Synthesis and structure of poly(N-propargylbenzamides) bearing chiral ester groups

Article abstract of DOI:10.1021/ma021532y

N-Propargylbenzamides having chiral ester groups on the benzene ring, 1-6, were polymerized with (nbd)Rh⁺[n⁶-C₆H₅^-(C₆ H₅)₃] to afford soluble polymers with moderate molecular weights (M_n = 34 000-100 000) in good yield. The ¹H NMR spectra demonstrated that the polymers have stereoregular structures (cis = 91-100%). The influence of the substitution position for the chiral ester group on the secondary structure was examined. From the comparison of the CD effects of poly(1)-poly(3), the meta-substituted polymer was proven to possess a one-handed helical conformation. When the chiral center was closer to the benzene ring or chiral ester group was bulkier, the polymers showed more intense chiroptical properties. A variable temperature CD spectroscopic study showed that the helical structure of poly(2) and poly(6) was thermally stable. However, the CD spectra of poly(4) and poly(5) were inverted in sign on temperature change in chloroform or toluene, meaning that the helix inversion took place. The temperature of helix inversion could be controlled by copolymerizing with N-propargyl-benzamide (7).

Full text of DOI:10.1021/ma021532y

Macromolecules 2003, 36, 573-577
573
Synthesis and Structure of Poly(N-propargylbenzamides) Bearing Chiral
Ester Groups
J u n ich i Ta bei, Ryoji Nom u r a , a n d Tosh io Ma su d a *
Department of Polymer Chemistry, Graduated School of Engineering, Kyoto University,
Kyoto 606-8501, J apan
Received September 27, 2002; Revised Manuscript Received December 3, 2002
ABSTRACT: N-Propargylbenzamides having chiral ester groups on the benzene ring, 1-6, were
polymerized with (nbd)Rh+[η6-C6H5B-(C6H5)3] to afford soluble polymers with moderate molecular weights
(Mn ) 34 000-100 000) in good yield. The 1H NMR spectra demonstrated that the polymers have
stereoregular structures (cis ) 91-100%). The influence of the substitution position for the chiral ester
group on the secondary structure was examined. From the comparison of the CD effects of poly(1)-poly-
(3), the meta-substituted polymer was proven to possess a one-handed helical conformation. When the
chiral center was closer to the benzene ring or chiral ester group was bulkier, the polymers showed more
intense chiroptical properties. A variable temperature CD spectroscopic study showed that the helical
structure of poly(2) and poly(6) was thermally stable. However, the CD spectra of poly(4) and poly(5)
were inverted in sign on temperature change in chloroform or toluene, meaning that the helix inversion
took place. The temperature of helix inversion could be controlled by copolymerizing with N-propargyl-
benzamide (7).
Sch em e 1
In tr od u ction
One of the representative secondary conformations of
polymer is the helical structure. Much attention has
been paid to helical polymers owing to their unique
functions such as molecular recognition ability and
catalytic ability for asymmetric synthesis.1 Previous
intensive efforts have made it possible to produce a
variety of well-ordered helical polymers. The helical
polymers are classified into two types. One is the
polymers with stable helical conformation,2 in which the
helix-helix or helix-coil transition does not occur at
all due to their rigid main chain. The other is the
polymers having dynamic helix,3 which have a stiff but
not rigid main chain. These polymers undergo helix-
helix and/or helix-random coil interconversion due to the
small energetic barriers for helix reversal. The poly-
meric materials that can change the helix sense by
external stimuli are attracting much attention owing
to the potential application in devices such as molecular
switches, data storage, and transmission.4
Recently, we showed that stereoregular poly(N-pro-
pargylalkylamides) adopt a helical structure which is
stabilized by the intramolecular hydrogen bonds be-
tween the pendant amide groups.5 Poly(N-propargyl-
alkylamides) undergo a helix-random coil transition
upon sensing external stimuli such as changes in
temperature and solvent because the intramolecular
hydrogen bonds can be readily broken by external
stimuli.6,7 The polymers also show thermochromism and
solvatochromism upon this conformational change.6 On
the contrary, the secondary conformation of poly(N-
propargylarylamides) has not been investigated in detail
because of their poor solubility.7 In the present study,
we synthesized poly(N-propargylbenzamides) having
chiral ester groups, poly(1)-poly(6) (Scheme 1). It has
been found that the position and structure of the chiral
ester group significantly influence the secondary struc-
ture. We further show that poly(4) and poly(5) undergo
the thermally driven inversion of the screw sense.
Exp er im en ta l Section
Ma ter ia ls. The solvents were distilled by usual methods
prior to use. Propargylamine (Aldrich), (S)-(-)-2-methyl-1-
butanol (Tokyo Kasei), (S)-(-)-2-butanol (Aldrich), (S)-(-)-2-
octanol (Azmax), phthaloyl chloride (Wako), isophthaloyl
chloride (Wako), terephthaloyl chloride (Tokyo Kasei), pyridine
(Wako), isobutyl chloroformate (Wako), and 4-methylmorpho-
line (Wako) were used without further purification. (nbd)Rh +-
[η6-C6H5B-(C6H5)3] was prepared as reported.8a
Mon om er Syn th esis. Synthesis of 1 is described as a
typical procedure. A mixture of (S)-(-)-2-methyl-1-butanol (5.4
mL, 49.3 mmol) and pyridine (8.7 mL, 98.7 mmol) was slowly
added to a THF solution (150 mL) of phthaloyl chloride (10 g,
49.3 mmol) at 0 °C. After the reaction mixture was refluxed
for 6 h, water (20 mL) was added, and the mixture was further
refluxed for 6 h. The mixture was washed with 2 M aqueous
HCl and then water and concentrated to give (S)-2-(2-meth-
ylbutoxycarbonyl)benzoic acid in 68% yield. Isobutyl chloro-
* Corresponding author: Tel +81-75-753-5613; Fax +81-75-753-
5908; e-mail masuda@adv.polym.kyoto-u.ac.jp.
10.1021/ma021532y CCC: $25.00 © 2003 American Chemical Society
Published on Web 01/09/2003

574 Tabei et al.
Macromolecules, Vol. 36, No. 3, 2003
formate (4.60 mL, 33.8 mmol) was added to a THF solution
(100 mL) of the resulting (S)-2-(2-methylbutoxycarbonyl)-
benzoic acid (7.97 g, 33.8 mmol) and 4-methylmorpholine (3.41
mL, 33.8 mmol) at 0 °C. After 15 min, propargylamine (2.33
mL, 33.8 mmol) was added to the solution. The solution was
stirred at room temperature for 1 h. After the white precipitate
was filtered off, the filtrate was concentrated. Ethyl acetate
(ca. 100 mL) was added to the residue, and the solution was
washed with 2 M aqueous HCl and saturated aqueous NaH-
CO3, dried over MgSO4, and concentrated. Monomer 1 was
isolated (1.62 g, 5.92 mmol, 12%) by flash column chromatog-
raphy on silica gel (hexane/AcOEt, 3/2, v/v). Monomers 2-6
were prepared in a similar way. The spectral data are as
follows.
P olym er iza tion P r oced u r es. A CHCl3 solution of the
monomers ([M]total ) 2 M) was added to a CHCl3 solution of
(nbd)Rh+[η6-C6H5B-(C6H5)3] ([monomer]/[cat] ) 100) under
dry nitrogen, and the solution was kept at 30 °C for 1 h. The
solution was poured into a large amount of methanol to
precipitate the polymers. The resulting polymers were collected
by filtation and dried under reduced pressure.
Poly(1): IR (CHCl3): 3308, 3020, 1717, 1647, 1538, 1219,
1084, 779 cm-1. 1H NMR (CDCl3): δ 0.78-0.99 (6H, CH(CH3)-
CH2CH3), 0.99-1.18 (1H, CH2CH3), 1.18-1.43 (1H, CH2CH3),
1.43-1.76 (1H, CH(CH3)CH2), 3.60-4.41 (4H, CHdCCH2, (Cd
O)OCH2), 5.90-6.39 (1H, CHdCCH2), 7.02-7.75 (5H, NH, Ar).
Poly(2): IR (CHCl3): 3335, 3027, 1719, 1632, 1545, 1219,
791 cm-1 1H NMR (CDCl3): δ 0.76-1.01 (6H, CH(CH3)-
.
1: yield 12%; mp 49 °C; [R]D ) +4.18°. IR (KBr): 3324, 2963,
CH2CH3), 1.01-1.22 (1H, CH2CH3), 1.22-1.43 (1H, CH2CH3),
1.43-1.78 (1H, CH(CH3)CH2), 3.78-4.51 (4H, CHdCCH2, (Cd
O)OCH2), 6.12-6.40 (1H, CHdCCH2), 7.10-7.48 (1H, Ar),
7.86-8.19 (1H, Ar), 8.19-8.44 (1H, Ar), 8.44-8.75 (1H, Ar),
8.75-9.24 (1H, NH).
1
1713, 1647, 1529, 1261, 1134, 794 cm-1. H NMR (CDCl3): δ
0.87 (t, 3H, J ) 7.32 Hz), 0.92 (d, 3H, J ) 6.84 Hz), 1.19 (m,
1H), 1.43 (m, 1H), 1.76 (m, 1H), 2.22 (d, 1H, J ) 2.44 Hz),
4.11 (d, 2H, J ) 4.39 Hz), 4.02 (d, 2H, J ) 6.81 Hz), 4.16 (d,
2H, J ) 2.44 Hz), 6.32 (s, 1H), 7.40-7.48 (m, 4H), 7.81 (d, 1H,
J ) 6.35 Hz). 13C NMR (CDCl3): δ 11.13, 16.34, 25.92, 33.99,
70.27, 71.81, 71.88, 79.16, 127.64, 129.38, 129.71, 129.94,
130.00, 131.75, 166.63, 168.79. Anal. Calcd for C16H19NO3: C,
70.33; H, 6.96; N, 5.13. Found: C, 70.28; H, 6.72; N, 5.31.
2: yield 35%; mp 34 °C; [R]D ) +4.61°. IR (KBr): 3333, 2968,
Poly(3): IR (CHCl3): 3312, 3020, 1717, 1638, 1541, 1211,
754 cm-1 1H NMR (CDCl3): δ 0.75-1.10 (6H, CH(CH3)-
.
CH2CH3), 1.10-1.36 (1H, CH2CH3), 1.36-1.58 (1H, CH2CH3),
1.58-1.96 (1H, CH(CH3)CH2), 3.71-4.31 (4H, CHdCCH2, (Cd
O)OCH2), 5.80-6.28 (1H, CHdCCH2), 7.22-7.95 (4H, Ar),
7.95-8.38 (1H, NH).
1713, 1649, 1541, 1263, 991, 738 cm-1
.
1H NMR (CDCl3): δ
Poly(4): IR (CHCl3): 3335, 3020, 1716, 1632, 1543, 1219,
0.82 (t, 3H, J ) Hz), 0.87 (d, 3H, J ) 6.83 Hz), 1.13 (m, 1H),
1.37 (m, 1H), 1.72 (m, 1H), 2.16 (d, 1H, J ) Hz), 4.02 (d, 2H,
J ) 4.40 Hz), 4.14 (d, 2H, J ) 2.44 Hz), 6.70 (s, 1H), 7.38 (1H,
J ) Hz), 7.86 (1H, J ) Hz), 7.92, (1H, J ) Hz), 8.25 (s, 1H).
13C NMR (CDCl3): δ 11.18, 16.41, 26.01, 34.15, 69.96, 71.84,
71.92, 79.23, 127.73, 128.77, 130.79, 131.74, 132.50, 134.00,
165.86, 166.21. Anal. Calcd for C16H19NO3: C, 70.33; H, 6.96;
N, 5.13. Found: C, 70.13; H, 7.04; N, 4.92.
777 cm-1 1H NMR (CDCl3): δ 0.64-0.99 (3H, CH(CH3)-
.
CH2CH3), 0.99-1.35 (3H, CH(CH3)CH2CH3), 1.35-1.77 (2H,
CH(CH3)CH2CH3), 3.80-4.71 (2H, CHdCCH2), 4.71-5.18 (1H,
CH(CH3)CH2CH3), 5.90-6.49 (1H, CHdCCH2), 7.28-7.51 (1H,
Ar), 7.81-8.19 (1H, Ar), 8.19-8.42 (1H, Ar), 8.42-8.77 (1H,
Ar), 8.77-9.31 (1H, NH).
Poly(5): IR (CHCl3): 3343, 3020, 1711, 1635, 1541, 1219,
779 cm-1 1H NMR (CDCl3): δ 0.76-1.01 (6H, CH(CH3)-
.
3: yield 18%; mp 102 °C; [R]D ) +5.24°. IR (KBr): 3308,
CH2CH3), 1.01-1.22 (1H, CH2CH3), 1.22-1.43 (1H, CH2CH3),
1.43-1.78 (1H, CH(CH3)CH2), 3.78-4.51 (4H, CHdCCH2, (Cd
O)OCH2), 6.12-6.40 (1H, CHdCCH2), 7.10-7.48 (1H, Ar),
7.86-8.19 (1H, Ar), 8.19-8.44 (1H, Ar), 8.44-8.75 (1H, Ar),
8.75-9.24 (1H, NH).
2964, 1724, 1641, 1541, 1275, 1018, 729 cm-1 1H NMR
.
(CDCl3): δ 0.86 (t, 3H, J ) 8.79 Hz), 1.00 (d, 3H, J ) 6.84
Hz), 1.27 (m, 1H), 1.49 (m, 1H), 1.85 (m, 1H), 2.27 (d, 1H, J )
2.44 Hz), 4.19 (d, 2H, J ) 5.85 Hz), 4.24 (d, 2H, J ) 2.44 Hz),
6.74 (s, 1H), 7.85 (d, 2H, J ) 8.30 Hz), 8.70 (d, 2H, J ) 8.30
Hz). 13C NMR (CDCl3): δ 11.21, 16.43, 26.07, 34.21, 69.92,
71.88, 72.06, 79.16, 127.04, 127.13, 129.69, 129.78, 133.27,
137.44, 165.79, 166.30. Anal. Calcd for C16H19NO3: C, 70.33;
H, 6.96; N, 5.13. Found: C, 70.17; H, 6.98; N, 5.15.
Poly(6): IR (CHCl3): 3346, 3019, 1715, 1634, 1539, 1217,
787 cm-1. 1H NMR (CDCl3): δ 0.68-0.95 (3H, CH2CH3), 0.95-
1.38 (11H, CH(CH3)CH2CH2CH2CH2CH2CH3), 1.38-1.52
(1H, CH(CH3)CH2CH2CH2CH2CH2CH3), 1.52-1.78 (1H, CH-
(CH3)CH2CH2CH2CH2CH2CH3), 3.81-4.62 (2H, CHdCCH2),
4.82-5.18 (1H, CH(CH3)CH2), 5.98-6.40 (1H, CHdCCH2),
7.30-7.58 (1H, Ar), 7.96-8.19 (1H, Ar), 8.19-8.42 (1H, Ar),
8.42-8.76 (1H, Ar), 8.76-9.18 (1H, NH).
Mea su r em en ts. Molecular weights and molecular weight
distributions of polymers were estimated by GPC (Shodex KF-
850L columns) calibrated by using standard polystyrenes in
chloroform solution. NMR spectra were recorded on a J EOL
EX-400 spectrometer. IR spectra were obtained with a Shi-
madzu FTIR-8100 spectrophotometer. UV-vis spectra were
recorded on a J ASCO V-500 spectrophotometer. Optical rota-
tion was measured with a J ASCO 600 spectropolarimeter. CD
spectra were recorded on a J ASCO V-530 spectropolarimeter.
4: yield 15%; mp 75 °C; [R]D ) +25.7°. IR (KBr): 3277, 2974,
1718, 1638, 1541, 1265, 733 cm-1. 1H NMR (CDCl3): δ 0.87 (t,
3H, J ) 7.19 Hz), 1.24 (d, 3H, J ) 6.39 Hz), 1.62 (m, 2H), 2.20
(d, 1H, J ) 2.44 Hz), 4.18 (d, 2H, J ) 2.44 Hz), 6.42 (s, 1H),
7.43 (t, 1H, J ) 7.59 Hz), 7.93 (d, 1H, J ) 7.59 Hz), 8.08 (d,
1H, J ) 7.59 Hz), 8.29 (s, 1H). 13C NMR: δ 10.56, 20.75, 29.44,
30.09, 72.43, 73.83, 79.43, 128.50, 129.72, 131.68, 132.24,
133.45, 134.58, 166.36, 155.73. Anal. Calcd for C15H17NO3: C,
69.50; H, 6.56; N, 4.75. Found: C, 69.56; H, 5.35; N, 6.64.
5: yield 35%; mp 38 °C; [R]D ) -82.5°. IR (KBr): 3290, 2957,
1716, 1649, 1458, 1259, 731 cm-1 1H NMR (CDCl3): δ 0.74
.
(d, 3H, J ) 6.84 Hz), 0.87 (t, 6H, J ) 6.84 Hz), 1.07 (q, 2H, J
) 11.72 Hz), 1.21 (d, 1H, J ) 7.33 Hz), 1.50 (t, 2H, J ) 11.71
Hz), 1.68 (m, 2H), 1.88 (m, 1H), 2.00 (m, 1H), 2.25 (d, H, J )
2.42 Hz), 4.23 (d, 2H, J ) 2.44 Hz), 4.91 (dt, 1H, J ) 4.39,
10.75 Hz), 6.47 (s, 1H), 7.49 (t, 1H, J ) 7.81 Hz), 7.98 (d, 1H,
J ) 7.81 Hz), 8.13 (d, 1H, J ) 7.81 Hz), 8.35 (s, 1H). 13C
NMR: δ 16.35, 20.71, 23.45, 26.45, 29.86, 31.44, 34.19, 40.82,
47.13, 71.98, 72.17, 79.213, 127.62, 128.83, 131.24, 131.86,
132.63, 133.97, 165.32, 166.17. Anal. Calcd for C21H27NO3: C,
73.90; H, 7.92; N, 4.11. Found: C, 73.72; H, 7.85; N, 3.95.
6: yield 32%; mp 25 °C; [R]D ) +30.3°. IR (KBr): 3361, 2932,
Resu lts a n d Discu ssion
P olym er Syn th esis. Polymerization was performed
using a rhodium catalyst, (nbd)Rh+[(C6H5)B-(C6H5)3],
which is effective for the polymerization of N-propar-
gylamides to afford stereoregular, cis-transoidal poly-
mers.5,6 The polymerization results of 1-6 which have
chiral ester groups on the phenyl ring are summarized
in Table 1. Polymers with moderate molecular weights
(Mn ) 34 000-100 000) were obtained in good yield.
Poly(N-propargylbenzamide), poly(7), did not dissolve
in common organic solvents such as toluene, CHCl3 and
THF. On the other hand, other polymers, poly(1)-poly-
(6), were soluble in organic solvents.
1
2127, 1716, 1651, 1537, 1261, 785 cm-1. H NMR (CDCl3): δ
0.82 (t, 3H, J ) 6.40 Hz), 1.13 (m, 11H), 1.56 (m, 1H), 1.70
(m, 1H), 2.32 (s, 1H), 4.24 (d, 2H, J ) 2.44 Hz), 5.13 (q, 1H, J
) 6.01 Hz), 6.84 (s, 1H), 7.49 (t, 1H, J ) 7.81 Hz), 8.01 (d, 1H,
J ) 7.81 Hz), 8.14 (d, 1H, J ) 7.81 Hz), 8.63 (s, 1H). 13C NMR
(CDCl3): δ 13.96, 19.94, 20.93, 22.48, 25.32, 29.03, 31.61, 35.89,
71.75, 72.28, 79.25, 127.68, 128.66, 131.19, 131.60, 132.58,
133.93, 165.35, 166.23. Anal. Calcd for C19H25NO3: C, 72.38;
H, 7.94; N, 4.44. Found: C, 72.66; H, 7.91; N, 4.38.
The stereoregularity of the polymers was examined
1
by H NMR spectroscopy. Poly(1)-poly(6) exhibited a
well-resolved signal at around 5.8 ppm, which can be

Macromolecules, Vol. 36, No. 3, 2003
Poly(N-propargylbenzamides) 575
a
Ta ble 1. P olym er iza tion of 1-6
Ta ble 2. IR Sp ectr a l Da ta of P oly(1)-P oly(6) in CHCl3
monomer
polymer
amide I (cm-1
)
amide II (cm-1
)
yield
(%)a
cis
b
b
[R]D (deg)
Mn
Mw/Mn
(%)c [R]D (deg)d
poly(1)
poly(2)
poly(3)
poly(4)
poly(5)
poly(6)
1647
1632
1637
1632
1635
1634
1537
1545
1541
1543
1541
1539
1
2
3
4
5
6
(+4.18)
(+4.61)
(+5.24)
(+25.7)
(-82.5)
(+30.3)
67
57
63
63
70
60
34 000
100 000
38 000
51 000
98 000
51 000
4.78
4.67
6.74
4.04
6.99
5.19
100
100
91
100
100
95
-29.4
+365.0
-51.1
+573.0
+380.0
+586.0
a
c ) 48 mM.
a
b
Methanol-insoluble part. Estimated by GPC (CHCl3, PSt
standards). c Determined by 1H NMR. c ) 0.0880-0.108 g/dL in
CHCl3.
d
to a conclusion that the main chain of poly(2) exists in
a helical conformation with an excess of one-handed
screw sense while poly(1) and poly(3) take a random
conformation. Therefore, introduction of a chiral ester
group at the meta position of the phenyl ring is suitable
to induce the helical structure.
Next, the CD spectra of poly(2), poly(4), and poly(5)
which have various chiral ester groups at the meta
position were compared. These polymers showed very
intense CD effects and large [R]D in CHCl3 at room
temperature, indicating that they have helical confor-
mation with a biased screw sense in solution. The
magnitude of the Cotton effects of poly(5) and poly(6)
was larger than that of poly(2). Thus, when the asym-
metric carbon is located closely to the main chain or
when the pendant group is bulky, the screw sense is
more effectively biased.
F igu r e 1. CD spectra of poly(1)-poly(6) in CHCl3 at room
temperature.
To investigate the nature of the hydrogen bonding of
the amide groups, IR spectra of poly(1)-poly(6) were
measured in CHCl3 (Table 2). Because the amide I band
of poly(1) (1647 cm-1) stayed at the high-frequency
region compared to other polymers, its amide group was
not effectively hydrogen-bonded. However, the amide II
band of poly(1) (1537 cm-1) was located in the hydrogen-
bonded region. These results indicate that the carbonyl
moiety of the ester group and N-H moiety of the amide
group form the cyclic hydrogen bond and that this cyclic
hydrogen bond hinders the intramolecular hydrogen
bond between the pendant amide groups.9 Thus, poly-
(1) cannot form the helical structure. Because the amide
I band of poly(3) (1637 cm-1) was observed at almost
the same position as the other polymers, the absence of
the helical conformation in poly(3) is not due to the lack
of the intramolecular hydrogen bond. Probably, the
rigidity of polymer backbone is not enough to take the
helical conformation.
Th er m a lly Dr iven Helix-Sen se In ver sion . To
examine the thermal stability of the helical conforma-
tion, we measured CD spectra of poly(2), poly(4), poly-
(5), and poly(6) in CHCl3 or toluene at various temper-
atures. The CD spectra of the polymers with long alkyl
chain on the phenyl ring such as poly(2) and poly(6)
showed increased molar ellipticity by lowering temper-
ature. However, the shape did not change (Figure 3).
On the other hand, the shape of the CD effects of poly-
(4) and poly(5) remarkably changed with changing
temperature (Figure 4). When the temperature was
lowered, for example, the Cotton effects of poly(4)
decreased in intensity, and the sign of the Cotton effect
was inverted reversibly at around -5 °C (Figure 4a).
The CD spectra at 55 °C was almost mirror-imaged to
that at -40 °C except for a slight shift of the absorption.
This means that the helix-sense inversion occurs by
lowering temperature. Because the screw sense of poly-
(4) at 55 and -40 °C was opposite to each other, these
two helical conformations are diastereomers. This is the
reason for the shift of the CD absorption wavelength of
poly(4) upon the change in temperature. On the other
F igu r e 2. UV spectra of poly(1)-poly(3) in CHCl3 at room
temperature.
assigned to the olefinic protons of the cis-transoidal
main chain. The cis contents were estimated to be 91-
100% by the integrated intensities of the 1H NMR
signals. The peaks of the protons close to the main chain
are very broad for poly(N-propargylalkylamides) due to
the limited main-chain mobility.5 On the other hand,
these poly(N-propargylbenzamides) displayed clear sig-
nals, indicating their main-chain flexibility.
Secon d a r y Con for m a tion . The CD and UV-vis
spectra of poly(1)-poly(6) are shown in Figures 1 and
2, respectively. To find the optimal structure to induce
the helical structure, the CD spectra of poly(1)-poly-
(3), which have the same chiral ester group at different
substitution positions, were compared. Poly(2) showed
a large [R]D (Table 1) and intense CD effect in the region
of the main-chain absorption in CHCl3 at room temper-
ature. On the other hand, poly(1) did not show a clear
CD, and poly(3) displayed only a weak CD (Figure 1).
We have reported that the absorptions of poly(N-
propargylalkylamides) at around 320 and 390 nm cor-
respond to the disordered and helical conformations,
respectively.6 Poly(2) showed the absorption centered
at 380 nm, while poly(1) and poly(3) displayed absorp-
tion maxima at 320 nm (Figure 2). These results lead

576 Tabei et al.
Macromolecules, Vol. 36, No. 3, 2003
F igu r e 3. Variable temperature CD spectra of (a) poly(2) and
(b) poly(6) in CHCl3.
F igu r e 5. Variable temperature CD spectra of poly(5-co-7)
(a) (5/7 ) 95/5) (b) (5/7 ) 78/22) in toluene.
F igu r e 6. (a) CD and (b) UV spectra of poly(5) in toluene. (1)
J ust after the thermally induced helix inversion, (2) after the
TFA treatment, and (3) after the reconstruction of the original
helical conformation.
F igu r e 4. Variable temperature CD spectra of (a) poly(4) in
CHCl3 and (b) poly(5) in toluene.
hand, the helix-sense inversion of poly(5) inverted upon
heating. As shown in Figure 4, the CD spectra of poly-
(5) decreased in intensity upon heating, and the helix-
sense transition occurred at 43 °C. Such a helix-helix
transition occurs as a result of the changes in enthalpy
and entropy terms in Gibbs free energy equation being
the same sign. Thus, the free energy difference between
the right- and left-handed conformations changes in sign
as a function of temperature.10 J ulian et al. reported
that a polysilane having chiral pendant groups under-
goes a thermally induced helix-helix transition. Further-
more, they have shown that the helix-helix transition
temperature can be controlled by the copolymerization
with an achiral comonomer.11 Such control of the helix-
sense transition was also possible by the copolymeriza-
tion of 5 with an achiral monomer 7. For example, the
transition temperatures of the homo- and copolymers
with a ratio of 5:7 ) 100:0, 95:5, and 78:22 were 43, 32,
and 5 °C, respectively (cf. Figure 5).12 When the ratio
of 7 was increased by 5%, the transition temperature
decreased by about 10 °C. From these results, we can
conclude that when the bulkiness of side group is
reduced, the transition temperature decreases. In other
words, the helix-helix transition temperature can be
adjusted by the composition ratio of the copolymer.
It is very interesting that, in contrast to poly(4), the
helix transition of poly(5) was irreversible. For example,
when a toluene solution of poly(5) was heated at 70 °C
for 30 min, the initially positive Cotton effect changed
to a negative one (Figure 6a-1). However, this negatively
signed CD signal did not revert to the positively signed
one even when the solution was cooled to 20 °C. Only a
few percent of the original spectrum was restored when
the solution was kept 20 °C for 24 h. A similar behavior
was also observed for poly(5-co-7) (5/7 ) 95/5). Because
poly(5) may be applied as a data storage material, we
tried to restore the original helix-sense conformation of
helix-inverted polymer according to the cycle described
in Scheme 2. In an attempt to deform helical conforma-
tion of helix-inverted poly(5), the polymer solution was
poured into methanol, which can destroy the helical
conformation of poly(N-propargylalkylamides). How-

Macromolecules, Vol. 36, No. 3, 2003
Poly(N-propargylbenzamides) 577
Sch em e 2
tion temperature can be adjusted by the composition of
copolymer of 5 with 7. The original helix-sense could
be restored when the helix with inverted screw sense
was deformed by acid and then dissolved in acid-free
solvent. This characteristic would allow poly(N-propal-
gylbenzamides) to be applied as a new data storage
material.
Refer en ces a n d Notes
(1) Nakano, T.; Okamoto, Y. Chem. Rev. 2001, 101, 4013-4038.
(2) (a) Okamoto, Y.; Susuki, K.; Ohta, K.; Hatada, K.; Yuki, H.
J . Am. Chem. Soc. 1979, 101, 4763-4765. (b) Corley, L. S.;
Vogl, O. Polym. Bull. (Berlin) 1980, 3, 211-217. (c) Kamer,
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6825. (d) Ito, Y.; Ihara, E.; Murakami, Y.; Sisido, M. Macro-
molecules 1992, 25, 6810-6813.
ever, the CD and UV spectrum of the resulting polymer
in toluene was almost the same as the spectrum of helix-
inverted polymer (Figure 6-1). This means that the
treatment with methanol could not destroy the helical
structure. This is in contrast to the previous results
about poly(N-propargylalkylamides); e.g., the helical
conformation of poly(N-propargyl-3-methylbutanamide)
is readily deformed by treating with methanol. Defor-
mation of poly(5) was achieved by adding a few drops
of trifluoroacetic acid (TFA). The CD and UV spectrum
of this TFA-treated sample are shown in Figure 6-2. The
CD effect disappeared, and the absorption shifted to 320
nm, which indicates that the helical conformation was
completely deformed. When the TFA-treated sample
was purified by reprecipitation and dissolved in toluene
again (Figure 6-3), it showed the CD effect identical in
shape and intensity to the original one. This means that
the original conformation is recovered by this acid-
catalyzed deformation process (Scheme 2).13
Con clu sion . Poly(N-propargylbenzamides) having
chiral ester groups at the meta position have proven to
exist in the helical conformation. When the chiral center
is located near the benzene ring or when the chiral ester
group is bulky, the screw sense of helix is more ef-
fectively biased. Some of poly(N-propargylphenyla-
mides) such as poly(4) and poly(5) undergo helix-sense
inversion driven by the change in temperature. The
helix-sense inversion of poly(4) was reversible, whereas
that of poly(5) was irreversible. The helix-helix transi-
(3) (a) Fujiki, M. J . Am. Chem. Soc. 1994, 116, 11976-11981.
(b) Green, M. M.; Andreola, C.; Mun˜oz, B.; Reidy, M. P.; Zero,
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(5) Nomura, R.; Tabei, J .; Masuda, T. J . Am. Chem. Soc. 2001,
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(6) Nomura, R.; Tabei, J .; Masuda, T. Macromolecules 2002, 35,
2955-2961.
(7) Nomura, R.; Yamada, K.; Masuda, T. Chem. Commun. 2002,
478-479.
(8) (a) Schrock, R. R.; Osbron, J . Inorg. Chem. 1970, 9, 2339-
2343. (b) Kishimoto, Y.; Itou, M.; Miyatake, T.; Ikariya, T.;
Noyori, R. Macromolecules 1995, 28, 6662-6666.
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(10) (a) Maeda, K.; Okamoto, Y. Macromolecules 1998, 31, 5164-
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(11) (a) J ulian, R. K.; Fujiki, M.; Motonaga, M.; Nakashima, H.
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(12) The molecular weights of poly(5-co-7) (5/7 ) 90/10 and 80/20
in feed) were 60 000 (Mw/Mn ) 2.96) and 45 000 (Mw/Mn
4.03), respectively.
)
(13) The molecular weight of poly(5) did not change, and isomer-
ization or degradation did not occur during the acid treatment.
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