Main-chain chiral polymer

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]. 

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 ... 

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... 

Polymers having configurational chirality in the side chain or the main chain can be prepared by asymmetric synthesis polymerization or enantiomer-selective polymerization. As for the main-chain chiral polymers, it is difficult to determine a definite chiral structure and rationally connect observed chiroptical properties with the structure for most polymers. Improved analytical techniques need to be evolved to address this issue. 

Recent developments in polymerisation by metallocene catalysts are found in general reviews w and more specialist monographs covering the role of agostic interactions, non-coordinating anions, applications to main-chain chiral polymers, and the analysis of polypropene by... 

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 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. 

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). 

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]. 

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. 

Utilization of the single hydrogen bond between pyridine and benzoic acids in SLCP s has been a source of inspiration for other groups in the development of main-chain supramolecular polymers based on diacids and dipyridines.53-56 Supramolecular rod-coil polymers have been developed by assembly of 4,4 -bipyridines and telechelic polypropylene oxide with benzoic acid end-groups, which show highly ordered liquid crystalline phases.57 The use of tartaric acid derivatives in combination with bipyridine units resulted in the formation of hydrogen-bonded, chiral main-chain LCP s, as has been shown by circular dichroism measurements, optical microscopy, and X-ray data.58,59... 

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... 

Waymouth and Coates employed the homogeneously catalyzed cyclopolymerization of 1,5-hexadiene giving poly (methylene-1,3-cyclopentane) as previously developed in his group in order to utilize the stereoselectivity of the monomer insertion for the construction of a polymer with main-chain chirality. The cyclopolymerization is a remarkable chain growth reaction during which a conventional... 

---