Isotactic chain propagation

Figure 3.46 Possible isotactic poly(a-olefin) chain propagation after the insertion of an achiral ethylene unit (a) chain end stereocontrol (b) enantiomorphic site stereocontrol during the propagation...

Fig. 14. The probabilities Pi, and of formation of isotactic, syndiotactic, and heterotactic triads, respectively, as a function, of o, the probability of isotactic placement of monomer units during chain propagation. Experimental points at the left are for methyl methacrylate polymers prepared with free radical initiators . those at the right for polymers prepared with anionic initiators ( O) t peaks ...

Consequently, in the free-cation propagation mechanism, conditions exist for the predominant formation of the isotactic chain structure. In order that... 

This has been attributed to the fact that the initiator is a 1 1 mixture of non-interconvertible enantiomers and that highly regular all-cw isotactic chains grow separately from each enantiomeric metal site. The chirality at the metal site, interacting with the chiral monomer, controls the rate of propagation, while the chirality of the chain end , i.e. the previously added monomer unit, independently controls the stereochemistry of the next monomer addition. This effect is not observed with 140b as monomer, where the rates of propagation at the two types of site are presumably not sufficiently different (O Dell 1994). 

Fig. 12 The three-site model of active species for MgCl2ATiCl4/Lewis base catalyst systems LI and L2 genetically denote chemisorbed Lewis base molecules, a, b and c are proposed to give rise to highly isotactic, poorly isotactic ( isotactoid ) and (chain-end-controlled) syndiotactic polypropylene chain propagation, respectively (adapted from [50])...

The influence of the medium is stronger. Polymers with a relatively high syndiotactic triad fraction are produced in highly polar solvents (see Table 18-5). The isotactic diad fraction increases with increasing apolarity of the solvent. The heterotactic triad fraction remains more or less constant. Even here, the increase in isotacticity obviously parallels the increase in ion pairs. This conclusion can be readily understood. The linkage in chain propagation by free ions is determined by the most sterically favorable transition state but... 

The mechanism of monomer insertion and steric control in polymerizations of a-olefins by the metallocene catalysts received considerable attention [293-297]. There is no consensus on the mechanism of polymerization. Many studies of chain propagation tend to support the Cossee-Arleman mechanism [293-297]. An example is work by Miyake et al. [294] who synthesized unsymmetrical ansa-metallocenes and separated them into threo and erythro isomers. Both isomers coupled with methylaluminoxane polymerize propylene in toluene to highly isotactic polymers of = 105,000. The isotactic placement is greater that 99.6% and the polymer melting point is 161°C. 

The Cl-symmetric active cation derived from 3 has two diastereotopic sites. To explain their observations, the authors postulated that propylene insertion is enantioselective at only one of the sites, and isotactic/atactic stereoblock chain propagation results from long sequences of monomer insertions at the two sites feolCsH ] 2> feinv and feaspCCsH ] < inv in Scheme 8.4). 

SCHEME 8.6 Proposed mechanism of isotactic/atactic stereoblock propylene polymerization with bis(benzamidinate) Zr(IV) complexes. Chain propagation at high monomer concentration would be site-controlled isotactic-selective, whereas at low monomer concentration it would become nonstereoselective owing to intramolecular growing chain epimerization. (Reprinted with permission from Volkis, V. Nelken-baum, E. Lisovskii, A. Hasson, G Semiat, R. Kapon, M. Botonshansky, M. Eishen, Y. Eisen, M. S. J. Am. Chem. Soc. 2003,125, 2179-2194. Copyright 2003 American Chemical Society.)... 

When isoselective polymerization is promoted by nickel catalysts, such as the Ni(acac)2/Et3N/MAO system, the insertion of the monomer is mainly secondary (2,1-). The steric control of the polymerization still arises from chiral sites (enantiomorphic-site control), but the configuration of the sites apparently changes rather often during chain propagation since iPS with lower isotacticity is obtained. 

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