Of metallocenes

C, b.p. 249 C. The prototype of metallocenes, a typical cyclopentadienylide. A sandwich compound. Oxidized to the blue ferricinium cation ((h —CsHjijFe)". ... 

The second aspect, predicting reaction dynamics, including the quantum behaviour of protons, still has some way to go There are really two separate problems the simulation of a slow activated event, and the quantum-dynamical aspects of a reactive transition. Only fast reactions, occurring on the pico- to nanosecond time scale, can be probed by direct simulation an interesting example is the simulation by ab initio MD of metallocene-catalysed ethylene polymerisation by Meier et al. [93]. 

Meier, R.J., Van Doremaele, H.J., larlori, S., Buda, F. Ab-initio molecular dynamics study of metallocene-catalysed ethylene polymerization. J. Amer. Chem. Soc. 116 (1994) 7274-7281. 

The preparation and structure determination of ferrocene marked the beginning of metallocene chemistry Metallocenes are organometallic compounds that bear cyclo pentadiemde ligands A large number are known even some m which uranium is the metal Metallocenes are not only stucturally interesting but many of them have useful applications as catalysts for industrial processes Zirconium based metallocenes for example are the most widely used catalysts for Ziegler-Natta polymerization of alkenes We 11 have more to say about them m Section 14 15... 

Metallocene Catalysts. Three types of metallocene catalysts are presentiy used ia industry Kaminsky, combination, and Dow catalysts. 

Metallocene Catalysts. Polymerization of cycloolefins with Kaminsky catalysts (combinations of metallocenes and methylaluminoxane) produces polymers with a completely different stmcture. The reactions proceeds via the double-bond opening in cycloolefins and the formation of C—C bonds between adjacent rings (31,32). If the metallocene complexes contain bridged and substituted cyclopentadienyl rings, such as ethylene(hisindenyl)zirconium dichloride, the polymers are stereoregular and have the i j -diisotactic stmcture. 

The introduction of metallocene and single-site polyethylene catalyst systems may eventually change the demand for higher olefins used as comonomers. Some sources indicate that their use will increase the demand for comonomers, but others feel that they will reduce comonomer use. At any rate, it is not expected that their introduction will have a significant effect on comonomer growth rates for the period 1992—1996 (see Olefin polymers). 

The focus of commercial research as of the mid-1990s is on catalysts that give desired and tailored polymer properties for improved processing. Development of metallocene catalyst systems is an example. Exxon, Dow, and Union Carbide are carrying out extensive research on this catalyst system for the production of polyethylene and polypropylene. 

Reduction of propylene oxide to propylene is accompHshed by use of metallocenes, such as Ti(AH5)2Cl2, and sodium amalgam (88). 

CyclopentadienylthaHium and its alkylated derivatives are used in the synthesis of metallocenes (qv) and other transition-metal cyclopentadienyl complexes (29). 

Mention has already been made in this chapter of metallocene-catalysed polyethylene (see also Chapter 2). Such metallocene catalysts are transition metal compounds, usually zirconium or titanium. Incorporated into a cyclopentadiene-based structure. During the late 1990s several systems were developed where the new catalysts could be employed in existing polymerisation processes for producing LLDPE-type polymers. These include high pressure autoclave and... 

The application of metallocene catalysis to the preparation of polypropylenes reached a commercial stage with the production by Exxon of their Achieve range in 1996 and in 1997 by Targor, the BASF-Hoechst joint venture with the introduction of Metocene. Such metallocene polypropylenes are, however, only a small proportion of the total polypropylene market, predicted at only about 3% of the total in 2005. 

Detailed modifications in the polymerisation procedure have led to continuing developments in the materials available. For example in the 1990s greater understanding of the crystalline nature of isotactic polymers gave rise to developments of enhanced flexural modulus (up to 2300 MPa). Greater control of molecular weight distribution has led to broad MWD polymers produced by use of twin-reactors, and very narrow MWD polymers by use of metallocenes (see below). There is current interest in the production of polymers with a bimodal MWD (for explanations see the Appendix to Chapter 4). 

This new edition not only includes information on the newer materials but attempts to explain in modifications to Chapter 2 the basis of metallocene polymerisation. Since it is also becoming apparent that successful development with these polymers involves consideration of molecular weight distributions an appendix to Chapter 2 has been added trying to explain in simple terms such concepts as number and molecular weight averages, molecular weight distribution and in particular concepts such as bi- cmd trimodal distributions which are becoming of interest. 

Fe Ultra fine particle Decomposition of metallocene 1060 Benzene 21... 

LLDPE with narrow molecular weight distribution exhibits a lower, a sharper melting point [3], better hot tack and heat seal properties as well as higher clarity and better impact resistance (Fig. 3), tensile strength (Fig. 4) [11], and lower levels of alkane-soluble components. The most distinguishing characteristic of metallocene-based LLDPEs is that they are not restricted by the current immutable property relationships that are... 

Narrow molecular weight distribution, which is characteristic of metallocene-based polyethylene (Fig. 7), causes processing difficulty in certain applications due to increased melt pressure, reduced melt strength, and melt fracture [14,15]. This problem can be overcome by blending the metallocene polymer with other prod-... 

Figure 3 Impact strength of metallocene LLDPE (Ethylene-hexene resin, 1 melt index). (From Ref. 11.)...

Some of the important features of metallocene-based isotactic polypropylene are as follows ... 

Table 12 Performance Data of Metallocenes in Polymerization of Liquid Propylene...

Table 13 Comparison of Metallocene-Based Isotactic Polypropylene ...

T. C. Chung, New utilities of metallocene catalysts and borane reagents in the functionalization and block/graft reactions of polyolefins, MetCon 95 Proceedings, USA, May 1995. 

---