Metallocenes chain migratory insertion mechanism

In summary, characterizations of stereosequences in polymers obtained by catalytic systems based on well-characterized metallocene complexes have produced a general acceptance of the chain migratory insertion mechanism and of models described in i-iii. 

Possible Back-Skip of Growing Chain. Several experimental facts relative to propene polymerization behavior of different metallocene-based catalytic systems can be rationalized by considering a disturbance of the chain migratory insertion mechanism due to a kinetic competition between the monomer coordination in the alkene-free state and a back-skip of the growing chain to the other possible coordination position (see Scheme 1.3). 

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]. 

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],... 

Because of the mechanism of enantioface selectivity and the two-site, chain migratory insertion mechanism, the microstructure of a poly(l-olefin) made with a given metallocene is, to a large extent, predictable. In a series of landmark papers, Ewen and co-workers and Kaminsky and co-work-ers described a series of stereoselective metallocene catalysts which define what are now referred to as Ewen s symmetry rules . These are summarized in Chart 2. When the metallocene molecule is C v, meso Cs-symmetric, or highly fluxional, an aspecific polymerization has to be expected. 

Syndiotactic polypropylene (sPP) with a. ..rrrrrmrrrrrmmrrrrr... mixed microstructure is obtained with the iPr[CpFlu]ZrCl2/HAO (MAO methylaluminoxane Cp cyclopentadienyl anion Flu - fluorenyl anion). The structures of the metallocene and the polymers are in accord with chain migratory insertion being the predominant mechanism of chain growth and with stereochemical control being provided by the alternating handedness of polymerization active, cationic Zr monoalkyls. 

Figure 12. Scheme of stereospecific 1-olefins polymerization with generic C2 and Cs symmetric metallocenes. In the framework of a regular chain migratory mechanism, the C2 and Cs symmetric catalysts lead to iso- and syndiotactic polymers, respectively. In fact, multiple insertions of the same enantioface occur with C2 symmetric metallocenes, while multiple insertions of alternating enantiofaces occur with Cs metallocenes. 

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. 

According to the Cossee mechanism, the two key steps in Ziegler—Natta polymerizations are monomer coordination and migratory insertion into the metal-polymer chain bond (Scheme 10). In analogy to heterogeneous Ziegler—Natta catalysis, a first-order reaction rate with respect to monomer concentration is generally assumed also for metallocene-based catalysts, that is. 

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