Mechanism of Stereocontrol

Since insertion occurs at two different sites on the metal center most often in an alternating fashion, an m dyad (two methyl groups on the same side of the polymer chain) results when the same enantioface of propylene is inserted consecutively (re after re and si after si) isotactic polymer is produced after many insertions. Similarly, when the two coordination sites of the catalyst prefer opposite enantiofaces (re after si and si after re), an r dyad (two methyl groups on opposite sides of the polymer chain) is formed and a syndiotactic polypropylene is produced. 

It should be noted that a mixture of active catalyst centers that insert either only the re or si enantioface will still produce iPP. Although the chain start and chain end of each polymer chain are generally not the same group, when iPP molecular weight is high, a pseudo plane of symmetry exists 

Stereoselective Polymerization with Single-Site Catalysts 

FIGURE 1.2 The ten possible combinations of five adjacent stereocenters (pentads) in polypropylene and the NMR spectrum of the methyl region of atactic polypropylene. 

FIGURE 1.3 SI and re coordination of propylene to a metal center (P = growing polymer chain). 

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A single step of the polymerization is analogous to a diastereoselective synthesis. Thus, to achieve a certain level of chemical stereocontrol, chirality of the catalytically active species is necessary. In metallocene catalysis, chirality may be associated with the transition metal, the ligand, or the growing polymer chain (e.g., the terminal monomer unit). Therefore, two basic mechanisms of stereocontrol are possible (145,146) (i) catalytic site control (also referred to as enantiomorphic site control), which is associated with the chirality at the transition metal or the ligand and (ii) chain-end control, which is caused by the chirality of the last inserted monomer unit. These two mechanisms cause the formation of microstructures that may be described by different statistics in catalytic site control, errors are corrected by the (nature (chirality) of the catalytic site (Bernoullian statistics), but chain-end controlled propagation is not capable of correcting the subsequently inserted monomers after a monomer has been incorrectly inserted (Markovian statistics). 

P. Corradini, G. Guerra, and L. Cavallo, Do New Century Catalysts Unravel the Mechanism of Stereocontrol of Old Ziegler-Natta Catalysts , Acc. Chem. Res. 37, 231-241 (2004). 

For stereospecific polymerization of a-olefms such as propene, a chiral active center is needed, giving rise to diastereotopic transition states when combined with the prochiral monomer and thereby different activation energies for the insertion (see Figure 2). Stereospecificity may arise form the chiral /0-carbon atom at the terminal monomer unit of the growing chain - chain end control - or from a chiral catalyst site - enantiomorphic site control . The microstructure of the polymer produced depends on the mechanism of stereocontrol as well as on the metallocene used [42-44]. 

It is well accepted that two mechanisms of stereocontrol (the chiral induction responsible for selecting the monomer enantioface) are operative in stereoselective a-olefm polymerizations. In the simpler cases, the discrimination between the two faces of the prochiral monomer may be dictated either by the configuration of the asymmetric tertiary C atom of the last inserted monomer unit or by the chirality of the catalytic site. These two different mechanisms of stereocontrol are named chain-end stereocontrol and enantiomorphic-site or site stereocontrol. In the case of chain-end stereocontrol, the selection between the two enantiofaces of the incoming monomer is operated by the chiral environment provided by the last inserted tertiary C atom of the growing chain, whereas in the case of site stereocontrol this selection is operated by the chirality of the catalytic site. The origin of stereocontrol in olefin polymerization has been reviewed extensively.162,172-178... 

Corradini, P. Guerra, G. Cavallo, L. Do new century catalysts unravel the mechanism of stereocontrol on old Ziegler-Natta catalysts Acc. Chem. Res. 2004, 37, 231-241. 

In 1962. Natta and Zambelli reported a heterogeneous. vanadium-based catalyst mixture which produced partially syndiotactic polypropylene at low polymerization temperatures. " The regiochemistry of the insertion was determined to be a 2.1-insertion of propylene, and a chain-end control mechanism determined the s mdiospecificity of monomer insertion. This catalyst system suffered from both low activity and low stereoselectivity. Highly active single-site olefin polymerization catalysts have now been discovered that make syndiotactic polypropylene with nearly perfect stereochemistry. Catalysts of two different symmetry classes have been used to make the polymer, with Cs-symmetric catalysts typically outperforming their Q -symmetric counterparts due to different mechanisms of stereocontrol (Figure 10). 

Collins has reported the synthesis of a related class of metallocenes (43), some of which form elastomeric, stereoblock polypropylene when activated by The elastomeric properties of the polymer formed using 43 (M = Hf, X = SiMe2 7)xn = 25 °C) were far superior to those formed by the other metallocenes in the study. The polymers made using 42 and 43 have similar microstructures, as well as physical and mechanical properties. However, after detailed microstructural analysis of the polymer the authors proposed an alternate mechanism of stereocontrol to Chien s site epimerization model (Scheme... 

Scheme 14. Proposed Mechanism of Stereocontrol for Syndiospecific Polymerization of Styrene Using CpTiCls/MAO...

Scheme 16. Proposed Mechanism of Stereocontrol in the Polymerization of Conjugated Dienes...

These materials have also been reported by Hoechst. On the basis of their crystallinity and simple NMR spectra in comparison to other alternated polymers of this type, it is proposed that they are isotactic. A possible mechanism of stereocontrol is alternated insertion between isospecific (nor-bomene insertion) and aspecific sites (ethylene insertion). Amdt-Rosenau has used MAO-activated Me2C(3- PrCp)(Fl)ZrCl2 to form this polymer with a melting point as high as 320 °C. ... 

E. Mechanisms of Stereocontrol in Primary Insertion (Site vs Chain-End ControO... 

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. 

As reported in section II.E, the two main mechanisms of stereocontrol in 1-olefin polymerization arise from the chirality of the catalytic site enantiomor-phic site control) and from the chirality of the last methine in the polymer chain (chain-end control). Two statistical models, based on these basic mechanisms, have been developed and used by different authors and are known as the enantiomorphic site modeP and the Bernoullian modeB ... 

Mechanisms of Stereocontrol. Stereochemistry of the olefin insertion step can be controlled by both the steric environment of the active site (enantiomorphic-site control) as well as the growing polymer chain (chain end control). In chain end stereocontrol, stereospecificity arises from the chiral )3-carbon atom of the last enchained monomer imit, which in turn influences the stereochemistry of monomer addition. Chain-end control is usually less effective than site control and has been observed for some achiral metallocenes at low polymerization temperatures. Partially iPP resulting from chain end stereocontrol has been obtained with Cp2TiPh2/MAO (56,272). The syndiospecific polymerization of 1-butene using the Cp 2MCl2/MAO (M = Zr, Hf) catalyst systems has been described (273). Predominantly sPP has been obtained under chain end control, using Brookhart s diimine nickel catalysts (274-277). 

Numerous theoretical studies on both early and late transition metal SSCs have appeared in recent years covering nearly all aspects of the olefin polymerization process (for reviews, see Refs. 282 and 283). The employed methodologies include molecular mechanics (284-291), ah initio electronic structure methods (292-297), density functional studies (298-303), as well as various hybrid techniques (304-308), such as the combination of quantum and molecular mechanics (QM/MM). A detailed description of these studies is outside the scope of this article nevertheless, these theoretical investigations have played a major role in elucidating the elementary steps of olefin complexation, chain propagation, and chain termination as well as the mechanisms of stereocontrol in catalytic olefin polymerization. 

High-field NMR studies by Busico et al. ultimately led to a better understanding of the polymer microstructure and to a substantial revision of the mechanism of stereocontrol, which was found to be critically dependent on the steric hindrance of the aryl substituent on the indenyl rings. 

The catalytic route closest to being completely managed and understood is stereoblock-isotactic polymerization in the presence of bis(2-Ar-indenyl) group 4 metallocenes with very bulky aryl substituents. The simple and clean mechanism of stereocontrol entails an oscillation of the active cation between the two enantiomorphous rac-like conformations. Unfortunately, the resulting polymers have fairly high crystallinity and thermoplastic properties which are of rather limited practical interest, compared with those in which the alternation of crystallizable and amorphous blocks results in materials behaving as TPEs. 

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