Syndiospecific 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.

Furthermore, studies of the microstructure of copolymers formed by the low-temperature copolymerisation of cis-1 -(2 H)-propene (or trans isomer) and perdeuteropropene in the presence of soluble vanadium-based Ziegler-Natta catalysts showed syndiospecific propagation to involve a monomer insertion of the cis type [27]. 

Syndiospecific Propagation Reaction Stereocontrol in the Presence of Soluble Vanadium-based Ziegler-Natta Catalysts... 

Stereocontrol of Syndiospecific Propagation with Chirotopic (Enantiotopic) 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.

We and others have revealed that syndiospecific propylene polymerization is exclusively initiated by 1,2-insertion followed by 2,1-insertion as the principal mode of polymerization [64]. This is the first example of a predominant 2,1-insertion mechanism for chain propagation exhibited by a group 4 metal-based catalyst. The unusual preference for 2,1-regiochemistry displayed by the Ti-FI catalysts compared with the Zr- and Hf-FI catalysts is apparently inconsistent with the crys-tallographically characterized structures, which indicate that the Ti is shielded more by the phenoxy-imine ligands and thus possesses higher steric compression. The reason for the unusual preference in the regiochemistry of Ti-FI catalysts is unclear at the present time. 

All the proposed models for syndiotactic propagation suppose that the active center is a vanadium-carbon bond and that the monomer first coordinates to the metal. Moreover, all of them attribute the stereospecificity to steric factors. However, different driving forces for the syndiospecificity have been proposed. 

The same conclusion as in the case of propylene homopolymerisation has been drawn considering IR [396] and NMR [389,395] spectra of ethylene/propylene copolymers obtained with vanadium-based syndiospecific catalysts. The type of propylene insertion depends on the kind of last inserted monomer unit secondary insertion [scheme (40)] occurs more frequently when the last monomeric unit of the growing chain is propylene, while primary propylene insertion [scheme (39)] is more frequent when the last monomeric unit of the growing chain is ethylene [2]. The above explains the microstructure of ethylene/propylene copolymers obtained with vanadium-based Ziegler-Natta catalysts. These copolymers contain both m and r diads when the sequence of propylene units is interrupted by isolated ethylene units i.e. a propylene insertion after an ethylene insertion is substantially non-stereospecific [327,390,397], The existence of a steric interaction between the incoming monomer molecule and the last added monomer unit is also confirmed by the fact that the propagation rate for the secondary insertion of propylene in syndiospecific polymerisation is lower than for primary insertion in non-stereospecific polymerisation [398],... 

Such a catalytic centre is chiral, but interconversion between enantiomeric complexes is assumed to occur, after each insertion step, when the V atom is pentacoordinated. The analysis of the non-bonded interactions at the catalytic site suggests an si insertion of the last monomer unit (which generates what is called an si chain) to favour the formation of a A complex. This complex should in turn favour the re coordination and insertion of the successive monomer unit, thus ensuring syndiospecific polypropylene chain propagation. In other words, the chirality of the growing chain (expressed by the configuration of the last inserted monomeric unit) imposes the chirality of the coordinating monomer... 

Considering the above stereochemical model for syndiospecific styrene polymerisation, one may conclude reasonably that tf coordination of the monomer at the active site could hardly be possible, and r 2 coordination would always be involved in the syndiospecific polymerisation of this monomer [87]. One should note that preliminary concepts concerning the stereoregulation mechanism of syndiospecific styrene polymerisation assumed the styrene monomer to undergo only t]4 coordination at the titanium centre, the propagating chain being anchored via a benzylic bond as an t]3 ligand at the titanium [44,55,70]. 

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. 

As regards the homogeneous vanadium-based catalytic complexes on which propagation is predominantly syndiospecific, similar coordi-... 

The bulkiness above M is more likely to differ from that below M when catalytic complexes are on a crystal surface. Consequently, isospedfic catalysts are more frequently heterogeneous and syndiospecific catalysts are more Irequently homogeneous. This does not rule out that both types of stereospecific propagation can occur on both heterogeneous and homogeneous complexes. 

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