Catalytic properties of diblock copolymers of N-vinylcaprolactam and N-vinylimidazole

Article abstract of DOI:10.1134/S0012500815110014

Thermoresponsive diblock copolymers of N-vinylcaprolactam (VCL) with N-vinylimidazole (NVI) have been synthesized for the first time through two-stage controlled reversible addition-fragmentation chain transfer (RAFT) polymerization. The resulting diblock

Full text of DOI:10.1134/S0012500815110014

ISSN 0012ꢀ5008, Doklady Chemistry, 2015, Vol. 465, Part 1, pp. 253–256. © Pleiades Publishing, Ltd., 2015.
Original Russian Text © A.I. Barabanova, I.V. Blagodatskikh, O.V. Vyshivannaya, T.P. Klimova, N.V. Grinberg, T.V. Burova, A.V. Muranov, V.I. Lozinskii, V.Ya. Grinberg,
A.S. Peregudov, A.R. Khokhlov, 2015, published in Doklady Akademii Nauk, 2015, Vol. 465, No. 2, pp. 178–181.
CHEMISTRY
Catalytic Properties of Diblock Copolymers
of NꢀVinylcaprolactam and NꢀVinylimidazole
A. I. Barabanova, I. V. Blagodatskikh, O. V. Vyshivannaya, T. P. Klimova,
N. V. Grinberg, T. V. Burova, A. V. Muranov, V. I. Lozinskii, V. Ya. Grinberg,
A. S. Peregudov, and Academician A. R. Khokhlov
Received June 23, 2015
Abstract—Thermoresponsive diblock copolymers of
(NVI) have been synthesized for the first time through twoꢀstage controlled reversible addition–fragmentaꢀ
tion chain transfer (RAFT) polymerization. The resulting diblock copolymers exhibit the same catalytic
activity toward ꢀnitrophenyl propionate (NPP) hydrolysis as “proteinꢀlike” copolymers of VCL and NVI.
The catalytic activity of the diblock copolymers is likely caused by their structure favorable for the manifestation
of catalytic properties, namely, by micelleꢀlike chain conformations with a relatively dense core formed by the
Nꢀvinylcaprolactam (VCL) with Nꢀvinylimidazole
p
more hydrophobic PVCL block and a periphery formed by the hydrophilic polyꢀNꢀvinylimidazole (PNVI).
DOI: 10.1134/S0012500815110014
Our recent theoretical and experimental studies units arranged on the outer side as far as is possible. It
have focused on the computer design [1–3], experiꢀ is evident that this arrangement should lead to an
increase in the local concentration of diphilic NPA
molecules near the NVI units and, as a result, to an
increase in hydrolysis rate.
mental synthesis, and study of catalytic properties of
synthetic proteinꢀlike polymers (PPs) [4–6]. In parꢀ
ticular, statistical heteroblock VCL–NBI copolymers
with the proteinꢀlike arrangement of hydrophilic
(NVI) and hydrophobic (VCL) units in the polymer
chain have been synthesized via radical copolymerizaꢀ
tion [4–6]. In dilute aqueous solutions at temperaꢀ
tures above the lower critical solution temperature
(LCST), PPs undergo the coil–globule transition
without phase separation. The thermal response of the
copolymers is ensured by the PVCL blocks for which
the LCST depends on the chain length, being 32–
45°C [7]. The PP globules containing the hydrophobic
core and the stabilizing hydrophilic shell are actually
nanoreactors with a high concentration of catalytically
active NVI groups at the surface, exhibiting the propꢀ
erties of highly selective catalysts of different chemical
reactions [5].
This work deals with the synthesis of diblock VCL–
NVI copolymers with different lengths of separate
blocks and with the study of the effect of the microꢀ
structure of these copolymers on their catalytic activꢀ
ity in NPP hydrolysis reactions, as well as with comꢀ
parison of the catalytic activity of statistical PPs and
diblock VCL–NVI copolymers.
In this work, diblock copolymers of VCL with NVI
were synthesized for the first time through RAFT
polymerization [9]. At the first stage, the PVCL block
was obtained through the RAFT polymerization of
VCL in bulk at 60°C for 20 h in the presence of azobisꢀ
isobutyronitrile (AIBN) ([AIBN] = 1.0
the initiator and (1ꢀ( ꢀethylxanthyl)ethyl)benzene
(EXEB) ([EXEB] = 5.6
10–2 mol/L) as the lowꢀ
×
10–2 mol/L) as
O
×
Thermoresponsive diblock copolymers of NVI and
molecularꢀweight chainꢀtransfer agent. EXEB was
synthesized as described elsewhere [10, 11].
Nꢀisopropylacrylamide (NIPA) also exhibit catalytic
activity toward the hydrolysis of ꢀnitrophenyl acetate
p
(NPA) [8]. In an aqueous medium with increasing
temperature, the diblock copolymers form micelles,
with hydrophobic polyꢀNꢀisopropylacrylamide blocks
located inside and hydrophilic catalytically active NVI
The numberꢀaverage (Mn) and weightꢀaverage
(
Mw) molecular weights and dispersity (Mw
/Mn) of the
resulting PVCL with the terminal 1ꢀ(Oꢀethylxanꢀ
thyl)ethyl group (PVCL–RAFT agent) were deterꢀ
mined by gel permeation chromatography (GPC)
1
(table). The Mn for PVCL was also determined by H
Nesmeyanov Institute of Organoelement Compounds,
Russian Academy of Sciences, ul. Vavilova 28, Moscow,
119991 Russia
NMR (table). These results are quite consistent with
the available data on the synthesis of PVCL through
RAFT polymerization [14].
eꢀmail: barabanova@polly.phys.msu.ru
253

254
BARABANOVA et al.
Synthesis conditions and characteristics of diblock VCL–NVI copolymers
1H NMR(1) GPC(3)
VCL : NVI, mol % g/mol Mw
HSꢀDSC(5)
t h, J/g
(4)
Sample
Т
, °С
ls
Mn(2)
,
g/mol
Mn
,
/
Mn
Тt
,
°
С
Δ
PVCL
4100
–
100 : 0
70 : 30
50 : 50
0 : 100
2180
2500
3400
8370
1.37
1.42
1.41
1.50
46 0.5
46 0.5
46 0.5
–
42.0
–
32.3
–
CP30
CP50
–
42.6
–
33.8
–
PNVI
–
(1) 1
H NMR spectra of the copolymers were recorded on a Bruker spectrometer (600 MHz). D O was used as a solvent.
n
2
(2)
M was determined from the integrated intensity ratio of the signal of the –CH groups of the main PVCL chain at 4.0–4.5 ppm to the signal
of the –CH methine group of the VCL unit bonded to the terminal 1ꢀ(Oꢀethylxanthyl)ethyl group at 5.35 ppm.
(3)
Experimental conditions: an Agilent 1200 Series system with a refractive index detector, PLmixC column, elution with 1ꢀmethylpyrroliꢀ
done, 50°C, flow rate 0.5 mL/min. Calibration was performed using polystyrene standards.
(4)
T is the temperature at which the light scattering intensity sharply increases. The experiments were carried out on a PhotoCor Complex
ls
spectrometer (λ = 633 nm). The heating rate was 0.3 K/min. Measurements were taken at 1 K intervals (equilibration time, 5 min; measureꢀ
ment time, 5 min).
(5)
T is the transition temperature (a maximum of the temperature dependence of the excess heat capacity); Δ h is the enthalpy of transition
t
t
(related to the weight of VCL units in the sample); solvent, water; polymer concentration, 6 mg/mL. Measurements were performed with
a DASMꢀ4 (Biopribor, Pushchino) differential adiabatic scanning calorimeter in the temperature range 10–110°C at an excess pressure of
0.25 MPa and a heating rate of 1 K/min. Data were collected using the COMPORT software (Institute of Organoelement Compounds, RAS).
At the second stage, diblock copolymers were synꢀ CP50 toward higher molecular weights as compared
thesized via RAFT polymerization of NVI in dimethꢀ with PVCL and closeꢀtoꢀunity Mw /Mn values (table)
ylformamide in the presence of AIBN and the PVCL–
RAFT agent. The polymerization was carried out under
the following conditions: [NVI] = 1.8 mol/L, [AIBN] =
points to the controlled (“living”) character of the
polymerization reaction.
The catalytic activity of CP30 and CP50 was studꢀ
ied for the NPP hydrolysis reaction as an example in
the temperature range 25–55°C at pH 7.0 and was
compared with the catalytic activity of PNVI. The
hydrolysis kinetics was studied by measuring the optical
density of a colored product, pꢀnitrophenol (408 nm),
at different temperatures as a function of time. Figure
2 shows the temperature dependence of the hydrolysis
2.3 × ×
10–3 mol/L, [PVCL] = 1.8 10–2 (base mol)/L, 60
and 80°C, and 24 h. The composition of copolymers
1
was determined using H NMR (table). The syntheꢀ
sized polymers were diblock copolymers with the same
molecular weight of the PVCL block containing 30
(CP30) and 50 mol % (CP50) of NVI units in the polyꢀ
mer chain.
The synthesized diblock NVI–VCL copolymers
are characterized by unimodal molecular weight disꢀ
rate (V) of NPP in the presence of PNVI, CP30, and
CP50 in the Arrhenius coordinates. As temperature
increases from 25 to 60°C, the reaction rate in the
presence of PNVI additions linearly increases (Fig. 2,
tribution curves (Fig. 1, curves
2 and 3). The shift of
the molecular weight distribution curves of CP30 and
curve
strate hydrolysis in the presence of copolymers (Fig. 2,
curves and ). The dependence of ln on 1/ is nonꢀ
1). Another situation is observed for the subꢀ
dW/d (logM)
2
3
V
T
3
1.5
1
linear: in the temperature range from 40 to 55°C, the
reaction rate increases faster than in the temperature
range from 25 to 40°C. Figure 2 also demonstrates
that, even at low temperatures (25–40°C), the diblock
copolymers exhibit somewhat higher catalytic activity
than PNVI and that the larger the number of NVI
units in the copolymer, the higher the catalytic activity.
A similar behavior has been observed for the NPA
hydrolysis in the presence of diblock copolymers of
NIPA with NVI [8].
2
1.0
0.5
0
We compared the catalytic activity of CP50 with
the available results of studying the catalytic activity of
PPs composed of VCL and NVI [5]. In [5], it has been
shown that the NPP hydrolysis rate in the presence of
VCL–NVI PPs (the PP concentration was
3.0
3.5
4.0 log
M
Fig. 1. Molecular weight distribution curves of (
) CP30, and ( ) CP50.
1) PVCL,
(2
3
DOKLADY CHEMISTRY Vol. 465
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2015

CATALYTIC PROPERTIES OF DIBLOCK COPOLYMERS
kHz
255
ln
V
I
,
–6.4
–6.8
–7.2
–7.6
2
3
3000
2000
1000
0
1
3
2
1
30
40
50
°С
3.0
3.1
3.2
3.3
3.4
103, K–1
T–1
×
T
,
Fig. 3. Intensity of scattered light at an angle of 90° in soluꢀ
tions of ( ) PVCL, ( ) CP30, and ( ) CP50 as a function of
temperature.
Fig. 2. NPP hydrolysis rate (
vs. 1/ at pH 7.0 in a 0.05 M K H PO /Na HPO solution
in the presence of (
concentration of NVI units in PNVI and copolymers is
2.4 mol/mL.
V
2
) ([NPP] = 0.4
μ
mol/mL)
1
2
3
T
2
4
2
4
1
) PNVI, (2) CP30, and (3) CP50. The
μ
reflecting the formation of submicron particles
(460 nm) of a new phase. The observed phase transiꢀ
tion is completely reversible with decreasing temperaꢀ
ture. An analogous structural rearrangement is also
observed in the CP30 copolymer.
0.5 µmol/mL as calculated for the NVI content)
increases fivefold with increasing temperature from 25
to 50°C at pH 7.3. Roughly under the same conditions
(pH 7.0), the hydrolysis rate in the presence of CP50
increases 5.2 times with increasing temperature from
Studying CP50 and PVCL by highꢀsensitivity difꢀ
25 to 50°C. Thus, the diblock copolymers exhibit the ferential scanning calorimetry (HSꢀDSC) has shown
same catalytic activity as PPs.
that both polymers undergo a phase transition involvꢀ
ing separation of the solution into two phases. The
enthalpies of phase transitions calculated for the
weight of VCL units in these samples are the same
(table). It is known that the major contribution to the
enthalpy of the transition of PVCL is made by dehyꢀ
To find a correlation between the catalytic properꢀ
ties of CP30 and CP50 and their ability to change the
conformation with increasing temperature, dilute
aqueous solutions of copolymers were studied at difꢀ
ferent temperatures by static and dynamic light scatꢀ
tering methods (SLS–DLS).
The cloud point Tls (table) corresponding to the
phase separation of PVCL, CP30, and CP50 solutions
was determined by the light scattering method as the
temperature at which a sharp increase in the light scatꢀ
tering intensity is observed (Fig. 3).
I/Im
1.5
1.0
0.5
0
Figure 4 shows the distribution of the scattered
light intensity over the hydrodynamic radius of partiꢀ
cles (Rh) in dilute solutions of CP50 at different temꢀ
peratures. The diblock copolymer at 25°C (Fig. 4,
curve 1) is a molecular solution with Rh = 2.7 0.2 nm.
As the temperature approaches Tls in the range 40–
3
2
1
45°C, supermolecular micelles of 70–80 nm in size
are formed (Fig. 4, curve 2). Thus, the deterioration of
solvent quality for the PVCL block with an increase in
temperature leads to structural rearrangements in a
CP50 solution to form micelles consisting of the more
hydrophobic core and the hydrophilic shell of NVI
units of the diblock copolymer. At 47°C, a sharp
increase in light scattering intensity is observed,
10–1
100
101
102
103
h, nm
R
Fig. 4. Scattered light intensity distribution over the partiꢀ
cle size in aqueous solutions of CP50 at ( ) 25, ( ) 43, and
) 47°C at a concentration of 2 mg/mL.
1
2
(
3
DOKLADY CHEMISTRY Vol. 465
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256
BARABANOVA et al.
dration of its chains accompanying the formation of
the concentrated phase. The above results indicate
that the hydrate structure of PVCL persists in CP50;
i.e., the hydrophilic block of the copolymer behaves
autonomously and does not disturb the cooperativity
of dehydration of PVCL blocks of the copolymer on
heating. This result confirms the block structure of the
synthesized copolymer.
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ACKNOWLEDGMENTS
This work was supported by the Russian Science
Foundation (project no. 14ꢀ13ꢀ00544).
Translated by G. Kirakosyan
DOKLADY CHEMISTRY Vol. 465
Part 1
2015