Metallocene catalysts polymerization mechanism

In this section, we will briefly examine the salient features of the basic classes of metallocene catalysts. Polymerization activity comparisons and the mechanisms that govern polymerization will be treated in subsequent sections. 

Collins, S. Ward, D. G Suddaby, K. H. Group-transfer polymerization using metallocene catalysts Propagation mechanisms and control of polymer stereochemistry. Macrotnolecules 1994, 27, 7222-7224. 

After the synthesis and employment of thousands of metallocenes, a vast amount of information has been obtained regarding catalyst stmcture-polymer property relationships. This information has been adroitly dissected by a large number of scientists to compile a rather detailed mechanistic understanding of the metallocene-mediated polymerization mechanism. Metallocenes have made possible detailed studies on initiation, propagation, termination, kinetics, and stereochemical control. These studies have been integrated to make possible many novel polyolefins with highly engineered micro-stmctures to meet a wide variety of applications - most of which would probably amaze the founders of ZN polymerization. 

A special case of the chain back skip polymerization mechanism and therefore an entirely different polymerization behavior was observed for differently substituted asymmetric complexes (for example catalyst 3). Although asymmetric in structure, these catalysts follow the trend observed for C2-symmetric metallocenes [20], Chien et al. [23] reported a similar behavior for rac-[l-(9-r 5-fluorenyl)-2-(2,4,7-trimethyl-l-ri5-indenyl)ethane]zirconium dichloride and attributed this difference in the stereoerror formation to the fact that both sides of the catalyst are stereoselective thus isotactic polypropylene is obtained in the same manner as in the case of C2-symmetric metallocene catalysts. 

Figure 11-7 Proposed mechanism for catalytic polymerization. An olefin and an alkyl group on an oiganometallic site react to add the olefin to the growing chain, Also shown are typical polymerization catalysts Ti/Si solid catalyst and organometallic metallocene catalysts.

It is generally adopted that the catalytically active species in the metallocene-catalysed polymerization is a 14-electron cation. As an example, the mechanism of activation of an unbridged zirconocene catalyst is presented in Fig. 9.5-4, top. In the first two steps the activation by MAO, resulting in the 14-electron cation, is shown. The same cation can be generated by N,N -dimethylanilinium-tetrakis(pentafluorophenyl)borate and methylated metallocenes. As side-products methane and an amine are formed. TiBA can also be involved in the activation, which is not shown in Fig. 9.5-4, bottom. On the other hand, TiBA acts as a scavenger in the polymerization. The above-mentioned reactions take place in the absence of the monomer and are performed before the catalyst is used in the polymerization process. 

Figure 9.5-5. Mechanism of ethylene polymerization with metallocene catalyst.

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. 

The mechanism for the dehydropolymerization of R2SnH2 by the Group 4 metallocene catalysts is expected to be similar to the mechanism for the dehydropolymerization of hydrosilanes by the same catalysts. The mechanism for the Rh-catalyzed polymerization remains to be determined. 

The importance and relevance of homogeneous catalysis in polymerization reactions have increased tremendously in the past few years for two reasons. First, from about the beginning of the early 1990s a special class of sandwich complexes has been used as homogeneous catalysts. These catalysts, often referred to as metallocene catalysts, can effect the polymerization of a wide variety of alkenes to give polymers of unique properties. Second, the molecular mechanism of polymerization is best understood on the basis of what is known about the chemistry of metal-alkyl, metal-alkene, and other related complexes. 

Two major mechanisms have been proposed for alkene polymerization. These are the Cossee-Arlman mechanism and the Green-Rooney mechanism. A modified version of the latter has also been considered to explain the behavior of homogeneous, metallocene catalysts. The original Cossee-Arlman mechanism was proposed for the TiCl3 based heterogeneous catalyst. In the following sections we discuss these different mechanisms in some detail. In the following discussion in accordance with the results obtained from the metallocene systems, the oxidation states of the active surface sites are assumed to be 4+. 

Figure 6.5 Mechanism and catalytic cycle for propylene polymerization with a model metallocene catalyst. Conversion of 6.16 to 6.17 and 6.19 to 6.20 involve insertion of (n + 1) propylene molecules.

The mechanical, thermal, optical, and other properties of a polymer depend on the structure of the monomer units. Where a copolymer is used, it depends additionally on the relative amounts and distribution of the two monomeric building blocks. Metallocene catalysts have four main advantages over the conventional polymerization catalysts. They can polymerize a very wide variety of vinyl monomers irrespective of their molecular weights and steric features. They also can polymerize mixtures of monomers to give polymers of unique properties. 

The third advantage associated with metallocene catalysts is that the predominant mechanism for chain termination is by /3-hydride elimination. This produces a vinyl double bond at the end of each polymer chain. Further functionalization of the vinyl group by graft polymerization with maleic anhydride and other functional monomers is far more effective than is typical for polyolefins obtained by conventional catalysts. 

Finally, as already mentioned, metallocene catalysts can polymerize a variety of olefins. In certain cases the structural features of the monomer lead to the formation of novel polymers. Two such examples are shown by reactions 6.6 and 6.7. It is clear that the polymerization processes involve considerable rearrangements of the bonds. Reactions 6.8 and 6.9 show the formal mechanisms of such rearrangements for 1,5-hexadiene and methylenecyclobutane, respectively. 

Cyclopentene has been polymerized by several metallocene catalysts (Table 22). For all of them the homopolymers were found to contain no 1,2-enchainments. While for the formation of cis-1,3 enchainments a mechanism similar to that proposed for the formation of 1,3-enchainments in polypropene is reasonable, there is no plausible explanation for the formation of trans structures. 

Without the third component no activity at all was observed. They proposed a mechanism similar to the one given by Yasuda et al. [216, 217] for polymerization of methylmethacrylate by lanthanocenes which are isoeleetronie with alkylzirconocenium ions. The role of the third component in this mechanism is not very clear. Nevertheless polymerization of polar monomers by metallocene catalysts is an open field of research and investigations are just beginning. 

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