Independent electrocyclization and oxidative chain cleavage along the backbone of cis-poly(phenylacetylene)

Article abstract of DOI:10.1021/ma051060y

cis-Poly(phenylacetylene) (PPA) and cis-poly(pentadeuteriophenylacetylene) (PPA-d₅) undergo similar thermally induced transformations in bulk and solution. The structures identified in thermally treated samples include cis - trans isomerization of the backbone, cyclohexadiene repeat units in the main chain, and extruded 1,3,5-triphenylbenzene (1,3,5-C₆H ₃Ph₃). Among these structures, cyclohexadiene sequences along the backbone formed by intramolecular cyclization are the principal products in all solvents regardless of temperature and the presence or absence of ambient light or O₂. We propose that cyclohexadiene structures form via 6π electrocyclization of triene sequences and, therefore, are an unavoidable consequence of cis-alkene units of the polyene backbone. Concurrent with cyclization, decrease of molecular weight and increase of molecular weight distribution are observed. Traces of acid lead to a dramatic increase in cyclization. Extrusion of 1,3,5-C₆H₃Ph₃ results from cyclohexadiene repeat units and is slower than electrocyclization, even in the presence of acid, light, or O₂. In the presence of ambient light, O₂ accelerates the rate of cyclization and extrusion of 1,3,5-C₆H₃Ph₃, the decrease of molecular weight, and the formation of carbonyl-containing products detected in a 5:1 molar ratio to extruded 1,3,5-C₆H₃Ph₃. Under these conditions, the most dramatic changes in molecular weight and molecular weight distribution are observed.

Full text of DOI:10.1021/ma051060y

Macromolecules 2005, 38, 7241-7250
7241
Independent Electrocyclization and Oxidative Chain Cleavage along the
Backbone of cis-Poly(phenylacetylene)
Virgil Percec* and Jonathan G. Rudick
Roy & Diana Vagelos Laboratories, Department of Chemistry, University of Pennsylvania,
Philadelphia, Pennsylvania 19104-6396
Received May 24, 2005; Revised Manuscript Received June 30, 2005
5
ABSTRACT: cis-Poly(phenylacetylene) (PPA) and cis-poly(pentadeuteriophenylacetylene) (PPA-d ) un-
dergo similar thermally induced transformations in bulk and solution. The structures identified in
thermally treated samples include cis-trans isomerization of the backbone, cyclohexadiene repeat units
6 3 3
in the main chain, and extruded 1,3,5-triphenylbenzene (1,3,5-C H Ph ). Among these structures,
cyclohexadiene sequences along the backbone formed by intramolecular cyclization are the principal
products in all solvents regardless of temperature and the presence or absence of ambient light or O
2
.
We propose that cyclohexadiene structures form via 6π electrocyclization of triene sequences and, therefore,
are an unavoidable consequence of cis-alkene units of the polyene backbone. Concurrent with cyclization,
decrease of molecular weight and increase of molecular weight distribution are observed. Traces of acid
lead to a dramatic increase in cyclization. Extrusion of 1,3,5-C
units and is slower than electrocyclization, even in the presence of acid, light, or O
ambient light, O accelerates the rate of cyclization and extrusion of 1,3,5-C Ph
molecular weight, and the formation of carbonyl-containing products detected in a 5:1 molar ratio to
extruded 1,3,5-C Ph . Under these conditions, the most dramatic changes in molecular weight and
molecular weight distribution are observed.
6
H
3
Ph
3
results from cyclohexadiene repeat
. In the presence of
, the decrease of
2
2
6
H
3
3
6
H
3
3
Introduction
takes place with an activation energy of ∼29 kcal/mol
in bulk.3
4,48
Little change is observed by H NMR, IR,
1
Poly(phenylacetylene) (PPA), a soluble analogue of
polyacetylene, has been of interest for its electrical
or UV-vis for as long as 2 years in the solid state at
0-25 °C.49 Even at 120-130 °C, cyclization occurs
without 1,3,5-C6H3Ph3 formation and a small decrease
2
1
2
conductivity, photoconductivity, and nonlinear optical
properties, especially in liquid crystalline phases.5-11
Cis-cisoidal, cis-transoidal, trans-cisoidal, and trans1-
transoidal PPA can each adopt a helical conformation,
and circular dichroism confirmed that cis-transoidal
3,4
of Mn.1
,34,35,37
In solution, the activation energy depends
on the initial cis-content and ranges from 13 to 24 kcal/
mol.3
4,48
In purified THF solution, the same has been
observed, and Mn is unchanged for samples up to 50
days.49 Above 160 °C, extrusion of 1,3,5-C6H3Ph3 is
significant in bulk and solution. Samples undergo large
1
2,13
PPA is a dynamic helical polymer.
Subsequently,
most research is directed toward PPAs as biological
10,11,14-19
17,20-31
mimics,
sensors to measure chirality,
and
3
7,38
decrease of Mn,
depletion of the long wavelength
chiral stationary phases for enantioselective separa-
absorption in UV-vis spectra,50 cyclization, and extru-
38
3
2
tion.
50
sion of 20-25% of 1,3,5-C6H3Ph3. Complete loss of the
Thermally induced reactions of PPA consist of isomer-
ization of cis-PPA to trans-alkenes and formation of
several cyclic sequences: 1,3-cyclohexadiene, 1,2-ter-
phenylene, 1,3-terphenylene, and 1,4-terphenylene.1,33-36
Terphenylene sequences form by the aromatization of
the 1,3-cyclohexadiene units with elimination of ben-
zene. The extrusion of triphenylbenzene (C6H3Ph3)
occurs by aromatization of the 1,3-cyclohexadiene se-
1
cis-polyene is observed by H NMR spectroscopy to-
gether with a significant decrease of Mn (by GPC) and
51
formation of 10% 1,3,5-C6H3Ph3 (1 h, 180 °C, bulk).
For a liquid crystalline polyarylacetylene, structural
changes were observed above 180 °C where 1,3,5-
triarylbenzene was found to be the only aromatic
product.6
1
,33,34
More complex results have been reported for thermal
quences and concomitant chain cleavage.
Scheme
treatments in air.4
9,51-55
The focus of such experiments
1
outlines the formation of 1,3-terphenylene units and
has been on changes in molecular weight and spectral
evidence for oxidation. Almost all reports agree that
molecular weight decreases for bulk and solution
samples. Vohlidal, Sedlacek, and co-workers have shown
that the decrease of molecular weight in THF solution
can bias the determination of absolute and relative
molecular weights.56 Short residence times (i.e., less
than 1 h with THF as eluent) on the GPC column
provide reproducible results. The cumulative time spent
in solution appears to be the most significant factor
related to the decrease of molecular weight. Air acceler-
ates the decrease of cis-polyene sequences and molecular
weight compared to samples in inert atmosphere.
Similar to anaerobic experiments, the decrease of rela-
tive molecular weight for solutions of PPA in CHCl3,
the extrusion of 1,3,5-C6H3Ph3 from a stereoregular
head-to-tail cis-transoidal PPA. Alternate isomers result
from head-to-head defects and stereoirregularity. De-
37,38
crease of molecular weight supports this mechanism.
3
5,39-41
Development of metathesis
and of Rh(I)-based
raised questions regarding the generality
42-44
catalysts
of the above observations.
3
8,45-47
A general framework
has emerged from bulk and solution samples investi-
gated in anaerobic (vacuum, Ar, or N2) and ambient
atmosphere.
In the absence of air, cyclization and aromatization
are the only observed processes. Loss of the cis-polyene
*
Corresponding author. E-mail: percec@sas.upenn.edu.
0.1021/ma051060y CCC: $30.25 © 2005 American Chemical Society
1
Published on Web 07/30/2005

7242 Percec and Rudick
Macromolecules, Vol. 38, No. 17, 2005
Scheme 1. Thermal Reactions of Stereoregular Head-to-Tail Cis-transoidal Poly(phenylacetylene) (PPA)
PhCl, and THF in air is not influenced by light.49,55
Masuda and co-workers have questioned the generality
of these results, observing an accelerated decrease of
molecular weight of PPA in THF and toluene solu-
tilled from Mg turnings under argon. RhCl
bicyclo[2.2.1]hepta-2,5-diene (nbd, Aldrich), n-butyllithium (2.5
M in hexanes, Aldrich), hexachloro-1,3-butadiene (C Cl , Acros),
and anhydrous potassium carbonate (K CO , Fisher) were used
as received. Triphenylphosphine (99%, PPh , Acros) was
recrystallized from hexanes. Phenylacetylene (98%, PA) was
vacuum-distilled from CaH (both from Acros). 4-(Dimethy-
lamino)pyridine (DMAP, Lancaster) was recrystallized from
toluene. Carbon tetrachloride (CCl , Acros) was fractionally
distilled from P O under argon. 2,2,6,6-Tetramethyl-1-pip-
3 2
‚xH O (Lancaster),
4
6
2
3
5
5
1
3
tions. Both IR and H NMR spectroscopy support the
formation of oxidation products in PPA solutions (i.e.,
THF, PhCl, and CHCl3) thermally treated in air.4
Bulk thermal treated PPA produced oxidation prod-
2
9,51,55
4
5
2,57
ucts.
By contrast, bulk samples annealed at 180 °C
2
5
for 1 h in air are indistinguishable from those under
vacuum.
eridinyloxy free radical (TEMPO, Acros) was sublimed under
vacuum. [Rh(nbd)Cl] was prepared according to a literature
procedure. Rh(CtCPh)(nbd)(PPh (Noyori’s catalyst) was
prepared according to a literature procedure
without recrystallization. Deuterated solvents CDCl
D), C (99.6% D), and THF-d (99.5% D) (all from Cambridge
Isotope Laboratories) were used as received. In CDCl , trace
acid and its removal by storage over K CO were qualitatively
confirmed using pH paper (Whatman).
5
1
2
5
9
3 2
)
Additives and impurities complicate efforts to draw
connections between experiments. Stable free radicals
and radical trapping agents retard the rate of molecular
weight decrease in THF, PhCl, and CHCl3 but do not
6
0,61
and used
(99.98%
3
D
6 6
8
3
4
9,53,55
prevent it.
purification, Mw of PPA determined by light scattering
20 °C) increases with time in THF solution, remains
constant in toluene solution, and decreases for a CHCl3
Using reagent grade solvents without
2
3
(
1
Techniques. H (500 MHz) NMR spectra were recorded on
a Bruker DRX-500 or Bruker DMX-600 spectrometer. Sealed
NMR sample tube experiments were performed in NMR
sample tubes equipped with a Schlenk-adapted Teflon screw
cap (New Era Enterprises). UV-vis spectra were recorded
using a Shimadzu UV-1601 spectrophotometer. Relative mo-
lecular weights were determined using a Shimadzu LC-10AT
liquid chromatograph equipped with a CTO-10A column oven
5
4
solution. A contrasting report, which does not specify
the quality or purification of THF, found Mw of PPA
determined by light scattering (25 °C) to decrease in
time. Addition of HCl to a CHCl3 solution of PPA
accelerates the rate of molecular weight decrease while
5
5
K2CO3 retards the process.
(
40 °C), a Nelson Analytical 900 Series integrator data station,
To clarify the role of O2 in thermal transformation of
PPA and provide a clear understanding of the general
process, we have undertaken mechanistic and structural
studies. Herein we will confirm the microstructural
changes in PPA prepared with Rh(I)-based catalysts,
demonstrate that the intramolecular cyclization rate is
solvent independent, and isolate the effects of ambient
light and O2. Thermal treatment of PPA-d5 clearly
shows that formation of cyclohexadiene repeat units is
the most prominent reaction, although it is accompanied
by extrusion of 1,3,5-C6H3Ph3 and cis-trans isomeriza-
tion. We clarify the role of base and radical scavengers
in CDCl3 solutions of PPA to confirm that the rate of
cyclization is solvent independent. Taken with the
general scheme above, the process is best described as
a 6π electrocyclization of triene sequences along the
polyene backbone. This description becomes more com-
plex when samples are thermally treated in acidic media
or in the presence of both O2 and ambient light.
2
and two Polymer Laboratories PL gel columns of 5 × 10 and
4
1
0 Å with THF as the eluent at 1 mL/min. Detection was by
UV absorbance at 254 nm. Under a N atmosphere THF is
2
distilled into the reservoir that supplies the GPC pump to
eliminate oxygen and peroxide contaminants. Relative weight-
average (M ) and number-average (M ) molecular weights
w n
were calculated using a calibration plot constructed from
polystyrene standards. Air-sensitive procedures were per-
formed under argon using standard Schlenk techniques.
Typical Polymerization of Phenylacetylene (PA) Us-
3 2
ing Rh(CtCPh)(nbd)(PPh ) . An oven-dried, argon-filled
round-bottom flask equipped with a Teflon-coated magnetic
stir bar was charged with 37.1 mg (0.045 mmol) of Rh(Ct
CPh)(nbd)(PPh3)2 and 56.4 mg (0.46 mmol) of DMAP. The
solids were degassed by three cycles of evacuation followed
by backfilling with argon. Via a glass syringe, 50 mL of dry
THF was added under argon and stirred until all solids were
dissolved. Another oven-dried, argon-filled Schlenk tube was
charged with 1.0 mL (9.01 mmol) of PA and 10 mL of dry THF.
This solution was degassed by three freeze-pump-thaw
cycles. The THF solution of PA was transferred to the catalyst
solution at 21 °C under argon using a glass syringe. The
reaction was stirred under argon for 3 h and quenched with 2
mL of glacial acetic acid. The reaction mixture was precipitated
into 500 mL of cold MeOH. The polymer was collected by
filtration and dried under vacuum to yield 780 mg (85%) of
Experimental Section
Materials. Diethyl ether (Et O, Fisher) and tetrahydrofu-
2
ran (THF, Fisher, Certified A.C.S.) were distilled from Na/
benzophenone ketyl under argon. Glacial acetic acid (Fisher)
was used as received. Methanol (MeOH, Fisher, Certified
A.C.S.) and ethanol (100%, EtOH, Pharmco) were each dis-
4
1
PPA. M
n
) 5.6 × 10 g/mol; M
w n 3
/M ) 1.54. H NMR (CDCl , δ,
ppm): 6.94 (m, 3H), 6.64 (d, J ) 6.89 Hz, 2H), 5.85 (s, 1H).

Macromolecules, Vol. 38, No. 17, 2005
cis-Poly(phenylacetylene) 7243
2
Typical Polymerization of PA Using [Rh(nbd)Cl] . A
flame-dried, argon-filled round-bottom flask equipped with a
Teflon-coated magnetic stir bar was charged with 39.2 mg
(
0.009 mmol) of [Rh(nbd)Cl]
cycles of evacuation followed by backfilling with argon. Via a
glass syringe, 6.0 mL of dry NEt was added under argon and
warmed to 30 °C. In a flame-dried, argon-filled Schlenk tube,
.0025 (9.81 mmol) of PA was degassed by three freeze-
pump-thaw cycles. Addition of 7.0 mL of dry NEt produced
2
. The solid was degassed by three
3
1
3
a yellow solution, which was immediately transferred via glass
syringe to the catalyst solution. The reaction was stirred under
argon for 20 h, and the resulting precipitate was collected by
filtration. Consecutive washings of the solid material with THF
separated the desired cis-transoidal fraction. Precipitation of
the THF washings into cold MeOH provided 240 mg (24%) of
4
PPA. M
n
) 9.4 × 10 g/mol; M
w n
/M ) 2.09.
Thermal Treatment of Cis-Transoidal PPA Solutions
at 60 °C in NMR Sample Tubes Sealed in Vacuum (in
the Dark). A solution of ∼7 mg of cis-PPA in 0.5 mL of CCl
4
(
or other solvent) was prepared while taking no precaution to
exclude to air. Samples were subjected to three freeze-pump-
thaw cycles, and the NMR sample tube was sealed. Degassed
samples were introduced into an NMR spectrometer at 60 °C.
After appropriate time intervals, spectra were measured at
Figure 1. 1H NMR spectra of PPA-d
as-prepared in CCl , (b) following thermal treatment at 60 °C
for 8 h in CCl , and (c) following thermal treatment at 60 °C
in CDCl . Thermal treatments were performed on degassed
samples in NMR sample tubes sealed under vacuum. Heating
was performed in an oil bath with no precaution to exclude
ambient light.
measured at 28 °C: (a)
5
4
4
60 °C.
3
Thermal Treatment of Cis-Transoidal PPA Solutions
at 60 °C in NMR Sample Tubes Sealed in Vacuum (in
the Dark). A solution of ∼7 mg of cis-PPA (or cis-PPA-d
5
) in
0
.5 mL of CCl (or CDCl ) was prepared while taking no
4 3
precaution to exclude to air. Samples were subjected to three
sample was subsequently diluted with THF and used for GPC
measurements.
In a similar experiment, CCl was sparged with O for 30
4 2
min prior to sample preparation. The NMR sample tubes were
sealed under an ambient atmosphere before heating.
freeze-pump-thaw cycles, and the NMR sample tube was
sealed. Degassed samples were introduced into an oil bath
1
equilibrated at 60 °C. After appropriate time intervals,
H
NMR spectra were measured at 30 °C. In some cases, the
sample was opened and diluted with THF after obtaining the
1
H NMR spectrum. The THF solution was used for GPC
Results and Discussion
measurements.
Thermal Treatment of Cis-Transoidal PPA in C
4
Cl
6
Numerical Model for Kinetic Analyses. In a
previous report51 we took for granted that cis-transoidal
Solutions at 120 or 160 °C in an NMR Sample Tube
Sealed in Vacuum (with Light). A solution of ∼26 mg of
PPA prepared with Noyori’s arylacetylene polymeriza-
cis-PPA in 3.0 mL of C
4
Cl
6
was prepared while taking no
60,61
tion catalyst
undergoes the same thermal transfor-
precaution to exclude to air. Portions were distributed among
several NMR sample tubes. Samples were subjected to three
freeze-pump-thaw cycles, and the NMR sample tube was
sealed. Degassed samples were introduced into an oil bath
equilibrated at the desired temperature. After appropriate
mations as PPA prepared with Ziegler-Natta type or
ill-defined metathesis catalysts. To develop a quantita-
tive measure for the observed transformations, we
prepared poly(pentadeuteriophenylacetylene) (PPA-d5)
and investigated its structure after thermal treatment.
1
time intervals, H NMR spectra were measured at 60 °C.
1
Figure 1 presents H NMR spectra for the as-prepared
2
Thermal Treatment of Cis-Transoidal PPA in O -
Saturated Solutions at 60 °C in Sealed NMR Sample
sample in CCl4 as well as samples sealed under vacuum
Tubes (in the Dark). Carbon tetrachloride (or CDCl
sparged with O for 30 min prior to preparing the sample
solution. A solution of ∼7 mg of cis-PPA was dissolved in 0.5
mL of CCl (or other solvent) was prepared while taking no
3
) was
and thermally treated at 60 °C for 8 h in CCl4 and
2
CDCl . Evident from the as-prepared sample is the lone
3
resonance at δ 5.80 ppm assigned to the cis-transoidal
4
1,33,61
stereostructure.
Downfield from this resonance are
precaution to exclude to air. The NMR sample tube was sealed
and introduced into an NMR spectrometer at 60 °C. Spectra
were measured at appropriate time intervals.
very weak resonances from incomplete deuteration of
the aromatic ring. Three new resonances are observed
in the thermally treated samples. At δ 7.68 ppm in CCl4
and δ 7.80 ppm in CDCl3 is the resonance for 1,3,5-C6H3-
Thermal Treatment of Cis-Transoidal PPA in Solution
with TEMPO at 60 °C in an NMR Sample Tube Sealed
in Vacuum (in the Dark). A solution of ∼8 mg of cis-PPA
(
Ph-d5)3. At δ 7.26 ppm in CCl4 and δ 7.36 ppm in CDCl3
is the resonance for trans-alkene residues along the
polyene backbone.1 Finally, a broad resonance at δ
3.90 ppm in CCl4 and δ 4.00 ppm in CDCl3 is the
3
and 4.5 mg (0.03 mmol) of TEMPO in 0.5 mL of CDCl was
,33
prepared while taking no precaution to exclude to air. Samples
were subjected to three freeze-pump-thaw cycles, and the
NMR sample tube was sealed. Degassed samples were intro-
duced into an NMR spectrometer at 60 °C. Spectra were
measured at appropriate time intervals.
methine proton of the cyclohexadiene structures formed
1,33,38,45,46
along the backbone.
Neither sample exhibits
resonances inconsistent with that previously described
for PPA prepared using other catalysts.
Thermal Treatment of Cis-Transoidal PPA Solutions
in CCl
4
at 60 °C (with Light and O
2
). Approximately 23 mg
with no precaution
To quantify the extent that each transformation takes
place during thermal treatment, we define the mole
fraction of alkene residues (Fa.r.) for each component
structure. Each cis or trans double bond constitutes a
single alkene residue. The cyclic structures cyclohexa-
diene and 1,3,5-C6H3Ph3 each account for three alkene
residues. From the spectra for PPA-d5 in Figure 1 these
of cis-PPA was dissolved in 1.5 mL of CCl
4
for the exclusion of air. Portions were distributed among
several NMR sample tubes. The NMR sample tubes were then
capped with a Teflon screw cap valve, but the valve was left
open to ambient atmosphere. Samples were introduced into
an oil bath equilibrated at 60 °C. After appropriate time
1
intervals, H NMR spectra were measured at 30 °C. The

7244 Percec and Rudick
Macromolecules, Vol. 38, No. 17, 2005
previously described for samples prepared with alter-
nate catalysts.1,6,33-35,37,38,47-56
To extend this analysis to nondeuterated polymers,
the aromatic ring protons need to be considered. It was
found empirically that the cis-PPA and trans-PPA could
not be calculated independently. Broadening of the
resonances for the aromatic ring protons in thermally
treated PPA and overlap with resonances from 1,3,5-
C6H3Ph3 prevents identification and integration of a
unique resonance for trans-alkene residues in PPA. As
such, there are too few independent variables to solve
the above system of equations. Consequently, we will
calculate Fa.r.(PPA) ) Fa.r.(trans-PPA) + Fa.r.(cis-PPA).
The proton resonances for the 1,3,5-C6H3Ph3 core and
the cyclohexadiene methine are readily isolated and
integrated (AC H Ph and AMethine, respectively) in all
6
3
3
solvents. The cis-polyene resonance is well-resolved but
is overlapped by the vinyl resonances from the cyclo-
hexadiene (Acis-PPA+diene), as noted above. Taking each
of these into consideration, the following expressions
were employed to monitor Fa.r. for each component:
Figure 2. Plots of the mole percentage of alkene residues (Fa.r.
100%) attributed to PPA (9), cyclohexadiene moieties (O),
and extruded 1,3,5-C Ph (b) for (a) CCl , (b) C Cl , (c) THF-
, and (d) C solutions thermally treated at 60 °C. Thermal
Fa.r.(PPA) )
×
1
- Fa.r.(cyclohexadiene) - Fa.r.(C H Ph )
6 3 3
H
6 3
3
4
4
6
d
8
6 6
D
treatments were performed on degassed samples in NMR
sample tubes sealed under vacuum. Heating was performed
in the NMR spectrometer.
Fa.r.(cyclohexadiene) )
3AMethine
)
(AC6H3Ph3 + Aaromatic + Acis-PPA+diene + Amethine)/6
values for 1,3,5-C6H3(Ph-d5)3 and trans-PPA-d5 are
directly measurable from the integrated area for each
assigned resonance. The cyclohexadiene structure con-
tains two vinyl protons that overlap with the cis-PPA
18Amethine
Atotal
3
8,45,46
resonance.
The integrated area for the methine
resonance is, therefore, proportional to (1/3)Fa.r.. Deter-
mination of Fa.r. for cis-PPA from the integration of the
assigned resonance requires subtraction of the area due
to the vinyl protons of the cyclohexadiene structure.
From sample thermally treated in CCl4 these values are
F
a.r.(C H Ph ) )
6 3 3
1
6
-
Fa.r.(cyclohexadiene) A
[
( )
3
]
C H Ph
3
6
3
A
+ Aaromatic + A
C H Ph
cis-PPA+diene
6
3
3
Fa.r.(trans-PPA-d ) )
Role of Solvent. Degassed solutions of PPA in CCl4,
5
C4Cl6, C6D6, and THF-d8 (Figure 2) were thermally
treated at 60 °C in the NMR spectrometer (i.e., in the
dark). The plots in Figure 2 illustrate the experimen-
tally determined mole fraction of alkene residues ex-
pressed as percentages for each reaction component over
time. The decrease of the polyene backbone is similar
in all four solvents. Concurrent with the loss polyene
backbone, an equivalent amount of cyclohexadiene
formed and only a trace of 1,3,5-C6H3Ph3 can be identi-
fied.
Aδ7.26
)
)
0.07
0.03
Aδ7.68 + Aδ7.26 + Aδ5.80 + Aδ3.90
Fa.r.(C H (Ph-d ) ) )
6
3
5 3
Aδ7.68
Aδ7.68 + Aδ7.26 + Aδ5.80 + Aδ3.90
Fa.r.(cyclohexadiene) )
In addition to thermal treatment in the NMR spec-
trometer, we have investigated how light influences
PPA solutions in CCl4 thermally treated at 60 °C.
Comparison of the results in Figure 2a to those values
determined for thermally treated PPA-d5 in CCl4 shows
good agreement. In the latter experiment, heating was
performed in an oil bath exposed to ambient light. The
amounts of PPA (Fa.r. ) 0.85), cyclohexadiene (Fa.r. )
3
Aδ3.90
)
)
0.13
0.77
Aδ7.68 + Aδ7.26 + Aδ5.80 + Aδ3.90
Fa.r.(cis-PPA-d ) )
5
Aδ5.80 - (2Aδ3.90
)
Aδ7.68 + Aδ7.26 + Aδ5.80 + Aδ3.90
0
.14), and 1,3,5-C6H3Ph3 (Fa.r. ) 0.01) after 8 h at 60 °C
For the sample thermally treated in CDCl3, the values
are Fa.r.(trans-PPA-d5) ) 0.04, Fa.r.(C6H3(Ph-d5)3) ) 0.02,
Fa.r.(cyclohexadiene) ) 0.36, and Fa.r.(cis-PPA-d5) ) 0.57.
The disagreement between experiments in CCl4 and
CDCl3 was also noted in kinetic experiments described
below. Nonetheless, both observations validate our
qualitative assessment that PPA prepared using Rh(I)-
based catalysts undergo the same transformations as
in CCl4 almost exactly match those observed in the
above experiment with PPA-d5. The first four entries
of Table 1 offer a second point of comparison. Anaerobic
CCl4 solutions of PPA thermally treated at 60 °C in light
show quantitative agreement with in the dark. It is
important to note that the cyclization phenomenon,
accompanied by marginal amounts of 1,3,5-C6H3Ph3
extrusion, results in measurable decreases of Mn and

Macromolecules, Vol. 38, No. 17, 2005
cis-Poly(phenylacetylene) 7245
Table 1. Structural Composition, Molecular Weight, and
Molecular Weight Distribution of PPA in CCl4 Solutions
at 60 °C under Various Atmospheres in Light
Fa.r. × 100%
time
(
cyclo-
1,3,5-
Mn ×
-3 a
h) treatment PPA hexadiene C6H3Ph3 10
Mw/Mn
b
b
96
96
92
87
92
83
77
86
79
76
4
4
0
0
2
3
2
5
8
2
3
6
62.3
51.3
41.5
30.0
11.3
2.8
1.44
1.56
1.64
1.93
2.9
1
4
8
1
4
8
1
4
8
vacuumc
vacuum
vacuum d
ambient
ambient
ambient
6
10
5
12
15
13
17
18
Figure 4. Plots of the mole percentage of alkene residues (Fa.r.
2.7
×
100%) attributed to PPA (9), cyclohexadiene moieties (O),
2.2
2.3
6 3 3 4 6
and extruded 1,3,5-C H Ph (b) for a C Cl solution thermally
e
O2
O2
O2
8.8
3.3
treated at 120 °C. Thermal treatments were performed on
degassed samples in NMR sample tubes sealed under vacuum.
Heating was performed in an oil bath with no precaution to
exclude ambient light.
3.5
2.6
2.5
2.3
a
Relative molecular weight determined by GPC in THF (1 mL/
b
min) calibrated using polystyrene standards. As-prepared sample
of PPA. c Samples were degassed by freeze-pump-thaw cycles
solutions of PPA in CHCl3 and the effects on the change
in Mn assessed.55 While acid accelerates the decrease
of Mn, K2CO3 and TEMPO show slower decreases of
d
and sealed under vacuum. Samples were open to air during
e
thermal treatment. Samples were prepared in O2-saturated CCl4
and sealed in air.
55
Mn. The inhibition observed in the TEMPO-containing
solution was used to substantiate a radical-based oxida-
tion mechanism.55 Addition of radical scavenging agents
to THF and PhCl solutions of PPA inhibits the decrease
of molecular weight.49 In light of the accelerated rate
of cyclization in CDCl3 compared to all other solvents,
we considered that acidity might be a critical factor to
influence the rate of cyclization. To validate our obser-
vation, we investigated the effect of K2CO3-pretreated
CDCl3 and addition of TEMPO in degassed solutions of
PPA. The results are illustrated in Figure 3b,c. In the
absence of O2, both additives inhibit the rate of cycliza-
tion. In fact, both experiments display kinetic behavior
similar to that observed in CCl4 and other solvents. As
both additives are competent to consume the acidic
deuteron,62 acceleration due to acid is eliminated.
1
No new molecular fragments are observed in H NMR
spectra of C4Cl6 solutions of PPA at 120 or 160 °C.
Figure 4 illustrates kinetics for the observed structural
transformations of PPA at 120 °C. Transformation of
PPA to cyclohexadiene repeat units is rapid, while no
more 1,3,5-C6H3Ph3 is formed than for experiments at
6
0 °C. Even after 8 h at 120 °C in C4Cl6, we observed
Figure 3. Plots of the mole percentage of alkene residues (Fa.r.
Fa.r.(C6H3(Ph-d5)3) ) 0.07, Fa.r.(cyclohexadiene) ) 0.37,
and Fa.r.(PPA) ) 0.56. After only 3 min at 160 °C in C4-
Cl6 the sample had the following composition: Fa.r.(C6H3-
×
100%) attributed to PPA (9), cyclohexadiene moieties (O),
and extruded 1,3,5-C
CDCl pretreated with K
TEMPO thermally treated at 60 °C. Thermal treatments
6
H
3
Ph
CO
3
(b) for solutions in (a) CDCl
3
, (b)
3
2
3
, and (c) CDCl containing 33 mol
3
(Ph-d5)3) ) 0.07, Fa.r.(cyclohexadiene) ) 0.28, and Fa.r.-
%
were performed on degassed samples in NMR sample tubes
sealed under vacuum. Heating was performed in the NMR
spectrometer.
(PPA) ) 0.65. Comparing the formation of 1,3,5-C6H3Ph3
at 120 and 160 °C indicates that the activation energy
for the extrusion process is dramatically higher than
that for cyclization. At 60 and 120 °C, extrusion of 1,3,5-
C6H3Ph3 is not significant.
broadening of the molecular weight distribution. The
cyclization process is independent of solvent and light.
Anomalous behavior is observed upon thermal treat-
ment of degassed solutions of PPA in CDCl3 (Figure 3a).
The transformation of the polyene backbone to cyclo-
hexadiene structures is significantly accelerated. In
addition to the increased rate of cyclization, there is an
increase in the amount of 1,3,5-C6H3Ph3 formed. The
amount of extruded benzene may be greater because of
the significant amount of cyclization. While the ac-
celeration was consistently observed, there was notice-
able variation in the amount of cyclization between
experiments. This variance is evident when comparing
the kinetic data in Figure 3a and the numerical analysis
from Figure 1c.
We have previously shown51 that annealing bulk
samples of PPA at 180 °C for 1 h in argon or air
produces comparable changes in structure and molec-
ular weight. In those experiments, ∼10% 1,3,5-C6H3-
Ph3 was identified in GPC chromatograms. Similar
experiments are reported in Table 2 for bulk samples
annealed at 100 °C for 1 h. No evidence of 1,3,5-C6H3-
1
Ph3 formation is observed by H NMR spectroscopy. The
transformations in bulk are the same as in solution.
Role of Molecular Oxygen (O2). Figure 5 illustrates
kinetic data for samples thermally treated in O2-
saturated CCl4 and CDCl3 solutions. Again, heating was
performed in the dark. Surprisingly, the CCl4 solution
of PPA behaved just like the degassed solution. Cycliza-
tion of PPA in O2-saturated CDCl3 solution is dramati-
Unspecified amounts of HCl, K2CO3, and the stable
free radical TEMPO have been added to open-air

7246 Percec and Rudick
Macromolecules, Vol. 38, No. 17, 2005
Table 2. Influence of Atmosphere on the Structural
Composition, Molecular Weight, and Molecular Weight
Distribution of PPA Thermally Treated for 1 h at 100 °C
in Bulk in Light
Fa.r. × 100%
cyclo-
1,3,5-
C6H3Ph3
Mn ×
10
-
3 a
treatment
PPA
hexadiene
Mw/Mn
b
95
90
94
5
10
6
0
0
0
6.7
6.2
5.6
1.34
1.38
1.51
c
ambient
argond
a
Relative molecular weight determined by GPC in THF (1 mL/
b
min) calibrated using polystyrene standards. As-prepared sample
of PPA. c Samples were annealed in air. d Samples were degassed
by freeze-pump-thaw cycles and sealed in argon.
1
Figure 6. Comparison of H NMR spectra of PPA in CCl
4
Figure 5. Plots of the mole percentage of alkene residues (Fa.r.
thermally treated at 60 °C for 8 h. (a) Degassed sample was
sealed under vacuum and heating was performed in the NMR
×
100%) attributed to PPA (9), cyclohexadiene moieties (O),
and extruded 1,3,5-C (b) for solutions in (a) CCl
2
b) CDCl saturated with O . Thermal treatments were per-
6
H
3
Ph
3
4
and
2
spectrometer. (b) O -saturated solution sealed under ambient
(
3
atmosphere and heating was performed in the NMR spectrom-
eter. (c) Degassed sample was sealed under vacuum and
formed on samples in NMR sample tubes sealed under an
ambient atmosphere. Heating at 60 °C was performed in the
NMR spectrometer.
2
heating performed in an oil bath. (d) O -saturated solution
sealed under ambient atmosphere. No precaution was taken
to exclude ambient light from samples during heating in an
oil bath.
cally accelerated compared to degassed CDCl3 and CCl4.
Decomposition of CDCl3 (or CHCl3) to form DCl (or HCl)
63
is accelerated by the presence of O2. While the cycliza-
tion step is accelerated under conditions that promote
decomposition of CDCl3 to DCl, there is no apparent
increase in the amount of 1,3,5-C6H3Ph3 compared to
the degassed sample. This suggests that acid exclusively
promotes the cyclization step. The enhanced quantities
of 1,3,5-C6H3Ph3 in both CDCl3 solutions of PPA com-
pared to the CCl4 solutions are more likely a conse-
quence of larger concentrations of cyclized triene se-
quences and resulting steric strain.
extrusion of 1,3,5-C6H3Ph3 in the same sample. After
72 h, several resonances in the same region were found
in spectra of a thermally treated PPA-d5 sample in
CDCl3. The relative intensity of these resonances changed
upon removal of solvent and drying under vacuum. After
drying under vacuum, the sample was precipitated from
1
CHCl3 into MeOH. H NMR spectra of both the MeOH-
soluble and MeOH-insoluble fractions contained reso-
nances attributable to aldehyde moieties. The oxidation,
therefore, involves the polyene carbon chain and results
in oxidized residues in the cleaved polymer chain and
low molecular weight materials.
While O2 or light does not directly affect the rate of
cyclization, we observe that in combination they result
in accelerated cyclization and decrease of molecular
weight. Table 1 provides comparative composition and
molecular weight data for thermally treated solutions
in CCl4 in light. As noted above, degassed solutions
thermally treated at 60 °C in an oil bath show the same
composition as a sample in the NMR spectrometer.
Samples open to air and in O2-saturated solvent sealed
in air undergo more rapid cyclization, extrusion of 1,3,5-
C6H3Ph3, and decrease of Mn when exposed to light. A
modest acceleration is noted between the ambient and
O2-saturated solutions.
Compression Treatment of PPA. Compression of
several substituted PPAs has been reported to induce
cis-trans isomerization of the polyene backbone.6
Because of the reactions described above, thermal
isomerization is not the method of choice for preparing
trans-PPA. Characterization of compressed samples has
focused on bulk analysis. Given the promiscuity of the
cyclization reaction, we have investigated a sample of
4-68
2
PPA compressed at 1500 kg/cm for 10 min. The polymer
was prepared by polymerization of PA using [Rh(nbd)-
Cl]2 in NEt3 at 20 °C. Effort was made to remove the
THF insoluble fraction (cis-cisoidal PPA). Table 3
tabulates relevant structural and molecular weight
characteristics for the as-prepared and compressed
samples.
Structural differences are also noticeable between the
different thermally treated samples. Figure 6 illustrates
1
H NMR spectra for PPA in CCl4 thermally treated at
0 °C for 8 h and compares the impact of light, O2, and
6
the combination of them. The appearance of resonances
between δ 10-9 ppm is indicative of aldehyde function-
alities due to oxidation. Integration of the two reso-
nances observed in Figure 6d (assuming that each is
the result of one proton) demonstrates that this oxida-
tive chain cleave occurs 5 times more frequently than
Compression of the initially orange powder produced
a deep-red disk. The compressed pellet was dissolved
in THF or CDCl3. Not all the material was soluble,
which suggests that cis-transoidal PPA crystallizes into
the cis-cisoidal form during compression.1
,69 1
H NMR
spectra of the soluble portion of the compressed pellet

Macromolecules, Vol. 38, No. 17, 2005
cis-Poly(phenylacetylene) 7247
Table 3. Influence of Compression on the Structural
Composition, Molecular Weight, and Molecular Weight
Distribution of PPA
Furthermore, it has been demonstrated that the rate
of cyclization and its activation parameters are solvent
71
independent. Cyclization along the polyene backbone
Fa.r. × 100%
of PPA appears to be the same.
PPA cyclohexadiene 1,3,5-C6H3Ph3 Mn × 10-
3 a
Mw/Mn
In the limit of a purely concerted reaction mechanism,
b
6π electrocyclization reactions are expected to be ste-
9
5
0
5
10
0
0
94.0
89.1
2.09
2.12
c
reospecific and abide by the Woodward-Hoffmann
9
rules.7
7,78
Theoretical approaches to such reactions
a
Relative molecular weight determined by GPC in THF (1 mL/
b
affirm that the process is concerted, though not neces-
sarily synchronous. In the absence of a stereochemical
bias, dynamic helical polymers undergo rapid isomer-
ization between P- and M-helical conformations.13 Heli-
cal triene sequences are predisposed to cyclization.
Evident from the experiments described above, cycliza-
tion occurs even in the absence of light and air. As a
consequence, this process may serve to distort the
observed dynamic helical behavior in helical PPAs. The
cyclohexadiene structures are capable of conformational
inversion, but not stereochemical inversion. Further-
more, the newly generated stereocenter adjacent to the
main chain may impart a strong thermodynamic input
to the adopted helical conformation.
min) calibrated using polystyrene standards. As-prepared sample
c
of PPA. Soluble fraction of PPA sample compressed at 1500 kg/
2
cm for 10 min.
In acidic media, cyclization may proceed as described
above. To our knowledge, there are no examples of acid-
catalyzed 6π electrocyclization reactions. As an alterna-
tive to explain the accelerated cyclization, we can invoke
an electrophilic reaction pathway (Scheme 3). This
pathway, though, is inconsistent with other observa-
tions. As pointed out above, there are no apparent
structural differences between degassed samples ther-
mally treated in CCl4 and CDCl3. The promiscuity of
carbocations to react with adventitious water and
-
4
Figure 7. UV-vis spectra in THF of (a) PPA (1.03 × 10 M)
and (b) the soluble fraction of PPA after compression (less than
-
5
7
.87 × 10 M).
and original polymer sample were indistinguishable.
Only a nominal difference in Mn and molecular weight
distribution are observed. Figure 7 illustrates UV-vis
spectra of the as-prepared and compressed PPA samples
as solutions in THF.
The lack of evidence for thermally induced transfor-
mations in the soluble portion of compressed PPA
confirms that this is a mild processing method. Crystal-
lization of cis-transoidal PPA into cis-cisoidal PPA
leaves uncertainty in structural characterization. Be-
cause no 1,3,5-C6H3Ph3 is identified in the compressed
samples, we can employ previously developed methods
1
undergo hydride abstraction is not apparent in the H
NMR spectrum of PPA-d5 thermally treated at 60 °C in
CDCl3 (Figure 1c).
While light and O2 do not independently affect the
structural and molecular weight changes reported herein,
ambient light and O2 synergistically cause dramatic
1
changes. Structural features observed by H NMR
spectroscopy suggest oxidative cleavage of the polyene
backbone. The mechanism by which oxidation occurs is
not clear. Photosensitization of singlet O2 by PPA is
1
,35,48
2
consistent with the dependence on both O and light.
for assessing the cis-content.
The small change in
from 96% to 92% indicates that very
1
,35,48
2
Reactions of singlet O preferentially undergo an “ene”-
cis-content
79
type reaction with an allylic hydrogen. We can envi-
sion the “ene” reaction at the methine associated with
the cyclohexadiene, but this appears to preclude extru-
little cis-trans isomerization occurs, which contrasts
the structural assignments based on Raman, ESR, and
diffuse reflectance UV-vis spectra of ring-substituted
64,65,67,68
6 3 3
sion of 1,3,5-C H Ph . When the “ene” pathway is
polyarylacetylenes.
Furthermore, we have shown
unavailable, [2 + 2] cycloaddition of singlet O2 to an
alkene occurs.80 The resulting 1,2-dioxetane can rear-
range to the corresponding dicarbonyl. These statements
hold for conformationally unlocked dienes.81 Cyclohexa-
diene is exclusively transformed to the endoperoxide,
which further transforms into an epoxy ketone or
diepoxide.81 Among the described reactivity, the only
path consistent with our observations is [2 + 2] cycload-
dition of singlet O2 with a backbone alkene to generate
a transient 1,2-dioxetane. The 1,2-dioxetane further
decomposes to generate aldehyde products (Scheme 4).
In addition to the formation of aldehyde functionalities,
ambient light and O2 accelerate cyclization and extru-
sion of 1,3,5-C6H3Ph3. The solvent dependencies re-
ported previously are likely connected to these processes
and not the electrocyclization pathway.
the sensitivity of molecular weight in PPA to small
structural changes. Such changes are not apparent from
our investigations of compressed samples of PPA.
Scope and Limitations. Given the solvent indepen-
dence of this process and the previously reported activa-
3
4,48
tion energy,
we envision cyclization to occur via 6π
electrocyclization (Scheme 2). This is based on analogy
to kinetic investigations of such reactions in cis-1,3,5-
trienes. The experimentally determined activation en-
ergy for conversion of 1,3,5-hexatriene to 1,3-cyclohexa-
7
0
diene is 29.9 kcal/mol. Substituted analogues have
7
1,72
comparable activation parameters.
Substitution of
the triene shows small electronic influence, although
substitution at the terminal carbons imposes a steric
7
3
penalty on the rate of cyclization. Computational
7
4
efforts validate this observation. This mechanistic
pathway offers an explanation for the observation that
ortho-substituents on the aromatic ring inhibit the loss
of cis-content and decrease of molecular weight.7
Solvent decomposition also affects the observed struc-
tural changes. The representative example of CDCl3 has
been shown herein. Other examples are evident in the
5,76

7248 Percec and Rudick
Macromolecules, Vol. 38, No. 17, 2005
Scheme 2. Illustration of Transoid-Cisoid Conformational Isomerization Followed by Concerted 6π
Electrocyclization of a Triene Sequence along the Polyene Backbone of PPA
Scheme 3. Illustration of Electrophilic Cyclization of a Triene Sequence along the Polyene Backbone of PPA in
the Presence of Acid (HCl or DCl)
Scheme 4. Illustration of Photosensitized [2 + 2]
Cycloaddition of Singlet O to PPA Followed by
Chain Cleavage Due to Decomposition of the
,2-Dioxetane to an Aldehyde and Ketone
The authors distinguish between a free radical autoxi-
dation of poly(methylacetylene) and a photooxidation
process. Free radical scavengers are found to inhibit and
retard the autoxidation process but do not affect the
photooxidation process in poly(methylacetylene).83 As
shown herein, the observed role of radical scavengers
can be obscured by the influence of solvent. In the case
of poly(methylacetylene), all experiments were per-
formed in bulk to avoid complications due to adventi-
tious solvent.83
2
1
Conclusions
Kinetic investigations of the structural and molecular
weight changes undergone by cis-PPA and cis-PPA-d5
in bulk and solution have been undertaken using a
1
combination of H NMR spectroscopy and GPC experi-
ments. In the absence of light and O2, the process is
best described as formation of cyclohexadiene repeat
units via cyclization of triene sequences along the
polyene backbone. When exposed to either light or an
O2-containing atmosphere, the process is the same.
Cyclization along the polymer backbone produces mod-
est decreases in molecular weight and broadening of the
molecular weight distribution. In the presence of O2 or
ambient light, or in the absence of both, the rate of
cyclization is solvent independent, so this process is
unavoidable. The cyclization event likely proceeds via
literature. Contradictory statements regarding results
in CHCl3, toluene, THF, and PhCl may find some
explanation therein. The results described in Figures 3
and 5 and Table 1 shed light on the observation55 that
light does not influence the rate of molecular weight
decrease of PPA in CHCl3 solution. The rate of 6π
electrocyclization is the same in all solvents, including
neutral CDCl3, and is independent of the presence of
light in degassed samples. On the other hand, light
influences the oxidative decomposition of CHCl3 (and
6
π electrocyclization because of the overall solvent
independence and activation energy.
6
3
CDCl3). 6π Electrocyclization and acid-catalyzed cy-
In CDCl3, which decomposes to form DCl, the rate of
cyclization appears to be dramatically faster than other
solvents. This phenomenon is the result of the acid
formed by decomposition of the solvent. Since light and
O2 promote decomposition of CDCl3 (or CHCl3), such
conditions should be avoided when handling PPA. We
observed a marked increase in the rate of cyclization in
O2-saturated CDCl3. Acid catalysis of electrocyclization
is the likely cause of this observation, though an
electrophilic cyclization mechanism has not been ex-
cluded. Oxidative decomposition of other solvents such
as THF may account for other discrepancies between
literature results.
clization in PPA may be more rapid than solvent
decomposition. Similarly, autoxidation of THF82 can
lead to divergent data sets.
Experiments in air with and without light present
have led to the presumption of a radical-based autoxi-
dative mechanism for decrease of molecular
4
9,52,53,55,56
weight.
Drawing on the speculative discussion
about the autoxidation mechanism of poly(methylacety-
8
3
lene), this view has gained acceptance. Some key
distinctions can be made between the two polymers
during oxidation. The initiation process for autoxidation
of poly(methylacetylene) directly involves O2.83 Clearly,
this is not the case for PPA where light is a necessary
component. While it is acknowledged that poly(methy-
lacetylene) can serve as a sensitizing agent to generate
the excited singlet state, the authors suggest that the
subsequent “ene” reaction pathway is insignificant.83
When samples are exposed to both O2 and light, yet
another process dominates the observed behavior. Cy-
clization and extrusion of 1,3,5-C6H3Ph3 are more rapid
under these conditions, but not as fast as in acidic

Macromolecules, Vol. 38, No. 17, 2005
cis-Poly(phenylacetylene) 7249
media. Decrease of molecular weight and broadening
of molecular weight distribution are dramatically ac-
celerated when light and O2 are present. Reliable
correlation of structure and function cannot be made
under such conditions. In contrast to previous explana-
tions for the transformations observed in light and air,
we do not advocate a free radical autoxidation mecha-
nism. Rather, we have noted distinct differences be-
tween PPA and poly(methylacetylene). Most important
is the necessity for both components. The results
reported here provide correct strategies for manipulat-
ing cis-PPA during solution phase investigations at
ambient temperature and solid state at elevated tem-
peratures.
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979, 17, 421-429.
1
tional Science Foundation (DMR-99-96288 and DMR-
(
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(
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Supporting Information Available: A full explanation
1980, 67, 43-71.
of the equations used to determine the mole fraction of alkene
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experiments with PPA. This material is available free of charge
via the Internet at http:\\pubs.acs.org.
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