UV-vis monitoring of radical polymerizations by spin trapping with chromophoric nitrones

Article abstract of DOI:10.1021/ma4025174

The application of chromophoric radical spin traps for highly sensitive UV-vis monitoring of conversion of radical enhanced spin capturing polymerizations (ESCP) is presented. In ESCP the growing macroradical is reacting with a nitrone providing the corresponding polymeric nitroxide radical spin trap which will react irreversibly with a second macroradical to eventually give a polymeric alkoxyamine. The progress of styrene and n-butyl acrylate ESC polymerizations in the presence of π-conjugated nitrones is successfully visualized via a significant change of absorbance of the nitrone chromophore in the 200-500 nm range due to the nitrone transformation into an alkoxyamine moiety upon incorporation into the polymer. Moreover, related studies on UV-vis monitoring of photochemical rearrangement of nitrones to their corresponding oxaziridines are discussed.

Full text of DOI:10.1021/ma4025174

Article
pubs.acs.org/Macromolecules
UV−Vis Monitoring of Radical Polymerizations by Spin Trapping with
Chromophoric Nitrones
Ralph Husmann,† Sebastian Wertz,† Constantin G. Daniliuc,† Sascha W. Schafer,‡ Ciaran B. McArdle,‡
́
̈
and Armido Studer*,†
†Institute of Organic Chemistry, Department of Chemistry, University of Munster, Corrensstraße 40, 48149 Munster, Germany
̈
̈
‡Henkel AG & Co. KGaA/Adhesives Technologies, Henkelstrasse 67, 40589 Dusseldorf, Germany
̈
S
* Supporting Information
ABSTRACT: The application of chromophoric radical spin traps for highly sensitive UV−vis monitoring of conversion of
radical enhanced spin capturing polymerizations (ESCP) is presented. In ESCP the growing macroradical is reacting with a
nitrone providing the corresponding polymeric nitroxide radical spin trap which will react irreversibly with a second macroradical
to eventually give a polymeric alkoxyamine. The progress of styrene and n-butyl acrylate ESC polymerizations in the presence of
π-conjugated nitrones is successfully visualized via a significant change of absorbance of the nitrone chromophore in the 200−
500 nm range due to the nitrone transformation into an alkoxyamine moiety upon incorporation into the polymer. Moreover,
related studies on UV−vis monitoring of photochemical rearrangement of nitrones to their corresponding oxaziridines are
discussed.
heterocycles derived from 1-pyrenyl-substituted nitrones were
shown to intercalate specific DNA domains and showed
promising antitumor activity.10 The use of nitrones as
electrophiles in cross-dehydrogenative coupling (CDC) reac-
tions with terminal alkynes and subsequent formation of 3,5-
disubstituted isoxazoles was shown by our group.11
Only recently, nitrones have been implemented in polymer
chemistry. To this end, bis-nitrones were utilized in 1,3-dipolar
polycycloaddition reactions with bis-maleimides to form the
corresponding polyaddition products.12 First reports on the
employment of nitrones as chain propagation regulators in
radical polymerization reactions appeared at the end of the
1990s.13 Constitutive studies along these lines revealed the in
situ formation of the corresponding nitroxides and alkoxy-
amines via radical trapping during polymerization.14 Still, for in
situ NMP a prereaction between nitrone and radical initiator,
either in the presence or absence of the monomer, is necessary.
The concept of direct control of the average chain length by
nitrones was coined “enhanced spin capturing polymerization
(ESCP)” by Barner-Kowollik et al.4e,15 This strategy is based on
INTRODUCTION
■
The concept of spin trapping was introduced by Janzen and
Blackburn in the late 1960s1 and has become a versatile method
for indirect detection and identification of structurally variable
free radicals via EPR spectroscopy.2 To this end, pronounced
interest has been devoted to nitroxides, nitroso compounds,
and nitrones which feature excellent physicochemical properties
to address efficient spin trapping3 and which have consequently
found widespread application in synthesis, polymer, and
materials science.4 Nitrones are of particular interest, since
they exhibit 2-fold radical scavenging properties via C-trapping
and subsequent O-trapping of radicals to afford alkoxyamines.
The latter substance class can be implemented for initiation/
regulation of controlled nitroxide-mediated polymerization
(NMP) processes.5 Derivatives of these types of structures
have also been used to study thermo-oxidative degradation in
polymers or as sensors to detect and investigate oxidative stress
in biological systems.6 Despite their importance as nitroxide/
alkoxyamine precursors, specific nitrones occupy biological
activity whose consequent potency as therapeutics has been
reviewed by Floyd et al.7 Furthermore, the versatility of
nitrones as valuable intermediates in organic synthesis becomes
apparent for their reactivity as 1,3-dipoles in cycloaddition
reactions affording for instance isoxazolidines.8,9 In this context,
Received: December 10, 2013
Revised: January 17, 2014
Published: January 30, 2014
© 2014 American Chemical Society
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Scheme 1. Enhanced Spin Capturing Polymerization (ESCP)
the irreversible trapping of growing radical chains by nitrones to
ideally generate midchain macroalkoxyamines with well-defined
size. ESC polymerization is initiated with a conventional
initiator such as α,α′-azobis(isobutyronitrile) (AIBN) in the
presence of a nitrone (Scheme 1). The growing macroradical
reacts with the nitrone to generate a macronitroxide, which in
turn reacts irreversibly as an in situ generated polymeric spin
trapping reagent with a second macroradical to eventually
afford a polymer containing an alkoxyamine moiety connecting
the two polymer chains. Importantly, it was shown that the
amount of added nitrone allows controlling the degree of
polymerization and hence the physical properties of the
resulting polymer: the nitrone spin trap enhances and also
controls cross-termination reactions during polymerization.
The concept of ESCP clearly differs from NMP since the
crucial dissociation of the alkoxyamine C−O bond in the latter
process is very unlikely to occur within typical temperature
ranges of ESCP (<100 °C).16 As compared to the conventional
polymerization in absence of a spin trapping nitrone, the
structure of the polymer bears an alkoxyamine moiety (see
Scheme 1). This structural variation changes the thermal
stability of the polymer. At temperatures above 120 °C, C−O
bond homolysis at the alkoxyamine site, a key step in NMP, will
occur, which will lead to polymer chain breaking at that
position via disproportionation forming a chain with a
hydroxylamine at the terminus and the other chain with an
alkene terminus. Moreover, nitroxide crossover in the
polymeric alkoxyamines is also possible at higher tempera-
ture.5d
Scheme 2. Nitroxide Spin Trapping and Spin Annihilation
To Afford Alkoxyamines
region becomes feasible whereas the reactivity of the nitrone is
adjustable by the variation of R′.
Choosing N-tert-butyl-α-phenylnitrone (PBN)commonly
preferred as regulator in ESCPs by the groups of Grishin13 and
Barner-Kowollik16as model substrate, we initially focused on
the application of nitrones derived from 1-pyrenecarboxalde-
hyde, expecting a distinct difference in the UV−vis absorbance
of the corresponding nitrones and alkoxyamines core scaffolds
that would be incorporated in the polymer according to the
above outlined spin trapping technique.
Nitrones 1a and 1b were successfully prepared by
condensing 1-pyrenecarboxaldehyde and the corresponding
hydroxylamine in 82% and 98% yield, respectively (Scheme
3).10 The sterically demanding nitrone 1c was obtained in
excellent yield (93%) by elevating the temperature over
prolonged reaction times under otherwise identical condi-
tions.26
Scheme 3. Preparation of 1-Pyrenecarboxaldehyde Derived
Nitrones 1a−c (*at 60 °C for 4 days)
The real-time monitoring of specific parameters in polymer-
ization processes17 is widely investigated with a broad array of
methods like calorimetry,18 light scattering,19 gas chromatog-
raphy,20 and fluorescence,21 infrared,22 Raman,23 or NMR
spectroscopy.24 Related UV−vis spectroscopic studies have
been performed for elucidation of certain aspects of polymer-
ization of different monomers under varying conditions.25
Herein, we present the use of chromophoric nitrones as spin
traps in radical polymerization reactions. As the nitrone is
covalently incorporated in the polymer chain, the change of
absorbance in the UV−vis spectra is used to monitor
polymerization progress.
UV−vis spectroscopy revealed distinct absorption patterns in
the 350−420 nm region for nitrones 1a−c (Figure 1, left)
which we expected to change significantly upon reaction with a
radical species due to disturbance of the conjugated π-system in
the chromophore moiety. Notably, during the course of our
investigation, we found that the UV−vis spectrum of authentic
samples of nitrone solutions in CHCl3 already changed
progressively when left to stand in ambient light. This
phenomenon is attributed to an intramolecular photoisomeri-
zation of nitrones to their corresponding oxaziridines.27
Representatively, the conversion of nitrone 1a upon light
exposure is illustrated by the depletion of its absorbance
maxima at 370 and 391 nm, whereas the emergence of new
maxima at 328 and 344 nm marks the formation of the
corresponding oxaziridine 2a (Figure 1, right).
The isosbestic point can be appointed at 350 nm, and
complete conversion was accomplished after irradiation with a
light-emitting diode (LED) at 365 nm for 5 h. Importantly, no
change in UV−vis absorbance was observed as long as solutions
were stored in the dark and as minimum exposure to any light
source was granted. In addition, isomerization to the
oxaziridine did not occur in the dark at elevated temperatures
(e.g., 70 °C in solvent), showing that the nitrones are thermally
RESULTS AND DISCUSSION
■
The purpose of our study was the identification of a low-cost
and readily synthesized chemical sensor for spectroscopic
monitoring of monomer conversion in polymerization
reactions. Along these lines, the use of chromophore-
conjugated nitrones seemed promising in radical polymer-
ization reactions since they will be incorporated in the polymer
chain by formation of an alkoxyamine midchain function (ideal
midchain polymers are obtained when intermediate nitroxides
instantly trap a second polymer chain, thus •P1 ≈ •P2), as
shown in Scheme 2. The chromophoric residue R of the
nitrone can be varied such that monitoring in the UV−vis
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Figure 1. Left: UV−vis spectra of 1-pyrenyl nitrones 1a−c in CHCl3 (35 μM). Right: UV−vis monitoring of 365 nm LED irradiation induced
photoisomerization of nitrone 1a to oxaziridine 2a in EtOAc (43 μM).
stable. These results seemed promising since oxaziridine
formation has a similar impact on the disruption of the
chromophoric system as envisioned for nitroxide and alkoxy-
amine formation during ESC-based polymerization. Never-
theless, we planned to study photoisomerization in greater
detail to be able to exclude interference of both effects (UV−vis
change due to oxaziridine formation or a nitroxide/alkoxyamine
formation) in our targeted polymer progress monitoring
system.
For this purpose, additional nitrones 1d−g with varying
substitution patterns were synthesized according to the
previously reported method in high yields and subjected to
photoisomerization conditions as summarized in Table 1.28
Figure 2. Crystal structure of oxaziridine 2a.
Table 1. Photoisomerization of Nitrones 1a−g to
clean conversions of the starting materials to the desired
Oxaziridines 2a−g
oxaziridines were achieved under the conditions depicted in
Table 1, and only trace amounts of the parent aldehyde were
identified as side products in some experiments (see
Supporting Information). Notably, increasing the steric bulk
of the N-substituent led to a more rapid isomerization (N-Me <
N-Bn < N-tBu). This trend was also observed for the electron
poor p-nitrophenyl-substituted nitrones 1d and 1e which gave
oxaziridines 2d (68%) and 2e (86%) upon prolonging
irradiation time (entries 4 and 5). However, p-cyanophenyl
nitrone 1f gave product 2f quantitatively after shorter reaction
time (entry 6), and naphthyl aldehyde derived nitrone 1g was
fully converted to heterocycle 2g (entry 7).
On the basis of these results, we next evaluated the effect of
nitrones 1a−c on the radical polymerization of styrene.
Reactions were initiated by 0.5 mol % of AIBN and were
performed in neat styrene at 70 °C in a Schlenk tube under an
argon atmosphere in the presence of a nitrone in the dark to
prevent photoisomerization of the nitrone. UV−vis spectra
were recorded at a concentration of 1 mg/mL in THF after
stopping the reaction.
When 0.1 mol % of N-methyl-substituted nitrone 1a was
used as an additive in the polymerization of styrene, a fast
decline of the characteristic nitrone peaks (392 nm, 372 nm)
was observed (Figure 3, left), indicating a very high
incorporation efficiency of 1a into the formed polystyrene
backbone. Nearly complete consumption of the nitrone took
place at low monomer conversion (below 55% conversion).
The number-average molar weight (ranging from 22.5 to 47.0
kg/mol) and PDI (constantly ∼1.5) of the polymer at different
a
b
entry
R
R′
product time [h] yield [%]
c
1
2
3
4
5
6
7
1-pyrenyl (1a)
1-pyrenyl (1b)
1-pyrenyl (1c)
4-NO2C6H4 (1d)
4-NO2C6H4 (1e)
4-CNC6H4 (1f)
1-naphthyl (1g)
Me
Bn
2a
2b
2c
2d
2e
2f
5
99, 67
c
3.5
1.5
83
tBu
Me
tBu
Me
Me
99
68
86
99
99
38
24
3
2g
7
a
Solvent choice based on optimal solubility; see Supporting
b
c
Information. Isolated yield after column chromatography. A 500
W light bulb was used instead of the 365 nm LED.
Although oxaziridine formation already took place in solution
under ambient light, using an LED emitting at 365 nm proved
to be more efficient. 1-Pyrenecarboxaldehyde derived nitrones
bearing methyl (1a), benzyl (1b), or tert-butyl (1c) substituents
at nitrogen gave the corresponding oxaziridines 2a−c in high
yields (entries 1−3). The structure of oxaziridine 2a was
unambiguously confirmed by X-ray structure analysis (Figure
2). Conversion of benzyl derivative 1b to 2b was achieved by
irradiation with a 500 W light bulb as a lower energy alternative
to the LED since considerable decomposition to the
corresponding pyrene aldehyde and N-benzylhydroxylamine
was observed upon using the LED light source. In general,
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Figure 3. Left: UV−vis spectra of polystyrene prepared with 1a (0.1 mol %) as an additive at various conversions. Right: Mn, PDI vs monomer
conversion (styrene, 0.5 mol % AIBN, 0.1 mol % 1a, 70 °C).
Figure 4. Left: UV−vis spectra of polystyrene prepared with 1c (0.1 mol %) as additive at various conversions. Right: Mn, PDI vs monomer
conversion (styrene, 0.5 mol % AIBN, 0.1 mol % 1c, 70 °C).
Figure 5. Left: UV−vis spectra of polystyrene prepared with 1b (0.1 mol %) as an additive at various conversions. Right: Mn, PDI vs monomer
conversion (styrene, 0.5 mol % AIBN, 0.1 mol % 1b, 70 °C).
conversions are displayed in Figure 3, right.29 Accordingly, the
mean molecular weight rapidly increased once the added
nitrone was nearly fully consumed (after >46% monomer
conversion), but PDI remained at around 1.5. These results
clearly show that nitrone 1a is too reactive toward styrene-
based macroradicals formed in the early stages of the
polymerization and gets too rapidly consumed. This particular
UV-sensor must therefore be considered as inappropriate for
comprehensive monitoring of styrene polymerization (monitor-
ing until only low conversion is possible), probably due to
insufficient steric demand of the N-substituent (methyl for 1a)
of the controlling/monitoring reagent 1a.
We consequently turned our attention to a sterically more
demanding alternative and applied the tert-butyl-substituted
nitrone 1c as UV-sensing ESCP reagent. The UV−vis spectra
of polystyrene prepared in the presence of 0.1 mol % tert-butyl-
substituted nitrone 1c and the corresponding number-average
molar weights and PDIs at various conversions are depicted in
Figure 4. In contrast to the polymerization in the presence of
1a (see above), no significant change in the UV−vis spectra was
observed up to approximately 44% styrene conversion. Hence,
this sterically more hindered nitrone was incorporated into the
polymer only to a small extend in the early stages of the
polymerization process. Since no isosbestic point could be
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Figure 6. Left: UV−vis spectra of poly(n-butyl acrylate) prepared with 1a (0.1 mol %) as additive at various conversions. Right: Mn, PDI vs
monomer conversion (n-butyl acrylate, 0.5 mol % AIBN, 0.1 mol % 1a, 70 °C).
Figure 7. Left: UV−vis spectra of poly(n-butyl acrylate) prepared with 1b (0.1 mol %) as an additive at various conversions. Right: Mn, PDI vs
monomer conversion (n-butyl acrylate, 0.5 mol % AIBN, 0.1 mol % 1b, 70 °C).
identified, the overall loss of intensity at higher conversions
(58−66%) was attributed to unspecified degradation of 1c with
extinction of absorbance in the 300−450 nm range. Up to 70%
conversion, the increase of the number-average molar weight
did not allow for unambiguous correlation between UV
absorbance and polymerization process. Notably, the PDI
remained around 1.5 throughout the whole reaction. We
therefore concluded that 1c is not a suitable nitrone for
monitoring of the polymerization of styrene by UV−vis
spectroscopy.
Whereas too little steric demand and concomitantly high
reactivity of the nitrone moiety lead to too rapid incorporation
of the UV−vis-monitoring entity into the growing polymers
(compare application of nitrone 1a, Figure 3), a nitrone bearing
a sterically demanding N-substituent as in 1c is not efficiently
incorporated into the polystyrene.
In order to balance reactivity issues, we next focused on the
application of N-benzylated nitrone 1b which should show
reactivity in between the methyl 1a and the tert-butyl congener
1c. Figure 5 shows the UV−vis spectra of polystyrene prepared
in the presence of 0.1 mol % of the N-benzyl substituted
nitrone 1b and the related values for number-average molar
weight and PDI. To our delight, UV−vis monitoring revealed a
decrease of peak intensity at 370 and 391 nm with increasing
conversion and emerging peaks at 328 and 344 nm over a
broad conversion range (Figure 5, left). The isosbestic point is
located at 350 nm. This indicates a conversion-dependent
incorporation of 1b into the growing polymers. Values for
mean molecular weights remained in a range of 30−40 kg/mol
and PDIs were found to be around 1.5 (Figure 5, right).
Encouraged by this result, we investigated the related radical
polymerization of n-butyl acrylate. UV−vis spectra were
recorded at a concentration of 1 mg/mL in THF after stopping
the reaction (unless otherwise stated).
The UV−vis spectra of the acrylate polymerization at
different conversions with nitrone 1a as an additive are
presented in Figure 6. According to the rapid decrease of
intensity of the characteristic nitrone absorbance maxima (392
and 372 nm), 1a was fully consumed at low monomer
conversion. Reaction with 1a was clean and the isosbestic point
could be located at 350 nm. Mean molecular weight (Mn) and
polydispersity index (PDI) were determined by SEC against a
poly(methyl methacrylate) standard. The Mn was ∼330−600
kg/mol, and PDI values were around 2.1, 2.3, and 1.5 at 79%,
82%, and 87% conversion (Figure 6, right). In analogy to the
polystyrene case, the reactivity of the methyl-substituted
nitrone 1a toward the growing macroradical is too high, and
consequently 1a is not a suitable UV-active nitrone for
monitoring the n-butyl acrylate polymerization.
As in the styrene case, we therefore switched to the sterically
more hindered nitrones 1b and 1c. However, in contrast to the
styrene polymerization, where the N-benzyl-substituted nitrone
1b showed ideal sensing properties (compare Figure 5), 1b
turned out to be too reactive for the n-butyl acrylate
polymerization. The distinct nitrone UV absorbance dimin-
ished at low conversion (<30%), suggesting full consumption of
the nitrone 1b at an early stage of the polymerization
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Figure 8. Left: UV−vis spectra of poly(n-butyl acrylate) prepared with 1c (0.1 mol %) as an additive at various conversions. Right: Mn, PDI vs
monomer conversion (n-butyl acrylate, 0.5 mol % AIBN, 0.1 mol % 1c, 70 °C).
Figure 9. Left: UV−vis spectra of poly(n-butyl acrylate) prepared with 1c (0.05 mol %) as an additive at various conversions (concentration of the
samples: 3 mg/mL in THF). Right: Mn, PDI vs monomer conversion (n-butyl acrylate, 0.5 mol % AIBN, 0.05 mol % 1c, 70 °C).
Figure 10. Left: UV−vis spectra of poly(n-butyl acrylate) prepared with 1c (0.05 mol %) as an additive at various conversions (concentration of the
samples: 8 mg/mL in THF). Right: corresponding monomer conversion and absorbance vs time (conditions: n-butyl acrylate, 0.5 mol % AIBN, 0.05
mol % 1c, 70 °C).
(isosbestic point: 350 nm; see Figure 7, left). The number-
average molar weight was around 220 kg/mol at 30%
conversion, and it further rose to 580 kg/mol at high
conversion (94%, Figure 7, right).
and 329 nm over a broad conversion range were observed with
an isosbestic point located at 350 nm. The number-average
molar weight ranged from 160 to 230 kg/mol, and PDI values
remained small (between 1.4 and 1.5, Figure 8, right).
We then switched to the least reactive tert-butyl-substituted
nitrone 1c (0.1 mol %) as an additive. Results are presented in
Figure 8. To our delight, a gradual decrease of the peaks at 392
and 372 nm and a simultaneous increase of the peaks at 345
On the basis of the promising results with 0.1 mol % tert-
butyl-substituted nitrone 1c, we next investigated n-butyl
acrylate polymerization at lower additive loading, and the
results are displayed in Figure 9. A decline of intensity of the
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characteristic absorbance maxima at 392 and 372 nm until
complete disappearance at high conversion (94%) was noted.
Moreover, the buildup of two new absorbance maxima at 345
and 329 nm indicated the incorporation of the nitrone into the
polymer (isosbestic point for this system is located at 350 nm).
The number-average molar weight of the polyacrylate was
significantly higher (335−416 kg/mol) as compared to those
obtained for the polymerization run with 0.1 mol % 1c. The
PDI values were in the 1.4−1.6 range (Figure 9, right). This
adjustment of the mean molecular weight by variation of the
nitrone concentration proves an ESCP-based control of the
polymerization process where a higher concentration of the
nitrone moiety leads to a decrease of the Mn. Thus, 1-pyrenyl
aldehyde derived nitrones can play a dual role since they
constitute (1) UV-sensors for polymerization progress
monitoring and (2) controlling agents for regulation of mean
polymer size distribution. Looking at Figure 10, which
illustrates additional experiments on n-butyl acrylate polymer-
ization in the presence of nitrone 1c with more data points
(sample concentrations at 8 mg/mL in THF), it becomes
apparent that successive incorporation of the nitrone over the
whole polymerization process is achieved, rendering this UV-
sensor/controlling agent ideal for monitoring and controlling
the radical polymerization of n-butyl acrylate.
ASSOCIATED CONTENT
* Supporting Information
Synthetic procedures, NMR and UV−vis spectra, X-ray crystal
structure analysis, and polymer characterization. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: studer@uni-muenster.de (A.S.).
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Henkel AG & Co. KGaA for support and
funding of this research. The authors are grateful to Henkel AG
& Co. KGaA for permission to publish this work.
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■
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Figure 10 (right) illustrates both the depletion of nitrone
absorbance at 393 nm and monomer conversion over time. It
should be noted that this decay is related to the incorporation
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CONCLUSIONS
■
UV−vis sensitive nitrones were shown to be applicable as
efficient monitoring probes for polymerization processes which
additionally allow for enhanced spin capturing control of mean
molecular weights. Importantly, specific tuning of the reactivity
of the applied nitrone by modification of steric demand resulted
in the identification of ideal additives for each monomer. To
this end, nitrones 1a−c were investigated as UV−vis-sensitive
ESCP reagents in the polymerization of styrene and n-butyl
acrylate under various conditions. Whereas 1a appeared to be
too reactive (generally high degrees of incorporation into
polymeric material after low monomer conversion), 1b was
identified as an optimal candidate for monitoring styrene
polymerization. Nitrone 1c, which is structurally closely related
to PBN, a frequently applied reagent in ESCP processes, was
successfully applied to the polymerization of n-butyl acrylate.
We believe that the identification of suitable UV-sensors can
also be achieved for other monomer types via fine-tuning of the
selected nitrone in future studies.
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