Helical conjugated polymers: Synthesis, stability, and chiroptical properties of poly(alkyl phenylpropiolate)s bearing stereogenic pendants

Article abstract of DOI:10.1021/ma048960j

Chiral poly(alkyl phenylpropiolate)s -{(C₆H ₅)C=C[CO₂(CH₂)₂OCOR*]} _n - with R* = (S)(+)-[1-(6-methoxy-2-naphthyl)ethyl (P1), (1R,2S,5A)-(-)-menthoxymethyl (P2), (S)-(+)-(α-acetoxy)benzyl (P

Full text of DOI:10.1021/ma048960j

Macromolecules 2004, 37, 6695-6704
6695
Helical Conjugated Polymers: Synthesis, Stability, and Chiroptical
Properties of Poly(alkyl phenylpropiolate)s Bearing Stereogenic
Pendants
J a ck y W. Y. La m , Yu p in g Don g, Kevin K. L. Ch eu k , Ch a r les C. W. La w ,
Lo Min g La i, a n d Ben Zh on g Ta n g*
Department of Chemistry and Open Laboratory of Chirotechnology, The Hong Kong University of
Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China
Received May 26, 2004; Revised Manuscript Received J uly 7, 2004
ABSTRACT: Chiral poly(alkyl phenylpropiolate)s -{(C6H5)CdC[CO2(CH2)2OCOR*]}n- with R* ) (S)-
(+)-[1-(6-methoxy-2-naphthyl)ethyl (P1), (1R,2S,5R)-(-)-menthoxymethyl (P2), (S)-(+)-(R-acetoxy)benzyl
(P3), and cholesteryloxy (P4) were synthesized, and their structures and properties were investigated.
The monomers [C6H5CtCCO2(CH2)2OCOR*; 1-4] were prepared by esterifications of stereogenic acids
8-10 or chloroformate (11) with 3-hydroxyethyl phenylpropiolate (7) in high yields. Polymerizations of
1-4 were effected by MoCl5-Ph4Sn at 60 or 80 °C, and polymers with high molecular weights (Mw up to
∼100 × 103) were obtained in moderate yields. The polymers were characterized by IR, NMR, TGA, UV,
and CD analyses. All of the polymers are stable, losing little of their weights when heated to g300 °C
and undergoing no chain scissions when annealed in air at g150 °C. The macromolecular chains take
helical conformations with preferred handedness, and their helical chirality can be reversibly tuned by
solvent or temperature to varying extents.
In tr od u ction
lenes will behave in a similar manner; that is, a helical
disubstituted polyacetylene will be more stable than its
monosubstituted counterpart.
Polyacetylene is the best-known conjugated polymer,
whose doped form shows high conductivity that stands
comparison with that of copper.1 Its notorious intrac-
tability and instability have, however, greatly detracted
from its potential for technological applications. Attach-
ments of appropriate pendants to the polyacetylene
backbone can not only help improve the processability
and stability but also generate new polyacetylene
derivatives with functional properties that are absent
in the parent form.2,3 An example in this regard is the
incorporation of stereogenic groups into polyacetylene
chains. Enthusiastic synthetic effort in the area has
resulted in the creation of a large variety of helical
substituted polyacetylenes, most of which are soluble,
some of which are stable, and all of which are optically
active, although the optical activity varies considerably
from one polymer to another.4-6
Most of the optically active polyacetylenes bear one
substituent in each of their monomer repeat units (and
are hence monosubstituted), with an overwhelming
majority of them being poly(phenylacetylene) and
polypropargyl derivatives.4-6 While the monosubstituted
polyacetylenes are more stable than their unsubstituted
polyacetylene parent, their stabilities are still of concern
for many practical applications.7,8 One way to further
enhance the polymer stability is to add one more
substituent into the repeat unit to make a disubstituted
polyacetylene, which is generally more stable than its
monosubstituted homologue.3a,9 For example, poly(1-
chloro-2-phenylacetylene), an achiral disubstituted poly-
acetylene, does not degrade when heated in air at 120
°C for 20 h, whereas poly(phenylacetylene) readily
decomposes when treated under the same conditions.9
It is envisioned that the chiral substituted polyacety-
Helical disubstituted polyacetylenes have, however,
been seldom prepared.10 The rareness of such polymers
is mainly due to the synthetic difficulty originating from
the lack of efficient polymerization systems for disub-
stituted acetylenes containing functional groups. While
TaCl5 and NbCl5 are the most widely used catalysts
for the polymerizations of nonfunctional or nonpolar
disubstituted acetylenes such as 1-phenyl-1-alkynes,2
they are intolerant of functional groups. Indeed, the
transition-metal halides are so incompatible to polar
moieties that they fail to initiate the polymerization of
a disubstituted acetylene containing even an ester unit,
a functional group often found in common organic
molecules.3a,11,12
In our previous work, we succeeded in polymerizing
a group of chiral disubstituted acetylenes bearing ester
functionality, i.e., phenylpropiolates (C6H5CtCCO2R*),
using an inexpensive WCl6-Ph4Sn mixture, a “classic”
metathesis catalyst.11,13,14 Our success may offer access
to a potentially large family of helical disubstituted
polyacetylenes because a great number of stereogenic
propiolates can be facilely prepared by simple esterifi-
cation reactions of propiolic acid, a commercially avail-
able compound, with chiral alcohols, which are syn-
thetically ubiquitous and naturally abundant. Indeed,
with ease, we have already prepared two groups of
propiolates by attaching naturally occurring sterols to
phenylpropiolate directly (Scheme 1, eq 1) or via an
aromatic ring (eq 2) and effected their polymerizations
using the W catalyst in our previous studies.11,15
To further demonstrate the versatility of the propi-
olate system and to explore its utility potential in the
synthesis of helical disubstituted polyacetylenes, in this
work, we attached stereogenic units to phenyl-propiolate
via a short aliphatic chain. Although the alkyl phenyl-
propiolates can be prepared through the route shown
* To whom correspondence should be addressed: phone +852-
2358-7375; Fax +852-2358-1594; e-mail tangbenz@ust.hk.
10.1021/ma048960j CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/13/2004

6696 Lam et al.
Macromolecules, Vol. 37, No. 18, 2004
Sch em e 1
Sch em e 2
Ta ble 1. P olym er iza tion of 2-{(S)-(+)-
[1-(6-Meth oxy-2-n a p h th yl)eth yl]ca r bon yloxy}eth yl
P h en ylp r op iola te (1)a
Ch a r t 1
temp yield
b
b
no.
catalyst
solvent
(°C)
(%)
Mw
Mw/Mn
c
1
2
3
4
5
[Rh(nbd)Cl]2
CH3CN
toluene
40
60
rtd
60
80
0
0
0
WCl6-Ph4Sn
MoCl5-Ph4Sn toluene
MoCl5-Ph4Sn toluene
MoCl5-Ph4Sn toluene
57.6
trace
74 200
2.7
in eq 3 (Scheme 1), we took a different synthetic
approach, as given in eqs 4-6 (Scheme 2). This synthetic
path is more “economic” because it subjects the more
expensive chiral compounds to just single steps of
reactions. The syntheses were straightforward, and the
products 1-4 were obtained in high yields. The mono-
mers were, however, not polymerizable by the W
catalyst, indicating that the catalytic process is substrate-
sensitive. In this report we tell how the new monomers
can be polymerized and what properties of the resultant
poly(alkyl phenylpropiolate)s (P1-P4; Chart 1) exhibit.
a
Carried out under nitrogen for 24 h; [M]0 ) 0.2 M, [cat.] )
b
[cocat.] ) 10 mM; for [Rh(nbd)Cl]2, [cat.] ) 0.2 mM. Determined
by GPC in THF on the basis of a polystyrene calibration. c nbd )
d
2,5-norborndiene. Room temperature (∼23 °C).
are white solids, 2 and 3 are pale yellow liquids. All of
the monomers were characterized by spectroscopic
methods, from which satisfactory analysis data were
obtained (see Experimental Section for details).
To polymerize the monomers, we tried different
catalyst systems. We first attempted to polymerize 1
using [Rh(nbd)Cl]2, a good catalyst for polymerization
of monosubstituted propiolates (HCtCCO2R).16 Stirring
a mixture of [Rh(nbd)Cl]2 and 1 in acetonitrile at 40 °C
for 24 h, however, yielded no polymeric product (Table
1, no. 1). Clearly the rhodium complex is incapable of
initiating the polymerization of 1, a disubstituted pro-
piolate. WCl6-Ph4Sn could not polymerize 1 either,
which is striking because the W mixture can effectively
initiate polymerizations of aryl phenylpropiolates.11
MoCl5-Ph4Sn, delightfully, polymerized 1 at 60 °C into
a polymer with a high molecular weight (∼74 × 103) in
a good yield (∼58%; Table 1, no. 4). No or trace amount
of polymer product was obtained when the reaction was
carried out at a lower (rt) or higher temperature (80 °C;
Table 1, nos. 3 and 5), indicative of thermally sensitive
nature of the propiolate polymerization.
Resu lts a n d Discu ssion
P olym er Syn th esis. The stereogenic alkyl phenyl-
propiolates 1-4 are, as discussed above, prepared by
the molecularly economic synthetic routes shown in
Scheme 2. Monomers 1-3 are synthesized by esterifi-
cation of phenylpropiolic acid (5) with ethylene glycol
(6) in dichloromethane (DCM) at room temperature (eq
4) followed by a successive esterification of 2-hydroxy-
ethyl phenylpropiolate (7) with stereogenic acids 8-10
in the presence of 1,3-dicyclohexylcarbodiimine (DCC),
p-toluenesulfonic acid (TsOH), and 4-(dimethylamino)-
pyridine (DMAP). Taking advantage of the commercial
availability of cholesteryl chloroformate (11), monomer
4 is prepared by esterification of 11 with 7 in the
presence of pyridine (eq 6). The reactions went smoothly,
and the desirable monomers were isolated in high yields
(∼84-97%) after column purifications. While 1 and 4
We then tried to polymerize monomers 2-4, and the
results are summarized in Table 2. None of the mono-

Macromolecules, Vol. 37, No. 18, 2004
Helical Poly(phenylpropiolate)s 6697
Ch a r t 2
Ta ble 2. P olym er iza tion s of Eth yl P h en ylp r op iola tes
the main structural difference between these two sets
of monomers are the groups attached to the propiolate
oxygen atoms (marked in color): ethyl chains in 1-4
vs phenyl rings in 13-15. It is amazing that such a
small structural difference has caused such a big
difference in the polymerizability of the monomers by
the different catalysts. Is this due to steric or electronic
effects? To get a clue to the question, we compared
the polymerizabilities of some propiolate monomers
with similar molecular structures. Monomer 12, which
possesses a hexyl chain, is polymerizable by the Mo
catalyst, whereas monomer 16, which possesses a naph-
thyl ring, is polymerizable by the W catalyst.17 This set
of data further proves the general trend that the Mo
and W mixtures are effective catalysts for the alkyl and
aryl phenylpropiolates, respectively, but still does not
provide a clear answer to the question raised above.
(2-4)a
b
b
no.
catalyst
temp (°C) yield (%)
Mw
Mw/Mn
2-[(1R,2S,5R)-(-)-Menthoxymethylcarbonyloxy]ethyl
Phenylpropiolate (2)
1
2
3
WCl6-Ph4Sn
MoCl5-Ph4Sn
MoCl5-Ph4Sn
60
60
80
trace
trace
17.8
60 900
2.0
2-{(S)-(+)-[(R-Acetoxy)benzyl]carbonyloxy}ethyl
Phenylpropiolate (3)
4
5
6
WCl6-Ph4Sn
MoCl5-Ph4Sn
MoCl5-Ph4Sn
80
60
80
0
trace
34.3
15 400c
1.5
3.2
2-(Cholesteryloxycarbonyl)ethyl Phenylpropiolate (4)
WCl6-Ph4Sn
MoCl5-Ph4Sn
7
8
80
80
trace
6.2
96 400
a
Carried out under nitrogen in toluene for 24 h; [M]0 ) 0.2 M,
b
[cat.] ) [cocat.] ) 10 mM. Determined by GPC in THF on the
basis of a polystyrene calibration. c Soluble fraction (completely
soluble in chloroform but only partially soluble in THF).
Monomer 17, however, can be polymerized by the W
catalyst18 but not the Mo mixture. Since the borneyl
group of 17 is an aliphatic but not aromatic ring, the
electronic effect can thus be excluded. The major
structural difference between monomer 17 and mono-
mers 1-4 as well as 12 is the bulkiness of the alkyl
groups. It can thus be concluded that the steric
effect has played an important role in determining the
polymerizabilities of the phenylpropiolate monomers. In
other words, the Mo and W mixtures are capable of
polymerizing the sterically less (1-4 and 12) and more
demanding phenylpropiolates 13-17, respectively. It is
noteworthy that the former catalyst cannot polymerize
the latter monomers, and vice versa, revealing the
stereospecific nature of the phenylpropiolate polymer-
izations.
mers could be polymerized by WCl6-Ph4Sn. The poly-
merizations of 2-4 catalyzed by MoCl5-Ph4Sn were
again temperature-sensitive, but the best results for the
polymerizations of these monomers were obtained at the
highest temperature we examined, that is, 80 °C. The
polymers of 2 and 4 as well as 1 are completely soluble
in THF, but that of 3 is only partially soluble in THF,
though it is completely soluble in halogenated solvents
such as chloroform and DCM. The characterization of
this polymer was thus conducted in the halogenated
solvents.
It becomes clear that WCl6-Ph4Sn is ineffective in
polymerizing the alkyl phenylpropiolates 1-4. In our
previous work, however, we have found that this
mixture works well for the polymerizations of the aryl
phenylpropiolates containing similar chiral units R*
(13-15).11 As can be easily recognized from Chart 2,
Str u ctu r a l Ch a r a cter iza tion . The polymeric prod-
ucts were characterized by spectroscopic methods, and
all the polymers gave satisfactory analysis data corre-
sponding to their expected molecular structures (see

6698 Lam et al.
Macromolecules, Vol. 37, No. 18, 2004
F igu r e 1. IR spectra of (A) monomer 1 and (B) its polymer
P1.
Experimental Section for details). An example of the IR
spectrum of P1 is shown in Figure 1, the spectrum of
whose monomer (1) is also given in the same figure for
comparison. The monomer shows absorption bands at
2232 and 2209 cm-1 associated with CtC stretching.
The vibration bands are strong due to the electronic
communication between the propiolate ester group and
the acetylene triple bond.19 No CtC stretching bands
are observed in the spectrum of P1. On the other hand,
a new band associated with CdC stretching of the
polyene backbone appears at 1573 cm-1, proving that
P1 is formed by the conversion of the acetylenic triple
bonds to the olefinic double bonds.
F igu r e 2. 1H NMR spectra of chloroform solutions of (A) 1
and (B) its polymer P1. The solvent peak is marked with an
asterisk (*).
1
Figure 2 shows the H NMR spectra of polymer P1
and its monomer 1 in chloroform. The protons of the
phenyl ring linked to the acetylene triple bond of 1
resonate at δ 7.43 and 7.37 (a; Figure 2A), which
disappear in the spectrum of P1 (Figure 2B). The
transformation of the acetylenic triple bond to the
olefinic double bond upfield shifts the absorptions of the
phenyl protons, whose resonance now occurs at δ 6.76
(a; Figure 2B).19 Interestingly, the resonance peaks of
the ethyl protons (b, c, e, and d) also upfield shift upon
polymerization, whereas those of the naphthyl (f) and
methoxy protons (g) slightly downfield shift. No unex-
pected signals are observed in the spectrum of the
polymer, and all the peaks can be readily assigned to
the resonance of appropriate protons as marked in
Figure 2B. The NMR analyses thus confirm that the
triple bonds have been consumed by the acetylene
polymerization and that the molecular structure of the
polymer is indeed P1, as shown in Chart 1.
Figure 3 shows the 13C NMR spectrum of P1 along
with that of its monomer 1. While the acetylene carbon
atoms of 1 resonate at δ 86.7 and 80.1, these peaks are
completely absent in the spectrum of its polymer (P1).
Instead, two new peaks appear at δ 150.8 and 140.4.
The olefinic carbons of poly(methyl propiolate) {-[HCd
C(CO2CH3)]n-} is known to resonate at δ 134 and 128.16
Since P1 can be viewed as a disubstituted congener of
poly(methyl propiolate), the new peaks at δ 150.8 and
140.4 can thus be assigned to the resonance of polyene
carbons of P1. This once again proves that the propiolate
polymerization is realized through the transformation
of the triple bonds to the double bonds. Similar to what
observed in the polymerizations of aryl propiolates 13-
15,11 the polymerization of 1 also causes a downfield
shift of the resonance peak of the carbonyl carbon
F igu r e 3. 13C NMR spectra of chloroform solutions of (A) 1
and (B) its polymer P1. The solvent peaks are marked with
asterisks (*).
directly linked to the acetylene triple bond (from δ 153.3
to 165.5).16
Th er m a l Sta bility. (Unsubstituted) polyacetylene
readily decomposes upon exposure to air at room tem-
perature because of its high reactivity with oxygenic
species.1-3 Polymers P1-P4 are polyacetylene deriva-

Macromolecules, Vol. 37, No. 18, 2004
Helical Poly(phenylpropiolate)s 6699
F igu r e 4. TGA thermograms of polymers P1-P4 recorded
under nitrogen at a heating rate of 20 °C/min.
F igu r e 5. UV spectra of poly(alkyl phenylpropiolate)s P1-
P4 in chloroform at room temperature.
Ta ble 3. Th er m olysis Resista n ce of P oly(2-{(S)-(+)-
[1-(6-Meth oxy-2-n a p h th yl)eth yl]ca r bon yloxy}eth yl
P h en ylp r op iola te) (P 1)a
of P1-P 4 because none of their monomers possesses a
chromophoric group that absorbs at wavelengths longer
than ∼300 nm.11,19 The λmax and ꢀmax values of the
backbone absorptions of the polymers suggest that their
effective conjugation lengths are short, being just a few
monomer repeat units, due to the steric effects of their
pendant groups (there exist two substituents in one
monomer repeat unit). This offers a circumstantial
evidence for their low reactivity and high resistance to
thermolytic and oxidative decompositions.
b
b
no.
temp (°C)
Mw
Mw/Mn
1c
2
3
4
5
∼23d
74 200
78 000
74 000
78 700
insoluble
2.7
2.3
2.4
2.5
80
100
150
200
a
b
Annealed in air at a given temperature for 2 h. Determined
by GPC in THF on the basis of a polystyrene calibration. c Starting
Ch ir op tica l P r op er ties. All of the poly(phenylpro-
piolate)s (P1-P4) are optically active. The specific
optical rotations ([R]20D) of P1-P3 (-544° to +511°) are
almost 1 order of magnitude higher than those of their
monomers (-60.0° to +52.1°) in the same solvents,
suggestive of chirality contribution from the polyene
backbones. In other words, the stereogenic pendants
have induced the polyacetylene chains to helically rotate
d
material. Room temperature.
tives, and it is of interest to learn how stable they are.
GPC analyses of the polymer samples stored under
ambient conditions (in air) for >2 years find no decrease
in their molecular weights, revealing that the polymers
are stable at room temperature. TGA analyses of the
polymers indicate that they are also stable at elevate
temperatures. As can be seen from Figure 4, P1 loses
merely 5% of its weight when heated to a temperature
as high as 357 °C (T5). The T5’s of P2, P3, and P4 are
333, 340, and 297 °C, respectively, which are well
comparable to that of polystyrene (330 °C),20 a stable
commodity polymer.
with an excess of one preferred handedness. The [R]20
D
values of P4 are lower than those of P1-P3, but the
change in its [R]20 with solvent is bigger, indicating
D
that the chain helicity of P4 is more susceptible to
the environmental change. It is, however, noteworthy
that, though the magnitudes of the [R]20D’s of the
polymers vary with solvent, their signs (+ or -) remain
unchanged. This is in contrast to what observed in
the helical monosubstituted polyacetylene systems,
where not only the magnitudes of their [R]20D’s change
vigorously but also their signs reverse with a change
To further examine the stability of the polymers, we
annealed P1 at different temperatures in air for 2 h.
Table 3 lists the molecular weight changes of the
polymer after the thermal treatment. Little change in
in solvent.4-7 For example, the [R]20 values of a
M
w is observed when P1 is annealed at temperature up
D
to 150 °C, while further heating to 200 °C leads to gel
formation. This confirms that the polymer is resistant
to thermal degradation caused by oxidative chain scis-
sion. The high stability of the polymer is possibly due
to the jacket effect of its pendant groups as well as the
electron-withdrawing effect of the ester groups in the
immediate vicinity of the polyene backbone. The former
effect sterically shields the polyene backbone from the
attack of the thermolytic species, and the latter effect
reduces the reactivity of the polymer through the
electronic interaction of the ester groups with the
polyene backbone.
poly(phenylacetylene) bearing l-alanine pendants are
+440.0° and -953.2° in THF and chloroform, respec-
tively (noting the sign inversion).21 A disubstituted
polyacetylene chain is generally more rigid than a
monosubstituted one,22 and it is believed that the chain
stiffness of P1-P4 has helped keep their helical-sense
preference unaltered by the solvent perturbation.
Acetylene polymerizations initiated by Mo- and W-
based catalysts are known to yield stereoirregular
polymers.13 Many research groups have found that
stereoregularity is a prerequisite for monosubstituted
polyacetylenes to take helical chain conformations.2,5,23
The stereoirregular poly(aryl phenylpropiolate)s bearing
stereogenic pendants (P13-P15) prepared in our previ-
ous work using WCl6-Ph4Sn as catalyst were chirop-
tically active,11 suggesting that the conformational
regularity is not necessarily a requirement for a disub-
stituted polyacetylene chain to rotate in a helical
Electr on ic Tr a n sition . The UV spectra of the
chloroform solutions of the polymers are shown in
Figure 5. The polymers exhibit absorption maxima (λmax
)
in the wavelength regions of ∼320-330 and ∼370-390
nm with varying molar absorptivities (ꢀmax). These
absorption peaks must be due to the polyene backbones

6700 Lam et al.
Macromolecules, Vol. 37, No. 18, 2004
F igu r e 6. Circular dichroism spectra of monomer 1 in THF
and its polymer P1 in different solvents. Concentration: 1.04-
1.44 mM.
fashion. Will the stereoirregular poly(alkyl phenylpro-
piolate)s take helical chain conformations? To answer
this question, we investigated the chiroptical properties
of P1-P4 using circular dichroism (CD) spectroscopy.
Figure 6 shows the CD spectra of P1 in different
solvents as well as that of it monomer 1 in THF. The
THF solution of the polymer exhibits strong Cotton
effects at 303 and 340 nm with molar ellipticities ([θ])
of -36 800 and +87 500 deg cm2 dmol-1, respectively.
Since the THF solution of 1 is CD-inactive at wave-
lengths longer than ∼285 nm, the CD bands at 303 and
340 nm must be due to the absorptions of the segments
of the polyene backbone of P1, thus unambiguously
confirming that its polyene chain possesses a helical
conformation with an excess of one-handedness. The
spectral pattern of P1 remains the same when its
solvent is changed from THF to chloroform or toluene,
indicating that the same screw sense dominates in the
chloroform or toluene solution. The Cotton effects of the
polymer are, however, intensified, suggesting that the
preferred handedness of its helical chain further pre-
vails over the opposite one in the chloroform or toluene
solution.
F igu r e 7. (A) Circular dichroism spectra of P2-P4 in
chloroform and (B) solvent dependence of Cotton effects of P2
at 340 nm, P3 at 340 nm, and P4 at 333 nm. Polymer
concentration: 0.76-1.47 mM.
its chain segments is spiraling in the preferred helical
sense in the chloroform solution. The reason for this
is unclear at present but may be associated with
the relative flexibility of the carbonate group in the
pendant, which makes the chirality transcription from
the stereogenic pendant to the polyene backbone a less
efficient process.6e
The CD spectral analyses of P2-P4 verify that all the
polymers, like their cousin P1, possess helical conforma-
tions. The chloroform solution of P2 exhibits a [θ] value
of -48 300 deg cm2 dmol-1 at 340 nm (Figure 7A). This
value is 7.5-fold higher (in absolute term) than that
(+6400) of poly[(1R,2S,5R)-(-)-menthyl propiolate]
(-{HCdC[CO2-(-)-Men]}n-; P18), a monosubstituted
polyacetylene bearing the same (-)-Men pendant and
prepared by a similar Mo catalyst.23 Stereoirregular P2
is prepared by a Mo catalyst, but its absolute [θ] value
is even higher than that of a stereoregular P18 prepared
by a Rh catalyst (+9100).23 Taking into account that the
main structural difference between P2 and P18 is their
substitution mode (di- vs monosubstitution), it can be
concluded that it is the molecular structure rather than
the chain stereoregularity that dictates the helical
chirality of the polypropiolates. Polymer P3 exhibits
even stronger Cotton effects with opposite signs, sug-
gesting that more chain segments of P3 take the
preferred helical conformation with a screw sense
opposite to that of P2. The CD activity of P4 is, however,
weak, suggesting that only a small statistic excess of
The molar ellipticities of P2-P4 also change with
solvent, as depicted in Figure 7B. The variations in [θ]
with solvent for the polymers share some similarities
with those in [R]20 (cf. Table 4). The orders of the [θ]
D
values of P2 and P4 (|[θ]chloroform| < |[θ]THF| < |[θ]toluene|)
are the same as those of the [R]20 values in the same
D
20
20
20
solvents (|[R]D chloroform| < |[R]D THF| < |[R]D toluene|).
The extent of variation is the biggest for P 4, with its
[θ] in toluene (+33 500) being 6.25-fold higher than that
in chloroform. However, in this case again, none of the
polymers (including P1; cf. Figure 6) changes its signs
of Cotton effects with solvent. A similar phenomenon
has been observed in the helical disubstituted poly(aryl
phenylpropiolate) (P13-P15) system.11 The steric crowd-
edness caused by the existence of two substituent groups
in one monomer repeat unit of a disubstituted poly-
acetylene imposes an energy barrier, which is high
enough to endow the polymer with a resistance to the
solvent-induced inversion in the helical-sense prefer-
ence.

Macromolecules, Vol. 37, No. 18, 2004
Ta ble 4. Sp ecific Op tica l Rota tion s of P oly(a lk yl p h en ylp r op iola te)s P 1-P 4 in Differ en t Solven tsa
[R]20 (c, g/dL)
Helical Poly(phenylpropiolate)s 6701
D
solvent
P1
P2
P3
P4
THF
CHCl3
toluene
+511.0 (0.042)
+625.0 (0.052)
+592.1 (0.048)
-544.0 (0.050)
-294.8 (0.046)
-565.1 (0.047)
b
+17.9 (0.058)
+12.6 (0.046)
+114.1 (0.051)
+871.3 (0.048)c
d
a
[R]20 values of THF solutions of monomers (concentration given in the parentheses): for 1: +22.4° (0.058 g/dL); for 2: -60.0° (0.061
D
g/dL); for 3: +52.1° (0.043 g/dL), for 4: -8.6° (0.051 g/dL). Partially soluble. c [R]20 in DCM: +883.0° (0.054 g/dL). Insoluble.
b
d
D
to the temperature close to the boiling point of the
solvent.24,25 When the solution is cooled to room tem-
perature, the original CD spectrum of the polymer is
completely reinstalled, demonstrating that the tuning
of the helical chirality by temperature is fully reversible.
Similarly, the molar ellipticity of P2 can be thermally
manipulated in a reversible fashion (Figure 8B). Re-
markably, although the Cotton effect of this polymer
drops to nearly zero or its chain conformation is almost
randomized when the solution is heated to 90 °C, its
CD spectrum returns to the original shape with similar
peak intensities when the solution is cooled to room
temperature. The helicity reversibility or “memory”
capability is thus a general property for all the helical
polypropiolates including the poly(alkyl phenylpropi-
olate)s synthesized in this work as well as their aryl
congeners prepared in our previous study.11 Careful
scrutiny of the temperature effects, however, reveals
some difference between the two groups of polypropi-
olates. The first Cotton effect of the poly(aryl phenyl-
propiolate) bearing (1R,2S,5R)-(-)-menthyl pendant
(-{(C6H5)CdC[CO2-C6H4-p-OCO-(-)-Men]}n-; P14) at
354 nm is decreased from +88 550 to +56 460 deg cm2
dmol-1 (with a |∆[θ]| of 32 090) when its toluene solution
is heated from 30 to 90 °C.11 For the same extent of
temperature change, P2 shows a 2.5-fold higher ellip-
ticity change (|∆[θ]| ) 80 500), noting that this polymer
bears the same chiral pendant of (-)-Men. The struc-
tural difference between P2 and P14 is that the former
has an alkyl (ethyl) spacer while the latter has an
aromatic (phenyl) ring between the two ester groups.
The internal plasticizing effect of the soft alkyl spacer
makes the polymer chain of P2 somewhat less stiff and
F igu r e 8. Circular dichroism spectra of toluene solutions of
(A) P1 and (B) P2 at different temperatures. Polymer concen-
hence more responsive to the thermal stimulus. On the
other hand, the rigid aromatic ring stiffens the polymer
tration: 1.22 (P1) and 1.16 (P2) mM. The CD spectra of the
chain of P14, making it more resistant to the thermal
polymers heated to 90 °C and then cooled to 30 °C are shown
agitation. The chain stiffening effect of rigid aromatic
rings is further evidenced by a recent work on helical
polycarbodiimides. The disubstituted polycarbodiimides
with rigid, bulky anthryl pendants possess extremely
stable helical conformations, whose chiroptical activities
remain constant even after the polymers have been
annealed in toluene at 80 °C for >34 h.26
in dotted lines and marked with start (*) symbols. Insets:
variations of the molar ellipticity at 340 nm with temperature
in toluene solutions of (A) P1 and (B) P2.
Helicity Tu n in g by Tem p er a tu r e. The preference
of the helical sense of the polypropiolates cannot be
changed by temperature either. However, the magni-
tudes of their molar ellipticities can be tuned to large
extents reversibly by the thermal stimulus. When the
temperature of a toluene solution24 of P1 is increased
from 30 to 40 °C, the intensity of its first Cotton effect
at 340 nm is decreased from +110 200 to +82 100 deg
cm2 dmol-1 (Figure 8A). The molar ellipticity of P1
continuously decreases with a further increase in tem-
perature, probably because a higher temperature causes
more chain segments to undergo conformational ran-
domization, like what happens in the thermally induced
denaturing or unfolding processes of proteins.6d How-
ever, even when the temperature is raised to 90 °C, the
spectral pattern of P1 remains unvaried, and its Cotton
effect is still strong (+32 300), indicating that the
polymer chain is not randomized even when it is heated
Con clu d in g Rem a r k s
Functional disubstituted acetylenes have been dif-
ficult to polymerize.2,3 The polymerizations of the alkyl
phenylpropiolates 1-4 were not easy, but through
catalyst selection and process optimization, we eventu-
ally succeeded in converting the stereogenic monomers
into high molecular weight polymers (P1-P4), adding
a group of new members to the short list of functional
disubstituted polyacetylene family.
Unlike their monosubstituted counterparts, the di-
substituted poly(alkyl phenylpropiolate)s are stable. The
polymers are thermolysis- and oxidation-resistant, los-
ing little of their weights when heated to g300 °C under
nitrogen and suffering from no decrease in their mo-

6702 Lam et al.
Macromolecules, Vol. 37, No. 18, 2004
was cooled to 0-5 °C with an ice-water bath, to which 2.0 g
(13.7 mmol) of 5 dissolved in 50 mL of DCM was added under
stirring via a dropping funnel. The reaction mixture was
stirred overnight. After filtering out the formed urea solid, the
solution was concentrated by a rotary evaporator. The product
was purified by a silica gel column using chloroform/acetone
(10:1 by volume) as the eluent. Colorless liquid of 7 was
isolated in 50.8% yield.
lecular weights when annealed in air at g150 °C.
Compared to the helical monosubstituted poly(phenyl-
acetylene)s, the disubstituted poly(alkyl phenylpropi-
olate)s are conformationally more stable, whose pref-
erence of helical sense is not alterable by such external
perturbations as solvent and temperature.
The bulkiness of the group linked to the propiolate
oxygen atom (marked in red or blue color in Chart 2) is
found to dramatically affect polymerization behavior of
the monomer and chiroptical property of the polymer.
Thus, the monomers with sterically less (1-4 and 12)
and more demanding groups (13-17) can only be
polymerized by one of the Mo and W catalysts, while
the optical activity of the polymers with sterically less
(P1-P4) and more demanding groups (P13-P15) can
be tuned by external stimuli to larger and smaller
extents, respectively. Inspired by the insights gained in
this study, we are currently working on the design and
synthesis of chiral poly(phenylpropiolate)s with longer
chains (e.g., octyl) and bulkier rings (e.g., anthryl) in
an effort to develop helical disubstituted polyacetylenes
with opposite attributes, that is, higher stimuli respon-
siveness and stronger perturbation resistance, respec-
tively.
2-{(S)-(+)-[1-(6-Meth oxy-2-n aph th yl)eth yl]car bon yloxy}-
eth yl P h en ylp r op iola te (1). This monomer was prepared by
esterification of 7 with (S)-(+)-6-methoxy-R-methyl-2-naph-
thaleneacetic acid (8). The synthetic procedure is similar to
that described above for the preparation of 7. White solid of 1
was isolated in 88.1% yield. IR (KBr), ν (cm-1): 2232 and 2209
(vs, CtC), 1726 and 1711 (vs, CdO). 1H NMR (300 MHz,
CDCl3), δ (ppm): 7.54 [m, 3H, Ar-H meta to OCH3 and meta
and ortho to CH(CH3)], 7.43 (m, 2H, Ar-H ortho to CtC), 7.37
[m, 4H, Ar-H para and meta to CtC and ortho to CH(CH3)],
7.05 (m, Ar-H ortho to OCH3), 4.35 [m, 4H, (OCH2)2], 3.85
[m, 1H, CH(CH3)], 3.76 (s, 3H, OCH3), 1.53 [d, 3H, CH(CH3)].
13C NMR (75 MHz, CDCl3), δ (ppm): 174.0 [CO2CH(CH3)],
157.4 (aromatic carbon linked with OCH3), 153.3 (≡CCO2),
135.1, 133.5, 132.7 (aromatic carbons ortho to CtC), 130.5
(aromatic carbon para to CtC), 129.1, 128.7, 128.3 (aromatic
carbons meta to CtC), 127.0, 125.9, 125.7, 119.1 (aromatic
carbon linked with CtC), 118.7, 105.3, 86.7 (ArC≡), 80.1
(≡CCO2), 63.1 [CH2OCOCH(CH3)], 61.8 (≡CCO2CH2), 54.7
(OCH3), 45.0 [CH(CH3)], 18.2 [CH(CH3)]. MS (CI): m/e 403.1
[(M + 1)+, calcd 403.1].
Exp er im en ta l Section
2-[(1R,2S,5R)-(-)-Men th oxym eth ylca r bon yloxy]eth yl
P h en ylp r op iola te (2). Its preparation was similar to that of
1, but (-)-menthoxyacetic acid (9), instead of 8, was used. Pale
yellow liquid of 2 was isolated in 96.8% yield. IR (KBr), ν
(cm-1): 2222 (vs, CtC), 1764 and 1715 (vs, CdO). 1H NMR
(300 MHz, CDCl3), δ (ppm): 7.60 (m, 2H, Ar-H ortho to Ct
C), 7.38 (m, 3H, Ar-H para and meta to CtC), 4.45 (m, 4H,
OCH2), 4.17 (m, 2H, OCOCH2O), 3.18 (ddd, 1H, OCH), 2.30
(m, 1H), 2.04 (m, 1H), 1.60 (m, 3H), 1.30 (m, 3H), 0.98-0.79
(m, 10H). 13C NMR (75 MHz, CDCl3), δ (ppm): 170.6
(OCOCH2O), 153.6 (≡CCO2), 133.0 (aromatic carbons ortho to
CtC), 130.8 (aromatic carbon para to CtC), 128.6 (aromatic
carbons meta to CtC), 119.4 (aromatic carbon linked with Ct
C), 87.2 (ArC≡), 80.3 (≡CCO2), 80.2 (OCOCH2O), 65.8 (OCH),
63.3 (CH2OCO), 62.0 (≡CCO2CH2), 48.1, 39.9, 34.4, 31.5, 25.5,
23.3, 22.2, 20.9, 16.2. MS (CI): m/e 387.1 [(M + 1)+, calcd
387.1].
2-{(S)-(+)-[(r-Acetoxy)ben zyl]ca r bon yloxy}eth yl P h en -
ylp r op iola te (3). It was prepared by the esterification of 7
with (S)-(+)-O-acetylmandelic acid (10) using a procedure
similar to that described above for the preparation of 7. Pale
yellow liquid of 3 was isolated in 96.7% yield. IR (KBr), ν
(cm-1): 2218 (vs, CtC), 1747 and 1713 (vs, CdO). 1H NMR
(300 MHz, CDCl3), δ (ppm): 7.61 (m, 2H, Ar-H ortho to CtC),
7.50-7.36 (m, 8H, Ar-H), 5.97 (s, 1H, CH), 4.37 (m, 4H,
OCH2), 2.20 (s, 3H, CH3). 13C NMR (75 MHz, CDCl3), δ (ppm):
170.2 (CH2OCOCH), 168.5 (OCOCH3), 153.4 (≡CCO2), 133.4
(aromatic carbon linked with CH), 133.0 (aromatic carbons
ortho to CtC), 130.8 (aromatic carbon para to CtC), 129.2
(aromatic carbons ortho to CHOCOCH3), 128.8 (aromatic
carbons meta to CH), 128.6 (aromatic carbons meta to CtC),
127.5 (aromatic carbon para to CH), 119.3 (aromatic carbon
linked with CtC), 87.0 (ArC≡), 80.1 (≡CCO2), 74.3 (CH), 62.9
(CH2OCO), 62.6 (≡CCO2CH2), 20.7 (CH3). MS (CI): m/e 367.1
[(M + 1)+, calcd 367.1].
Gen er a l In for m a tion . Toluene (BDH) was predried over
4 Å molecular sieves and distilled from sodium benzophenone
ketyl immediately prior to use. DCM and acetonitrile (Lab-
Scan) were dried over molecular sieves and distilled over
calcium hydride. Except for molybdenum(V) chloride (Acros),
all other reagents and solvents were purchased from Aldrich
and used as received.
The IR spectra were measured on a Perkin-Elmer 16 PC
FT-IR spectrophotometer. The NMR spectra were recorded on
a Bruker ARX 300 NMR spectrometer using chloroform-d as
the solvent and tetramethylsilane (δ ) 0) or chloroform (7.26)
as the internal reference. The UV spectra were measured on
a Milton Roy Spectronic 3000 Array spectrophotometer, and
the molar absorptivities (ꢀ) of the polymers were calculated
on the basis of their monomer repeat units. The mass spectra
were recorded on a Finnigan TSQ 7000 triple quadrupole mass
spectrometer operating in a chemical ionization (CI) mode
using methane as the carrier gas. The molecular weights of
the polymers were estimated by a Waters Associates GPC
system. Degassed THF was used as the eluent at a flow rate
of 1.0 mL/min. A set of monodisperse polystyrene standards
covering the molecular weight range of 103-107 was used for
molecular weight calibration.
The thermal stability of the polymers was evaluated on a
Perkin-Elmer TGA 7 under dry nitrogen at a heating rate of
20 °C /min. The CD spectra were measured on a J asco J -720
spectropolarimeter in 1 mm quartz curettes using a step
resolution of 0.2 nm, a scan speed of 50 nm/min, a sensitivity
of 0.1°, and a response time of 0.5 s. Each spectrum was the
average of 5-10 scans. The molar concentrations of the
polymer solutions were calculated on the basis of the repeat
units of the polymers.
Mon om er Syn th eses. The alkyl phenylpropiolates 1-3
were prepared by the esterification of phenylpropiolic acid (5)
with ethylene glycol (6) (Scheme 2, eq 4) followed by another
esterification of 2-hydroxyethyl phenylpropiolate (7) with
stereogenic acids 8-10 (eq 5) using DCC as the dehydrating
agent. The cholesterol-containing phenylpropiolate 4 was
prepared by the esterification of 7 with cholesteryl chlorofor-
mate (11) in the presence of pyridine. Typical experimental
procedures for the syntheses of the monomers are given below.
2-Hyd r oxyeth yl P h en ylp r op iola te (7). In a 500 mL two-
necked flask under nitrogen were dissolved 4.5 g (72.5 mmol)
of 6, 4.0 g (19.5 mmol) of DCC, 0.5 g (2.5 mmol) of TsOH, and
0.3 g (2.5 mmol) of DMAP in 200 mL of dry DCM. The solution
2-(Ch olester yloxyca r bon yl)eth yl P h en ylp r op iola te (4).
In a 250 mL two-necked flask under nitrogen was dissolved
5.7 g (12.8 mmol) of cholesteryl chloroformate (11) in 100 mL
of dry DCM. The solution was cooled to 0-5 °C with an ice-
water bath, to which 2.5 g (13.2 mmol) of 7 and 1.4 g (17.2
mmol) of pyridine in 25 mL of DCM was injected through a
syringe. The mixture was slowly warmed to room temperature
and stirred overnight. DCM was evaporated using a rotary
evaporator. The solid residue in the flask was dissolved in 50
mL of chloroform, and the solution was washed with water
and dried over anhydrous magnesium sulfate. The crude

Macromolecules, Vol. 37, No. 18, 2004
Helical Poly(phenylpropiolate)s 6703
product was purified on a silica gel column using chloroform
as the eluent. Recrystallization from ethanol/water mixture
(4:1 by volume) gave 5.6 g of white powdery product of 4 (yield
84.4%). IR (KBr), ν (cm-1): 2221 (vs, CtC), 1750 and 1720
P4: Yellow powdery solid; yield 6.2%. Mw: 96 400; Mw/Mn:
3.2 (GPC, Table 2, no. 8). IR (KBr), ν (cm-1): 1746 and 1717
1
(vs, CdO). H NMR (300 MHz, CDCl3), δ (ppm): 6.63 (Ar-H
ortho, para, and meta to CdC), 5.38 (dCH), 4.39 (OCH2), 2.29,
1.53, 1.11, 0.96, 0.86, 0.78, 0.69. 13C NMR (75 MHz, CDCl3), δ
(ppm): 164.2 (dCCO2), 154.2 (ArCd), 139.1 (dCCO2 and Cd
CH), 126.5 (aromatic carbons ortho, para, meta, and linked to
CdC), 122.8 (CdCH), 77.9 (OCH), 64.5, 56.8, 56.3, 50.1, 42.4,
39.7, 38.1, 36.6, 36.4, 36.0, 32.1, 28.1, 24.5, 24.2, 23.1, 23.0,
22.8, 22.7, 21.3, 19.5, 18.9, 12.1. UV (CHCl3, 1.6 × 10-4 mol/
1
(vs, CdO). H NMR (300 MHz, CDCl3), δ (ppm): 7.58 (m, 2H,
Ar-H ortho to CtC), 7.35 (m, 3H, Ar-H para and meta to
CtC), 5.39 (d, 1H, dCH), 4.41 (m, 4H, OCH2), 2.34 (t, 2H),
1.99-0.67 (m, 41H). 13C NMR (75 MHz, CDCl3), δ (ppm): 154.2
(≡CCO2), 153.6 (OCO2), 139.2 (CdCH), 133.0 (aromatic car-
bons ortho to CtC), 130.7 (aromatic carbon para to CtC),
128.5 (aromatic carbons meta to CtC), 123.0 (CdCH), 119.4
(aromatic carbon linked with CtC), 87.1 (ArC≡), 80.2 (≡CCO2),
78.3 (OCH), 64.7, 63.3, 56.6, 56.1, 49.9, 39.7, 39.5, 37.9, 36.8,
36.5, 36.1, 35.7, 31.84, 31.78, 28.2, 28.0, 27.6, 24.2, 23.8, 22.8,
22.5, 21.0, 19.2, 18.7, 11.8. MS (CI): m/e 603.6 [(M + 1)+, calcd
603.6].
P olym er iza tion Rea ction s. All the polymerization reac-
tions and manipulations were carried out under nitrogen using
Schlenk techniques in a vacuum line system or an inert-
atmosphere glovebox (Vacuum Atmospheres), except for the
purification of the polymers, which was done in an open
atmosphere. A typical experimental procedure for the polym-
erization of 1 is given below.
Into a baked 20 mL Schlenk tube with a stopcock in the
sidearm was added 322.0 mg (0.80 mmol) of 1. The tube was
evacuated under vacuum and then flushed with dry nitrogen
three times through the sidearm. Freshly distilled toluene (2
mL) was injected into the tube to dissolve the monomer. The
catalyst solution was prepared in another tube by dissolving
10.9 mg of molybdenum(V) chloride and 17.2 mg of tetra-
phenyltin in 2 mL of toluene. The catalyst solution was aged
at 60 °C for 15 min, into which the monomer solution was
added using a hypodermic syringe. The reaction mixture was
stirred at 60 °C under nitrogen for 24 h. The solution was then
cooled to room temperature, diluted with 5 mL of chloroform,
and added dropwise to 500 mL of acetone through a cotton
filter under stirring. The precipitate was allowed to stand
overnight, which was then filtered with a Gooch crucible.
Polymer P1 was washed with acetone and dried in a vacuum
oven to a constant weight.
L), λmax: 315 nm; ꢀmax: 0.46 × 104 mol-1 L cm-1
.
Ack n ow led gm en t. The work described in this paper
was partially supported by the Research Grants Council
(603304, N_HKUSR606_03, 604903, HKUST6085/02P,
6121/01P, and 6187/99P) and the University Grants
Committee of Hong Kong through an Area of Excellence
(AoE) Scheme (AoE/P-10/01-1A).
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Ch a r a cter iza tion Da ta . P1: Yellow powdery solid; yield
57.6%. Mw: 74 200; Mw/Mn: 2.7 (GPC, Table 1, no. 4). IR (KBr),
ν (cm-1): 1734 and 1716 (vs, CdO). 1H NMR (300 MHz,
CDCl3), δ (ppm): 7.63 [Ar-H meta to OCH3 and meta and
ortho to CH(CH3)], 7.15 (Ar-H ortho to OCH3), 6.76 (Ar-H
ortho, para, and ortho to CdC), 4.12 (OCH2), 3.93 (OCH3), 3.41
[CH(CH3)], 1.37 [CH(CH3)]. 13C NMR (75 MHz, CDCl3), δ
(ppm): 174.5 [CO2CH(CH3)], 165.5 (dCCO2), 157.3 (aromatic
carbon linked with OCH3), 150.8 (ArCd), 140.4 (dCCO2),
135.6, 135.0, 133.5, 129.1, 128.6, 126.9, 126.2, 125.9, 118.8,
105.5, 63.3 (OCH2), 55.2 (OCH3), 44.8 [CH(CH3)], 18.8
[CH(CH3)]. UV (CHCl3, 2.98 × 10-4 mol/L), λmax
: 332 nm;
ꢀmax: 0.55 × 104 mol-1 L cm-1
.
P2: Yellow powdery solid; yield 17.8%. Mw: 60 900; Mw/
Mn: 2.0 (GPC, Table 2, no. 3). IR (KBr), ν (cm-1): 1763 and
1714 (vs, CdO). 1H NMR (300 MHz, CDCl3), δ (ppm): 6.54
(Ar-H ortho, para, and meta to CdC), 4.15 (OCH2), 3.80
(OCOCH2O), 3.10 (OCH), 2.28, 1.94, 1.64, 1.28, 0.83. 13C NMR
(75 MHz, CDCl3), δ (ppm): 170.3 (OCOCH2O), 165.3 (dCCO2),
133.6 (aromatic carbons ortho to CdC), 131.0 (aromatic carbon
para to CdC), 125.0 (aromatic carbons meta to and linked with
CdC), 79.8 (OCOCH2O), 65.7 (OCH), 63.1 (CH2OCOCH2 and
dCCO2CH2), 47.8, 39.8, 34.3, 31.4, 25.4, 23.3, 22.1, 20.7, 16.5.
UV (CHCl3, 2.9 × 10-4 mol/L), λmax: 320 nm; ꢀmax: 0.43 × 104
mol-1 L cm-1
.
P3: Yellow powdery solid; yield 34.3%. Mw: 15 400; Mw/
Mn: 1.5 (GPC, Table 2, no. 6). IR (KBr), ν (cm-1): 1746 and
1714 (vs, CdO). 1H NMR (300 MHz, CDCl3), δ (ppm): 7.21
(Ar-H), 6.62-6.47 (Ar-H ortho, para, and meta to CdC), 5.68
(CH), 4.12 (OCH2), 2.10 (CH3). 13C NMR (75 MHz, CDCl3), δ
(ppm): 169.8 (CH2OCO), 168.6 (OCOCH3), 165.6 (dCCO2),
150.6 (ArC)), 140.2 ()CCO2), 135.4, 133.2, 129.1, 128.7, 127.7,
126.8, 125.8, 74.3 (CH), 63.5 (CH2OCO), 62.2 (dCCO2CH2),
20.6 (CH3). UV (CHCl3, 2.4 × 10-4 mol/L), λmax: 325 nm; ꢀmax
:
0.30 × 104 mol-1 L cm-1
.

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MA048960J