Optically active polymers substituted

Incorporation of the (.S )-2-mcthyloctoxy group afforded optically active polymers with preferential helical screw sense (see Section 3.11.6.1). The observed helicity was corroborated by force field calculations, which indicated similar helical conformations for both dialkoxy- and dialkyl-substituted polymers. Based on their similar conformational properties, it was suggested that the origin of the spectral red shift was electronic, due to a a-n mixing interaction, as for polymers 76 above, rather than conformational. 

In the polymer field, reactions of this type are subject to several limitations related to the structure and symmetry of the resultant polymers. In effect, the stereospecific polymerization of propylene is in itself an enantioface-diflferen-tiating reaction, but the polymer lacks chirality. As already seen in Sect. V-A there are few intrinsically chiral stractures (254) and even fewer that can be obtained from achiral monomers. With two exceptions, which will be dealt with at the end of this section, optically active polymers have been obtained only from 1- or 1,4-substituted butadienes, fiom unsaturated cyclic monomers, fiom substituted benzalacetone, or by copolymerization of mono- and disubstituted olefins. The corresponding polymer stmctures are shown as formulas 32 and 33, 53, 77-79 and 82-89. These processes are called asymmetric polymerizations (254, 257) the name enantiogenic polymerization has been recently proposed (301). 

The first enzymatic polymerizations of substituted lactones were performed by Kobayashi and coworkers using Pseudomonas fluorescens lipase or CALB as the biocatalyst [90-92]. A clear enantiopreference was observed for different lactone monomers, resulting in the formation of optically active polymers. More recently, a systematic study was performed by Al-Azemi et al. [93] and Peelers et al. [83] on the ROP of 4-alkyl-substituted CLs using Novozym 435. Peelers et al. studied the selectivity and the rates as a function of the substituent size with the aim of elucidating the mechanism and the rate-determining step in these polymerizations. Enantio-enriched polymers were obtained, but the selectivity decreased drastically with the increase in substituent size [83]. Remarkably for 4-propyl-e-caprolactone, the selectivity was for the (R)-enantiomer in a polymerization, whereas it was S)-selective in the hydrolysis reaction. Comparison of the selectivity in the hydrolysis reaction (Fig. 10b) with that of the polymerization reaction (Scheme 8a) revealed that the more bulky the alkyl substituent, the more important the deacylation step becomes as the rate-determining step. 

In the presence of catalysts such as ZnEt2/H20 or AlEt3 20, optically active 2-substituted -propio-lactones readily polymerize to give optically active stereoregidar polyesters exhibiting quite unique properties compared with the corresponding racemic polymers (Scheme 4). 

In 1961, Natta reported one of the first examples of enantioselective catalysis using a transition metal catalyst. In this reaction, an optically active polymer was formed from 1,3-pentadiene using a chiral organoaluminum/VClj catalyst [62]. The optical activity of this polymer results from the main-chain chiraHty of polymer, where the methyl-substituted stereogenic centers are predominantly of one absolute configuration. Since this initial study, significant advances in the enantioselective synthesis of main-chain chiral polymers have been reported using ionic and metal-based techniques. 

In 1960, Natta reported the first direct synthesis of an optically active polymer from an achiral monomer, where methyl sorbate was polymerized using (R)-2-pentyllithium [95]. Ozonolysis of the polymer (under conditions possibly allowing epimerization) produced (S)-methyl succinic acid in 5% ee, which provides evidence of asymmetric induction and absolute configuration of the polymer main chain. Since this initial report, a remarkable void in the Hterature exists concerning the synthesis of main-chain chiral polymers from achiral monomers using anionic initiators. Okamoto and Oishi have polymerized N-substituted maleimides with chiral anionic initiators (Scheme 14) [96,97]. The polymer is assumed to have predominantly a frans-diisotactic microstructuxe, which adopts a secondary helical structure. The absolute configuration of the main chain has... 

It was also observed that conjugated polymers that are also electrical conductors (see Chap. 10) exhibit optical activity that depends critically on their structural organization [78]. Thus, strong chiroptical properties can be obtained firom substituted polythiophene [79] (Chap. 10) with optically active side chains, especially when the monomers are coupled within the polymer in a regioregular head-to-tail fashion. Actually, optical activity of these materials is only found when the polymers are aggregated at low temperature, in poor solvent, or in solution cast films. This contrasts with other optically active polymers, like polypeptides, poly(l-alkynes) and polyisocyanates that show an optically active conformation of the main chain in the absence of supramolecular association. 

The N-substituted maleimide derivative (7 in Fig. 3) is the monomer that can produce an optically active polymer by asynunetric polymerization with chiral initiators such as ( )-sparteine-/i-BuLi [29]. The monomeric unit of the polymer is chiral if the polymerization proceeds in frans-addition by predominantly forming either an (R,R) or (S,S) center. The polymer 7 with a high optical activity exhibits a chiral recognition [30]. 

DCA and apoCA can serve as effective host components for asymmetric inclusion polymerization of prochiral monomers such as 1-substituted butadienes. We reported previously the preparation of optically active polymers with extremely high specific optical rotation of arbitrary sign from (E)- or (Z)-2-methyl-l,3-pentadiene by inclusion polymerization in the canals [7,12-14]. Moreover we have found that butadiene derivatives with polar groups such as cyano or carbomethoxy group can be polymerized to yield optically active polymers. The [ajp values of the resulting polymers were much higher than those of polymers obtained by other known polymerization method. 

Nucleic acids have a chiral center adjacent to the nucleic acid base, which is believed to provide the polymers with stereoregular structures. To confer this feature to the analogues, optically pure a-nucleic acid base substituted propanoic acids were prepared by the conventional optical resolution method. They were coupled with the functional polymers via amide or ester bonds to yield the optically active polymers [4-7]. Functionalization and coupling reactions of nucleobases with the polymers were well summarized in the reviews [2, 3]. 

Stereospecific polymerization of substituted conjugated dienes. Stereoregular polytactic polymers have been obtained from a number of substituted dienes, including one optically active 1,3-substituted propadiene, and various 1- or 1,4-substituted butadienes. (R)-penta-2,3-diene has been polymerized by means of 7T-allyl-Ni-iodide to an optically active polymer, to which an interesting stereoregular structure has been attributed (Scheme 26) (224). Some of the stereoregular polymers... 

Starting from these substituted dienes, asymmetric syntheses of optically active polymers are possible, since the chirality of the asymmetric carbon atoms of the main chain is determined by the local environment. Indeed these syntheses have been successfully carried out with many of them. The synthesis of optically active polysorbates by Natta et al. dates back to 1960 (228), and is the first example of an asymmetric synthesis of homopolymers. A conclusive proof of the asymmetric induction was obtained by oxidative degradation of the polymers to succinic acid derivatives (229). This synthesis, as well as those performed with trans 1,3-pentadiene (230), 1-phenyl-butadiene (231), and l-phenyl-4-methy1-butadiene (146) have been carried out using optically active initiators. A new kind of asymmetric polymerization was obtained by Farina et al (232) by y-irradiation of trans-1,3-pentadiene included in (-)perhydrotriphenylene (XIX). 

When compared, optically pure and racemic polymers reveal some significant differences in their properties such as crystallization, solubility or crystalline structure. The way of racemiza-tion of a polymer could be realized either by intercrystallite compensation, as in the case of polymethylthiirane (56) or by formation of a racemic lattice, as observed for monomers with bulky substituents such as t-butylthiirane (57). The properties of the racemic polymer are then very different of those of the optically pure one, for example, the melting points could differ of more than 50 C. A similar behaviour was recently observed in the case of substituted 6 propiolactones (58-60). Therefore the preparation of pure optically active polymers remains of inte-... 

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. 

Two types of monomers A and B, have been distinguished on the basis of the relationship between their structure and polymer chirality. To the former type belong monomers, such as vinyl and vinylidene monomers, which need to be chiral in order to give optically active polymers, while monomers of the latter type such as suitably substituted dienes suffice to be prochiral. 

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... 

Synthesis of optically active polymers by polymerization of prochiral substituted 1,3-butadienes in the presence of optically active catalytic systems... 

In contrast to homo-polymerization of vinyl monomers, where the main chain becomes pseudo-asymmetric and optically inactive, in the homo-polymerization of diene monomers and a, 3-substituted olefins asymmetry can be introduced into the main chain, thus leading to optically active polymers... 

Accordingly, the predicted structure for these two classes of polyamides PIP and DMPIP) results in a rigid rod in the cases of both racemic and optically active polymers, although the two rods may be different in the two cases. The optically active polymer may, in fact, possess a definite sense of spiralization due to the conformational purity of the substituted cyclohezane ring, while this is unlikely in the case of the racemic polymer. 

Circular dichroism (CD) is a very sensitive spectroscopic tool for probing chain conformations in optically active polymers. For example, with peptides, CD spectroscopy has been widely employed to estimate the proportion of the chain present as the alternative a-helix, P-sheet and random coil conformations. Following our recent discovery of both electrochemical - and chemicaP i routes to chiral PAn.HCSA and related ring-substituted emeraldine salts, we have employed CD spectroscopy extensively to (a) distinguish between "extended coil" and "compact coil" PAn conformations, (b) probe redox and pH switching in PAn, (c) characterize conformational changes in solvatochromism and thermochromism for PAn, and (d) distinguish unequivocally between the conformations/structures of electrochemically and chemically prepared PAn. Similar valuable information on poly-... 

Varying the substitution of the P-Iactone, by using racemic a-ethyl-a-methyl-P-propiolactone or a-propyl-a-methyl-P-propiolactone, led to the production of an optically active polymer which obeyed stereoselection in the presence of a diethylzinc/(R)-(-)-3,3-dimethyl-l,2-butanediol catalytic system, whereas the diethylzinc/methanol system led only to atactic polymers [56-58]. These findings indicated that the enantiomorphic sites of the zinc coordination catalyst were unable to recognize the chirality of the lactone monomer used for the polymerization. It should also be noted here that an antisteric type of steroselection in the polymerization of racemic a-propyl-a-methyl-P-propiolactone was reported when dimethylcadminium/(R)-(-)-3,3-dimethyl-l,2-butanediol was used as catalyst [56]. 

The introduction of a substituent at the lactone ring inevitably generates a chiral center. As the action of lipases relies on a two-step mechanism with an acylation step and a deacylation step, involving a covalent acyl-enzyme intermediate vide supra), both the acylation and deacylation step can occur enantioselectively when using a (chiral) substituted lactone [41]. It is well known that lipases such as CALB show a pronounced selectivity for (R)-secondary alcohols in the deacylation step [42, 43]. Although less elaborately studied, the acylation step can also occur enantioselectively [44-50], and therefore the enzymatic ROP of substituted lactones may result in optically active polymers, since selectivity for one of the enantiomers can be expected. 

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. ... 

It is possible that the helicity is a result of the chiral substitution itself and that the polymers with achiral substituents have, in fact, all-anti conformations. While this possibility cannot be directly ruled out, comparison of the spectroscopic data for the polymers with chiral substituents and achiral substituents, for example, 47 and 48, respectively, indicates similar main-chain dihedral angles, since the UV absorption maxima are so similar. Both polymers should therefore be latent helical, that is, contain segments of opposite screw sense separated by strong kinks (helix reversal points), with the difference being that in the case of 47 the overall numbers of P and M turns are equal, whereas for 48, one of the screw senses predominates, resulting in net helicity and optical activity. 

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