Editorial

Abstract

Asymmetric syntheses of optically active polymethacrylate, polyacrylate, polyacrylamide, and polyisocyanate with helical conformation and their chiral recognition abilities are described. 1-Phenyldibenzosuberyl methacrylate (PDBSMA) gave a purely onehanded-helical, optically active polymer ([α]₃₆₅ +1670 ～ +1780º) with almost perfectly isotactic structure by anionic polymerization using optically active initiators. Radical polymerizations of PDBSMA using chiral initiators, chain transfer agents, and additives also afforded optically active polymers with a prevailing onehanded helicity. Triphenylmethyl acrylate yielded an optically active, helical polymer ([α]₃₆₅ +102º) having a dyad isotacticity of 70% using an optically active anionic initiator. Although the polyacrylate demonstrated chiral recognition ability as a chiral stationary phase for HPLC, the ability was low mainly because of the low degree of one-handedness. N-(3-Chlorophenyl)-N-phenylacrylamide gave an optically active, helical polymer ([α]_365–343º) in the asymmetric anionic polymerization; the polymer had a dyad tacticity of 77%. Optically active polyisocyanates with a predominantly one-handed helical conformation were prepared in homo-and co-polymerization of optically active phenyl isocyanate derivative. These polyisocyanates showed the ability to discriminate enantiomers in solution.     

Stereospecific and asymmetric polymerization of 1-phenyldibenzosuberyl methacrylate with radical and anionic initiators

Stereospecific and asymmetric (helix-sense-selective) polymerization of 1-phenyldibenzosuberyl methacrylate (PDBSMA) was performed with radical and anionic initiators. A highly isotactic polymer having triad isotacticity greater than 97% was obtained by radical polymerization with (i-PrOCOO)₂ at 40°C. The radical polymerization of PDBSMA in (+)- and (-)-menthol gave (-)-and (+)-polymers, respectively, whose optical activity is ascribed to the prevailing one-handed helical conformation of a polymer chain. The radical copolymerization of PDBSMA with a small amount of an optically active monomer, (+)-phenyl-2-pyridyl-o-tolylmethyl methacrylate, afforded an optically active copolymer with the prevailing one-handed helical structure of PDBSMA sequences. Asymmetric anionic polymerization of PDBSMA was carried out with the complex of N, N′-diphenylethylenediamine monolithium amide and a chiral ligand, (+)-1-(2-pyrrolidinylmethyl)pyrrolidine in toluene at −78°C. The obtained polymer was highly isotactic and optically active due to nearly 100% one-handed helical structure.     

Asymmetric polymerization of methacrylates

The syntheses of optically active polymers having helical conformation from bulky methacrylates are reviewed focusing on selected topics. The monomers include triphenylmethyl methacrylate and its analogues. Asymmetric anionic polymerization of the monomers gives isotactic, optically active polymers having a helical structure with excess helicity. The isotactic content and the extent of helical‐sense excess depend on the monomer structure and the reaction conditions. In the case of methacrylates, completely isotactic and single‐handed helical polymers can be produced by asymmetric anionic polymerization (helix‐sense‐selective polymerization). Asymmetric radical polymerization is also possible for this class of monomer. Some of the helical polymers show chiral recognition ability toward a wide range of racemic compounds. Polymers having main‐chain configurational chirality are also discussed.     

Asymmetric polymerization of aromatic isocyanates with optically active anionic initiators

The asymmetric anionic polymerization of o-, m-, and p-methylphenyl isocyanates, p-methoxyphenyl isocyanate, p-chlorophenyl isocyanate, 2,6- and 3,4-dimethylphenyl isocyanates, and 1-naphthyl isocyanate was carried out using chiral anionic initiators such as the lithium salts of (?) -menthol, (?) -(2-methoxymethyl) pyrrolidine, and (+) -1-(2-pyrrolidinylmethyl) pyrrolidine. Although o-methylphenyl isocyanate gave an insoluble polymer and 2,6-dimethylphenyl isocyanate afforded no polymer, the other monomers gave soluble polymers, which showed optical activity due to the prevailing helicity of the polymer chain induced by chiral initiator residues attached to the α-end of the polymer chain. The molecular mechanics conformational calculation for a tetramer of m-methylphenyl isocyanate supported the helical conformation of the main chain. The optical rotation of the polymers depended significantly on temperature. © 1994 John Wiley & Sons, Inc.     

Helix-induced asymmetric polymerization

Asymmetric polymerization could be induced by an already formed optically active living prepolymer with one-handed screw sense helix conformation. The usually formed anionic active centre on the prepolymer could be changed to cationic, radical and even of Ziegler-Natta type. These living prepolymers with various kinds of active centre were all effective to induce a consequent asymmetric polymerization of a monomer which may be other than that in the prepolymer, to afford an optically active helical chain with the same screw sense as that of the prepolymer. Eight monomers have been used in the work. Optical rotation, circular dichroism and gelpermeation chromatography have been taken to prove the helix-induced asymmetric polymerization.     

(−)-Sparteine: The compound that most significantly influenced my research

A chiral diamine alkaloid, (−)-sparteine (Sp), has been found to be very effective as a ligand for Grignard reagents when used for the enantiomer-selective polymerization of racemic RS-1-phenylethyl methacrylate. The enantiomeric excess of the initially polymerized monomer is 93%, and at about a 60% conversion, nearly optically pure R-monomer is recovered. This enantiomer selectivity is today the highest in polymer chemistry. Triphenylmethyl methacrylate (TrMA) is a unique monomer that gives a highly isotactic polymer even during radical polymerization. When TrMA is polymerized with the Sp complex with n-butyllithium in toluene at −78 °C, an optically active, isotactic polymer [poly(triphenylmethyl methacrylate) (PTrMA)] with a one-handed helical conformation is obtained. The helical structure is maintained even at room temperature in solution. Analogous helical polymethacrylates that show various conformational changes have also been found. One-handed helical PTrMA exhibits high chiral recognition to a variety of racemates as a chiral stationary phase (CSP) for high-performance liquid chromatography. This finding has led to the development of very powerful CSPs based on polysaccharides, such as cellulose and amylose. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4480–4491, 2004     

(-)-双(薄荷基甲酰基)苯基氧化膦的合成及在自由基光聚合中的螺旋选择性

以苯基二氯化膦和薄荷基甲酸为原料, 合成了一种手性双酰基膦氧化物光引发剂(-)-双(薄荷基甲酰基)苯基氧化膦. 采用核磁共振谱、 质谱及元素分析确定了该化合物的结构, 并测定了其螺旋诱导能力. 结果表明, (-)-BMPPO在引发自由基光聚合大位阻的单体甲基丙烯酸-1-苯基二苯并环庚酯(PDBSMA)的反应中具有螺旋选择性. 所得聚合物具有高度全同结构并显示出光学活性, 说明该聚合物形成了稳定的单手性螺旋链构象. 降低单体与引发剂的投料比及提高聚合温度有利于增加反应的螺旋选择性. 在手性薄荷醇溶剂中, 由(2,4,6-三甲基苯甲酰基)二苯基氧化膦(TPO)引发单体PDBSMA的光聚合反应也具有螺旋选择性, 但螺旋选择性很小.     

Stereocontrol in radical polymerization

The stereospecific radical polymerization of vinyl esters, methacrylates, and alpha-substituted acrylates was studied. Fluoroalcohols, as a solvent, have remarkable effects on the stereoregularity of the radical polymerizations of vinyl acetate, vinyl pivalate, and vinyl benzoate, affording polymers rich in syndiotacticity, heterotacticity, and isotacticity, respectively. This method was successfully applied to the polymerization of methacrylates to give syndiotactic polymers. The steric repulsion between the entering monomer and the chain-end monomeric unit bound by the solvent through hydrogen bonding is important for the stereochemical control in these systems. Lewis acid catalysts, such as lanthanide trifluoromethanesulfonates and zinc salts, were also effective for the stereocontrol during the radical polymerization of methyl methacrylate, to reduce the syndiotacticity and alpha-(alkoxymethyl)acrylates to synthesize isotactic and syndiotactic polymers. Radical polymerization of the methacrylates bearing a bulky ester group, such as the triphenylmethyl methacrylate derivatives, gave highly isotactic polymers, as in the case of anionic polymerization. In addition, the control of one-handed helical conformation was attained in the radical polymerization of 1-phenyldibenzosuberyl methacrylate using chiral neomenthanethiol or cobalt(II) complexes as an additive.     

丙烯酸三苯甲酯及其衍生物的不对称聚合反应

含有大位阻基团的1,1-双取代α,β-不饱和酯,诸如甲基丙烯酸三苯甲酯(TrMA)、甲基丙烯酸邻甲苯基二苯基甲酯(DPIMA),在不对称聚合反应中,能够形成具有单手螺旋构象的旋光性聚合物。这类聚合物因具有独特的手性识别能力而一直为人们所瞩目。现有的研究结果表明,在这类单体的不对称聚合反应中,手性配体如鹰爪豆碱[(—)-Sp]决定着聚合产物     

Stereospecific radical polymerization of N-triphenylmethylmethacrylamides leading to highly isotactic helical polymers

N-Triphenylmethylmethacrylamide (TrMAM) and N-[di(4-butylphenyl)phenylmethyl]methacrylamide (DBuTrMAM) were polymerized using radical initiators. The polymer PTrMAM obtained from TrMAM was insoluble in the common solvents, but was dissolved in sulfuric acid to quantitatively give polymethacrylamide. The 1H NMR spectrum of PTrMAM in sulfuric acid-d2 indicated that the derived polymethacrylamide was nearly 100% isotactic. On the other hand, DBuTrMAM yielded the polymer PDBuTrMAM, which was soluble in THF and chloroform and whose NMR pattern in sulfuric acid-d2 was similar to that of PTrMAM. The polymerization of TrMAM and DBuTrMAM in an optically active solvent, (-)- or (+)-menthol, afforded the optically active polymers having a prevailing one-handed helical structure. The monomers were not solvolized in a methanol-chloroform mixture. TrMAM was not polymerized by anionic initiators.     

Helix‐sense‐selective copolymerization of triphenylmethyl methacrylate with chiral 2‐isopropenyl‐4‐phenyl‐2‐oxazoline

The asymmetric induction leading to a one‐handed helix was investigated in the anionic and radical copolymerization of triphenylmethyl methacrylate (TrMA) and (S)‐2‐isopropenyl‐4‐phenyl‐2‐oxazoline ((S)‐IPO), and highly isotactic copolymers with a reasonable optical activity were obtained. In the anionic copolymerization, the optical activity of the obtained copolymers depended on the polarity of solvents, and a highly optically active copolymer was produced in the copolymerization in toluene. The chiral oxazoline monomer functioned not only as a comonomer but also as a chiral ligand to endow the polymer with large negative optical rotation in the copolymerization with TrMA. The copolymers with small positive optical rotation were obtained in THF, indicating that IPO unit may work only as the chiral monomer that dictates the helical sense via copolymerization with TrMA. The isotacticity of the obtained copolymers depended on the contents of TrMA units in the copolymers, but was almost independent of the solvent for copolymerization. In the radical copolymerization, the obtained copolymers exhibited small optical activities. It seemed that the chiral monomer cannot induce one‐handed helical structure of TrMA sequences even if the sequences probably have a high isotacticity. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 441–447     

Optically active synthetic polymers as chiral stationary phases in HPLC

Synthetic, optically active polymers used as CSP are reviewed. The polymers are classified into three major categories, namely, addition polymers, condensation polymers, and cross-linked gels. The emphasis lies on polymethacrylates having helical conformation belonging to the first category. Helical polymethacrylates are synthesized using asymmetric anionic or radical polymerization techniques and show resolving ability towards a wide range of racemate.     

圆偏振光在手性聚合物合成与结构调控中的应用

手性是自然界中普遍存在的有趣现象之一,在生命体中手性大分子特有的不对称结构在维持生命过程、新陈代谢和进化等面均起着决定性作用.受此启发,合成具有新型结构的光学活性聚合物,研究其独特的物理化学性质和功能已成为当今高分子领域研究的热点.左旋和右旋的圆偏振光已被广泛应用于氨基酸衍生物的不对称光合成、光分解和去消旋化反应,以及诱导含有偶氮苯或三苯胺等特定功能基团的超分子组装体或无机纳米粒子形成稳定螺旋结构.本文详细地介绍了圆偏振光辐照在手性聚合物合成与螺旋结构调控中的应用,初步揭示了圆偏振光的作用机制以及优势,归纳总结了已取得的研究进展,并对圆偏振光在手性聚合物合成与结构调控中的应用及发展进行了简单的评述和展望.     

Asymmetric polymerization of triphenylmethyl methacrylate by C2-chiral catalysts

Asymmetric polymerization of triphenylmethyl methacrylate (TrMA) was investigated with optically active anionic catalysts in toluene at ?78°C. The catalysts were prepared in various combinations of organolithium compounds with C²-chiral tertiary diamines ( 1 – 6 ). Tetramethyl-ethylenediamine derivative bearing an axially dissymmetric biphenyl moiety ( 1 ) and the binaphthyl analogue ( 4 ) were found to provide efficient catalysts for the preparation of highly isotactic poly(TrMA)s of very large optical rotations whose signs depended on the configurations of the diamines. Especially, the catalysts consisting of 1 gave nearly pure one-handed helical polymers soluble in tetrahydrofuran (THF) in excellent yields, regardless of the kind of the lithium compounds used. The BuLi- 1 catalyst caused the metallation of toluene used as the solvent, and hence the resulting polymer had a benzyl moiety as an initiator fragment. The polymerization was also discussed with respect to the mole ratio of 1 to BuLi. Circular dichroism (CD) spectra of the (?)- and (+)-polymers, which were obtained with the BuLi-(R)- 1 and -(S)- 1 catalysts, respectively, were virtually complete mirror images of each other. High performance liquid chromatography (HPLC) using the THF–soluble, optically active poly(TrMA) as chiral adsorbent realized the complete resolution of racemic compounds.     

Optically active helical structure and magnetic interaction of poly(phenylacetylene)-based polyradicals

We synthesized optically active polyradicals possessing an excess of one-handed helical backbone by helix-sense-selective polymerization (HSSP) of achiral monomers which was promoted by rhodium complex catalyst in the presence of (R)- or (S)-1-phenylethylamine. The monomer, which had two hydroxy groups and a hydrogalvinoxyl unit, gave the corresponding polymer with an optically active helical conformation stabilized by intramolecular hydrogen bonding. The chemical oxidation of the polymer yielded the corresponding optically active helical polyradical with high spin concentration. The static magnetic susceptibility of the chiral polyradical was measured using a SQUID magnetometer. We have found that the stronger antiferromagnetic interaction was observed for the polyradical synthesized by HSSP in comparison with the polyradical via polymerization in the presence of racemic phenylethylamine.     

接枝于聚薄荷基丙烯醇的手性氨基醇的合成及其对Henry反应的不对称催化作用的研究

一种新型的旋光活性的氨基醇类聚合物被合成出来。以旋光活性聚薄荷基乙烯基酮为原料, 将聚薄荷基乙烯基酮用氢化铝锂还原后得到聚薄荷基丙烯醇(poly-MPO),poly-MPO与氢化钠反应后与环氧氯丙烷反应得到固载环氧丙烷的poly-MPO。将固载环氧丙烷的poly-MPO与各种类型的胺反应得到接枝于聚薄荷基丙烯醇的手性氨基醇(poly-MPO)-APO。将其应用在催化不对称Henry反应中得到了高产率、中等选择性的产物。     

Recent advances of optically active helical polymers as adsorbents and chiral stationary phases for chiral resolution

Chiral resolution is very important and still a big challenge due to different biological activity and same physicochemical property of one pair (R)- and (S)-isomer. There is no doubt that chiral selectors are essentially needed for chiral resolution, which can stereoselectively interact with a pair of isomers. To date, a large amount of optically active helical polymers as chiral selectors have been synthesized via two strategies. First, the target helical polymers are derived from natural polysaccharide such as cellulose and amylose. Second, they can be synthesized by polymerization of chiral monomers. Alternatively, an achiral polymer is prepared first followed by static or dynamic chiral induction. Furthermore, a part of them is harnessed as chiral stationary phases for chromatographic chiral separation and as chiral adsorbents for enantioselective adsorption/crystallization, resulting in good enantioseparation efficiency. In summary, the present review will focus on recent progress of the polymers with optical activity for chiral resolution, especially the literature published in the past 10 years. In addition, development prospects and future challenges of optically active helical polymers will be discussed in detail.     

Control of helix sense by composition of chiral-achiral copolymers of N-propargylbenzamides

N-Propargylbenzamides 1-7 were polymerized with (nbd)Rh(+)[eta(6)-C(6)H(5)B(-)(C(6)H(5))(3)] to afford polymers with moderate molecular weights (M(n) = 26,000-51,000) in good yields. The (1)H NMR spectra demonstrated that the polymers have fairly stereoregular structures (81-88 % cis). The optically active polymers, poly(1) and poly(2), were proven by their intense CD signals and large optical rotations to adopt a stable helical conformation with an excess of one-handed screw sense when heated in CHCl(3) or toluene. The sign of Cotton effect could be controlled by varying the content in the copolymers of either chiral bulky 1 and achiral nonbulky 3, or chiral nonbulky 2 and achiral bulky 7. The smaller the pendant group in the copolymerization of achiral monomers with 1, the more easily did the preferential helical sense change with the copolymer composition. However, the copolymers of chiral nonbulky 2 and achiral nonbulky 3 did not change the helical sense, irrespective of the composition. The free energy differences between the plus and minus helical states, as well as the excess free energy of the helix reversal, of those chiral-achiral random copolymers were estimated by applying a modified Ising model.     

L-丙烯酸薄荷酯与2,5-二(4'-己氧基苯基)苯乙烯无规共聚物的合成和旋光性质

通过自由基共聚合制备了一系列L-丙烯酸薄荷酯( L-MtA)与2,5-二(4'-己氧基苯基)苯乙烯(BHPSt)的共聚物L-MtA(x)-co-BHPSt(1-x).利用UV-Vis和1H-NMR光谱表征了共聚物的组成,根据Kelen-Tüd(o)s (K-T)方法计算得到的L-MtA和BHPSt的竞聚率分别为0.36...     

Emulsion copolymerization of substituted acetylenes for constructing optically active helical polymer nanoparticles. Synergistic effects and helicity inversion

Copolymerizations of substituted acetylenes have been intensively studied in solution polymerization for constructing chirally helical polymers, while emulsion copolymerizations of the kind of monomers have been only scarcely reported. In the present study, chiral substituted acetylene monomer containing cholic acid group underwent emulsion copolymerizations with an achiral acetylenic monomer in the presence of rhodium catalyst, providing optically active helical copolymer nanoparticles. Synergistic effects were found in the resulting helical copolymers, enabling one certain copolymer to show the maximum CD signal intensity. Moreover, the helicity of the helical copolymers in nanoparticle state was opposite to that in solution state. This is the first demonstration that synergistic effects and helicity inversion simultaneously occurred in helical copolymers prepared by emulsion copolymerization process. To deepen the understanding of the unique phenomena, corresponding solution copolymerization and emulsification process were also investigated. Different from “Sergeant and Soldiers rule” approach, the present study provides a new strategy for preparing chirally helical polymer particles by making full use of achiral monomers. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1679–1685