Peroxy radical activated addition of tert-butylcatechol to 2,6-Di-tert-butyl-7-substituted quinone methide polymerization retarders

Article abstract of DOI:10.1021/op200217q

Peroxy radicals formed by the autoxidation of styrene react with 4-tert-butylcatechol (TBC) to give a tert-butyl-semiquinone radical. When the TBC radical is generated in the presence of a 2,6-di-tert-butyl-7-substituted quinone methide (QM), the result is o-TBC addition at the 7-position of the QM. The effects of TBC/QM addition are observed during styrene polymerization retarder testing under aerobic conditions for 2-(3,5-di-tert-butyl-4- oxocyclohexa-2,5-dien-1-ylidene)acetonitrile (QM-CN), 2,6-di-tert-butyl-4- (methoxymethylene)cyclohexa-2,5-dienone (QM-OMe), and 4-benzylidene-2,6-di-tert- butylcyclohexa-2,5-dienone (QM-Ph). Increasing the concentrations of QM-Ph and TBC during aerobic batch styrene polymerization allowed for silica gel chromatography isolation of 5-(tert-butyl)-3-((3,5-di-tert-butyl-4- hydroxyphenyl)(phenyl)methyl)benzene-1,2-diol, a novel compound. Radicals generated by the autoxidation of cumene and by homolysis of dicumylperoxide also activate TBC/QM addition. TBC/QM interaction causes a reduction in the performance of QMs as styrene polymerization retarders under aerobic conditions. Under anaerobic test conditions, a better simulation of industrial styrene purification, the TBC/QM interaction leads to only minimal reduction in retarder performance.

Full text of DOI:10.1021/op200217q

Article
pubs.acs.org/OPRD
Peroxy Radical Activated Addition of tert-Butylcatechol to
2,6-Di-tert-butyl-7-Substituted Quinone Methide Polymerization
Retarders
Andrew R. Neilson* and Christopher F. Morrison
Nalco Energy Services, Downstream Petrochemicals Research, 7705 Highway 90-A Sugar Land, Texas 77478, United States
S
* Supporting Information
ABSTRACT: Peroxy radicals formed by the autoxidation of styrene react with 4-tert-butylcatechol (TBC) to give a tert-butyl-
semiquinone radical. When the TBC radical is generated in the presence of a 2,6-di-tert-butyl-7-substituted quinone methide (QM), the
result is o-TBC addition at the 7-position of the QM. The effects of TBC/QM addition are observed during styrene polymerization
retarder testing under aerobic conditions for 2-(3,5-di-tert-butyl-4-oxocyclohexa-2,5-dien-1-ylidene)acetonitrile (QM-CN), 2,6-di-tert-
butyl-4-(methoxymethylene)cyclohexa-2,5-dienone (QM-OMe), and 4-benzylidene-2,6-di-tert-butylcyclohexa-2,5-dienone (QM-Ph).
Increasing the concentrations of QM-Ph and TBC during aerobic batch styrene polymerization allowed for silica gel chromatography
isolation of 5-(tert-butyl)-3-((3,5-di-tert-butyl-4-hydroxyphenyl)(phenyl)methyl)benzene-1,2-diol, a novel compound. Radicals generated
by the autoxidation of cumene and by homolysis of dicumylperoxide also activate TBC/QM addition. TBC/QM interaction causes a
reduction in the performance of QMs as styrene polymerization retarders under aerobic conditions. Under anaerobic test conditions, a
better simulation of industrial styrene purification, the TBC/QM interaction leads to only minimal reduction in retarder performance.
retarders in the presence of TBC, identify a mechanism of
QM/TBC addition, and describe how conditions required for
QM/TBC addition are not present in styrene production.
INTRODUCTION
■
Thermal self-initiation of styrene polymerization1−3 creates
physical and economic complications for styrene production,
because styrene, whether prepared from ethylbenzene dehydro-
genation or methylbenzylalcohol dehydration, is purified by
distillation at temperatures of 70−120 °C.4,5 In order to mitigate
in-process polymerization, producers add inhibitors and retarders
to the distillation train. Industrial inhibition practices have
evolved from the use of elemental sulfur4,5 to hydroxylamine6
or nitroxide7 inhibitors in combination with nitrophenol
retarders.8,9 While it is possible to operate using only inhibitors,
nearly all producers prefer to also add a retarder for protection
in areas of low flow and during unexpected plant shutdowns.
Toxic nitrophenol polymerization retarders, such as the current
commodity of choice, 2,6-dinitro-4-sec-butylphenol (DNBP), are
candidates for government restrictions on production, impor-
tation, and use.10,11 These restrictions have created a need for a
new environmentally compatible and economically viable poly-
merization retarder for the styrene industry.
RESULTS AND DISCUSSION
■
Merrill16 reported that 15 ppm of TBC renders 100 ppm of a
7-aryl quinone methide ineffective towards preventing styrene
polymerization during a 120 °C test, under conditions described
as “in the absence of oxygen.” In order to determine the validity
of these claims, ensure the safety and performance of QMs in
styrene production, and better understand potential QM/TBC
interaction mechanisms, similar testing was performed on a series
of QMs (Figure 1). QM-Ph, QM-OMe, and QM-CN were
chosen because of their reported effectiveness as styrene
polymerization retarders, and to determine if previously reported
2,6-Di-tert-butyl-7-substituted quinone methides (QMs) have
endured extensive research because of their antipolymerant and
antioxidant characteristics,12−15 and they are the leading candi-
dates to replace DNBP. While QM chemistry has been applied
successfully in field trials, there remain concerns over how the
chemistry interacts with other styrene additives.16 In addition to
the inhibition chemicals added to the bottoms of the distillation
towers, 4-tert-butylcatechol (TBC) is added into the overhead
of the styrene finishing tower, where it continues on with the
styrene product for polymerization protection during trans-
portation and storage.4 If the styrene tower overhead is designed
with a partial recycle or the need arises to rework off specifica-
tion styrene, some of this TBC may be recycled back into the
tower where the QM retarder is present. Here we report the
polymerization testing of three quinone methide polymerization
Figure 1. Retarder performance testing was carried out on the above
three 7-substituted quinone methides.
Received: August 9, 2011
Published: November 11, 2011
© 2011 American Chemical Society
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differences in reactivity caused by varying the 7-substituent change
with oxygen. When equimolar amounts of TBC and QM-Ph are
dissolved in a minimal amount of styrene, and the mixture is
heated to 80 °C while sparging with oxygen, 44% conversion to
an adduct is observed by GC. Silica gel chromatography allows
for the isolation of 5-(tert-butyl)-3-((3,5-di-tert-butyl-4-hydroxy-
phenyl)(phenyl)-methyl)benzene-1,2-diol (1), a compound that
has not previously been reported. TBC addition products for
QM-OMe and QM-CN were observed by GC−MS as the
elimination product: 2,6-di-tert-butyl-4-(5-(tert-butyl)-2,3-
dihydroxybenzylidene)cyclohexa-2,5-dienone. However, because
the yields of adduct were too low to allow for isolation and
further characterization, it was not possible to determine if the
elimination occurs also during retarder testing or if it is a function
of ionization in the mass spectrometer. Under the same
conditions described above, if styrene is replaced with ethyl-
benzene, no reaction is observed. When the solvent is changed to
cumene, which readily undergoes autoxidation,22 adduct yields
comparable to those in styrene are obtained. This is consistent
with proposed TBC antioxidant mechanisms where TBC reacts
with the autoxidation product of a hydrocarbon, not with
oxygen.23 Further evidence for peroxide-induced TBC activation
causing adduct formation is observed when dicumylperoxide is
added to equimolar TBC/QM solution in ethylbenzene at 80 °C
to give a 23% yield of compound 1. On the basis of these results
a proposed mechanism for TBC/QM addition is shown in Figure
3. The first step is the reaction of TBC with a peroxy radical to
generate a semiquinone radical intermediate. Quinone methides
preferentially react with carbon radicals, which accounts for the
addition to the unhindered carbon ortho to the semiquinone.
Hydrogen abstraction by the hindered phenoxy radical, followed
by rearomatization of the TBC ring, affords compound 1.
While the peroxide-induced TBC addition explains the
results obtained by Merril,16 it will have little effect on the per-
formance of QMs as retarders for styrene production. Styrene is
distilled at high vacuum, which greatly reduces the solubility of
oxygen in hydrocarbons. Published experimental data for
toluene indicates that at 50 °C and 0.33 atm the saturation
limit for oxygen is less than 0.11 ppm.24 The absence of soluble
oxygen in the styrene purification train is demonstrated by the
the interaction with TBC.13,17
While all three compounds were prepared according to
published synthetic routes,18−21 the hydrolytic instability of QM-
OMe required recrystallization prior to each test. We observed a
10% conversion to 3,5-di-tert-butyl-4-hydroxybenzaldehyde when
crystalline QM-OMe is stored for 24 h under nitrogen. It is
common industry practice to test antipolymerants on the basis
of equal weight fractions, because that is how they are dosed into
a production facility.17 However, the polymerization testing
described within contains equimolar retarder concentrations to
eliminate performance differences from variations in initial molar
concentration (Table 1). The concentration of TBC (15 ppm)
was chosen because it represents the highest levels of TBC
present in any portion of the styrene plant. The results of the
retarder testing are shown in Figure 2.
QM/TBC retarder testing was carried out under anaerobic
conditions by sparging the solutions with nitrogen prior to
heating (Figure 2d). While the presence of TBC caused a
minor decrease in retarder performance compared to the results
without TBC (Figure 2c), a complete loss of performance as
reported by Merrill16 was not observed. To probe the effect
that oxygen has on QM performance in the presence of TBC,
the polymerization experiments were repeated without sparging
the solutions with nitrogen (Figure 2a,b). Without TBC, the
QM retarders maintain their performance despite the increased
polymerization rate caused by the dissolved oxygen. However,
the loss of performance when TBC and dissolved oxygen are
present is consistent with the results reported by Merrill.16 This
indicates that in order for interaction to take place between
TBC and a QM, the system must contain dissolved oxygen.
The small loss of QM performance and the small performance
improvement of the blank observed under anaerobic conditions
(Figure 2d) are likely due to the presence of styrene autoxi-
dation products present in the system prior to sparging.
To better understand the nature of QM/TBC interaction, the
concentration of the inhibitors was increased from ppm levels to
percent levels, and instead of relying on dissolved oxygen already
present in the styrene, the reactions were sparged continuously
Table 1. Styrene polymerization retarder test results
c
d
polymer concentration (wt %) at time (min)
a
b
retarder
TBC (ppm)
O2
15
30
45
60
75
90
QM-Ph
QM-Ph
QM-Ph
QM-Ph
−
15
−
−
−
yes
yes
−
0.03
0.01
0.02
0.01
0.00
0.00
0.00
0.00
0.05
0.00
0.00
0.00
0.31
0.00
0.41
0.00
0.23
0.16
0.17
0.08
0.07
0.06
0.05
0.06
0.32
0.19
0.52
0.06
1.50
0.44
2.80
0.05
0.66
0.44
0.60
0.43
0.13
0.09
0.08
0.09
0.98
1.11
1.23
0.16
2.97
2.71
6.07
1.00
1.45
1.48
1.88
2.62
0.16
0.88
1.53
2.77
1.65
2.06
2.14
3.10
5.45
4.72
10.08
5.30
2.07
2.69
2.84
6.03
0.84
3.36
4.07
5.72
3.16
4.15
3.67
7.06
7.30
7.02
3.14
4.80
3.34
15
11.45
2.73
QM-CN
QM-CN
QM-CN
QM-CN
QM-OMe
QM-OMe
QM-OMe
QM-OMe
−
−
15
−
−
5.69
yes
yes
−
6.26
15
11.57
4.27
−
15
−
−
6.14
yes
yes
−
−
yes
5.02
15
10.45
12.80
10.59
17.59
14.81
−
15
−
−
−
−
14.16
9.66
15
yes
a
b
Retarder concentration was kept constant at 0.31 μM, a typical plant dosage. Dissolved oxygen: all reactions were carried out under nitrogen
atmosphere; soluble oxygen was removed by sparging with N2. Polymer concentrations were determined by methanol precipitation/absorbance
(ASTM D2121). Results are displayed as an average of four separate reactions.
c
d
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Figure 2. Equimolar QM retarder testing at 120 °C. (a) Nitrogen atmosphere only, dissolved oxygen levels not consistent with plant operation: QM
retarder performance is maintained. (b) Retarder performance is decreased substantially in the presence of dissolved oxygen and TBC. (c) Anaerobic
conditions, no TBC: dissolved oxygen levels consistent with plant operation. (d) Anaerobic conditions with 15 ppm of TBC: maximum potential
TBC concentration in styrene production.
sparging contains much higher soluble oxygen than is present in
the plant. Styrene saturated with air contains 50 ppm of oxygen,25
and heating the tubes increases the pressure inhibiting oxygen
diffusion into the gas phase. Styrene inhibitor testing must be
carried out in a rigorously maintained anaerobic environment if
the goal is to recreate plant conditions.
CONCLUSIONS
■
The styrene polymerization retarder performance of three 2,6-
di-tert-butyl-7-substituted quinone methides, leading candidates
to replace DNBP, was measured in the presence of TBC at
varying levels of dissolved oxygen. In the presence of dissolved
oxygen, TBC has a negative effect on QM styrene polymer-
ization retarder performance. Autoxidation products of styrene
react with TBC, forming a semiquinone radical that adds to
the 7-position of the QM to form an adduct that does not
inhibit styrene polymerization. Evidence for the mechanism of
addition was found by preparing 5-(tert-butyl)-3-((3,5-di-tert-
butyl-4-hydroxyphenyl)(phenyl)methyl)benzene-1,2-diol
with both styrene and cumene as solvents, and through
Figure 3. Proposed mechanism for TBC addition to QMs.
reaction with dicumylperoxide. TBC does not affect QM
retarder performance under anaerobic conditions found in
styrene production.
ineffectiveness of phenolic antioxidants like TBC as in-process
inhibitors without constant oxygen injection into the system.5
The pressure tube retarder testing performed without nitrogen
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reflux condenser, gas inlet, magnetic stirbar, and rubber septa,
was added QM-Ph (20 g, 0.068 mol), TBC (11.25 g, 0.068 mol),
and styrene (30 g, 0.288 mol). The mixture was heated to 80 °C
while sparging continuously with oxygen. The reaction was
monitored by GC, and after 2 h a new peak was observed at
15.8 min. After 24 h of continuous heating/sparging/stirring,
the yield of the new peak was 21%. The reaction stalled after
36 h at a final GC yield of 44%. Silica gel chromatography (1%
MeOH in CH2Cl2) followed by rotary evaporation on 10 g of
the reaction mixture gave 5-(tert-butyl)-3-((3,5-di-tert-butyl-4-
hydroxyphenyl)(phenyl)-methyl)benzene-1,2-diol (1) as a sticky
yellow solid (1.68 g, 3.6 mmol, 32% isolated yield). Mp 78−
80 °C. 1H NMR (300 MHz, CDCl3) δ: 7.29−7.21 (m, 3H), 7.19
(t, J = 7.3 Hz, 2H) 6.95 (s, 2H), 6.82 (s, 1H), 6.42 (s, 1H), 5.57
(s, 1H), 5.12, (b, 1H), 4.73 (b, 2H), 1.35 (s, 18H), 1.14 (s, 9H).
13C NMR (75 MHz, DMSO-d6) δ: 152.39, 143.89, 143.68,
143.09, 138.71, 135.83, 132.48, 131.03, 129.13, 128.44, 126.51,
125.91, 119.01, 110.78, 51.39, 34.33, 34.17, 31.34, 30.26. Anal.
Calcd for C31H40O3: C, 80.83; H, 8.75; O, 10.42. Found: C,
80.54; H, 8.65.
EXPERIMENTAL SECTION
■
Materials. Prior to use, styrene was passed over alumina to
remove the TBC that is packaged with the product. 2-(3,5-Di-
tert-butyl-4-oxocyclohexa-2,5-dien-1-ylidene)acetonitrile (QM-
CN),18 2,6-di-tert-butyl-4-(methoxymethylene)cyclohexa-2,5-
dienone (QM-OMe),19,20 and 4-benzylidene-2,6-di-tert-butyl-
cyclohexa-2,5-dienone (QM-Ph)21 were prepared according to
previous literature reports. Due to its hydrolytic instability,
QM-OMe had to be recrystallized from hexanes immediately
prior to each use to remove 3,5-di-tert-butyl-4-hydroxybenzal-
dehyde generated during storage.26
Instrumentation. Gas chromatography (GC) was carried
out on a Hewlett-Packard 5890 with a Hewlett-Packard 3396
series II. GC measurements were quantified using 2,6-di-tert-
butyl-4-methylphenol as an internal standard. Absorbance mea-
surements for ASTM D212127 were performed using a Hach
DR/2010 UV−vis spectrophotometer. 1H and 13C spectra were
recorded on a Varian Oxford 300 NMR in CDCl3 with a TMS
standard. GC−MS was completed using an Agilent 6890 GC
with a 5973 mass detector. Midwest Microlab, LLC performed
carbon and hydrogen combustion analysis with an Exeter
Analytical, Inc. CE440 elemental analyzer.
Anaerobic Retarder Test. QM-Ph (0.100 g, 0.34 mmol)
and styrene (1.000 g, 9.6 mmol) were weighed into a 20-mL
scintillation vial. TBC (0.250 g, 1.5 mmol) and styrene (10.0 g,
96 mmol) were weighed into a different 20-mL scintillation vial;
0.250 g of the QM-Ph solution, 0.375 g of the TBC solution,
and 250 g of styrene were added to a 400-mL glass jar to give a
0.31 μM (100 ppm) QM-Ph and a 0.08 μM (15 ppm) TBC
solution. Twenty-four Ace Glass #15 threaded pressure tubes
equipped with PTFE screw caps and fluoroelastomer (FETFE)
O-rings were filled with 8.0 g of 0.31 μM (100 ppm) QM-Ph
styrene solution. Each tube was sparged with nitrogen for 2 min
using a gas dispersion tube and then sealed under nitrogen. The
24 tubes were loaded into a test tube heating block preheated to
120 °C. At 15-min intervals, four tubes were removed from the
block, cooled in an ice bath, and placed into a freezer (−17 °C)
to quench the reaction. Methanol precipitation and absorbance
spectroscopy (ASTM D2121) were used to measure the polymer
concentrations in each of the tubes, and the four data points for
each time were averaged.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details for all polymerization retarder tests, di-
cumylperoxide activated synthesis of 5-(tert-butyl)-3-((3,5-di-
tert-butyl-4-hydroxyphenyl)(phenyl)-methyl)benzene-1,2-diol
(1), GC−MS of 2,6-di-tert-butyl-4-(5-(tert-butyl)-2,3-
dihydroxybenzylidene)cyclohexa-2,5-dienone, and NMR char-
acterization of compound 1. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
arneilson@nalco.com.
ACKNOWLEDGMENTS
■
We thank Nalco Research and Development for funding this
work. We also thank Nalco Diagnostic Solutions members
Terry Street and Mark Ward for their work on GC−MS and
NMR characterization.
Aerobic Retarder Test. QM-Ph (0.100 g, 0.34 mmol) and
styrene (1.000 g, 9.6 mmol) were weighed into a 20-mL
scintillation vial. TBC (0.250 g, 1.5 mmol) and styrene (10.0 g,
96 mmol) were weighed into to a different 20 mL scintillation vial.
0.250 g of the QM-Ph solution, 0.375 g of the TBC solution and
styrene (250 g) were added to a 400 mL glass jar to give a
0.31 μM (100 ppm) QM-Ph and a 0.08 μM (15 ppm) TBC
solution. Twenty-four Ace Glass #15 threaded pressure tubes
equipped with PTFE screw caps and fluoroelastomer (FETFE)
O-rings were filled with 8.0 g of 0.31 μM (100 ppm) QM-Ph
styrene solution. The vapor space of each tube was swept with
nitrogen for 2 min and then sealed under nitrogen. The 24 tubes
were loaded into a test tube heating block preheated to 120 °C. At
15-min intervals, four tubes were removed from the block, cooled
in an ice bath, and placed into a freezer (−17 °C). Methanol
precipitation and absorbance spectroscopy (ASTM D2121) were
used to measure the polymer concentrations in each of the tubes,
and the four data points for each time were averaged.
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