Metallocene catalysts heterogeneous

Most catalysts for solution processes are either completely soluble or pseudo-homogeneous all their catalyst components are introduced into the reactor as Hquids but produce soHd catalysts when combined. The early Du Pont process employed a three-component catalyst consisting of titanium tetrachloride, vanadium oxytrichloride, and triisobutjlalurninum (80,81), whereas Dow used a mixture of titanium tetrachloride and triisobutylalurninum modified with ammonia (86,87). Because processes are intrinsically suitable for the use of soluble catalysts, they were the first to accommodate highly active metallocene catalysts. Other suitable catalyst systems include heterogeneous catalysts (such as chromium-based catalysts) as well as supported and unsupported Ziegler catalysts (88—90). 

Eluidized-bed reactors are highly versatile and can accommodate many types of polymerization catalysts. Most of the catalysts used for LLDPE production are heterogeneous Ziegler catalysts, in both supported and unsupported forms. The gas-phase process can also accommodate supported metallocene catalysts that produce compositionaHy uniform LLDPE resins (49—51). 

In the early 1990s, solution processes acquired new importance because of their shorter residence times and abiUty to accommodate metallocene catalysts. Many heterogeneous multicenter Ziegler catalysts produce superior LLDPE resins with a better branching uniformity if the catalyst residence time in a reactor is short. Solution processes usually operate at residence times of around 5—10 min or less and are ideal for this catalyst behavior. Solution processes, both in heavy solvents and in the polymer melt, are inherently suitable to accommodate soluble metallocene catalysts (52). For this reason, these processes were the first to employ metallocene catalysts for LLDPE and VLDPE manufacture. 

The sterically unencumbered catalyst active site allows the copolymerization of a wide variety of olefins with ethylene. Conventional heterogeneous Ziegler/Natta catalysts as well as most metallocene catalysts are much more reactive to ethylene than higher olefins. With constrained geometry catalysts, a-olefins such as propylene, butene, hexene, and octene are readily incorporated in large amounts. The kinetic reactivity ratio, rl, is approximately... 

A characteristic of polyolefins synthesized with metallocene catalysts is their significantly lower polydispersity compared to one obtained by using heterogeneous Ziegler-Natta catalysts. Such narrower molecular mass distributions can lead to different mechanical properties of the resulting material. 

These are some key advantages that the metallocene catalysts have over conventional Ziegler-Natta catalysts and hence it is highly probable that inter-and intra-chain heterogeneity expected in ethylene-a-olefins copolymers can be controlled through the use of the metallocene system. 

Copolymerization. See also Copolymers of acrylonitrile, 11 202-204 anionic, 7 624-626 catalytic, 7 627-632 cationic, 7 626-627 chloroprene-sulfur, 19 833-834 of cyclic olefins, 16 112-113 with depropagation, 7 617-619 free-radical, 7 611-624 heterogeneous, 11 203-204 homogenous, 11 202-203 with metallocene catalysts, 16 111... 

In heterogeneous polymerizations in bulk, the formed polymer is insoluble in its monomer and the polyreaction is performed below the softening point of the polymer. On an industrial scale, this type of process is especially utilized for chain polymerizations, for example, the radical polymerization of liquid vinyl chloride, the polymerization of liquid propylene with Ziegler-Natta or with metallocene catalysts, and the polymerization of molten trioxane. 

Stereospecific Polymerization. In the early 1950s, Ziegler observed that certain heterogeneous catalysts based on transition metals polymerized ethylene to a linear, high density material at modest pressures and temperatures. N atta showed that these catalysts also could produce highly stereospecific poly-a-olefins, notably isotactic polypropylene, and polydienes. They shared the 1963 Nobel Prize in chemistry for their work. More recently, metallocene catalysts that provide even greater control of molecular structure have been introduced. 

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],... 

Pathways (a)-(b)-(c)-(d) and (a )—(b )—(c )—(d ) correspond to the original mechanism proposed by Cossee [268,276,277] and are still valid, apart from some minor modifications [1], for heterogeneous catalysts. For metallocene-based catalysts of classes II and partially V, this mechanism gives rise to successive additions at the same site (from a configurational point of view) and is known as the chain stationary insertion mechanism ( chain skipped insertion or site isomerisation without insertion mechanism) [143, 146, 345], The (a)-(b)-(c)-(a )—(b )—(c ) pathway corresponds to the chain migratory insertion mechanism found in the case of metallocene catalysts of classes I, III, IV and partially V [143, 146]. 

Soga, K., Kaminaka, M., Kim, H. J. and Shiono, T., Heterogeneous Metallocene Catalysts , in Ziegler Catalysts, Springer-Verlag, Berlin, 1995, pp. 333-342. 

Styrene, which can be treated formally as an a-olefin branched in the 3-position, forms copolymers with ethylene and a-olefins (as well as with /i-olefins, involving isomerisation copolymerisation). Both heterogeneous Ziegler-Natta catalysts and a single-site metallocene catalyst promote the copolymerisation. 

Soga, K., Arai, T., Nozawa, H. and Uozumi, T., Recent Development in Heterogeneous Metallocene Catalysts , Macromol. Symp., 97, 53-62 (1995). 

It is worthwhile mentioning that 1,3-homopolymerisation is possible using homogeneous metallocene catalysts but impossible with heterogeneous Ziegler Natta catalysts which are not capable of the isomerisation illustrated by scheme (3). 

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