Enantiomorphic site stereocontrol

Scheme 1.1 Typical steric defects in a (mainly) isotactic poly-l-alkene chain (adapted Fisher projections) for (a) chain-end-stereocontrol (b) enantiomorphic site stereocontrol.

Both possibilities, i.e. enantiomorphic site stereocontrol (in the case of an optically inactive catalyst it consists of a racemic mixture of enantiomorphic sites) and chain end stereocontrol, have been verified, depending on the kind of catalyst. These two essential types of stereocontrol mechanism operating in propylene polymerisation with various stereospecific Ziegler-Natta catalysts are presented in Table 3.3. 

Figure 3.47 Typical steric defects (pentad distribution of stereoerrors) in a (mainly) syndiotactic poly(a-olefin) chain (a) isolated m diad, characteristic of chain end stereocontrol (b) pair of m diads, characteristic of enantiomorphic site stereocontrol during the propagation...

Enantiomorphic site stereocontrol with error correction XVI... 

Scheme 2 (a) Chain-end stereocontrol mechanism, (b) Enantiomorphic site stereocontrol mechanism, (c) Tactic polymers via chain-end control mechanism, (d) Isotactic polymers via enantiomorphic site control mechanism. L pM-OR is an enantiomerically pure metal alkoxide complex that prefers R-monomer L n is an enantiomerically pure, chiral ligand. 

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... 

The enantiomorphic site control model attributes stereocontrol in isoselective polymerization to the initiator active site with no influence of the structure of the propagating chain end. The mechanism is supported by several observations ... 

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. 

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). 

Over the years two mechanisms have been proposed as being responsible for the stereocontrol 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. In the chain-end control mechanism the last monomer inserted determines how the next molecule of 1-alkene will insert. Several Italian schools have supported the latter mechanism. For heterogeneous catalysts it would seem that site control was strongly gaining preference [32], at least until the more detailed work on homogeneous catalysts became known. 

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... 

Stereocontrol in polymerization can arise from either chain-end control, where the stereochemistry of the chain-end determines the configuration of the incoming monomer, or enantiomorphic site control, where the chirality of the ligand imparts stereoselectivity. However, these two mechanisms are not mutually exclusive, and often act in concert in a given system. 

Scheme 1. Chziin-End and Enantiomorphic Site Mechzinisms of Stereocontrol...

Considering each insertion as an independent event (which implies enantiomorphic site control, i.e., the chirality of the catalyst is the dominating factor for stereocontrol). Hart and Rappe then calculated the intensities for the mmmm pentad by multiplication of the individual probabilities. The calculated intensities were in excellent agreement with those from NMR data for a series of substituted ethylene-bridged, C2-symmetric zirconocenes. The use of this new and more quantitative approach to analysis of the relationship between the molecular structure of the catalyst and the polymer microstructure was, however, restricted to comparison of intensities for the mmmm pentad. Application to error pentads or prediction of pentad distributions for new or modified catalysts was not attempted. 

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). 

SCHEME 24.2 Chain-end (a) and enantiomorphic-site (b) mechanisms of stereocontrol in epoxide polymerization (L. M-OR is an enantiomerically pure catalyst that prefers /J-monomer). 

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