Conformational asymmetric polymerization

Another result of great importance—the conformational asymmetric polymerization of triphenylmethyl methacrylate realized in Osaka (223, 364, 365)— has already been discussed in Sect. IV-C. The polymerization was carried out in the presence of the complex butyllithium-sparteine or butyllithium-6-ben-zylsparteine. The use of benzylsparteine as cocatalyst leads to a completely soluble low molecular weight polymer with optical activity [a]o around 340° its structure was ascertained by conversion into (optically inactive) isotactic poly(methyl methacrylate). To the best of my knowledge this is the first example of an asymmetric synthesis in which the chirality of the product derives finom hindered rotation around carbon-carbon single bonds. 

Asymmetric Polymerization. Polymerization of methacrylate derived from (1) affords optically active polymer of helical conformation of single screw sense. ... 

These experiments revealed the important stereochemical features of polyisocyanide. First, racemic polyisocyanides consisting of achiral repeating units exist as a 1 1 mixture of right- and left-handed helices. Second, the right- or left-handed helical conformation is stable enough to exist as either enantiomer at ambient temperature in solution, although the stability may depend upon the AT-substituent as well as the degree of polymerization. Based on these ideas, efficient systems for asymmetric polymerization of isocyanides have been further pursued. 

Optically active oligomers of methyl methacrylate were obtained by the asymmetric polymerization of triphenylmethyl methacrylate, followed by the substitution of methyl for triphenylmethyl, and subsequent GPC separation275 and optical resolution.276 Detailed study on the stereostructure of the oligomer made it possible to discuss precisely the mechanism of polymerization leading to the formation of polymer with one-handed helical conformation. 

Some isotactic polymers such as polychloral and poly(triphenylmethyl methacrylate)289 are known to exist only in purely helical conformation. The helical structure of the polymers is rigid even in solution, owing to the bulkiness of the side-groups. This has been demonstrated by the measurement of high optical activity of the polymers prepared by asymmetric polymerizations the optical activity is based on a one-handed helical conformation of the polymer chain. 

Introduction of relatively weak functional groups, such as carbonyl, hydroxyl, nitro, amide, etc., in the nanochannels of PCPs would affect the monomer alignment, which may lead to precision control of stereoselectivity and regioselectivity of the resulting polymers. In particular, PCPs with either helical or chiral structures on the pore surface are of intense interest in chemistry and such porous solids are potentially useful to find applications in enantioselective sorption/separation and catalysis [34, 38 0, 42, 45]. Of considerable interest is the use of the chiral channels to affect asymmetric polymerizations such as asymmetric selective polymerization of racemic monomers as well as asymmetric polymerization of prochiral monomers, which may give helical polymer conformations. 

Stereocontrol in the polymerization of these acrylates was more difficult compared with that in the polymerization of methacrylates. The specific rotations ([ajses ) of poly(TrA) and poly(PDBSA) obtained by the asymmetric polymerization were much smaller than those of the corresponding polymethacrylates prepared under similar conditions and were up to+102° (ligand PMP, diad isotacticity 70%) and -94° (ligand DDB, diad isotacticity 61%), respectively. The isotactic part of the polymers is considered to have a helical conformation with excess helicity. 

More recently, direct coupling (hyphenation) of MALLS and RI detectors to flow FFF channel has been used to provide direct, accurate determination of molecular weights, Rg and conformation of polymeric wheat proteins. Problems such as overloading the FFF channel in order to provide sufficient signal for RI/MALLS have been addressed for both asymmetrical " and symmetrical" " FFF. 

As described in X-ray Crystal Analyses section, DBF oligomers have been shown to have a jc-stacked, single-handed helical conformation in crystal. A jc-stacked structure was indicated also for poly(PDBF) by remarkable hypochromicity in absorption and exclusive dimer emission in fluorescence spectra (Fig. 34). Therefore, it will be reasonably assumed that the CD absorptions observed in this work are based on a helical conformation of poly(PDBF) with excess single handedness. Because the polymers showing clear CD bands in film did not show chiroptical properties in solution, molecular aggregation may amplify and stabilize the single-handed helical conformation induced by the asymmetric polymerization possibly due to intermolecular cooperative effects in the solid state. Atomic force microscopic (AFM) analyses supported this assumption (Fig. 39). The samples were prepared by... 

Nakano T, Satoh Y, Okamoto Y. Asymmetric polymerization of l-(3-pyridyl)diben-zosuberyl methacrylate and chiral recognition by the obtained optically active polymer having single-handed hehcal conformation. Polym J. 1998 30 635-640. 

One of the most important results of inclusion polymerization is the synthesis of optically active polymers from nonchiral compounds. Asymmetric polymerization of /ra 5-pentadiene in PHTP has been reported. The optical purity of the polypentadiene is about 7%. DCA and ACA, as natural hosts, induce a greater asymmetric polymerization. The cis and rra/i5-2-methyl-pentadiene gave the highest asymmetric polymerization values [88]. The optical rotatory power disappeared with temperatures higher than 70°C, indicating that this process is reversible and results from a conformational transformation. 

After the Natta s discovery of highly stereospecific polymerization processes, the interest in the preparation and properties of optically active polymers has greatly increased. In fact, the use of asymmetric catalysts or monomers to obtain optically active polymers may supply interesting informations on the mechanism of steric control in stereo-specific polymerization furthermore optical activity is an useful tool to study the polymer stereoregularity and the chain conformations of polymers in the molten state or in solution. 

Aldehyde Polymers Asymmetric anionic polymerization can lead trichloroacetalde-hyde (chloral) to a one-handed helical, isotactic polymer having a 4/1-helical conformation with... 

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