Metallocene-Based Ziegler-Natta Catalysts

The earliest Ziegler-Natta catalysts were combinations of titanium tetrachloride (T1CI4) and diethylalummum chloride [(CH3CH2)2A1C1] but these have given way to more effective zirconium based metallocenes the simplest of which is bis(cyclopentadi enyl)zirconmm dichlonde (Section 14 14)... 

B.P. Chemicals [81] have also reported the new metallocene-based LLDPE (with 1-hexene comonomer) having much superior properties than those produced by using the Ziegler-Natta Catalyst. 

Muhlhaupt R, Fischer D, JUngling S (1993) Donor- and acceptor-modified metallocene-based homogeneous Ziegler-Natta catalysts. Makromol Chem Macromol Syp 66 191-202... 

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... 

Since the discovery of Ziegler-Natta catalysts, polyolefin industries have been developed mainly by using titanium-based catalysts. However, after the appearance of metallocene catalysts, much emphasis has been placed on those based on zirconium owing to the superiority of their performance over titano-cene catalysts. 

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],... 

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]. 

It must be emphasised that copolymers produced by metallocene-based catalysts display comonomer distributions essentially independent of the chain lengths the copolymers obtained with these catalysts contain larger fractions of higher a-olefins than those obtained with heterogeneous Ziegler-Natta catalysts under comparable conditions [30]. 

Figure 3.59 Life cycles of catalysts for olefin coordination polymerisation (a) early-generation Ziegler-Natta catalysts for ethylene and propylene polymerisation (b) Phillips catalysts for ethylene polymerisation (c) fourth-generation Ziegler-Natta catalysts for ethylene polymerisation (d) fourth-generation Ziegler-Natta catalysts for propylene polymerisation (e) metallocene-based catalysts for olefin polymerisation leading to polymers of various stereoregularity...

What are the advantages of half-sandwich metallocene-based catalysts as compared with heterogeneous Ziegler-Natta catalysts in styrene polymerisation What are the possible consequences of this for developing industrial processes ... 

In contrast to heterogeneous Ziegler-Natta catalysts, homogeneous catalysts based on biscyclopentadienyl derivatives of group 4 transition metals, which contain cationic metallocene species of formally d° 14-electronic structure, hardly promote the polymerisation of conjugated dienes, since the diene can act as a donor of four electrons rather than of two electrons as in monoolefin polymerisation (let us recall that the polymerisation of conjugated dienes is catalysed by half-sandwich metallocene-based catalysts). However, it has been reported [162] that statistical copolymers of ethylene and butadiene were obtained with the Cp2ZrCl2— [Al(Me)0]x catalyst. 

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