2,6-Bis(3,5-di-tert-butyl-4-hydroxybenzyl)cycloalkanone: Synthesis and prospects for use as stabilizer for ethylene-propylene rubber

Article abstract of DOI:10.1134/S1070427214010066

A phenolic antioxidant was synthesized by the reaction of N,N-dimethyl(3,5-di-tert-butyl-4-hydroxybenzyl) amine with cyclohexanone. The infl uence of temperature, reactant ratio, pressure, and time on the yield of the product, 2,6-bis(3,5-di-tert-butyl-4-hydroxybenzyl)cyclohexanone, was examined. The antioxidant obtained, taken both separately and in synergistic mixtures with alkylphenol and phosphites, showed high performance in inhibition of the thermal aging of ethylene-propylene rubber. Pleiades Publishing, Ltd., 2014.

Full text of DOI:10.1134/S1070427214010066

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2014, Vol. 87, No. 1, pp. 42−47.  Pleiades Publishing, Ltd., 2014.
Original Russian Text  E.N. Cherezova, F.B. Balabanova, D.P. Shalyminova, S.Sh. Saigitbatalova, A.G. Liakumovich, 2014, published in Zhurnal Prikladnoi
Khimii, 2014, Vol. 87, No. 1, pp. 46−51.
ORGANIC SYNTHESIS
AND INDUSTRIAL ORGANIC CHEMISTRY
2,6-Bis(3,5-di-tert-butyl-4-hydroxybenzyl)cycloalkanone:
Synthesis and Prospects for Use as Stabilizer
for Ethylene–Propylene Rubber
E. N. Cherezova, F. B. Balabanova, D. P. Shalyminova,
S. Sh. Saigitbatalova, and A. G. Liakumovich
Kazan National Research Technological University, ul. Karla Marksa 68, Kazan, Tatarstan, 420015 Russia
e-mail: saygitbatalova@mail.ru
Received August 20, 2014
Abstract—A phenolic antioxidant was synthesized by the reaction of N,N-dimethyl(3,5-di-tert-butyl-4-hydroxy-
benzyl)amine with cyclohexanone. The influence of temperature, reactant ratio, pressure, and time on the yield of
the product, 2,6-bis(3,5-di-tert-butyl-4-hydroxybenzyl)cyclohexanone, was examined. The antioxidant obtained,
taken both separately and in synergistic mixtures with alkylphenol and phosphites, showed high performance in
inhibition of the thermal aging of ethylene–propylene rubber.
DOI: 10.1134/S1070427214010066
Phenolic antioxidants (PAOs) are widely used as
nonstaining stabilizers for polymers [1–6]. These agents
terminate oxidation chains by the reaction of phenolic
hydroxyl with peroxy radicals (InH + ROO· → In· +
ROOH). Despite the structural diversity of the pheno-
lic compounds described, only a very limited number
of them are used as PAOs because of the difficult avail-
ability of the raw materials and lack of commercially
acceptable production technologies. Therefore, it is nec-
essary to search for effective PAOs that can be prepared
by easy-to-implement processes from readily available
raw materials.
benzyl)amine (Mannich Base, MB). Our approach was
based on the concept of priority formation of bisphe-
nolic PAOs, which are considerably less volatile than
monophenolic AOs and exhibit high performance.
As a rule, dialkyl-substituted phenolic fragments are
linked in bisphenolic AOs by bridges of various struc-
tures. As starting reagent for the formation of the bridge,
we used commercially available cyclohexanone (CH). Its
incorporation into the stabilizer structure can be expected
to enhance the stabilizer performance owing to the addi-
tionally arising inhibition route, scavenging of alkyl radi-
cals initially generated in the course of the polymer aging,
because it is known that lactones and cyclic ketones react
with alkyl radicals at high rates [9, 10].
The majority of PAOs contain a 3,5-di-tert-butyl-
4-hydroxyphenyl fragment in their structure [7]. Such
fragment can be introduced, in particular, using N,N-
dimethyl(3,5-di-tert-butyl-4-hydroxybenzyl)amine as
reactive agent [8]. A series of effective PAOs have been
developed on its basis. Syntheses involving this com-
pound occur under relatively mild conditions with good
yield.
EXPERIMENTAL
The reaction of MB with CH was performed in a
round-bottomed flask equipped with a stirrer, a bubbler,
a reflux condenser, and a thermometer. The flask was
charged with the calculated amount of the reactants,
and the temperature was elevated to 135–145°С. The
This study is aimed to expand the range of PAOs pre-
pared using N,N-dimethyl(3,5-di-tert-butyl-4-hydroxy-
42

2,6-BIS(3,5-DI-tert-BUTYL-4-HYDROXYBENZYL)CYCLOALKANONE
43
reaction mixture was stirred at this temperature for
2–3 h in a nitrogen flow at atmospheric or reduced
pressure. The reaction completion was judged from
the disappearance of the spot of the starting MB on
the thin-layer chromatogram (Silufol UV-254 plates;
eluent: hexane–acetone, 5 : 1 by volume). After the
reaction completion, an alkanol (С1–С4) was added,
and the bottoms were cooled to room temperature. The
precipitate that formed was filtered off, washed with
cooled alkanol, and dried to constant weight. A white
crystalline powder with mp 220–221°С was isolated.
The elemental analysis of the isolated product was
performed with a CHNS-O Elementar Vario El cube
universal elemental analyzer.
The induction period of oxidation of ethylene–
propylene rubber (EPDM) was determined on
a manometric oxidation setup in an oxygen atmosphere
[р(О2) = 250 mmHg] at 170–175°С [1] as the time of
the onset of attaining the stationary rate of the polymer
oxidation.
RESULTS AND DISCUSSION
The IR spectra were recorded with a Specord-61
NIR spectrophotometer using attenuated total internal
reflection (ATIR) technique, and the H NMR spectra
were taken with an AvanceIII-400 device (Bruker,
400 MHz) at 25°С from solutions in acetone-d6. The
chemical shift was determined relative to signals of the
residual protons of the deuterated solvent.
The reaction of MB with CH was performed
at varied values of the MB : CH ratio (from 2 : 1 to
2 : 1.2), reaction time, and temperature. The reaction of
N,N-dimethyl(3,5-di-tert-butyl-4-hydroxybenzyl)amine
with cyclohexanone can be described by the following
scheme:
1
–NH(CH3)2
I
The IR spectrum of the isolated product contains
stretching vibration bands of the HO group at 3665 cm–1,
bending vibration bands of the aromatic ring in the re-
gion of 735 cm–1, and stretching vibration bands of the
С=O group in the region of 1705 cm–1.
Because the reaction involves the release of
dimethylamine, it seemed appropriate to remove it from
the reaction mixture to accelerate the process. To this
end, 1 h after the start of the reaction, the pressure in
the system was reduced. However, this did not affect
1Н NMR spectrum (acetone-d6, 400 MHz, ppm,
J, Hz): 1.41 s (36H, СН3), 2.26 dd (2H, СH, 3J = 8.7, 2J =
Y, %
2
14), 2.78 d (4Н, СН2, J = 6.6), 2.64 m (2H, CHC=O,
3
2
3J = 8.7, CH, J = 8.7, JAM = 14), 3.09 dd (2H, CH,
3J = 8.7, 2J = 14), 5.81 s (2Н, OН), 6.98 s (4Н, Нarom).
Found, %: С 80.39, Н 10.07. С36Н54О3. Calculated, %:
С 80.90, Н 10.11.
T, °C
The data obtained are consistent with the structure of
2,6-bis(3,5-di-tert-butyl-4-hydroxybenzyl)cyclohexa-
none (I).
x, mol per 2 mol of MB
The target product yield increases with an increase in
the reactant ratio, MB : CH, from 2 : 1 to 2 : 1.1 (Fig. 1)
and in the reaction temperature from 135 to 145°С.
Fig. 1. Influence of temperature Т and reactant ratio on the
yield Y of target product I. Reaction time 3 h, Р = 760 mmHg.
(х) CH amount.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 87 No. 1 2014

44
CHEREZOVA et al.
Table 1. Influence of the conditions on the target product yield in the reaction of 3,5-di-tert-butyl-4-hydroxy-N,N-
dimethylbenzylamine with cyclohexanone. Reaction time 3 h
Solvent for product
MB : CH molar ratio
Т, °С
P, mmHg
Yield of I, %
precipitation
n-C4H9OH
n-С4Н9ОН
2 : 1.1
2 : 1.1
145
135
135
760
760
660
86
76
75
2 : 1.1
2 : 1.1
2 : 1.2
n-С4Н9ОН
135
135
140
140
140
140
140
660
700
660
660
660
660
660
n-С4Н9ОН
n-С4Н9ОН
СН3ОН
63a
85
84
80
76
80
63
2 : 1.1
2 : 1.1
С2Н5ОН
2 : 1.1
i-С3Н7ОН
n-С4Н9ОН
n-С4Н9ОН
2 : 1.1
2 : 1.0
a Reaction time 2 h.
significantly the reaction rate and the target product
yield (Table 1).
The experimental data (Fig. 2) showed that the sug-
gested stabilizer I taken in a concentration of 0.2 wt %
well competes in the performance with Agidol 2 com-
mercial stabilizer [bis(2-hydroxy-5-methyl-3-tert-butyl-
phenyl)methane, II].
The synthesized product I was tested as EPDM
stabilizer. The stabilizer performance was evaluated
by the length of the induction period of the polymer
oxidation in the course of thermal oxidative aging.
An important way to enhance the performance of
stabilizing additives is the use of mixed formulations that
exhibit the synergistic effect [1, 7]. Synergistic effects
are observed most frequently in mixtures of substituted
phenols with phosphites [8, 9].
To examine the possibility of using the stabilizer un-
der consideration more efficiently, we formulated such
mixtures with tris(2,4-di-tert-butylphenyl) phosphite
(III) as the phosphite component. Compound III is one
of the most effectiveAOs of this class. The data obtained
(Fig. 3) show that there is a strong synergistic effect at I
to III weight ratios from 1 : 3 to 3 : 1.
The synergistic effect also takes place in phenol–phe-
nol mixtures in the case when the rate constants of the
reactions of the inhibitors with peroxy radicals differ by
no less than an order of magnitude. Gurvich et al. [10]
presented a series of rate constants of the reactions of
peroxy radicals with phenolic inhibitors. It is seen from
τ, min
Fig. 2. Kinetic curves of oxidation of EPDM rubber containing
various phenolic antioxidants. р(О2) = 250 mm Hg, сAO
0.2 wt %, Т = 170–175°С; the same for Figs. 3 and 4. [Δр(О2)]
pressure change and (τ) time; the same for Fig. 6. Antioxidant:
(1) I and (2) II.
=
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2,6-BIS(3,5-DI-tert-BUTYL-4-HYDROXYBENZYL)CYCLOALKANONE
45
Table 2. Influence of the antioxidant ratio on the time τ before the onset of EPDM oxidation. Т = 165–170°С, р(О2) = 250 mmHg,
сАО = 0.2 wt %
Content, wt %
Practical synergism,a
τ, min
Spr = τsm/τ10
I
II
III
0
0
0
0
0.2
90
160
260
–
–
0.2
0
0.133
0.067
0
0.067
0.133
0
1.625
0
210
170
220
180
440
360
420
1.313
–
0.2
0.133
0.067
0.067
0.067
0.133
0
0.067
0.133
0.067
0
1.294
1.060
2.588
2.250
2.471
0
0.067
0.133
0.067
0
a (τ10) Induction period of polymer oxidation in the presence of the more effective mixture component at its molar concentration equal to that
of the sum of the inhibitors.
also tris(nonylphenyl) phosphite (Polygard, IV) as
phosphite component. The stabilizers were taken in
1 : 1 : 1 weight ratio corresponding to the strongest
synergistic effect in mixtures of III with phenolic AOs.
these data that, for 2,6-di-tert-butylphenol stabilizers,
the logarithms of the rate constants of these reactions,
log k, are 4.17–4.37, whereas for Agidol 2 (II) log k is
5.6. Thus, with mixtures of AOs I and II, we can expect
the synergistic effect.As seen from Fig. 4, the synergistic
effect is indeed observed at the I to II weight ratios from
1 : 3 to 3 : 1.
τ, min
Because the synergistic effects were observed in both
phenol I–phosphite III and phenol I–phenol II mixtures,
we suggested that, with the ternary mixture I + II + III,
the two synergistic effects could be combined.
To confirm this assumption and find the optimum
composition of the ternary stabilizing mixture, we used a
three-factor planning matrix (Table 2). The experimental
results showed that the induction period of the polymer
oxidation considerably increased in the ternary mixture,
reaching a maximum at the AO weight ratio of 1 : 1 : 1
(Fig. 5).
[Phosphite], wt %
[PAO], wt %
Fig. 3. Induction period τ of EPDM oxidation as a function of
the stabilizer type and concentration.Antioxidant: (1) III, (2) I,
and (3) mixture of I and III; (4) straight line corresponding to
the additive stabilizing effect.
To determine how the structure of the phosphite affects
the oxidation resistance of the polymer, we formulated
mixtures containing, along with phenolic AOs I and II,
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 87 No. 1 2014

46
τ, min
CHEREZOVA et al.
0.0
0.2
0.0
0.2
0.1
cII, wt %
0.1
0.0
cI, wt %
Fig. 4. Induction period τ of EPDM oxidation as a function of
PAO weight ratio in the formulation. Antioxidant: (1) I, (2) II,
and (3) mixture of I and II; (4) straight line corresponding to
the additive stabilizing effect.
τ, min
Fig. 6. Kinetic curves of oxidation of EPDM containing
antioxidant mixtures. р(О2) = 250 mmHg, Т = 170°С, сАО
=
0.2 wt %. AO mixture (weight ratio): (1) I + IV (2 : 1), (2) I +
II + IV (1 : 1 : 1), and (3) I + II + III (1 : 1 : 1).
(2) The ways of using 2,6-bis(3,5-di-tert-butyl-
4'-hydroxybenzyl)cyclohexanone for stabilization of
ethylene–propylene rubber against thermal aging were
examined.Theproductsynthesizedinhibitstheoxidation
on the same level as does Agidol 2 commercial stablizer.
(3) Binary mixtures of 2,6-bis(3,5-di-tert-butyl-4-
hydroxybenzyl)cyclohexanone with an alkylphenol
and a phosphite and ternary mixtures 2,6-bis(3,5-
di-tert-butyl-4-hydroxybenzyl)cyclohexanone–
Agidol 2–tris(2,4-di-tert-butylphenyl) phosphite [or
tris(nonylphenyl) phosphite] exhibit a synergistic effect
in inhibition of the thermal aging of ethylene–propylene
rubber.
Fig. 5. Lines of equal time level (min) before the onset
of oxidation in the presence of stabilizers I (τ1 vertex),
II (τ2 vertex), and III (τ3 vertex). сАО = 0.2 wt %, р(О2) =
250 mmHg.
REFERENCES
In this case, we also observed a supersynergistic AO
effect (Fig. 6).
1. Piotrovskii, K.B. and Tarasova, Z.N., Starenie i stabili-
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phosphite, I–alkyl-substituted phenol, and I–alkyl-substi-
tuted phenol–triaryl phosphite, the best results in EPDM
stabilization were achieved with the I-alkyl-substituted
phenol–phosphite ternary mixtures.
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CONCLUSIONS
(1) Methods for preparing 2,6-bis(3,5-di-tert-butyl-
4'-hydroxybenzyl)cyclohexanone by the reaction of
3,5-di-tert-butyl-4-hydroxy-N,N-dimethylbenzylamine
and cyclohexanone in the bulk were optimized. The
highest yield of the desired product, 86 wt %, was
reached when the reaction was performed at 135–145°С
for 3 h.
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