Nitroxide-mediated polymerization of methyl methacrylate by 4,4′-dimethoxydiphenyl-based alkoxyamine

Article abstract of DOI:10.1039/c6ra19114b

Nitroxide-mediated polymerization of methyl methacrylate (MMA) was carried out using 4,4′-dimethoxydiphenyl nitroxide-based alkoxyamine as a mediator. This alkoxyamine can be easily prepared and is able to control the nitroxide-mediated polymerization of

Full text of DOI:10.1039/c6ra19114b

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PAPER
Nitroxide-mediated polymerization of methyl
methacrylate by 4,40-dimethoxydiphenyl-based
Cite this: RSC Adv., 2016, 6, 73842
alkoxyamine†
*
Zhecheng Zhu, Guorong Shan and Pengju Pan
Nitroxide-mediated polymerization of methyl methacrylate (MMA) was carried out using 4,40-
dimethoxydiphenyl nitroxide-based alkoxyamine as a mediator. This alkoxyamine can be easily prepared
and is able to control the nitroxide-mediated polymerization of MMA up to a conversion of 65%. A linear
increase of the number-average molecular weight with conversion was observed and polymers with
narrow molecular weight distributions (PDI ¼ 1.2–1.4) were obtained. The living character was also
confirmed by chain extension from the prepared poly(methyl methacrylate) macroinitiator with MMA or
styrene. ESR (electron spin resonance) results indicate that the living chain fraction is more than 85%.
Additionally, the MMA polymerization cannot be well controlled by the diphenylnitroxide-based
alkoxyamine because of the unwanted cross combination reaction between two nitroxides; the prepared
polymer shows a bimodal and broad molecular weight distribution.
Received 28th July 2016
Accepted 29th July 2016
DOI: 10.1039/c6ra19114b
www.rsc.org/advances
nitroxide with propagating macroradicals would be minimized.
Grubbs et al. developed a methodology based on the addition of
Introduction
C-centered radicals obtained from alkyl halides precursors to
nitroso to prepare the corresponding N-phenyl alkoxyamines
compounds, and moderate control at low monomer conversion
was obtained with this alkoxyamines.20,21 Several sterically
hindered imidazoline nitroxides and their corresponding
alkoxyamines bearing a spiro cyclic moiety were prepared by
Edeleva et al. Although the good control was achieved up to
a conversion of about 55%, the synthesis of alkoxyamines is
complicated.22 Yoshida investigated the photo-living radical
polymerization of MMA mediated by TEMPO using a photo-acid
generator, bis(alkylphenyl)iodonium hexauorophosphate
(BAI), PMMA with a narrow molecular weight distribution was
produced but the mechanism remained unclear as the role of
the BAI was not elucidated.23,24
Until now, the use of diarylnitroxides as control radicals in
NMP has been seldom studied. Being different from nitroxides
usually used in NMP, the stability of these radicals is generally
attributed to electron delocalization. Diphenylnitroxide (DPN)
was the rst diarylnitroxide reported by Wieland et al.25 But the
use of DPN in NMP is restricted because of the high electron
density on the carbon atom in para-position, this would cause
a cross combination reaction (Scheme 1). Haghighat and co-
workers have utilized DPN to control the polymerization of
1,3-butadiene, while a bimodal molecular weight distribution of
the nal product was observed.26 This is ascribed to the
dimerization reaction occurred during the oxidation process of
aromatic diphenylamine. 4,40-Dimethoxydiphenyl nitroxide
(DMDPN) was later synthesized by Meyer et al.27 The presence of
methoxy groups in para position of phenyl rings makes DMDPN
Nitroxide-mediated polymerization (NMP) is a powerful tech-
nique to prepare well-dened (co)polymers with controllable
molar masses, narrow molar mass distribution and end group
functionalities.1–6 NMP operates by the reversible trapping of
growing polymer chains with nitroxides and is controlled by the
so-called persistent radical effect which was established by
Fischer and Fukuda.7,8 Compared with other controlled/living
radical polymerization (CLRP) techniques like atom transfer
radical polymerization (ATRP)9 and reversible addition-
fragmentation chain transfer (RAFT),10 NMP has always been
considered as a more simple method due to a simple control
system, a thermal activation mechanism and no purications
for the polymer except simple precipitation to remove unreac-
ted monomer. 2,2,6,6-Tetramethyl-1-piperidinyloxy nitroxide
(TEMPO) was rst successfully used to control the polymeriza-
tion of styrene.11,12 Now, a variety of nitroxides with different
structures have been introduced in NMP to extend the range of
monomers,13–16 while methyl methacrylate (MMA) is still
a challenging one.17,18 A big breakthrough was made by Guilla-
neuf et al. who utilized 2,2-diphenyl-3-phenylimino-2,3-
dihydroindol-1-yloxyl (DPAIO)-based alkoxyamines as a medi-
ator for NMP.19 Because the presence of a phenyl ring allows the
delocalization of free radical, the cross-disproportionation of
State Key Laboratory of Chemical Engineering, College of Chemical and Biological
Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China. E-mail:
shangr@zju.edu.cn; Tel: +86-571-87951334
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra19114b
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Scheme 1 Instability of the diphenylnitroxide.
much more stable than its parent radical DPN. Considering the (60 mL), and 4,40-dimethoxydiphenyl amine (0.9160 g, 4 mmol)
high stability, simple preparation, and delocalization effect of were added into a 250 mL ask and stirred vigorously at 0 ꢀC. A
DMDPN, we intended to investigate the capability of DMDPN solution of oxone (10 g, 16.3 mmol) in water (90 mL) was then
and its corresponding alkoxyamine in the controlling radical added to the mixture drop-wise over a period of 1 h. The pH of
polymerization of MMA.
reaction mixture was monitored by a pH-meter and kept
constant at pH 7.5–8 by adding 2 mol Lꢁ1 solution of KOH. The
reaction mixture became deep red within several minutes. Upon
completion of the addition, the reaction was allowed to proceed
Experimental
Materials
ꢀ
at 0 C for 2 h under stirring. The CH2Cl2 layer was separated
4,40-Dimethoxydiphenyl amine (99%), potassium perox- and the aqueous phase was extracted with CH2Cl2 for three
ymonosulfate (oxone, 99%), tetra-n-butylammonium hydrogen times. The organic phase was dried with anhydrous MgSO4 and
sulfate (99%), ethyl 2-bromoisobutyrate (99%), and PMDETA concentrated by vacuum distillation. The crude product was
(99%) were purchased from J&K Chemical Reagent and used as puried by silica gel column chromatography (hexane : ethyl
received. Potassium hydroxide (KOH, >96%), methylene chlo- acetate ¼ 3 : 1). The nitroxide (0.83 g, yield ¼ 85%) was obtained
ride (CH2Cl2, 99%), acetone (99%), ethanol (99%), acetonitrile as red crystals. Anal. calc. for C14H14NO3: C, 68.85%; H, 5.74%;
(MeCN, 99%), ethyl acetate (99%), and hexane (99%) were N, 5.74%. Found: C, 68.53%; H, 6.01%; N, 5.69%.
purchased from Sinopharm Chemical Reagent (SCRC,
Shanghai, China) and used as received. 2,20-Azobis(isobutyr-
Synthesis of ethyl 2-((bis(4-methoxyphenyl)amino)oxy)-2-
onitrile) (AIBN, 98%) was purchased from SCRC and puried by
recrystallization in ethanol. MMA and styrene were purchased
from SCRC and distilled before use. Glass tubes (custom-built,
length ¼ 400 mm, inner diameter ¼ 4 mm, outer diameter ¼ 6
mm) were used in polymer synthesis.
methylpropanoate
DMDPN (1.32 g, 5.4 mmol), copper powder (0.172 g, 2.5 mmol),
ethyl 2-bromoisobutyrate (0.98 mg, 5 mmol) and MeCN (10 mL)
were added into a 25 mL ask and stirred under continuous
stirring. The ask was sealed with a rubber septum, placed in an
oil bath at 30 ꢀC, and degassed by bubbling with argon for
20 min. PMDETA (0.5 g, 3 mmol) was added by syringe. Aer
Characterization
1H NMR spectra were recorded on a Bruker Avance 300M NMR 3 h, the mixture was diluted by 100 mL of ethyl acetate, and
spectrometer using deuterated chloroform as the solvent and washed with 10% HCl, saturated NaHCO3, and saturated NaCl,
tetramethylsilane as the reference. The pH value was probed by respectively. The organic phase was dried with anhydrous
a pH-meter (LEICI PHS-2C) with a micro-pH electrode (E201-4). MgSO4 and concentrated by vacuum distillation. The crude
The average molecular weight and molecular weight distribu- product was puried by silica gel column chromatography
ꢀ
tion of polymers were measured at 30 C using a Waters 1525/ (hexane : ethyl acetate ¼ 5 : 1). The alkoxyamine (1.69 g, yield ¼
2414 gel permeation chromatography (GPC) system (Waters, 87%) was light red crystals. 1H NMR: d 1.30 (t, 3H; CH3–CH2O),
3
˚
Milford) equipped with PLgel 10 mm columns (500 A, 10 and d 1.50 (s, 6H; CH3), d 3.77 (s, 6H; O–CH3), d 4.17 (s, 2H; CH2),
104) (molecular weight ranges: 100 to 800 000 g molꢁ1) and d 6.83 (d, 4H; Ar–H), d 6.67 (d, 4H; Ar–H); anal. calc. for
refractive index detector. Poly(methyl methacrylate)s with
C20H25NO5: C, 70.19%; H, 6.96%; N, 3.90%. Found: C, 69.98%;
narrow molecular weight distributions were used as standards H, 7.08%; N, 4.02%.
(Mn/PDI of the standards: 645/1.29, 1970/1.10, 5000/1.09,
10 900/1.04, 27 600/1.02, 60 150/1.03, 138 600/1.02, 298 900/
1.02, 625 500/1.03, 1 677 000/1.08). THF was used as the
eluent at a ow rate of 1 mL minꢁ1. Electron spin resonance
Typical polymerization experiment
MMA (20 g, 0.2 mol) and alkoxyamine (0.18 g, 0.5 mmol) were
mixed and stirred until the alkoxyamine was fully dissolved. The
solution was then degassed by N2 bubbling for 20 min and
added into several glass tubes. The tubes were sealed and
placed into a preheated oil bath at 110 ꢀC. Aer polymerization
for a certain period, the tube was removed from the oil bath and
(ESR) experiments were carried out on
spectrometer.
a Bruker A300
Synthesis of 4,40-dimethoxydiphenyl nitroxide
Acetone (80 mL), CH2Cl2 (60 mL), tetra-n-butyl ammonium immersed into ice-water to stop the reaction. The polymer was
hydrogen sulfate (0.085 g, 0.25 mmol), phosphate buffer isolated by precipitating the reaction mixture into cold ethanol.
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Conversion was determined gravimetrically. The average were linear which have shown promise in controlling MMA
molecular weight and polydispersity index were determined by polymerization with DMDPN. The poor controllability over
GPC.
MMA polymerization under bimolecular initiating system was
also reported by Guillaneuf for DPAIO/PERKADOX 16 (bis(tert-
butylcyclohexyl)peroxydicarbonate) pair.19 Because the unim-
olecular system generally have better controllability over
molecular weight and polydispersity than the bimolecular
initiating system,30 DMDPN-based alkoxyamine was then used
as a mediator in the MMA polymerization.
Chain extension polymerization
Chain extension polymerization initiated by a DMDPN-capped
PMMA macroinitiator was conducted for both MMA and
styrene. The DMDPN-capped PMMA macroinitiator was
precipitated in methanol twice before use. For the MMA chain
extension, the macroinitiator was ꢀdissolved in MMA (1000
equiv.) and allowed to react at 110 C for 3 h. For the styrene
chain extension, the macroinitiator wasꢀ dissolved in styrene
(4000 equiv.) and allowed to react at 125 C for 6 h.
As displayed in Table 1, when the DMDPN-based alkoxy-
amine was used, 39% conversion was achieved in 9 h (entry 2).
The measured molecular weights can be well t by the theo-
retical ones and the molecular weight distributions were narrow
under different monomer/alkoxyamine ratios; this suggests
a “living” character of this polymerization (entries 2 and 3). The
ln[M]0/[M] vs. time and molecular weight vs. conversion plots
are shown in Fig. 2. The kinetic plot was pseudo-rst-order and
the molecular weight increased linearly with conversion up to
a conversion of about 65% (Fig. 2a). The measured molecular
weights were consistent well with the theoretical values and the
PDIs were small at different conversions (Fig. 2b). The poly-
Results and discussion
DMDPN was synthesized according to the method reported by
Brik.28 The synthetic route of DMDPN-based alkoxyamine is
illustrated in Scheme 2. 4,40-Dimethoxydiphenyl amine was
oxidized to DMDPN by oxone under biphasic conditions
(CH2Cl2/H2O) rapidly; this reaction showed high yield (85%)
aer 3 h. The alkoxyamine was prepared using the reported
method of Harrisson.29 Ethyl 2-bromoisobutyrate was used to
generate the corresponding alkyl radicals in this study.
A series of polymerization experiments were conducted
under different conditions to study the controllability of
DMDPN and its alkoxyamine to control the polymerization of
MMA (Table 1). In a preliminary study, we conducted the
polymerization of MMA with DMDPN under a bimolecular
initiating system using AIBN as initiator at 110 ꢀC (entry 1,
Fig. 1). When the DMDPN/AIBN feed ratio is 1.6 and the tar-
geted Mn is 80 kg molꢁ1, a large discrepancy was observed
between the theoretical and calculated molecular weights. The
PDIs were higher than 1.8 at different conversions, which is
above the limit of controlled polymerization. However, the plots
of ln[M]0/[M] vs. time and molecular weight (Mn) vs. conversion
ꢀ
merization was also conducted at 90 C (entry 4); it proceeded
quite slowly at this temperature and the origin of the linear
correlation did not go through 0 (Fig. 2a). The measured
molecular weights were higher than the theoretical values; the
PDIs were relatively large but still smaller than 1.5 (Fig. 2b). This
decreased controllability may be due to the slow initiation rate
ꢀ
of the alkoxyamine at 90 C.
Chain extension polymerization of MMA initiated by the
DMDPN-capped PMMA macroinitiator was conducted to check
the living character. To prepare the DMDPN-capped PMMAꢀ mac-
roinitiator, the bulk MMA polymerization reaction at 110 C was
stopped and quenched at a conversion of 13%; the polymer was
puried by precipitation in methanol twice (Mn ¼ 5200 g molꢁ1
,
PDI ¼ 1.25). 1 g of the macroinitiator was dissolved in 19.25 g of
ꢀ
MMA monomer (1000 equiv.) and allowed to react at 110 C for
3 h. The result is shown in Fig. 3. The GPC trace shi obviously to
Scheme 2 Synthesis of DMDPN-based alkoxyamine.
Table 1 Summary of experimental conditions and results for MMA polymerization in bulk
Entry Initiating system
Temperature (ꢀC) Targeted Mn (kg molꢁ1
)
Time (h) Convn (%) Mn,GPC (kg molꢁ1
)
Mnth (kg molꢁ1
)
PDIb
1
2
3
4
DMDPN/AIBN ¼ 1.6 110
80
7
9
10
22
51
39
32
18
64.5
16.3
27.1
13.2
20.4
15.6
25.6
6.0
1.88
1.25
1.3
Alkoxyaminea
Alkoxyamine
Alkoxyamine
110
110
90
40
80
40
1.4
a
b
Alkoxyamine represents the DMDPN-based alkoxyamine. Determined by GPC.
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Fig. 1 (a) Evolution of ln[M]0/[M] vs. time for the polymerization of MMA at 110 ꢀC under a bimolecular initiating system. DMDPN/AIBN ¼ 1.6,
targeted Mn ¼ 80 kg molꢁ1. The solid line is the line of best fit. (b) Evolution of Mn ( ) and PDI ( ) vs. conversion. The dashed line corresponds to
the theoretical Mn.
Fig. 2 (a) Evolution of ln[M]0/[M] vs. time for the polymerization of MMA mediated by DMDPN-based alkoxyamine at 110 ꢀC ( ) or 90 ꢀC ( )
targeted Mn ¼ 40 kg molꢁ1. (b) Evolution of Mn (full symbol) and PDI (empty symbol) vs. conversion.
the smaller elution time, indicating that the DMDPN-capped
The DMDPN-capped PMMA macroinitiator was also used for
PMMA macroinitiator had living chain ends and could be the chain extension polymerization of styrene, which allows for
reinitiated.
the preparation of diblock copolymer. Considering the poor
solubility of PMMA in styrene, 1 g of the macroinitiator was
dissolved in 80.1 g of styrene monomer (4000 equiv.). The
polymerization was conducted at a higher temperature of
ꢀ
125 C to increase the conversion. The GPC curve of prepared
diblock copolymer is shown in Fig. 4. As shown in this gure,
when styrene was used as the second monomer, the PMMA-b-PS
diblock copolymer with high molecular weight was prepared.
Although the PDI of prepared PMMA-b-PS was broad (PDI ¼
2.21), there was no low-molecular-weight peak or shoulder in
the GPC trace aer precipitation. This means that nearly all the
DMDPN-capped PMMA macromolecules were converted to the
block copolymers.
ESR measurements were carried out to conrm the living
nature of prepared DMDPN-capped PMMA. The DMDPN-
capped PMMA macroinitiator, which was the same as that
used in the chain extension polymerization, was dissolved in
tert-butyl benzene to get 0.1 mmol Lꢁ1 solution with oxygen as
the scavenger. Aer heating to 110 ꢀC, the ESR spectra of
DMDPN was observed and its signal increased with time (Fig. 5).
Fig.
3 Chain extension polymerization of MMA initiated by the
DMDPN-capped PMMA macroinitiator. Macroinitiator: Mn ¼ 5200 g
molꢁ1, PDI ¼ 1.25 (solid line). After polymerization for 1.5 h: Mn
¼
12 300 g molꢁ1, PDI ¼ 1.31 (dashed line). After polymerization for 3 h:
Mn ¼ 21 300, PDI ¼ 1.35 (dotted line).
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ring plays an important role in the polymerization of MMA with
diarylnitroxides.
Conclusions
While most of the nitroxides used in nitroxide-mediated poly-
merization were dialkylnitroxides, with at least one hindered
alkyl group linked to the N atom, diarylnitroxides which have
different stability mechanism were seldom studied in NMP. In
this work, DMDPN was synthesized by oxidation of 4,40-dime-
thoxydiphenyl amine with oxone in high yield and its corre-
sponding alkoxyamine was employed as a mediator in NMP of
MMA. At 110 ꢀC, the living polymerization of MMA up to about
65% conversion was achieved, showing the linear increase of Mn
vs. conversion and the narrow molecular weight distributions
(PDI ¼ 1.20–1.4) of prepared polymers at the different
monomer/alkoxyamine feed ratios. An induction period
appeared when the polymerization was conducted at 90 ꢀC, due
to the slow initiation rate of alkoxyamine. The prepared
DMDPN-capped PMMA could be reinitiated by MMA or styrene,
which conrmed the living character of the chain ends. The
diblock polymer was successfully prepared by using styrene as
the second monomer in the chain extension polymerization, in
spite of its relatively large PDI. The high living chain fraction
was evidenced by ESR. For comparison, the diphenylnitroxide-
based alkoxyamine was synthesized and used to mediate the
polymerization of MMA. A bimodal molar weight distribution
with PDI > 3 was observed for the prepared polymer, suggesting
the importance of the para-substitution on the phenyl ring
when dialkylnitroxides were used in NMP of MMA.
Fig. 4 Chain extension polymerization of styrene initiated by the
DMDPN-capped PMMA macroinitiator. Macroinitiator: Mn ¼ 5200 g
molꢁ1, PDI ¼ 1.25 (solid line). After polymerization for 6 h: Mn ¼ 52 300
g molꢁ1, PDI ¼ 2.21 (dashed line).
Aer the macroinitiator was fully decomposed, the quantica-
tion of the signal showed that the concentration of the DMDPN
released was 0.0856 mmol Lꢁ1, which means that the fraction of
living chains is more than 85%.
In order to assess the effect of the para-substitution on the
phenyl ring on polymerization, DPN-based alkoxyamine was
also synthesized inꢀthe same way and used for the polymeriza-
tion of MMA (110 C, targeted Mn ¼ 40 kg molꢁ1). Aer poly-
merization under the same conditions, a bimodal molecular
weight distribution with large PDI (>3) was observed for the
prepared polymers. This bimodal distribution has been attrib-
uted to the formation of difunctional nitroxide during the
oxidation process26 and disappeared aer a reduction treat-
ment. The same reduction experiment of prepared PMMA was
also conducted; however, no difference in GPC traces was
observed (see ESI†), meaning that the difunctional nitroxide
formation mechanism was not responsible for the poor
controllability of DPN to MMA polymerization. The decompo-
sition path way of DPN exhibited in Scheme 1 might be the
main reason, in which diphenylamine and N-phenyl-p-benzo-
quinoimine N-oxide were formed and retarded the polymeri-
zation. This suggests that the para-substitution on the phenyl
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