Synthesis of Single-Handed Helical Polymethacrylates from Designed

Downloaded by UNIV MASSACHUSETTS AMHERST on September 1, 2012 | http://pubs.acs.org Publication Date: March 4, 2002 | doi: 10.1021/bk-2002-0812.ch002

Chapter 2

Synthesis of Single-Handed Helical Polymethacrylates from Designed Bulky Monomers by Anionic and Free Radical Catalyses 1

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Yoshio Okamoto , Kumiko Tsunematsu , Kiyoko Ueda , Yasuaki Hidaka , Naotaka Kinjo , and Tamaki Nakano 1

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DepartmentofApplied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), Takayama-cho 8916-5, Ikoma, Nara 630-0101, Japan

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This chapter describes asymmetric anionic and free-radical polymerization of bulky methacrylates leading to highly isotactic, optically active polymers having a helical conformation with excess screw sense (helix-sense-selective polymerization). The monomers used in this work include 1phenydibenzosuberyl methacrylate, racemic and optically active 10,11-O-isopropylidene-/trans-10,11-dihydroxy-5-phenyl-10,11-dihydro-5H-dibenzo[a,d|cyclo-heptene-5-yl methacrylate, (l-methylpiperidin-4-yl)diphenylmethyl methacrylate, and 10,10-dibuty l-9-phenylanthracen-9-yl methacrylate. These monomers gave highly isotactic, helical polymers not only by anionic mechanism but also by radical mechanism. Helix-sense selection was realized on the basis of chirality of monomer or initiator or additive. Enantiomer selection was also attained in the polymerization of the chiral monomer.

Helical synthetic polymers, especially optically active ones with single screw sense, have been drawing attention not only because the synthesis and the structure are interesting from an academic view but also because this class of polymers has a wide variety of potential application including chiral recognition

© 2002 American Chemical Society

In Synthetic Macromolecules with Higher Structural Order; Khan, I.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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and liquid crystal formation. " Helical polymers having excess screw sense are produced from monomers including acrylic compounds, isocyanides, isocyanates, chloral, and acetylene derivatives. We have been extensively studying helix-sense-selective polymerization of methacrylates and related monomers. Bulky methacrylates such as triphenylmethyl methacrylate (TrMA) afford highly isotactic, optically active polymers having a singlehanded helical conformation of the main chain by asymmetric (helix-senseselective) anionic polymerization The anionic polymerization is carried out using a complex of an organolithium and a chiral ligand such as (-)-sparteine, (+)-1 -(2-pyrrolidiny lmethyl Pyrrolidine (PMP), and (+)-2,3-dimethoxy-1,4bis(dimethylamino)butane (DDB). The optical activity of the polymers is based mostly on the helical structure; therefore, poly(methyl methacrylate)s (PMMAs) derived from the polymers show very low optical activity based only on the asymmetric centers in the vicinity of the chain terminals. The single-handed helical polymethacrylates show chiral recognition ability toward a wide range of racemic compounds and some of them have been successfully commercial lized. In recent years, we are focusing on the synthesis of helical, optically active polymethacrylates via free-radical mechanism as well as anionic mechanism. Although free-radical reaction is generally much more difficult to control compared with anionic reaction, it has a major advantage over anionic reaction that severe dry conditions are usually not necessary and a much wider variety of monomers can be polymerized. This means that single-handed helical polymethacrylate bearing functional groups may be synthesized under relatively mild reaction conditions via radical mechanism if the reaction and monomer structure are adequately designed. Through our efforts towards this goal, helixsense selection has been realized in polymerization of several monomers on the basis of monomer design or using chiral additive. In this chapter, we discuss the polymerization of four monomers. 12

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Helix-Sense-Selective Radical Polymerization of 1-Phenyldibenzosuberyl Methacrylate (PDBSMA) PDBSMA gives a highly isotactic, optically active polymer ([ot] +1778°) with completely single-handed helical structure by anionic polymerization using the complexes of N,N -diphenylethylene monolithium amide (DPEDA-Li) with Sp, DDB, and PMP in toluene at -78°C. This monomer affords a highly isotactic, helical polymer also by free-radical polymerization regardless of reaction condition. The free-radical polymerization product is considered to be an equimolar mixture of right- and left-handed helices. The high 365

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PDBSMA


13 stereospecificity in the PDBSMA polymerization by radical catalysis may arise from the rigid structure of the 1-phenyldibenzosuberyl moiety with the two phenyl groups bridged with an ethylene group leading to a rigid helical conformation of growing radical. 1 -(2-Pyridy 1)- and l-(3-pyridyl)dibenzosuberyl methacrylates having the same backbone as that of PDBSMA also afford highly isotactic polymers by radical polymerization. The highly isotactic structure of the radical polymerization product indicates that introduction of chirality into polymerization reaction may result in helix-sense selection by radical mechanism. Based on this idea, production of either helix in excess was first achieved using an optically active initiator, chaintransfer agent, and solvent (additive) leading to a ratio of enantiomeric helix of up to7:3. We recently found that helix-sense-selective radical polymerization of PDBSMA is possible with higher efficiency using an optically active cobalt (II) complex (1). Since the complex 1 is a d species (radical), it can interact with growing radical in the polymerization system. Selected conditions and results


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1 of polymerization in a CHCl -pyridine mixture are shown in Table I. The polymerization products were fractionated into tetrahydrofuran (THF)-insoluble (higher-molecular-weight) and -soluble (lower-molecular-weight) parts. The polymerization in the presence of 1 resulted in optically active polymers (THFsoluble part) though the presence of 1 reduced polymer yield and molecular weight of the products. No polymer was obtained when [Co] was 0.13 M or in the absence of pyridine. The optically active polymers showed circular dichroism (CD) absorptions with a spectral pattern similar to that of a completely single-handed poly(PDBSMA) prepared by the anionic polymerization, indicating that the optical activity of the radically obtained polymers is based on excess screw sense of the main chain helix. SEC analysis of a dextrorotatory polymer using Polarimetrie and UV detectors provided important information on the chiral structure of the polymer (Figure 1), that is, the higher-molecular-weight fractions showed higher optical activity. This indicates that helical sense excess is larger for higher-molecular-weight fractions. Chromatographic fractionation of the high-molecular-weight part (shaded area in Figure 1 A) gave 8wt% of the THF-soluble polymer with a DP of 43 whose Polarimetrie and UV chromatograms had a very similar shape and the same peak position, indicating that optical activity does not depend on molecular weight in this range (Figure IB). Specific rotation of this fraction was estimated to be fa] +1600° and in addition, CD spectral pattern and intensity of this 3

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Table I. Helix-Sence-Selective Radical Polymerization of PDBSMA with AIBN in the Presence of 1 in a Chloroform-Pyridine Mixture at 60°C a


THF-soluble part b

Yield (%) DP Mw/Mn

Run

Wo (M)

1 2 3 4

0 0.011 0.039 0.057

c

C

86 59 39 16

170

2.78

78

1.60

Yield (%) DP

d

3 2 3 4

d

Mw/Mn

19 19 19 19

(deg)

1.27 1.20 1.28 1.19

f

+270 +550 +160

Conditions: PDBSMA = 0.5 g, [PDBSMA] = 0.44-0.45 M , [AIBN] = 0.029-0.031 M, [pyridine] = 0.51-0.54 M , time = 24 hr. MeOH-insoluble part of the products, determined by GPC of PMMA derived from poly(PDBSMA). determined by GPC of poly(PDBSMA). estimated based on GPC curves obtained by UV and Polarimetrie detections. DP = 20 (Mw/Mn = 1.14) as determined by GPC of PMMA. SOURCE: Reproduced from Reference 12. Copyright 1999 American Chemical Society. 0 b

0

0

f

Ίο

4

ίο · ΙΟ ΙΟ Molecular Weight (vs. polystyrene) 3

5

3

4

ίο · 3

5

ΙΟ

3

Figure 1. GPC curves obtained by Polarimetrie (top) and UV (bottom) detectors of the THF-soluble polymer of run 3 in Table I (A) and the high-molecular-weight fraction separated from the THF-soluble polymer (B). SOURCE: Reproduced from Reference 12. Copyright 1999 American Chemical Society.



15 fraction were quite similar to those of the anionically obtained single-handed helical polymer. This means that the polymer of this fraction has almost singlehanded conformation. The results described so far indicate that the Co complex 1 induced a single-handed helical conformation in the radical polymerization of PDBSMA. This induction is probably based on the interaction between 1 and growing polymer radical. Right- and left-handed helical radicals may have different strength of interaction or binding constant with 1, that is, 1 may undergo stronger interaction with one helix than with the other, leading to different apparent propagation rates of the two radicals (Figure 2). This assumption will result in dependence of optical activity on molecular weight. The complex 1 was recently shown to induce configurational chirality of the main-chain in the polymerization of iV-phenyl and iV-cyclohexyl maleimides. 13

monomer addition

Figure 2. Proposed mechanism of helix-sense-selective radical polymerization of PDBSMA using an optically active Co(II) complex. Polymerization of a Chiral PDBSMA Analogue (±)- and (+)-10,l l-O-Isopropylidene-rmra-lO,! l-dihydroxy-5-phenyl10,1 l-dihydro-5//-dibenzo[tf,

Synthesis of Single-Handed Helical Polymethacrylates from Designed

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