Thermosensitive imidazole-containing polymers as catalysts in hydrolytic decomposition of p-nitrophenyl acetate

Article abstract of DOI:10.1021/ma0487453

Poly(N-vinylcaprolactam-co-1-vinylimidazole) (PVCL-Vim) and poly(N-isopropylacrylamide-co-1-vinylimidazole) (PNIPA-Vim) are thermosensitive in water and water-2-propanol solutions. These copolymers are soluble at room temperature; however, they undergo ph

Full text of DOI:10.1021/ma0487453

Macromolecules 2004, 37, 7879-7883
7879
Thermosensitive Imidazole-Containing Polymers as Catalysts in
Hydrolytic Decomposition of p-Nitrophenyl Acetate
Iva n M. Ok h a p k in ,†,‡ Lyu d m ila M. Br on stein ,§ Elen a E. Ma k h a eva ,
Va len tin a G. Ma tveeva ,# Esth er M. Su lm a n ,# Mik h a il G. Su lm a n ,# a n d
Alexei R. Kh ok h lov*,†,‡,
Department of Polymer Science, University of Ulm, Albert-Einstein-Allee 11, D-89069 Ulm, Germany;
Nesmeyanov Institute of Organoelement Compounds, Vavilov Street 28, 119991 Moscow, Russia;
Chemistry Department, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47405;
Physics Department, Moscow State University, 119992 Moscow, Russia; and
Tver Technical University, 170026 Tver, Russia
Received J une 23, 2004; Revised Manuscript Received J uly 28, 2004
ABSTRACT: Poly(N-vinylcaprolactam-co-1-vinylimidazole) (PVCL-Vim) and poly(N-isopropylacrylamide-
co-1-vinylimidazole) (PNIPA-Vim) are thermosensitive in water and water-2-propanol solutions. These
copolymers are soluble at room temperature; however, they undergo phase transition at higher
temperatures (Ttransition > 35 °C). Catalytic properties of PVCL-Vim and PNIPA-Vim as well as of
1-methylimidazole and poly(1-vinylimidazole) (PVim) in hydrolysis of p-nitrophenyl acetate were studied.
Variation of the reaction rate with temperature was investigated at constant concentrations of substrate
and catalyst (imidazole groups). It was found that the reaction catalyzed by the copolymers does not
follow Arrhenius-type behavior: as temperature is increased over the aggregation temperature, the
reaction rate increases faster than it could be predicted from the Arrhenius law. This was explained via
the assumption that the substrate having an affinity to polymer/solvent interface can be adsorbed at the
surfaces of polymer aggregates. This leads to higher concentration of substrate around the catalytically
active groups. Theoretical consideration supports such a possibility.
In tr od u ction
The problem of mimicking the enzymes with the
synthetic polymers bearing corresponding groups (for
example, imidazole, acidic, or others) has received
considerable attention starting from the 1960s and
continued in the late 1990s due to both increasing
interest to very selective enzyme catalysis and attempts
to overcome limitations of the enzymes such as require-
ments of narrow pH range and subtle temperature
control.1-8 Recently, a polymeric catalyst for ester
hydrolysis allowing one to switch on and off the catalytic
activity was designed on the basis of a polymer gel
consisting of two types of monomers: N-isopropyl-
acrylamide (major component) and 4(5)-vinylimidazole
(minor component).9 In that paper the authors used the
ability of poly(N-isopropylacrylamide) (PNIPA) to swell
or shrink reversibly with the change of temperature or
solvent and observed significant increase of catalytic
activity in the collapsed gel, suggesting that hydrophobic
PNIPA network increases affinity for amphiphilic sub-
strate, thus enhancing the catalytic activity. However,
macrosized gel catalysts may be plagued with transport
limitations and excessive sensitivity of properties to the
synthesis conditions. In addition, a very low fraction of
catalytically active units (1.6% mol) may limit the
overall catalytic activity.
F igu r e 1. Chemical structures of monomers and catalytic
reaction studied.
(N-isopropylacrylamide-co-1-vinylimidazole) (Figure 1)
by switching its morphological state via a phase transi-
tion in otherwise homogeneous solutions. Namely, upon
temperature increase these macromolecules exhibit a
coil-to-globule transition with subsequent formation of
globular aggregates of submicrometer sizes. Normally
these changes occur above 30-32 °C for homopolymers
of N-vinylcaprolactam and N-isopropylacrylamide.10-12
Introducing hydrophilic units into the polymer back-
bone, one may increase the transition temperature.
In this paper we explore the idea of controlling the
catalytic activity of imidazole-bearing linear copolymers
poly(N-vinylcaprolactam-co-1-vinylimidazole) and poly-
† University of Ulm.
The aggregates of thermosensitive copolymers are
very stable in aqueous solutions and have developed
surface areas, thus providing conditions of microhetero-
geneous catalysis with negligible diffusion limitations
and with high concentration of the catalytically active
‡ Nesmeyanov Institute of Organoelement Compounds.
§ Indiana University.
Moscow State University.
Tver Technical University.
#
* Corresponding author: e-mail khokhlov@polly.phys.msu.ru.
10.1021/ma0487453 CCC: $27.50 © 2004 American Chemical Society
Published on Web 09/21/2004

7880 Okhapkin et al.
Macromolecules, Vol. 37, No. 21, 2004
Ta ble 1. Ch a r a cter istics of th e P olym er s
groups at the aggregate interfaces. Also, 1-vinylimida-
zole units are amphiphilic, so they tend to position
themselves in the polymer/water interfacial layers.
Vim
Vim
Mw,
fraction, %, fraction, %,
g/mol
T
transition, °C titration
1H NMR
Exp er im en ta l Section
PVim
46 000
PNIPA-Vim-11a 48 000
PNIPA-Vim-13a 79 000
PVCL-Vim-20a 32 000
PVCL-Vim-29a 41 000
35
35
40
42
11
13
20
30
Ma ter ia ls. N-Vinylcaprolactam and N-isopropylacrylamide
(Polysciences) were recrystallized from ethanol solution, 2,2′-
azoisobisbutyronitrile (AIBN, Merck) was recrystallized from
methanol solution, p-nitrophenyl acetate (NPA, Fluka) was
recrystallized from 2-propanol solution, 1-vinylimidazole (Vim,
Aldrich) was distilled under reduced pressure, and 1-methyl-
imidazole (Aldrich) was used as received.
P olym er Syn th esis. We synthesized two types of random
copolymers containing both catalytically active 1-vinylimida-
zole and thermosensitive N-vinylcaprolactam or N-isopropyl-
acrylamide units using free radical polymerization in solution.
Also, 1-vinylimidazole homopolymer was synthesized in the
same way.
19
28
a
The polymers are marked according to the Vim content.
temperature. The correlation functions obtained were analyzed
using the CONTIN program.
All samples for light scattering studies were made by
dissolution of the freeze-dried polymers in a water-2-propanol
mixture (10 vol %) at room temperature for 12 h. The solutions
were buffered with 2 mM of phosphate buffer (KH2PO4/Na2-
HPO4) at pH 7.4.
Initial monomer solutions in absolute ethanol containing
AIBN as initiator were incubated in argon atmosphere at 50
°C for 48 h. Then the reaction solution was 5-fold diluted with
ethanol and poured into diethyl ether. White precipitate was
collected. It was dissolved in deionized water and dialyzed for
72 h. Water was exchanged every 24 h. After dialysis, polymer
solutions were freeze-dried for 20 h.
Kin etic Mea su r em en ts. All kinetic measurements were
performed at Vim and NPA concentrations of 2 mM in water-
2-propanol solutions (10 vol %). The solutions were buffered
with 2 mM of phosphate buffer (KH2PO4/Na2HPO4) at pH 7.4.
The progress of catalytic reaction was monitored by tracing
the rate of optical density increase at 402 nm due to the release
of p-nitrophenol in a deprotonated form.
Some measurements were performed in turbid solutions.
Hence, the optical density values were dependent on both
turbidity and p-nitrophenol absorption. In the standard pro-
cedure of kinetic measurements (e.g. ref 13) a small amount
of concentrated substrate solution is injected to the polymer
solution and then mixed by shaking, and finally the optical
density is recorded as a function of time. This method did not
show good results, since injection and mixing made the
turbidity and overall optical density values unstable during
the measurement. To address this issue, a special procedure
was developed to obtain constant and reproducible optical
density values of turbid solutions.
Equal volumes (1 mL) of substrate solution (4 mM) and
polymer solution (4 mM of Vim units and 4 mM of phosphate
buffer) were kept at constant (to within 0.2 °C) temperature
until full equilibration. The solutions of substrate were kept
in quartz cuvettes. Both catalyst and substrate solutions were
equipped with a stirrer.
Polymer solutions for LLS measurements were clarified by
filtration through Millipore GS 0.22 µm pore size filters before
use. For DLS measurements, the polymer concentration was
0.5 mg/mL.
Static light scattering (SLS) measurements were carried out
for Mw determination of the polymers using the Debye plot
method. The scattering angle was 90°, and polymer concen-
trations varied from 0.7 to 3.0 mg/mL. Refractive index
increments were measured with an Otsuka Photal RM-102
differential refractometer operating at λ ) 628 nm.
P olym er Ch a r a cter iza tion . Polymer characteristics are
given in Table 1. Average molecular weights of the synthesized
polymers were determined by SLS. The Vim unit fraction in
the copolymers was determined by standard titration and by
1H NMR. Potentiometric measurements for titration were
carried out with a HANNA pH-300 pH-meter at 25 °C. The
solutions of nonionized copolymers with a concentration of Vim
units of 0.002 M were loaded with an excess of NaOH and 0.2
M NaCl. They were titrated with 0.03 M HCl. NaCl was used
for the increase of pKa of Vim units and better resolution of
the equivalence point. The HCl/Vim concentration ratio was
taken to be high since dilution is not beneficial for good
resolution of the equivalence point.
1
The H NMR spectra were recorded with a Bruker Avance
DRX 400 MHz spectrometer using deuterated methanol as a
solvent. A polymer concentration was 1 wt %. The spectra were
collected at 25 °C. Chemical shifts for the proton spectra were
referenced to deuterated methanol at 3.31 ppm. The results
of the titration corroborated the 1H NMR determination (Table
1).
Rea ction Ra te Ca lcu la tion . The calculation of reaction
rate was performed using eq 1:
Then the catalyst solutions were promptly injected into
quartz cuvettes containing substrate. No changes of solution
temperature were detected to within 0.2 °C after that proce-
dure. After the injection, the solutions in quartz cuvettes were
stirred at constant temperature for time tn, and the optical
density at 402 nm of the solution was detected. A Hewlett-
Packard 8452A single-beam spectrophotometer was used for
that purpose. A solution stirred for t0 ) 25 s was used as a
baseline solution. The initial slope of optical density D vs (tn
- t0), which can be well fitted with linear model, was used to
calculate the hydrolysis rate.
Constant stirring and recording of the baseline in 25 s after
injection eliminated the possible influence of mixing and
injection on absorbance values of turbid solutions. The studies
of nonturbid solutions followed the same procedure.
La ser Ligh t Sca tter in g (LLS) Exp er im en ts. The laser
light scattering experiments were performed with an ALV-
Instruments ALV/CGS-8F laser goniometer system and an
ALV-5000 multi-τ digital correlator. The light source was a
J DS Uniphase helium/neon 22 mW laser operating at λ ) 633
nm. In the dynamic light scattering (DLS) experiments time-
intensity correlation functions in the beating mode were
dD 1 1
dt ꢀ F
V )
(1)
where dD/dt is the initial slope of optical density variation with
time, ꢀ is the extinction coefficient of p-nitrophenol (deproto-
nated form), ꢀ ) 18 500 L/(mol cm), and F is the fraction of
p-nitrophenol (deprotonated form) at pH 7.4 (F ) 0.6, as was
found by spectrophotometry).
Resu lts a n d Discu ssion
As was already mentioned, PVCL-Vim and PNIPA-
Vim are thermosensitive copolymers with the LCST in
the interval 35-45 °C. It is challenging to compare the
catalytic activity of these polymers in aggregated and
nonaggregated states in order to understand how the
aggregation affects catalytic properties. One cannot
compare those properties directly since the increase of
temperature normally leads to the enhancement of
catalytic activity, and the effects due to the increase of
temperature and aggregation can overlap.
measured at
a scattering angle of 90° as a function of

Macromolecules, Vol. 37, No. 21, 2004
Imidazole-Containing Polymers 7881
F igu r e 2. Reaction rate of hydrolysis of NPA catalyzed with
polymeric catalysts as a function of inverse temeperature.
Concentrations of Vim groups and NPA are equal to 2 mM in
each case.
Ta ble 2. P a r a m eter s of Regr ession of th e log(V)-T-1
Dep en d en cies for th e Ca ta lysts Stu d ied (Lin ea r Mod el Is
Ap p lied )
F igu r e 3. Distributions of hydrodynamic diameter for co-
polymers in water solutions at various temperatures: (a)
PNIPA-Vim-11; (b) PVCL-Vim-29.
catalyst
Ra
error, % N, degrees of freedom
1-methylimidazole
PVim
PNIPA-Vim-11
PNIPA-Vim-13
PVCL-Vim-20
PVCL-Vim-29
0.996
0.988
0.945
0.937
0.951
0.943
6
11
24
26
23
20
3
3
3
3
3
4
ylimidazole and PVim the correlation coefficient is close
to 0.99, whereas for copolymers it normally does not
exceed 0.95. It can also be seen that the relative error
of the slope obtained by least-squares fitting is larger
than 20% for copolymers whereas for 1-methylimidazole
and PVim it is much lower. These facts mean that the
linear model fit is less probable for the copolymers than
for 1-methylimidazole and PVim in the temperature
interval 20-55 °C due to deviations observed at tem-
peratures around 35-45 °C.
To associate the catalytic properties of the copolymers
with the aggregation properties, we have performed a
DLS study of copolymer particle sizes as a function of
temperature. Figure 3 shows the distributions of hy-
drodynamic diameter for several copolymers at various
temperatures.
At the temperatures below 35 °C the copolymers exist
in the form of coils whose hydrodynamic diameter does
not exceed 20 nm at the average. The aggregates with
a larger hydrodynamic diameter are formed upon heat-
ing (see Figure 3). Aggregation of PNIPA-Vim-11 starts
at 35 °C, whereas for PVCL-Vim-29 aggregates emerge
at 42 °C, as seen from Figure 3. The aggregates are
growing larger with the temperature increase. When
just formed (for both PNIPA-Vim-11 and PVCL-Vim-
29), their size is about 200 nm. After temperature is
further raised, the size attains 400 nm in diameter.
The measurements of cloud point values for PNIPA-
Vim-13 showed that for this copolymer the aggregation
temperature is close to that of PNIPA-Vim-11, whereas
for PVCL-Vim-20 it was found to be 40 °C.
The comparison of the data presented in Figures 1
and 3 shows that the temperature intervals of aggrega-
tion precede the temperature interval of rapid growth
of the reaction rate in the region of 35-45 °C. Indeed,
for PNIPA-Vim the reaction rate starts growing faster
than exponentially above 35 °C, whereas the reaction
catalyzed by PVCL-Vim undergoes additional accelera-
tion at temperatures higher than 40 °C.
a
R is the correlation coefficient.
It is well-known that for the majority of temperature
activated reactions the reaction rate is an exponential
function of inverse temperature (Arrhenius-type behav-
ior). When plotted in semilogarithmic (Arrhenius) co-
ordinates, reaction rate gives a linear plot. If there are
some effects that can influence the reaction rate along
with the temperature, we should be able to observe a
deviation from the linear law. Our main interest is to
obtain a temperature-responsive catalyst, which would
stimulate the reaction rate growth which is faster than
exponential due to more effective catalysis by polymer
aggregates than by separate polymer chains.
Figure 2 shows the change of the reaction rate with
a temperature increase for the four copolymer catalysts,
1-methylimidazole and PVim, at identical concentra-
tions of imidazole groups (2 mM) for each catalyst. The
dependencies are presented in Arrhenius coordinates.
From Figure 2 one can see that for 1-methylimidazole
and PVim the dependencies are quite linear, showing
that these catalysts follow Arrhenius type behavior.
PVim appears to be less effective catalyst than its low-
molecular-weight analogue 1-methylimidazole.
The copolymer catalysts show different behavior: the
rate-temperature dependencies are not linear in Ar-
rhenius coordinates. In the temperature range 35-45
°C the growth law is faster than the linear one.
However, when temperature is further raised, one may
observe an opposite effectsthe reaction rate slows down.
This type of behavior can be seen in Figure 2.
In Table 2 statistical analysis of the curves of Figure
2 is performed. For each curve, a linear model is
assumed, and the correlation coefficient together with
the relative error of slope is presented. Both for 1-meth-

7882 Okhapkin et al.
Macromolecules, Vol. 37, No. 21, 2004
F igu r e 4. Interfacial layer of copolymer aggregates as a catalytic nanoreactor.
Thus, we can state that the observed acceleration of
the reaction is closely connected with the aggregation
and is due to the higher catalytic activity of the
aggregates compared to that of the coils.
The catalytic activity of the copolymers, however, does
not exceed that of 1-methylimidazole at equal concen-
trations of Vim groups. For PVCL-Vim copolymers, at
certain temperature it can be as high as that of
1-methylimidazole, and in all cases it is much higher
than for PVim.
The property of enhanced catalytic activity in the
presence of copolymer aggregates reveals itself under
the conditions where the coil-to-globule transition fol-
lowed by aggregation takes place. Since we observe the
F igu r e 5. Michaelis-Menten-type kinetics for decomposition
of NPA by PNIPA-Vim-11 at 45 °C.
formation of aggregates rather than separate globules
(Figure 3), catalysis is thus promoted by a microhet-
erogeneous phase consisting of polymer aggregates. In
this context, it is appropriate to explain the phenomenon
of enhanced catalytic activity in the following terms.
Because of the existence of many hydrophobic copoly-
mer/solvent interfaces in the solution of aggregates, one
should expect that NPA will adsorb at these interfaces
(Figure 4). The interfacial concentration of the substrate
can be high since NPA is a good surfactant. We have
investigated the interfacial activity of NPA at hexane/
water boundary, which can model the copolymer/solu-
tion interface. When NPA molecules are partitioned
between hexane and water (log P ) 0.87 at 25 °C), the
free energy of adsorption of these molecules from water
to hexane/water boundary is 9.7kT (24 kJ /mol), indicat-
ing that NPA has a significant affinity to water/organic
interfaces.
Also, amphiphilic Vim units of the copolymer should
be at the interface because they tend to interact with
molecules of water and minimize the interfacial energy
of aggregates. Indeed, the Vim units are capable of
positioning themselves at interfaces. Our investigation
of the interfacial activity of Vim monomer shows that
if partitioned between hexane and water, it can be
adsorbed at the hexane/water boundary with the free
energy of adsorption equal to 5.8kT (14 kJ /mol). Fur-
thermore, Vim has a larger affinity to water than to
hexane (log P ) -1.26 at 25 °C), and this can be an
additional factor for Vim units to tend to leave partially
the hydrophobic surrounding of the aggregates.
of both Vim and NPA local concentrations, thus leading
to the effective growth of the catalytic activity of the
copolymer. In this sense, the interfacial nanolayer of
aggregates can be considered as a catalytic nanoreactor.
In enzymatic catalysis, the catalytic act is preceded
by the complex formation between catalyst and sub-
strate. Because of the complex formation, enzymatic
reactions follow Michaelis-Menten-type kinetics:14
V0[S]
V )
(2)
Km + [S]
where V0 ) kcat[E0], kcat is the first-order rate constant
for breakdown of the substrate-catalyst complex, [E0]
is the concentration of catalyst, and [S] is the concentra-
tion of substrate.
One may expect that if the reaction rate vs substrate
concentration measurements yields Michaelis-Menten-
type kinetics in our case, it can be a good indication that
indeed NPA (substrate) adsorbs to the interfaces of
aggregates, forming a kind of “complex” with surface-
active catalytic groups. Such measurements were per-
formed for PNIPA-Vim-11 at 45 °C (Figure 5). For that
copolymer a kinetic curve was obtained which could be
well fitted with eq 2, giving the parameters V0 ) 8.6 ×
10-7 mol/(L s) and Km ) 0.0105 M. The concentration
of the catalyst was taken 10 times lower than in the
temperature experiments (0.2 mM) to get an excess of
substrate over catalyst, which is the condition for
observing Michaelis-Menten kinetics. As one may see,
the catalysis by PNIPA-Vim-11 indeed follows the
kinetics of Michaelis-Menten type, although the Km
Thus, the interface appears as an area where the
concentration of active species (both catalyst and sub-
strate) is larger than in the bulk of the solution. At the
interface the reaction progresses faster due to increase

Macromolecules, Vol. 37, No. 21, 2004
Imidazole-Containing Polymers 7883
parameter is rather large when compared, for example,
to that of R-chymotrypsin (for the same substrate, Km
) 1.6 × 10-4 M).15
is similar to a stage of enzymatic catalysis. Indeed,
before a catalytic act takes place substrate is usually
sorbed by the active center of the enzyme. In some
enzymes, like acetylcholinesterase, pepsin, and R-chy-
motrypsin, the substrate binding site contains hydro-
phobic structures which can effectively interact with
hydrophobic moieties of the substrate, contributing to
the formation of a relatively stable complex. However,
in our system, that complex is somewhat less stable
than those formed by enzymes, judging by Michaelis-
Menten analysis of the kinetics of catalysis. Also,
catalytic activity in our case is much lower than for
enzymes: the catalytic activity of our most active
polymer catalyst (PVCL-Vim-20) toward NPA hydroly-
sis at 43 °C is over 200 times lower than that of
R-chymotrypsin at 25 °C. The latter was estimated from
the data of ref 15. Indeed, other factors inherent to
enzymes (e.g., geometric correspondence between active
site of an enzyme and substrate) which increase the
stability of the complex and catalytic activity of enzyme
are not involved in the substrate-catalyst interactions
in our system.
Vasilevskaya et al.16 explored the theoretical possibil-
ity of acceleration of the reaction if both catalyst and
substrate can adsorb at the oil/water interfaces in mini-
emulsions. It was shown that at certain optimum size
of miniemulsion droplets (normally around several
hundred nanometers) a significant acceleration of the
reaction can take place compared to the case where
the size of droplets is too small or too large as well as
to the case of complete phase separation. Furthermore,
it was shown that for the optimum size droplets a
Michaelis-Menten profile of the reaction rate vs sub-
strate concentration is typical when the surface of
miniemulsion droplets is saturated with substrate spe-
cies. Therefore, an increase of the reaction rate due to
concentrating of the substrate at the interface of ag-
gregates and the fact that the reaction at the interface
of aggregates obeys Michaelis-Menten-type kinetics are
theoretically justified.
There can be some additional factors that enhance
catalytic activity at the surfaces of globules or ag-
gregates. First, both catalyst and substrate are specif-
ically oriented due to polymer concentration gradient
in the interfacial layer. This factor can stimulate mutual
orientation of the interacting species which in some
cases can be beneficial for an elementary act of the
catalytic reaction. Second, the substrate species are
subjected to an additional high stress due to the polymer
concentration gradient. The second factor obviously can
decrease the activation energy of the reaction since the
stress increases the ground state energy of the sub-
strate, resulting in energy-rich species which can be
more readily converted into the product.
Ack n ow led gm en t. The financial support from the
Alexander von Humboldt Foundation, Program for
Investment in the Future, is highly appreciated.
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Con clu sion
Temperature-sensitive poly(N-vinylcaprolactam-co-1-
vinylimidazole) and poly(N-isopropylacrylamide-co-1-
vinylimidazole) are catalytically active in the reaction
of hydrolysis of p-nitrophenyl acetate.
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The reaction rate does not follow Arrhenius-type
behavior upon temperature increase: in the tempera-
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exponential, while further increase of the temperature
decreases the reaction rate. This type of behavior is
observed when copolymers form aggregates in aqueous
solution. The temperature of aggregation precedes the
temperature of rapid growth of reaction rate. This
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