Stereoselective polymerizations

Chiral polymers have been applied in many areas of research, including chiral separation of organic molecules, asymmetric induction in organic synthesis, and wave guiding in non-linear optics [ 146,147]. Two distinct classes of polymers represent these optically active materials those with induced chirality based on the catalyst and polymerization mechanism and those produced from chiral monomers. Achiral monomers like propylene have been polymerized stereoselectively using chiral initiators or catalysts yielding isotactic, helical polymers [148-150]. On the other hand, polymerization of chiral monomers such as diepoxides, dimethacrylates, diisocyanides, and vinyl ethers yields chiral polymers by incorporation of chirality into the main chain of the polymer or as a pedant side group [151-155]. A number of chiral metathesis catalysts have been made, and they have proven useful in asymmetric ROM as well as in stereospecific polymerization of norbornene and norbornadiene [ 156-159]. This section of the review will focus on the ADMET polymerization of chiral monomers as a method of chiral polymer synthesis. 

Development of the elucidation of the catalytic reaction mechanism and the structure-reactivity relationships proceeded much more slowly. By the mid-1960s Wilke [17], Porri [18], and Dolgoplosk [19] had already shown that allyl-transition metal complexes can catalyze the butadiene polymerization stereoselectively and quite probably represent the real catalysts. In particular the allylnickel(II) complexes [Ni(C3Hs)X]2 (X = I [20], CF3CO2 [21]) and more recently the cationic complexes [Ni(C3H5)L2]PFe, with L = P(OPh)3, etc. [22, 23], were also used to explore the catalytic reaction mechanism. 

Butadiene polymerization in water as a reaction medium catalyzed by rhodium salts was already reported in the 1960s by Rinehart et al. and by Canale and coworkers. Utilizing RhCl3 3 H20 as a catalyst precursor, butadiene is polymerized stereoselectively in aqueous emulsion to semicrystalline trans-1,4-polybutadiene exclusively (Eq. 2 > 99% trans) [28, 29]. 

Keyword polysilanes, regiospedfic polymerization, stereoselectivity, masked disilene... 

A soln. of 8-benzyloxy-2,6-dimethyl-2,3-epoxyoctane in ether added to /-Pr2NH and 4 eqs. H2O in the same solvent under a balloon of SiF4 at 0°, the mixture stirred for 1 h, then quenched with aq. KF -> 8-benzyloxy-2,6-dimethyl-2-fluorooctan-3-ol. Y 91%. The method is short and is undertaken with conventional glass apparatus olefins, ethers and 1,2-disubstd. oxiranes (aliphatic) were unaffected, while mono-subst. aliphatic oxiranes polymerized. Stereoselectivity is illustrated by the formation of 5y -fluorohydrins. F.e.s. M. Shimizu, H. Yoshioka, Tetrahedron Letters 29, 4101 (1988) l-fluoro-2-hydroxysilanes s. Tetrahedron Letters 30, 967-70 (1989). 

Busico, V. Cipullo, R. Caporaso, L. Angelini, G Segre, A.L. Cz-symmetric ansa-metallocene catalysts for propene polymerization Stereoselectivity and enantioselectivity. J. Mol. Catal., Part A. 1998,128, 53-64 and references therein. 

Both catalyst activity and polymerization stereoselectivity increases and PDI decreases while we are moving from the 3rd to the 5th generation. However, this is not the case for succinate-based systems. 

In continuation of the investigation on the chain-effect polymerization, stereoselectivity of this process was examined. The set of polymerizations were conducted with polypeptide initiator with similar composition but different chiral stmeture. Obtained results indicated that the activated NCA-typ>e polymerization is stereoselective. 

Thus, the propagation steps are consecutive and rapid insertions of coordinated monomer into the growing polymer chain. Both the position of the coordinated monomer and the location of the regenerated vacant site can significantly impact the catalyst polymerization stereoselectivity, an issue that will be discussed in Section 3.21.5. While there is just one possibility for ethylene coordination and insertion, there are four possible modes for 1-alkene (a-olefin) coordination and insertion. Selection among these modes determines the... 

The focus of this section is the polymerization stereocontrol made possible by ligand modification of metallocenes. Thus, propylene will be the monomer of focus since the stereochemistry of polypropylene is the best tmderstood of any polyolefin," if not of any synthetic polymer ever studied. The observed correlation between a catalyst s stmcture/symme-try and a catalyst s stereoselectivity is often referred to as Ewen s Symmetry Rules.Metallocenes have been manipulated to a remarkable degree to direct the enantiomorphic site control mechanism for polymerization stereoselectivity. ... 

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