Chain migratory insertion

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). 

Figure 6 Relationship between catalyst symmetry and transition states of chain-migratory insertion of propylene monomer.

Soluble catalysts again may exhibit unique selectivities. Hafnium and zirconium fluorenyl metallocenes with methylaluminoxanes give syndiotactic polymers in high yields. The microstructure of the products indicates site stereochemical control with chain migratory insertion resulting in site isomerization with each monomer... 

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],... 

Remember that the polymerisation mechanism devised by Cossee [268] implies two main steps coordination of the monomer at the titanium vacant site with the double bond parallel to the Ti-C bond, and chain migratory insertion of the coordinating monomer molecule (with migration of the growing polymer chain to the position previously occupied by the coordinating monomer molecule) isospecificity of the active site is assumed only if the polymer chain skips back to the original position before further insertion [scheme (50)]. 

Therefore, according to Razavi et al. [127,143], chain migratory insertions and chain stationary insertions would follow each other alternately, probably involving a pen-penultimate mechanism dictated by the periodicity of the helices of the growing chain [413]. If such a coordination site switching mechanism intervenes periodically, the resulting polypropylene chain would expose a syndioisoblock microstructure. 

Chain Migratory Insertion, Chain Stationary Insertion, and Site Isomerization... 

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