Isospecific propagation

Figure 1.23 Transition states for secondary insertion of styrene into secondary growing chain presenting si chirality (that is, generated secondary insertion of. si-coordinated styrene), (a) Model for unlike (syndiospecific) propagation includes fluxional site of R chirality at metal atom, which imposes re-propene coordination, while (b) model for like (isospecific) propagation includes fluxional sites of S chirality at metal, which imposes. si-propene coordination. Syndiospecific transition state (a) is favored because smallest substituent on C atom of chain, the H atom, can be pointed toward Cp ligand, whereas isospecific transition state (b) is of higher energy because Cp of growing chain is oriented toward Cp ring.

Isospecific Propagation Reaction Stereocontrol in the Presence of Heterogeneous Ziegler-Natta Catalysts... 

Stereocontrol of Isospecific Propagation with Achirotopic Catalysts... 

Figure 24. Molecular mechanics minimum energy geometry for re and si propene coordination on the Cp2Ti-(growlng chain) model. The growing chain Is labeled as si-chaln, since the chirality of its tertiary carbon atom closest to the metal has been obtained by a primary insertion of a si-coordinated propene. For the model corresponding to isospecific propagation (a) the chain (atoms C3, C4...) points away from the olefin, while for the model corresponding to syndiospecific propagation (b) it points toward the olefin.

The parameter /j(re re) represents the isospecific propagation at this site (two successive reinsertions), while p re si) is the probability of error correction after a stereoerror. This model is called the asym metric Markovian model and the mathematical expressions for pentad distribution are collected in Table 14. 

In propylene polymerization using titanium chloride catalysts, chain propagation takes place via primary (1,2-) insertion of the monomer. For isospecific propagation, there must be only one coordination vacancy and the active site must be chiral. Corradini and co-workers have demonstrated that the asymmetric... 

Stereocontrol of Isospecific, Hemiisospecific, Isospecific-Aspecific and Syndiospecific Propagation with Chirotopic (Diastereotopic) Catalysts... 

Although the isospecific polymerisation of styrene monomers has much less steric demands for the Ziegler-Natta catalysts than that of x-oldins, it proceeds with much lower propagation rate constants by comparison with the polymerisation of x-olefins for example, on a molar basis, styrene is ca 100 times less reactive than propylene in the polymerisation [30,33], Also, compare the relatively slow polymerisation of styrene and other vinylaromatic monomers with the relatively fast polymerisation of vinylcyclohexane [20,31,34-36]. 

Using a similar catalyst, Kashiwa 83 n8> also noticed an increase of the isotactic production rate after the addition of EB, although the total productivity was lowered. Moreover, he found that the molecular weight of the isotactic polymer increased, while the number of the isospecific sites did not change, and hence concluded that the effect of the Lewis base was to increase the propagation rate constant (kp) of the stereospecific sites. 

Moreover, application of the above law to the formation rates of isotactic and atactic fractions showed that the overall rate equation is the result of two equations characterized by different values of kA (200 1 mol-1 for the isospecific centers and 40 1 mol 1 for the non-specific centers). Thus, the kinetic behavior of the polymerization was rationalized on the basis of a two-center polymerization model. Furthermore, based on an approximate estimate of the partition function of the transition state involving propagating chain and coordinated monomer, monomer insertion was proposed as the rate determining step. 

Zam belli and Tosi have extensively studied the stereochemistry of the propagation step in propylene polymerization on Ziegler-Natta catalysts. Specific features of this process are shown in Table 4. Cis-addition of the olefin to the active metal-carbon bond has been observed both in isospecific and syndiospecific polymerization. The olefin addition to the active bond proceeds with the participation of the primary (L,(Mt—CH2—CHR—P) and secondary (L,Mt—CHR—CH2—P) carbon atoms of the growing polymer chain using isospecific and syndiospecific catalysts, respectively. 

Since for various catalytic systems only the relative content of different fractions changes (e.g. from 25 to 98.5% for a fraction insoluble in boiling n-heptane without changing their stereoregularity, the composition of catalytic systems influences the relative amount of isospecific and non-stereospecific centers. The reactivities of these centers (rate constants of propagation of isotactic and atactic polymers) for the titanium chloride-based catalysts are similar (Table 2 and Ref >). 

However, the comparative data on (Table 1) and the stereoregularity of polymer fractions (Table 6) for one- and two-component catalysts based on titanium chlorides indicate that the cocatalyst does not influence the reactivity and stereospecificity of the propagation centers. Its effect on the overall polymerization rate is apparently due to the change in the total number of active centers and the ratio of isospecific and non-stereospecific centers. 

Propagation rates of first order in monomer concentration have been reported for ethylene and for propylene in the case of aspecific metallocenes266 as well as for propylene polymerization with isospecific metallocenes activated with MAO, B(C6F5)3, and [PhsCHBlCftFsL].290 297 Moreover, first-order kinetics were also observed for 1-hexene polymerization with the [ra(r-G2H4( 1 -Ind)2ZrMc [ McB(Gf,l 3)3. 156... 

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