Thermoreversible reactions on inorganic nanoparticle surfaces: Diels-alder reactions on sterically crowded surfaces

Article abstract of DOI:10.1021/cm303049k

Organically surface-functionalized nanoparticles are important cross-linkers for nanocomposites. In the past, many cross-linking reactions were based on simple radical additions. However, novel smart materials require reversible reactions. These reactions, such as the Diels-Alder reaction, often have a specific sterical demand, e.g., a six-centered transition state. In this study, < 5 nm silica particles were functionalized with maleimide groups, and their reactivity with regard to Diels-Alder reactions were investigated, applying various techniques. A new method for the surface modification of silica nanoparticles is presented, minimizing agglomeration in organic solvents and thus increasing the accessibility of the functional groups on the particle surface. Kinetic studies of substituted model compounds were carried out to evaluate the reactivity of the maleimide functionality. The Diels-Alder reaction between 2,5-dimethylfuran and N-propylmaleimide, N-ethyl(N-propylcarbamato) maleimide, and N-phenylmaleimide was followed by UV/Vis spectroscopy. The reaction rate increases in this order, showing the effect of maleimide substitution. Afterwards N-((3-triethoxysilyl)propyl)maleimide was used to graft maleimidopropyl functional groups onto the nanoparticle surface. 3-Aminopropyltriethoxysilane, which could then be reacted with 1,1′-(methylenedi-4,1-phenylene)bismaleimide, was used to attach phenyl-substituted maleimide functionality to the surface. 3- Isocyanatopropyltriethoxysilane introduced the electron-drawing carbamato functionality into the system. The surface coverage of the samples was characterized applying CHN analysis, TGA-FTIR coupling, and FTIR spectroscopy. All analytical methods revealed that the functional groups are covalently bonded to the silica surface and the maleimide rings remain intact. Diels-Alder reactions of the surface groups show that the reactivity of the molecules attached to the particles depends on sterical crowding, but the reaction rate is not significantly changed by surface effects.

Full text of DOI:10.1021/cm303049k

Article
pubs.acs.org/cm
Thermoreversible Reactions on Inorganic Nanoparticle Surfaces:
Diels−Alder Reactions on Sterically Crowded Surfaces
Tom Engel and Guido Kickelbick*
Inorganic Solid State Chemistry, Saarland University, Am Markt Zeile 3, 66125 Saarbrucken, Germany
̈
S
* Supporting Information
ABSTRACT: Organically surface-functionalized nanoparticles
are important cross-linkers for nanocomposites. In the past,
many cross-linking reactions were based on simple radical
additions. However, novel smart materials require reversible
reactions. These reactions, such as the Diels−Alder reaction,
often have a specific sterical demand, e.g., a six-centered
transition state. In this study, < 5 nm silica particles were
functionalized with maleimide groups, and their reactivity with
regard to Diels−Alder reactions were investigated, applying
various techniques. A new method for the surface modification of silica nanoparticles is presented, minimizing agglomeration in
organic solvents and thus increasing the accessibility of the functional groups on the particle surface. Kinetic studies of substituted
model compounds were carried out to evaluate the reactivity of the maleimide functionality. The Diels−Alder reaction between
2,5-dimethylfuran and N-propylmaleimide, N-ethyl(N-propylcarbamato)maleimide, and N-phenylmaleimide was followed by
UV/Vis spectroscopy. The reaction rate increases in this order, showing the effect of maleimide substitution. Afterwards N-((3-
triethoxysilyl)propyl)maleimide was used to graft maleimidopropyl functional groups onto the nanoparticle surface. 3-
Aminopropyltriethoxysilane, which could then be reacted with 1,1′-(methylenedi-4,1-phenylene)bismaleimide, was used to attach
phenyl-substituted maleimide functionality to the surface. 3-Isocyanatopropyltriethoxysilane introduced the electron-drawing
carbamato functionality into the system. The surface coverage of the samples was characterized applying CHN analysis, TGA-
FTIR coupling, and FTIR spectroscopy. All analytical methods revealed that the functional groups are covalently bonded to the
silica surface and the maleimide rings remain intact. Diels−Alder reactions of the surface groups show that the reactivity of the
molecules attached to the particles depends on sterical crowding, but the reaction rate is not significantly changed by surface
effects.
KEYWORDS: nanoparticles, surface-functionalization, thermoreversible reaction, Diels−Alder
points prevents the cross-linked nanocomposite from being
reprocessed and reshaped.
INTRODUCTION
■
Inorganic nanoparticles are important building blocks for many
novel materials. The incorporation of nanoparticles in polymer
matrixes can dramatically change their properties. Particularly
the mechanical and thermal properties are influenced by the
incorporated inorganic phase. In many cases it is necessary to
provide a surface-functionalization to enhance the compatibility
and the dispersion of the nanoparticles in an organic matrix.1,2
Changing the polarity and the availability of functional groups
has a significant effect on the dispersibility in the monomer or
polymer matrix and allows a tight bonding to the latter by the
formation of cross-linked networks. The grafting of polymer-
izable functional groups onto the nanoparticle surface allows
covalent bonding with the matrix, improving the compatibility
of the phases and preventing phase separation and agglomer-
ation of particles.2,3 Methacryloxy and epoxy functionalities
have been used to incorporate silica nanoparticles into organic
matrixes through copolymerization.3 This increases homoge-
neity of the particle distribution and also mechanical strength of
the resulting nanocomposite. A drawback of highly cross-linked
materials is crack formation and crack propagation. In addition,
the often observed absence of glass transition and melting
In recent years, compounds with reversible bonding became
the focus of materials scientists, due to self-healing properties
and the possibility to reprocess and reshape cross-linked
nanocomposites.4−8 One of the major mechanisms used in this
respect is the Diels−Alder (DA) reaction, applying maleimide
and furan functional groups as dienophiles and dienes. Due to
the thermoreversible reaction type these materials show also a
self-healing ability.
Wudl et al. were the first to synthesize a thermally
remendable macromolecule and to investigate its self-healing
properties.4 They found that cuts or cracks could be healed by a
simple thermal treatment. Our aim is the combination of
reversible DA chemistry with the advantages of nanocomposite
materials.
By using reversible DA cross-linking, the inorganic particles
can be separated from the polymer matrix by a simple thermal
Received: September 21, 2012
Revised: December 7, 2012
© XXXX American Chemical Society
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treatment. Beyer et al. used reversible DA chemistry to develop
polymer films in which a change in optical clarity can be
thermally triggered.9 Rahman et al. grafted free maleimide
functional groups through nucleophilic addition of a
commercially available bismaleimide onto the surface of
amino-functionalized silica nanoparticles.10 They investigated
the reactivity of the maleimide groups to undergo Michael
reaction on the surface of the particles.
Dynamic light scattering measurements before and after the
solvent exchange showed that the particle suspensions
remained stable during this procedure (Figure 1).
In the same spirit we investigated the surface functionaliza-
tion of silica nanoparticles with maleimide functional groups.
An important aspect of our work was the investigation of the
reactivity of the dienophiles on the surface of nanoparticles.
Because of the sterical hindrance and the demand of a six-
membered transition state for the DA reaction, it is important
to judge the effect of sterical crowded surfaces on this reaction
type. Therefore, different coupling agents with maleimide
functionality were synthesized and grafted onto particles, and
the surface coverage was determined. A model reaction with
furan was used to investigate the influence of steric hindrance
on DA and retro-Diels−Alder (retro-DA) chemistry.
Figure 1. DLS size distribution of silica particles before and after
solvent exchange and after suspension in toluene.
RESULTS AND DISCUSSION
Synthesis and Characterization of Silica Nanopar-
■
The particles in the original alcoholic dispersion had a
diameter of 4.8 0.8 nm. The particles transferred to IBMK
showed a diameter of 5.4 1.2 nm (DLS). In addition, Figure
1 shows the size distribution plot of the same silica particles if
suspended in toluene. Agglomerates with a diameter of more
than 300 nm were formed in toluene.
Coupling Agents. Different coupling agents for the surface
modification of the nanoparticles were used. N-((3-
Triethoxysilyl)propyl)maleimide 1, 2-(2,5-dioxo-2,5-dihydro-
1H-pyrrol-1-yl)ethyl(3-(triethoxysilyl)propyl)carbamate 2, and
1-(4-{[4-(2,5-dioxo-3-{[3-(triethoxysilyl)-propyl]amino}-
pyrrolidin-1-yl)phenyl]methyl}phenyl)-2,5-dihydro-1H-pyr-
role-2,5-dione 3. These molecules are composed of a
triethoxysilane anchor group, a spacer, and a maleimide
functional group used as the dienophile functionality for DA
reaction.
ticles. Spherical amorphous silica nanoparticles were prepared
following a modified Stober process.11 Tetraethyl orthosilicate
̈
(TEOS) was first hydrolyzed and then condensed in alcoholic
solution, applying ammonia as base. Transmission electron
microscopy (TEM) (Figures S1 and S2, Supporting Informa-
tion) and dynamic light scattering (DLS) measurements of the
obtained nanoparticles in methanol revealed a diameter of 3.8
0.9 nm and 4.8 0.8 nm, respectively. Nitrogen sorption
measurements were performed to calculate the surface area of
particles according to Brunauer, Emmett, and Teller (BET).12
The nitrogen sorption measurement shows a specific surface
area of 750 m2 per gram of silica nanoparticles. When assuming
a spherical shape of the particles and a density for silica of 2.2 g
cm−3, the calculated diameter is 3.6 nm, which corresponds to
the diameter measured in the TEM experiment. Some
discrepancy may be explained by porosity. The physisorption
isotherm revealed a characteristic hysteresis loop, which is
associated with capillary condensation taking place in
mesopores (IUPAC Type IV H2).13
1 was synthesized according to a literature procedure
(Scheme 1)15 with an overall yield of 96%.
a
Scheme 1. Reaction Scheme for the Preparation of 1
Surface functionalization was carried out by applying a
solvent-exchange methodology, which allows for higher surface
coverage and very low agglomeration of the particles.14 Isobutyl
methyl ketone (IBMK) was used as the suspension medium for
the silica particles during the modification step because of its
relatively high dielectric constant, ε = 13.1, which results in
ionized hydroxyl groups on the nanoparticle surface. Another
advantage of this solvent is the relatively high boiling point of
115 °C, which favors the elimination of ethoxy groups and
condensation of the silane coupling agents during functional-
ization reactions.
In previous studies, toluene was the preferred solvent
because of the high boiling point, but its low dielectric constant
of ε = 2.4 made it a poor choice because the particle suspension
is not stable in toluene and the particles tend to agglomerate.
Green et al. isolated the bare silica particles and redispersed
them in IBMK after drying.14 In this work, it was examined if
the solvent could be changed without isolation of the particles.
Therefore, IBMK was added to a freshly prepared silica
suspension in methanol, and the two solvents were codistilled
by rotary evaporation. This step was repeated two times.
a
Reagents and conditions: (a) dry DCM, RT, 1 h; (b) ZnCl, HMDS,
dry toluene, 80 °C, 5 h.
2 was prepared according to a modified literature procedure
via a dibutyltin dilaurate-catalyzed addition of protected 2-
hydroxyethylmaleimide with N-(3-isocyanoato-propyl)-
triethoxysilane (Scheme 2).16 The protection of maleimide
functionality with furan via DA reaction is necessary because 2-
hydroxyethylmaleimide tends to polymerize via Michael
addition of the hydroxyl group to the maleimide double
bond. Deprotection takes place during the surface modification
step of the silica nanoparticles. The boiling point of the
suspension medium IBMK is sufficiently high to provoke retro-
DA reaction with liberation of furan.
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a
Scheme 2. Reaction Scheme for the Preparation of 2
a
Reagents and conditions: (a) Toluene, rt, 24 h; (b) ethanolamine, dry
MeOH, reflux, 24 h; (c) isocyanatopropyltriethoxysilane, dibutyltin
dilaurate, acetone, reflux, 12 h.
Figure 2. Dynamic light scattering size distributions of 1@SiO2 in 2-
propanol, 2@SiO2 in 2-propanol, and 3@SiO2 in DMF.
Commercially available APTES was used to react with 1,1′-
(methylendi-4,1-phenylene)bismaleimide via Michael addition
as shown in Scheme 3 to form 3.
Surface Modification and Characterization. Dynamic
Light Scattering. DLS was used to investigate the dispersibility
of the isolated particles, which is an important proof of the
availability of the functional groups at the surface. The particles
were mixed with different solvents, and they were sonicated for
10 min. For 1@SiO2 and 2@SiO2, the best results were
achieved with 2-propanol as suspension medium (Figure 2).
The other solvents resulted in agglomerates of more than 100
nm. Only suspension in 2-propanol resulted in a narrow
distribution around 15 and 60 nm, respectively. This could be
explained by the duality of the 2-propanol properties. On the
one hand, it has a hydroxyl group, which can undergo hydrogen
bonding with either the particle surface or the carbonyl groups
of the maleimide ring, and on the other hand, it has an
isopropyl chain, which can develop weak interactions with the
surface functionalities. 3@SiO2 was not dispersible in 2-
propanol and most other suspension mediums. The best result,
a relatively broad size distribution of 100 nm, was achieved in
dimethylformamide (DMF). This study shows how critical the
suspension medium is for the accessibility of the functional
groups.
FTIR Analysis. The success of functionalization was studied
by FTIR spectroscopy (Figure 3). All samples showed multiple
peaks in the “fingerprint”-region of silica between 460 and 1200
cm−1: A strong asymmetric Si−O−Si vibration at about 1100
cm−1, weaker asymmetric Si−OH vibration at 957 cm−1,
symmetric Si−O−Si vibration at 797 cm−1, and two additional
peaks at 573 and 460 cm−1.17 The FTIR spectrum of pure silica
shows the presence of molecular water through scissor bending
vibration at ∼1630 cm−1. The broad signal between 3300 and
3700 cm−1 is a superimposition of H-bonded molecular water
Figure 3. Infrared spectra of bare silica and the sample modified with
1.
and hydroxyl terminals of silanol groups. An O−H stretching
vibration, characteristic of free surface silanol groups, was only
detected in the pure silica sample at 3746 cm−1.18 CH2 and
CH3 (2907, 2945, and 2984 cm−1) vibrational modes can be
assigned to the remaining ethoxy groups on the surface of the
Scheme 3. Reaction Scheme for the Preparation of BMPTES
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nanoparticles as well as the weak signals at 1484, 1450, and
1394 cm−1, which rise from C−H bending modes..
ethoxy groups not reacted during condensation. This mass loss
totaled 2.0%. The second step from 150 °C to 310 °C released
CO2, ammonia, water, carbon monoxide, and an organic acid,
pointing to the thermal decomposition of the maleimido
functionality. During this step, 3.3% of the initial mass was lost.
The third decomposition step released 14.0% of CO2, water,
CO, and ethylene. No residual carbon was burned under
oxygen. According to the TGA measurements, about 17% of
the mass of the 2@SiO2 sample could be assigned to the
coupling agent.
Attenuated total reflection infrared (ATR-FTIR) measure-
ments of the 1-functionalized particles revealed characteristic
signals for organic coupling agents (Figure 3). The strong
asymmetric stretch of the carbonyls could be detected at 1704
cm−1, and even the weak symmetric stretching mode is visible
at 1770 cm−1. The symmetric C−N−C stretching vibration at
1369 cm−1 is another proof of the successful functionalization
with maleimide functionality. Another indication for the high
surface modification is the peak at 696 cm−1 assigned to
maleimide ring deformation.19
Elemental Analysis. The percentage of nitrogen is the
perfect tool to calculate the number of functional groups on the
surface of the silica particles, because the nitrogen atoms are
only present in the coupling agent and not in residual solvent
or ethoxy groups. Equation 1 gives the molar amount of
functional groups per gram of particles. This calculation gave
1.5 0.2, 0.3 0.1, and 1.3 0.1 mmol of maleimide per gram
of particle for 1@SiO2, 2@SiO2, and 3@SiO2, respectively.
By taking into account the surface area per gram of silica, the
number of functional groups per surface area could be
calculated (eq 2) and resulted in the following surface
coverages: 1.2 0.2 functional groups per nm2 for 1@SiO2;
The FTIR spectrum of 3@SiO2 (Figure S4, Supporting
Information) shows signals for both the symmetric and
asymmetric stretching modes of the conjugated maleimide
carbonyl groups at 1773 cm−1 (very weak) and 1714 cm−1
(strong). The last mentioned signal is stronger than all the
other carbonyl signals because the surface molecules are
composed of two imides. The signal at 1513 cm−1 could be
assigned to aromatic C−C stretching mode of the benzene
rings and the peak at 1388 cm−1 is caused by a symmetric C−
N−C stretching vibration within the maleimide ring as shown
by Parker et al.19
In the spectrum of 2@SiO2 (Figure S3, Supporting
Information), both symmetric and asymmetric stretching
modes of carbonyls at 1773 cm−1 and 1710 cm−1 were present
but the signals are weaker because of lower surface coverage.
Weak C−H bending modes could be observed at 1439 cm−1
and 1409 cm−1. A signal at 1546 cm−1 could arise from
maleimide CC stretching or N−H bending of the urethyl
group. The high amount of surface modification is also verified
by the maleimide ring deformation at 697 cm−1.
Thermogravimetric Analysis. TGA was coupled with
infrared spectroscopy to investigate the thermal stability of
modified particles. The three-dimensional IR plots are shown in
Figures S5−S7, Supporting Information. The temperature-
dependent mass loss of 1@SiO2 revealed two discernible steps.
The first one at 110−300 °C released mainly IBMK adsorbed
to the particle surface and ethanol from excess ethoxy groups
not reacted during condensation. This mass loss totaled 6%.
The second step from 300 °C to 700 °C released CO2,
ammonia, water, carbon monoxide, and ethylene, pointing to
the thermal decomposition of the maleimidopropyl function-
ality. During this step 17.5% of the initial mass was lost.
Burning of residual carbon under oxygen released additional
6.5%. According to the TGA measurements, 24% of the mass of
the 1@SiO2 sample could be assigned to the coupling agent.
The temperature-dependent mass loss of 3@SiO2 showed
three discernible steps. The first one at 30−200 °C released
water adsorbed to the particle surface and ethanol from excess
ethoxy groups not reacted during condensation. This mass loss
totaled 1.2%. The second step from 200 °C to 360 °C released
CO2, ammonia, water, carbon monoxide, and an organic acid,
pointing to the thermal decomposition of the maleimido-
functionality. During this step 3.2% of the initial mass was lost.
The third decomposition step released 22.4% of CO2, water,
CO and ethylene. Burning of residual carbon under oxygen
released additional 13.7%. According to the TGA measure-
ments, about 39% of the mass of the 3@SiO2 sample could be
assigned to the coupling agent.
0.3
functional groups per nm2 for 3@SiO2.
0.1 functional groups per nm2 for 2@SiO2; 1.0
0.1
Diels−Alder Reaction on the Surface of Nano-
particles. The particles were reacted with furan at room
temperature to prove their ability to carry out DA reactions. All
three samples were isolated and were dispersed in furan by
sonication. The suspensions were stirred for 2 days at RT.
Afterward the particles were collected by centrifugation, washed
three times with diethyl ether, and dried in vacuum. ATR-
FTIR, TGA-IR, and elemental analysis were used to character-
ize the samples F-1@SiO2, F-2@SiO2, and F-3@SiO2.
Afterward retro-DA reaction was carried out at 150 °C in a
vacuum-oven. TGA measurements were used to determine the
difference in mass loss between the samples before and after
retro-DA reaction.
Figure 4 shows a comparison between the IR spectra of 1@
SiO2 and F-1@SiO2. The first difference is the small red-shift of
Figure 4. Comparison of 1@SiO2 and F-1@SiO2 by infrared
spectroscopy.
the carbonyl peak from 1704 to 1698 cm−1, which could be
explained by breaking the conjugation between the carbonyls
and the maleimide double bond during DA reaction. On the
other hand, the maleimide ring deformation signal at 696 cm−1
disappeared almost completely. The integration of this signal
The temperature dependent mass loss of 2@SiO2 showed
three discernible steps. The first one at 30−150 °C released
water adsorbed to the particle surface and ethanol from excess
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was used to follow the conversion over time of the maleimide
system to the DA product.
on the surface of nanoparticles should be fulfilled. This 1.62%
equals 0.24 mmol furan per gram of particles reacted via DA
reaction to the maleimide surface groups. Comparing this value
with the number of functional groups per gram of particles, the
percentage of groups reacting with furan equals 16%.
TGA of F-2@SiO2 gave rise to an additional mass loss of
0.75% between 100 °C and 200 °C (Figure S8, Supporting
Information). Taking into account the number of functional
groups on the surface of the particles, we could estimate a
conversion of 37% of the 2 moieties. F-3@SiO2 liberates 1.50%
furan, meaning that 17% of the maleimde groups reacted with
furan (Figure S9, Supporting Information).
These results suggest that the percentage of maleimide
moieties reacting with furan depends on the surface coverage of
the particles and not on the type of surface modification. The
more the surface of the particles is crowded, the less of the
functional groups are able to react. In total, 1@SiO2 shows the
highest amount of furan reacted via DA reaction because these
samples show the highest surface coverage of 1.2 molecules per
nm2, but relative to surface coverage it exhibited the lowest
reactivity. 2@SiO2, which presented a much lower coverage of
0.3 molecules per nm2, showed higher reactivity with respect to
the number of functional groups. 1@SiO2 expressed four times
as many maleimides on the surface than 2@SiO2 but only twice
as much furan reacted via DA reaction. This observation
suggests that there is an ideal number of surface molecules per
nm2 where both effects are compensated.
F-2@SiO2 showed the same vibrational changes, but the
changes were more difficult to observe. The red-shift of the
carbonyl peak is partly overlapped by the signal of the
carbamate. Furthermore, the maleimide ring deformation
disappeared during the reaction, but initially the signal was
very weak.
No clear differences in the spectra of F-3@SiO2 and 3@SiO2
could be observed. The red-shift is covered by carbonyl
stretching modes of the additional imide and several signals in
the region between 700 and 650 cm−1 overlap with the
deformation of the maleimide ring, making it impossible to
distinguish between these signals.
A comparison of the thermal decomposition between 1@
SiO2 and F-1@SiO2 showed a larger mass loss in the sample
reacted with furan (Figure 5). The difference between the two
Elemental analysis results could not be analyzed as easily
because the different decomposition steps cannot be distin-
guished and only carbon, hydrogen, and nitrogen composition
can be obtained. The nitrogen content followed the same trend
as the TGA results. For all three samples, the relative nitrogen
content was lower if the samples were reacted with furan.
Kinetic Studies of Differently Funtionalized Malei-
mides. The DA reaction between maleimides with various
pendant groups and dimethylfuran (MF) was studied with UV
spectroscopy and compared to the surface-functionalized
nanoparticles. All the reactions followed the same scheme as
that shown in Scheme 4.
Figure 5. Comparison of TGA measurements of 1@SiO2 and F-1@
SiO2.
samples represented 1.62%. This difference was determined
from 100 °C to 200 °C because IR measurements of the
decomposition gases showed that furan is liberated in between
these temperatures. Furan, released by retro-DA reaction, was
detected by IR spectroscopy showing the typical strong signal
at 745 cm−1 characteristic for furan (Figure 6). The maximum
release of furan from the sample was detected at 135 °C. Above
this temperature, ideal reaction conditions for the rDA reaction
Scheme 4. General Reaction Scheme for Model DA
Reactions
The proportions of exo and endo products are not relevant
in the context of the use of DA reaction as a cross-linking
reaction between nanoparticles and macromolecular systems.
Kinetic studies were only performed to compare reactivity of
the maleimdes and to study the effects of different
functionalities on the rate constant k. Given the structures of
the used coupling agents for particle functionalization, we chose
three different model compounds for preliminary kinetic
Figure 6. IR Spectra of the decomposition gases at 135 °C of samples
1@SiO2 and F-1@SiO2.
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studies. These model compounds were N-propylmaleimide M1,
N-ethyl(N-propylcarbamato)maleimide M2, and N-phenyl-
maleimide M3, respectively. Whereas the UV spectrum of
MF in 1,2-dichloroethane did not show any absorption above
270 nm, M1 in the same solvent (c = 0.02 M) displayed a
characteristic maximum at about 290 nm (ελ = 656 M−1 cm−1)
arising from the excitation of the conjugated system of CC
and CO bonds. During the reaction and as the DA products
form, this conjugation breaks, which results in the hipsocromic
shift of the absorption corresponding now to the excitation of
the remaining carbonyls. The progress of DA reaction could
therefore be followed by UV spectroscopy by keeping track of
the decrease of the absorbance at 290 nm (for M1) with time.
This was carried out for the three molecules at different
temperatures, specifically 50 °C, 60 °C, and 70 °C. The
maximum absorptions for M2 and M3 were at 298 nm (ελ =
348 M−1 cm−1) and 295 nm (ελ = 457 M−1 cm−1), respectively.
Figures S10−S12 (Supporting Information) show the results of
kinetic measurements of the three molecules at 60 °C with a
30-fold excess of MF. The high excess of MF was chosen to
guarantee pseudo-first-order kinetics with regard to the
maleimides and to minimize reaction time. A first experiment
with a 10-fold excess of MF lasted several hours, so the
concentration of MF was tripled.
used for the propylmaleimide 1-modified samples because
integration of the signal at 696 cm−1 over time gave no
evaluable results for the other samples. A 200 mg amount of
1@SiO2 was dispersed in 20 mL of 2-propanol, and 1 mL of
furan was added. Samples were withdrawn from this suspension
after various time intervals. The samples were dried
immediately, and ATR-IR measurements were carried out
(Figure 7). The spectra were normalized using the strongest
Figure S10 shows the natural logarithm of the conversion
over time of the three different model maleimides. Table 1 lists
Table 1. Reaction Rate Constants k for Temperatures 50 °C,
60 °C, and 70 °C for Model DA Reactions (confidence
interval 95%)
model
compound
k [10−4 s−1],
T = 50 °C
k [10−4 s−1],
T = 60 °C
k [10−4 s−1],
T = 70 °C
M1
M2
M3
3.8 0.1
9.0 0.6
11.9 0.6
6.2 0.1
13.3 1.0
21.6 1.7
12.0 0.3
15.8 2.7
28.8 0.4
the reaction rate constants k for each temperature as calculated
from the slope of the graphics in Figures S13−15, Supporting
Information. Only the linear domain was used to calculate the
rate constants because the reactions are not quantitative. The
back reaction takes place in different amounts depending on the
temperature and the nature of maleimides. As we were only
interested in a comparison of the rate at which the reactions
take place, we do not take into account measurements near the
reaction equilibrium.
Analysis at the k values at 50 °C showed that M1 is the
slowest reactant for the conversion with MF because the propyl
chain increases the electron density in the maleimide system,
raising the energy level of the LUMO of the dienophile and
therefore decreasing energy compatibility of reactive orbitals.
The urethyl functionality in M2 is withdrawing electron density
from the maleimide system, therefore lowering the energy level
of the LUMO and increasing the interaction with the HOMO
of the diene. The phenyl ring of M3 withdraws even more
electron density from the system, which results in even a higher
reaction rate constant with regard to M1.
Kinetic Studies at the Surface of Particles with Furan
at RT. To investigate if the reaction rate is drastically changed
at the surface of nanoparticles, the reaction rate constant was
estimated by monitoring the maleimide ring deformation signal
in the infrared spectra during DA reaction on the surface of
silica nanoparticles. Unfortunately, this method could only be
Figure 7. (a) Infrared spectra of 1@SiO2 at different time intervals
during the reaction with furan. (b) Conversion over time plot. (c) Plot
for first-order kinetics as a function of the linear fit and reaction rate
value k.
unchanged signal, the asymmetric Si−O−Si stretching, as
reference to obtain quantitative information. After a basis line
correction, the area under the signal at 696 cm−1 was used to
plot the conversion against reaction time. The conversion
follows exponential decay, as in the case for first-order kinetics
(Figure 7). The kinetic rate constant is in the order of 10−5 s−1.
This value is in the same order of magnitude as that for the
reaction rate in the case of molecular systems at room
temperature, meaning that the reaction rate is not significantly
F
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cleaning bath, adding one drop (30 μL) on the copper grid and
evaporating the solvent.
altered by surface effects. UV/Vis spectroscopic measurements
could not be performed because of the light scattering
properties of the silica nanoparticles.
Dynamic light scattering (DLS) measurements were carried out by
noninvasive backscattering on an ALV/CGS-3 compact goniometer
system with an ALV/LSE-5003 correlator and multiple tau correlator
at a wavelength of 632.8 nm (He−Ne Laser) and at a 90° goniometer
angle. The dispersing media were purified before use with a syringe
filter (200 nm mesh). The determination of the particle radius was
carried out by the analysis of the correlation function via the g2(t)
method followed by a linearized number-weighting (n.w.) of the
distribution function.
Nitrogen sorption measurements were performed on a Sorptomatic
1900 instrument from Fisons Instruments at 77 K. The samples were
degassed under vacuum at 60 °C for at least 2 h prior to measurement.
The surface area was calculated according to Brunauer, Emmett, and
Teller (BET).
CONCLUSIONS
■
This study ascertained that the conditions chosen to conduct
surface modification of nanosized silica particles, involving an
exchange of the suspension medium from methanol to IBMK,
ensures high surface coverage and low agglomeration of the
particles. The low dielectric constant of toluene makes it a poor
choice because ionization of surface silanols is unlikely.
Another goal of this work was to determine the reactivity of
differently substituted maleimides. It was shown in molecular
systems that phenyl-substituted maleimides are three times
more reactive than propyl-substituted maleimdes. Furthermore,
it has been demonstrated that the amount of active sites for the
DA reaction on the surface of silica nanoparticles is dependent
on the grafting density of the dienophile species. The higher the
grafting density and the sterical effects near the reaction site,
the lower the conversion of DA reaction. Already reacted
furan/maleimide groups interfere with other approaching furan
molecules, hindering the continuation of DA reaction. Finally, a
kinetic study of DA reactions between maleimide moieties on
surfaces and molecular furan has shown that surface effects do
not dramatically decrease the reaction rate.
Materials. Tetraethyl orthosilicate (TEOS) was provided by
Wacker Silicones. 3-Aminopropyltriethoxysilane (APTES), 3-isocya-
natopropyltriethoxysilane, 1,1′-(methylenedi-4,1-phenylene)-
bismaleimide, isobutyl methyl ketone, dry toluene, dry dichloro-
methane, furan, dimethylfuran, hexamethyldisilazane, and dibutyltin
dilaurate (95%) were purchased from Sigma-Aldrich and were used as
received. All other solvents and chemicals were purchased from the
central chemical depot of the Saarland University and dried according
to standard procedures if necessary. Zinc chloride was purified in
boiling dioxane with zinc dust and recrystallized from dioxane.
Synthesis of Silica Nanoparticles. The silica nanoparticles were
synthesized according to a modified literature procedure.20 In a 250
mL round-bottom flask, methanol (100 mL) was mixed with 51 mg
(1.0 mmol) of 33% ammonia and 1.98 g (110 mmol) of water and
stirred for 10 min. Then 10.41 g (50 mmol) of TEOS were added. The
solution was stirred for 2 days, and a part of the particles was isolated
to characterize the unmodified sample. The particles were precipitated
with 100 mL of hexane and 50 mL of diethyl ether. Afterwards they
were isolated and washed three times with ethanol by centrifugation at
13000 rpm and dried overnight in a vacuum oven (≈50 mbar) at 60
°C. Yield: 1.5 g silica particles; TEM: diameter 3.8 0.9 nm; DLS:
diameter 4.8 0.8 nm; surface area: BET 750 m2 g−1.12
Synthesis of N-((3-Triethoxysilyl)propyl)maleimide 1. The
synthesis was carried out following a literature procedure.15 The
reaction was carried out under argon atmosphere. In a 250 mL three-
neck round-bottom flask equipped with a dropping funnel, 1.73 g
(17.6 mmol) of maleic anhydride were dissolved in 60 mL of dry
dichloromethane. A mixture of 3.90 g (17.6 mmol) of 3-
(aminopropyl)triethoxysilane (APTES) and 20 mL of dry dichloro-
methane were added through a dropping funnel while stirring. After
allowing the mixture to stand for 1 h at room temperature, the solvent
was removed under vacuum. The intermediate product was collected
as a white powder. 1H NMR (200.13 MHz, CDCl3, 25 °C) δ = 0.68 (t,
3J = 7.7 Hz, 2H), 1.23 (t, 3J = 7.0 Hz, 9H), 1.74 (p, 3J = 7.7 Hz, 2H),
3.39 (q, 3J = 6.5 Hz, 2H), 3.84 (q, 3J = 7.0 Hz, 6H), 6.33 (s, 2H), 7.83
(br s, 1H). 13C NMR (50.32 MHz, DMSO-d6, 25 °C) δ = 7.80
(CH2Si), 18.24 (CH3CH2O), 21.95 (CH2CH2CH2), 42.64 (CH2NH),
58.59 (CH3CH2O), 131.54 (CHCONH), 136.08 (CHCOOH),
165.56 (COOH), 166.03 (CONH).
EXPERIMENTAL SECTION
■
Methods. Fourier transform infrared spectroscopy (FT-IR)
measurements were performed on a Bruker Vertex 70 Spectrometer
under ambient air (40 scans at a resolution of 4 cm−1) in attenuated
total reflectance (ATR) mode. Thermogravimetric analysis (TGA)
were performed on a Netzsch Iris TG 209 C in an alumina crucible
heating from room temperature to 700 °C under nitrogen followed by
heating to 800 °C under oxygen with a rate of 20 K min−1. Differential
scanning calorimetry (DSC) measurements were carried out on a
Netzsch DSC 204 F1 Phoenix calorimeter in aluminum crucibles with
pierced lids, heating under nitrogen with a rate of 15 K min−1. Liquid-
state NMR spectra were recorded on a Bruker AC 200F spectrometer
(1H at 200.13 MHz, 13C at 50.32 MHz). Liquid-state 29Si NMR
spectra were recorded on a Bruker Avance 300 spectrometer at 69.63
MHz. Elemental analysis was carried out on a Leco 900 CHN
Analysator. The percentage of nitrogen was used to calculate the
number of functional groups on the surface of the silica particles.
Equation 1 gives the molar amount of functional groups per gram of
particles:
ΔmN
MN
100
Δm
0.01
N
cFG
=
×
×
SiO2
(1)
where cFG is the molar concentration of functional groups per gram
[mol g−1], ΔmSiO is the residual mass of SiO2 from TGA measurement
2
[%], ΔmN is the mass of nitrogen from EA [%], MN is the molar mass
of nitrogen [g mol−1], and N is the number of nitrogen atoms per
functional group.
By taking into account the surface area per gram of silica, the
number of functional groups per surface area NFG‑A can be calculated
according to eq 2:
In the next step, the intermediate product was dissolved in 60 mL of
dry toluene, and 2.40 g (17.6 mmol) of zinc chloride were added at
once. After the reaction mixture was heated to 80 °C, a solution of
2.84 g (17.6 mmol) of hexamethyldisilazane with 20 mL of toluene
was added dropwise. The temperature was held during 5 h at 80 °C.
After cooling, the solution was filtered to remove zinc chloride and the
solvent was removed in vacuum. Yield: 5.12 g (17.0 mmol; 96.5%) of a
colorless oily liquid. 1H NMR (200.13 MHz, CDCl3, 25 °C) δ = 0.55−
0.64 (m, 2H), 1.22 (t, 3J = 6.9 Hz, 9H), 1.62−1.78 (m, 2H), 3.51 (t, 3J
1
A
NFG‐A = cFA × N ×
× 10−18
A
(2)
where NFG‑A is the number of functional groups per area [nm−2], nFG is
the molar concentration of functional groups per gram [mol g−1], NA
is Avogadro’s number, and A is the specific surface area of the
nanoparticles per gram [m2 g−1].
Transmission electron microscopy (TEM) images were recorded on
a JEOL JEM-2010 microscope. The samples were attached to Plano
S160-3 copper grids by dispersing them in ethanol using an ultrasound
3
= 7.37 Hz, 2H), 3.81 (q, J = 6.9 Hz, 6H), 6.68 (s, 2H). 13C NMR
(50.32 MHz, CDCl3, 25 °C) δ = 7.49 (CH2Si), 18.25 (CH3CH2O),
25.06 (CH2CH2CH2), 45.36 (CH2N), 58.44 (CH3CH2O), 134.21
(HCCH), 171.10 (CO). 29Si NMR (59.63 MHz, CDCl2, 25 °C)
δ = −46.34. IR (cm−1): 3099, 2974, 2932, 2887, 1770, 1703, 1440,
G
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Chemistry of Materials
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1406, 1365, 1255, 1220, 1166, 1101, 1072, 956, 829, 783, 756, 694,
453.
mL of chloroform. 1.1 g (5 mmol) APTES were added, and the
mixture was stirred for 12 h at RT. The crude product was used
without further purification. 1H NMR (200.13 MHz, CDCl3, 25 °C) δ
Synthesis of Protected Maleic Anhydride 3,6-Epoxy-1,2,3,6-
tetrahydrophthalic Anhydride 4. The synthesis was carried out
following a modified literature procedure.16 In a 250 mL three-neck
round-bottom flask under argon atmosphere, 20 g (0.204 mol) of the
maleic anhydride and 14.04 g (0.206 mol) of furan were dissolved in
100 mL of toluene. The mixture was stirred for 24 h at room
temperature. A white precipitate formed during this time. The solid
was collected by filtration and washed two times with cold diethyl
ether. The filtrate was reduced by rotary evaporation to 20 mL and
cooled to 4 °C overnight. A second crop crystallized which was again
collected by filtration and washed with diethyl ether. Finally, the
crystals were dried in vacuum (≈10−2 mbar) overnight. Yield: 27.36 g
3
3
= 0.69 (t, J = 8.0 Hz, 2H), 1.24 (t, J = 7.0 Hz, 9H), 1.72 (m, 2H),
3
2.72 (m, 2H), 3.03−3.42 (m, 2H), 3.84 (q, J = 7.0 Hz, 6H), 3.99 (s,
1H), 4.05 (s, 2H), 6.86 (s, 2H), 7.29 (m, 8H). 13C NMR (50.32 MHz,
CDCl3) δ = 7.62, 18.09, 23.91, 36.62, 40.95, 46.48, 53.58, 58.11,
126.31, 126.51, 129.27, 129.51, 134.02, 140.02, 140.81, 168.16, 175.19,
176.31. 29Si NMR (59.63 MHz, CDCl2, 25 °C) δ = −46.03. IR
(cm−1): 3346, 3039, 2974, 2931, 2885, 1776, 1703, 1670, 1512, 1438,
1382, 1307, 1168, 1101, 1074, 1020, 952, 856, 811, 773, 671, 605, 513,
466, 408.
Solvent Exchange. Methanol was exchanged with isobutyl methyl
ketone (IBMK) by adding 20 mL of IBMK to 50 mL of the particle
suspension. A first fraction of methanol and excess ammonia were
removed by rotary evaporation to reach a volume of approximately 10
mL. Then a second volume of 10 mL of IBMK was added, and the
suspension was again concentrated to approximately 10 mL. This
procedure was repeated two times. Afterwards the suspension was
diluted with 90 mL of fresh IBMK.
Surface Functionalization of Silica Particles in IBMK. In a 250
mL three-neck round-bottom flask with a reflux condenser, 2 g of 1, 3
g of 2, or 3 g of the crude 3 was added to the silica particle suspension
in IBMK and heated to 115 °C for 24 h. Afterwards the suspension
was cooled to room temperature, and half of the solvent was removed
by rotary evaporation. The particles were isolated by centrifugation at
13000 rpm for 10 min. Afterwards they were washed three times with
acetone and dried overnight in a vacuum oven (≈50 mbar) at 80 °C.
The particles were stored in a desiccator over P2O5. Yield: 0.8 g of
yellowish particles.
1
(0.165 mol; 80.6%) of a white solid. H NMR (200.13 MHz, CDCl3,
25 °C) δ = 3.01 (s, 2 H), 5.40 (s, 2 H), 6.57 (s, 2 H). 13C NMR (50.32
MHz, CDCl3, 25 °C) δ = 47.52 (CH), 81.40 (CHO), 136.67 (CC),
175.29 (CO). IR (cm−1): 3606, 3143, 3099, 3089, 3066, 3033, 3000,
2991, 1857, 1780, 1309, 1282, 1230, 1211, 1193, 1145, 1083, 1018,
948, 921, 902, 877, 848, 821, 800, 732, 690, 674, 634, 574, 428. Onset
of decomposition (DSC, N2, 15 K min−1): 117.9 °C. Elemental
analysis (%): Calcd for C8H6O4: C 57.84, H 3.64, N 0.00; Found: C
56.75, H 3.71, N 0.00.
Synthesis of Protected 2-Hydroxyethylmaleimide 5. The
synthesis was carried out following a modified literature procedure.16
In a 100 mL round-bottom flask equipped with a reflux condenser,
5.80 g (35 mmol) of 4 were dissolved in 30 mL of dry MeOH.
Afterwards 2.14 g (35 mmol) of ethanolamine were added dropwise at
0 °C. The mixture was refluxed for 24 h. The MeOH was removed by
rotary evaporation, and the crude product was recrystallized from
diethyl ether at −20 °C. The crystals were collected by filtration and
washed with cold diethyl ether. Reducing the filtrate and recrystallizing
again gained a second crop. The final product was dried in vacuum
(≈10−2 mbar) overnight. Yield: 5.75 g (18.5 mmol; 52.7%) of a white
solid. 1H NMR (200.13 MHz, CDCl3, 25 °C) δ = 0.86 (bs, 1H), 2.90
(s, 2H), 3.76 (m, 4H), 5.30 (s, 2H), 6.54 (s, 2H). 13C NMR (50.32
MHz, CDCl3, 25 °C) δ = 41.74 (CHOCHCO), 47.46 (NCH2),
60.22 (CH2OH), 80.95 (HCO), 136.49 (CC), 176.74 (CO). IR
(cm−1): 3473, 3097, 3026, 3006, 2993, 2972, 2931, 2895, 1766, 1681,
1469, 1434, 1404, 1386, 1334, 1317, 1286, 1267, 1218, 1168, 1155,
1099, 1053, 1033, 1012, 958, 937, 916, 873, 850, 806, 771, 721, 703,
653, 594, 563, 530, 487, 428. Onset of decomposition (DSC, N2, 15 K
min−1): 138.1 °C. Elemental analysis (%): Calcd for C10H11NO4: C
57.41, H 5.30, N 6.70; Found: C 57.39, H 5.43, N 6.59.
ASSOCIATED CONTENT
* Supporting Information
■
S
Supplementary infrared spectra, TGA/FTIR data, and UV/Vis
spectra. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: kickelbick@mx.uni-saarland.de.
■
Notes
The authors declare no competing financial interest.
Synthesis of Protected 2-(2,5-Dioxo-2,5-dihydro-1H-pyrrol-
1-yl)ethyl 3-(Triethoxysilyl)propylcarbamate 2. The reaction was
carried out under argon atmosphere. In a 100 mL three-neck round-
bottom flask, 935 mg (3 mmol) of protected hydroxyethyl maleimide
5 was suspended in 10 mL of dry acetone. 890 mg (3.6 mmol) 3-
Isocyanotopropyltriethoxysilane were added via a syringe while
stirring. The solution was stirred at reflux overnight followed by the
addition of 3 drops of dibutyltin dilaurate catalyst. The reaction
mixture was refluxed for an additional 1 h. The crude mixture was
ACKNOWLEDGMENTS
■
This work is supported by the German Research Society
(DFG) through the priority program “Design and Generic
Principles of Self-healing Materials” SPP 1568 (http://www.
spp1568.uni-jena.de/).
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3
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I
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