Controlled radical polymerization of alkyl acrylates in the presence of the tri-n-butylborane–p-quinone system

Article abstract of DOI:10.1007/s11172-016-1521-x

The reactivities of different p-quinones in the radical polymerization of methyl and tert-butyl acrylates were studied. The inhibitory effect of p-quinones decreases witn an increase in the volume and number of substituents. The radical polymerization of alkyl acrylates in the presence of tri-n-butylborane and p-quinones proceeds without gel effect by the “living” polymerization mechanism. UV spectroscopy showed that the reaction between the growth radical and p-quinone proceeds with different regioselectivity and depends on the nature of the latter. The obtained polyacrylates possess the capability of reinitiating polymerization. The reinitiation mechanism was studied by mass spectrometry (MALDI-TOF) and ESR spectroscopy. Gel permeation chromatography showed that, depending on the nature of p-quinone, macroinitiator polymers exhibit different activity in post-polymerization.

Full text of DOI:10.1007/s11172-016-1521-x

Russian Chemical Bulletin, International Edition, Vol. 65, No. 7, pp. 1859—1866, July, 2016
1859
Controlled radical polymerization of alkyl acrylates in the presence
of the triꢀnꢀbutylborane—pꢀquinone system*
D. V. Ludin,a Yu. L. Kuznetsova,a I. D. Grishin,a V. A. Kuropatov,b and S. D. Zaitseva
aN. I. Lobachevsky Nizhny Novgorod State University,
23 prosp. Gagarina, 603950 Nizhny Novgorod, Russian Federation.
Fax: +7 (831) 462 3085. Eꢀmail: kyul@yandex.ru
bG. A. Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences,
49 ul. Tropinina, 603137 Nizhny Novgorod, Russian Federation.
Fax: +7 (831) 462 7497
The reactivities of different pꢀquinones in the radical polymerization of methyl and tertꢀ
butyl acrylates were studied. The inhibitory effect of pꢀquinones decreases witn an increase in
the volume and number of substituents. The radical polymerization of alkyl acrylates in the
presence of triꢀnꢀbutylborane and pꢀquinones proceeds without gel effect by the "living"
polymerization mechanism. UV spectroscopy showed that the reaction between the growth
radical and pꢀquinone proceeds with different regioselectivity and depends on the nature of the
latter. The obtained polyacrylates possess the capability of reinitiating polymerization. The
reinitiation mechanism was studied by mass spectrometry (MALDIꢀTOF) and ESR spectroꢀ
scopy. Gel permeation chromatography showed that, depending on the nature of pꢀquinone,
macroinitiator polymers exhibit different activity in postꢀpolymerization.
Key words: triꢀnꢀbutylborane, pꢀquinone, radical polymerization, alkyl acrylate.
In our previous works,1—4 it was shown that homopoꢀ
lymerization of styrene and methyl methacrylate (MMA)
in the presence of the catalytic system based on triꢀnꢀ
butylborane (Bu3B) and pꢀquinones proceeds as controlled
radical polymerization by the mechanism of reversible inꢀ
hibition.2—4 The resulting macroinitiatorꢀtype polymers
possess the capability of reinitiating polymerization upon
addition of a new monomer portion. For example, postꢀ
polymerization of MMA affording highꢀmolecularꢀweight
(MW) products5 and block copolymerization of styrene
with MMA were carried out. The block copolymer obꢀ
tained by the second method was characterized by high
physical and mechanical properties.6 This catalytic sysꢀ
tem can be used also for the synthesis of a new class of
polymer materials, viz., gradient copolymers. The possible
synthesis of such copolymers was shown by the example of
copolymerization of styrene and vinyl acetate.7 Compared
to methacrylates and styrene, alkyl acrylates possess
a higher activity in the radical polymerization. For this
reason, the control of the process rate and the MWs of
polyacrylates is a complex problem.
Experimental
Organic solvents were purified according to known procedures.8
Methyl acrylate (MA, Reakhim, 98%) was dried over calcium
hydride and distilled under atmospheric pressure (101.3 kPa,
80 °C). tertꢀButyl acrylate (TBA, Acros, 99%) was distilled under
reduced pressure (8.0 kPa, 61—63 °C). Azoisobutyronitrile
(AIBN) was recrystallized from methyl tertꢀbutyl ether and dried
until constant weight; its purity was controlled by NMR spectroꢀ
scopy. Dicyclohexyl peroxydicarbonate (DCPD) was obtained
by the reaction of cyclohexyl chloroformate and sodium perꢀ
oxide9 and recrystallized from methanol. Its purity was deterꢀ
mined by iodometric titration. Tributylborane was prepared by
the reaction between boron trifluoride etherate and nꢀbutyl magꢀ
nesium bromide in diethyl ether according to the earlier deꢀ
scribed procedure10 and distilled under reduced pressure (1.07 kPa,
90 °C). Phenyl Nꢀtertꢀbutylnitrone (AlfaꢀAesar, 98%), duroꢀ
quinone (DQ, Aldrich, 97%), and 2,5ꢀdiꢀtertꢀbutylbenzoquinoꢀ
ne (DTBQ, Aldrich, 99%) were used as received. Naphthoquinoꢀ
ne (NQ, Reakhim, 97%) was purified by recrystallization from
petroleum ether. Benzoquinone (BQ) was obtained by oxidation
of hydroquinone in the presence of potassium bichromate and
sulfuric acid followed by recrystallization from petroleum ether11
1
The aim of the present work was to study whether the
controlled synthesis of polyacrylates in the presence of
a Bu3B—pꢀquinone catalytic system can be realized.
(the yield was 86%). H NMR (CDCl3), δ: 6.78 (s, 4 H). Diꢀ
methylbenzoquinone (DMBQ) was synthesized from 2,3ꢀdiꢀ
methylhydroquinone (Aldrich, 97%) by oxidation in the presꢀ
ence of CrO3 and acetic acid followed by recrystallization from
petroleum ether11 (the yield was 60%). 1H NMR (CDCl3), δ: 2.03
(s, 6 H); 6.71 (s, 2 H).
* Based on the materials of the International Conference
"Organometallic and Coordination Chemistry. Problems and
Advances" (VI Razuvaev Readings) (September 18—23, 2015,
Nizhny Novgorod, Russia).
Polymerization mixture was prepared in an ampule dissolvꢀ
ing the required amounts of the initiator and pꢀquinone in the
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1859—1866, July, 2016.
1066ꢀ5285/16/6507ꢀ1859 © 2016 Springer Science+Business Media, Inc.

1860
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Ludin et al.
The molecular weight characteristics of polymers were
determined by GPC in THF at 40 °C (the flow rate of the eluent
was 0.7 mL min–1) on a Prominence LCꢀ20VP liquid chromꢀ
atograph (Shimadzu) using Tosoh Bioscience polystyrene
gelꢀpacked columns with a pore size of 1•105 and 1•104 Å.
The detector was a differential refractometer. Chromatograms
were processed using the LCsolution software. Calibration was
performed using the PMMA narrowꢀdisperse standards
(MWs 1•103—2.5•106).
BQ, DQ, DTBQ, DMBQ
NQ
BQ: R1 = R2 = R3 = R4 = H
DQ: R1 = R2 = R3 = R4 = Me
DTBQ: R1 = R3 = But; R2 = R4 = H
DMBQ: R1 = R2 = H; R3 = R4 = Me
Results and Discussion
pꢀQuinonesꢀinhibited polymerization of acrylates. At the
first stage, we studied the inhibiting ability of differentꢀ
nature quinones in the radical polymerization of MA and
TBA. There are few studies in this field.13,14 For example,
the kinetic regularities of inhibited polymerization of MA
and ethyl acrylate in the presence of BQ, its halogen deꢀ
rivatives, and DQ have been studied earlier. To estimate
the reactivity of quinones with regard to acrylate growth
radicals, we studied the initial rate of MA and TBA polyꢀ
merization, as well as the molecular weight characteristics
of polymers synthesized in the presence of differentꢀstrucꢀ
ture quinones (Table 2). As is known, the increase in the
inhibition constant (kz), which is the ratio of the inhibiꢀ
tion rate constant to the chain propagation rate constant,
results in a decrease in the initial polymerization rate (V)
and a decrease in the MW. The aboveꢀmentioned effects
depend on the presence and size of a substituent in the
quinone molecule.15 The steric hindrance of pꢀquinones
was estimated using the Charton steric constants (ΣV). It
is seen from Table 2 that, with an increase in the number
and volume of substituents, the inhibition constant deꢀ
creases. For example, the highest rate of MA and TBA
polymerization is observed in using DQ and the lowest
rate and the lowest MW were observed when BQ was addꢀ
ed to the system. In general, pꢀquinones can be arranged
by the reactivity with regard to the acrylate macroradicals
in the following order: BQ > DMBQ > NQ > DQ. As
expected, this order does not depend on the nature of
growth radicals. According to expectations, this order does
not depend on the nature of growth radicals. From the
data on polymerization rate and MW, one can conclude
that pꢀquinones more efficiently trap the MA macroradiꢀ
cals compared to the TBA ones.
monomer. The required amount of a solution of Bu3B in hexane
from a vacuum lineꢀconnected burette was added to another
ampule. Hexane was removed from the solution by distillation
under reduced pressure. Both ampules were degassed by sixꢀfold
freezing in vacuo. The contents of the ampules was mixed fast
and frozen in liquid nitrogen. The ampules were sealed. Prior to
polymerization, they were stored in liquid nitrogen. After deꢀ
frost, the ampule was placed in a thermostat set to either 60 °C
(for the AIBNꢀinitiated polymerization) or 25 °C (for the DCPDꢀ
initiated polymerization). In a given time after initiation of polyꢀ
merization, the ampules were removed from the thermostat and
cooled with liquid nitrogen. The resulting polymers were isolatꢀ
ed by rapid precipitation in petroleum ether (in the case of PMA)
or ethanol—water (9 : 1) (in the case of PTBA). Polymerization
kinetics was studied by gravimetry or dilatometry. To study the
postꢀpolymerization, a solution of the PMA macroinitiator
(5 wt.%) in the monomer was prepared, placed in an ampule,
degassed, and kept in a thermostat at 30 °C.
The dried polymer samples were dissolved in chloroform
and studied by UV spectroscopy on a Shimadzu UVꢀ1650 spectroꢀ
meter. The MALDIꢀTOF analysis of polymers was performed
under linear conditions on a Bruker Microflex LT instrument
equipped with a nitrogen laser (λ = 337.1 nm). The spectrometer
was calibrated by the peaks [PMMA + Na]+ of narrowꢀdisperse
PMMA standards (Waters, Mn = 2580 and 8200). The calibraꢀ
tion dependence was approximated by a thirdꢀpower polynomial.
The experimental data were processed by the Bruker flexControl
and flexAnalysis software. The matrix was transꢀ2ꢀ[3ꢀ(4ꢀtertꢀ
butylphenyl)ꢀ2ꢀmethylꢀ2ꢀpropenylidene]malonitrile. Test samꢀ
ples were prepared mixing a a solution of polymer (10 mg mL–1
,
5 L), a solution of the matrix (20 mg mL–1, 10 L), and sodium
trifluoroacetate (5 mg mL–1, 3 L ) in THF. The resulting soluꢀ
tion (2 L) was applied onto a stainless steel support and anaꢀ
lyzed. ESR spectra were recorded on a Bruker EMX Xꢀrange
spectrometer. The molecular weights of polyacrylates were calꢀ
culated from the values of intrinsic viscosity:
Polymerization of alkyl acrylates in the presence of triꢀ
nꢀbutylborane and pꢀquinones. Introduction of pꢀquinones
together with Bu3B into polymerization of alkyl acrylates
results in an increase in its rate compared to the inhibited
polymerization. From the data given in Table 3, it is seen
that addition of Bu3B increases the polymerization rate on
the average twoꢀfold. For TBA, the rate of the process was
higher than that for MA.
The increase in the process rate upon addition of
pꢀquinones is due to the increase in the amount of reacꢀ
tion centers formed by the active chain transfer to Bu3B.1
With an increase in the concentration of pꢀquinone in the
a
[η] = K•Mη
.
The measurement conditions and values of Mark—Kuhn—
Houwink constants (K, a) are given in Table 1.
Table 1. Mark—Kuhn—Houwink constants for PMA and PTBA12
Compound
Solvent
T/°C
K•104/dL g–1
а
PMA
PTBA
Chloroform
Acetone
30
25
3.22
4.70
0.678
0.750

Controlled radical polymerization
Russ.Chem.Bull., Int.Ed., Vol. 65, No. 7, July, 2016
1861
Table 2. Data from polymerization of MA and TBA initiated by AIBN (0.10 mol.%) at 60 °C in the presence of
pꢀquinones (0.25 mol.%)
Monomer
Quinone
kz•10–3
/L mol–1 s–1 15
ΣV
V•103
(% s–1
)
Time/h
Conversion
(%)
Mη•10–4
MA
TBA
MA
ТБА
МА
TBA
MA
DQ
DQ
NQ
0.92
0.92
10.3
10.3
22.1
22.1
108
1.08
1.08
0.52
0.52
0.52
0.52
0
10.5
17.7
16.9
19.5
13.4
13.7
11.4
11.3
0.5
0.3
2.0
2.1
3.8
3.5
4.6
4.5
22.0
24.0
24.1
29.5
29.4
27.9
23.7
25.4
102.0
147.0
40.1
96.0
22.8
68.9
15.9
14.3
NQ
DMBQ
DMBQ
BQ
TBA
BQ
108
0
Scheme 1
system, the initial polymerization rate of acrylates increases
(see Table 3).
It is known that, upon polymerization of acrylates,
there occurs a significant chain transfer onto the polymer,
especially, upon a deep conversion, which results in the
formation of branched and crossꢀlinked structures. In adꢀ
dition, the conversion upon polymerization of acrylates
corresponding to the beginning of selfꢀacceleration is obꢀ
served even at early stages. To avoid the aboveꢀmentioned
adverse processes, we decided to decrease the synthesis
temperature to 25 °C and to use the "lowꢀtemperature"
initiator DCPD. The decrease in the synthesis temperaꢀ
ture also resulted in a change in the ratio between the
inhibition and chain propagation constants in favor of
more efficient inhibition.
In Ref. 1, we found that the addition of MMA macroꢀ
radicals to quinone can proceed by two pathways (Scheme 1).
Pathway 1 results in the formation of adducts responꢀ
sible for the "living" polymerization. The cumulative
data obtained in the present work allow us to state that,
as in the case of MMA, the reaction of the growth radiꢀ
cals with pꢀquinone can proceed by two pathways (see
Scheme 1).
The addition of macroradical to the C=O bond of
pꢀquinone results in the formation of the phenoxyl radical
which reacts with Bu3B to form terminal boraryloxy groups
(1) (see Scheme 1). Similar adducts can reinitiate polyꢀ
merization by the mechanism of reversible inhibition
(Scheme 2).4—6 The addition of the macroradical to the
C=C bond of pꢀquinone followed by the SR2ꢀsubstitution
at the boron atom (see Scheme 1, pathway 2) results in the
formation of "dead" quinoid derivatives 2 which cannot
reinitiate polymerization. Their formation can widen the
molecular weight distribution of the polymer.
Figure 1 shows that polymerization of alkyl acrylates
initiated by DCPD with addition of Bu3B and pꢀquinones
proceeds without selfꢀacceleration. The kinetic regulariꢀ
Table 3. Rates of MA and TBA polymerization initiated
by AIBN (0.10 mol.%) at 60 °C in the presence of Bu3B
(0.80 mol.%) and different amounts of naphthoquinone
Monomer
MA
[NQ] (mol.%)
V•103 (% s–1
)
0.25
0.50
0.75
12.0
21.3
45.1
Scheme 2
TBA
0.25
0.50
0.75
18.5
36.0
75.0

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Russ.Chem.Bull., Int.Ed., Vol. 65, No. 7, July, 2016
Ludin et al.
Conversion (%)
was observed upon polymerization of styrene in the presꢀ
ence of the aboveꢀmentioned pꢀquinones.4 In our opinꢀ
ion, this can be due to the nature of the monomer, namely,
due to steric hindrances during addition of the tertꢀbutylꢀ
acrylate radical to the quinone molecule and a lower polꢀ
arity of TBA compared to MA. Polymerization of TBA in
the presence of BQ tends to be retarded. This can be due to
a decrease in the rate of dissociation of "living" adducts.
As far as Bu3B in the system becomes exhausted, polymerꢀ
ization kinetics starts to take the form of inhibited polyꢀ
merization. In using pꢀquinones with a lower inhibiting
ability (DQ, DTBQ), the gel effect cannot be suppressed
(Table 4). Within 15 min, the degree of MA conversion
reaches 60% and the weightꢀaverage MW (Mw) of PMA in
using DQ reaches more than 500 kDa.
a
100
1
2
80
60
40
20
3
2
4
6
t/h
To obtain more detailed data on the mechanism of
polymerization of alkyl acrylates in the presence of the
pꢀquinone—Bu3B system, we studied the molecular weight
characteristics of resulting polymers. The numberꢀaverꢀ
age MW of polyacrylates (Mn) obtained using the Bu3B—
pꢀquinone system increases linearly with an increase in
the degree of conversion (Fig. 2). The pattern of the relaꢀ
tionship between the MW and the degree of conversion
is influenced by the portion of the "living" process and
the addition of the growth radical to the C=C bond of
quinone. The highest conribution of the "dead" polymer 2
to the MWD takes place when the strongest inhibitor, BQ,
was used. On going to the weakest inhibitor (NQ), the
slope of the linear relationship between the numberꢀaverꢀ
age MW and the degree of conversion increases. This is
quite natural, since, when using NQ, the time to the inꢀ
hibition act is longer and, correspondingly, higher MWs
are achieved. In the case of BQ, this can be due to the
difficulty of "animation" of reversible inhibition adducts
obtained using this pꢀquinone. Earlier, it was shown by
the example of MMA that similar adducts virtually cannot
dissociate.2 The highest gain in the MW with conversion
occurs in using DMBQ, despite its higher reactivity comꢀ
pared to NQ. In this system, reversible inhibition domiꢀ
nates over "usual" polymerization initiated by DCPD.
The polydispersity parameters of both PMA and PTBA
obtained using different pꢀquinones tend to decrease durꢀ
ing the process. Figure 3 shows the MWD curves for PMA
isolated at different degrees of conversion. In general,
PTBA is characterized by the analogous regularities (see
Fig. 2, b). Also, the highest gain in the MW is achieved
upon addition of DMBQ (see Fig. 2, b, curve 1). When
using NQ, uniform narrowing of the MWD modes for
Conversion (%)
b
100
80
60
40
20
2
3
1
1
2
3
4
t/h
Fig. 1. Kinetic curves for polymerization of MA (a) and TBA (b)
initiated by 0.10 mol.% of DCPD at 25 °C in the presence of
0.80 mol.% of Bu3B and 0.25 mol.% of pꢀquinones: NQ (1);
DMBQ (2); and BQ (3).
ties of MA polymerization in the presence of the Bu3B—
pꢀquinone catalytic system are arranged in accordance
with the reactivity of quinones (see Fig. 1, a). The highest
polymerization rate is observed in using NQ (see Fig. 1, a,
curve 1) and the lowest rate is observed in using BQ (see
Fig. 1, a, curve 3). The analogous regularities take place
upon polymerization of TBA; however, this effect is less
pronounced. The kinetic curves of TBA polymerization in
the presence of NQ (see Fig. 1, b, curve 1) and DMBQ
(see Fig. 1, b, curve 2) virtually coincide. A similar effect
Table 4. Data from polymerization of MA in the presence of Bu3B (0.80 mol.%) and pꢀquinones (0.25 mol.%)
pꢀQuinone
Conversion (%)
Time/h
Mη•10–4
Mn•10–4
Mw•10–4
Mw/Mn
DQ
DTBQ
56.7
63.4
0.25
0.25
35.0
11.6
15.6
7.69
61.2
13.8
3.92
1.79

Controlled radical polymerization
Russ.Chem.Bull., Int.Ed., Vol. 65, No. 7, July, 2016
1863
Mn•10–4
8
81.2 (1.50)
41.5 (1.75)
a
94.0 (1.46)
34.1 (1.88)
6
4
2
20.9 (2.14)
1
2
3
20
40
60
80 Conversion (%)
3.0
3.5
4.0
4.5
5.0
5.5
lgM
Fig. 3. MWDs of the PMA samples obtained in the presence of
0.10 mol.% of DCPD, 0.80 mol.% of Bu3B, and 0.25 mol.% of
NQ at 25 °C. The degree of conversion and the polydispersity
parameters are shown near the curves (in parentheses).
Mn•10–4
8
b
31.3 (1.59)
6
4
2
55.3 (1.73)
70.1 (2.12)
20
40
60
80 Conversion (%)
Fig. 2. Mn as a function of the degree of conversion for PMA (a)
and PTBA (b) obtained in the presence of 0.10 mol.% of DCPD,
0.80 mol.% of Bu3B, and 0.25 mol.% of pꢀquinones: DMBQ (1);
NQ (2); and BQ (3). T = 25 °C.
4.0
5.0
6.0 lgM
Fig. 4. MWDs of the PTBA samples obtained in the presence of
10 mol.% DCPD, 0.80 mol.% of Bu3B, and 0.25 mol.% of BQ at
25 °C. The degree of conversion and the polydispersity parameꢀ
ters are shown near the curves (in parentheses).
both PMA and PTBA (see Fig. 3) occurs with conversion.
The analogous case is observed upon substitution of
DMBQ for NQ. When a strong inhibitor (BQ) is used,
only a slight increase in the MW with conversion upon
polymerization of both MA and TBA is observed (Fig. 4),
which can be due to the presence of a great amount of the
"dead" polymer (2) and the difficulty of "animation" of the
reversible inhibition adducts (1).
The addition products of growth radicals to pꢀquinoꢀ
nes were analyzed by UV spectroscopy. The UV spectra of
the PTBA samples synthesized in the presence of DMBQ
and BQ (Fig. 5, curves 3 and 4, respectively) do not conꢀ
tain the absorption bands of the starting pꢀquinones (see
Fig. 5, curves 1 and 2, respectively). In the case of DMBQ,
the absorption bands of the aromatic fragments of polyꢀ
mer 1 (242 nm) partially overlap with the weakly resolvꢀ
able band corresponding to the quinoid structures 2 (see
Fig. 5, curve 3), the absorption maxima of quinone and
the "dead" polymer 2 are considerably remote from each
other. In the case of BQ, the absorption bands of the
starting quinone (283 nm) and "dead" polymer 2 (288 nm)
differ only by 5 nm (see Fig. 5, curves 2 and 3). However,
in both cases the polymer contains the aromatic structures 1
which correspond to the absorption maximum at 242 nm
(see Fig. 5, curves 3 and 4). Adducts with this terminal

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Russ.Chem.Bull., Int.Ed., Vol. 65, No. 7, July, 2016
Ludin et al.
A (a.u.)
"living" polymerization methods is the ability to reꢀ
start the process upon addition of a new portion of monoꢀ
mer. Before starting this work, we have studied the strucꢀ
ture of the macroinitiator polymers by MALDIꢀTOF. The
spectrum shown in Fig. 6 display two types of "triads",
polymer products differing in the type of terminal groups.
These triads correspond to polymers obtained by the addiꢀ
tion of the cyclohexyloxyl radical formed upon decomꢀ
position of DCPD or the butyl radical formed in the reꢀ
action of oxygenꢀcentered radicals with Bu3B. In both
cases, there occurs chain termination on NQ followed
by SR2ꢀsubstitution. Within each "triad", the peaks differ
by 16 units, which corresponds to the oxygen atom being
incorporated to the chain by oxidation of boron—carꢀ
bon bonds inevitably occurring at the step of isolaꢀ
tion (Scheme 3). The results of analysis of this spectrum
are given in Table 5.
1.0
0.8
0.6
0.4
0.2
1
2
3
4
240
280
320
360
400 λ/nm
Fig. 5. UV spectra of solutions of DMBQ (1), BQ (2), and PTBA
obtained using 0.10 mol.% of DCPD, 0.80 mol.% of Bu3B, and
0.25 mol.% of DMBQ (3) or BQ (4) in chloroform.
As Table 5 shows, the molecular structure of the macꢀ
roinitiator contains both the boralkyl fragments and their
oxidized forms. Earlier, it was shown by the example of
polystyrene macroinitiators6 that oxidation of boron—carꢀ
bon bonds adversely affects the initiating ability of the
macroinitiator. Insertion of the oxygen atom at the B—C
bond upon oxidation results in a decrease in the stability of
the aryloxyl radical formed upon decomposition of macꢀ
roinitiator 1 (see Scheme 2). Similar semiꢀoxidized and
oxidized forms of the macroinitiator accumulate during
air storage. For this reason, the contacts of macroinitiator
and air oxygen should be minimized.
The polymers isolated after postꢀpolymerization were
analyzed by GPC. From the data given in Table 6, it folꢀ
lows that, within 1.5 h of the postꢀprocess, the MW of
PMA increases and the MWD narrows. The highest inꢀ
crease in the MW (1.16ꢀfold) is observed upon initiation
group direct polymerization by the mechanism of reversꢀ
ible inhibition.
Thus, polymerization of MA and TBA in the presence
of the Bu3B—pꢀquinone catalytic system bears the signs of
a "living" process. Quinones (NQ and DMBQ) act as the
most effective mediators of chain propagation holding an
intermediate position in the reactivity order. The use of
weak inhibitors, DQ and DTBQ, does not result in the
disappearance of the gel effect (see Table 4). Upon addiꢀ
tion of the strong inhibitor, BQ, the progress of “living”
polymerization is hindered due to the formation of a great
amount of the "dead" polymer (2) and the difficulty of
animation of the macroinitiator polymer (1).
Postꢀpolymerization initiated by macroinitiators. One
of the important properties of polymers obtained by the
1702.8
1788.9
1804.3
1819.7
1718.2
1733.5
1759.9
1846.5
1861.9
1743.9
1730.6
1775.9
1700
1740
1780
1820
1860
m/z
2000
4000
6000
8000
10000
12000
m/z
Fig. 6. Mass spectrum of PMA obtained using 0.10 mol.% of DCPD, 0.80 mol.% of Bu3B, and 0.25 mol.% NQ at 25 °C. The degree of
conversion was 20.9% and Mn = 1.95•104.

Controlled radical polymerization
Russ.Chem.Bull., Int.Ed., Vol. 65, No. 7, July, 2016
1865
Scheme 3
Table 5. Data from the massꢀspectral analysis
of the PMA samples obtained in the presence
of DCPD (0.10 mol.%), Bu3B (0.80 mol.%),
and NQ (0.25 mol.%) at 25 °C
onyl group of NQ. The analogous situation is observed
also for macroinitiators obtained by polymerization in the
presence of DMBQ (see Fig. 7, b).
Since initiation proceeds involving the macroinitiator
containing unoxidized boronꢀcarbon bonds, to confirm
the presumable mechanism of postꢀpolymerization, one
should exclude the possibility of reinitiation by means of
residual air oxygen. To establish the mechanism of reinitiꢀ
Polymerꢀ
structure
m/z
MWteor
1a
1b
1c
2a
2b
2c
1702.7
1718.2
1733.5
1743.9
1759.9
1775.9
1696.0
1712.0
1727.0
1739.0
1755.0
1771.0
a
1
2
4.0
5.0
6.0 lgM
1, 2: R = OBBu2 (a), OB(OBu)Bu (b), OB(OBu)2 (c)
b
2
of polymerization with the DMBQꢀbased macroinitiator.
Approximately identical increase in the MW for the same
time is observed when macroinitiators obtained involving
BQ and NQ were used. The MWDs of postꢀpolymerizaꢀ
tion products have different patterns (Fig. 7, a, b). The
postꢀpolymer obtained in the presence of the BQꢀbased
macroinitiator is characterized by a bimodal MWD (see
Fig. 7, a, curve 2). The appearance of the second mode
suggests a considerable amount of the inactive polymer 2
formed due to the addition of the macroradical to the
C=C bond of BQ. The highꢀmolecularꢀweight mode corꢀ
responds to the polymer formed during postꢀpolymerizaꢀ
tion due to dissociation of active adducts. These results
allow assuming that the addition of the growth radicals to
BQ is characterized by a low regioselectivity. When the
NQꢀbased macroinitiator was used, a regular shift in the
MWD curves in favor of higher MWs accompanied by
a decrease in the polydispersity parameter was observed. In
this case, one can suggest the polymer chains to contain
predominantly the boraryloxyl terminal groups formed
upon the reaction between the macroradical and the carbꢀ
1
4.0
4.5
5.0
5.5
lgM
Fig. 7. MWDs of the PMA macroinitiators (1) obtained at 25 °C
in the presence of 0.10 mol.% of DCPD, 0.80 mol.% of Bu3B,
and 0.25 mol.% of pꢀquinones: BQ (a) and DMBQ (b); as well as
MWDs of the postꢀpolymerization products (2).

1866
Russ.Chem.Bull., Int.Ed., Vol. 65, No. 7, July, 2016
Ludin et al.
Table 6. Characteristics of PMA macroinitiators and postꢀPMA obtained in the presence
of macroinitiators (5 wt.%) within 1.5 h at 30 °C
pꢀQuinone
Macroinitiator
Mn•10–4
Mw/Mn
Postꢀpolymer
Conversion (%)
Mn•10–4
Mw/Mn
BQ
DMBQ
NQ
5.95
7.02
7.52
1.85
1.78
1.53
5.62
6.90
5.91
9.77, 6.16
8.16
1.80
1.61
1.53
8.50
This work was financially supported by the Ministry of
Education and Science of the Russian Federation (State
Task No. 4.1537.2014K).
a
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ation, we performed the ESR study in the presence of the
spin trap, PBN. Earlier,16 it was shown that alkyl and
alkoxyl radicals are detected in the reaction between Bui3B
and air oxygen in the MMA medium. During postꢀpolyꢀ
merization, the chain propagation radicals whose signal
intensity increased over 30 min were detected only (Fig. 8).
Thus, reinitiation in postꢀpolymerization is realized
only through dissociation of aromatic fragments by the
reaction shown in Scheme 2. The presence of such strucꢀ
tures was proved by MALDIꢀTOF and UV spectroscopy.
The PMA macroinitiators obtained involving DMBQ and
NQ possess the highest activity. The postꢀpolymers obꢀ
tained on the macroinitiator using BQ are characterized
by the bimodal MWD, which suggests the presence of
a considerable portion of the "dead" polymer.
Received December 31, 2015;
in revised form March 9, 2016