Metallocene catalysts active center

The mixture of metallocene and co-catalyst is soluble. Its active center, which is chiral, induces with a very low rate of defects only one type of monomer linkage ( single site catalysts )- That is why high activities (some 1,000 kg polymer/g... 

Further evidence for the existence of cationic centers is given by the activation of metallocene catalysts for olefin polymerization by the use of anionic counterions such as tetraphenylborate (CeFIs B-, carborane (C2B9H12), or fluorinated borate. The use of (CgF5)4B counterions by Hlatky et al. (83), Sishta et al. (84), and Zambelli et al. (85) leads to highly active metallocene catalysts, which are formed by the reaction of a dealkylated zirconocene with dimethylaniliniumtetra(/us,-perfluorophenyl)borate ... 

Generally, metallocenes favor consecutive primary insertions as a consequence of their bent sandwich structures. Secondary insertion also occurs to an extent determined by the structure of the metallocene and the experimental conditions (especially temperature and monomer concentration). Secondary insertions cause an increased steric hindrance to the next primary insertion. The active center is blocked and therefore regarded as a resting state of the catalyst (138). The kinetic hindrance of chain propagation by another insertion favors chain termination and isomerization processes. One of the isomerization processes observed in metallocene-catalyzed polymerization of propylene leads to the formation of 1,3-enchained monomer units (Fig. 14) (139-142). The mechanism originally proposed to be of an elimination-isomerization-addition type is now thought to involve transition metal-mediated hydride shifts (143,144). 

Natta postulated that for the stereospecific polymerization of propylene with Ziegler-Natta catalysts, chiral active sites are necessary he was not able to verify this hypothesis. However, the metallocene catalysts now provide evidence that chiral centers are the key to isotacticity. On the basis of the Cossee-Arlman mechanism, Pino et al. (164,165) proposed a model to explain the origin of stereoselectivity The metallocene forces the polymer chain into a particular arrangement, which in turn determines the stereochemistry of the approaching monomer. This model is supported by experimental observations of metallocene-catalyzed oligomerization. 

For stereospecific polymerization of a-olefms such as propene, a chiral active center is needed, giving rise to diastereotopic transition states when combined with the prochiral monomer and thereby different activation energies for the insertion (see Figure 2). Stereospecificity may arise form the chiral /0-carbon atom at the terminal monomer unit of the growing chain - chain end control - or from a chiral catalyst site - enantiomorphic site control . The microstructure of the polymer produced depends on the mechanism of stereocontrol as well as on the metallocene used [42-44]. 

Using deuterium-labeling experiments, about 100% of the metal was shown to be active in 1-hexene polymerizations with the [rar -C2H4( 1 -lnd)2ZrMc [McB((4,f5)3] catalyst,922 and the reactivity of M-(secondary alkyl) bonds at —80 °C was comparable to that of primary alkyl metallocenes.302 These relative monomer insertion rates appear strongly ligand specific. However, when these comparisons are made, it must be borne in mind that different authors use very different catalysts, as well as different definitions of the term active center Landis defined the active... 

Compared to conventional heterogeneous Ziegler-Natta systems in which a variety of active centers with different structures and activities usually coexist, homogeneous metallocene-based catalysts give very uniform catalyt-ically active sites which possess controlled, well-defined ligand environments [37]. Consequently, the polymerization processes in homogeneous systems are often more simple, and kinetic and mechanistic analyses for these systems are greatly simplified [38]. 

These variations in efficiency are usually ascribed, particularly for metallocenes, to variations in steric crowding at the active center, consistent with the larger size of the comonomer. Alternatively, the greater ability of a-olefins to donate electron density to the active metal could also play a role in discriminating between ethylene and comonomer. That is, catalyst environments that result in lower electron density on the Cr atom, such as through strained bonding or bonding to acidic supports, could be more favorable to coordination of comonomer and its subsequent incorporation. 

As an alternative to the suspension process. Witco GmbH developed (1995) a technique which immobilizes the active compounds on a spray-dried silica support by utilizing a fluidized bed reactor. They claimed to produce supported metallocene catalysts with a controllable distribution of active centers achieved by using the three different supporting methods. Controlling the addition of tri-methylaluminum and water in the different reactor... 

---