From nonconjugated diynes to conjugated polyenes: Syntheses of poly(1-phenyl-7-aryl-1,6-heptadiyne)s by cyclopolymerizations of asymmetrically α,ω-disubstituted alkadiynes

Article abstract of DOI:10.1021/ma0475481

The synthesis of Poly(1-phenyl-7-aryl-1, 6-heptadiyne) by Cyclopolymerizations of asymmetrically α, ω distributed alkadiynes was discussed. The alkadiynes were prepared by a synthetic route involving consecutive palladium-catalyzed cross coupling reactions. Substitution of the olefinic hydrogen in cyclic polyene by an aromatic ring was used for the polymer stability. It was found that the polymer stability increases with a decrease in the number of the active hydrogen atoms.

Full text of DOI:10.1021/ma0475481

660
Macromolecules 2005, 38, 660-662
From Nonconjugated Diynes to Conjugated
Polyenes: Syntheses of
Poly(1-phenyl-7-aryl-1,6-heptadiyne)s by
Cyclopolymerizations of Asymmetrically
r,ω-Disubstituted Alkadiynes
Charles C. W. Law, Jacky W. Y. Lam, Yuping Dong,
Hui Tong, and Ben Zhong Tang*
Department of Chemistry, Center for Display Research,
The Hong Kong University of Science & Technology,
Clear Water Bay, Kowloon, Hong Kong, China
Received November 28, 2004
1,6-Heptadiyne can be polymerized by MoCl5-based
catalysts into a polyene of cyclic structure.1 The polymer
is, however, insoluble in common solvents and sensitive
to air oxidation. Introduction of substituents to the
4-position can help enhance the processability and
stability of the polyene, and a large variety of (a)-
symmetrically 4,4-disubstituted poly(1,6-heptadiyne)s
have been synthesized (Supporting Information, Scheme
S1), some of which have been found to show such
functional properties as liquid crystallinity, photocon-
ductivity, optical nonlinearity, and photorefractivity.2,3
In contrast to the active research on the internally
disubstituted polyenes, little work has been done on
their terminally disubstituted congeners, mainly due to
the involved synthetic difficulty. Among a few scattered
reports in the area, polymerizations of 1,7-bis(trimeth-
ylsilyl)-4-oxa-1,6-heptadiyne gave insoluble powders
with partially desilylated structural defects in very low
yields (e13%),2,4 and living polymerizations of diethyl
2,2-di(2-butynyl)malonate were catalyzed by delicate
ternary mixtures of MoOCl4 + organometallic cocata-
lysts + alcohols.5
If a “simple”, effective polymerization system for
terminally disubstituted diynes can be developed, it will
be very rewarding. Chemically, it is easy to attach
groups to the triple-bond terminals via versatile cou-
pling reactions,6 which will open a wide avenue to the
creation of a great number of new functional polyenes.
Substitution of the olefinic hydrogen in the backbone
of the cyclic polyene by an aromatic ring will boost the
polymer stability because it has been well understood
in the linear polyene system that (1) the polymer
stability increases with a decrease in the number of the
active hydrogen atoms in the repeat unit: -(RCd
CR′)n- > -(RCdCH)n- > -(HCdCH)n- and (2) that,
among the disubstituted polyenes, the polymers carry-
ing aromatic pendants are more stable than those
bearing alkyl ones: e.g., poly(1-phenyl-1-propyne)
-[(C6H5)CdC(CH3)]n- does not degrade after annealing
at high temperatures (e.g., 160 °C) in air for 20 h, while
poly(2-octyne) -[(C5H11)CdC(CH3)]n- decomposes to
oligomeric species when heated at much lower temper-
atures (e.g., 100 °C).7,8
Figure 1. (A) Syntheses of poly(1-phenyl-7-aryl-1,6-hep-
tadiyne)s P1(3)-P4(3) and 13C NMR spectra of chloroform-d
solutions of (B) monomer 1(3) and (C) its polymer P1(3)
(sample taken from Table 1, no. 2). The solvent peaks are
marked with asterisks (/).
its spacer length (m): only can the diynes with m ) 3,
viz., the 1,6-heptadiyne derivatives, be effectively po-
lymerized. The terminally disubstituted 1,6-heptadiynes
[1(3)-4(3)] show polymerization behaviors distinctly
different from those of their internally disubstituted
cousins: the former cannot be effectively polymerized
by the Mo-based mixtures, which are good catalysts for
the polymerizations of the latter. Delightfully, however,
we find that WCl6-Ph4Sn, a simple metathesis catalyst,
can convert the aryl-disubstituted diynes into high
molecular weight, cyclic polyenes consisting of exclu-
sively six-membered rings. The polyenes with their
labile olefinic hydrogen atoms replaced by the stable
aromatic rings are, as anticipated, processable and
stable.
The alkadiynes (1-4) were prepared by a synthetic
route involving consecutive palladium-catalyzed cross-
coupling reactions9 (Figure 1A) and all the products give
satisfactory analysis data (see Supporting Information
for details). Whereas many internally disubstituted 1,6-
heptadiynes can be effectively cyclopolymerized by the
MoCl5-based catalysts, the Mo-catalyzed reaction of 1(3),
In this work, we designed and synthesized a group of
R,ω-disubstituted alkadiynes with different aryl rings
and spacer lengths [1(m)-4(3); Figure 1A). The polym-
erizability of the diyne is found to be very sensitive to
* Corresponding author. Telephone: +852-2358-7375. Fax:
+852-2358-1594. E-mail: tangbenz@ust.hk.
10.1021/ma0475481 CCC: $30.25 © 2005 American Chemical Society
Published on Web 01/04/2005

Macromolecules, Vol. 38, No. 3, 2005
Communications to the Editor 661
Table 1. Polymerizations of
1-Phenyl-7-aryl-1,6-heptadiynesa
b
b
no.
catalyst
temp (°C) yield (%)
Mw
Mw/Mn
1-Phenyl-7-(1-naphthyl)-1,6-heptadiyne [1(3)]
1
2
3
MoCl5-Ph4Sn
WCl6-Ph4Sn
WCl6-Ph4Sn
60
60
80
10.1
99.0
99.0
12 600
60 900
56 300
1.8
2.3
2.7
1-Phenyl-7-(9-phenanthryl)-1,6-heptadiyne [2(3)]
4
5
WCl6-Ph4Sn
WCl6-Ph4Sn
60
80
82.0
68.2
16 000
14 100
2.5
2.0
1-Phenyl-7-(2-fluorenyl)-1,6-heptadiyne [3(3)]
WCl6-Ph4Sn 60 40.0 23 000
6
2.0
1-Phenyl-7-[(4-pentyl-4′-biphenylyl)]-1,6-heptadiyne [4(3)]
WCl6-Ph4Sn 60 95.0 83 500 3.1
7
a Carried out under nitrogen in toluene for 24 h; [M]0 ) 0.2 M,
[cat.] ) [cocat.] ) 10 mM. b Determined by GPC in THF on the
basis of a polystyrene calibration.
a terminally disubstituted 1,6-heptadiyne, proceeds
sluggishly, giving a polymeric product in a low yield
(∼10%; Table 1, no. 1). The monomer can, however, be
nicely polymerized by WCl6-Ph4Sn, producing a com-
pletely soluble polymer with a high molecular weight
in a practically quantitative yield.
The congeners of 1(3) with longer alkyl spacers, that
is 1(5) and 1(6), cannot be polymerized by the Mo and
W catalysts (Supporting Information, Table S1), due to
the difficulty in forming larger alkenyl rings. On the
other hand, the cousins of 1(3) with the same length of
alkyl spacer, viz., 2(3)-4(3), can all be polymerized by
the W catalyst under the similar reaction conditions
(Table 1, nos. 4-7), confirming the key role the spacer
length plays in determining the stability of the alkenyl
rings and hence the ease of their formation in the diyne
cyclopolymerization. The result of the polymerization
of 4(3) is especially impressive: a polymer with a high
molecular weight (Mw 83500) is produced in a very high
yield (95%).
All the polymers readily dissolve in common organic
solvents such as toluene, THF, and chloroform, enabling
structural characterizations by various spectroscopic
methods. An example of the IR spectrum of P1(3), along
with that of its monomer 1(3), is given in Figure S1.
The CtC stretching band observed at ∼2224 cm-1 in
the spectrum of 1(3)10 is absent in that of P1(3),
indicating that the polymer contains no triple bonds in
its repeat unit. The 1H NMR spectrum of P1(3) is
composed of two very broad peaks centered at δ ∼7.1
and ∼1.6, which are assignable to the resonances of the
protons of the aromatic rings and alkyl spacer, respec-
tively. No information about the cyclic polyene structure
can be obtained from the 1H NMR analysis, because the
polymer is a disubstituted polyene that contains no
olefinic protons.
Figure 2. UV spectra of THF solutions of polymers P1(3)
(sample taken from Table 1, no. 2), P2(3) (Table 1, no. 4), P3(3)
(Table 1, no. 6), P4(3) (Table 1, no. 7), and poly(1-phenyl-1-
octyne) (PPO).
Accompanying the yne-ene transformation, the carbon
atoms of the methylene groups at R (c, e) and â positions
(d) shift up- and downfield, respectively (Figure 1C). In
the cyclopolymerizations of some internally disubsti-
tuted 1,6-heptadiynes, both six- and five-membered
alkenyl rings are formed.2 The resonances of the me-
thylene carbon atoms of the former occur in the higher
chemical shift region (downfield) than those of the
latter,3a,5 due to the ring strain involved in the smaller
pentenyl ring. The R-methylene carbon atoms in the
five-membered ring of P1(3), if formed, are expected to
resonate at δ ∼ 35-40. No resonance peaks are,
however, observed in this chemical shift region in the
spectrum of P1(3), revealing that the five-membered
rings are not formed in the W-catalyzed cyclopolymer-
ization of 1(3). In other words, P1(3) are comprised of
exclusively six-membered rings.
The UV spectra of the polyenes are shown in Figure
2. Since none of their monomers absorbs at λ > 300 nm,
the peaks of the polymers in the visible spectral region
thus must be due to the absorptions of their polyene
backbones. The molar absorptivities are larger and the
band edges are redder than those of poly(1-phenyl-1-
octyne) (PPO), a linear cousin of the cyclic polyenes,
revealing that the backbones of P1(3)-P4(3) are more
electronically conjugated. The twisting of the alternating
double bonds is hampered by the alkenyl rings, enabling
the polyene backbone to take a more planar conforma-
tion. While PPO emits strong blue light,7 all the cyclic
polyenes are weak fluorophores, whose fluorescence
quantum yields (ΦF) are low, ranging from 0.04% to
0.23% (Supporting Information, Figure S2; cf., ΦF,PPO
) 43%12). This once again proves that the backbones of
the cyclic polyenes are more π-conjugated than that of
the linear PPO, considering that polyacetylene, a linear
polyene with an extended π-conjugation, is totally
nonluminescent.7
The 13C NMR spectra are more informative. Again,
no resonance peaks of acetylenic triple bonds are
observable in the spectrum of P1(3), although they are
clearly seen in that of its monomer 1(3) in the chemical
shift region of δ ) 94.2-79.3 (Figure 1). A new broad
peak appears in the spectrum of P1(3) at δ ∼ 140, which
is assignable to the resonance of the olefinic carbon in
the polyene backbone, taking into account that many
internally disubstituted poly(1,6-heptadiyne)s bearing
no aromatic pendants resonate in this chemical shift
region (δ ∼ 138-142).3,5,11 This spectroscopically con-
firms the conversion of the triple bonds in 1(3) to the
double bonds in P1(3) by the W-catalyzed diyne cyclo-
polymerization.
The polymers are not only soluble but also stable.
None of them loses any weight when heated to ∼350
°C under nitrogen (Figure 3). Polymer P1(3) retains
∼95% of its original weight at ∼390 °C, and the 5%
weight loss for P2(3) occurs at ∼400 °C, thanks to its
“bigger” phenanthryl pendant. Even when the polymer
is heated to 500 °C, still more than half of its weight

662 Communications to the Editor
Macromolecules, Vol. 38, No. 3, 2005
functional groups, in an effort to add useful function-
alities to the processable and stable polyenes.
Acknowledgment. This work was partly supported
by the Research Grants Council of Hong Kong (603303,
604903, HKUST6085/02P and 6121/01P), the University
Grants Committee of Hong Kong through an Area of
Excellence Scheme (AoE/P-10/01-1-A), the National
Science Foundation of China (N_HKUST606_03), and
the High Impact Area Grant of our University.
Supporting Information Available: Text describing
detailed synthetic procedures and analytical data of the
monomers [1(m)-4(3)] and polymers [P1(3)-P4(3)], Table S1,
listing the results of the attempted polymerizations of 1(5) and
1(6), Scheme S1, showing the reactions, and Figures S1, S2,
and S3, showing the IR spectra of 1(3) and P1(3), the PL
spectra of P1(3)-P4(3), and the GPC chromatograms of the
annealed P1(3), respectively. This material is available free
of charge via the Internet at http://pubs.acs.org.
References and Notes
Figure 3. TGA thermograms of polymers P1(3) (sample taken
from Table 1, no. 2), P2(3) (Table 1, no. 4), P3(3) (Table 1, no.
6), and P4(3) (Table 1, no. 7) recorded under nitrogen at a
heating rate of 20 °C/min. Inset: Changes in molecular weights
of P1(3)-P4(3) after heating in air for 2 h.
(1) Jang, M. S.; Kwon, S. K.; Choi, S. K. Macromolecules 1990,
23, 4135.
(2) For a review, see: Choi, S. K.; Gal, Y. S.; Jin, S. H.; Kim,
H. K. Chem. Rev. 2000, 100, 1645.
(3) E.g.: (a) Fox, H. H.; Wolf, M. O.; Odell, R.; Lin, B. L.;
Schrock, R. R.; Wrighton, M. S. J. Am. Chem. Soc. 1994,
116, 2827. (b) Kim, S. H.; Jin, S. H.; Jang, M. S.; Moon, S.
B.; Gal, Y. S.; Moon, D. K.; Park, J. W.; Choi, S. K. Mol.
Cryst. Liq. Cryst. 2001, 371, 297. (c) Sivakumar, C.; Va-
sudevan, T.; Gopalan, A.; Wen, T. C. Polymer 2002, 43, 1781.
(d) Krause, J. O.; Zarka, M. T.; Anders, U.; Weberskirch,
R.; Nuyken, O.; Buchmeiser, M. R. Angew. Chem., Int. Ed.
2003, 42, 5965.
(4) Gal, Y. S.; Lee, H. J.; Choi, S. K. Macromol. Rep. 1996, A33,
57.
(5) Kubo, H.; Hayano, S.; Misumi, Y.; Masuda, T. Macromol.
Chem. Phys. 2002, 203, 279.
(6) Metal Catalyzed Cross-Coupling Reactions; Diederich, F.,
Stang, P. J., Eds.; Wiley-VCH: New York, 1998.
(7) For a review, see: Lam, J. W. Y.; Tang, B. Z. J. Polym. Sci.,
Part A: Polym. Chem. 2003, 41, 2607.
(8) Masuda, T.; Tang, B. Z.; Higashimura, T.; Yamaoka, H.
Macromolecules 1985, 18, 2369.
(9) Ha¨ussler, M.; Zheng, R.; Lam, J. W. Y.; Tong, H.; Dong, H.;
Tang, B. Z. J. Phys. Chem. B 2004, 108, 10645.
(10) Dong, Y. P.; Lam, J. W. Y.; Peng, H.; Cheuk, K. K. L.; Kwok,
H. S.; Tang, B. Z. Macromolecules 2004, 37, 6408.
(11) Jin, S. H.; Kim, S. H.; Cho, H. N.; Choi, S. K. Macromolecules
1991, 24, 6050.
(12) (a) Lam, J. W. Y.; Dong, Y.; Cheuk, K. K. L.; Luo, J.; Xie,
Z.; Kwok, H. S.; Mo, Z.; Tang, B. Z. Macromolecules 2002,
35, 1229. (b) Lam, J. W. Y.; Kong, X.; Dong, Y.; Cheuk, K.
K. L.; Xu, K.; Tang, B. Z. Macromolecules 2000, 33, 5027.
(c) Lam, J. W. Y.; Luo, J.; Dong, Y.; Cheuk, K. K. L.; Xu, K.;
Tang, B. Z. Macromolecules 2002, 35, 8288.
(∼53%) is retained. Its high stability is due to the “jacket
effect” of the bulky and stable aromatic pendants,7,12
which wrap the labile polyene backbone and shield it
from the attack by thermolytic species. Since polymeric
materials are normally used in air, it is of importance
to check the resistance of the cyclic polyenes to oxidative
degradation. After P1(3) was annealed at 100 and 150
°C in air for 2 h, neither cross-linking nor chain scission
occur to a significant extent, as verified by the hardly
changed GPC traces given in Figure S3 (Supporting
Information) and the almost flat Mw-T plot shown in
the inset of Figure 3. The Mw-T plots for other polyenes
are similar, that is, all the polymers are resistant to the
oxidative thermolysis.
In summary, in this work, we succeeded in converting
a group of terminally aryl-disubstituted diynes into
cyclic polyenes of high molecular weights in high yields
by the cyclopolymerization catalyzed by a simple cata-
lyst system. Our structural design paid off: the replace-
ment of the active olefinic hydrogen atoms by the bulky
and stable aromatic rings endowed the polyenes with
processability and stability. Following the versatile
synthetic route developed in this study, we are now
working on the preparations of cyclic poly(1,6-hep-
tadiyne)s (a)symmetrically R,ω-disubstituted by various
MA0475481