Unimolecular ligand-initiator dual functional systems (ULIS) for low copper ATRP of vinyl monomers including acrylic/methacrylic acids

Article abstract of DOI:10.1039/c2cc16663a

A novel approach for ATRP has been developed which enables the polymerization of vinyl monomers including those bearing carboxylic acid groups such as acrylic/methacrylic acid in the free acid form with ppm amounts of copper. The quantity of copper used i

Full text of DOI:10.1039/c2cc16663a

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COMMUNICATION
Unimolecular ligand–initiator dual functional systems (ULIS) for low
copper ATRP of vinyl monomers including acrylic/methacrylic acidsw
Satyasankar Jana, Anbanandam Parthiban* and Foo Ming Choo
Received 27th October 2011, Accepted 21st February 2012
DOI: 10.1039/c2cc16663a
A novel approach for ATRP has been developed which enables
the polymerization of vinyl monomers including those bearing
carboxylic acid groups such as acrylic/methacrylic acid in the
free acid form with ppm amounts of copper. The quantity of
copper used in the polymerization is comparable to those left in
purified polymers obtained by a conventional ATRP process.
may proceed more homogeneously and the active–dormant cycles
of the ATRP process may become intramolecular in nature. Also,
the energy barrier for the cleavage of the C–X bond leading to the
formation of radicals as well as that of the red–ox process
involving CuI/CuII transformation can be expected to be different
in the case of ULIS as compared to the conventional ATRP
process due to its intramolecular nature.
Atom transfer radical polymerization (ATRP)1 is one of the
most robust and powerful controlled polymerization techniques
for the preparation of well defined polymeric materials of diverse
architectures and functionality.1 It is a promising technological
development which initially suffered from drawbacks like high
amounts of metal residue, inability to polymerize polar vinyl
monomers bearing free carboxylic acids and non-conjugated
vinyl monomers, etc. The residual metal content has been
dramatically brought down by developments like initiators for
continuous activator regeneration (ICAR)2 and activators
generated by electron transfer (ARGET).3 However, direct homo-
or copolymerization of polar vinyl monomers like acrylic or
methacrylic acid is not yet possible by ATRP with the exception
of sodium methacrylate.4a Poisoning of catalyst by acidic vinyl
monomers has been proposed as a possible cause of this inability.4b
However, this proposition has recently been disputed.5
Even though, at first look, it appeared that this process
could retain all the metal salt employed in the polymerization since
the ligand will be preserved as the o-chain end of the polymer, the
possibility of polymerization proceeding homogeneously actually
allows the use of metal salt in lower quantities.
(Scheme S1 in ESIw compares the polymerization by conven-
tional ATRP with ULIS promoted ATRP). Unlike the case of
ATRP where the metal catalyst remains isolated during active–
dormant cycles, in ULIS promoted ATRP, the catalyst is part of
the polymer chain during the active–dormant cycles. Even though
the complexing agent (i.e. ligand) and the initiating moiety have
been part of the same species before,8 it has only been used as an
initiator with additional catalyst systems. We have observed that
ULIS are capable of polymerizing vinyl monomers using CuBr in
quantities which are comparable to the residual metal impurities
in purified polymers. The metal impurities in purified polymers
obtained by ATRP have been reported to vary from hundreds to
thousands of ppm.9 Also, it is possible to directly homo- and
copolymerize free carboxylic acid bearing vinyl monomers using
ULIS as reported here. For this work we have synthesized many
unimolecular ligand–initiator systems (ULIS),6 three of which are
presented here in Fig. 1, namely, SBLI, TALI and TDLI, with
different ligand components by employing various synthetic
protocols (see the experimental section in ESIw).
In order to address these issues, we recently designed and
developed unimolecular ligand–initiator systems (ULIS) whereby
the ligand used for complexing the metal and the initiator employed
for initiating polymerization are part of the same species (Fig. 1).6
Through this modification, we visualized that the polymerization
At first, when SBLI was used as the dual functional ligand–
initiator for the polymerization of styrene (St) in toluene at 110 1C
at a ratio of St : SBLI : CuBr = 250 : 1 : 1 ([CuBr] = 4000 ppm)
without any further use of an external ligand, the polymerization
(P1 in Table 1) proceeded via normal ATRP with good control
and yielded a polymer with narrow polydispersity index (PDI).
Surprisingly the use of much lesser amount of CuBr (P2 in
Table 1) where [SBLI] : [CuBr] = 50 : 1 did not disturb the
living nature of the polymerization significantly as seen in the
kinetics study (see Fig. S1 in ESIw). The molecular weight
(Mn,GPC) of the resultant polymer (characterized by GPC
without any purification or precipitation) increased linearly
with the conversion of monomers. However, the use of lesser
Fig. 1 Different unimolecular ligand–initiator systems (ULIS) synthesized
for this study.
Institute of Chemical and Engineering Sciences, Agency for Science,
Technology and Research (A*STAR), 1, Pesek Road, Jurong Island,
Singapore, 627833. E-mail: aparthiban@ices.a-star.edu.sg;
Fax: +65 6316 6184; Tel: +65 6796 3910
w Electronic supplementary information (ESI) available: Synthesis of
ULIS, NMR and HRMS spectra, polymerization procedure and
photographs of polymers. See DOI: 10.1039/c2cc16663a
c
4256 Chem. Commun., 2012, 48, 4256–4258
This journal is The Royal Society of Chemistry 2012

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Table 1 Polymerization of St and MMA using SBLI and ppm amounts of CuBr
GPC resultsd
c
Polymer code ULIS system
Monomera
St
Target DP [CuBr], ppm Time/h Conv.b (%) Mn,Theo Mn,GPC
PDI
P1
P2
SBLI
SBLI
250
250
4000
80
2
7
2
19.8
38.3
17.6
34.2
82.3
25.9
49.2
88.3
—
5200
8800
1.25
1.28
1.86
1.76
1.59
2.02
2.09
1.76
—
1.32
1.30
1.31
1.43
1.47
1.43
1.54
1.89
1.83
1.83
1.66
1.49
1.59
1.93
2.03
10 600
5100
9500
16 000
10 500
15 900
33 400
16 200
25 400
45 800
No polymer
12 900
27 400
30 400
18 300
29 500
16 400
18 200
19 100
30 500
32 800
39 000
34 200
65 300
14 900
21 400
St
St
7
30
2
21 400
7300
P3
SBLI
250
40
7
13 400
23 600
—
10 900
22 800
24 400
7400
20 900
5500
11 900
9700
23 000
13 600
23 100
17 900
34 600
2200
24
19
1
4
6
1
6
6
26
6
24
2
6
6
24
2
P4
P5
SBLI
SBLI
MMA
MMA
250
250
0
200
41.5
65.2
95.2
27.5
81.3
19.8
45.4
36.9
90.3
52.0
90.1
86.5
42.8
3.8/14.2
57.7/84.7
P6
SBLI
MMA
250
250
250
250
100
100
100
100
P7
SBLI + EBIB (1 : 4) MMA
P8
TALI
TDLI
SBLI
TDLI
MMA
MMA
P9
P10e
P11g
MMA
St
250
375
75
50f
St + MMA (3 : 1) 187/63
100
22
17 200
a
St was polymerized in a 11 : 42 monomer to toluene ratio at 110 1C and MMA was polymerized in a 11 : 41 monomer to toluene ratio at 100 1C
without any external ligand; target DP is equal to monomer to initiator ratio. Calculated from 1H NMR spectra. Including ULIS as the end
b
c
d
group. As obtained from the polymerization and without further purification except for polymer P1 where the polymer was purified by passing
e
through an alumina column. Sequential polymerization of MMA and St to synthesize PMMA-b-PS. With respect to total amount of MMA
f
g
and St. Random copolymerization of St and MMA using monomer mixture.
amount of CuBr under similar polymerization conditions (P3 in
Table 1), although showed living nature and increase in Mn,GPC
with the conversion, produced polymers of broader PDI.
Indeed we have applied the same methodology for the
polymerization of methyl methacrylate (MMA) in toluene at
100 1C (P4–P6 in Table 1). However, no significant polymeri-
zation was observed using SBLI without the use of CuBr,
which rules out the possibility of self-dissociation of the
ligand–initiator, SBLI (P4 in Table 1). Use of 200 ppm of
CuBr (P5) in MMA polymerization showed good control and
use of 100 ppm of CuBr (P6) yielded reasonably well controlled
PMMA with respect to both Mn,GPC and PDI. Interestingly,
use of SBLI together with the traditional ATRP initiator ethyl
a-bromoisobutyrate (EBiB) showed moderate control in the
polymerization of MMA (P7). Polymerization of MMA using
100 ppm of CuBr in the presence of TALI and TDLI (P8 and
P9 respectively, in Table 1) produced polymers with relatively
higher PDI values. The living nature of the polymerization of
vinyl monomers using SBLI and ppm amounts of copper was
further confirmed by the synthesis of a diblock copolymer by
chain extension reaction (P10 in Table 1 and Fig. S3 in ESIw).
Most interestingly these unimolecular ligand–initiator systems
(ULIS) were able to polymerize acid monomers with o100 ppm
of copper. Polymerization of methacrylic acid (MAA) using
TALI in toluene yielded polymers via dispersion polymerization
(P12 and P13 in Table 2) reaction. The polymers produced were
completely white and were analysed by THF GPC after methyl-
ation reaction using (trimethylsilyl)diazomethane (TMSDM)10
solution in CH3OH–THF mixture. Polymerization in these cases
was uncontrolled and yielded polymers with much higher Mn,GPC
than expected although the PDI values were moderately low.
Solution polymerization of MAA in D2O (P14 in Table 2) using
TALI showed time dependent increase in Mn,GPC with conver-
sion like typical living radical polymerization. However the
Mn,GPC obtained were much higher than the theoretical values.
Polymerization of MAA using TDLI (P17) produced a polymer
with very high conversion with reasonable control with respect to
both molecular weight and PDI value. Methylation of P17
produced completely white PMMA (P17m). Polymerization of
acrylic acid (AA) in the presence of TDLI (P18) produced a very
high molecular weight polymer with a reasonable PDI. Random
copolymerization of St and MAA at a 3 : 1 molar ratio in
toluene using TDLI was completely homogeneous throughout
polymerization and the St-r-MAA copolymer produced was
collected by precipitating from methanol. Methylation of this
copolymer by TMSDM10 yielded St-r-MMA with a St/MMA
molar ratio of 1.71 : 1 (obtained from the 1H NMR spectrum of
the polymer) which is different from the copolymerization of
St and MMA under similar conditions (P11 in Table 1 where
the St/MMA molar ratio in the polymer was 2.67 : 1).
The successful homo- and copolymerization of vinyl monomers
bearing a free carboxylic acid group by ULIS is most likely due to
their unique structure which results in the formation of stable
inner-sphere complexes. From the results presented here, it is clear
that catalyst poisoning does not occur. Copper(I) and copper(II)
complexes exist in different geometries, namely, four-coordinate
tetrahedral and distorted octahedra respectively.7 Thus during the
red–ox transitions of the active–dormant cycles, the ligand cage
has to undergo such structural transformation as well. This
structural change may be more difficult in the polymerization of
c
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Chem. Commun., 2012, 48, 4256–4258 4257

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Table 2 Polymerization of methacrylic acid (MAA) and acrylic acid (AA) using TALI or TDLI and ppm amounts of CuBr
GPC resultsd
Solventa,b (status)
Target DP Temp./1C Time/h Conv.c (%) Mn,Theo
Mn,GPC PDI
c
Polymer code Monomer, ULIS system
P12
P13
P14
MAA,
TALI
MAA,
TALI
MAA,
TALI
Toluene
(precipitate)
Toluene
(precipitate)
D2O
(solution)
50
110
110
100
17
17
62.0
27.8
3600
71 600
94 200
1.81
1.60
600
250
17 200
1
2
5
24
30.4
44.6
72.7
3.7
8100
37 400
42 800
59 900
NA
2.42
2.18
3.41
NA
11 700
18 700
NA
P15
P16
P17
P18
P19e
MAA,
TDLI
MAA,
TDLI
MAA,
TDLI
AA,
Toluene
(solution)
Methanol
(solution)
D2O
(solution)
D2O
(solution)
250
200
250
250
70
80
30
2
18.6
84.4
60.2
NA
NA
NA
1.59
1.73
95
21 700
15 600
32 700
75 400
95
2
TDLI
St + MAA (3 : 1), TDLI Toluene (solution) 187/63
100
2
22
19.1/38.3
67.1/96.2
6700
19 700
21 400
29 700
1.70
1.95
a
(M)AA was polymerized in a 1 : 1 monomer to solvent ratio of mentioned solvent except P15 where a 1 : 1.6 monomer to solvent ratio was used;
b
target DP is equal to monomer to initiator ratio. [CuBr] used were 80–90 ppm (mol molÀ1). Calculated from 1H NMR spectra.
d
c
e
Of corresponding methylated polymer (i.e. P(M)MA). Random copolymerization of St and MAA using monomer mixture.
(M)AA thereby resulting in more of CuI species which could shift
the equilibrium more towards the active state. This may be the
reason for why the polymerization is very fast and not well
controlled nevertheless living. It has been reported that poly-
dispersity for a living polymerization exceeds 1.5 when the rate of
exchange between the active and dormant species is slow.11 The
contribution of viscosity of the medium too could be one of the
reasons for the high PDI observed. The large deviation from
theoretical molecular weight could also be due to poor efficiency
of ULIS. It is worthwhile to note that the amount of CuBr used is
far lower than that of the initiator. This leads to the formation of
complexed and uncomplexed initiators which would exhibit
different reactivity and hence varying efficiency to initiate polymeri-
zation. It has been confirmed by employing 14C-radiolabelled
initiators that in a conventional ATRP process continued initiation
of new chains occur throughout polymerization and some
unreacted initiators are left at the end of the polymerization even
after achieving very high conversion of monomers.12 The end group
fidelity of TDLI promoted synthesis of PMAA (P17) and its
methylated derivatives i.e. PMMA (P17m) was further confirmed
by the presence of aromatic protons corresponding to the ligand
portion of TDLI in their 1H NMR spectra (see Fig. S7 in ESIw).
In summary, we have reported a novel approach using
unimolecular ligand–initiator systems (ULIS) for the ATRP
process which broadens the scope of the existing process by
way of homo- and copolymerizing vinyl monomers including
acidic monomers like (meth)acrylic acid. The polymerizations
were performed using ppm amounts of copper salt. The control
on polymerization was dependent on the structure of ULIS and
also the solvent used for polymerization. As the polymerization
works well in water as well,6 we believe that this system is highly
adaptable for emulsion polymerization in the conventional manner.
The ability to make super absorbent polymers is another attractive
feature of this modified process.6
Notes and references
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This work was supported by the Science and Engineering
Research council of A*STAR (Agency for Science, Technology
and Research), Singapore.
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4258 Chem. Commun., 2012, 48, 4256–4258
This journal is The Royal Society of Chemistry 2012