Chain migratory insertion mechanism

Chain Migratory Insertion Mechanism. For a given catalytic model, the stereoselectivity of each insertion step does not assure its stereospecificity (i.e., to lead to a stereoregular polymer). In fact, the possible presence as well as the kind of stereospecificity depends on possible differences between stereostructures of transition states of two successive insertion steps. 

In the framework of the chain migratory insertion mechanism (Scheme 1.3), the stereospecific behavior of the model sites depends on the relationship between the two situations obtained by exchanging, in the coordination step, the relative positions of the growing chain and the incoming monomer. Depending on the local symmetry of the coordinated bridged n-ligand, these two situations can be as follows ... 

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],... 

Every active metal atom has two available coordination sites (the two locks), which can both insert the olefin, and that can be different in either shape or chirality. In the framework of the chain migratory insertion mechanism, the monomer has to be inserted alternately on each site, and the structure of the resulting polypropene depends on the combination of the regio- and enantioselectivity of the two... 

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. 

The Two Coordination Positions Are of Similar Energy. In the hypothesis that the chain migratory insertion mechanism is still prevailing, the model of these catalytic systems would be isospecific or syndiospecific, if the two situations originated from propene coordination on the two coordination positions are enantioselective in favor of the same or opposite propene enantiofaces, respectively. If only one situation is enantioselective, the corresponding catalytic system is hemiisospecific. 

A chain migratory insertion mechanism for ethylene/norbomene copolymerization utilizing both A and B coordination sites in an alternating fashion is implied by the derived parameter set... 

After the chain migratory insertion mechanism, the newly formed polymer chain occupies the position that previously was the vacant coordination site, where the monomer coordinated. After a chain stationary insertion mechanism, it is predicted that the newly formed polymer chain occupies its original position. Although a distinction between these two mechanisms may not be important for ethylene polymerization, it is cmcial to understanding the polymerization of 1-alkenes, where stereochemistry is highly relevant. 

The chain migratory insertion mechanism is universally accepted and the original mechanistic description originated with Cossee and Arlman ° for heterogeneous ZN catalyst polymerizations. Figure 10 shows the Cossee-Arlman mechanism... 

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