Copolymerization of chiral amino acid-based acetylenes and helical conformation of the copolymers

Article abstract of DOI:10.1021/ma021432s

The present study deals with the copolymerization of an amino acid-based acetylene, N-(tert-butoxycarbonyl)-L-alanine N′-propargylamide (L-1A) with the optical isomer (D-1A), achiral hexanoic acid N-propargylamide (2A), and pivalic acid N-propargylamide (3A), and chiroptical properties of the copolymers. The copolymerization catalyzed by (nbd)Rh⁺[η⁶-C₆H₅B^- (C₆H₅)₃] afforded copolymers with number-average molecular weights over 10⁴ in good yields. Nonlinear relationships were observed in the specific rotation and CD and UV-vis spectroscopies with respect to the L-1A content in feed. For instance, poly(L-1A-co-D-1A)s incorporated with 12.5% of either optical isomer (75% ee) exhibited almost the same optical properties as those of the homopolymers. Chiral amplification was observed in the copolymerization of L-1A with the achiral monomers. The specific rotation of poly(L-1A-co-2A) with 45% L-1A unit was practically the same as that of poly(L-1A). The copolymers of L-1A and 3A with an L-1A content in feed as low as 12% exhibited nearly the same helix content as the homopolymer of L-1A did.

Full text of DOI:10.1021/ma021432s

3938
Macromolecules 2003, 36, 3938-3943
Copolymerization of Chiral Amino Acid-Based Acetylenes and Helical
Conformation of the Copolymers
Gu a n gzh en g Ga o, F u m io Sa n d a ,* a n d Tosh io Ma su d a *
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University,
Kyoto 606-8501, J apan
Received September 4, 2002; Revised Manuscript Received February 10, 2003
ABSTRACT: The present study deals with the copolymerization of an amino acid-based acetylene, N-(tert-
butoxycarbonyl)-L-alanine N′-propargylamide (L-1A) with the optical isomer (D-1A), achiral hexanoic acid
N-propargylamide (2A), and pivalic acid N-propargylamide (3A), and chiroptical properties of the
copolymers. The copolymerization catalyzed by (nbd)Rh+[η6-C6H5B-(C6H5)3] afforded copolymers with
number-average molecular weights over 104 in good yields. Nonlinear relationships were observed in the
specific rotation and CD and UV-vis spectroscopies with respect to the L-1A content in feed. For instance,
poly(L-1A-co-D-1A)s incorporated with 12.5% of either optical isomer (75% ee) exhibited almost the same
optical properties as those of the homopolymers. Chiral amplification was observed in the copolymerization
of L-1A with the achiral monomers. The specific rotation of poly(L-1A-co-2A) with 45% L-1A unit was
practically the same as that of poly(L-1A). The copolymers of L-1A and 3A with an L-1A content in feed
as low as 12% exhibited nearly the same helix content as the homopolymer of L-1A did.
Ch a r t 1
In tr od u ction
Amino acids and peptides are biologically important
and useful substances for chiral auxiliaries and building
blocks in organic synthesis.1 A wide variety of amino
acid- and peptide-based synthetic polymers have been
developed, some of which exhibit biocompatibility, bio-
degradability, and unique optical properties based on
higher order structures such as helix.2 On the other
hand, acetylenic polymers have alternating single and
double bonds along the main chain, which induces
characteristic optical, electrical, and magnetic proper-
ties.3 It is expected that introduction of chiral amino
acid into polyacetylene would lead to the formation of
higher order structures such as helix and self-assembly.
In macro- and supramolecular chemistry, the control of
helicity attracts much attention because polymers with
regulated higher order structures are applicable to
stimuli-responsive materials4 and enantioselective cata-
lysts.5 Some polyacetylenes with chiral substituents
exhibit strong circular dichroism (CD) signals and large
optical rotations based on the helical conformation.
Ciardelli et al. have prepared helical polyacetylenes by
the polymerization of optically active 1-alkynes.6 Moore,
Gorman, and Grubbs have prepared helical polyacety-
lenes by the ring-opening metathesis polymerization of
monosubustituted cyclooctatetraenes with chiral side
groups.7 Aoki, Oikawa, and co-workers have synthesized
helical poly(phenylacetylene) derivatives with optically
active menthyl8 and pinanyl groups9 and have reported
that the membranes of those polymers show enantiose-
lective permeability. Shinohara et al. have succeeded
in measuring the right-handed double helix structure
of a poly(phenylacetylene) bearing menthoxycarbonyl-
amino groups by scanning tunneling microscopy.10
Yashima, Okamoto, and co-workers have induced a
helical structure in optically inactive poly((4-carboxy-
phenyl)acetylene)11 and poly(propiolic acid)12 upon com-
plexation with optically active amines, amino alcohols,
and amino acids. They have also induced CD in several
polyacetylenes bearing phosphonophenyl13 and ami-
nophenyl groups14 by the assistance of optically active
peptides and carboxylic acids, respectively. Akagi,
Shirakawa and co-workers have synthesized helical
polyacetylene under an asymmetric reaction field con-
sisting of chiral nematic liquid crystals.15 Among several
reports concerning one-handed helical polyacetylenes,
Tang and co-workers have synthesized some amino acid-
containing polyacetylenes and tuned the helicity by pH
changes.16 We have synthesized several poly(N-propar-
gylamide)s with an excess of one-handed screw sense.17
The helix is stabilized by the steric repulsion and
intramolecular hydrogen bonds between the pendent
amide groups. The helical structure is deformed into
randomly coiled state by external stimuli such as
heating and addition of polar solvents. In the course of
our study on helical poly(N-propargylamide)s, we have
recently reported the synthesis and polymerization
of N-(tert-butoxycarbonyl)-L-alanine N′-propargylamide
(L-1A, Chart 1).18 It satisfactorily undergoes polymeri-
zation with (nbd)Rh+[η6-C6H5B-(C6H5)3] (nbd ) 2,5-
norbornadiene) as a catalyst to afford the corresponding
polymer [poly(L-1A)], which exhibits a large specific
rotation and a CD signal, indicating that it takes a
helical conformation.
* Corresponding author: Fax: +81-75-753-5908. E-mail:
masuda@adv.polym.kyoto-u.ac.jp.
10.1021/ma021432s CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/08/2003

Macromolecules, Vol. 36, No. 11, 2003
Copolymerization of Chiral Acetylenes 3939
Sch em e 1
The present study deals with the copolymerization of
L-1A with the optical isomer, D-1A, and achiral mono-
mers, hexanoic acid N-propargylamide (2A), and pivalic
acid N-propargylamide (3A), and analysis of the chi-
roptical properties of the copolymers.
Exp er im en ta l Section
Mea su r em en ts. 1H and 13C NMR spectra were recorded
in chloroform-d (CDCl3) on a J EOL EX-400 spectrometer. IR
spectra were measured using a Shimadzu FTIR-8100 spectro-
photometer. The number- and weight-average molecular weights
(Mn and Mw) of polymers were determined by gel permeation
chromatography (GPC) on a J asco Gulliver system (PU-980,
CO-965, RI-930, and UV-1570) equipped with polystyrene gel
columns (Shodex columns K804, K805, and J 806), using THF
as an eluent at a flow rate of 1.0 mL/min, calibrated by
polystyrene standards at 40 °C. CD and UV-vis spectra were
recorded in a quartz cell (thickness: 1 cm) at room tempera-
ture using a J asco J 600 or J 800 spectropolarimeter and a
Shimadzu UV-2200 spectrophotometer, respectively. Melting
points (mp) were measured on a Yanaco micro melting point
apparatus. Specific rotations ([R]D) were measured on a J asco
DIP-1000 digital polarimeter with a sodium lamp as a light
source.
Ma ter ia ls. CH2Cl2 was distilled by the standard procedures.
All the reagents in monomer synthesis were used as purchased
without purification. (nbd)Rh+[η6-C6H5B-(C6H5)3] was pre-
pared by the reaction of [(nbd)RhCl]2 with NaB(C6H5)4 as
described in the literature.19 L-1A and hexanoic acid N-
propargylamide (2A) were synthesized according to our previ-
ous method.17c,18
Syn th esis of N-(ter t-Bu toxyca r bon yl)-D-a la n in e N′-
P r op a r gyla m id e (D-1A). Di-tert-butyl dicarbonate (24 g, 0.11
mol) was added to a solution of D-alanine (8.91 g, 0.10 mol)
and triethylamine (14 mL, 0.10 mol) in 1,4-dioxane/H2O
(volume ratio: 1/1, 100 mL) at 0 °C, and the resulting mixture
was stirred at room-temperature overnight. It was washed
with AcOEt (100 mL). 5 N HCl was added to the solution to
adjust its pH at 2, and then AcOEt (100 mL) was added to
the solution. The organic layer was washed with saturated
aqueous sodium chloride, dried over anhydrous Na2SO4, and
concentrated by rotary evaporation. The residue was purified
by recrystallization from AcOEt to obtain solid N-tert-butoxy-
carbonyl-D-alanine (Boc-D-alanine) in 75% yield. D-1A was
prepared from Boc-D-alanine and propargylamine in a manner
similar to l-1A in 62% yield. Mp: 137 °C. [R]D 50.7 ° (c ) 1.00
g/dL, in CHCl3 at room temperature). 1H NMR (400 MHz,
CDCl3): δ 1.37-1.38 (m, 3H, CH3), 1.45 [s, 9H, (CH3)3], 2.23
(s, 1H, CtCH), 4.04 (s, 2H, CH2), 4.17 (s, 1H, H3CCHNH), 4.96
(s, 1H, NHCOO), 6.51 (s, 1H, NHCO). 13C NMR (100 MHz,
CDCl3): δ 18.24 (CH3), 28.26 [(CH3)3], 29.07 (CH2), 49.81
(H3CCH), 71.46 [C(CH3)3], 79.31 (HCt), 80.22 (HCtC), 155.57
(NHCOO), 172.46 (CONH). IR (cm-1, KBr): 3360 (N-H), 3297
(H-Ct), 2963, 3045, 2980, 2973, 2932, 1675 (CdO), 1550 (N-
H, C-N), 1453, 1389, 1368, 1302, 1266, 1229, 1175, 1123, 1080,
1040, 1024, 932, 874, 787, 756, 743, 693, 664, 617, 586, 524.
Anal. Calcd for C11H18N2O3: C, 58.37; H, 8.02; N, 12.38.
Found: C, 58.25; H, 7.84; N, 12.50.
was added to (nbd)Rh+[η6-C6H5B-(C6H5)3] (10.6 mg, 0.02
mmol), and the resulting mixture was vigorously stirred. It
was kept in a water bath at 30 °C for 1 h. Then, acetic acid
(30 µL) was added to the reaction mixture. The resulting
mixture was poured into n-hexane (200 mL) to precipitate a
copolymer. It was separated by filtration using a membrane
filter (ADVANTEC H100A047A), and dried under reduced
pressure. Yield: 210 mg (93%).
Sp ectr oscop ic Da ta of th e P olym er s. P oly(D-1A). 1H
NMR (400 MHz, CDCl3): δ 1.41 [br s, 12H, CH3, (CH3)3], 4.15
(br s, 3H, CHCH3, CH2), 5.70 (br s, 1H, NH), 6.16 (br s, 1H,
-CHdC), 6.60 (br s, 1H, NH). 13C NMR (100 MHz, CDCl3): δ
19.38 (CH3), 28.46 [(CH3)3], 50.35 (CHCH3), 79.23 (CH2), 125.50
(HCd), 137.70 (dCCH2), 155.22 (NHCOO), 175.57 (NHCO).
IR (cm-1, KBr): 3310 (N-H), 2980, 2936, 1670 (CdO), 1520
(N-H, C-N), 1460, 1393, 1368, 1252, 1169, 1026.
P oly(L-1A0.50-co-D-1A0.50). 1H NMR (400 MHz, CDCl3): δ
1.46 [br s, 12H, CH3, (CH3)3], 4.37 (br s, 3H, CHCH3, CH2),
6.15 (br s, 1H, -CHdC), 7.92 (br s, 1H NH). 13C NMR (100
MHz, CDCl3): δ 19.41 (CH3), 28.70 [(CH3)3], 44.81 [C(CH3)3],
50.32 (CHCH3), 79.76 (CH2), 126.49 (HCd), 136.93 (dCCH2),
155.80 (NHCOO), 174.33 (NHCO). IR (cm-1, KBr): 3320 (N-
H), 2980, 2936, 1670 (CdO), 1520 (N-H, C-N), 1460, 1393,
1368, 1250, 1050, 1026, 854.
P oly(L-1A0.50-co-2A0.50). 1H NMR (400 MHz, CDCl3): δ 0.88
(br s, CH3 of 2A), 1.31 (br s, CH2 of 2A), 1.42 [br s, CH3 and
(CH3)3 of L-1A], 1.60 (br s, CH2 of 2A), 2.24 (br s, CH2 of 2A),
3.93 (br s, COCH2 of 2A), 4.24 (br s, CH2 of L-1A), 6.09 (br s,
NH of L-1A and 2A), 7.82 (br s, NH of l-1A). 13C NMR (100
MHz, CDCl3): δ 13.91 (CH3 of 2A), 19.08 (CH3 of L-1A), 22.39,
25.78 (CH2 of 2A), 28.39 [(CH3)3 of L-1A], 29.12, 31.50, 36.13
(CH2 of 2A), 44.80 [C(CH3)3 of L-1A], 50.61 (CHCH3 of L-1A),
79.43 and 79.58 (CH2 of L-1A and 2A), 125.77 (HCd of L-1A
and 2A), 136.33 and 137.22 (dCCH2 of L-1A and 2A), 155.33
(NHCOO of L-1A), 173.95 and 174.60 (NHCO of L-1A and 2A).
IR (cm-1, KBr): 3310 (N-H), 3090, 2975, 2940, 2850, 1650
(CdO), 1540 (N-H, C-N), 1453, 1380, 1368, 1250, 1057, 1024,
857.
Syn th esis of P iva lic Acid N-P r op a r gyla m id e (3A). The
title compound was synthesized from pivalic acid instead of
Boc-L-alanine in a manner similar to L-1A in 50% yield. Mp:
66.5-67.0 °C. 1H NMR (400 MHz, CDCl3): δ 1.20-1.29 [m,
9H, C(CH3)3], 2.24 (s, 1H, HC≡), 3.99-4.10 (m, 2H, CH2), 5.89
(s, 1H, NH). 13C NMR (100 MHz, CDCl3): δ 27.70 [C(CH3)3],
29.65 (CH2), 38.89 [C(CH3)3], 71.75, 71.86 (HCt), 80.09 (HCt
C), 178.35 (NHCO). IR (cm-1, KBr): 3320 (H-Ct), 2969, 2360
(CtC), 1642 (CdO), 1530 (N-H, C-N), 1480, 1366, 1352,
1298, 1264, 1227, 1211, 1010, 627, 559. Anal. Calcd for
P oly(L-1A0.50-co-3A0.50). 1H NMR (400 MHz, CDCl3): δ 1.16
(br s, (CH3)3 of 3A), 1.41 [br s, CH3, (CH3)3 of L-1A], 1.60 (br s,
CH2 of 3A), 2.18 and 4.75 (br s, CH2 of L-1A and 3A), 6.08 (br
s, NH of L-1A and 3A). 13C NMR (100 MHz, CDCl3): δ 19.52
(CH3 of L-1A), 27.99 [(CH3)3 of l-1A], 28.74 [(CH3)3 of 3A], 50.39
and 51.09 [C(CH3)3 of L-1A and 3A], 79.58 and 79.84, (CH2 of
L-1A and 3A), 125.80 (HCd of L-1A and 2A), 138.63 (dCCH2
of L-1A and 3A), 155.13 (NHCOO of L-1A), 178.30 and 180.00
(NHCO of L-1A and 3A). IR (cm-1, KBr): 3320 (N-H), 2979,
2920, 1610 (CdO), 1520 (N-H, C-N), 1458, 1368, 1250, 1169.
C
12H20N2O3: C, 59.98; H, 8.39; N, 11.66. Found: C, 59.99; H,
8.11; N, 11.59.
Cop olym er iza tion (Typ ica l P r oced u r e). All the poly-
merizations were carried out in a Schlenk tube equipped with
a three-way stopcock under nitrogen. A solution of L-1A (113
mg, 0.5 mmol) and D-1A (113 mg, 0.5 mmol) in CH2Cl2 (5 mL)
Resu lts a n d Discu ssion
Mon om er Syn th esis. Scheme 1 illustrates the syn-
thetic routes for the monomers used, L-1A, D-1A, 2A,

3940 Gao et al.
Macromolecules, Vol. 36, No. 11, 2003
Sch em e 2
Ta ble 1. Cop olym er iza tion of L-1A w ith D-1Aa
Ta ble 3. Cop olym er iza tion of L-1A w ith 3Aa
d
d
monomer feed
ratio L-1A:D-1A
yieldb
(%)
[R]D
monomer feed
ratio (L-1A:3A)
yieldb
(%)
[R]D
Mnc/103
Mw/Mn
(deg)
run
Mnc/103
Mw/Mn
(deg)
c
c
run
1
2
3
4
5
6
7
8
9
100:0
88:12
75:25
62:38
50:50
38:62
25:75
12:88
0:100
83
92
96
90
93
87
94
95
91
13 900
21 000
13 200
16 700
13 500
18 700
13 500
22 700
15 300
2.61
2.26
1.62
1.76
1.63
1.81
1.65
2.51
2.20
-1393
-1356
-506
-163
2
159
473
1334
1381
1
2
3
4
5
6
7
8
9
0:100
12:88
25:75
38:62
50:50
62:38
75:25
88:12
100:0
43
43
45
51
60
74
82
88
95
e
10 900
11 200
11 600
13 300
15 100
17 300
17 600
21 000
e
1.94
1.85
1.83
1.94
2.03
2.28
2.59
2.29
-1529
-1644
-1507
-1443
-1416
-1483
-1406
-1376
a
a
Polymerization time: 1 h. Temperature: 30 °C. [M]0 ) 0.20
mol/L in CH2Cl2. [M]0/[Cat] ) 50. Catalyst: (nbd)Rh+[η6-C6H5B--
(C6H5)3], nbd ) norbornadiene. The monomer conversion was
quantitative in every case, which was determined by 1H NMR.
Polymerization time: 1 h. Temperature: 30 °C. [M]0 ) 0.20
mol/L in CH2Cl2. [M]0/[Cat] ) 50. Catalyst: (nbd)Rh+[η6-C6H5B--
(C6H5)3], nbd ) norbornadiene. n-Hexane-insoluble part. c De-
termined by GPC calibrated by polystyrene standards. Eluent:
b
n-Hexane-insoluble part. c Determined by GPC calibrated by
polystyrene standards. Eluent: THF. Measured by polarimetry
THF. Measured at room temperature, c ) 0.11-0.12 g/dL, in
b
d
CHCl3. e Insoluble.
d
at room temperature, c ) 0.10-0.12 g/dL in CHCl3.
Ta ble 2. Cop olym er iza tion of L-1A w ith 2Aa
monomer copolymer
feed ratio compositionb yieldc
[R]D
e
(%) Mnd/103 Mw/Mn
(deg)
d
run (L-1A:2A) (L-1A: 2A)
1
2
3
4
5
6
7
8
9
0:100
12:88
25:75
38:62
50:50
62:38
75:25
88:12
100:0
0:100
14:86
23:77
45:55
52:48
68:32
84:16
88:12
100:0
78 25 200
2.11
0
71 15 000f 2.71f
68 10 400f 3.14f
-207
-952 to -528g
85 12 600f 2.74f -1427
60 21 700
2.11 -1463
93 11 900f 2.32f -1620
83 18 500
96 16 100
83 13 900
2.91 -1606
3.77 -1469
2.61 -1393
a
Polymerization time: 1 h. Temperature: 30 °C. [M]0 ) 0.20
mol/L in CH2Cl2. [M]0/[Cat] ) 50. Catalyst: (nbd)Rh+[η6-C6H5B--
(C6H5)3], nbd ) norbornadiene. Determined by 1H NMR. c n-
b
d
Hexane-insoluble part. Determined by GPC calibrated by poly-
styrene standards. Eluent: THF. e Measured by polarimetry at
room temperature, c ) 0.103-0.149 g/dL, in CHCl3. f Multimodal.
F igu r e 1. Relationship between enantiomeric excess of D-1A
unit over L-1A unit in poly(L-1A-co-D-1A) and the [R]D of the
copolymer measured by polarimetry at room temperature in
CHCl3, c ) 0.10-0.13 g/dL (samples of Table 1).
g
Ranged during the measurement.
and 3A. They were prepared by the reaction of propar-
gylamine with the corresponding carboxylic acids using
1-(3-(dimethylamino)propyl)-3-ethylcarbodiimide hydro-
chloride (EDC‚HCl) and 4-(dimethylamino)pyridine
(DMAP) as condensation agents, as shown in Scheme
1. The monomers were identified by 1H NMR, 13C NMR,
and IR spectroscopies besides elemental analysis.
Cop olym er iza tion . Scheme 2 and Table 1 sum-
marize the conditions and results of the copolymeriza-
tion of L-1A with D-1A catalyzed by (nbd)Rh+[η6-
C6H5B-(C6H5)3] in CH2Cl2 at 30 °C for 1 h. Monomers
L-1A and D-1A satisfactorily underwent copolymeriza-
tion to afford the corresponding copolymers with the Mn
in the range from 13 200 to 22 700 in good yields, which
were soluble in THF, CH2Cl2, and CHCl3, but insoluble
in n-hexane. The color of the homopolymers of L-1A and
D-1A was yellow, and that of the copolymers gradually
became white with the incorporation of the optical
isomer. The difference of the polymer colors will be
commented on later in conjunction with the conforma-
tion. The molecular weight distributions of the copoly-
mers tended to be smaller than those of the homopoly-
mers.
Table 2 summarizes the results of the copolymeriza-
tion of L-1A with 2A using the Rh+ catalyst in CH2Cl2
at 30 °C for 1 h. In all cases, the monomers quantita-
tively converted to afford the copolymers with Mn in the
range from 10 400 to 25 200 in high yields, which were
soluble in THF, CH2Cl2, and CHCl3, but insoluble in
n-hexane. The color of the homopolymer of 2A was
white, while that of the copolymers with L-1A was
yellow. The color of the copolymers became more yel-
lowish with increase of L-1A content. The molecular
weight distributions of the copolymers were rather
broad (2.11-3.77) compared to those of the homopoly-
mers, and some copolymers exhibited multimodal GPC

Macromolecules, Vol. 36, No. 11, 2003
Copolymerization of Chiral Acetylenes 3941
F igu r e 3. [A] CD and [B] UV-vis spectra of poly(L-1A-co-
2A) measured in CHCl3 at room temperature, c ) 1.14-2.33
× 10-4 mol/L (samples of Table 2).
F igu r e 2. [A] CD and [B] UV-vis spectra of poly(L-1A-co-D-
1A) measured in CHCl3 at room temperature, c ) (1.18-1.83)
× 10-4 mol/L. The figures in the spectra represent the
enantiomeric excess (%) of D-1A unit over L-1A unit in the
copolymer (samples of Table 1).
specific rotation, while incorporation of 25% of the
isomer (50% ee) resulted in a large change of specific
rotation. The difference of specific rotation between the
copolymers of 25% and 0% ee was relatively small. As
we have reported in our preceding article, poly(L-1A)
takes a helical conformation in a CHCl3 solution.18 The
result of specific rotation suggests that the helicity of
the copolymers does not simply correspond to the unit
ratio. The CD spectra of the homopolymers exhibit the
mirror image Cotton effect at 400 nm, which indicates
that poly(L-1A) and poly(D-1A) take helical structures
with opposite directions (Figure 2). The copolymers with
75% ee exhibited almost the same CD spectral patterns
as in the homopolymers. On the other hand, the
copolymers with 50 and 25% ee displayed no clear CD
signal at 400 nm, but a relatively small CD signal at
320 nm. This means that the copolymerization of L- and
D-1A obeys the “majority rule”, which has been eluci-
dated by Green et al. concerning helical polyisocy-
antes.20
The UV-vis spectra of the homopolymers and copoly-
mers with 75% ee showed an absorption maximum at
400 nm, which agrees with the wavelength of the CD
signals. On the other hand, the main absorption was
seen not at 400 nm but at 320 nm in the other
copolymers. This indicates that these copolymers are
less conjugated than the homopolymers are. Hence, the
random structure of these polymers seems to reduce the
π-conjugation compared with the helical structure.
As described in the results of copolymerization, the
homopolymers of L-1A and D-1A were yellow, while the
traces. The copolymer compositions, which were deter-
1
mined by H NMR spectroscopy, were almost the same
as the monomer feed ratios. It can be considered that
random copolymerization took place. The specific rota-
tions of the copolymers ranged from 0 to -1620°.
Table 3 summarizes the conditions and results of the
copolymerization of L-1A with 3A catalyzed by (nbd)-
Rh+[η6-C6H5B-(C6H5)3] in CH2Cl2 at 30 °C for 1 h. The
copolymers obtained were soluble in THF, CH2Cl2, and
CHCl3, while the homopolymer of 3A was insoluble in
these solvents. The copolymer yield increased from 43
to 95% and the molecular weight of the copolymers
increased from 10 900 to 21 000 with the increase of
monomer L-1A in feed. The specific rotation of the
copolymer reached -1529° when the L-1A content in
feed was as low as 12%, and it reached a maximum
when L-1A in feed was 25%. The color of the copolymers
was yellow, while the color of poly(3A) was white.
Con for m a tion of Cop olym er s in Solu tion . The
conformation of the copolymers was examined by spe-
cific rotation and CD and UV-vis spectroscopic meth-
ods. The specific rotations of poly(L-1A-co-D-1A) ranged
from -1393 to +1381° at room temperature as displayed
in Table 1. They did not exhibit a linear relationship
with respect to the enantiomeric excess of the monomer
unit in the copolymers (Figure 1). Incorporation of 12.5%
of the optical isomer (75% ee) scarcely affected the

3942 Gao et al.
Macromolecules, Vol. 36, No. 11, 2003
F igu r e 4. [A] CD and [B] UV-vis spectroscopic changes of
poly(L-1A0.23-co-2A0.77) measured in CHCl3 at room tempera-
ture, c ) 1.56 × 10-4 mol/L (run 3 in Table 2). The sample
solution was prepared just before the measurement.
F igu r e 5. [A] CD and [B] UV-vis spectra of poly(L-1A-co-
3A) measured in CHCl3 at room temperature, c ) 1.18-1.57
× 10-4 mol/L (samples of Table 3).
copolymers were white. From the UV-vis spectroscopic
data, we can conclude that the yellow color is not due
to impurities in the polymer, but due to the UV-vis
absorption around 400 nm (Figure 2), which is assign-
able to the main chain π-π conjugation of a helical
polyacetylene backbone. As shown in Figure 2, the
polymers without a CD signal at 400 nm do not exhibit
UV-vis absorption at the same position. In this case,
it is considered that the polymer takes a random coil
structure and the color is white, because the main chain
conjugation of random-coiled polyacetylenes is shorter
than that of helical ones.17
The specific rotations of the homopolymers and co-
polymers of L-1A with 2A and 3A exhibited nonlinear
relationships with respect to the monomer unit ratio.
The specific rotation of poly(L-1A-co-2A) with L-1A unit
of 45% was as large as that of poly(L-1A). Not poly(L-
1A) but poly(L-1A0.68-co-2A0.32) showed the largest spe-
cific rotation. On the other hand, the copolymer incor-
porated with L-1A of 25% exhibited the largest specific
rotation among the poly(L-1A-co-3A)s. Thus, we can
conclude that the copolymers effectively take helical
conformations in the presence of relatively small amounts
of the chiral monomer (L-1A) unit, especially in the case
of comonomer 3A. The bulky tert-butyl group is favor-
able to induce the tight stable helical structure, as
similarly reported by Yashima et al.21 The CD and UV-
vis spectra gave similar tendencies to each other in the
poly(L-1A-co-2A)s. This indicates that the conjugation
along the main chain becomes short and the helical
conformation changes into a random coil with the
increase of monomer 2A unit (Figure 3).
The specific rotation and CD, and UV-vis spectra of
poly(L-1A0.23-co-2A0.77) were unstable in CHCl3, and the
values and patterns changed during the measurements
(Figure 4), which was not observed with the other
copolymer samples of L-1A with 2A. J ust after prepara-
tion of the polymer solution in CHCl3, the copolymer
did not show a clear pattern of helical conformation, but
the CD signal and UV-vis absorption at 400 nm
gradually increased with time. This appears to be due
to a delicate balance between helical and random coil
structures. Similar phenomena have been reported
concerning the helical properties of polymethacrylates.22
Figure 5 depicts the CD and UV-vis spectra of poly-
(L-1A-co-3A)s obtained by the copolymerization at vari-
ous monomer feed ratios, which confirms easy formation
of helix irrespective of the unit ratios. Poly(L-1A0.12-co-
3A0.88) showed almost the same CD spectral patterns
as poly(L-1A), which means that the copolymerization
of L-1A and 3A obeys the “sergeants and soldiers rule”
by Green et al.20 The UV-vis spectra agreed with the
CD results and confirmed the large main chain conjuga-
tion in all cases. Hence, we can conclude that bulky tert-
butyl group of 3A is effective to achieve the long helix
persistence length.21

Macromolecules, Vol. 36, No. 11, 2003
Copolymerization of Chiral Acetylenes 3943
(8) Aoki, T.; Kokai, M.; Shinohara, K.; Oikawa, E. Chem. Lett.
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Macromolecules 1999, 32, 79.
Su m m a r y
We have performed the copolymerization of the L-
alanine-derived N-propargylamide L-1A with the optical
isomer D-1A, achiral hexanoic acid N-propargylamide
2A, and pivalic acid N-propargylamide 3A and have
examined the chiroptical properties of the formed
copolymers. It was confirmed that the copolymerization
of L-1A with D-1A obeyed the “majority rule”, judging
from the specific rotation and CD signals of the formed
copolymers. The [R]D of the copolymers exhibited non-
linear relationships against the content of L-1A. It was
found that incorporation of 68% and 25% of L-1A in the
copolymers with 2A and 3A, respectively, was enough
to induce practically the same helical structures with
an excess of one-handed screw sense as that of poly(L-
1A). This means that the copolymerization obeys the
“sergeants and soldiers” rule.
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