Catalytic-site control

An excellent way to treat such data is to use reaction probability models.(1,2) In the NMR analysis of tacticity, it is frequently possible to distinguish whether the configuration is chain-end controlled or catalytic-site controlled during polymerization. Various statistical models have been proposed. The chain-end controlled models include Bemoullian (B), and first- and second-order Markovian (Ml and M2) statistics.(1) The simplest catalytic-site controlled model is the enantiomorphic site (E) model.(3) The relationship between the chain-end and catalytic-site controlled models and possible hybrid models have been delineated in a recent article.(4)... 

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). 

Figure 19 Stereoselective insertions of propylene (grey) under catalytic-site control, mediated by the oc,[3 segment of the growing polymer chain (black), for isospecific polymerization by a C2-symmetric catalyst (A, left) and for syndiospecific polymerization by a Cs-symmetric catalyst (B, right).

Figure 28 Stereoerror pentads expected for essentially isotactic poly propylenes, generated under catalytic-site control (top) and under chain-end control (bottom).

While the detailed structures of most catalyst sites are still unknown, it was established that stereoselectivity does not come from the chirality of the growing chain end. Rather, it is built into the catalyst site itself. Normal preparations of the catalysts give equal numbers of (/ ) and (5) chiral catalyst sites. These coordinate selectively with (R) and (5) monomers, respectively, in the process of catalytic-site control. ... 

Figure 11 Stereoselectivity via chain-end and catalytic site control.

Again, NMR spectroscopy represents the method of choice to distinguish between chain-end and catalytic-site control mechanisms using the PP microstructure as fingerprint. Recently, Randall reported the characterization of steric defects in PP by means of NMR spectroscopy for iPP as well as sPP [27]. As outlined in Figiue 12, false insertion affords a change in chirality of the stereogenic carbon atom of... 

Fig. 4 Transition state TS for favored si-facial olefin insertion into a Zr-polymeryl bond according to the model of chain-segment-mediated catalytic site control [11,12] (left) and reaction complex RC, which is thought to precede transition state formation [7] (right)...

Abstract Metallocene complexes that serve as stereoselective olefin polymerization catalysts are described. The polymerization of propylene, styrene, methyl methacrylate, 1,3-dienes, non-conjugated dienes and cycloolefins is discussed. The stereochemistry of monomer insertion is governed by the chiral steric environment of catalysts derived from a ligand structure (catalytic-site control) or a chiral center in the polymer chain (chain-end control). The mechanism of formation of isotactic and syndiotactic polymers in each monomer and catalyst is explained. Non-metallocene catalysts for stereospecific polymerization are also mentioned. 

Fig. 3 NMR spectroscopy of isotactic polypropylene produced under catalytic-site control...

Stereoselectivity in the insertion of olefins is brought about by a chiral environment of the coordination sphere. When the selectivity is predominantly governed by chirality of the catalyst itself, stereospecificity shows catalytic-site control , also called catalyst control or enantiomorphic-site control . Catalysts must have chiral structures (but need not be homochiral) in this case. Even though a catalyst is achiral, an inserted a-olefin can make a chiral center at the end of a polymer chain. When this chirality controls the stereoselectivity of the next monomer insertion, it is called chain-end control . 

These two mechanisms can be distinguished by observation of the stereoerrors found in the polymers. Catalytic-site controlled isotactic polymers have an error described in Fig. 4. One misinsertion has little effect on the face selectivity of the next insertion, because it is governed by the catalyst structure. It results in a sequence of mmmrrmmm. Thus mmmr, mmrr and mrrm pentads are found besides mmmm. On the other hand, in chain-end control, once a monomer is misinserted, the opposite chirality governs the next monomer insertion. It gives an mmmrmmm sequence, as shown in Fig. 4, and mmmr and mmrm appear in the spectra. 

Fig. 4 Stereoerrors in polyolefins catalytic-site control and chain-end control...

Not only bridged metallocenes but also non-bridged lanthanocenes 36,37 give iso-rich poly(MMA). There are racemo and meso isomers in 36, and both 36 and 37 have several kinds of rotamers. Although the rotation is too fast to be detected by NMR at low temperature, stereomultiblock polymers were obtained [ 129]. It was also described that the tacticity is not consistent with either catalytic-site control or chain-end control. In regard to these reports, isospecific MMA polymerization seems not simply regulated as in the case of propylene polymerization. 

Chen et al. showed that stereoregularity greatly depends on co-catalysts 44 and 46 with borane give isotactic poly(MMA) by catalytic-site control, while these catalysts with the alane A1(C6F5)3 give syndio-rich poly(MMA) by chain-end control [144]. They achieved iso-b-syndio stereoblock poly(MMA) using the borane and the alane in one pot [145]. 

More detailed studies have revealed that the stereochemical propagation can occur by several different mechanisms. Two general kinds of mechanisms can be identified one is chain-end controlling and the other is catalytic-site controlling. Free radical, anionic and most cationic polymerizations are chain end controlled, whereas many Ziegler-Natta polymerizations are catalytic-site controlled [19]. 

A polymer having only racemic diads gives syndiotactic polymer, whereas one having only meso diads gives isotactic polymer. The control of stereoregularity is once again by (1) catalytic site control and (2) chain end control, which is caused by the chirality of the previous monomer inserted. 

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