Synthesis and Properties of N-Arylpyrrole-Functionalized Poly(1-hexene- alt-CO)

Article abstract of DOI:10.1021/acs.macromol.8b01629

The 1,4-polyketone derived from 1-hexene and carbon monoxide, poly(1-hexene-alt-CO) (1), can undergo Paal-Knorr cyclization with substituted anilines to yield N-pyrrole-functionalized polyketones. A variety of pendant functionalities are introduced to 1,

Full text of DOI:10.1021/acs.macromol.8b01629

Article
pubs.acs.org/Macromolecules
Cite This: Macromolecules XXXX, XXX, XXX−XXX
Synthesis and Properties of N‑Arylpyrrole-Functionalized Poly(1-
hexene-alt-CO)
Evan M. Samples, Jeremy M. Schuck, Padmanabh B. Joshi, Katherine A. Willets,
and Graham E. Dobereiner*
Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, United States
S
* Supporting Information
ABSTRACT: The 1,4-polyketone derived from 1-hexene and
carbon monoxide, poly(1-hexene-alt-CO) (1), can undergo
Paal−Knorr cyclization with substituted anilines to yield N-
pyrrole-functionalized polyketones. A variety of pendant
functionalities are introduced to 1, including azobenzene (6)
and triarylamines (5a−e). The extent of incorporation depends
on the nucleophilicity of the parent aniline. Polymer 6 exhibits a
UV-induced trans → cis photoisomerization and has lower rates
of photoinduced decomposition relative to parent 1. Triaryl-
amine-containing polymers 5a−e are redox-active, forming
optically active radical cations upon chemical oxidation. Compared with model N-arylpyrrole compounds 9a−e, polymers
5a−e demonstrate hypsochromic shifts in UV−vis absorptions and exhibit features between +0.5 and +1.2 V (vs Ag/AgCl) in
solution-phase differential pulse voltammetry.
Scheme 1. Postpolymerization Paal−Knorr Cyclizations of
INTRODUCTION
■
1,4-Polyketones
1,4-Polyketones have attracted attention for the low cost of
monomer feedstock (carbon monoxide and olefin), the ease of
producing perfectly alternating copolymer structures, and
valuable properties including chemical resistance, mechanical
resilience, hydrocarbon impermeability, and photodegradabil-
ity of the polymer.1−3 Polyketones have been used as
adhesives, coatings, and fibers4−6 and have potential use in
liquid crystals, drug delivery, ion conduction, and membrane
applications.2,7 A further feature is facile postpolymerization
reactions at carbonyls, yielding a vast range of polymer
structures with diverse functionality.6 Polyketones react with
LiAlH4 to form polyols,8 undergo acid-promoted cyclization to
generate conjugated furans,9 and condense with hydroxylamine
to yield polyketoximes10,11 or with hydrazine to form N-
heterocycles.12 Introduction of five-membered heterocycles via
postpolymerization Paal−Knorr cyclization is particularly
effective (Scheme 1A), offering furans, thiophenes,8 and
pyrroles13−16 via addition of phosphorus pentoxide, Law-
esson’s reagent, and primary amines, respectively.
olefins (larger than propene). Because both backbone R group
and carbonyl functionalization reaction can be tuned, system-
atic study of polyketone functionalization could yield versatile,
modular platforms for applications such as photoactive
materials.
Robust, versatile polymers containing triarylamines are
particularly appealing targets for their potential use as hole-
transport layers in perovskite solar cell (PSC) devices and
OLEDs.27 Beyond the widely available poly(triarylamine)
Academic13,14 and industrial16−21 research groups have
modified polymer properties via Paal−Knorr pyrrole synthesis;
for example, condensation of polyketones and alkyl diamines
effectively cross-links oligomers of Shell’s Carilon thermo-
plastics to form Shell’s Carilite Resins.15,22,23 Yet despite
demonstrated synthetic versatility, the properties and applica-
tions of functionalized polyketones have only been sporadically
evaluated. Functionalization examples have nearly exclusively
focused on the more common poly(ethylene-alt-CO),24
poly(propylene-alt-CO),25 and poly(styrene-alt-CO)26 poly-
mers without evaluating copolymers constructed from higher
Received: July 30, 2018
Revised: November 2, 2018
© XXXX American Chemical Society
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(PTAA) and poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)-
diphenylamine) (TFB; Chart 1), triarylamines have been
HTMs,29,39 and high Tg is desirable to prevent microcrystalline
states from forming in the HTM.39 We hypothesized that a
tacky aliphatic polyketone that has been functionalized with
redox-relevant side chains may allow for independent control
of pore filling and HOMO levelkey to developing a rational
HTM design scheme.40
Chart 1. Representative Triarylamine-Containing Polymers
and Molecular Spiro-OMeTAD
While pTAAs are widespread throughout optoelectronic
research (Chart 1), most polymer designs require laborious
syntheses of specialized monomers. Herein, we instead
introduce redox-active pendants postpolymerization. Building
upon prior methods13,17 we present a synthetic strategy for
transforming a 1,4-polyketone into a library of functionalized
polymers (Scheme 1B). A facile Sc(OTf)3-catalyzed Paal−
Knorr cyclization of poly(1-hexene-alt-CO) with aromatic
amines allows for introduction of triarylamine pendants,
azobenzenes, and other aromatic functionality. The incorpo-
rated groups yield polymers with specialized properties;
azobenzene-derived polyketones are markedly more resistant
to UV degradation than unfunctionalized polymers and
undergo facile photoisomerization, while polyketones bearing
triarylamine pendants form stable radical cations.
RESULTS AND DISCUSSION
■
Polymerization Optimization. While typical polyketones
are composed of ethylene or propylene comonomers, we chose
the less common41,42 1-hexene-derived polymer 1 (Scheme
1B) to maintain maximum organic solubility in successive
functionalization reactions. Depending on catalyst, prepara-
tions of poly(1-hexene-alt-CO) can yield mixtures of
interconvertible polyketone and polyspiroketal.43,44 [Pd-
1,44−46
(dppp)(MeCN)2](BF4)2
was used to ensure generation
of an exclusively alternating polyketone structure.47 The
structure of polymer 1 was confirmed using 1H NMR
(methylene protons, 3.25−2.75 ppm in CDCl3), 13C NMR
(carbonyl carbons, 212 ppm), ATR-FTIR (carbonyl νCO, 1704
cm−1), and GPC (molecular weights of the copolymer
averaged Mn = 2.2 × 104 and Mw = 3.7 × 104 with dispersity
of 1.6).
Postpolymerization Functionalization. The presence of
regularly spaced carbonyls allows the polyketone to be
functionalized with an amine via Paal−Knorr condensation.6
Catalysts can be used to promote mild (room temperature)
Paal−Knorr reactions.13,14 Montmorillonite clay, iodine, iron
phosphate, microwaves, and various Lewis acids have all
previously been used as catalysts or promoters for small-
molecule Paal−Knorr condensations.48−50 But even with a
catalyst, polymer-based reactions are expected to be more
challenging than molecular counterparts. Long polymer chains
undergo cyclization at random places along the polymer
backbone, leaving randomly spaced ketones trapped between
the pyrroles. 86% conversion of the 1,4-diketone moieties to
pyrroles is the statistical maximum.13,51
Reaction of aniline with 1 achieves 27% conversion
compared to the 60% previously reported with poly-
(ethylene-alt-CO).13 Since Sen found amine reactivity to
depend on steric bulk, we first attempted a condensation of
halogenated anilines and polyketone using a Paal−Knorr50
condensation, followed by a Buchwald−Hartwig amination of
resulting N-(haloaryl)pyrrole units with functionalized anilines.
The latter reaction failed using p-toluidine or 2-pyrrolidinone,
and so an alternative path was chosen where functionalized
amines are used directly in Paal−Knorr cyclizations.
Buchwald−Hartwig amination was used to join Boc-protected
incorporated in the backbone or introduced as stryenic
substituents.28−34 To date, PSC devices built with triaryl-
amine-based hole transport polymers have lower power
conversion efficiencies than those employing molecular
Spiro-OMeTAD (>20% power conversion efficiency), spurring
continued interest in viable polymeric alternatives.27,35−37
The modular properties of polyketones make them
compelling candidates for polytriarylamine-based (pTAA)
hole transport polymers. A major challenge in hole transport
material (HTM) design is finding routes to rationally maximize
the extent of pore filling in device assembly.38,39 Good pore-
filling and a stable amorphous state are desirable features for
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4-bromoaniline 2 and commercially available diarylamines 3a−
e to form amines 4a−e, targeted for their steric bulk and
inherent radical stability (Scheme 2). 4a−e were then
deprotected with 30% HCl and were stored as hydrochloride
salts and neutralized as needed.
polymers was achieved by repeated precipitation from
methylene chloride with methanol. The NMR assay used to
ascertain the percent incorporation of functionalized units is
described in the Supporting Information. No obvious
correlation between percent incorporation and amine proper-
ties (sterics and electronics) is clear from the data collected.
Functionalized Polymer Properties. Azobenzene-
Derived 6. Covalently incorporating azobenzene functionality
postpolymerization can introduce photodegradation resistance,
birefringence, and nonlinear optical effects52−56 while avoiding
the expense of producing azobenzene-derived mono-
mers.53,57,58 Nozaki and co-workers had previously prepared
a similar polyketone with copolymerization of a 4-(5-
hexenyloxy)azobenzene.53 Nozaki did not comment on the
appearance of the material. Here, the high molar extinction
coefficient of azobenzene-functionalized 6 resulted in a brightly
colored orange polymer despite relatively low percent
incorporation (2%). Since the photoisomerization of azoben-
zene is known,59 we studied the optical activity of 6 to observe
a reversible trans → cis isomerization via irradiation with UV
and visible light (Scheme 3). Azo π−π* and azo n−π*
excitations trigger trans → cis and cis → trans isomerizations,
respectively.53
Scheme 2. Preparation of 4-Aminotriarylamines 4a−e for
Polyketone Functionalization
Subsequent reaction of 4a−e with 1 forms triarylamino-
derived polymers 5a−e (Chart 2). Similar reactions of
Chart 2. Functionalized Polymers Prepared via Paal−Knorr
Scheme 3. Reversible Photoisomerization of Azobenzene
Pendants of Polymer 6
a
Reactions with 1
To induce photoisomerization, 6 was dissolved in ethyl
acetate and irradiated for 60 min with a medium pressure
mercury lamp (400 W; 280−600 nm with a Pyrex filter). The
polymer π−π* absorption band52 (ca. 338 nm) decreases after
60 min of irradiation, indicating conversion to the cis isomer
(Figure 1A).52,58,60 Irradiating this sample subsequently with
visible light (5.88 W blue LED, 440−460 nm) promoted the
cis−trans isomerization (dashed, green trace), with a partial
recovery of the π−π* trans absorption band. UV degradation
was also assessed by measuring the change in polymer
molecular weight after a period of irradiation (Figure 1B,C).
Based on gel permeation chromatography (GPC) analysis, a
sample of 1 irradiated by a 400 W mercury lamp rapidly
decomposes, reaching a 33% reduction in molecular weight
within 1 h. By comparison, the molecular weight of 6 is
essentially unchanged after 1 h of irradiation, with a more
gradual decomposition thereafter (Figure 1B,C), consistent
with prior findings where azobenzene chromophores are added
to polymers to prevent UV degradation.60 Covalently linking
the azo moiety to the polymer scaffold offers greater polymer
stability and maximum photoresponsiveness of the azobenzene
chromophore.60
a
Percent incorporation is calculated from 1H NMR spectra,
integrating pyrrole 1H resonances relative to an internal standard.
See the Supporting Information for details.
functionalized anilines with 1 yield azobenzene-derived 6,
dimethoxyphenyl 7, and para-substituted phenyl 8. A typical
reaction with an amine and the polyketone consisted of
addition of 0.5 equiv of aminechosen based on the reaction
stoichiometry of two carbonyls per one amineto a solution
of polyketone in methylene chloride followed by addition of
Sc(OTf)3 (5 mol %). The reaction was heated to 40 °C for 24
h. When the reaction was halted, catalyst solids were removed
via filtration and solvents removed under vacuum to yield the
crude polymer. Purification of the pyrrole-functionalized
Triarylamine-Functionalized Polymers 5a−e. Triaryl-
amines are common components of photovoltaic devices61 and
OLEDs62 because of their facile oxidation, stability, and low
crystallinity. In our hands, triarylamine-derived polymers 5a−e
all form stable radical cations upon oxidation with AgSbF6.
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voltammetry (CV) and differential pulse voltammetry (DPV)
have both been used previously to investigate electrochemical
behavior of polymers bearing triarylamines.63−70 Here we
relied on the more sensitive DPV to study the oxidation
features of 1 and 5a−e (Figure 3). Changes in current (ΔI) for
a given polymer indicate the relative amount of charge
transferred, but a quantitative comparison of polymer electro-
activities is complicated by effective triarylamine concen-
tration; for a given polymer mass, a higher percent
incorporation of a triarylamine will result in a higher absolute
ΔI value. Each functionalized polymer 5a−e have distinct
electrochemical signatures absent in the parent polyketone 1
between 0.5 and 1.2 V (vs Ag/AgCl). Electrochemical behavior
of the triarylamine-functionalized polyketones depends upon
the structure of the polymer’s pendant group, indicating the
oxidations are occurring at or around the triarylamine moieties.
In addition, the oxidation features of the triarylamine polymers
are similar to the behavior of molecular model compounds
(vide infra).
Voltammetric features of polymers 5a−e resemble oxidation
behavior of other triarylamines. For example, the strong
influence of pendant functional groups upon oxidation
potentials is consistent with profiles for other polymeric
triarylamines (pTAAs).68,71−75 Oxidation of the pTAA nitro-
gen centers typically occurs 0.6−1.2 V vs Ag/AgCl, depending
on aryl substituent. Monomeric triphenylamine itself under-
goes oxidation at ca. 1.0 V76 (vs Ag/AgCl unless otherwise
noted), and polymers incorporating a N,N-diphenylamino
substituent show oxidations between 0.8 and 1.1 V.76,77 The
features of 5a and 5e voltammograms just above 1.0 V are
therefore consistent with N-centered triarylamine oxidation.
Methyl-substituted pTAAs oxidize at slightly lower potentials
(ca. 0.8 V74,75); similarly, methyl-substituted 5c and 5d show
oxidations at ca. 0.85 V. One of the two features of 5b between
0.6 and 0.9 V could be N-centered triarylamine oxidation;
methoxy-substituted pTAAs oxidize at ca. 0.65 V.75
Additional oxidation features of polymers 5a−e are
apparent, suggesting more complex behavior than found in
other molecular and polymeric triarylamines. In the case of
unsubstituted or p-methyl-substituted pTAAs, oxidation can
induce intra- and intermolecular coupling between the aryl
rings, forming carbazole units or dimers, which themselves can
be oxidized.72 For intermolecular coupling, this can cause cross-
linking of polymer chains and precipitation of the polymer, but
in our hands, no solubility changes are observed upon
oxidation. While the functionalization of 5b is random and
limited to ∼15% incorporation, a fraction of pendants could be
close enough to one another to influence oxidation behavior,
either as neighbors along a single polyketone chain or via
through-space interactions between polymer chains. The
additional oxidation features of 5a, 5b, and 5e (ca. 0.5 V)
could be a result of chromophore stacking, which reduces
oxidation potentials for aromatic polymers and DNA
sequences.66,71,78−82 It has been shown previously that π
stacking has a marked influence on lowering oxidation
potentials of thiophene oligomers,83 poly(dibenzofulvene)-
s,66,71,78 and similar pTAAs.68 Therefore, precise redox
potentials would be influenced by π stacking of the
triarylamine residues of our materials.66,83,84 This phenomenon
is evidenced when single molecules are contrasted against the
polymeric forms (vide infra). We note that pyrrole oxidation is
possible and may further complicate electrochemical behavior.
An oxidizing environment could form radical cations centered
Figure 1. UV−vis spectrum of (A) azobenzene polymer 6 before UV
exposure (solid blue trace), post-UV irradiation (dotted red trace),
and post-visible irradiation (dashed green trace). Polyketone 1 before
and after UV radiation is shown in the inset. GPC chromatograms of
6 (B) and 1 (C) during UV irradiation.
Before oxidation, the polymers resemble the other amine-
functionalized polymers (dark brown, glassy solids). The UV−
vis spectra obtained from dilute solutions of 5a−e had
absorption maxima in the UV (250−350 nm) similar to
other pTAAs and are assignable to π−π* transitions from the
triarylamine pendants (Figure 2).31−33 To oxidize the
polymers, 2 mL of AgSbF6 in methylene chloride (26 mM)
was added dropwise to dilute solutions of each polymer
(∼0.01% w/v in DCM). Each polymer solution initially
became dark yellow; within a few minutes each developed its
own color (a, tan/yellow; b, yellow-gray; c, tan/yellow; d, blue-
gray; e, teal). All of the oxidized triarylamines absorbed at
∼466 nm, and polymers bearing −OMe groups (5b, 5d, 5e)
showed additional absorptions at wavelengths >650 nm.
Electrochemical studies were conducted to complement the
findings from chemical oxidation. Solution-phase cyclic
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Figure 2. UV−vis spectra for polymers 5a−e (A) prior to and (B) after reaction with AgSbF6 in dichloromethane: 5a, purple, dashed; 5b, blue,
dash-dot; 5c, green, square dot; 5d, red, round dot; 5e, blue, solid.
Figure 3. Differential pulse voltammograms for parent polyketone 1 and triarylamine-functionalized 5a−e.
at the pyrrole rings, which could couple to oxidized
triarylamine pendants within the same N-arylpyrrole unit or
to radical cations nearby.70 Cai et al.77 propose complex
oxidation behavior at N-(triarylamino)pyrrole units because of
oxidations at both the pyrrole and triarylamine centers.
To further deconvolute the voltammogram of 5b, we
prepared mixed polymers from 1, aniline, and the deprotected
4b aniline, resulting in a random tricopolymers trico1, trico2,
and trico3 (Scheme 4). Despite varying the stoichiometry of
aniline and triarylamino aniline, the previously optimized
conditions consistently provided an approximate 3:1 phenyl to
triarylamine ratio. The overall percent incorporation (the
amount of 1,4-diketones converted to an N-arylpyrrole) ranged
from 10 to 25%, allowing for a comparison of overall
triarylamine incorporation into the polymer. With increased
percent incorporation of the triarylamine unit, there is an
increase in ΔI of the shoulder at 0.75 V. This feature may
reflect a localized decrease in oxidation potential due to
proximity effects between triarylamines. Figure 4 compares the
voltammograms of trico1-3 and 5b with the N-phenylpyrrole-
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Scheme 4. A Random Tricopolymer from Condensation of
1, Aniline, and 4b
Scheme 5. Preparation of Compounds 9a−e
a
Table 1. Absorption Maxima for Functionalized Polymers
and Molecular Model Compounds before (Neutral) and
after (Oxidized) Treatment with AgSbF6
a
The relative amounts of amine added in the preparation of each
tricopolymer had little influence on the actual ratio of pyrrole
substitutions (n:o) but allowed for testing a range of percent
incorporation of triarylamine.
neutral (λmax, nm)
polymers molecules
5a−e 9a−e
oxidized (λmax, nm)
polymers
molecules
9a−e
functionalized poly(1-hexene-alt-CO) (8a). While triaryl-
amine-containing polymers demonstrate a pronounced oxida-
tion feature near 0.9 V (vs Ag/AgCl), 8a exhibits a ΔImax at a
higher potential (1.15 V). Since 8a does not exhibit features
0.8−1.0 V, the pyrrole ring and ketone functional groups are
not alone responsible for the oxidation features observed with
triarylamine polymers. 8a does show a minor feature at 0.5 V,
similar to 5b, and so this feature may be due to backbone or N-
arylpyrrole oxidation.
5a−e
diphenyl (5a, 9a)
dimethoxy (5b, 9b)
dimethyl (5c, 9c)
methoxymethyl
(5d, 9d)
293
295
302
300
303
298
304
300
463
465, 767
465
445
445, 767
441
465, 739
470
methoxy (5e, 9e)
300
302
465, 785
448, 766
Properties of Substituted Pyrrole Model Compounds.
To gain insight into the redox behavior of the triarylamine
pendants of the functionalized polymers 5a−e, we prepared
molecular models 9a−e of the redox-active N-arylpyrrolyl units
and explored their oxidation behavior in the absence of the
polyketone polymer scaffold. The oxidation effects could then
be compared to polymers 5a−e to understand the origins of
oxidative features in DPV electrochemistry as well as the UV−
vis behavior of the chemically oxidized polymers. The
preparation of the model compounds was straightforward,
accessed via coupling N-(p-bromophenyl)pyrrole with com-
mercially available diarylamines to yield compounds 9a−e in
moderate to excellent yields (Scheme 5). Once prepared, 9a−e
were chemically oxidized with AgSbF6. The oxidized polymers
5a−e and the oxidized molecular analogues 9a−e all absorb
strongly at ∼450 nm (Table 1). UV−vis spectra for polymers
5b, 5d, and 5e and molecules 9b and 9e feature an additional
absorption band at ∼750 nman absorption observed in
other −OMe-substituted triarylamines.33 For oxidized materi-
als, the triarylamine absorptions show negligible change upon
immobilizing onto the polymer backbone. The substitution
pattern and oxidized absorption maxima do not follow a clear
trend. The −OMe-substituted materials were expected to be
more red-shifted than the alkyl-substituted materials (red
absorption wavelength in the order of dimethoxy >
methoxymethyl > methoxy),33,85 but this relationship is not
observed, perhaps due to the additional influence of the
pyrrole moiety.
The electrochemical oxidation (DPV) features of the
triarylamine model compounds differ from the corresponding
polymer. As a representative example, the voltammograms for
5b and 9b (Figure 5) clearly indicate substantial changes in
properties of triarylamine upon polymer incorporation. The
more complex features of polymer 5b (marked by an asterisk)
are absent in molecule 9b. One potential cause is poor
oxidative stability of molecular pyrroles relative to moieties
isolated in the polymer scaffolds.86 Additionally, low
concentrations of molecule 9b do not permit interactions
Figure 4. Differential pulse voltammograms for tricopolymers trico1, trico2, and trico3 and polymers 8a and 5b.
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(e.g., CDCl3 corrected to 7.26 ppm) or external standards (31P NMR,
85% phosphoric acid solution), and chemical shifts reported as ppm.
Apparatus. All NMR spectra were collected at room temperature
on a Bruker 400 spectrometer (400 MHz) and a Bruker Avance 500
(500 MHz). Infrared spectra were collected as ATR-FTIR spectra
using a Thermo Scientific Nicolet is5 with a id5 ATR attachment.
UV−vis spectra were collected using a Shimadzu UV-1800 double-
beam spectrophotometer. Polymerizations were performed in a 100
mL Parr Instrument Co. 4590 microstirred reactor. GPC was
performed with a Shimadzu LC 20-AT chromatograph using three
PolarGel-M (300 × 7.5 mm2) columns and THF as an eluent.
Calibration was done using standard PMMA samples (Mp range:
202−71800 Da). Solvents were purified by the Pure Process
Technology purification system.
Electrochemical Measurements. Differential pulse voltammetry
(DPV) measurements were performed using a 650E CH Instruments
potentiostat. Polymers/molecular analogues of interest were dissolved
in dichloromethane with 0.1 M tetrabutylammonium hexafluoro-
phosphate (NBu4PF6) as a supporting electrolyte. Glassy carbon,
silver wire, and platinum wire were used as a working, reference, and
counter electrode, respectively. DPV was conducted by applying
potential pulses of 50 mV amplitude for 50 ms duration over 500 ms
period (potential waveform figure included in the Supporting
Information).
Figure 5. Voltammograms comparing the electrochemical behavior
5b and 9b using DPV.
between triarylamines, as we propose for the 5b polymer (vide
supra).
During our study of pyrrole-functionalized triarylamines we
have found that oxidation of the polymeric triarylamines is
more facile than molecular analogues. This is not unexpected;
appending chromophores to polymer backbones is known to
reduce the anodic potentials at which they oxidize.66,68,71,78,83
Previous studies have also reported a dependence on
substitution pattern for the relative lifetimes of the formed
radicals before intermolecular coupling reactions quench
radical species.72,87 We, too, observe a dependence on
substitution and oxidizing potential (see the Supporting
Information). Oxidative events occurring between 0.25 and
0.75 V are reasonably assigned to the oxidation of the pyrrole
ring itself86 with the later features assigned to the oxidation of
the triarylamines. Pyrrole oxidation potentials are strongly
dependent on substituent and factor strongly into the
potentials required for pyrrole electropolymerization.86
Chemicals. Chemicals were used from Sigma-Aldrich Chemical
Co., Oakwood Chemicals, Airgas, Kodak, and Tokyo Chemical
Industry Co. without further purification with the exception of p-
bromoaniline (Kodak) which was sublimed prior to use. Solvents used
during air-free reactions were purified on a solvent purification
system.
Preparation of [(Ph)2P(CH2)3P(Ph)2Pd(NCMe)2](BF4)2.45,88 To
89
a 100 mL flask [(NCMe)4Pd](BF4)2 (0.533 g, 1.2 mmol),
diphenylphosphinopropane (0.494 g, 1.2 mmol) and a magnetic stir
bar were added. Acetonitrile (50 mL) was then added to the flask to
produce a clear, bright yellow solution. The reaction was left to stir
rapidly for 24 h. Once complete, the solvent was removed under
vacuum to produce [(Ph)2P(CH2)3P(Ph)2Pd(NCMe)2](BF4)2 as a
bright yellow film (0.831 g, 89.3% yield). 1H NMR (400 MHz,
CDCl3): δ 7.67 (dd, 7H), 7.49 (d, 3H), 2.93 (s, 4H), 2.30 (s, 2H),
1.90 (s, 6H). 31P NMR (202 MHz, CD2Cl2): δ 12.47.
CONCLUSION
■
Copolymerization of 1-Hexene and Carbon Monoxide (1).
The procedure was adapted from Abu-Surrah et al.42 A pressure
reactor was charged with [(Ph)2P(CH2)3P(Ph)2Pd(NCMe)2](BF4)2
(0.1 mol %, 0.0457 g, 0.0595 mmol) followed by dichloromethane
(10 mL), 1-hexene (7.43 mL, 59.5 mmol), and methanol (358 μL).
The reactor was sealed and removed from the glovebox. The reactor
was pressurized with carbon monoxide (1000 psi), and the reaction
stirred at room temperature for 48 h. Once complete, the reaction
solution was depressurized and quenched with an excess of methanol
followed by removal of catalyst residues via silica gel plug. Solvent was
removed under vacuum to yield the product as a translucent gel (2.05
In this report we have prepared a polyketone from 1-hexene
and carbon monoxide to functionalize postpolymerization.
This investigation has led to a host of polymers with N-
substituted pyrrolic functionality. With the synthesis of several
aminotriarylamines we were able to prepare polymers with
triaylamine functionality (5a−e). These polymers’ UV−vis
spectra were drastically changed upon reaction with AgSbF6,
and their oxidized spectra support the formation of radical
cationic species. Differential pulse voltammetry for each
triarylamine polymer (5a−e) yielded a distinct electrochemical
signature, confirming incorporation of optically and redox-
active functionality into polyketone-based macromolecules.
These observations are encouraging as they give potential to
the search for redox-active polymers from readily accessible
substrates with minimal synthetic effort. The modest
incorporation of the arylamine pendants may require
alternative routes to increase incorporation, however. We
found that even a slight incorporation (2%) of azobenzene into
the polyketone led to observable trans → cis photoisomeriza-
tion for polymer 6 and significantly reduced photoinduced
polymer decomposition. Further efforts in our laboratories will
explore potential applications of functionalized polyketones.
1
g, 31% yield). H NMR (500 MHz, CDCl3): δ 3.09 (br), 2.90 (br),
2.65 (br), 2.57 (br), 2.38 (br), 2.36 (br), 1.61 (br), 1.25 (br), 0.88
(br).
tert-Butyl (4-Bromophenyl)carbamate (2). Exclusion of air was
not necessary. To a 250 mL round-bottom flask equipped with a stir
bar freshly sublimed 4-bromoaniline (12.6 g, 73.1 mmol) was added
followed by THF (80 mL). Once a homogeneous solution was
formed, a solution of di-tert-butyl dicarbamate (17.7 g, 81.1 mmol) in
THF (70 mL) was added dropwise via addition funnel over the
course of 30 min. After addition was complete the addition funnel was
exchanged for a condenser, and the reaction was refluxed for 24 h or
until judged complete via TLC (1:10 ethyl acetate:hexanes). The
product was then purified via flash chromatography to afford 3 as a
1
white solid (4.18 g, 86.4% yield). H NMR (500 MHz, CD2Cl2): δ
7.43−7.23 (m, 4H), 6.57 (s, 1H), 1.49 (s, 9H).
EXPERIMENTAL SECTION
N-(4-Bromophenyl)-2,5-dimethylpyrrole. Prepared via the
work of Chen et al. without exclusion of air.50 2,5-Hexanedione (5
mmol), 4-bromoaniline (5 mmol), and Sc(OTf)3 (0.05 mmol, 1 mol
%) were added to a round-bottom flask equipped with a stir bar. The
■
General Remarks. All reactions were performed in an inert
atmosphere glovebox or using standard Schlenk techniques unless
otherwise noted. NMR spectra are referenced to deuterated solvent
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DOI: 10.1021/acs.macromol.8b01629
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reaction stirred at 30 °C for 30 min or until complete. After the
required time a tan solid was collected via filtration and washed with
cold water to yield an off-white solid (1.04 g, 67% yield). H NMR
(400 MHz, CD2Cl2): δ 7.60 (d, 2H), 7.09 (d, 2 H), 5.83 (s, 2H), 2.00
(s, 6H).
ACKNOWLEDGMENTS
■
1
Acknowledgment is made to the Donors of the American
Chemical Society Petroleum Research Fund for partial support
of this research. Support for the NMR facility at Temple
University by a CURE grant from the Pennsylvania Depart-
ment of Health is gratefully acknowledged. This work was
supported by a Targeted Research Grant from the Temple
University Office of the Vice Provost for Research. This
material is based upon work supported by the U.S. Department
of Energy, Office of Science, Office of Basic Energy Sciences
under Award DE-SC0010307. The authors acknowledge
Eromon O. Asikhia for his synthetic contribution to this
work and Dr. Brenden P. Derstine for experimental assistance.
General Cross-Coupling Procedure for Triarylamines (4 and
9). To a Schlenk flask 3 (1.37 g, 5.03 mmol), diphenylamine (0.425 g,
2.52 mmol), tris(dibenzylideneacetone)dipalladium (0.184 g, 0.201
mmol, 4 mol %), tri(tert-butyl)phosphine (0.163 g, 0.805 mmol, 16
mol %), potassium tert-butoxide (1.13 g, 10.1 mmol), and toluene (15
mL) were added. The flask was removed from the glovebox and
heated to 111 °C under nitrogen for 24 h or until judged complete by
TLC. Once completed, the reaction was cooled to room tempertaure
and extracted via aqueous work-up. The crude product was purified
via flash chromatography (1:10 ethyl acetate:hexanes) and dried
under vacuum to afford 4a as an opalescent foam (0.624 g, 70%
1
yield). H NMR (500 MHz, CD2Cl2): δ 7.33−7.19 (m, 6H), 7.11−
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.8b01629.
1H NMR spectra of starting materials, catalyst, and
polymer; calculation of percent conversion of polymer
functionalization; UV−vis spectra of 9a−e before and
after AgSbF6 oxidation (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: dob@temple.edu.
ORCID
Katherine A. Willets: 0000-0002-1417-4656
Graham E. Dobereiner: 0000-0001-6885-2021
Notes
The authors declare no competing financial interest.
H
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DOI: 10.1021/acs.macromol.8b01629
Macromolecules XXXX, XXX, XXX−XXX