Synthesis of Polyisocyanide Derived from Phenylalanineand Its Temperature-Dependent Helical Conformation

YUKI YAMADA, TADASHI KAWAI, JIRO ABE, TOMOKAZU IYODA

Department of Applied Chemistry, Graduate School of Engineering, Tokyo Metropolitan University, 1-1, Minami-Osawa,Hachioji, Tokyo, 192-0397, Japan

Received 22 June 2001; accepted 9 November 2001

ABSTRACT: Screw-sense-selective polymerization of the chiral isocyanide monomersderived from phenylalanine with NiCl2 as a catalyst in methanol to yield helical-conjugated polyisocyanide was investigated with respect to the thermal stability of itshelical conformation. Poly(1-tert-butoxycarbonyl-2-phenylethyl isocyanide) (poly1c)took a stable helical conformer independent of the polymerization temperature. Inpoly(1-ethoxycarbonyl-2-phenylethyl isocyanide) (poly2c), which had slightly smallerside groups, the helical conformation was thermally destabilized. The specific rotationand circular dichroism of poly2c prepared at temperatures greater than 40 °C wereconsiderably depressed in comparison with the values for poly2c prepared at or belowroom temperature. Additionally, poly2c prepared at low temperatures exhibited revers-ible temperature-dependent specific rotation and circular dichroism, whereas poly1cshowed few changes. It is suggested that polyisocyanide derived from phenylalaninetakes various helical conformers (i.e., from tightly to loosely coiled helices), the thermalstability of which depends on the size of the side group. © 2001 John Wiley & Sons, Inc. JPolym Sci Part A: Polym Chem 40: 399–408, 2002Keywords: polyisocyanide; amino acid; helical conjugation; NiCl2; helical conforma-tion; stimuli-sensitive polymers; nonlinear polymers; gel permeation chromatography

INTRODUCTION

Polyisocyanide has attracted much attention forthe helical conformation of its conjugated mainchain since Millich’s first preparation1–3 and thenickel-catalyzed polymerization by Drenth andNolte.4–11 These studies have driven syntheticchemists to improve the polymerization reactionsand to achieve living polymerization with a pal-ladium catalyst,12–14 a �-allyl nickel catalyst,15–18

and a Pt–Pd �-ethynediyl catalyst.19–21 Continu-ous efforts have been spent on both monomer andcatalyst designs to stabilize the helical conforma-

tion, which is believed to show larger specific ro-tations and stronger circular dichroism (CD).22–27

A helical conformation is described by severalstructural parameters. Unfortunately, a directcorrelation between the parameters and the spe-cific rotations or CD has not been well understoodso far, even in several theoretical works.28–30 Thiskind of structural analysis for a realistic pictureof the helical conformation will be more importantbecause such helical polymers are being appliedto molecular recognition or separation,31 macro-molecular building blocks in nanostructured ma-terials,32 the memory effect of helicity,33 the con-trol of helical structures by external stimuli,34–36

liquid crystals,37–39 the design of functional sidechains of helices,40–43 and so on. NMR, light scat-tering, and other physicochemical techniques44–47

have been used as powerful tools to investigate

Correspondence to: T. Iyoda (E-mail: iyoda-tomokazu@c.metro-u.ac.jp)Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 40, 399–408 (2002)© 2001 John Wiley & Sons, Inc.DOI 10.1002/pola.10127

399

stable helical conformations. At the same time,polymer designs for adequately destabilizing he-lical conformations into metastable conforma-tions (i.e., for taking various possible conformers)will also be required as good targets by preciselytuning the size of the side group of the monomer.This was our motivation for this study.

We synthesized helical-conjugated polyisocya-nides derived from phenylalanine-based chiralisocyanides and realized various types of helicalconformations by tuning the size of the sidegroup. This helical polymer should take severalmetastable conformations as local minima in acomplicated potential surface between two intrin-sic conformers, from the tightly coiled helix to theextended planar form. Therefore, a temperatureeffect on the specific rotations and CD spectra wasexamined to provide a more realistic picture ofthe main-chain conformation. As a result, severalpossible helical conformations frozen at the poly-merization temperature and a partly reversibleconformational change were observed in poly(1-ethoxycarbonyl-2-phenylethyl isocyanide) (poly2c).

EXPERIMENTAL

Chemicals

Optically active isocyanides were derived fromthe corresponding amino acids according to thesynthetic route shown in Scheme 1.

Phenylalanine and alanine were obtained fromWako. NiCl2 � 6H2O, used as the polymerizationcatalyst, was purchased from Kojundo ChemicalLaboratory. Unless otherwise indicated, otherchemicals were obtained from commercial suppli-ers and were used without further purification.

(S)-1-tert-Butoxycarbonyl-2-phenylethylIsocyanide (1c)

The tert-butyl ester of L-phenylalanine (1a) wassynthesized according to the literature.48

Yield: 8.16 g (85%). [�]58920 : �51.9 [c 2.0, ethanol

(EtOH)]. IR (KBr, cm�1, �): 1735 (CAO). 1H NMR(270 MHz, ppm, CDCl3, �): 1.43 (s, 9H, CH3), 2.84(dd, 1H, 3JAB � 7.75 Hz, 2JBB� � 13.5 Hz,CHBHB�), 3.03 (dd, 1H, 3JAB� � 5.77 Hz, 2JBB�� 13.5 Hz, CHBHB�), 3.61 (dd, 1H, 3JAB� 7.75 Hz,

3JAB� � 5.77 Hz, CHA), 6.63 (br, 2H, NH2),7.26 (m, 5H, aromatic). ELEM. ANAL. Calcd. forC13H19NO2: C, 70.56%; H, 8.65%; N, 6.33%.Found: C, 71.32%; H, 8.90%; N, 6.11%.

Because conventional N-formylation with ace-tic formic anhydride gave a very low yield, Meine-hofer’s method49 with formic anhydride was ap-plied to the synthesis of 1b. A 45-mL CHCl3 so-lution containing a 3.4 mL (90 mmol) solution offormic acid was added dropwise to a CHCl3 solu-tion (57 mL) containing dicyclohexylcarbodiimide(9.3 g, 45 mmol) at 0 °C with stirring. The mixturewas further stirred for 5 min to form the anhy-dride and then was added over a period of 30 minto 57 mL of an ice-cooled pyridine solution of thetert-butyl ester 1a (4.8 g, 22 mmol) with stirring.The stirring was continued for 4 h in an ice bath.Evaporation of the solvent was followed by theaddition of ether. The resulting 1,3-dicyclohex-ylurea precipitate was removed by filtration andwashed with ether. The combined filtrate wasconcentrated into a crude viscous liquid, whichwas purified by column chromatography on silicagel with CHCl3 followed by CHCl3/CH3OH (96/4)as the eluent. The main fraction yielded 3.1 g of1b as colorless oil.

Scheme 1. Synthetic route of chiral isocyanide mono-mers derived from alanine and phenylalanine: (a)esterification with EtOH or tert-butyl acetate, (b) N-formylation with acetic formic anhydride or formic an-hydride, and (c) dehydration with bis(trichloro-methyl)carbonate.

400 YAMADA ET AL.

Yield: 3.1 g (56%). [�]58920 : �18.0 (c 1.9, EtOH).

IR (neat, cm�1, �): 3300 (NH), 1735 (CAO of es-ter), 1669 (CAO of formamide). 1H NMR (270MHz, ppm, CDCl3, �): 1.42 (s, 9H, CH3), 3.13 (m,2H, CH2), 3.81 (m, 1H, CH), 6.07 (br, 1H, NH),7.28 (m, 5H, aromatic), 8.18 (s, 1H, CHO). ELEM.ANAL. Calcd. for C14H19NO3: C, 67.45%; H, 7.68%;N, 5.62%. Found: C, 67.30%; H, 7.46%; N, 5.64%.

The isocyanide 1c was synthesized by dehydra-tion of the N-formamide 1b according to themethod of Nolte and Drenth.22 At �10 °C, bis(tri-chloromethyl)carbonate (Wako; 0.60 mg, 2 mmol)in 10 mL of dry CH2Cl2 was introduced into 15mL of dry CH2Cl2 containing 1.5 g (6 mmol) of 1bover a period of approximately 2 h with stirring.Just after the reaction was completed for approx-imately 3 h, 50 mL of water was added to themixture. The organic layer was quickly washedwith 150 mL of a 7.5% NaHCO3 aqueous solutionand dried over MgSO4. The crude product waspurified by column chromatography on silica gelwith CH2Cl2 as the eluent.

Yield: 64%. [�]58920 : �9.1 [c 0.66, methanol

(MeOH)]. IR (neat, cm�1, �): 2149 (N'C), 1748(CAO). 1H NMR (270 MHz, ppm, CDCl3, �): 1.45(s, 9H, CH3), 3.12 (dd, 1H, 3JAB � 8.08 Hz, 2JBB�� 13.9 Hz, CHBHB�), 3.22 (dd, 1H, 3JAB� � 5.28Hz, 2JBB� � 13.9 Hz, CHBHB�), 4.33 (dd, 1H, 3JAB� 8.08 Hz, 3JAB� � 5.28 Hz, CHA), 7.30 (m, 5H,aromatic). 13C NMR (68 MHz, ppm, CDCl3, �):27.8 [C(CH3)3], 39.0 (CH2), 58.7 (CH), 84.0[C(CH3)3], 127.6 (C-4 of Ph), 128.6 (C-2 of Ph),129.3 (C-3 of Ph), 134.5 (C-1 of Ph), 159.5(N'C),164.8 (CAO).

1-Ethoxycarbonyl-2-phenylethyl Isocyanide (2c)

L-Phenylalanine was esterified with EtOH anddry HCl according to a standard procedure.50

Yield: 95%. [�]58920 :�7.8 (c 2.0, MeOH). IR (neat,

cm�1, �): 1740 (CAO). 1H NMR (270 MHz, ppm,CDCl3, �): 1.15 (t, 3H, 3JHH � 7.09 Hz, CH3), 3.40(m, 2H, CH2OPh), 4.12 (q, 2H, 3JHH � 7.09 Hz,CH2OCH3), 4.94 (m, 1H, CH), 6.16 (br, 2H, NH2),7.24 (m, 5H, aromatic).

The N-formylation of 2a with acetic formic an-hydride51 afforded 2b as a colorless liquid; thiswas followed by distillation under reduced pres-sure (156 °C at 2 mmHg).

Yield: 64%. [�]58920 : �18.6 (c 10.5, MeOH). IR

(neat, cm�1, �): 3296 (NH), 1740 (CAO of ester),1669 (CAO of formamide). 1H NMR (270 MHz,ppm, CDCl3, �): 1.25 (t, 3H, 3JHH� 7.09 Hz, CH3),3.14 (m, 2H, CH2OPh), 4.18 (q, 2H, 3JHH � 7.09

Hz, CH2OCH3), 4.93 (m, 1H, CH), 6.23 (br, 1H,NH), 7.20 (m, 5H, aromatic), 8.14 (s, 1H, CHO).ELEM. ANAL. Calcd. for C12H15NO3: C, 65.14%; H,6.83%; N, 6.33%. Found: C, 64.87%; H, 6.89%; N,6.52%.

The isocyanide 2c was synthesized from 2b ina manner similar to that used for 1c, and thecrude product was purified by column chromatog-raphy on silica gel with CH2Cl2 as the eluent togive (S)-2c.

Yield: 73%. [�]58920 : �25.4 (c 0.83, MeOH). IR

(neat, cm�1): 2148 (N'C), 1765 (CAO). 1H NMR(270 MHz, ppm, CDCl3, �): 1.26 (t, 3H, 3JHH� 7.09 Hz, CH3), 3.02 (dd, 1H, 3JAB � 7.59 Hz,2JBB� � 13.7 Hz, CHBHB�OPh), 3.14 (dd, 1H,3JAB� � 4.95 Hz, 2JBB� � 13.7 Hz, CHBHB�OPh),4.22 (q, 2H, 3JHH � 7.09 Hz, CH2OCH3), 4.68(dd,1H, 3JAB � 7.59 Hz, 3JAB� � 4.95 Hz, CHA), 7.18(m, 5H, aromatic). 13C NMR (68 MHz, ppm,CDCl3, �): 24.9 (CH2OCH3), 38.8 (CH2OPh), 57.9(CH2OCH3), 62.6 (CH), 127.5 (C-4 of Ph), 128.6(C-2 of Ph), 129.0 (C-3 of Ph), 134.1 (C-1 of Ph),160.5 (N'C), 165.8 (CAO).

(R)-2c was synthesized from D-phenylalaninein the same manner as (S)-2c.

Yield: 83%. [�]58920 : �25.0 (c 1.0, MeOH).

(S)-1-Ethoxycarbonylethyl Isocyanide (3c)

As reported in the literature,22,50 the isocyanide3c was synthesized from L-alanine and purified bycolumn chromatography on silica gel withCH2Cl2.

Yield: 25%. IR (neat, cm�1, �): 2146 (N'C),1752 (CAO). 1H NMR (270 MHz, ppm, CDCl3, �):1.33 (t, 3H, 3JHH � 7.26 Hz, CH2OCH3), 1.66 (d,3H, 3JHH � 6.93 Hz, CHOCH3), 4.28 (q, 2H, 3JHH� 7.26 Hz, CH2), 4.37 (q, 1H, 3JHH � 6.93 Hz,CH).

Polymerization

MeOH as the polymerization solvent was purifiedby distillation from aluminum trimethoxide, re-fluxing for 6 h with freshly dehydrated CuSO4 (2g/L) under a dry N2 stream, and distillation beforeuse. Just before the polymerization, the isocya-nide monomer was synthesized and purified bycolumn chromatography. An aliquot of MeOHcontaining an appropriate amount of NiCl2 � 6H2O(0.5 mol % for the monomer) was loaded into areaction vessel, and the solvent was flushed. Thepolymerization started with the addition of theMeOH solution containing the isocyanide mono-

POLYISOCYANIDE DERIVED FROM PHENYLALANINE 401

mer and stirring at specified temperatures. After1.5 days, the reaction solution became viscousand dark brown. After evaporation of the solvent,the concentrated reaction mixture was addeddropwise to an excess amount of vigorouslystirred MeOH/water (1/4 v/v). The brown precip-itate was collected by filtration and dried in vacuoat ambient temperature. The polymer was iden-tified and characterized by Fourier transform in-frared, 1H NMR, gel permeation chromatography(GPC), ultraviolet–visible (UV–vis) spectra, spe-cific optical rotation, and CD spectra.

Measurements

Optical rotation was measured on a Union PM-101 polarimeter. Elemental analyses were per-formed on a PerkinElmer 2400 CHN elementalanalyzer. CD spectra were measured with a JascoJ-720 spectrometer. UV–vis spectra were takenon a Shimadzu UV-3100S. 1,2-Dichloropropane(Wako) was used as the solvent for temperature-controlled CD and UV spectral measurements.The solution in a quartz cell was sealed in vacuo.The volume change in the 1,2-dichloropropanedue to thermal expansion was not corrected inthis study because it was within 5.3% in a rangeof 5–70 °C.52 13C and 1H NMR spectra were takenon a JEOL JNM-EX270 spectrometer with tetra-methylsilane as the internal standard in CDCl3 atroom temperature. IR spectra were recorded on aBio-Rad FTS-3000MX spectrometer. The molecu-lar weight of the polymer was determined by GPCmeasurements on a Jasco instrument equippedwith an Elmer UV detector and GPC columnsserially connecting a Shodex KF-802 and a Jaigel3H-AF with tetrahydrofuran as the eluent at 30°C. The molecular weight of the polymer was de-termined with a calibration curve obtained withstandard polystyrenes (Tosoh).

RESULTS AND DISCUSSION

Screw-Sense-Selective Polymerization of 2c

Figure 1 shows typical time courses of the mono-mer conversion and molecular weight of isolatedpoly2c in the polymerization of the isocyanidemonomer (S)-2c in MeOH at room temperature.The polymerization within 1.5 days reached amonomer conversion of 83% and gave a brownpolymer with an isolated yield of about 72% byreprecipitation.

Figure 2(a) shows CD and UV spectra of themonomers derived from both enantiomers of 2c inCH3CN. Both monomers showed oppositelysigned CD as mirror images in the region of lessthan 250 nm, and their optical purities were iden-tical. The corresponding polymers also exhibitedalmost opposite Cotton effects [Fig. 2(b)], as-signed to the helical conformations of the mainchain conjugated with the imino moiety, inthe longer wavelength region of up to 380 nm.As Nolte and coworkers demonstrated previ-ously,6,7,22,53–55 an amino acid is one of the beststarting materials derivatized to the isocyanidemonomers for giving one-handed helical polyiso-cyanides with respect to the following:

1. A wide variety of optically pure amino ac-ids as starting materials are commerciallyavailable.

2. The corresponding isocyanide monomercan be easily prepared in a high yield.

3. The steric factor of the monomer, whichwill delicately influence the helical confor-mation, can be easily tuned by the appro-priate choice of the starting amino acid andderivatization of the carboxylic acid moi-ety, here as the ester with the alcohol dif-ferent in size.

Temperature Effect on Polymerization

The temperature effect on polymerization was ex-amined (Table I). For polymerization tempera-tures of 25–68 °C, both monomers, (S)-1c and

Figure 1. Time courses of the monomer conversion(circles) and the molecular weight of the polymerizationof the isocyanide monomer (S)-2c in MeOH at roomtemperature ([2c] � 1.0 mol/L; [2c]/[Ni(II)] � 77). Boththe conversion and molecular weight were determinedby GPC.

402 YAMADA ET AL.

(S)-2c, were efficiently polymerized to approxi-mately 80–90% conversions to give poly(1-tert-butoxycarbonyl-2-phenylethyl isocyanide (poly1c)and poly2c. The isolated polymers had relativelynarrow polydispersity (PD). At 5 °C, the polymer-ization achieved a slightly lower conversion, butthe resulting polymers still had higher molecularweights.

The specific optical rotations of poly1c andpoly2c prepared by polymerizations at varioustemperatures were quite interesting. The isolatedpolymer precipitate was dissolved in CHCl3 atroom temperature, and the optical rotation wasmeasured at 20 °C. All the poly1c solutionsshowed large positive specific rotations indepen-dent of the polymerization temperature. Thelarge specific rotation of poly1c, compared with

that of the monomer 1c, may be attributed to thechirality that resulted from the helical mainchain. These specific rotations, independent of thepolymerization temperature, suggest that poly1cwould take a thermally stable helical conformerin the polymerization. However, a strong depen-dence of the specific rotation on the polymeriza-tion temperature was found in poly2c. The poly-mers prepared at 5 and 25 °C, labeled poly2c(5)and poly2c(25), showed large negative specific ro-tations, whereas the specific rotation of the poly-mer decreased with increasing polymerizationtemperature. The value of poly2c(68) reachedonly a sixth but was still 4 times larger than thatof the monomer 2c. These behaviors demon-strated that poly2c prepared at different temper-atures should take various helical conformers(e.g., tightly or loosely coiled helices) in the poly-merization at the specified temperatures andcould be frozen at least in this isolation procedureand the optical rotation measurement.

A consistent temperature effect was observedin the CD spectra of both polymers. The CHCl3solution of poly1c showed a large induced CD inthe 250–350 nm region corresponding to the n–�*transition of the imino moiety conjugated withthe main chain.56 The magnitude of the CD wasalso independent of the polymerization tempera-ture. Poly2c(5) showed oppositely signed negativeCD with absolute values similar to those ofpoly1c. However, poly2c(25) showed only half ofthe magnitude, and the polymers prepared attemperatures greater than 40 °C lost any dichro-ism, as Figure 3 shows.

Again, a strong dependence of the polymeriza-tion temperature on the helical conformationswas observed in poly2c. Only a slight difference insize between tert-butyl and ethyl esters broughtsuch a contrastive result of the thermal stabilityof the helical conformation. Additionally, poly[(S)-1-ethoxycarbonylethyl isocyanide] (poly3c) bear-ing still smaller substituents showed less than98° of specific rotation and little CD, even whenthe polymerization was performed at 5 °C.57,58

Thermal Stability of the Helical Conformation inPoly2c

The next interesting aspect of poly2c is the ther-mal stability of the helical conformation deter-mined beforehand in the polymerizations at lowtemperatures. Figure 4 shows the CD intensitychanges averaged in the 280–300 nm region ofthe 1,2-dichloropropane solution of poly2c(5)

Figure 2. (a) UV–vis and CD spectra of the mono-mers (R)-2c (dashed lines) and (S)-2c (solid lines) atroom temperature in CH3CN and (b) UV–vis and CDspectra of the polymers poly(R)-2c (dashed lines) andpoly(S)-2c (solid lines) at room temperature in CHCl3.

POLYISOCYANIDE DERIVED FROM PHENYLALANINE 403

when the solution temperature was elevated to40, 55, or 70 °C, held there for 1.5 days, and thenlowered to room temperature. The CD intensitydecreased by 7% at 40 °C, 14% at 55 °C, and 30%at 70 °C with a delay of several hours and wasback to the original values within 1 h. However,

less than a 10% decrease in the CD intensity ofpoly1c was observed even at 70 °C. Poly1c andpoly2c were likely to take tightly and looselycoiled helical conformers, respectively. Even theabsolute value (����) of poly2c(5), which decreasedmost with the solution temperature kept at 70 °C,was larger than that of poly2c(68).

A consistent temperature-dependent helicalconformational change was more clearly observedin the specific rotation measurements of poly2c(5)and poly2c(25) in 1,2-dichloropropane, as Figure5 shows. The polymer solution was kept at 70 °Cfor a specified time and cooled for 10 min; this wasfollowed by specific rotation measurements at 20°C.59 Both poly2c(5) and poly2c(25) showed re-versible large changes in the specific rotation be-tween the treatments at 70 and 25 °C. However,poly2c(5) and poly2c(25) at 70 °C showed 349 and190°, respectively, much different specific rota-tions. Therefore, the resulting poly2c(5) andpoly2c(25) should take different metastable con-formers. However, the observed specific rotationswere still larger than that of poly2c(68). Thisobservation suggests that poly2c should have plu-ral metastable helical conformers, even in thehelical one defined so far in contrast to the ex-tended one.60,61 Interestingly, these different con-

Table I. Temperature Dependence of the Polymerization of Optically Active Isocyanides Derived fromL-Phenylalanine in MeOH with NiCl2 � 6H2Oa

Monomer

Polymer

Temperature(°C)

Conversion(%)b

IsolatedYield(%)c Mw

b PDb [�]58920 d

Poly1c(5) 1c 5 63 60 5.3 � 103 1.1 �561Poly1c(25) 1c 25 83 72 4.9 � 103 1.1 �467Poly1c(42) 1c 42 84 74 4.2 � 103 1.1 �555Poly1c(55) 1c 55 86 78 4.2 � 103 1.1 �413Poly1c(68) 1c 68 80 60 3.5 � 103 1.1 �5171c — — — — — — �9.1e

Poly2c(5) 2c 5 63 62 7.5 � 103 1.5 �651Poly2c(25) 2c 25 88 85 8.5 � 103 1.4 �451Poly2c(42) 2c 42 84 80 4.4 � 103 1.4 �125Poly2c(55) 2c 55 82 65 4.5 � 103 1.4 �120Poly2c(68) 2c 68 87 62 4.0 � 103 1.3 �1002c — — — — — — �25.4f

a [1c]/[Ni(II)] � 195; [1c] � 1 mol/L; [2c]/[Ni(II)] � 77; [2c] � 1 mol/L; reaction time � 1.5 days.b Determined by GPC (polystyrene standard).c Isolated polymers were reprecipitated in MeOH/H2O (1/4 (v/v).d Specific rotation (deg cm2 g�1) were measured in a 1-cm cell at concentrations (g cm�3) of 0.1–0.3 g/mL in CHCl3.e In CH3OH. c � 0.66.f In CH3OH. c � 8.26.

Figure 3. UV–vis and CD spectra of CHCl3 solutionsof poly2c(5), poly2c(25), poly2c(40), poly2c(55), andpoly2c(68), which were polymerized at 5, 25, 40, 55,and 68 °C, respectively. The measurements were car-ried out at room temperature. The details of the isola-tion procedure and sample preparation are described inthe text.

404 YAMADA ET AL.

formational changes were also determined before-hand by the polymerization temperature, whichmay be called a kind of memory effect.33

The helical conformation mentioned in thisstudy is briefly discussed with an energy diagram(Scheme 2). The surface is much simplified intotwo main parabolic curves, which are two intrin-sic conformers, an extended and a helical one.

The thermal stability of the helical conformersdecreased in the order of poly1c, poly2c, andpoly3c, on the basis of the magnitude of the spe-cific rotation and CD. Poly2c especially took var-ious stable or metastable conformers, amongwhich only helical ones could be argued on thebasis of specific rotations and CD. The helicalconformer could be relatively stabilized to reducethe considerable steric hindrance of the side

groups in the extended conformer. For the helical-conjugated polyisocyanide, the distortion of theconjugated main chain out of coplanarity toshorten an effective conjugation length was alsoserious in the extended conformer. The activationenergy between both conformers should also be

Figure 4. (a) Temperature profiles of a poly2c(5) so-lution against time at 70 (solid line), 55 (dashed line),and 40 °C (dotted line) and (b) plots of the rate of thedecreasing CD intensity of a 1,2-dichloroproapane so-lution of poly2c(5) with the solution temperature raisedto 70 (circles), 55 (squares), and 40 °C (triangles) for 1day and then lowered to room temperature.

Figure 5. Specific rotation changes of 1,2-dichloro-propane solutions of poly2c(5) with thermal treatment[poly2c(5) (circles) and poly2c(25) (squares)]. The solu-tion corresponding to each measurement point waskept at 70 °C for a fixed time and cooled for 10 min; thiswas followed by measurements at 20 °C.58

Scheme 2. Schematic energy diagram of the confor-mations of polyisocyanide. Several metastable helicalconformers are shown as local minima in the potentialsurface of a family of helical conformations. The ther-mal stability of the conformers should be discussedwith respect to both the energy diagram and the indi-vidual activation energy.

POLYISOCYANIDE DERIVED FROM PHENYLALANINE 405

considered with respect to the kinetics when thethermal stability of individual conformers is dis-cussed. Although a further computational studywill be required to obtain conclusive energetics,the following qualitative explanation may be stillvalid:

1. The helical conformer of poly1c is ther-mally stabilized because of the large acti-vation energy between both conformers.

2. Poly2c takes various metastable helicalconformers with small activation energies,which are determined beforehand by thepolymerization temperature.

3. Local minima in the complex potentialcurves between two intrinsic conformers inpoly2c would cause the reversible temper-ature-dependent conformational change,which will be extended to the coming iso-thermal conformational control by externalstimuli.

CONCLUSION

More than has been pointed out so far,27 it shouldbe remarked that the appropriate bulkiness of theside group stabilizes the helical conformation ofthe conjugated main chain of the polyisocyanide.The thermal stability of the helical conformationcould be tuned by only a slight change in the sizeof the substituent of the side group, tert-butyl andethyl esters. The metastable helical conformationdemonstrated in poly2c will provide us a signifi-cant strategy as a key polymer to isothermallyand reversibly control the helical conformation(e.g., between tight and loose helices) by substi-tuting a photoelectrochemically responsive group.A strong correlation between the conformationand the electronic interactive pathways along themain chain can be expected in the polyisocyanide,consisting of the tilted pz orbitals of the sp2 car-bons, unlike natural helical polymers such asDNA, RNA, and �-helices of oligopeptides andother nonconjugated helical synthetic polymers.This tunable helical conformation coupled withelectronic interaction is now being explored as thenext target in our project.

This work was partially supported by Grants-in-Aid forScientific Research (11305061) and Scientific Researchon Priority Area of Development of Molecular Conduc-tors and Magnets by Spin-Control (730/11224207) fromthe Ministry of Education, Science, Sports, Science and

Technology of Japan. T. Iyoda and J. Abe acknowledgethe financial support of the Tokyo Ohka Foundationand the Yamada Science Foundation, respectively.

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56. For poly(arylisocyanide) prepared by well-designedscrew-sense-selective polymerization with a Pt–Pd�-ethynediyl catalyst,19–21 large CD induced bythe helical main chain was observed in the 300–500-nm region consistent with the absorptionband, which could be assigned to the n–�* transi-tion of the imino moiety. However, the absorptiontails observed in poly1c and poly2c were extendedto 600 nm, much longer than the wavelength re-gion in which the CD appeared. The absorption tailin the visible region was thought to result from theconjugated main chain but showed no CD. Oneplausible explanation for this inconsistency mightbe the contribution of an extended structure con-sisting of the transoid-rich conformation of the con-jugated main chain. A less tilted (i.e., more planar)conjugated main chain would show a redshiftedabsorption with little CD. The other possibilitiesare (1) visible light scattering due to submicrome-ter polymer aggregates, which Fujiki et al.62

pointed out in chiral polysilane, and (2) d–� tran-sitions due to complexation of the residual Ni(II)with the imino moiety. The former possibility wasruled out by the consistent UV spectrum of theCHCl3 solution filtered by a filter 0.2 �m in pore size[Whatman 13mm GD/X syringe filters, polytetrafluo-roethylene (PTFE) filter media]. The latter was alsodenied by little UV spectral change in the solution,which was washed four times with a 2 N HCl aque-ous solution. However, further analysis is required toexplain such a broad absorption with a long tail.

57. Although van Beijnen et al.22 reported [�]57822 ��280

for poly3c prepared with the same polymerization,the poly3c we prepared exhibited smaller specificrotation and less reproducible CD. We failed to repro-duce the reported specific rotation and CD, althoughthe polymerization conditions, including the temper-ature, monomer concentration, monomer/catalystratio, and polymerization time, were optimized. How-ever, both van Beijnen et al.’s values and our valueswere still smaller than those for poly2c; therefore,little correction is required for our conclusion that asmall substituent in the monomer should destabilizethe helical conformation of the main chain.

58. Basically, the steric structure of polyisocyanide isdetermined by both configurational and conforma-tional isomers. The configurational isomers aresyn- and anti-forms of the imine moiety along thepolymer backbone, and the conformational isomersare s-cis and s-trans of the main chain, respec-tively. Helical conformation arises from s-cis geom-

POLYISOCYANIDE DERIVED FROM PHENYLALANINE 407

etry of the main chain and has the potential toinduce syn- and anti-forms as a tacticity in theworld of polymer chemistry. Unfortunately, be-cause of strongly restricted motions, the 1H NMRspectra showed peaks too broad to be analyzed withrespect to the steric structures previously men-tioned. Further analysis of the imino moiety withan isotope-labeling technique is now under way.

59. In situ measurements at 70 °C could not beplanned because of instrumental difficulty. Thecooling process before each measurement might

cause the observed specific rotation to be underes-timated. However, this effect is not so serious be-cause of a delay of a few hours in the cooling pro-cess, as Figure 5 shows.

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