Chain skipped insertion

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]. 

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). 

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)]. 

For the discussed model sites, the two situations presenting the outward and inward propylene coordination are enantioselective and non-enantioselective respectively. Hence, these model sites are isospecific only under the assumption that propylene always coordinates at a given coordination position (i.e. that the chain skips back to the starting position after each monomer insertion and prior or simultaneously to the coordination of the new propylene molecule) [345]. 

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. 

In distinction to C2-symmetric, iso-specific catalysts, however, stereoerrors can arise here also when the growing chain moves from its original coordination site to the other, without the intervention of an olefin insertion. These skipped insertions become frequent at low olefin concentrations for syndiospe-cific catalysts shown in Figure 19, since then site-exchange of the polymer chain without insertion becomes competitive with further chain growth. 

The invention of syndiospecific Cj-symmetric metallocenes has marked the turning point in the understanding of the mechanism of stereocontrol with metallocene catalysts. Again, the presence of isolated insertion errors of the type rrrrmmrrr is consistent with site control (Scheme 27). In the case of the syndiospecific Me2C(Cp)(9-Flu)ZrCl2 catalyst, in which the two sites are enantiotopic, occasional skipped insertions produce a minor amount of insertion errors of the type rrrrmrrrr, which are identical to those produced by chain-end control. In the case of isospecific C2-symmetric metallocenes, skipped insertions would not be observable due to the presence of two homotopic sites. 

The Two Coordination Positions Are of Substan tially Different Energy. For models of these catalytic systems, the sequence of chain migratory insertion steps can be altered. In fact, the growing chain in successive coordination and insertion steps can often occupy the same coordination position. That is, after each migratory insertion step, in the absence of a coordinated monomer molecule, the growing chain could swing back to the previous coordination position (back-skip of the growing chain). 

Heso dyad defects portrayed in Structure II have previously been associated with classical chain end controlled mechanisms of stereoregulation. However, the m errors are ambiguous. There are several other reaction mechanisms that can lead to them. The m placements can, for example, be due to a skipped insertion step in site controlled polymerizations. 

The polymerization mechanism for the dual-side catalysts is totally different from the C2-symmetric complexes. Due to their geometry, the dual-side complexes show different stereoselectivities for monomer coordination and insertion. It was shown that the introduction of the stereoerror formation by the 5-substituted asymmetric catalysts originates predominately from the kinetic competition between chain back-skip and monomer coordination at the aspecific side of the catalyst [9],... 

Chien already postulated that C,-symmetric ansa-bridged complexes exist in two isomeric states, which interconvert during the course of the polymerization reaction [14, 15, 21, 22], Different stereoselectivities for monomer coordination and insertion are found for the two coordination sites of the asymmetric metallocene catalysts (Fig. 6,1 and IV). The migration of the polymer chain to the monomer, coordinated at the isoselective site f I—>11), followed by a consecutive chain back-skip (at higher temperatures) to the sterically less hindered side (II >111) leads to isotactic [mmmm] sequences [11],... 

Scheme 1.3 Polymerization scheme showing the migratory insertion mechanism as well as the possible occurrence of the chain back-skip. The possible formation of agostic Mt-H bonds is...

The possible occurrence of a back-skip of the chain for catalytic systems based on C2-symmetric metallocenes would not change the chirality of the transition state for the monomer insertion and hence would not influence the corresponding polymer stereostructure. On the contrary, for catalytic systems based on Cs-symmetric metallocenes, this phenomenon would invert the chirality of the transition state for the monomer insertion, and in fact it has been invoked to rationalize typical stereochemical defects (isolated m diads) in syndiotactic polypropylenes.9 376 60 This mechanism of formation of stereoerrors has been confirmed by their increase in polymerization runs conducted with reduced monomer concentrations.65 In fact, it is reasonable to expect an increase in the frequency of chain back-skip by reducing the monomer concentration and hence the frequency of monomer insertion. 

Note, in this connection, that it is incorrect to ascrise the formation of small amounts of syndiotactic polypropylene fraction in the presence of heterogeneous Ziegier-Natta catalysts to the operation of a mechanism assuming a lack of back skip of the chain in the last stage of insertion in the presence of these... 

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