Optically Active Monomers

Scheme 4 Monomer + optically active low molecular weight compound - optically active derivative of monomer optically active derivative of polymer -> optically active low molecular weight compound + optically active polymer...

The crystal structures of polyesters based on o-hydroxy and 3-hydroxyacids have been extensively studied since the realization that a family of high melting polymers was achievable based on these structures (29,30). By suitable substitution of the monomers, optically active polymers can be prepared and the deliberate adjustment of the relative optical antipode content and distribution leads to steric copolymers which can present all the characteristics of isotactic, syndiotactic and stereoblock structures now familiar in the poly-a-olefin series. 

However, according to what happens for type A monomers, optically active polymers can also be obtained starting with chiral monomers according to the methods outlined in the previous sections. In the case of 2-substituted 1,3-dienes also, the 1,4-polymer does not contain asymmetric carbon atoms in the main chain, therefore in order to obtain optically active polymers the substituent must be dissymmetric [55, 56], the relation between macromolecule chirality and monomer structure being the same as for A monomers. In the case of 1,3-dienes bearing a chiral substituent in position 1, a second chiral center is formed per monomeric residue and this occurs under the possible stereochemical control of the asymmetric center already present in the monomer (Scheme 7) [57]. The complex... 

Monomer Optically active catalyst Polymer structure [01 Ref. 

Oligomer (Section 14 15) A molecule composed of too few monomer units for it to be classified as a polymer but more than in a dimer trimer tetramer etc Oligonucleotide (Section 28 6) A polynucleotide containing a relatively small number of bases Oligosaccharide (Section 25 1) A carbohydrate that gives three to ten monosacchandes on hydrolysis Optical activity (Section 7 4) Ability of a substance to rotate the plane of polanzed light To be optically active a sub stance must be chiral and one enantiomer must be present in excess of the other... 

Terpenes, specifically monoterpenes, are naturally occurring monomers that are usually obtained as by-products of the paper and citms industries. Monoterpenes that are typically employed in hydrocarbon resins are shown in Figure 2. Optically active tf-limonene is obtained from various natural oils, particularly citms oils (81). a and P-pinenes are obtained from sulfate turpentine produced in the kraft (sulfate) pulping process. Southeastern U.S. sulfate turpentine contains approximately 60—70 wt % a-pinene and 20—25 wt % P-pinene (see Terpenoids). Dipentene, which is a complex mixture of if,/-Hmonene, a- and P-pheUandrene, a- and y-terpinene, and terpinolene, is also obtained from the processing of sulfate Hquor (82). 

For the 1,2- and 3,4-addition, a chiral carbon (marked by an asterisk) is formed which has an R or 3 configuration, but there is no net optical activity, because equal amounts of the R and S configurations are formed. The R and S configurations along the polymer chains lead to diastereomeric isomers called isotactic, syndiotactic, and atactic. In isotactic polyisoprene all monomer units have the same configuration as illustrated for isotactic... 

The most important reaction with Lewis acids such as boron trifluoride etherate is polymerization (Scheme 30) (72MI50601). Other Lewis acids have been used SnCL, Bu 2A1C1, Bu sAl, Et2Zn, SO3, PFs, TiCU, AICI3, Pd(II) and Pt(II) salts. Trialkylaluminum, dialkylzinc and other alkyl metal initiators may partially hydrolyze to catalyze the polymerization by an anionic mechanism rather than the cationic one illustrated in Scheme 30. Cyclic dimers and trimers are often products of cationic polymerization reactions, and desulfurization of the monomer may occur. Polymerization of optically active thiiranes yields optically active polymers (75MI50600). 

Transition metal coupling polymerization has also been used to synthesize optically active polymers with stable main-chain chirality such as polymers 33, 34, 35, and 36 by using optically active monomers.29-31 These polymers are useful for chiral separation and asymmetric catalysis. For example, polymers 33 and 34 have been used as polymeric chiral catalysts for asymmetric catalysis. Due... 

Extensive studies of stereoselective polymerization of epoxides were carried out by Tsuruta et al.21 s. Copolymerization of a racemic mixture of propylene oxide with a diethylzinc-methanol catalyst yielded a crystalline polymer, which was resolved into optically active polymers216 217. Asymmetric selective polymerization of d-propylene oxide from a racemic mixture occurs with asymmetric catalysts such as diethyzinc- (+) bomeol218. This reaction is explained by the asymmetric adsorption of monomers onto the enantiomorphic catalyst site219. Furukawa220 compared the selectivities of asymmetric catalysts composed of diethylzinc amino acid combinations and attributed the selectivity to the bulkiness of the substituents in the amino acid. With propylene sulfide, excellent asymmetric selective polymerization was observed with a catalyst consisting of diethylzinc and a tertiary-butyl substituted a-glycol221,222. ... 

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. 

Some progress has also been achieved in the use of chiral polymer films at electrodes. Conductive polythiophenes containing optically active substituents in the 3-positions were prepared by electropolymerization of suitable monomers without apparent lc s of optical activity The polymer of exhibits distinct... 

FIGURE 3.11 (See color insert following page 336.) Optically active F -oligomer unit, built from eight optically inactive (W)-3.15 monomers. The calculated spectra of this octamer is in accordance with both the experimental VIS and CD spectra. (Reprinted from Foss, B.J. et al., Chem. Eur. J., 11, 4103, 2005b. With... 

Marvel, Dec, and Cooke [J. Am. Chem. Soc., 62 (3499), 1940] have used optical rotation measurements to study the kinetics of the polymerization of certain optically active vinyl esters. The change in rotation during the polymerization may be used to determine the reaction order and reaction rate constant. The specific rotation angle in dioxane solution is a linear combination of the contributions of the monomer and of the polymerized mer units. The optical rotation due to each mer unit in the polymer chain is independent of the chain length. The following values of the optical rotation were recorded as a function of time for the polymerization of d-s-butyl a-chloroacrylate... 

Enzymatic enantioselective oligomerization of a symmetrical hydroxy diester, dimethyl /Lhydroxyglutarate, produced a chiral oligomer (dimer or trimer) with 30-37% ee [24]. PPL catalyzed the enantioselective polymerization of e-substituted-e-hydroxy esters to produce optically active oligomers (DP < 6) [25]. The enantioselectivity increased with increasing bulkiness of the monomer substituent. Optically active polyesters with molecular weight of more than 1000 were obtained by the copolymerization of the racemic oxyacid esters with methyl 6-hydroxyhexanoate. 

Polyacetylenes are the most important class of synthetic polymers containing conjugated carbon-carbon double bonds. Some optically active monomers have been used with the following conclusions. Polymers of 1-alkynes having a branched side-chain assume in solution a helical conformation. A chiral side-chain induces a predominant screw sense in these helices. In particular, for alkyl branching, it has been shown that (S) monomers lead to a left-handed screw sense. 

This enantioselective mechanism is also in accordance with the elegant analysis and optical activity measurements by Pino et al.44,45 on the saturated propene oligomers obtained under suitable conditions with this kind of catalysts, proving that the re insertion of the monomer is favored in case of (R, R) chirality of coordination of the C2H4(1-Ind)2 ligand. 

Another significant cooperativity effect in preferential helical screw sense optically active copolymers is the majority rule phenomenon.18bl8q In this case, the screw sense of a helical main chain with unequal proportions of opposite chirality enantiopure chiral side groups is controlled by the enantiomeric excess only. Since this phenomenon was first reported from poly-a-olefins made of vinyl co-monomers bearing nonenantiopure chiral moieties by Green et al.8b and Pino et al.,16b this majority rule has been established in... 

Moller and co-workers co-polymerized dichlorodi- -pentylsilane with either dichloro-bis-(d )-2-methylbutylsilane or dichloro-(d )-2-methylbutyl- -pentylsilane in various ratios and found a linear dependence of optical activity on mole fraction of chiral co-monomer.313 On the other hand, studies by Fujiki on co-polymers 109 formed by the copolymerization of achiral (racemic) dichlorohexyl-2-methylbutylsilane and chiral dichlorohexyl-(d )-2-methylbutylsi-lane or dichlorohexyl-(l )-2-methylbutylsilane have shown that a preferential helical screw sense can be induced by even as little as 0.6 mol% of chiral co-monomer, and that at 5 mol%, the helicity, as gauged by the gabs value, is essentially the same as that of the chiral homopolymer, as shown in Figure 40. This indicates a positive non-... 

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