Enantiomeric isotactic forms

Substitution at the 2 position (li) yields a chiral center and stereoselection in the polymerization step could be visualized leading to the formation of two enantiomeric isotactic forms ... 

Polymers containing cyclophane-type moieties were also prepared for their unique functionalities and potential applications. Thus, isotactic poly(para-phenylene) ladder polymer (56) was prepared from the corresponding bromide and borate (55) in enantiomerically pure forms and subjected to the CD spectral measurement [62],... 

In usual polymerizations of vinyl monomers leading to isotactic polymers, the enantiomeric polymer molecules (A) and (B) are formed in equal amounts, and the polymerizations are not asymmetric but racemate-forming chirogenic polymerizations. 

Certain chiral organic compounds create crystalline environments and act as enantio-controlling media (7) even though they do not function as true catalysts. Natta s asymmetric reaction of prochiral trans-1,3-pentadiene, which was included in the crystal lattice of chiral perhydro-triphenylene as a host compound, to form an optically active, isotactic polymer on 7-ray irradiation, is a classic example of such a chiral molecular lattice (Scheme 1) (2). Weak van der Waals forces cause a geometric arrangement of the diene monomer that favors one of the possible enantiomeric sequences. 

D,L-copolymerization of enantiomerically imbalanced mixtures of 6,8-dioxybicy-clo[3.2. l]octane (1) has revealed that an isotactic sequence along the polymer chain is preferentially formed by the enantiomer selection at the chiral growing chain end. [21] If this is generally the case, asymmetric copolymerization should occur when a racemic bicyclic acetal is allowed to copolymerize with an optically... 

By enantioselective polymerization polymer chains, each containing only one configurational kind of monomeric unit, are produced from a mixture of stereoisomeric monomer molecules. The number of kinds of polymer chain generated therefore equals the number of various stereoisomers in the monomer mixture. In the course of propagation, the enantiomeric composition of the polymer and unreacted monomer remains identical to the intial composition. When optically active monosubstituted cyclic monomers are polymerized, stereoregular polymers are formed with both isotactic polyR and polyS chains... 

As discovered by Natta, Pino and coworkers(16,17), isotactic polymerization of chiral cc-olefins is stereoselective (e.g., isotactic poly-(R,S)-3-methyl-l-pen-tene consists of enantiomeric macromolecules which can be partially resolved.) If one considers that the isotactic steric control arises from the enantiose-lectivity of the chiral catalytic sites toward the enantiotopic carbon of the monomer(7 11) then stereoselectivity simply means that the insertion is dia-stereoselective. In other words, the diastereotopic faces of the monomer must have a different reactivity in the insertion (Figure 3)- Figure 4 shows the NMR spectrum of isotactic poly-(R,S)-3-methyl-l-pen-tene obtained in the presence of TiCl3 Al (sample 8). The resonances at 13-2l, 13.57 15-09 and 15-27 ppm are due to the - CI s of 2 - -uD-2,3-dimethyl-pentyl end groups formed in the initiation step(l8). 

In contrast, polymers with prochiral monomeric units form helices which are enantiomeric to each other. In this case, left- and right-handed helices are energetically equivalent, and so, equally probable. Thus isotactic poly(pro-pylene) in the crystalline state forms equal amounts of left- and right-handed helices, with the conformations (TG ) and (TG ) , respectively. 

Unsymmetrically substituted monomers have enantiomeric forms. An isotactic polymer of a single enantiomer will necessarily have an all HT structure while a syndiotactic... 

More importantly, while it is very difficult to control the nature of the site t)rpes on conventional heterogeneous Ziegler-Natta catalysts, metallocene catalysts can be designed to synthesize PP with different chain microstructures. PP chains with atactic, isotactic, isotactic-stereoblock, atactic-stereoblock and hemiisotactic configurations can be produced with metallocene catalysts (Figure 2). It is also possible to s)mthesize PP chains that have optical activity by using only one of the enantiomeric forms of the catalyst. Additionally, copolymers of propylene, ethylene and higher a-olefins made with metallocene catalysts have random (or near random) comonomer incorporation and narrow chemical composition distributions (CCD). 

The catalyst OsCls (in a 1 1 by volume mixture of ethanol/chlorobenzene) converts racemic 1-MeNB into an atactic, all-trans polymer with predominantly head-tail structures at low monomer concentrations (0.2 mol/L). The ratio of (HT + TH)/(HH + TT) signals is 9 1. By comparison, polymerization of a single isomer of 1 -MeNB gives isotactic poly( 1 -MeNB), as the selective head-tail enchainment necessarily leads to the meso stereochemistry when enantiomerically pure monomer is used (Scheme 20.8). At an increased monomer concentration (1.5 mol/L), a polymer with 16% cis units is formed. This polymer does not contain any cis-head-head sequences. 

Polymerization of a single enantiomer of methyl-(V-(l-phenylethyl)-2-azabicyclo[2.2.1]hept-5-ene-3-carboxylate (Scheme 20.23) by the hexafluoro-tert-butoxide-based Mo complex 16e produces high-cis polymer (>98% cis) with an all-head-tail structure. This product is isotactic as a result of the head-tail enchainment of enantiomerically pure monomer. By comparison, the all-head-tail polymer formed from a racemic mixture of this azabicyclic olefin monomer is atactic. 

Since according to this mechanism each enantiomer R or S, independently, would produce isotactic chains, and yet exclusively syndiotactic polymer chains are formed, it has to be concluded that the active enantiomeric species enantiomerize via an intramolecular mechanism and interconvert individually after each monomer insertion. 

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