Chain stationary insertion mechanism

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 3.36 Schematic representation of the chain stationary insertion mechanism of propylene polymerisation with the bridged mesogenic metallocene-based catalyst. Reproduced by permission from Ref. 402. Copyright 1995 Wiley-YCH Weinheim...

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. 

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. 

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