"dormant" inhibitors of urethane type as controllers of temperature modes of styrene polymerization

Article abstract of DOI:10.1134/S1070427208100224

Urethanes based on a series of monoisocyanates and phenolic inhibitors of styrene radical polymerization, which release the initial phenolic inhibitor at elevated temperatures, were prepared and characterized. Some of these compounds meet the requirements to "dormant" inhibitors: they sharply decelerate thermally initiated high-temperature polymerization of styrene and do not noticeably affect the rate of low-temperature polymerization in the presence of an inhibiting substance. The possibility of preventing uncontrollable emergency situations by adding these compounds to the initial monomer was confirmed experimentally.

Full text of DOI:10.1134/S1070427208100224

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2008, Vol. 81, No. 10, pp. 1821–1830. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © A.L. Aleksandrov, E.R. Badamshina, G.A. Gorbushina, A.A. Grishchuk, A.S. Dzhalmukhanova, V.V. Komratova, I.S. Kochneva,
A.I. Kuzaev, V.P. Lodygina, E.V. Stovbun, T.N. Fedotova, Ya.S. Estrin, 2008, published in Zhurnal Prikladnoi Khimii, 2008, Vol. 81, No. 10, pp. 1699–1709.
MACROMOLECULAR CHEMISTRY
AND POLYMERIC MATERIALS
“Dormant” Inhibitors of Urethane Type as Controllers
of Temperature Modes of Styrene Polymerization
A. L. Aleksandrov, E. R. Badamshina, G. A. Gorbushina, A. A. Grishchuk,
A. S. Dzhalmukhanova, V. V. Komratova, I. S. Kochneva, A. I. Kuzaev,
V. P. Lodygina, E. V. Stovbun, T. N. Fedotova, and Ya. S. Estrin
Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, Russia
Received February 28, 2008
Abstract—Urethanes based on a series of monoisocyanates and phenolic inhibitors of styrene radical polym-
erization, which release the initial phenolic inhibitor at elevated temperatures, were prepared and characterized.
Some of these compounds meet the requirements to “dormant” inhibitors: they sharply decelerate thermally
initiated high-temperature polymerization of styrene and do not noticeably affect the rate of low-temperature
polymerization in the presence of an inhibiting substance. The possibility of preventing uncontrollable emer-
gency situations by adding these compounds to the initial monomer was confirmed experimentally.
DOI: 10.1134/S1070427208100224
One of the most extensively used procedures for
preparing polystyrene is polymerization in the bulk in
large reactors, which is characterized by the complex
behavior owing to the nonlinear dependence of the reac-
tion rate on the temperature and reactant concentra-
tions. The exothermic heat effect of polymerization has
a positive feedback, i.e., both the reaction rate and heat
release increase with temperature, which, in turn,
causes further use in temperature. As a result, unstable
states of the process and emergency situations arise,
which are accompanied by an uncontrollable rise in
the temperature of the reaction mass by tens of degrees
and by a significant excess of the monomer conversion
over the level permissible for the reactor.
emergency situations is typical of the first reactor ow-
ing to the highest monomer concentration in the reac-
tion mixture. Three main procedures are used to pre-
vent the emergency with an uncontrollable temperature
increase in the reactor: cooling of the reaction mixture by
lowering the coolant temperature in the reactor jacket,
addition of inhibitors that sharply decrease the polym-
erization rate, and emergency discharge of the reaction
mixture into an external vessel with a solvent.
However, all the above procedures are noticeably
inertial, i.e., they are characterized by slow elimination
of the thermal and concentration gradients in the large
reactor. Owing to the high inertia, prevention of the
emergency situations in the case of local overheating
may become impossible. The use of so-called “dormant”
inhibitors (DIs) added to the reaction mixture before
polymerization can be probably the optimal solution to
this problem.
In the commercial production of polymers, vari-
ous mathematical models of the reactors are used,
which allows development of automatic control sys-
tems providing the maximal productivity of the proc-
ess. However, owing to the risk of uncontrollable
emergency situations, the process is frequently per-
formed under modes that are far from the optimum.
Continuous polymerization of styrene is carried out in
a cascade of reactors (with the volume of tens of cubic
meters) to provide a gradual increase in temperature as
the content of the monomer in the reaction mixture de-
creases [1]. The greatest danger of uncontrollable
The effect of these inhibitors is as follows. A DI
is inert at the polymerization temperatures. Upon an
uncontrollable rise in the temperature above the per-
missible level, a DI exhibits the inhibition activity via
its decomposition to components among which at least
one component acts as a polymerization inhibitor or
isomerization to give an active inhibitor. In this case,
it is not necessary to supply the inhibitor to the over-
1821

1822
ALEKSANDROV et al.
heated area because DI is uniformly distributed in the re-
action mixture from the very begining.
the kinetics of styrene polymerization under the iso-
thermal conditions and on the thermal modes under
the conditions of restricted heat exchange which
model parameters of the heat exchange in large-vol-
ume reactors.
DIs were first suggested for controlling the photo-
initiated polymerization of methyl methacrylate (MMA)
in the form of large blocks [2]. As DIs were studied
various compounds, such as nitroso compounds, hy-
drazine derivatives, saturated and unsaturated halogen
derivatives, nitrites, and ammonium salts, which are
activated and then inhibit polymerization even at 50–
60°C. However, these compounds cannot be used in
the case of styrene, because its polymerization is car-
ried out at temperatures no less than 50°C at which all
the above compounds act as polymerization inhibitors
lowering the process rate and molecular weight of
the polymer.
EXPERIMENTAL
Styrene containing 99.9% main compound and
stabilized with p-tert-butylpyrocatechol (0.001%) was
washed to remove the inhibitor with a 10% NaOH so-
lution (4 : 1 weight ratio). Then the monomer was
washed with distilled water to neutral reaction. Washed
styrene was dried over anhydrous CaCl2. The dried
monomer was distilled in a vacuum (44–45°C, 20 mm Hg)
and stored in an argon atmosphere at –18°C. The mono-
mer purity was determined by the kinetic method [8].
The reaction order of styrene polymerization with re-
spect to initiator was found to be 0.5, which corre-
sponds to the pure monomer.
As is known, phenols and polyphenols (in particu-
lar, di- and triphenols) are efficient inhibitors of radical
polymerization of styrene [3–5]. The inhibiting effect
of these compounds is due to the fact that propagating
polystyrene radicals detach a labile hydrogen atom
from the hydroxy group of the inhibitor molecule and
are thereby transformed into “dead” polymer mole-
cules. The resulting oxyphenyl radicals do not interact
with the monomer and thus cannot initiate the propaga-
tion of new polymer chains. If active hydrogen in the in-
hibitors can be blocked so that it will be activated only
at relatively high temperatures, then its reactivity with
respect to the polymer radicals at lower temperatures
will be suppressed.
Phenyl isocyanate (I-I), m-chlorophenyl isocy-
anate (I-II), o-chlorophenyl isocyanate (I-III), 2-eth-
ylphenyl isocyanate (I-IV), 2,6-dimethylphenyl isocy-
anate (I-V), o-tolyl isocyanate (I-VI), and 6-chloro-
hexyl isocyanate (I-VII), all purchased from Aldrich,
were distilled in a vacuum and stored in sealed glass
ampules.
In this study, we used the following phenols:
2,5,7,8-tetramethyl-2-(4',8',12'-trimethyltridecyl)-6-
chromanol (α-tocoferol) (P-I) purchased from Aldrich,
was used without additional purification; 2,2,5,7,8-
pentamethyl-6-chromanol, pure grade (P-II), mp 94–
96°C, was recrystallized from petroleum ether; 1,2-
dihydroxybenzene (pyrocatechol), pure grade (P-III),
and 1,3-dihydroxybenzene (resorcinol), pure grade (P-
IV), were sublimed in a vacuum at 100°C, mp 105–
106 and 109–110°C, respectively; 1,4-dihydroxyben-
zene (hydroquinone), pure grade (P-V), was succes-
sively recrystallized from xylene, water, and acetoni-
trile, mp. 173.5–175.0°C, 1,4-dihydroxy-2,5-di-tert-
butylbenzene (2,5-di-tert-butylhydroquinone), pure
grade (P-VI), was precipitated with water from metha-
nol and reprecipitated from ether with toluene, mp
217–219°C; and 4-tert-butyl-1,2-dihydroxybenzene
(p-tert-butylpyrocatechol), pure grade (P-VII), was
recrystallized from petroleum ether, mp 55–56°C.
The most appropriate blocking procedure is con-
version of phenols into urethanes in the reaction with
isocyanates (DI of urethane type). The reaction of ure-
thane formation can be represented as follows:
←
RNCO + HOR'
RNHC(O)OR'.
(1)
→
At elevated temperatures, the reaction equilibrium
is shifted to the left, and the temperature at which
the equilibrium concentrations of isocyanate and phe-
nol become noticeable depends on the structure of
radicals R and R' [6].
Hydrogen atoms are more strongly bound with
nitrogen atoms in the urethane group than with oxygen
atoms in phenols. Hence, urethanes should not exhibit
significant inhibiting activity at moderate tempera-
tures [7].
Urethanes were prepared by the reaction of mono-
isocyanates with hydroxy compounds by reaction (1),
using two standard procedures: (a) at [NCO]=[OH]
at room temperature to complete consumption of
Here, we describe the synthesis of some promis-
ing DIs of the urethane type and present characteristics
of the resulting compounds and data on their effect on
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“DORMANT” INHIBITORS OF URETHANE TYPE
1823
NCO groups and (b) at threefold excess of the isocy-
anate groups in the melt at 170°C, with subsequent
distillation of excess isocyanate in a vacuum. In both
procedures, the reaction was carried out in an inert gas
atmosphere in the presence of 10–5 M urethane-form-
ing catalyst (dibutyltin dilaurate, DBTDL). The result-
ing compounds were analyzed by IR spectroscopy for
the reaction completeness judged from the absence of
an intense absorption band at 2270 cm–1, characteristic
of the free isocyanate group. These compounds are
crystalline compounds with mp from 20 to 271°C, ex-
cept U-IIIb (Table 1). Urethanes were purified by re-
crystallization from toluene, the degree of purification
was analyzed by reversed-phase chromatography.
TP performance) with a water-methanol mixture as
eluent. The optimal composition of the eluent provid-
ing determination of the maximal number of impurities
at a sufficiently high resolution of the peaks was deter-
mined for each compound in the gradient mode, and
analysis was carried out in the isocratic mode. Depend-
ing on the polarity of the compounds analyzed, the
methanol : water ratio in the isocratic mode was varied
from 30 : 70 to 90 : 10 at a flow rate of the eluent of
100 μl min–1. The concentration of the compound ana-
lyzed in solution (in eluent or methanol) was up to
10 mg ml–1 at a sample volume of 8 μl. The detection
was performed in the single- or double-wave mode at
210 and 260 nm. For preliminarily identification of
the impurities (if necessary), the spectra of each chro-
matographic peak in the 190–360 nm range were re-
corded in the stopped-flow mode.
The IR spectra were recorded on a Specord M-82
spectrophotometer: the spectra of compounds U-Ia–U-
Id, U-IIa–U-IIf, and U-IIIa, in KBr pellets and those
of compounds U-IIIb and U-IIIc, in capillary films
prepared from solutions in chloroform.
The effect of potential DIs was preliminarily
evaluated from the data on the kinetics of styrene po-
lymerization at 50–70°C with azobis(isobutyronitrile)
(AIBN) initiator and at 125 and 150°C without initia-
tor (thermal initiation), in the presence of DI and with-
out it. The measurements were carried out in isother-
mal calorimeters of the Tian–Calvet type. The reaction
mixture in glass ampules was thoroughly degassed,
sealed, and placed in calorimeter cells [9]. The signals
of the calorimeters were recorded on a computer using
an E-24 analog-digital converter (ADC); the signal
from the E-24 ADC was recorded using a BEG pro-
gram developed by Yu.A. Sokolov.
The structures of the resulting urethanes are as
follows:
OCONHR
OCONHR
OCONHR
-Bu
-Bu
t
t
R1
U-(IIa–IIf ):
R = Ph, R1 = H (a);
R = Ph, R1 = t-Bu (b);
RNHCOO
U-(Ia–Id):
R = 3-ClPh (а),
2-ClPh (b),
2-EtPh (c),
R = 3-ClPh, R1 = t-Bu (c);
R = 2-ClPh, R1 = t-Bu (d);
R = 2-MePh, R1 = t-Bu (e);
R = 6-Cl(CH2)6, R1 = t-Bu (f)
Nonisothermal thermal modes of the styrene po-
lymerization under the conditions of restricted heat
exchange in the absence and in the presence of DI
were studied on a minireactor with restricted heat re-
moval. The monomer containing either an initiator
only or the initiator and DI was degassed and poured in
a vacuum into a glass ampule 30 mm in diameter,
equipped with a thermocouple well. The ampule was
filled with argon, sealed off, and placed in the installa-
tion preliminarily heated to the temperature of an ex-
periment. The thermal electromotive force of the cop-
per–constantan thermocouple, whose cold ends were
placed in a Dewar vessel containing an ice–water
mixture, was recorded with E-24 ADC.
2,6-(Me)2Ph (d)
RNHCOO
H3C
CH3
O
R
1
CH3
U-(IIIa–IIIc):
R = 3-ClPh, R1 = CH3 (a);
R = 3-ClPh, R1 = [CH2CH2CH2CH(CH3)]3CH3 (b);
R = 2,6-(Me)2Ph, R1 = [CH2CH2CH2CH(CH3)]3CH3 (c)
The characteristics of the urethanes prepared are
listed in Table 1.
Nonisothermal thermal modes of the styrene po-
lymerization were also simulated using 1- and 8-liter
metallic reactors equipped with a stirrer and modified
so as to provide the conditions of restricted heat ex-
change; the ratio of the heat-exchange area to the vol-
ume of the reaction mixture was about 1.5 m–1, whereas
The chromatographic analysis of the initial com-
pounds, urethanes, and diurethanes was carried out on
a Milikhrom chromatograph equipped with a UV de-
tector, using the reversed-phase procedure (2 × 80-mm
column packed with Separon SGX C18, 6 μm, 5500
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ALEKSANDROV et al.
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“DORMANT” INHIBITORS OF URETHANE TYPE
1825
for 15 m3 industrial reactors this ratio is close to 2 m–1.
Experimentally, we selected the conditions at which
the system passes into the high-temperature mode in
the absence of DI. In the nonisothermal tests we also
used AIBN initiator at a common concentration of
0.02 M.
Table 2. Inhibition parameters φ0 and φm of phenols in
styrene polymerization
φ0
φm
Potential
inhibitor
at indicated temperature, °С
60
125
2.5
1.5
1.40
0.9
1.2
2.0
1.5
60
а*
125
20.6
15.2
3.0
In all the cases, unless otherwise indicated, the con-
centrations of the inhibitors and potential DIs were
also 0.02 M.
P-I
1.05
2.03
2.08
1.35
1.68
b**
P-II
61.2
26.32
0.4
4.3
b**
P-III
P-IV
P-V
The molecular-weight distribution (MWD) of poly-
styrenes was determined at 25°C on a Waters chro-
matograph with a refractometric detector [10] and a tetra-
hydrofuran eluent at 1.2 ml min–1 flow rate. The vol-
ume of the sample was 2 ml, and the concentration of
the compound being analyzed in solution was 0.3–
0.4 wt %. We used three Styrogel columns (10 ×1000 mm)
calibrated using polystyrene references with a porosity
of 3 × 103, 3 × 104, and 3 × 105 Å.
1.1
1.3
P-VI
P-VII
10.0
7.1
b**
b**
* Reaction is terminated at the conversion α = 0.3.
** Strong inhibition, almost no polymerization is observed.
A distinctive feature of styrene is that it is capable of
thermally initiated polymerization at a rather high tem-
perature (above 90–100°C) [11, 12]. This means that,
if the control over the process (performed at moderate
temperatures in the presence of initiating substance) is
lost, further polymerization and, thus, uncontrollable
temperature increase can occur even after complete
consumption of the initiator. Just in these cases the use
of DIs may become the only way to avoid uncontrolla-
ble emergency situations.
of the inhibitor, respectively. The parameters of the in-
hibiting power of phenols in styrene polymerization are
listed in Table 2.
As can be seen, phenols P-IV and P-V do not no-
ticeably inhibit polymerization of styrene at tempera-
tures of 60°C and higher. Compounds P-III, P-VI, and
P-VII more strongly inhibit the process in the case of
initiated polymerization at 60°C, whereas at thermally
initiated polymerization (125°C and higher) chro-
manols P-I and P-II are the most effective.
Apparently, DIs of the urethane type can be syn-
thesized only from such proton-donor compounds that
rather effectively decelerate radical polymerization.
Therefor, we estimated the inhibiting power of a series
of phenols.
Based on the data on the inhibiting power of the
above compounds (Table 2), we chose phenols P-I–
P-III, P-VI, and P-VII for the synthesis of urethane-
type DIs.
It should be noted that polymerization of styrene
is, as a rule, accompanied by the gel effect, which is
manifested in a manyfold increase in the reaction rate
in the course of the process passing from the bimolecu-
lar chain termination to the diffusion-controlled stage.
Transition of the polymerization to the high-tempera-
ture uncontrollable mode is the most probable just in
the step of the gel effect, when the polymerization
rate is the highest, whereas the intensity of the heat
exchange decreases owing to the rapid increase in
the viscosity of the reaction mixture [13]. Therefore,
the inhibiting power of the compounds in question
was evaluated using two parameters: φ0 = W0/W0x and
φm = Wm/Wmx, where W0 and W0x are the reduced initial
rates of polymerization and Wm and Wmx are the maxi-
mal reduced rates of polymerization (at similar conver-
As shown above, the decomposition of urethane-
type DIs with liberation of the active inhibitor should
occur in a certain temperature range. We have shown
previously [14] that neat urethanes based of phenols
start to decompose to the initial components at a no-
ticeable rate [reaction (1)] at temperatures higher than
200°C. Apparently, such compounds cannot be used as
DIs. At the same time, as shown in [14], decomposi-
tion of these compounds with water [reactions (2)–(4)]
or aliphatic alcohols [reaction (5)], especially in the pres-
ence of tin-containing organic catalysts (e.g., DBTDL),
can occur at significantly lower temperatures with the
release of the initial phenol. In these reactions, the iso-
cyanates are converted into urea derivatives (reaction
with water) or alkyl urethanes (reaction with alcohols),
and reactions (2)–(5) are almost irreversible.
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1826
ALEKSANDROV et al.
Table 3. Parameters of inhibition of styrene polymerization
with urethanes at 60°C
of a compound in question. The ideal DI should not
affect the process at moderate temperatures, but should
decelerate polymerization at elevated temperatures.
Com-
pound
Com-
pound
ϕ0
ϕm
ϕ0
ϕm
4.35
The second procedure requires that similar com-
parative experiments should be performed in reactors
with restricted heat exchange, in which nonisothermal
modes of polymerization are provided and uncontrolla-
ble emergency situations are simulated. In this case,
polymerization in the presence and absence of DIs should
be performed in similar thermal modes of polymerization
in a comparatively low-temperature range. In the high-
temperature range, a certain decrease in temperature
should be observed in styrene polymerization in the pres-
ence of DI as compared to the control experiment.
U-Ia
U-Ib
U-Ic
1.11
1.08
1.25
1.39
1.25
1.11
1.07
1.43
1.23
2.67
2.80
1.85
2.56
1.33
U-IId
U-IIe
U-IIf
1.78
1.04
1.33
1.0
1.98
1.15
1.54
U-Id
U-IIa
U-IIb
U-IIc
U-IIIa
U-IIIb
U-IIIc
1.1
>100*
>100*
1.0
*_Polymerization is virtually terminated at 30–35% conversion.
The inhibition parameters of styrene polymeriza-
tion at 60°C for the urethanes prepared are listed in
Table 3. As can be seen, at 60°C almost all the ure-
thanes only slightly affect the initial and maximal rate
of polymerization, except urethanes U-IId, U-IIIb,
and U-IIIc. For example, urethane U-IId suppresses
the gel effect and does not noticeably affect the initial
polymerization rate, whereas urethanes U-IIIb and U-
IIIc do not affect the initial rate and strongly inhibit
polymerization at a conversion of about 30%. Neverthe-
less, the thermal polymerization of styrene in the pres-
ence of urethanes U-IIIb and U-IIIc was also studied.
(2)
(3)
(4)
(5)
RNHCOOAr + H2O → RNHCOOH + ArOH,
RNHCOOH → CO2 + RNH2,
RNHCOOAr + RNH2 → RNHCONHR + ArOH,
RNHCOOAr + HOR' → RNHCOOR' + ArOH.
Taking into account that the monomers for radical
polymerization are not thoroughly dried to remove
moisture, we assumed that the activation of DIs can
occur, depending on their structure and operation con-
ditions, in the acceptable temperature range.
The data on the effect of urethanes U-IIIb and
U-IIIc on thermal polymerization, presented in Fig. 1,
indicate that compound U-IIIb more effectively inhib-
its styrene polymerization at 125°C than does com-
pound U-IIIc; it permanently decreases the polymeriza-
tion rate and completely terminates the process at a 70%
conversion. The inhibiting power ofprocess at urne
U-IIIb is greater than that of U-IIIa at 150°C.
The inhibiting activity of DIs was evaluated by
two procedures. In the first procedure, we compared
the isothermal kinetics of polymerization of the poly-
mer (in particular, styrene) in the presence and absence
Our experimental data show that, at 125°C, ure-
thanes U-IIc and U-IId do not noticeably affect the
initial rate and noticeably decelerate the polymeriza-
tion in the course of the process, whereas compound
U-IIa only slightly affects the reaction rate throughout
the process. Urethane U-IIb at elevated temperatures
first significantly raises the initial rate, but after a 30%
conversion is attained, it does not noticeably affect the
polymerization rate. Hence, urethanes U-IIa and U-IIb
are unsuitable as DIs. Compound U-IIf does not affect
the initial rate of styrene polymerization, but it sup-
presses the gel effect, and the highest conversion at-
tained in this case is 85%.
Fig. 1. Reduced rate of thermal polymerization of styrene
W/[M ] vs. conversion X (1, 1') in the absence of additives
and in the presence of urethanes (2) U-IIIc, (3) U-IIIb,
(2') U-IIIa, and (3') U-IIIb; concentration of urethanes
2×10–2 M. Temperature, °C: (1–3) 125 and (1'–3') 150.
As noted above, the decomposition of urethanes
with the release of the free inhibitor at temperatures
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“DORMANT” INHIBITORS OF URETHANE TYPE
1827
Fig. 2. Reduced rate of thermal polymerization of styrene
Fig. 3. Reduced rate of thermal polymerization of styrene
W/[M ] vs. of conversion X (1) in the absence of additives
and in the presence of (2) urethane U-Id, (3) inhibit-
ing formulation based on urethane U-Id, (4) urethane
U-IIc, and (5) inhibiting formulation based on urethane
U-IIc. T = 125°C, [U] = [BuOH] = 2 ×10–2 M, [DBTDL] =
10–5 M.
W/[M ] vs. conversion X (1) in the absence of additives and
in the presence of (2) urethane U-IIIa, (3) inhibiting
formulation based on urethane U-IIIa, (4) urethane U-IIe,
and (5) inhibiting formulation based on urethane U-IIe.
T = 150°C, [U] = [BuOH] = 2 ×10-2 M, [DBTDL] = 10–5 M.
lower than 200°C proceeds at a significant rate only in
the exchange reactions with proton-donor compounds
and, in particular, with water and alcohols. This fact sug-
gests that the inhibiting effect of urethanes on the styrene
polymerization at 125–150°C is probably due to the ex-
change reactions with water present in the monomer.
tions in the absence of urethanes, neither n-butanol nor
DBTDL affect the rate of styrene polymerization in the
entire temperature range studied (60–150°C).
Since the MWD parameters are the decisive factors
affecting the polymer properties, we studied the effect
of some inhibitors and urethane-type DIs on the molecu-
lar weight (MW) of the resulting polystyrene (Table 4). As
can be seen, the additives inhibiting the styrene poly-
The temperature at which inhibitors of the pheno-
lic type are liberated from urethanes can be addition-
ally lowered (i.e., the DI performance can be im-
proved) by using the “inhibiting formulation” con-
taining a urethane-type DI, a proton-donor compound,
and a catalyst.
Table 4. MWD parameters of polystyrenes prepared in
the presence of some inhibitors and DIs
Mn×10–3 Mw×10–3
60°C (initiated polymerization)
Mz×10–3
Additive
Mw/Mn
Mz/Mw
To confirm the above assumption, we studied
the isothermal polymerization of styrene as influenced
by the inhibiting compositions based on urethanes
U-Id, U-IIc, U-IIe, and U-IIIa, containing n-butan-
ol (BuOH) taken in equimolar amount to urethane
(~0.02 M) and DBTDL (~10–5 M). At 60°C, these in-
hibiting formulations do not noticeably affect the ini-
tial rate of polymerization, but slightly diminish its
maximal rate (i.e., decrease the gel effect).
–
P-V
128
110
145
102
195
177
177
528
290
313
490
422
550
560
4.13
2.64
2.15
4.80
2.15
3.11
3.16
1027
540
530
1093
713
1050
1060
1.95
1.84
1.70
2.23
1.69
1.91
1.89
P-III
P-VII
U-IIa
U-IIc
U-IId
125°C (thermal polymerization)
The data on the inhibiting effect of some of the above
formulations at 125 and 150°C (Fig. 2, curves 2 and 3,
4 and 5; Fig. 3, curves 2 and 3) show that, compared to
urethanes, they are actually more effective DIs of the
isothermal polymerization of styrene. At the same
time, under the given experimental conditions, ure-
thane U-IIe exhibits the highest activity in the absence
of proton donor, which does not noticeably affect the
performance of this urethane (Fig. 3, curves 4, 5). Our
special tests showed that, under the isothermal condi-
–
P-I
145
64
390
161
133
225
177
361
313
295
253
2.69
2.52
3.97
2.50
2.68
2.40
2.49
2.63
2.67
640
–
1.64
–
P-VI
P-III
P-VII
U-IIa
U-IIc
U-IId
U-IIIb
34
102
66
150
133
112
95
276
453
332
608
594
546
–
2.07
1.77
1.88
1.68
1.79
1.85
–
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1828
ALEKSANDROV et al.
Fig. 5. Temperature curves of styrene polymerization in
glass ampules in the mode of restricted heat exchange
(1, 1') without additives and in the presence of (2) urethane
U-IIIb, (3) inhibiting composition based on urethane U-IIIb,
(2') urethane U-IIc, (3') inhibiting composition based on
urethane U-IIc. T0 = 75°C, [AIBN] = 2.2×10–2 M, [U] =
[BuOH] = 2×10–2 M, [DBTDL] = 10–5 M. (1'–3') Reac-
tion mixture before experiment was rapidly heated to 75°C;
(1–3) initial temperature of the reaction mixture was 20°C.
Fig. 4. Temperature curves of styrene polymerization in
glass ampules in the mode of restricted heat exchange.
(T ) Temperature and (τ) time; the same for Figs. 5 and 6.
T0, °C: (1) 59, (2) 60.5, (3) 62, (4) 65, (5) 70, and (6) 75. In
the insert: dependence of the maximal temperature Tm un-
der the conditions of restricted heat exchange, [AIBN] =
1.1×10–2 M.
merization reduce the polymer MW and this effect is
the most pronounced for polymerization at elevated
temperatures. At 125°C, the effect of the dormant in-
hibitors on Mn of the resulting polystyrene is signifi-
cantly weaker as compared to the initial inhibitors, and
at 60°C they cause even a slight increase in the poly-
mer molecular weight.
tiation. This is the mode of high-temperature initiated
polymerization. However, if the temperature in the
reactor reaches the values providing relatively rapid
thermal initiation of polymerization, the system can
pass into the high-temperature mode of thermal initia-
tion.
The thermal curves of styrene polymerization, re-
corded using sealed glass ampules at various T0 on the in-
stallation with restricted heat exchange, are shown in
Fig. 4. All the three modes are observed in this figure:
low-temperature mode (T0 59.0 and 60.5°C), high-
temperature mode of initiated polymerization (T0 62,
65, and 70°C), and high-temperature mode of thermal
polymerization (T0 75°C). The dependence of the max-
imal temperature in the ampule on T0, which is shown
in the inset, indicates that transition from one mode
to another occurs in a narrow T0 range, especially in
the region of thermally initiated polymerization.
Thus, our experimental data on styrene polymeri-
zation under isothermal conditions suggest that ure-
thanes U-Id, U-IIc, U-IIe, U-IIf, and U-IIIa or their
mixtures with alcohol and a catalyst of exchange reac-
tions can be promising DIs. However, the effect of DI
can be demonstrated only in the experiments simulat-
ing uncontrollable transition of the polymerization
process to the high-temperature mode.
Under the conditions of restricted heat exchange,
initiated polymerization of styrene can proceed in three
different modes, depending on the initial conditions.
At a coolant temperature T0 lower than a certain criti-
cal value, the process occurs in the low-temperature
mode at which the difference between the temperatures
of the reaction mixture and thermostat, T – T0, is small
and the temperature curve exhibits a flat maximum. At
T0 higher than the critical temperature, T – T0 rapidly
increases to tens degrees and the temperature curve is
characterized by a sharp maximum. Rapid decrease
in the temperature after passing the maximum is due to
the consumption of the initiator and termination of ini-
The effect of DI on the styrene polymerization was
studied under the conditions nearly critical (T0 75°C).
As can be seen from Fig. 5 (curves 1' and 2', 3'), which
illustrates the typical results of the experiments on the ef-
fect of DI on polymerization of styrene under condi-
tions of an uncontrollable reaction, addition of the in-
hibiting formulation based on urethane U-IIc apprecia-
bly depresses the temperature of maximal overheating
of the reaction mixture in the step of thermal polymeri-
zation and does not noticeably affect the process at
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“DORMANT” INHIBITORS OF URETHANE TYPE
1829
lower temperatures. In this case, the degree of po-
lymerization is significantly smaller in the presence
of DI than that in its absence and the reaction mixture
remains fairly fluid.
A similar behavior was observed for the inhibiting
formulation based on urethane U-IIe. As can be seen,
the kinetics of polymerization in the presence of this
formulation is close to the kinetics of uninhibited po-
lymerization at temperatures of up to 145°C. At higher
temperature, polymerization sharply decelerates and
the maximal temperature in the reactor decreases by
35° as compared to the uninhibited process.
Under the nonisothermal conditions, the highest
performance is exhibited by urethane U-IIIb, which
lowers the maximal temperature in the reactor by 50°
at a concentration of 0.02 M as compared to the con-
trol experiment. At the same time, up to transition to
the high-temperature mode, this urethane does not no-
ticeably affect the reaction rate. Additions of BuOH
and DBTDL do not appreciably affect the inhibiting
activity of this DI (Fig. 5, curves 2 and 3).
Fig. 6. Temperature curves of styrene polymerization in
a 1-l metallic reactor (1) without additives and (2) in
the presence of urethane U-IIIb, and in an 8-l metal-
lic reactor (3) without additives and (4) in the presence
of an inhibiting formulation based on urethane U-IIIa.
(1, 2) [AIBN] = 2.2 ×10–2 M, (2) [U] = 1.4 ×10–2 M;
(3, 4) [DIA] = 2 ×10–2 M, (4) [U] = [BuOH] = 1.5 ×10–2 M,
[DBTDL] = 10–5 M. Amount of the monomer loaded, ml:
(1, 2) 500 and (3, 4) 5000. Moments of the addition of
the cold solvent in the control experiments are shown by
the arrows.
The data on the effect of the urethane-type DI on
styrene polymerization, determined under the condi-
tions of restricted heat exchange using glass ampules,
were qualitatively confirmed by the experiments car-
ried out in 1- and 8-l metallic reactors equipped with
stirrers. The reactors were modified so as to provide
the minimal ratio of the heat exchange surface to the
volume of the reaction mixture, which allowed us to
find the conditions at which the initiated polymeriza-
tion passes to the high-temperature mode.
Under the same conditions, we performed experi-
ments with DI additives that were tested previously
using glass ampules (Fig. 6, curve 2). As can be seen,
urethane U-IIIb is less efficient under the given condi-
tions than in the case of the ampule experiments. In its
presence, the rate of thermal polymerization even some-
what increases as compared to the control experiment.
However, even in this case, the temperature maximum
is lower by 5° than that in the control experiment, and,
after attainment of the maximum, the temperature of
the reaction mixture sharply decreases and the polym-
erization virtually terminates. The final product is
fluid, and it can be discharged from the reactor without
dilution with a solvent, as in the control experiment.
Initially we found that, at a constant coolant tem-
perature of 90°C, the system passes to the high-
temperature mode as the temperature of the reaction
mixture increases to 140–150°C. However, further
passing to the high-temperature mode of thermal initia-
tion (similar to that in glass ampules) was not ob-
served. This is probably due to a sharp increase in
the heat loss through the nonthermostated inlet for
the stirrer in the reaction lid. Hence, it is impossible to
evaluate the DI performance under these conditions.
As a result, the thermal mode of the process was
changed so that after attainment of 90°C in the reactor,
the temperature of the thermostat was switched to
120°C. In this case, the system passed to the high-
temperature mode of thermal polymerization of styrene
as the temperature of the reaction mixture increased to
more than 200°C (Fig. 6, curve 1). As a result, the de-
gree of the monomer conversion was close to unity and
the resulting mass could be unloaded from the reactor.
The results obtained for a 1-l reactor were taken
into account when choosing the process modes of sty-
rene polymerization in an 8-l reactor. This reactor was
also modified to diminish the heat exchange in order to
provide the possibility of attainment of the high-
temperature mode of thermal polymerization. In these
experiments, the temperature control unit of the ther-
mostat was switched from 90 to 120°C when the tem-
perature of the reaction mixture reached 150°C. The ther-
mal modes in the course of styrene polymerization in
an 8-l reactor are shown in Fig. 6 (curves 3, 4).
In the control experiment, when the temperature
in the reactor exceeded 200°C, cold xylene was poured
to lower the temperature of the reaction mixture and to
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1830
ALEKSANDROV et al.
prevent uncontrollable emergency situation. Other-
wise, the stirrer could be blocked and it could become
impossible to unload the reaction mixture from the
reactor because of the high viscosity at a conversion
close to 100%. In the experiments with addition of the
inhibiting formulation based on U-IIIa, the maximal
temperature decreased by no less than 30° as compared
to the control experiment, especially if we take into
account that the control experiment was terminated at
the temperature of the reaction mixture of 202°C, be-
fore the maximal temperature was attained.
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ACKNOWLEDGMENTS
The study was financially supported by the In-
ternational Science and Technology Center (project
no. 1529).
14. Dzhalmukhanova, A.S., Badamshina, E.R., Komrato-
va, V.V., et al., Kinet. Katal., 2008, vol. 49, no. 1,
pp. 57–63.
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