Use of precipitons for copper removal in atom transfer radical polymerization [1]

Article abstract of DOI:10.1021/ma020155m

The use of precipitons for copper removal in atom transfer radical polymerization (ATRP) was discussed. Nitrogen ligands bearing precipitons were prepared and used to mediate ATRP. The results showed that the precipiton-bound ligands successfully mediated

Full text of DOI:10.1021/ma020155m

Volume 35, Number 13
J une 18, 2002
© Copyright 2002 by the American Chemical Society
Communications to the Editor
easier approach to catalyst removal.5 Zhu and co-
Use of P r ecip iton s for Cop p er Rem ova l in
Atom Tr a n sfer Ra d ica l P olym er iza tion
workers have reported the use of polyethylene-poly-
(ethylene glycol)-bound ligands.6 They also report an
improvement in the use of silica-supported multidentate
amine ligands in ATRP, affording polymers with low
polydispersity, good molecular weight control, and high
conversion for methyl methacrylate (MMA) reactions.7
Use of silica supported ligands with a poly(ethylene
glycol) spacer has been investigated by Zhu et al.,8
demonstrating that the length of the spacer affects the
rate of polymerization and molecular weight control.
Mica l E. Hon igfor t a n d Willia m J . Br itta in *
Department of Polymer Science, University of Akron,
Akron, Ohio 44325
Tod d Bosa n a c a n d Cr a ig S. Wilcox*
Department of Chemistry and The Combinatorial
Chemistry Center, University of Pittsburgh,
Pittsburgh, Pennsylvania 15260
Here we report an investigation using precipitons for
copper catalyst removal in ATRP. Precipitons were
developed to provide a convenient method for isolating
solutes from homogeneous reaction media. These isomer-
izable compounds are attached to a reactant and after
a reaction is complete they can be isomerized to cause
precipitation of the attached product.9-11 The cis-form
of the stilbene is soluble whereas the trans-form is
insoluble in common organic solvents. The precipitated
product can be isolated by filtration or centrifugation.
To test the usefulness of this strategy when applied to
the removal of catalysts in ATRP, nitrogen ligands
bearing precipitons were prepared and used to mediate
ATRP. This strategy enables homogeneous reaction
conditions to be combined with a facile method for
removal of the copper catalyst. We expected that after
the polymerization was complete, the polymer solution
could be exposed to UV light to induce precipitation of
the precipiton ligand/CuBr complex. The ATRP ligands
3 and 5 were synthesized starting from the bis-OH 1
and isocyanate 4 precipitons, respectively.
Received J anuary 29, 2002
Revised Manuscript Received May 3, 2002
Atom transfer radical polymerization (ATRP) methods
have been improved in recent years and provide a
promising set of tools for the controlled synthesis of
polymers.1 One drawback of ATRP, arising when stan-
dard homogeneous catalysts are used, is contamination
of the polymer by the ligand/metal complex. The colored
catalyst complex is typically removed by passing through
a column of alumina, followed by precipitation of the
polymer. Post-polymerization processing to remove cata-
lyst increases the cost of polymer production. Methods
that generate a clean polymer solution after reaction
and allow for catalyst recycling are desirable.
Surface-immobilized catalysts offer a convenient
method for catalyst recovery and have been applied to
ATRP.2-4 The catalyst is removed by simple filtration
or decantation, but comparison of these methods to
traditional ATRP conditions shows the surface-im-
mobilized catalysts often provide products with higher
molecular weights than predicted and broader molecular
weight distributions, and sometimes have longer reac-
tion times. It has been hypothesized that the sluggish
reaction rates seen with surface-immobilized catalysts
may be due to retarded diffusion of the polymer chain
to the silica or polystyrene bead surface. We have
investigated polyethylene-bound ATRP ligands because
this strategy allows polymerization to be conducted
under homogeneous reaction conditions and offers an
Bis-OH precipiton 1 was prepared as described in the
literature7 and was reacted with acryloyl chloride. The
resultant bis-acrylate underwent a Michael reaction
with N,N,N′,N′-tetraethyldiethylenetriamine (TEDETA)
to afford ligand 3 in 65% yield (Scheme 1). The iso-
cyanate functional precipiton 4 was prepared as de-
scribed in the literature.12 Ligand 5 was synthesized in
a one-step reaction of compound 4 with TEDETA
(Scheme 1) to afford 5 quantitatively.
10.1021/ma020155m CCC: $22.00 © 2002 American Chemical Society
Published on Web 05/17/2002

4850 Communications to the Editor
Macromolecules, Vol. 35, No. 13, 2002
Sch em e 1. Syn th esis of Liga n d 3 Sta r tin g fr om Diol 1
a n d Syn th esis of Liga n d 5 fr om Isocya n a te 4
F igu r e 1. Mn vs percent monomer conversion for ATRP of
MMA.
Ta ble 1. Resu lts for P olym er iza tion of MMA a t 90 °C in
50% (v/v) Tolu en e Usin g Liga n d s 3 a n d 5 ([MMA]:[Eth yl
2-Br om oisobu tyr a te]:[Liga n d ] ) 100:1:1.5)
ligand time (min) conv (%) Mn(exptl) Mn(theor) PDI
3
3
3
3
3
3
5
5
5
5
5
5
120
300
480
600
660
800
120
240
360
510
600
720
40
54
74
78
91
93
15
27
51
68
78
90
4600
5800
7700
8100
10 400
13 700
4000
5400
7400
7800
9100
9300
1500
2700
5100
6800
7800
9000
1.45
1.45
1.44
1.42
1.42
1.40
F igu r e 2. Kinetic plot for ATRP of MMA using precipiton
ligands.
3200
6100
7500
8200
10 100
1.22
1.20
1.20
1.19
1.19
reuse or recycle the ligand in this catalyst system is an
undesirable feature. In other supported catalyst sys-
tems, the ligand can be recovered and used again, but
the precipitons in our experiments cannot be recycled.
If ongoing efforts to develop recyclable precipitons are
successful, this system will provide a general and
economically attractive way to remove metals from
ATRP systems.
Ligands 3 and 5 were successfully used for ATRP of
MMA mediated by CuBr, using toluene as the solvent
and ethyl 2-bromoisobutyrate as the initiator. Upon
completion of the polymerization, the solution was
cooled and exposed to UV radiation for 2 h. The
precipiton ligand precipitated and remained complexed
with the Cu catalyst. The precipitated product can be
isolated by decantation, filtration, or centrifugation.
Copper content of the polymer solution was determined
by UV spectroscopy and indicated no detectable copper
based on the lack of absorbance at 680 nm. ICP analysis
for copper content indicates <1% of original copper in
the PMMA obtained using both ligands 3 and 5. The
PMMA from this reaction required no purification other
than simple decantation.
Compared to the various heterogeneous copper re-
moval techniques, this system offered somewhat better
control of molecular weight and molecular weight
distribution in reasonable reaction times. The results
are summarized in Table 1 for the reactions using
ligands 3 and 5. The plot of Mn vs monomer conversion
(Figure 2) shows that Mn(exptl) is in good agreement
with Mn(theor) until high conversion. At high conver-
sions, Mn(exptl) is greater than expected, indicating
possible radical-radical termination. The reaction fol-
lowed pseudo-first-order kinetics (Figure 2) and achieved
90-93% conversion in 12 h while polydispersity nar-
rowed over the course of the reaction. Better control over
polydispersity was obtained using ligand 5.
Exp er im en ta l Section . Ma ter ia ls. Reagents were
purchased from Aldrich and used as received unless
otherwise noted. N,N,N′,N′-Tetraethyldiethylenetri-
amine (TEDETA, 90%) and acryloyl chloride (96%) were
vacuum distilled. Methyl methacrylate (MMA) was
passed through a basic alumina column then vacuum
distilled from CaH2. CuBr (98%) was purified by the
method described by Keller and Wycoff.13
1
Ch a r a cter iza tion . H NMR spectra were recorded
on a Varian Gemini-300 spectrometer. Molecular weight
analysis was performed by gel permeation chromatog-
raphy (GPC) using a Waters 501 pump, guard column,
Waters HR2 and HR4 Styragel columns, a Waters 410
differential refractometer, and a Viscotek T60A dual
light scattering and viscosity detector. The eluent was
THF and flow rate was 1.0 mL/min. Mn and Mw were
determined using universal calibration. UV spectroscopy
was performed on a HP 8453 UV-visible spectropho-
tometer. ICP analysis for copper was performed by
Galbraith Laboratories.
ATRP P r oced u r e. A Schlenk flask was charged with
CuBr (1 equiv, 0.17 mmol, 0.02 g) and precipiton ligand
(for ligand 3, 0.5 equiv, 0.085 mmol, 0.075 g; for ligand
5, 1 equiv, 0.17 mmol, 0.10 g) and degassed with three
vacuum/argon cycles. Via syringe, deoxygenated toluene
(2 mL) and MMA (100 equiv, 0.017 mol, 1.7 g) were
added. The solution was heated in an oil bath at 90 °C.
Ethyl 2-bromoisobutyrate (1 equiv, 0.17 mmol, 25 µL)
was added and the solution heated for 12 h. Aliquots
were removed to determine conversion by 1H NMR and
The precipiton-bound ligands successfully mediated
the ATRP of MMA and allowed for easy and fast
removal of the copper catalyst by exposure of the
solution to a UV light source. The present inability to

Macromolecules, Vol. 35, No. 13, 2002
Communications to the Editor 4851
molecular weight. After cooling, the polymer solution
was transferred to a quartz tube and irradiated with
ultraviolet light from a xenon arc lamp (UV output )
2.8 W) for 2 h. The polymer solution was decanted from
the solid ligand/CuBr complex, collected by removal of
solvent in vacuo and analyzed by UV spectroscopy.
Copper content of the polymer without further purifica-
tion was determined by ICP analysis.
NMR (CDCl3): δ 0.9-1.0 (t, 12 H), 2.4-2.6 (m, 12 H),
3.3 (t, 4H), 4.3 (d, 2H), 6.6 (s, 2H), 7.0-7.6 (m, 17 H),
8.4 (s, 1 H). 13C NMR (CDCl3): δ 140.6, 140.4, 139.8,
139.0, 136.6, 130.2, 129.9, 129.6, 128.9, 127.4, 127.3,
127.2, 127.0, 126.9, 126.8, 52.9, 52.5, 48.0, 47.3, 12.0.
Ack n ow led gm en t. The financial support of ICI is
gratefully acknowledged (M.E.H. and W.J .B.).
Syn th esis a n d NMR Da ta for P r ecip iton Acr y-
la te 2. Bis-OH precipiton 1 was prepared as described
in the literature.7 The precipiton (1 equiv) was dissolved
in anhydrous THF and cooled to 0 °C. Triethylamine
(1.5 equiv) was added followed by dropwise addition of
acryloyl chloride (1.5 equiv), and the mixture was stirred
overnight. Volatiles were removed in vacuo and the
precipiton ligand was washed 10 times with water,
collected by centrifugation, and dried in vacuo to afford
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental details and characterization data and a figure showing
a UV spectral comparison of the PMMA/toluene solution
compared with a Cu-containing standard solution.. This mate-
rial is available free of charge via the Internet at http://
pubs.acs.org.
Refer en ces a n d Notes
1
(1) Matyjaszewski, K. Controlled Radical Polymerization; Ameri-
can Chemical Society: Washington, DC, 1998; Vol. 685.
(2) Kickelbick, G.; Paik, H.-j.; Matyjaszewski, K. Macromol-
ecules 1999, 32, 2941-2947.
(3) Haddleton, D. M.; Kukulj, D.; Radigue, A. P. Chem. Com-
mun. 1999, 99.
(4) Haddleton, D. M.; Duncalf, D. J .; Kukulj, D.; Radigue, A.
P. Macromolecules 1999, 32, 4769.
(5) Liou, S.; Rademacher, J . T.; Malaba, D.; Pallack, M. E.;
Brittain, W. J . Macromolecules 2000, 33, 4295.
(6) Shen, Y.; Zhu, S.; Zeng, F.; Pelton, R. H. Macromolecules
2000, 33, 5427.
(7) Shen, Y.; Zhu, S.; Pelton, R. H. Macromolecules 2001, 34,
3182.
(8) Shen, Y.; Zhu, S.; Pelton, R. H. Macromolecules 2001, 34,
5812.
(9) Bosanac, T.; Yang, J .; Wilcox, C. S. Angew. Chem., Int. Ed.
2001, 40, 1875.
(10) Bosanac, T.; Wilcox, C. S. Chem. Commun. 2001, 1618.
(11) Bosanac, T.; Wilcox, C. S. Tetrahedron Lett. 2001, 4309.
(12) Bosanac, T.; Wilcox, C. S. J . Am. Chem. Soc. 2002, 124, 4194.
(13) Keller, R. N.; Wycoff, H. D. Inorg Synth. 1947, 2, 1.
yellow solid (65%). H NMR (CDCl3): δ 5.2 (s, 4 H), 5.9
(d, 2 H), 6.1-6.4 (q, 2 H), 6.5 (d, 2 H), 6.6 (s, 4 H), 7.1
(s, 4 H), 7.2 (s, 8 H). 13C NMR (CDCl3): δ 164.1, 137.5,
136.2, 134.9, 130.5, 129.2, 129.0, 128.5, 126.9.
Syn th esis a n d NMR Da ta for P r ecip iton Liga n d
3. Bis-acrylate precipiton 2 was dissolved in excess
TEDETA and stirred overnight. TEDETA was removed
via Kugelrohr distillation to afford a brownish solid
(quantitative). 1H NMR(CDCl3): δ 0.9-1.0 (t, 24 H),
2.4-2.6 (m, 36 H), 2.8 (t, 4 H), 5.1 (s, 4 H), 6.6 (s, 4 H),
7.1 (s, 4 H), 7.2 (s, 12 H). 13C NMR (CDCl3): δ 137.5,
136.2, 134.9, 130.5, 129.2, 128.5, 126.9, 52.9, 52.5, 48.0,
47.3, 12.0.
Syn th esis a n d NMR Da ta for P r ecip iton Liga n d
5. Isocyanate 4 was placed in a Schlenk flask and
purged with argon. THF (anhydrous) was added to
dissolve the solids. TEDETA was added dropwise and
the reaction was stirred overnight. THF was removed
in vacuo, and TEDETA was removed via Kugelrohr
distillation to afford a yellow powder (quantitative). 1H
MA020155M