, the coordination of the olefin molecule at the cationic metallocene species, due to n bond formation (Figure 2.1), leads to lowering of the energy of the resultant n complex, e.g. the [Cp 2Mt(R)-olefin]1 [Alx(R)x OxX2] complex, which results in activation of the catalyst Mt-C bond and olefin C=C bond for the insertion reaction [136]. 

At the end of considerations of the role of alkylaluminoxanes as activators for metallocenes as Ziegler-Natta olefin polymerisation catalysts, it should be noted that the analogy between methylaluminoxane and simple acidic moieties does not appear to hold. For example, highly Lewis acidic perfluorinated boranes, such as Bf CgFs, show enhanced activity as activators when compared with [Al(Me)0]x, but the stability of the resulting zirconocene-based catalyst is drastically lower than that of the catalyst formed with [Al(Me)0]x [98]. 

Since NHC are excellent phosphane mimics, the use of functionalised carbenes as ligands in olefin polymerisation catalysts would be self evident. An excellent model reaction is the commercially used Shell Higher Olefin Process (SHOP), initially developed by Keim [100-102], Here, enolisable ketophosphanes are used as ligands for nickel(II) catalysts (see Figure 2.13). A variant of this is the use of phosphino phenol ligands introduced by Heinicke [103-105]. 

Tris(pentafluorophenyl)borane, a compound first made by Stone et al. in 1963, has in recent years become one of the most widely applied boron reagents, as a highly effective activator for metallocene-based olefin polymerisation catalysts. There are several reasons for this it is a strong Lewis acid, unlike boron trihalides it is resistant to hydrolysis, and it possesses strong B-C and C-F bonds which make it essentially inert to chemical attack for example, it can be recovered unchanged from neat elemental bromine. 

Radical anions and carbanions as donors in electron-transfer processes Organometallic complexes olefin polymerisation catalysts... 

Iron-Eased and Cobalt-Eased Olefin Polymerisation Catalysts, Springer,... 

In 1957, PP was commercially produced by Montecatini as Moplen. Recently, metallocenes have attracted widespread attention as the new generation of olefin polymerisation catalysts. Metallocene catalysts provide enhanced control over the molecular make up of PP, and grades with extremely high isotacticity and narrow molecular weight distribution (MWD) are possible. Properties of metallocene-polymerised PP are further discussed in Section 2.5. 

Gibson VC, Solan GA (2009) Iron-based and cobalt-based olefin polymerisation catalysts. In GuanZ (ed) Topics in organometallic chemistry. Springer, Berlin, doi 10.1007/3418 2008 10... 

---