New acetylenic monomers and polymers from 4,5-dicyanoimidazole

Article abstract of DOI:10.1021/ma035920r

An efficient synthesis of 1-methyl-2-ethynyl-4,5-dicyanoimidazole via Sonogashira coupling is described. Our optimized method generates 1-methyl-2-ethynyl-4,5-dicyanoimidazole in almost 80% yield from 1-methyl-2-bromo-4,5-dicyanoimidazole. Syntheses of th

Full text of DOI:10.1021/ma035920r

5900
Macromolecules 2004, 37, 5900-5910
New Acetylenic Monomers and Polymers from 4,5-Dicyanoimidazole
Cr ysta l G. Den sm or e†,§ a n d P a u l G. Ra sm u ssen *,†,‡
Macromolecular Science and Engineering Center and Department of Chemistry,
University of Michigan, Ann Arbor, Michigan 48109-1055
Received December 16, 2003; Revised Manuscript Received May 19, 2004
ABSTRACT: An efficient synthesis of 1-methyl-2-ethynyl-4,5-dicyanoimidazole via Sonogashira coupling
is described. Our optimized method generates 1-methyl-2-ethynyl-4,5-dicyanoimidazole in almost 80%
yield from 1-methyl-2-bromo-4,5-dicyanoimidazole. Syntheses of the ethyl, propyl, and p-methoxybenzyl
derivatives of 2-ethynyl-4,5-dicyanoimidazole are also reported. The polymerization of 1-methyl-2-ethynyl-
4,5-dicyanoimidazole with transition-metal catalysts and triethylamine is described. Variation of catalyst
concentration, reaction time, and temperature is also described. A more complete mechanistic picture of
acetylene polymerizations, especially those with electron-withdrawing substituents, is presented. Solid
samples of 1-methyl-2-ethynyl-4,5-dicyanoimidazole oligomers are EPR-active, and the number of free
spins was quantified. Cyclic voltammetry was used to investigate the reduction of 1-methyl-2-ethynyl-
4,5-dicyanoimidazole oligomers and other dicyanoimidazole derivatives.
Sch em e 1. P olym er iza tion of
2-Vin yl-4,5-d icya n oim id a zole a n d Its Alk yl Der iva tive
In tr od u ction
The HCN tetramer diaminomaleonitrile (DAMN) has
proved to be a very convenient reagent for the synthesis
of heterocyclic monomers.1-3 There has been no work
on acetylenic polymers based on DAMN, although our
group has recently reported vinylic polymers derived
from DAMN.4-8 High molecular weight polymers from
both 2-vinyl-4,5-dicyanoimidazole (1) and 1-methyl-2-
vinyl-4,5-dicyanoimidazole (3) were prepared using
AIBN as an initiator in DMF at 70 °C.5,6 See Scheme 1.
By comparison, the acetylenic monomer and polymer
analogues should have a high degree of conjugation and
unique properties due to the dicyanoimidazole electron-
withdrawing groups. We focus here on the anionic and
transition-metal polymerization of acetylenic dicyanoim-
idazoles. This work parallels earlier work on the anionic
polymerization of cyanoacetylene with butyllithium and
triethylamine reported by Gorman and co-workers;
however, they did not propose initiation, propagation,
or termination mechanisms.9
There have also been a number of reports on the
polymerization of acetylene derivatives using late-
transition-metal catalysts because they are typically
more tolerant of a variety of functional groups.9-25
Catalysts based on molybdenum, tungsten, niobium,
and tantulum are generally very air and moisture
sensitive,12 whereas rhodium-, palladium-, and nickel-
based catalysts are less sensitive and can be used with
a variety of solvents.
duced stereoregular cis-transoidal polymers of different
phenylacetylene derivatives.10,12 Tabata and co-workers
have done extensive work on ortho-, meta-, and para-
substituted phenylacetylene derivatives using rhodium
catalysis and have achieved unprecedented molecular
weights of stereoregular cis-transoidal polyacetylene
derivatives.21-25,27 They have also demonstrated that
triethylamine serves as a cocatalyst in the polymeriza-
tion reaction and promotes catalyst dissociation.22-24
Few polyacetylenes reported have contained electron-
withdrawing groups. D’Amato and Tabata reported the
polymerization of p-nitrophenylacetylene using [Rh-
(nbd)Cl]2 and [Rh(cod)Cl]2, but the polymer’s molecular
weight was 1200-2100 g/mol.25 This molecular weight
was much lower than other substituted poly(phenyl-
acetylenes) they have reported.
The polymerization of cyanoacetylene illustrates the
difficulties with electron-withdrawing substituents, and
thus far, a variety of polymerization methods have not
produced soluble, high molecular weight polymers.
Wallach and Manassen polymerized cyanoacetylene
using anionic initiators.28 Gorman and co-workers have
found that nickel and palladium catalysts were effective
in the polymerization of cyanoacetylene, giving molec-
ular weights on the order of 104 g/mol.9
Using solid-state NMR, Hirao and co-workers have
suggested that polymerization of phenylacetylene with
rhodium catalysts proceeds via an insertion mecha-
nism.16 Poly(phenylacetylene) was made using rhodium-
(I) complexes with norbornadiene or 1,5-cyclooctadiene
by Cataldo.26 Tang and co-workers have prepared poly-
(phenylacetylene) using water-soluble rhodium com-
plexes, Rh(nbd)(tos)(H2O) and Rh(cod)(tos)(H2O).12 They
also reported that [Rh(nbd)Cl]2 and [Rh(cod)Cl]2 pro-
† Macromolecular Science and Engineering Center.
‡ Department of Chemistry.
In general, very little has been said about the poly-
acetylene polymerization mechanism. Kishimoto and co-
workers proposed an insertion mechanism for rhodium-
catalyzed polymerization of phenylacetylenes.15 A termi-
§ Current address: Chemistry Division, Applied Chemical
Technologies Group (C-ACT), Los Alamos National Laboratory,
Los Alamos, NM 87545.
10.1021/ma035920r CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/14/2004

Macromolecules, Vol. 37, No. 16, 2004
Monomers and Polymers from 4,5-Dicyanoimidazole 5901
Sch em e 2. Su ccessfu l Son oga sh ir a Cou p lin g To Give
1-Meth yl-2-eth yn yl-4,5-d icya n oim id a zole
F igu r e 1. Polymerization of 1-methyl-2-ethynyl-4,5-dicy-
anoimidazole (8).
attention turned to the late-transition-metal catalysts,
[Rh(nbd)Cl]2 and [Rh(cod)Cl]2. These catalysts have
been shown to tolerate functional groups and exhibit
increasing catalytic activity in more polar organic
solvents.10,12 We selected nickel- and palladium-based
catalysts because Gorman and co-workers found them
to be successful in the polymerization of cyanoacety-
lene.9 Dimethylformamide was used as a solvent for the
initial polymerizations, and the following catalysts were
examined: [Rh(nbd)Cl]2, [Rh(cod)Cl]2, PdCl2(PPh3)2,
NiCl2(PPh3)2, and triethylamine.
Sch em e 3. Dep r otection of
1-(p-Meth oxyben zyl)-2-eth yn yl-4,5-d icya n oim id a zole
(9)
Palladium, nickel, and rhodium catalysts were com-
pared using DMF as the solvent and a 100:1 monomer-
to-catalyst ratio. Although initial yields were low and
high molecular weight polymer was not produced, Table
1 does show that these catalysts produced oligomers.
Because the sample sizes and yields were so small in
11-B, 11-C, 11-D, and 11-E, the Mn was not determined.
Oligomer was also produced using rhodium catalysis at
both room temperature and 65 °C. Since the rhodium
catalyst was much more soluble in common organic
solvents, it could be removed easily. For these reasons,
additional polymerizations focused largely on [Rh(nbd)-
Cl]2.
The monomer-to-catalyst ratio, reaction time, and
temperature were varied using DMF as a solvent (11-
G,A,H). There was a very slight increase in the polymer
yield by increasing the amount of catalyst from a 100:1
to 20:1 monomer-to-catalyst ratio. By extending the
reaction time from 24 to 48 h at room temperature, the
yield more than doubled. By increasing the temperature
to 65 °C, the yield increased to 65% (11-J ).
Tabata and co-workers suggest that TEA may serve
as a cocatalyst with rhodium complexes because it
promotes dissociation of the dimeric rhodium com-
plex.22-24 The role of TEA alone and with [Rh(nbd)Cl]2
was investigated with 1-methyl-2-ethynyl-4,5-dicyanoim-
idazole (8). It is clear from samples 11-K,L,M,N that
TEA alone leads to polymerization of 1-methyl-2-ethy-
nyl-4,5-dicyanoimidazole (8). At room temperature and
a reaction time of 18 h, the yield increases with the more
polar solvent DMF compared to toluene. Using DMF as
a solvent in 11-M, the yield increased to over 60% when
the temperature was increased and the polymerization
time was extended. The various yields listed for 11-N
reflect multiple polymerizations using acetonitrile as a
solvent, a monomer-to-catalyst ratio of 20:1, and a 24 h
reaction time at 65 °C. The properties of the these
oligomers (11-K,L,M,N) are similar. They are dark
brown-black solids and have similar IR and NMR
features.
The role of TEA with [Rh(nbd)Cl]2 in the polymeri-
zation of 1-methyl-2-ethynyl-4,5-dicyanoimidazole (8)
was also investigated. Using TEA as a cosolvent in 11-O
resulted in an increase in yield and molecular weight.
Whether TEA acts as a cocatalyst with [Rh(nbd)Cl]2 is
unclear since TEA alone leads to polymerization. There
are perhaps two competing polymerization mechanisms,
and if so, it would be very difficult to distinguish
nation mechanism was not proposed. Zhan and Yang
addressed the polymerization mechanism of acetylenes
using palladium and nickel acetylide catalysts.29 They
propose that the initial activation step, and also the
rate-determining step, involve coordination of a nickel
or palladium acetylide catalyst with an acetylene. Zhan
and Yang also discussed polymer termination as a result
of particularly acidic acetylenic monomers.29
Resu lts a n d Discu ssion
Mon om er Syn t h esis. Sonogashira coupling has
become widely used in recent syntheses of acetylenic
monomers. Tabata and co-workers have prepared a
variety of ortho-, meta-, and para-substituted aromatic
acetylenes synthesized by Sonogashira coupling.21-25,27
The successful synthetic approach to 2-ethynyl-4,5-
dicyanoimidazole derivatives used palladium-catalyzed
Sonogashira coupling of 2-bromo-4,5-dicyanoimidazole
derivatives with trimethylsilylacetylene. The reaction
is illustrated in Scheme 2. The silyl-protected product
was readily deprotected using potassium fluoride dihy-
drate in THF to give 1-methyl-2-ethynyl-4,5-dicyanoimi-
dazole (8) after extensive work-up. Details of our
optimized procedures are reported elsewhere.30 Synthe-
sis of the ethyl and propyl derivatives of 2-ethynyl-4,5-
dicyanoimidazole (10) can be found in the Supporting
Information.
In an effort to produce the unprotected 2-ethynyl-4,5-
dicyanoimidazole (10), Sonogashira coupling with 2-bro-
mo-4,5-dicyanoimidazole and trimethylsilylacetylene
was attempted. This reaction led to a mixture of
products that were extremely difficult to separate. The
target product, 2-(trimethylsilylacetylene)-4,5-dicyanoim-
idazole, could not be isolated from the mixture. Com-
plications with this coupling were largely attributed to
the acidity of the 1-position hydrogen. To avoid this
problem, protection and deprotection chemistry was
investigated,31-35 although the choice of protecting
groups for cyanoimidazoles is limited due the strong
electron-withdrawing nature of nitriles.36-38 Deprotec-
tion of 1-(p-methoxybenzyl)-2-ethynyl-4,5-dicyanoimi-
dazole (9) to give 2-ethynyl-4,5-dicyanoimidazole (10)
is shown in Scheme 3.
P olym er iza tion . The goal of this work was to
prepare acetylenic polymers of dicyanoimidazoles. See
Figure 1. With the successful synthesis of monomer our

5902 Densmore and Rasmussen
Macromolecules, Vol. 37, No. 16, 2004
Ta ble 1. Va r ia tion of Tr a n sition -Meta l Ca ta lyst
catalyst
M:cat
solvent
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
toluene
DMF
time (h)
temp
yield (%)
Mn(GPC)
11-A
11-B
11-C
11-D
11-E
11-F
11-G
11-H
11-I
[Rh(nbd)Cl]2
Ni(PPh3)Cl2
Pd(PPh3)Cl2
[Rh(nbd)Cl]2
Ni(PPh3)Cl2
Pd(PPh3)Cl2
[Rh(nbd)Cl]2
[Rh(nbd)Cl]2
[Rh(nbd)Cl]2
[Rh(nbd)Cl]2
TEA
TEA
TEA
TEA
[Rh(nbd)Cl]2
[Rh(nbd)Cl]2
100:1
100:1
100:1
100:1
100:1
100:1
20:1
250:1
20:1
20:1
20:1
20:1
20:1
20:1
20:1
20:1
24
24
24
24
48
24
24
24
48
48
18
18
24
24
24
24
r.t.
r.t.
r.t.
65 °C
65 °C
65 °C
r.t.
r.t.
r.t.
65 °C
r.t.
r.t.
65 °C
65 °C
r.t.
15
0
0
2200b
32a
10a
21a
20a
0
1100b
1800b
47a
65a
7
1500b
1900b
11-J
11-K
11-L
11-M
11-N
11-O
11-P
31
DMF
63a
62-79a
80a
40
3100c
2000c
3200c
1500b
CH3CN
4:1 DMF/TEA
TEA
r.t.
a
Crude yield. Contains DMF according to 1H NMR. b GPC in THF relative to polystyrene standards. c GPC in NMP relative to polystyrene
standards.
Ta ble 2. Oligom er iza tion of 1-(p-Meth oxyben zyl)-4,5-d icya n oim id a zole (12)
catalyst
M:cat
solvent
time (h)
temp
yield (%)
Mn(GPC)
12-A
12-B
[Rh(nbd)Cl]2
TEA
20:1
20:1
DMF
CH3CN
24
24
r.t.
r.t.
50
65
900a
700a
a
GPC in NMP relative to polystyrene standards.
Sch em e 4. Oligom er iza tion of
1-(p-Meth oxyben zyl)-2-eth yn yl-4,5-d icya n oim id a zole
(9)
between the two in this experiment. In 11-P , TEA was
used as the polymerization solvent. It is suspected that
the yield decreased compared to 11-O because TEA is
a poor solvent and the oligomer precipitated from
solution.
Along with 1-methyl-2-ethynyl-4,5-dicyanoimidazole
(8), the polymerization of 1-(p-methoxybenzyl)-2-ethy-
nyl-4,5-dicyanoimidazole (9) was also investigated. The
monomer 2-ethynyl-4,5-dicyanoimidazole (10) could be
produced by deprotecting 1-(p-methoxybenzyl)-2-ethy-
nyl-4,5-dicyanoimidazole (9) with ceric ammonium ni-
trate (CAN), and polymerization was avoided. 1-(p-
Methoxybenzyl)-2-ethynyl-4,5-dicyanoimidazole (9) was
polymerized using TEA and rhodium-based catalysis.
The product consisted largely of dimer and trimer in
both cases. The results are illustrated in Table 2.
Following isolation of 1-(p-methoxybenzyl)-4,5-dicy-
anoimidazole oligomers (12), the sample was depro-
tected using CAN in 3:1 (CH3CN:H2O) to give 2-ethynyl-
4,5-dicyanoimidazole oligomers (13). Elements of hydro-
gen bonding were evident in the FT-IR spectra of (13);
however, the deprotection byproduct, p-anisaldehyde,
was difficult to remove from the oligomer.
UV-Vis Ch a r a cter iza tion . The UV-vis spectrum
for 1-methyl-2-vinyl-4,5-dicyanoimidazole (3) was com-
pared to the spectra of oligomers of 1-methyl-2-ethynyl-
4,5-dicyanoimidazole (11). Compared to 3, the UV
absorbance of 1-methyl-2-ethynyl-4,5-dicyanoimidazole
oligomers (11) “tail off” toward longer wavelengths. The
molar absorptivity is considerably higher for the oligo-
mers (11) compared to the model compound (3). This
may be attributed to conjugation and enhanced delo-
F igu r e 2. UV-vis spectra of model compound and oligomers.
calization of the nonbonded electrons in multiple dicy-
anoimidazole pendant groups along the oligomer chain.
The UV-vis spectra exhibited similar tailing for all
oligomers of 1-methyl-2-ethynyl-4,5-dicyanoimidazole
(11) independent of how they were produced. Such
tailing was also observed by Tang and co-workers for
poly(phenylacetylene) derivatives.10 A similar absorp-
tion profile was observed by Gorman and co-workers for
poly(cyanoacetylene) polymerized using TEA or pal-
ladium catalysts.39 In oligomers of 1-methyl-2-ethynyl-
4,5-dicyanoimidazole (11), absorption at longer wave-
lengths clearly represents transitions due to the conju-
gated backbone.
NMR Ch a r a ct er iza t ion . A key element of the
characterization was to establish the presence of double
bonds in the backbone of 1-methyl-2-ethynyl-4,5-dicy-
anoimidazole oligomers (11). Broad peaks observed in
1H NMR between 7 and 8 ppm were labeled as the
olefinic backbone protons based on comparison to model
compounds (Figure 3). The model compound, 1-methyl-
2-styryl-4,5-dicyanoimidazole (15), was synthesized for
NMR comparison. The vinyl derivative, 1-methyl-2-
vinyl-4,5-dicyanoimidazole (14), was also used as a
model compound. The 1H NMR shifts illustrated in

Macromolecules, Vol. 37, No. 16, 2004
Monomers and Polymers from 4,5-Dicyanoimidazole 5903
the original CP-MAS spectrum and the bold line rep-
resents the spectrum with delayed decoupling. Peaks
between 120 and 130 ppm were assigned to the back-
bone carbons in the oligomer (11-N). The results are
summarized in Table 3.
Oligomers of 1-(p-methoxybenzyl)-2-ethynyl-4,5-dicy-
anoimidazole (12-A) made with [Rh(nbd)Cl]2 in DMF
were also analyzed using solid-state NMR. On the basis
of solution NMR of 12-A, DMF remains in the sample.
DMF was also observed in the solid-state NMR. In
addition, because the oligomer is largely composed of
dimer and trimer, peaks from the possible norborna-
diene end group from the rhodium catalyst were also
observed.
F igu r e 3. 1H NMR shifts of oligomers in DMSO-d6 at 400
MHz.
In Figure 6, peaks at approximately 15-20 ppm were
assigned to the methylene carbon in the norbornadiene
end group. The peaks at 30-40 ppm were assigned to
the bridgehead carbons in the norbornadiene end group.
The broad peaks around 110-125 ppm were assigned
to the CdC in norbornadiene, backbone carbons in the
oligomer chain, and the cyano carbons in the dicy-
anoimidazole pendant group. After decoupling, a peak
remains around 114 ppm that was assigned to the cyano
carbons. The results are summarized in Table 4. Ad-
ditional details about this technique and peak assign-
ments for 11 and 12 can be found in the Supporting
Information.
In the solid-state and solution NMR of 1-(p-methoxy-
benzyl)-2-ethynyl-4,5-dicyanoimidazole dimers and tri-
mers (12-A), norbornadiene was easily observed in the
short-chain oligomers. The confirmation of norborna-
diene in NMR coupled with rhodium analysis supports
our proposal that the catalyst remains as an end group
on the oligomer chain.
F igu r e 4. 1H NMR shifts of model compounds in DMSO-d6
at 400 MHz.
Elem en ta l An a lysis. Elemental analysis was used
to characterize the 1-methyl-2-ethynyl-4,5-dicyanoimi-
dazole oligomers (11) initiated with both triethylamine
and rhodium complexes. For 11-N initiated with TEA,
an anionic polymerization is proposed; however, there
are few anionic polymerizations that are initiated by
neutral nucleophiles such as amines.40 In the case of
the monomer 1-methyl-2-ethynyl-4,5-dicyanoimidazole
(8), it seems more likely that triethylamine would act
as a base rather than as a nucleophile in the first step
of initiation. If this is the case, the oligomer may have
the composition shown in Figure 7.
The calculated C, H, N percentages are in good
agreement with the experimental data. On the basis of
the model illustrated in Figure 7 and the data in Table
5, it is unlikely that TEA is incorporated into the
oligomeric product. This would be the case if TEA acted
as a nucleophile to initiate the anionic polymerization.
F igu r e 5. Solid-state NMR of 11-N with CP and MAS at 5
kHz followed by 1H-13C decoupling; *denotes spinning side-
band.
Figure 4 support the assignment of olefinic backbone
proton shifts in the oligomers (11). Oligomers of 1-(p-
methoxybenzyl)-2-ethynyl-4,5-dicyanoimidazole (12) and
2-ethynyl-4,5-dicyanoimidazole (13) also show broad,
For oligomers made with rhodium catalysis, rhodium
analysis was used to confirm its presence in the oligo-
mers of 1-methyl-2-ethynyl-4,5-dicyanoimidazole (11)
and 1-(p-methoxybenzyl)-2-ethynyl-4,5-dicyanoimida-
zole (12). On the basis of the number-average molecular
1
multiple H NMR peaks between 7 and 8 ppm.
Solid-state NMR combined with magic angle spinning
and decoupling was used to characterize oligomers of
1-methyl-2-ethynyl-4,5-dicyanoimididazole (11) and 1-(p-
methoxybenzyl)-2-ethynyl-4,5-dicyanoimidazole (12).
Sample 11-N was analyzed first with magic angle
spinning (MAS) at 5 kHz and cross-polarization (CP).
Following the CP-MAS experiment, a delayed decou-
pling pulse was applied to probe carbon-hydrogen
connectivity. The solid-state NMR spectrum is illus-
trated in Figure 5. A 50 µs delay was introduced
following cross-polarization but before decoupling data
acquisition began. In Figure 5, the dotted line represents
1
weight from GPC, H NMR, and mechanistic aspects
proposed by Kishimoto15 and Zhan and Yang,29 an
oligomer model was created for the initiation of 2-ethy-
nyl-4,5-dicyanoimidazole derivatives using [Rh(nbd)Cl]2.
All oligomer samples were rinsed thoroughly with
methylene chloride to remove unreacted monomer and
catalysts. However, methylene chloride may displace
chlorine as a ligand in the rhodium complex.21,22,25,27
Combining all these factors, an oligomer model is
illustrated in Figures 8.

5904 Densmore and Rasmussen
Macromolecules, Vol. 37, No. 16, 2004
Ta ble 3. NMR Sh ift Com p a r ison s
Mech a n istic Asp ects. As noted above, the anionic
polymerization of cyanoacetylene with butyllithium and
triethylamine was reported by Gorman and co-work-
ers.39 In our work, we observed that 1-methyl-2-ethynyl-
4,5-dicyanoimidazole (8) could be polymerized using
triethylamine (TEA). A proposed anionic polymerization
mechanism is illustrated in Figure 9. The primary
modes of termination are likely to be chain transfer to
monomer or a solvent such as methanol (Figure 10). In
ionic polymerizations, chain transfer to monomer is one
of the most important chain-breaking reactions for most
monomers, limiting the molecular weight, especially
when the polymerization temperature is higher than
approximately 20 °C.40 It is important to note, however,
that the kinetic chain is not terminated since a new
propagating species is formed.
The polymerization 1-methyl-2-ethynyl-4,5-dicyanoim-
idazole (8) was investigated using rhodium-based cata-
lysts, and the presence of the rhodium-norbornadiene
end group was verified for the first time. The presence
of such end groups was previously suggested by Kish-
imoto and co-workers but not confirmed.15 Thus, a more
complete mechanistic picture of rhodium-catalyzed acety-
lene polymerizations can now be proposed (Figure 11).
F igu r e 6. Solid-state NMR of 12-A with CP and MAS at 5
1
kHz followed by H-13C decoupling.
There is clearly a significant amount of rhodium
present in 11-Q made with [Rh(nbd)Cl]2 in DMF. If the
complex is present as an end group, the rhodium
percentage corresponds to an average of six monomer
repeats per oligomer chain. This agrees reasonably well
with the number-average molecular weight of 1400
g/mol determinined by GPC in N-methylpyrrolidinone
(NMP). See Table 6.
The dimeric rhodium catalyst [Rh(nbd)Cl]2 dissociates
to monomeric form when dissolved in the polymerization
solvent (usually DMF or acetonitrile). The acetylene
then coordinates with the rhodium center. The resulting
vinylic complex is eliminated with the loss of H+. The
polymeric chain begins and grows by insertion of the
π-coordinated acetylene monomer with the metal-
carbon σ bond in the metal acetylide catalyst. Termina-
tion is likely via monomer chain transfer. Transfer may
occur when the acidic acetylenic hydrogen is transferred
from the π-coordinated monomer to the propagating
chain (Figure 12).
A second sample of 1-methyl-2-ethynyl-4,5-dicyanoim-
idazole oligomers (11-R) made with [Rh(nbd)Cl]2 in CH3-
CN was also analyzed, and the rhodium percentage
corresponds to an average of three monomer repeats per
oligomer chain. This agrees reasonably well with the
number-average molecular weight of 1000 g/mol deter-
minined by GPC in N-methylpyrrolidinone (NMP). See
Table 7.
Oligomers of 1-(p-methoxybenzyl)-2-ethynyl-4,5-dicy-
anoimidazole (12) made with [Rh(nbd)Cl]2 in DMF were
also analyzed. The rhodium content was 11.21%, roughly
corresponding to dimer or trimer with the rhodium
complex as an end group. GPC analysis in NMP of the
oligomers showed two peaks corresponding to dimer and
trimer.
J ust as in the case of anionic polymerization, chain
transfer to monomer limits the polymer molecular
weight, especially when the polymerization temperature
is increased.40 The acidic acetylenic hydrogen of 1-meth-
yl-2-ethynyl-4,5-dicyanoimidazole (8) implies that mono-
mer chain transfer is very facile. Thus, the molecular
weight is more severely limited, and the oligomeric

Macromolecules, Vol. 37, No. 16, 2004
Monomers and Polymers from 4,5-Dicyanoimidazole 5905
Ta ble 4. NMR Sh ift Com p a r ison of 1-(p-Meth oxyben zyl)-2-eth yn yl-4,5-d icya n oim id a zole Oligom er s
Ta ble 6. Elem en ta l An a lysis Da ta of 11-Q
theoretical n ) 6
Rh(nbd)Cl
end group
Rh(nbd)CH2Cl2
end group
experimental
C
H
57.22
2.67
56.25
2.98
53.02, 52.77
2.85, 2.84
N
29.66
7.78
97.33
1.93
28.26
7.41
94.90
1.99
27.04, 27.01
7.65, 7.65
90.56, 90.27
1.96, 1.95
Rh
total
C/N
Ta ble 7. Elem en ta l An a lysis Da ta of 11-R
theoretical n ) 3
Rh(nbd)Cl
end group
Rh(nbd)CH2Cl2
end group
F igu r e 7. Oligomers of 1-methyl-2-ethynyl-4,5-dicyanoimi-
dazole (11-N).
experimental
C
H
N
Rh
total
C/N
54.85
2.71
26.24
12.05
95.85
2.09
53.17
2.79
24.80
11.39
92.15
2.14
51.04, 51.37
2.93, 2.78
25.47, 25.82
11.94, 11.94
91.38, 91.91
2.00, 1.99
Ta ble 5. Elem en ta l An a lysis Da ta of 11-N
theoretical
expermental
C
H
N
total
C/N
61.54
2.58
35.88
100
61.09. 61.20
2.98, 2.87
34.55, 34.78
98.62, 98.85
1.77, 1.76
samples has been debated since the early 1980s.41-44
Tabata suggests a radical mechanism for the temper-
ature- and compression-promoted isomerization of the
cis-transoidal to trans-transoidal microstructure based
on electron spin resonance measurements.22,25 Tabata
asserts that the g value can be used to determine the
cis or trans geometrical conformation.27 Tabata has also
suggested that the unpaired electrons are generated by
“rotational scission of the CdC bond in the cis form
during and/or after the polymerization.”21
1.72
product results. This is consistent with other poly-
(phenylacetylene) derivatives reported in the literature.
Phenylacetylenes with electron-withdrawing substitu-
ents (Cl or NO2), and consequently more acidic acety-
lenic hydrogens, have lower molecular weights than
other poly(phenylacetylene) derivatives.23,25
EP R Ch a r a cter iza tion a n d Qu a n tita tion . The
presence of unpaired electrons in undoped polyacetylene
was detected using EPR over 20 years ago, but exactly
how unpaired electrons are generated in undoped
EPR quantitation experiments are complicated by a
variety of factors such as differences in standards,
sample preparation, and sample relaxation times.45

5906 Densmore and Rasmussen
Macromolecules, Vol. 37, No. 16, 2004
Cyclic Volta m m etr y. Cyclic voltammetry was used
to investigate trends in reduction potentials for a variety
of dicyanoimidazole derivatives. Because of extended
conjugation and the electron-withdrawing ability of
dicyanoimidazole pendant groups, the reduction of the
backbone in 1-methyl-2-ethynyl-4,5-dicyanoimidazole
oligomers (11) should be observed at more positive
potentials compared to other dicyanoimidazole deriva-
tives. For comparison purposes, several dicyanoimida-
zole derivatives were studied using cyclic voltammetry.
The reduction potentials for these dicyanoimidazole
derivatives were compared to 11 made using both
rhodium-based complexes and amine initiators.
The reduction potential of 1-methyl-2-ethyl-4,5-dicy-
anoimidazole (16) occurs at E1/2 ) -3.46 V (vs fer-
rocene). The reduction potentials for 1-methyl-2-vinyl-
4,5-dicyanoimidazole (14) are more positive and can be
observed at E1/2 ) -1.21 and -3.06 V (vs ferrocene).
The reduction of 1-methyl-2-ethynyl-4,5-dicyanoimida-
zole oligomers (11-N) occurs at even more positive
potentials and can be observed E1/2 ) 0.41 and -2.39 V
(vs ferrocene). Reduction at a more positive potential
reflects the ease of reduction for 1-methyl-2-ethynyl-4,5-
dicyanoimidazole oligomers (11).
F igu r e 8. Model of 1-methyl-2-ethynyl-4,5-dicyanoimidazole
oligomers (11-Q, 11-R).
F igu r e 9. Proposed anionic initiation and propagation for
1-methyl-2-ethynyl-4,5-dicyanoimidazole (8).
Trends in reduction potentials for nitrile-substituted
ethylenes illustrate the effect of electron-withdrawing
substituents on reduction potential. The reduction
potential becomes more positive with additional electron-
withdrawing nitrile groups. Reduction potential trends
are listed in Table 9.
Con clu sion s
In this work, the synthesis and optimization of the
monomer 1-methyl-2-ethynyl-4,5-dicyanoimidazole (8)
are reported for the first time. The role of catalyst,
reaction time, and temperature were investigated.
Increasing the amount of catalyst and extending the
reaction time generally increased the polymer yield. A
more complete mechanistic picture has been proposed
for the transition-metal-catalyzed polymerization of
acetylenes with electron-withdrawing substituents, and
termination modes and end group possibilities are
addressed. For the [Rh(nbd)Cl]2 polymerization of 1-meth-
yl-2-ethynyl-4,5-dicyanoimidazole (8), we propose ter-
mination occurs via chain transfer to the acidic acety-
lenic monomer or solvent. We also propose that the
rhodium-norbornadiene group serves as an end group
on the polymer chain. In addition, we verified and
quantified unpaired electrons in 1-methyl-2-ethynyl-4,5-
dicyanoimidazole oligomers (11) and related their pres-
ence to chain length and temperature. Finally, the
reduction potentials of 1-methyl-2-ethynyl-4,5-dicy-
anoimidazole oligomers (11) were observed at more
positive potentials than the monomeric reference com-
pounds.
F igu r e 10. Proposed chain transfer termination.
Despite these difficulties, EPR has been used by Maty-
jaszewski and Zhu to quantify radical species in atom
transfer radical polymerizations using paramagnetic
standards such as copper(II) trifluoroacetylacetonate or
copper(II) acetylacetonate.46-50
Variable temperature EPR and quantitation experi-
ments were carried out on 11. The temperature was
increased from room temperature to 200 °C in 20 °C
increments. There was a very slight decrease in the
amplitude of the EPR signal upon heating. The decrease
in EPR signal with increasing temperature for 1-methyl-
2-ethynyl-4,5-dicyanoimidazole oligomers (11) can be
related to chain length. As the temperature increases,
the number of unpaired electrons increases. However,
because the chain lengths are short, the unpaired
electrons can recombine, thus decreasing the EPR signal
with increasing temperature.
For the quantitation of unpaired electrons in 11,
copper(II) acetylacetonate was used as a standard. EPR
spectra of the standard and quantitation details can be
found in the Supporting Information. Table 8 list
oligomers analyzed with EPR. For every unpaired
electron in all three 1-methyl-2-ethynyl-4,5-dicyanoimi-
dazole oligomer samples, there are approximately 103
repeat units or 102 oligomer chains. The concentration
of unpaired spins is relatively small but easily observ-
able. On the basis of these observations, we conclude
that unpaired electrons are generated from carbon-
carbon double bond twisting and possibly localized cis-
to-trans isomerization in the chain backbone. Complete
chain isomerization seems rather unlikely, especially for
long polymer chains.
Exp er im en ta l Section
1
Meth od s. H NMR spectra were collected with Varian 300
and 400 MHz field instruments and referenced to the residual
solvent resonance. 13C NMR were collected at 100 MHz and
referenced to the residual solvent resonance. All solid-state
NMR spectra were collected using a Bruker DSX 300 MHz
spectrometer and a 4 mm double-resonance MAS probe. Solid
samples were packed in 4 mm o.d. zirconia rotors and spun
about the magic angle at 5-6 kHz. FT-IR spectra were
collected with a Perkin-Elmer Spectrum BX FT-IR system.
Mass spectra were collected with a Micromass V6 70-250-5
magnetic sector mass spectrometer, using electron impact

Macromolecules, Vol. 37, No. 16, 2004
Monomers and Polymers from 4,5-Dicyanoimidazole 5907
F igu r e 11. Proposed rhodium-catalyzed polymerization mechanism for 1-methyl-2-ethynyl-4,5-dicyanoimidazole (8).
F igu r e 12. Proposed termination via monomer chain transfer.
Ta ble 8. Oligom er s for EP R Stu d y
160U or UV-1601 spectrometer with baseline correction. GPC
was used to determine molecular weights and molecular
weight distributions of samples with respect to polystyrene
standards. The GPC system uses NMP as a solvent and 100
and 3000 Å PSS GRAM gel columns. Viscosity measurements
were made using a Cannon (100-388H) viscometer. Melting
points were collected with a Mel-Temp and are uncorrected.
Elemental analyses were collected with a Perkin-Elmer CHN
2400. Rhodium analysis was contracted to Galbraith Labora-
tories. TLC was performed on Whatman silica gel plates with
a 250 µm layer and fluorescent indicator. TGA data were
collected with a Perkin-Elmer TGA 7. DSC data were collected
with a Perkin-Elmer DSC-7.
catalyst
TEA
[Rh(nbd)Cl]2
[Rh(nbd)Cl]2
M:cat
solvent
time (h) temp (°C)
11-N
11-Q
11-R
20:1
20:1
20:1
CH3CN
DMF
CH3CN
24
48
48
65
0
0
Ta ble 9. Tr en d s in Red u ction P oten tia ls for
Nitr ile-Con ta in in g Eth ylen es
THF was dried with sodium/benzophenone before use.
Acetonitrile was distilled from calcium hydride before use.
DMSO-d6 and DMSO were normally used with residual water
present; if dryness was required, the DMSO was frozen, and
the unfrozen residual water was decanted. Trimethylsilyl-
acetylene was purchased from GFS Chemicals. All rhodium-,
palladium-, and nickel-based transition-metal catalysts were
purchased from Strem Chemicals. All other reagents and
solvents were purchased from Aldrich or Fisher Scientific.
a
The reported potentials were converted to the ferricenium/
ferrocene (Fc+/Fc) ) 0 V reference using standard reduction
potentials vs NHE: 0.24 V (SCE), 0.40 (Fc+/Fc), and 0.80 (Ag+/
Ag).54
Ch ar acter ization . 1-Meth yl-2-(tr im eth ylsilylacetylen e)-
4,5-d icya n oim id a zole (7). A three-neck, 500 mL round-
bottom flask fitted with a magnetic stir bar and condenser was
charged with 9.00 g (42 mmol) of 1-methyl-2-bromo-4,5-
dicyanoimidazole (5). Upon addition of 225 mL of freshly
distilled THF, the mixture was stirred to dissolve the dicy-
anoimidazole solid. Via syringe, 9.0 mL (63 mmol) of triethyl-
amine was added to the flask along with 9.45 mL (63 mmol)
of trimethylsilylacetylene. To an angled solid addition funnel,
252 mg (1.3 mmol) of copper(I) iodide and 0.900 g (1.3 mmol)
of PdCl2(PPh3)2 were measured. The solid addition funnel was
attached to the round-bottom flask, and a nitrogen/vacuum
line was attached to the remaining open neck. The flask was
evacuated with reduced pressure and then purged with
nitrogen gas for a minimum of 15 min. The flask was immersed
in a 65 °C oil bath, and the solids were added. The reaction
was allowed to stir under nitrogen for 24 h. The reaction
mixture was evaporated down to a dark oil under reduced
(70 eV), fast atom bombardment, or chemical ionization
(ammonia) for ionization. GCMS spectra were collected using
a HP 5890 GC system coupled with a Finnigan automated GC/
EI-CI mass spectrometer system. EPR spectra were collected
using a Varian system with a regulated magnetic power
supply, a Varian V-4540 variable temperature controller, and
a Bruker ER 041MR microwave bridge controller. CV was done
using a Princeton Applied Research potentiostat/galvanostat
model PAR 263A interfaced with Powersuite software. The
solvent was dry acetonitrile or DMSO, and the supporting
electrolyte was 0.1 M tetrabutylammonium hexafluorophos-
phate (NBu4+PF6-) using a Pt button working electrode, a Pt
wire counter electrode, and a Ag/AgNO3 (0.01 M) reference
electrode. UV spectra were collected with a Shimadzu UV-

5908 Densmore and Rasmussen
Macromolecules, Vol. 37, No. 16, 2004
pressure. To the dark oily residue, 300 mL of 10% acetone in
ether was added along with activated charcoal. The mixture
was stirred at room temperature for approximately 2 h. The
mixture was filtered through a 1.5 in. pad of neutral alumina
and rinsed with copious amounts of 10% acetone in ether. The
filtrate was evaporated under reduced pressure to give a brown
residue. For additional purification, the solid was stirred with
warm ethanol and activated charcoal. The mixture was filtered
through Celite. The combined yield was approximately 75%.
Note: this product cannot be recrystallized by standard means
as it will spontaneously deprotect and polymerize.
FW (calcd) 228.3 g/mol; mp 106-110 °C; TLC Rf 0.69 (50/
50 hexanes/ethyl acetate). 1H NMR (400 MHz; DMSO-d6) δ
(ppm): 0.29 (s, 9H, (CH3)3), 3.80 (s, 3H, CH3). IR (KBr Pellet,
cm-1): 2963, 2904, 2239, 1628, 1456, 1391, 1252, 875, 848, 764.
CHN Analysis: Anal. Calcd for C11H12N4Si: C, 57.86; H, 5.30;
N, 24.54. Found: C, 57.91; H, 5.30; N, 24.85.
1-Meth yl-2-eth yn yl-4,5-d icya n oim id a zole (8). A 250 mL
Erlenmeyer flask fitted with a magnetic stir bar was charged
with 5.0 g (22 mmol) of 1-methyl-2-(trimethylsilylacetylene)-
4,5-dicyanoimidazole (7). To dissolve the solid, 150 mL of
distilled tetrahydrofuran was added to the flask. Upon adding
4.13 g (44 mmol) of potassium fluoride dihydrate to the flask,
the deprotection process was monitored by TLC. The reaction
was stirred at room temperature for no more than 2 h. The
mixture was filtered through a pad of neutral alumina, and
the filtrate was evaporated down under reduced pressure to
give a brown solid consisting of the target product and
oligomers. For purification, the crude monomer was rinsed
with copious amounts of 20% ethyl acetate in hexanes. The
mixture was filtered, and the filtrate was evaporated down to
give a 90% combined yield of light yellow solid. Note: this
product cannot be recrystallized by standard means as it will
polymerize. Very small amounts of monomer can be recrystal-
lized with ether or sublimed carefully. Any heating (>40 °C)
appears to polymerize the monomer. In addition, evidence
suggests that the monomer may undergo photopolymerization.
The product should be stored in an amber vial.
FW (calcd) 156.15 g/mol; mp 97-98 °C; TLC Rf 0.38 (50/50
hexanes/ethyl acetate). 1H NMR (400 MHz; DMSO-d6) δ
(ppm): 3.82 (s, 3H, CH3), 5.16 (s, 1H, CH). 13C NMR (100 MHz;
DMSO-d6) δ (ppm): 34.33 (CH3), 70.20 (â), 88.58 (R), 108.25
(C5), 111.90 (C4), 114.04 (CN at C5), 120.01 (CN at C4), 134.99
(C2). IR (KBr pellet, cm-1): 3268, 2953, 2242, 2131, 1469, 1394,
1323, 726, 714, 697, 667. CHN Analysis: Anal. Calcd for
C8H4N4: C, 61.54; H, 2.58; N, 35.88. Found: C, 61.60; H, 2.91;
N, 33.94. UV/vis (CH3CN) λmax (ꢀ) (nm, M-1 cm-1): 256.0
(19 200)
2-Eth yn yl-4,5-d icya n oim id a zole (10). A 100 mL Erlen-
meyer flask fitted with a magnetic stir bar was charged with
0.350 g (1.33 mmol) of 1-(p-methoxybenzyl)-2-(ethynyl)-4,5-
dicyanoimidazole (9). The solid was dissolved in 15 mL of 3:1
acetonitrile/water. In a separate 20 mL glass vial, 2.93 g (5.34
mmol) of ceric ammonium nitrate (CAN) was dissolved in 5
mL of 3:1 acetonitrile/water. The ceric ammonium nitrate
solution was added to the Erlenmeyer flask and heated at 50
°C for 17 h. The reaction was monitored with TLC. The
presence of p-anisaldehyde confirmed the reaction was pro-
gressing. After the reaction was complete, the acetonitrile was
evaporated off under reduced pressure. The residue was
treated with ethyl acetate and water and rinsed with a
saturated solution of sodium bicarbonate (3 × 20 mL). The
aqueous layer was back-extracted with ethyl acetate (2 × 20
mL). The white ceric hydroxide precipitate was then filtered
from the combined aqueous layer. The aqueous filtrate was
acidified slowly with hydrochloric acid. The acidified solution
was then extracted with ethyl acetate (3 × 20 mL). The
combined organics were dried over magnesium sulfate. The
drying agent was filtered, and the resulting filtrate was
evaporated under reduced pressure to give a 80% yield of
yellow waxy solid. Note: p-anisaldehyde continues to contami-
nate the monomer. Additional purification did not completely
remove the aldehyde.
CH). IR (KBr pellet, cm-1): 3200-2700 hydrogen-bonding
complexity, 2248, 2132, 1383, 1262, 1097, 1022, 800.
1-(p-Met h oxyb en zyl)-2-et h yn yl-4,5-d icya n oim id a zole
(9). A 500 mL Erlenmeyer flask fitted with a magnetic stir
bar was charged with 5.00 g (14.9 mmol) of 1-(p-methoxyben-
zyl)-2-(trimethylsilylacetylene)-4,5-dicyanoimidazole. To dis-
solve the solid, 250 mL of distilled tetrahydrofuran was added
to the flask, and the solution was stirred until the solid
dissolved. Upon adding 2.80 g (29.8 mmol) of potassium
fluoride dihydrate to the flask, the deprotection process was
monitored by TLC. The reaction was stirred at room temper-
ature for no more than 2 h. The mixture was gravity filtered
through a pad of neutral alumina, and the filtrate was
evaporated down under reduced pressure. For purification, the
oil was repeatedly rinsed with 10% ethyl acetate in hexanes.
The solution was evaporated under reduced pressure to give
the first portion of amber oil. The second and third portions
were recovered by repeated washings with 20-30% ethyl
acetate in hexanes. The combined portions of amber oil gave
a 73% yield. Further purification can be achieved by recrys-
tallization with methanol to give a tan yellow solid. This
recrystallization proved to be quite difficult and did not work
very well on larger scales. The purity of the amber oil is quite
good, however, as indicated by NMR. The oil product was used
for additional reactions.
FW (calcd) 262.27 g/mol, generally amber oil; mp 70-74 °C
(recrystallized from methanol); TLC Rf 0.48 (50/50 hexanes/
1
ethyl acetate). H NMR (400 MHz; acetone-d6) δ (ppm): 3.73
(s, 3H, CH3), 5.13, (s, 1H, CH), 5.44 (s, 2H, CH2), 6.95, 6.98 (d,
2H, aromatic ring), 7.28, 7.31 (d, 2H, aromatic ring). IR (NaCl
plate, cm-1): 3267, 2959, 2838, 2241, 2128, 1611, 1585, 1514,
1467, 1407, 1345, 1306,1254, 1179, 1118, 1031, 847, 807.
Typ ica l Oligom er iza tion of Acetylen ic Mon om er s w ith
TEA. A 25 mL three-neck round-bottom flask fitted with a
stir bar was charged with 0.67 mmol of acetylenic monomer
(8 or 9). To dissolve the monomer, 7.5 mL of distilled
acetonitrile was added to the flask. A vacuum/nitrogen line,
condenser, and rubber septum were attached to the flask and
secured. The flask was evacuated with reduced pressure and
purged with nitrogen for 15 min. Via syringe, 0.14 mL (0.96
mmol) of triethylamine was injected through the rubber
septum and into the solution. The flask was immersed in a 65
°C oil bath and stirred for 24 h. The apparatus was removed
from heat and allowed to cool to room temperature. The
acetonitrile was evaporated off under reduced pressure to give
a dark residue. The residue was washed with 10% acetone in
ether to extract residual monomer in the polymer sample. The
dark solid was dried overnight under reduced pressure.
Typ ica l Oligom er iza tion of Acetylen ic Mon om er s w ith
[R h (n bd )Cl]2. A clean, dry two-neck, 25 mL round-bottom
flask was fitted with a stir bar and vacuum/nitrogen line and
charged with 0.76 mmol of acetylenic monomer (8 or 9). The
solid was dissolved in 1.8 mL of DMF, and the open neck was
sealed with a rubber septum. The flask was evacuated under
reduced pressure and purged with nitrogen several times. A
clean, dry vial was charged with 0.017 g (0.038 mmol) of [Rh-
(nbd)Cl]2, and 2 mL of DMF was added. The mixture was
stirred until the catalyst completely dissolved. After 10 min,
the catalyst solution was injected into the monomer solution
and allowed to stir for 48 h in an ice-water bath. After the 48
h period, the reaction solution was concentrated under reduced
pressure. Ether or methylene chloride was added to the
residue, and the mixture was centrifuged (or gravity filtered).
After decanting off the solution, the solid was again rinsed
with ether or methylene chloride and centrifuged. The dark
brown/black product was dried under reduced pressure over-
night.
Dep r otection of P oly[1-(p-m eth oxyben zyl)-2-eth yn yl-
4,5-d icya n oim id a zole] (12). A 50 mL Erlenmeyer flask fitted
with a magnetic stir bar was charged with 0.050 g of poly[1-
(p-methoxybenzyl)-2-(ethynyl)-4,5-dicyanoimidazole] (12). The
solid was dissolved in 20 mL of 3:1 acetonitrile/water. In a
separate 20 mL glass vial, 0.417 g (0.76 mmol) of ceric
ammonium nitrate was measured. The ceric ammonium
nitrate was added to the Erlenmeyer flask and heated at 55
FW (calcd) 142.12 g/mol; TLC Rf 0.05 (50/50 hexanes/ethyl
1
acetate). H NMR (400 MHz; DMSO-d6) δ (ppm): 4.72 (s, 1H,

Macromolecules, Vol. 37, No. 16, 2004
Monomers and Polymers from 4,5-Dicyanoimidazole 5909
Sch em e 5. Dep r otection of
P oly[1-(p-m eth oxyben zyl)-2-eth yn yl-4,5-
d icya n oim id a zole] (12)
Curtis for cyclic voltammetry assistance and Erin Wim-
mers of Prof. Larry Beck’s lab for solid-state NMR
assistance. We also acknowledge support from the
donors of the Petroleum Research Fund, administered
by the American Chemical Society, and from the Gen-
eral Motors Corp.
Su p p or tin g In for m a tion Ava ila ble: Details on the syn-
thesis of additional dicyanoimidazole derivatives, EPR spectra
of standards and oligomers, and cyclic voltammagrams. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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F igu r e 14. Summary of reduction potentials of dicyanoimi-
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°C for 19 h. The reaction was monitored with TLC. The
presence of p-anisaldehyde in TLC confirmed the reaction was
progressing as it is a product of oxidative p-methoxybenzyl
deprotection. After the reaction was complete, the acetonitrile
was evaporated off under reduced pressure. The residue was
treated with ethyl acetate and water and rinsed with a 10%
solution of potassium bicarbonate (3 × 20 mL). The aqueous
layer was back-extracted with ethyl acetate (2 × 20 mL). The
white ceric hydroxide precipitate was then filtered from the
combined aqueous layer. The aqueous filtrate was acidified
slowly with hydrochloric acid. The acidified solution was then
extracted with ethyl acetate (3 × 20 mL). The combined
organics were dried over magnesium sulfate. The drying agent
was filtered, and the resulting filtrate was evaporated under
reduced pressure, yielding (13).
EP R P a r a m eter s. The variable temperature EPR spectra
were collected under the following conditions: gain ) 2.5 ×
105, modulation ) 2.5 Gpp, time constant ) 200 ms, center field
) 3365 G, sweep width ) 50 G, modulation frequency ) 100
kHz, phase ) 0°, sweep time ) 50 s, microwave frequency )
9.41 GHz, microwave power ) 208 µW, attenuation ) 30 dB,
and temperature ) r.t. to 200 °C in 20° increments. To quantify
the unpaired electron, copper(II) acetylacetonate was used as
a standard. The EPR spectrum of solid copper(II) acetylaceto-
nate was collected under the following conditions (the same
conditions were used for oligomer samples): gain ) 2.5 × 105
and 10 × 105, modulation ) 5 Gpp, time constant ) 200 ms,
center field ) 3300 G, sweep width ) 1000 G, modulation
frequency ) 100 kHz, phase ) 0°, sweep time ) 50 s,
microwave frequency ) 9.74 GHz, microwave power ) 208 µW,
attenuation ) 30 dB, and temperature ) r.t.
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