Chiral site stereocontrol, chirality polymerization

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 polymer chain end control model is supported by the observation that highly syndiotactic polypropene is obtained only at low temperatures (about —78°C). Syndiotacticity is significantly decreased by raising the temperature to —40°C [Boor, 1979]. The polymer is atactic when polymerization is carried out above 0°C. 13C NMR analysis of the stereoerrors and stereochemical sequence distributions (Table 8-3 and Sec. 8-16) also support the polymer chain end control model [Zambelli et al., 2001], Analysis of propene-ethylene copolymers of low ethylene content produced by vanadium initiators indicates that a syndiotactic block formed after an ethylene unit enters the polymer chain is just as likely to start with an S- placement as with an R-placement of the first propene unit in that block [Bovey et al., 1974 Zambelli et al., 1971, 1978, 1979]. Stereocontrol is not exerted by chiral sites as in isotactic placement, which favors only one type of placement (either S- or R-, depending on the chirality of the active site). Stereocontrol is exerted by the chain end. An ethylene terminal unit has no preference for either placement, since there are no differences in repulsive interactions. 

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... 

Ziegler-Natta Various types of Ziegler-Natta catalysts Polymerization mechanism and applications of molecular mechanics to the study of enantioselectivity of some stereospecific catalytic systems having chiral site stereocontrol Mechanism of enantioselectivity and its relevance to catalytic systems 56... 

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. 

The chain-end stereocontrol for olefin polymerizations leads generally to lower stereoselectivities (differences in activation energy for insertion of the two enantiofaces generally lower than 2 kcal/mol) than the chiral site stereo-control.18131132 For this reason, the corresponding catalytic systems have not reached industrial relevance for propene homopolymerization. However, some of them are widely used for propene copolymerization with ethene. 

The mechanisms of stereoselectivity which have been proposed for chain-end stereocontrolled polymerizations involving secondary monomer insertion also present a general pattern of similarity. In fact, molecular modeling studies suggest that for olefin polymerizations (both syndiospecific and isospecific, Section 4.1.2) as well as for styrene polymerization (syndiospecific, Section 4.2), the chirality of the growing chain would determine the chirality of a fluxional site, which in turn would discriminates between the two monomer enantiofaces. 

When the stereocontrol occurs by a chain end control mechanism, a stereochemical defect results in the stereochemistry of the defect being propagated along the chain until the next defect occurs (polymer 1 in Fig. 7). If a stereochemical defect occurs in a polymerization using an initiator that exhibits enantiomeric site control, the mistake will be rectified with the next incoming lactide unit (polymer 2 in Fig. 7). This is because it is the chirality of the metal centre which determines the PLA tacticity and not that of the last inserted lactide unit. 

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). 

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]. 

The classical heterogeneously catalyzed propene polymerization as discovered hy Natta is a stereospecific reaction forming a polymer with isotactic microstructure. During the development of single-site polymerization catalysts it was found that C2-symmetric chiral metallocene complexes own the same stereospecificity. An analysis of the polymer microstructure hy means of NMR spectroscopy revealed that misinsertions are mostly corrected in the next insertion step, which suggests stereocontrol (Figure 6) hy the coordination site, as opposed to an inversion of stereospecificity hy control from the previous insertion steps (chain-end control). In addition, it was found that Cs-symmetric metallocene catalysts lead to syndio-tactic polymer since the Cosee-Arlmann chain flip mechanism induces an inversion of the stereospecificity at every insertion step. This type of polymer was inaccessible by classical heterogeneous systems. 

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). 

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