Main chain chirality

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... 

Incorporation of chiral units into polymers generates optically active polymers.27 Two types of optically active polymers could be obtained according to where the chiral units reside optically active polymers with chirality derived from chiral side chains and optically active polymers with chirality derived from tire chiral main chain. The circular dichroism (CD) measurement of 32, an optically active polymer with chiral side chains, showed that the chiral substituents have induced main-chain chirality. The induced main-chain chirality disappeared at higher temperature and appeared upon cooling. This type of chiral conjugated polymer is potentially useful in reversing optical recording28 ... 

Although only several reports appeared employing these reactions for polymerization, they could potentially be used for the synthesis of different types of polyamines and polyketones, including optically active ones with main-chain chirality. 

Although main-chain chirality refers to both polymers with stereogenic centers in the main chain (configurational main-chain chirality) and polymers with main chains consisting of helical stereogenic bonds induced by chiral side groups or end termini (conformational main-chain chirality), in this chapter we mainly focus on polysilanes exhibiting the latter type of chirality. 

Most optically active polysilanes owe their optical activity to induced main-chain chirality, as outlined above. However, backbone silicon atoms with two different side-chain substituents are chiral. Long-chain catenates, however, are effectively internally racemized by the random stereochemistry at silicon, and inherent main-chain chirality is not observed. For oligosilanes, however, inherent main-chain chirality has been demonstrated. A series of 2,3-disubstituted tetrasilanes, H3Si[Si(H)X]2SiH3 (where X = Ph, Cl, or Br), were obtained from octaphenylcyclote-trasilane and contain two chiral main-chain silicon atoms, 6.16 These give rise to four diastereoisomers the optically active S,S and R,R forms, the activity of which is equal but opposite, resulting in a racemic (and consequently optically inactive) mixture and the two meso-forms, S,R and R,S, which are optically inactive by internal compensation. It is reported that the diastereoisomers could be distinguished in NMR and GC/MS experiments. For the case of 2-phenyltetrasilane, a racemic mixture of (R)- and (A)-enantiomers was obtained. 

Preferential helical screw sense polysilanes with main-chain chirality are essentially of two types those in which PSS chirality is induced by an internal chiral field, for example, incorporation of enantiopure chiral (unichiral) side chains or end groups, and those which are inherently achiral, in which PSS chirality is induced by an external chiral field, such as a unichiral additive or solvent. 

Bis(oxazoline) ligands have also been used to produce polymers containing main chain chirality. Some examples include those by Wagner and co-workers in which /-pr-box 45 is used to mediate the copolymerization of tert-butylstyrene 192 with carbon monoxide to achieve a polymer of type 193 with stereoregularity up to 98%, ° ° Oishi and co-workers polymerization of A-substimted maleimides... 

Natta carried out the anionic polymerization of methyl sorbate, a 1,3-diene, with an optically active initiator and obtained an optically active homopolymer with main-chain chirality. The high molecular weight crystalline polymer produced with (P)-2-methylbutyllithium had a tritactic (di-iso-rra/w-tactic) structure. This was probably the first metal-catalyzed asymmetric polymerization 134). Polymerization of other dienes was attempted by using various asymmetric methods 135). 

In 1997 Pu reported a new type of main chain chiral polymer derived from BINOLs [24]. Polymer 16 catalyzed enantioselective ethylation using diethylzinc to give secondary alcohols in up to 94% ee. It is noteworthy that 16 is a derivative of chiral BINOL but the addition of Ti(IV) is unnecessary unlike other reported chiral monomeric diols. In 1998, Pu reported that polymer 17, which has a phenylene spacer between two BINOL moieties, results in better ees of up to 98% [24]. 

Figure 7.5. Possible modes of regiochemistry and main chain chirality in the stereoregular altemat ing alkene-CO copolymers.

The catalyst [Pd(Me-DUPHOS)(MeCN)2](BF4)2 was also effective in the alternating asymmetric copolymerization of aliphatic a-olefins with carbon monoxide [27,28]. The polymer synthesized in a CH3N02-CH30H mixture has both 1,4-ketone and spiroketal (10) units in the main chain. The propylene-CO copolymer consisting only of a 1,4-ketone structure shows [ ]D +22° (in (CF3)2CHOH), and the optical purity of the main chain chiral centers is over 90% as estimated by NMR analysis using a chiral Eu shift reagent. 

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]. 

Asymmetric induction to main-chain chiral centers can also be achieved by the radical polymerization of sorbates having chiral ester groups [80,81] and 1,3-butadiene-l-carboxylic acid complexed with optically active amines [82,83]. 

Among the different approaches to immobilization, main chain chiral polymer catalysts are different from the traditional polymer catalysts prepared by anchoring monomeric chiral catalysts to an achiral polymer backbone (Pu, 1998). The three classes of synthetic main chain chiral polymers include ... 

L. Pu, Recent developments in asymmetric catalysis using synthetic polymers with main chain chirality, Tetrahedron Asymm. 1998, 9, 1457-1477. 

However, the particular synthetic requirements in the preparation of conjugated polymers have thus far severely limited the number of similarly hierarchically structured examples. Pu et al. reported different types of conjugated polymers with fixed main-chain chirality containing binaphthyl units in their backbone which exhibited, for example, nonlinear optical activity or were used as enantioselective fluorescent sensors [42—46]. Some chirally substituted poly(thiophene)s were observed to form helical superstructures in solution [47-51], Okamoto and coworkers reported excess helicity in nonchiral, functional poly(phenyl acetylenejs upon supramolecular interactions with chiral additives, and they were able to induce a switch between unordered forms as well as helical forms with opposite helical senses [37, 52, 53]. 

One may have a question of whether an optically active helical polymer obtained from an enantiopure monomer adopts a purely P- (or M-) screw sense helical main chain in solution at a given temperature, or is composed of an ensemble of pseudo-diastereomeric mixed helical motifs containing P- and M-screw senses. Fluorescence (FL) studies combined with circular dichroism (CD), UV, and NMR spectra of the main chain constitute a powerful probe in identifying the main chain chirality (screw sense, uniformity, and rigidity) and optical purity of helical polymers, since the photoexcited energy above... 

In contrast to polyolefins such as polypropene, polyketones possess true stereo-genic centers along the polymer backbone. Therefore, poly ketones present a unique opportunity to use simple monomers in combination with chiral, enantio-merically pure palladium catalysts to prepare highly isotactic, optically active polymers (or oligomeric compounds) with main-chain chirality. 

The polymers with trans-fused five-membered rings linked with a diisotactic head-to-tail sequence have chirality, although the polymers composed of the cis-fused ring are achiral. Scheme 10 summarizes the structures of the stereoisomeric polymers. The optically active zirconocene complex with a C2 symmetric structure catalyzes the enantioselective cyclopolymerization of 1,5-hexadiene (Eq. 20) [98, 99]. Although the polymer contains not only trans-fused ring but also cis-fused ring units (ca. 68 32), it shows optical rotation due to the main chain chirality. 

Tian M, Zhang B, Meng F et al (2006) Main-chain chiral smectic liquid-crystalline ionomers containing sulfonic acid groups. J Appl Polym Sci 99 1254-1263... 

Keywords Macromolecular stereochemistry. Main-chain chiral polymer, Atropisomeric polymer, Kinetic resolution. Anionic polymerization. Cationic polymerization. Insertion polymerization... 

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... 

---