Growing chain back-skip mechanism

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). 

SCHEME 1.7 Synthesis of iPP by a Cj-symmetric catalyst through the chain back skip mechanism (P = growing polymer chain). (Adapted with permission from Resconi, L. Cavallo, L. Fait, A. Piemontesi, F. Chem. Rev. 2000,100, 1253-1345. Copyright 2000 American Chemical Society.)... 

An analysis of molecular mechanics using model metallocene complexes, as possible intermediates for propylene polymerization was also reported by Guerra et al. [298]. The two coordination positions available for the monomer and the growing chain are diastereotopic. The conclusion was that the energy difference between the corresponding diastereoisomeric pre-insertion intermediates appear to be relevant for the model complexes [298]. It was also concluded [391] that energy differences can be related to an increased probability of a back-skip of the growing chain toward the outward coordination position after the monomer insertion and prior to the coordination of a new olefin molecule. 

Cl-symmetric catalysts such as 9 and 10 bear a Cp ring substituent pointing in the same direction as one of the chlorine atoms on the Zr center. In the active catalyst species, the chlorine is replaced by the growing polymer chain or a n-complex-bonded olefin. The coordination site on the same side as the Cp substituent (site B) is more sterically hindered than the other coordination site (site A). After insertion of a monomer coordinating at site B, the polymer chain is walked back to less sterically hindered site A by the next monomer insertion (alternating/chain migratory mechanism). But in some cases, the polymer chain can back skip to site A before the next monomer insertion (site epimerization). Norbornene, as a bulky olefin, can be rr-bonded only at site (see Chapter 2 for a further discussion of the alternating and site epimerization mechanisms with catalyst 9). 

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)]. 

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