Isotactic structures optical activity

An interesting aspect of the benzofuran cationic polymerization was uncovered by Natta, Farina, Peraldo and Bressan who reported in 196160,61 that an asymmetric synthesis of an optically active poly(benzofuran) could be achieved by using AlCl2Et coupled with (-)j3-phenylalanine, (+)camphorsulphonic acid or with (-)brucine. The optical activity was definitely due to the asymmetric carbon atoms in the polymer chain, indicating that at least some of the polymer s macromolecules possessed a di-isotactic structure, v/ z.62 ... 

Okamoto and his colleagues60) described the interesting polymerization of tri-phenylmethyl methacrylate. The bulkiness of this group affects the reactivity and the mode of placement of this monomer. The anionic polymerization yields a highly isotactic polymer, whether the reaction proceeds in toluene or in THF. In fact, even radical polymerization of this monomer yields polymers of relatively high isotacticity. Anionic polymerization of triphenylmethyl methacrylate initiated by optically active initiators e.g. PhN(CH2Ph)Li, or the sparteine-BuLi complex, produces an optically active polymer 60). Its optical activity is attributed to the chirality of the helix structure maintained in solution. 

Similarly the term isotactic was applied by Price and Osgan (78) to the crystalline polymer obtained from optically active and racemic propylene oxide. The zigzag and Fischer representations of an isotactic poly(propylene oxide) are shown in 36 and 37 (Scheme 7). Their different appearance is due, as already explained in a similar case, to the odd number of chain bonds existing in each monomer unit. Formula 36 presents alternately substituents on both sides of the chain and is very similar to the actual structure observed in the crystal state (79). 

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. 

The sp3 stereocenter (i.e., C ) in XII is chirotopic, like the case of poly(propylene oxide), since the first couple of atoms of the two chain segments are considerably different. The isotactic structures are optically active while the syndiotactic structures are not optically active. 

The polymers of the optically active and racemic 4-methyl-1-hexene and the poly-(S)-5-methyl-l-heptene have isotactic structure (115) the same structure seems probable also in the case of the other polymers prepared till now from optically active or racemic a-olefins. 

The polymers have isotactic structure with helix conformations in the solid state (78) polymers of optically active and racemic (1-methyl-propyl)-vinyl-ether seem to have the same crystalline structure (Table 12). 

Such a model is in agreement with all the experimental findings till now ascertained in the field of optically active vinyl-polymers in fact it explains, in the case of polymers having asymmetric carbon atoms in a or j position with respect to the principal chain, the relationships between absolute structure of monomers and sign of the rotatory power of polymers, and the high rotatory power observed in isotactic polymers. The rapid and reversible variation of the optical rotation with temperature (105) is probably connected with the existence of a conformational equilibrium that is rapidly attained at each temperature. 

I, 3-diene polymerization. Monomer molecules are included in chiral channels in the matrix crystals, and the polymerization takes place in chiral environment. The y-ray irradiation polymerization of trans- 1,3-pentadiene included in 13 gives an optically active isotactic polymer with a trans-structure. The polymerization of (Z)-2-methyl-1,3-butadiene using 15 as a matrix leads to a polymer having an optical purity of the main-chain chiral centers of 36% [47]. 

Anionic Catalysis Several bulky methacrylates afford highly isotactic, optically active polymers having a single-handed helical structure by asymmetric polymerization. The effective polymerization mechanism is mainly anionic but free-radical catalysis can also lead to helix-sense-selective polymerization. The anionic initiator systems can also be applied for the polymerization of bulky acrylates and acrylamides. The one-handed helical polymethacrylates show an excellent chiral recognition ability when used as a chiral stationary phase for high-performance liquid chromatography (HPLC) [97,98]. 

Signals are split into two closely located singlets and comparison of the C-NMR spectrum of the racemic polymer, obtained from the racemic monomer, with that of the optically active polymer (prepared from (+)-( R, 5S)-6,8-dioxabicyclo[3,2,l]oc-tane indicate, that the lower field, smaller peak of each signal pair comes from the D-L (syndiotactic) dyad. The hi er field signal can thus be ascribed to the dyad structures of D-D and L-L consecutive units (isotactic dyads). 

Polymers with ring structures, interspaced with CH2 groups, can be obtained by polymerization of 1,5-dienes. 1,2-Insertion of the terminal double bond into the zireonium-carbon bond is followed by an intramolecular cyclization forming a ring. Waymouth describes the cyclopolymerization of 1,5-hexadiene to poly (methylene-1,3-cyclopentane) [67]. Of the four possible microstructures, the optically active trans-, isotactic structure (Figure 4) is predominant (68%) when using a chiral pure enantiomer of [En(IndH4)2Zr](BINAP)2 and MAO. 

An isotactic structure is one in which the optically active centers of the repeat units all have the same absolute stereochemistry (G. Natta, P. Pino, P. Corradini, F. Danusso, E. Mantica, G. Mazzanti, G. Moraglio, J. Am. Chem. Soc. 1955, 77, 1708) in a syndiotactic polymer, neighboring units have opposite stereochemistry. If an isotactic polyolefin is drawn in its extended conformation, it will have all its substituents pointing in the same direction. If an isotactic polyketone is drawn in its extended conformation, the substituents will alternately point up and down. 

The isotactic polyolefins prepared using a Ziegler— Natta catalyst form a helical conformation in the solid state (crystalline regions).11 38,42 This helical structure persists in solution, but because of fast conformational dynamics, only short segments of the helix exist among disordered conformations. When an isotactic polyolefin is prepared from an optically active monomer having a chiral side group, the polymer shows the characteristic chiroptical properties which can be ascribed to a helical conformation with an excess helicity.12,43-46 The chiroptical properties arise in this case predominantly from the helical conformation of the backbone. 

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. 

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