Site control mechanism

In a system with site control ideally the mistake leads to a single odd insertion in the chain the site enforces for instance the growth of an isotactic chain with all m configurations and after one mistake has occurred it will return to producing the same configurations. In other words, an isospecific catalyst will produce a polymer chain with all methyl groups pointing towards us (as in 

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An FI catalyst normally assumes a C2-symmetric trans-O, ds-N, and d.s-Cl configuration as the predominant isomer. In addition, DFT calculations suggest that a catalytically active species derived from an FI catalyst favors a C2-symmetric configuration with a trans-O, cis-N, and d.v-polymer chain/coordinated olefin arrangement. Thus, FI catalysts have been targeted as catalysts capable of producing iPP via a site-control mechanism. 

The Tj, and the isospecificity obtained with these FI catalysts represent some of the highest values for iPPs ever synthesized. The isospecific propylene polymerization proceeds via a 1,2-insertion with a site-control mechanism. 

Site control versus chain-end control. Over the years two mechanisms have been put forward as being responsible for the stereo-control of the growing polymer chain firstly the site-control mechanism and secondly the chain-end control mechanism. In the site control mechanism the structure of the catalytic site determines the way the molecule of 1-alkene will insert (enantiomorphic site control). Obviously, the Cossee mechanism belongs to this class. As we have seen previously, propene is prochiral and a catalyst may attack either the re-face or the, v/-facc. If the catalyst itself is chiral as the one drawn in Figure 10.2, a diastereomeric complex forms and there may be a preference for the... 

Summarising, in the chain-end control mechanism the last monomer inserted determines how the next molecule of 1-alkene will insert. Several Italian schools [7] have supported the latter mechanism. What do we know so far Firstly, there are catalysts not containing a stereogenic centre that do give stereoregular polymers. Thus, this must be chain-end controlled. Secondly, whatever site-control we try to induce, the chain that we are making will always contain, by definition, an asymmetric centre. As we have mentioned above, the nature of the solid catalysts has an enormous influence on the product, and this underpins the Cossee site-control mechanism. Thus both are operative and both are important. Occasionally, chain-end control alone suffices to ensure enantiospecifity. 

However, there are numerous reported instances of stereocontrol by a site-control mechanism involving chiral metal catalysts. That is, Nozaki and coworkers first illustrated the asymmetric alternating copolymerization of cyclohexene oxide and CO2 employing a chiral zinc catalyst derived from an amino alcohol (Fig. 2a) [13-16]. This was soon followed by studies of Coates and coworkers utilizing an imine-oxazoline zinc catalyst (Fig. 2b) [17]. Both investigations provided isotactic poly(cyclohexene carbonate) (Fig. 3) with enantiomeric excess of approximately 70%. 

The mechanical properties of PLA rely on the stereochemistry of insertion of the lactide monomer into the PLA chain, and the process can be controlled by the catalyst used. Therefore, PLAs with desired microstructures (isotactic, heterotactic, and S3mdiotactic) can be derived from the rac- and W50-Iactide depending on the stereoselectivity of the metal catalysts in the course of the polymerization (Scheme 15) [66]. Fundamentally, two different polymerization mechanisms can be distinguished (1) chain-end control (depending on stereochemistry of the monomer), and (2) enantiomorphic site control (depending on chirality of the catalyst). In reality, stereocontrolled lactide polymerization can be achieved with a catalyst containing sterically encumbered active sites however, both chain-end and site control mechanisms may contribute to the overall stereocontrol [154]. Homonuclear decoupled NMR analysis is considered to be the most conclusive characterization technique to identify the PLA tacticity [155]. Homonuclear... 

Metal complexes which initiate rac-LA ROP with a high degree of stereocontrol are currently an area of major research interest and have the potential to produce a spectrum of different materials [19, 21], Much attention focuses on iso-selectivity as this can enable production of PLA of good thermal resistance (isotactic, stereoblock or even stereocomplex PLA). There are two mechanisms by which an initiator can exert iso-selectivity in rac-LA ROP (1) an enantiomorphic site control mechanism or (2) a chain end control mechanism. Enantiomorphic site control occurs using chiral initiators (Fig. 6) it is the chirality of the metal complex which... 

Arnold and colleagues have reported a series of chiral homoleptic yttrium and lanthanide fra(alkoxide) complexes [49, 50], These initiators (including complex 1) show high degrees of iso-selectivity and rapid rates, even at low temperatures. Thus, using the racemic mixture of the lanthanide initiator, stereoblock PLA was produced with a P, of 0.81 so far, this is the only known type of yttrium initiator able to exert such stereocontrol and a very exciting finding. Analysis of the stereoerrors indicates that an enantiomorphic site control mechanism is responsible for the iso-selectivity. 

Under an exclusively enantiomorphic site control mechanism, the symmetry of the catalyst would mandate homofacial insertion and cyclisation to yield cis... 

The enantiomorphic catalyst sites control mechanism was found to operate in the stereospecific polymerisation of tiiranes. Sigwalt et al. [79,153] found that... 

When activated with MAO complex 6, under pressure, polymerizes propylene producing isotactic polymer (Table 5). The fact that in the 13C-NMR spectrum of the polymers no mrmm signals were found, can testify about the formation of the polymers exclusively by the site control mechanism. Increasing the temperature induces an increase of the reaction rate, but the molecular weight... 

Scheme 1 introduces the parameters that are used to describe the stereoselectivity of the monomer enchainment process. For chain-end control, the parameters Pm and refer to the probability of meso and racemic placements, respectively (the Bovey formalism is a convenient way to describe polymer tacticity, with a small m for meso, and a small r for racemic relationships between adjacent stereogenic centers). A Pm equal to unity indicates isotac-ticity, while a P equal to unity signifies syndiotac-ticity. For site-control mechanisms, the parameter a represents the degree of enantiotopic selectivity of the enchainment. When a is either 1 or 0 an isotactic polymer forms, while an a parameter of 0.5 produces an atactic polymer. Polymer architectures relevant to this review are shown in Figure 1. 

The ring-opening polymerization of a simple cyclic olefin such as cyclooctene yields two structures of maximum order, which are distinguished by the configuration (cis or irans) of their main-chain olefins. In contrast, polymers made from bicyclic olefins such as norbornene are inherently more complicated and have four structures of maximum order (Scheme 24). In addition to cis- and trans-olefins, the polymers can also be isotactic or syndiotactic. The stereochemistry of these polymers becomes even more complicated when the monomer is asymmetric, since head—head, head—tail, and tail—tail regioisomers are possible. Nevertheless, single-site metathesis catalysts have been developed that can control polymer stereochemistry to an impressive degree by both chain-end and site-control mechanisms. ° ° ... 

On the basis of what we have discussed above, as far as enantioface selectivity is concerned, C2-sym-metric chiral metallocenes should not sense on what side, and how often, a monomer approaches the metal center. Indeed, the site control mechanism of these catalysts requires that the stereochemistry of insertion is independent from the previous insertion. The loss of stereospecificity with the decrease in monomer concentration has been accounted for, from a kinetic standpoint, with an equilibrium between active sites having a coordinated monomer (C M) and sites without coordinated monomer Q (Scheme 31). 

Table 1 lists the discrete group 4 metallocene or related initiators that are either known or presumed to produce PMMA by this mechanism. In the case of chiral or prochiral initiators, often the microstructure of the resulting polymer, particularly if it is consistent with a site-control mechanism, is a reliable indicator of mechanism. In other cases, this assumption is less certain, and often the mechanism is presumed to be analogous. In the case of the CGCTi complexes studied by Chen and co-workers, the unimolecular pathway is followed, but syndiotactic PMMA is produced by a chain-end control mechanism. 

A-isomer being in dynamic equilibrium with the A-isomer. The chiral zir-conium(iv) complexes produce moderate isotactic enchainment (P =0.8 in solution), the mechanism is presumably an enantiomorphic site-controlled mechanism. This was further confirmed by the fact that the J ,J -complex was more active for d-LA than l-LA and the 5, 5-complex more active for l-LA However, the meso complex is incredibly active for the ROP of rac-LA - more so than the chiral ligands - and is significantly more active and selective (Pm = 0.86). To achieve conversion in a reasonable timeframe the chiral complexes have to be heated to 70 °C, whereas the meso complex is active at room temperature. This is believed to be related to the fluxionality of the complex. Figure 8.8. 

The relative configuration of the monomer units can be controlled by the structure of the catalyst or by the configuration of the last inserted unit. These two scenarios are called site control and chain-end control. The isotactic polypropylene generated by the types of stereodefined metallocene catalysts presented in this chapter results from site control. More sophisticated architectures are possible by site-control mechanisms than chain-end control mechanisms, as illustrated by the variety of polyolefins prepared by homogeneous catalysts. 

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