Polymerisation with Metallocenes

Metallocenes such as Cp2TiCl2 and Cp2ZrCl2 alone are capable of polymerising styrene to an atactic polymer (involving a free radical propagation mechanism) [97]. The same metallocenes activated with methylaluminoxane form active catalysts for the polymerisation of styrene their productivity and syn-diospecificity, however, are not very high. In contrast, when activated with aluminium alkyls, these metallocenes do not afford catalysts that might be active in the polymerisation of styrene [98,99]. 

Mechanistic studies on u/Mu-titanocene methylaluminoxane systems have revealed the formation of intermediate compounds [60,61]  

These compounds exhibited fairly high catalytic activity for the syndiospecific polymerisation of styrene. 

It is worth mentioning that a rar.- / ,v -titanocene methylaluminoxane catalyst, such as rac.-Ph2C(Cp)(Ind)TiCl2—[Al(Me)0]x, which yields an isotactic polymer in propylene polymerisation, promotes the syndiospecific polymerisation of styrene [73,100]. This is the first example where two different stereoregular polymers, isotactic and syndiotactic, can be obtained using the same catalyst in the case of two different monomers. 

Heterogeneous non-supported or supported catalysts that have been used successfully for the syndiospecific polymerisation of styrene are presented in Tables 4.3 and 4.4 [62,63,66-69,101-103], 

---

High pressure polymerisation with metallocene catalysts 527... 

Figure 3.19 Chain migratory insertion mechanism for olefin polymerisation with metallocene-based catalysts...

The described chain migratory insertion mechanism, which operates in olefin polymerisation with metallocene-based single-site catalysts, follows that proposed by Cossee [268,277,278] for olefin polymerisation with heterogeneous catalysts there is, however, no back skip of the polymer chain to the previously occupied position prior to the coordination of the next monomer molecule, but rotation of the chain around the axis of the Mt-CH2 bond takes place (Figure 3.19) [358],... 

As regards the insertion mechanism in a-olefin polymerisation with metallocene-based catalysts, one should recall that a chain migratory mechanism is operating, but occasional skipped insertion or a constant skipped insertion mechanism may also be operating (Figure 3.17), depending on the kind of catalyst. 

As regards higher 7-olefins, their polymerisation with metallocene-based catalysts of class IV with Cs symmetry affords highly syndiotactic polymers as in the case of propylene [117]. This is a consequence of enantiomorphic site control over the polymerisation stereochemistry. 

Syndiotactic polymers of higher a-olefins such as 1-butene and 4-methyl-1-pentene are produced by homogeneous metallocene-based catalysts [117, 429, 430], In contrast to polymerisation with metallocene-based catalysts, higher a-olefins are much less reactive in polymerisation with soluble vanadium-based catalysts, and already in the case of 1-butene polymerisation only yield trace amounts of low molecular weight syndiotactic polymer [394]. 

Cyclopentadienyl ligands have become extremely important in catalysis for metal such as Ti, Zr, and Hf (Chapter 10) and in academic studies of related elements such as Ta. Ethylene polymerisation with the use of CpiTiCE (alkylated with aluminium alkyl compounds) has been known for many decades, but the intensive interest in derivatives of these compounds started in the early 1980 s following the discovery of MAO (methaluminoxane - see chapter 10) which boosted metallocene catalyst activities by several orders of magnitude. Commercial interest focussed on ethylene copolymers (LLDPE where more homogeneous comonomer incorporation resulted in greatly improved copolymer properties) and in enantiospecific polymerisations for propene, styrene, etc. 

Brintzinger, H. H., Fischer, D., Miilhaupt, R., Rieger, B. and Waymouth, R. M., Stereospecific Olefin Polymerisation with Chiral Metallocene Catalysts , Angew. Chem., Int. Ed. Engl., 34, 1143-1170 (1995). 

In the case of olefin polymerisation with homogeneous Ziegler-Natta catalysts, especially with single-site (metallocene) catalysts, the kinetic analysis may become simpler than in the case of polymerisation with heterogeneous catalysts, and in some instances can serve as a very useful tool for uncovering the true polymerisation mechanism [30,243],... 

The Mw/Mn ratio is usually equal to 5-10 for polyethylene [49,64,66,67, 123,244-247], A much lower polydispersity is displayed by polymers obtained in polymerisation with homogeneous metallocene catalysts the Mw/Mn ratio usually does not significantly exceed a value of 2 [22,95,101,112,138,140], By polymerising propylene with soluble vanadium-based Ziegler-Natta catalysts at low temperature, a very narrow molecular weight distribution of the polypropylene has been found (the Mw/Mn ratio usually reaches values of 1.15-1.25) and a linear increase in its Mn with time has been observed, indicating a noticeable living character of the polymerisation [75,76,241],... 

Chain transfer reactions in homogeneous olefin polymerisation systems with metallocene catalysts, which terminate individual polymer chains, in some instances can also terminate the polymerisation kinetic chain. For example, chain transfer with the monomer in propylene oligomerisation or polymerisation, which involves a bond metathesis reactions between the Mt-C species of the growing polymer chain and the C H species of methyl [scheme (45)] or vinyl [scheme (46)] groups in the monomer, gives rise to temporally inactive metal allyl or metal-vinyl species respectively [177, 241, 264] ... 

At the end of considerations of olefin polymerisation with homogeneous metallocene catalysts, it should be added that it is necessary to differentiate between the soluble catalyst system itself and the polymerisation system. For... 

Polymerisation with Homogeneous Metallocene Single-site Catalysts... 

Ethylene polymerisation with alkylaluminium-activated metallocene catalysts involves a propagation step characterised by the appearance of species alternating between an inactive ( dormant ) state and an active state in which the chain growth actually occurs [330-333] ... 

The mechanism that is commonly considered to operate in the polymerisation of ethylene and a-olefins in the presence of group 4 metallocene-based catalysts is that devised by Cossee [268, 276, 277] for propylene polymerisation with heterogeneous Ziegler-Natta catalysts, though modifications invoking effects such as a-agostic hydrogen interactions with the metal centre have been proposed [343,344]. 

Figure 3.36 Schematic representation of the chain stationary insertion mechanism of propylene polymerisation with the bridged mesogenic metallocene-based catalyst. Reproduced by permission from Ref. 402. Copyright 1995 Wiley-YCH Weinheim...

Advances towards the goal of polymerising polar monomers with coordination catalysts have been made with metallocene-based catalysts, especially aluminium-free cationic metallocene catalysts such as [Cp 2ZrMe]+[B(C6 Fs)4] or [Cp 2ZrMe]+[MeB(C6F5)3p. Waymouth et al. [500] found these... 

---