Polymerization helix-sense-selective

Helix-sense-selective polymerization is the reaction that yields polymers with a helical conformation with excess screw sense. Right- and left-handed helices are mirror images (atropisom-ers), and if one of the two is preferentially synthesized for a polymer chain, the polymer can be optically active (1-9]. 

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]. 

TABLE 11.2. Helix-Sense-Selective Polymerization of IYY1A with (-)-Sp-, (+)-DDB, and (+)-PMP-FILi Complexes in Toluene at -78°Ca... 

Helix-sense-selective polymerization of methyl, benzyl, and f-butyl methacrylates was attempted by using the complexes of chiral crown ethers, 91 and 92, and that of a chiral diamine 93 with n-BuLi however, these esters seem to be too small to form and maintain helical conformation [138,139], The complexes of BuLi with 58a and 59 failed in producing an optically active, helical polymer in the polymerization of methyl and benzyl methacrylates [104b]. 

Various optically active polymers have also been used as CSPs (Figure 3). We can produce the one-handed helical polymer 14 by helix-sense-selective polymerization... 

Vinyl polymers with a stable helical conformation are obtained from methacrylates with a bulky side group by isotactic specific anionic or radical polymerization.13,34 This type of polymer was first synthesized by asymmetric anionic polymerization (helix-sense-selective polymerization) of triphenylmethyl methacrylate (TrMA, 5) using a complex of n-BuLi with (—( sparteine (Sp, 6).13 Although, as discussed... 

Since the finding of the helix-sense-selective polymerization of TrMA, various other bulky monomers have been designed to find parallels to this behavior. The examples that appeared after our last review34 are discussed in this section. 

Helix-sense-selective polymerization has also been attempted for several bulky monomers including an acrylonitrile derivative105 and a-substituted acrylates.106107 Triphenylmethyl crotonate (TrC, 35) affords optically active, helical polymers by the polymerization using DDB—FILi and PMP—FILi complexes.108109 The polymers possess a nearly completely threo-diisotactic structure. Although the polymers indicate relatively small specific rotation ([(x]d +5.6° and +7.4° for the samples with DP =15 and... 

Figure 8. Helix-sense-selective polymerization of 1,2-diisocyanobezene derivatives.

Several procedures have been established in the past for the helix-sense-selective polymerization of isocyanides.169 For the introduction of a chiral bias, required for the discrimination of the otherwise enantiomeric helical senses, three distinct approaches can be applied. One is the use of a chiral monomer, which makes the left- and right-handed helices diastereotopic, and as a result one of the two will be energetically favorable (Figure 5). A second approach involves the use of a chiral catalyst or a chiral initiator, in which case during the initiation of the polymerization there will be a preference for the formation of one particular helical sense that will be retained upon propagation due to the helix inversion barrier. The former approach has been employed in numerous cases (vide infra) the accounts of the use of optically active Ni catalysts are limited.189-192 The... 

Homopolymers and block copolymers of poly(2,3-quinoxaline)s containing functional side chains (85) have been prepared employing the Pd(II)-catalyzed helix-sense-selective polymerization.233 235 Recently is was reported that side chains can be modified without the quenching of the living site of the polymer.236-237 The helical induction by the 1,1 -binaphth-2-ylpalladium(II) complexes has been investigated in detail, mainly using NMR techniques. 

Helical polymer stmctures can be classified into three categories. The first is a rigid helix having a stable existence at room temperature. In this case, one may expect to obtain an optically active polymer with an excess of a screw sense through the polymerization process using a chiral initiator or catalyst. This kind of polymerization has been called helix-sense-selective polymerization (lUPAG nomenclature asymmetric... 

Poly (PDBF) s synthesized by helix-sense-selective polymerization using FlLi-Sp and FlLi-DDB complexes did not show detectable CD or optical activity in solution. However, a thin film made by drop-casting the solution onto a quartz plate indicated intense CD absorptions. The spectra of the polymers synthesized with (+)- and (-)-DDB were almost mirror images. The CD spectra are not due to film anisotropy but to conformational chirality of polymer chains. Because the polymers showing clear CD bands in film did not show chiroptical properties in solution, molecular aggregation may amplify and stabilize the single-handed helical conformation induced... 

Helix-sense-selective polymerization has been achieved for several other optically active isocyanide monomers. In the polymerization of l-105 with NiCl2, helix sense was controlled by solvent and temperature. The spectra of poly-105 obtained in nonpolar solvents such as CCI4 and toluene were almost mirror images of those of poly-105 obtained in polar THF, indicating that the polymers have opposite helical senses, that is, the polymers are diastereomeric helices. The poly-105s with opposite helical senses showed cholesteric LC phases with opposite twist senses. 

Helix-sense-selective polymerization of achiral carbodii-mides 159 and 160 was performed using optically active titanium(IV) complex (161). " ° The anthracene-containing poly-159 slowly racemized in toluene at 80 °C. Below the racemization temperature, poly-159 exhibited reversible switching of chiroptical properties driven by temperature and solvent. ° The switching is due to a change in the orientation of pendent anthracene moieties, and the kinetically controlled helicity during the polymerization seems to remain unchanged. 

Triphenylmethyl methacrylate (TrMA) is a unique monomer, which affords a highly isotactic polymer (PTrMA) even by a radical process, and the polymer with more than a 95% triad isotacticity can be obtained by the anionic polymerization with butyllithium (BuLi) [10]. In 1979, we found that optically active PTrMA is formed during the anionic polymerization of TrMA with the complex of ( )-sparteine-n-BuLi at —78°C (Fig. 5) [11, 12]. This is the first example of helix-sense-selective polymerization preferentially producing a stable one-handed helical polymer through the polymerization process. The results clearly indicated that the existence of a stable helical polymer even in solution is possible on a vinyl polymer without chiral side groups. The helical structure is maintained due to the steric hindrance of the bulky triphenylmethyl groups. Therefore, when the ester groups are hydrolyzed with an acid, the optical activity of the polymer disappears. 

Fig. 5 Helix-sense-selective polymerization of triphenylmethyl methacrylate (TrMA) with (—)-sparteine-n-BuLi complex. Reprinted by permission from Chemical Society of Japan [5]...

Okamoto Y, Mohri H, Hatada K (1988) Highly helix-sense-selective polymerization of diphenyl-2-pyridylmethyl methacrylate. Chem Lett 17 1879-1882... 

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