Mechanism of Syndiospecific Polymerization

The stereorigid bridged metallocene dichloride 1, as depicted in Fig. 2, is a prochiral molecule and possesses bilateral symmetry. The activatirm of metallocene dichloride 1 leads to the formation of enantiomeric metallocenium-monoaUcyl cationic species that are both electron-deficient and coordinatively unsaturated and structurally very similar to the enantiomeric molecule depicted in Fig. 3. These enantiomeric metallocenium-monoalkyl species have the necessary vacant coordination position and energetically low lying, accessible empty fragment orbitals to be used for coordination and activation of incoming propylene molecules via interaction with their available n orbitals. 

The cationic metallocenium-monoalkyl species [21] are composed of equal numbers of R and S mirror-image related enantiomers and have monomer re si jt-face selective properties (Fig. 5). 

Since according to this mechanism each enantiomer R or S, independently, would produce isotactic chains, and yet exclusively syndiotactic polymer chains are formed, it has to be concluded that the active enantiomeric species enantiomerize via an intramolecular mechanism and interconvert individually after each monomer insertion. 

The systematic transformation of the R and S antipodes into one another implies that the relative positions of at least two of the four ligands or ligand moieties surrounding the transition metal center must be changed systematically. 

The meso triad-based, enantiomorphic site control type of stereochemical errors, mm, are formed whenever the said balanced nonbonded steric interaction is perturbed, and the correct alignment of the substituents of the three main participants (ligand, polymer chain and monomer) is not materialized perfectly. In such a case, a monomer with wrong face is inserted and a unit with inverted configuration is enchained. The ability of occasional reverse face selectivity emanates firom the inherent structural factors of the metallocene-based catalyst. It is independent of monomer concentration and cormter-ion pairing nature however, it is dependent on polymerization temperature. 

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Fig. 8 Representation of mechanism of syndiospecific polymerization and formation of syndiotactic polypropylene (top). Fischer projection of a perfect syndiotactic chain sequence (bottom)...

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