Design of effective systems for controlled radical polymerization of ...

Macromol. Rapid Commun. 20, 351–355 (1999) 351
Design of effective systems for controlled radical polymerization
of styrene: Application of 4,49-dimethyl and 5,59-dimethyl 2,29-
bipyridine copper(II) complexes
Ulrich S. Schubert*, Georg Hochwimmer, Christian E. Spindler, Oskar Nuyken*
Lehrstuhl für Makromolekulare Stoffe, Technische Universität München, Lichtenbergstr. 4,
D-85747 Garching, Germany
ulrich.schubert@ch.tum.de
(Received: October 29, 1998; revised a : March 4, 1999)
SUMMARY: [Cu(II)(4,49-dimethyl-2,29-bipyridine) 3 ](PF 6 ) 2 (2) and [Cu(II) (5,59-dimethyl-2,29-bipyridine)
3 ](PF 6 ) 2 (4) were used together with aluminium isopropoxide and (1-bromoethyl)benzene in the controlled
radical polymerization of styrene resulting in polystyrenes with predetermined molecular weight and narrow
molecular weight distribution. The received polymers are colorless with a content of copper lower than
210 ppm. The substitution pattern at the bipyridine ligands has a distinct influence on the polymerization.
The rate of polymerization of styrene using 2/[(CH 3 ) 2 CHO] 3 Al/C 6 H 5 CH(CH 3 )Br is two times larger than utilizing
4/[(CH 3 ) 2 CHO] 3 Al/C 6 H 5 CH(CH 3 )Br.
Introduction
Radical polymerization processes are of great importance
in polymer chemistry for the preparation of high molecular
weight polymers 1) . However, deficits in control of
both the obtained molecular weight and the resulting
structure of the polymers are major drawbacks in conventional
radical polymerization. This is mainly due to chain
transfer and termination processes. In the last years it has
been successfully demonstrated that controlled (“living”)
free radical polymerization techniques can be utilized to
prepare polymers of predetermined molecular weights
with narrow polydispersities. Among the promising
investigations in this field are nitroxide-mediated polymerizations
2–4) , ruthenium(II) and cobalt(II)-mediated
polymerizations 5–7) , triazolinyl-based polymerizations 8)
and atom transfer radical polymerization (ATRP) 9–18) .
Especially, ATRP seems to be suitable for the preparation
of polystyrenes and polyacrylates with controlled molecular
weights and narrow molecular weight distributions.
The favored system for this kind of polymerization of
styrene and acrylates contains besides alkyl halides bipyridine
ligands and copper(I) salts 9–15) (sometimes bipyridines
are replaced by 1,10-phenanthroline 19) ). However,
this system has several disadvantages: The resulting polymers
are quite often colored due to the high amount of
copper in these polymers which is difficult to remove.
Combined with the sometimes slow rates of polymerization
this is contradictionary to a potential application in
polymer industry. The present paper describes the first
results of a systematic approach to overcome some of
these problems by application of well-defined metallosupramolecular
complexes with special N-heterocyclic
ligands instead of bipyridine/metal salts mixtures.
Experimental part
Materials
4,49-Dimethyl-2,29-bipyridine (1) was used as received
(Aldrich and Reilly Tar & Chem. Corp). 5,59-Dimethyl-2,29-
bipyridine (3) was synthesized utilizing a novel procedure
starting from 2-amino-5-methylpyridine 20) . The Cu(CH 3 -
COO) 2 6H 2 O was used as received (Aldrich). Styrene was
purified by distillation under reduced pressure after destabilizing
by passing through an alumina column. (1-Bromoethyl)benzene
(Aldrich), aluminium isopropoxide
(Aldrich), 1,4-benzoquinone (Fluka) and acetonitrile (HPLC
grade, Fluka) were used as received.
Instruments
Gel permeation chromatography (GPC) analysis was performed
on a Waters Liquid Chromatograph system using
Shodex GPC K-802S columns and the Waters Differential
Refractometer 410 with chloroform as eluent. Calibration
was conducted with polystyrene standards. UV/VIS measurements
were recorded using a Varian Cary 3.
Synthesis of the supramolecular complexes
[Cu(II)(4,49-dimethyl-2,29-bipyridine) 3 ](PF 6 ) 2 (2): 4,49-Dimethyl-2,29-bipyridine
(1) was suspended in MeOH/H 2 O
(1:1) under nitrogen. A solution of Cu(CH 3 COO) 2 6H 2 Oin
H 2 O was added and the blue colored solution was refluxed
for 6 h. After addition of an excess of a saturated solution of
a
Delayed due to patent application.
Macromol. Rapid Commun. 20, No. 6 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 1999 1022-1336/99/0606–0351$17.50+.50/0

352 U. S. Schubert, G. Hochwimmer, Ch. E. Spindler, O. Nuyken
NH 4 PF 6 in CH 3 OH immediately a blue solid precipitated.
The solid was filtered off, washed with MeOH, H 2 O and
diethylether, dissolved in acetone and precipitated in diethyl
ether. After filtration the solid was dissolved in a small
amount of acetone. By slow diffusion of diethyl ether into
the solution blue crystals of 2 (93%) were obtained.
UV/VIS (CH 3 CN): k max /nm (e/(L N mol –1 N cm –1 )) = 285
(3270).
C 36 H 36 N 6 CuP 2 F 12 62H 2 O (942.2)
Calc. C 45.98 N 8.92 H 4.28
Found C 45.97 N 8.81 H 4.21
[Cu(II)(5,59-dimethyl-2,29-bipyridine) 3 ](PF 6 ) 2 (4): 5,59-Dimethyl-2,29-bipyridine
(3) was dissolved in CH 3 OH under
nitrogen. A solution of Cu(CH 3 COO) 2 6H 2 O in H 2 O was
added and the blue solution was refluxed for 5 h. After addition
of an excess of a saturated solution of NH 4 PF 6 in
CH 3 OH immediately a blue colored solid appeared. The mixture
was refluxed for another 5 min and was then allowed to
cool to room temperature. The remaining solid was filtered
off, washed with MeOH, H 2 O and diethyl ether, dissolved in
acetone and precipitated in diethyl ether. After filtration the
solid was dissolved in a small amount of acetone and crystallized
in diethyl ether to yield 84% of 4 as blue crystals.
UV/VIS (CH 3 CN): k max /nm (e/(L N mol –1 N cm –1 )) = 251
(47830), 299 (39420).
C 36 H 36 N 6 CuP 2 F 12 (906.2)
Calc. C 47.72 N 9.27 H 4.00
Found C 47.68 N 9.29 H 3.96
Polymerizations
A typical procedure was as follows: [(CH 3 ) 2 CHO] 3 Al
(59.64 mg, 0.292 mmol) and styrene (5.32 g, 51 mmol) were
added to a solution of the copper bipyridine complex
(0.073 mmol, 2: 68.78 mg, 4: 66.15 mg) in CH 3 CN (1.6 mL)
under argon. After addition of (1-bromoethyl)benzene
(13.44 mg, 0.073 mmol) the reaction mixture was degassed
by three freeze/pump cycles. Then the mixture was heated to
758C (oil bath) and kept at this temperature during polymerization.
Samples (0.1 mL) were taken in distinct time intervals
and quenched with 1,4-benzoquinone in CH 2 Cl 2 (2 mL,
27 g/L, 0.25 mol/L). The polymer samples were precipitated
in methanol and dried in vacuo at 508C. The conversion was
determined gravimetrically.
Results and discussion
The main focus of our approach in controlled radical
polymerization is the design of new effective systems
which circumvent some of the drawbacks of established
systems. In typical procedures a ratio of 2:1:1:100
(bipyridine:copper(I) salt:alkyl halide:monomer) is
used 9–11) (also a 3:1:1:100 ratio is often applied, see e.g.
ref. 21) ). Copper(I) chloride or copper(I) bromide is dispersed
in monomer and a bipyridine is added separately.
It is assumed that a bipyridine complex is formed in situ.
Fig. 1. Wireframe models of the complexes: left: [Cu(II)(4,49-
dimethyl-2,29-bipyridine) 3 ](PF 6 ) 2 (2); right: [Cu(II)(5,59-
dimethyl-2,29-bipyridine) 3 ](PF 6 ) 2 (4) (MAC Spartan Plus, level
MM2, PF 6 counter ions are omitted)
The resulting polymers are quite often colored, due to a
rather high residual metal concentration in the resulting
polymer 22) . Therefore, it was worthwhile to test welldefined,
already preformed bipyridine metal complexes
for this polymerization. We see an advantage over its in
situ formation by mixing together the components. In our
investigations we have further studied the influence of
the bipyridine substitution pattern (4-, 5- and preliminary
the 6-positions) for the polymerization process.
We first choose two different types of ligands. 4,49-
Dimethyl-2,29-bipyridine (1) is commercially available,
whereas the corresponding 5,59-dimethyl-2,29-bipyridine
(3) was synthesized in high yield using Stille type carbon-carbon
bond forming reaction utilizing organo-tin
intermediates (72% from commercial starting materials
20) ). The copper complexes of the bipyridines 1 and 3
could be obtained by reaction with an appropriate copper(II)
salt yielding blue crystalline materials (2: 93%, 4:
84%, isolated as PF 6 salts). The wireframe models of the
octahedral complexes 2 and 4 are shown in Fig. 1. These
metallo-supramolecular systems were used for the polymerization
of styrene (Scheme 1). The monomer and aluminium
isopropoxide were added to solutions of the complexes
in acetonitrile (1:700 complex to monomer ratio).
After addition of an alkyl halide (e.g. (1-bromoethyl)benzene,
1:1 complex to initiator ratio) the reaction mixtures
were heated to 758C. Samples were taken at different
times and quenched with 1,4-benzoquinone. The role of
the aluminium isopropoxide is yet not clear. We assume
that it acts as Lewis acid lowering the dissociation energy
Scheme 1:

Design of effective systems for controlled radical polymerization of styrene: ... 353
Tab. 1. Molecular weight data for the investigated polystyrenes
in the presence of 2 (aluminium isopropoxide: 0.292 mmol; styrene:
51 mmol; ratio 2: (1-bromoethyl)benzene = 1:1; ratio 2:
monomer = 1:700)
Time
in min
Conversion
in %
M n …GPC†
g=mol
M w …GPC†
g=mol
M — w/M — n
Fig. 2. Dependence of monomer conversion as function of
time in the polymerization of styrene at 758C using 2
([(CH 3 ) 2 CHO] 3 Al: 0.292 mmol; styrene: 51 mmol; ratio 2: (1-
bromoethyl)benzene = 1:1; ratio 2: monomer = 1:700)
140 3.0 2220 5770 2.60 a)
210 4.6 3340 6 010 1.80 a)
295 7.1 5220 7200 1.38 a)
530 13.0 7500 8250 1.10
665 19.4 13220 14410 1.09
1310 34.6 21860 24710 1.13
1585 42.2 21130 23880 1.13
1905 49.1 26120 30820 1.18
2880 69.8 35000 49700 1.42
a)
A second small peak at higher molecular weights was
referred to uncontrolled polymerization of styrene and not
used in the calculation.
Fig. 3. Dependence of molecular weights (f) and polydispersities
(9) as function of monomer conversion in the polymerization
of styrene at 758C using 2 ([(CH 3 ) 2 CHO] 3 Al: 0.292 mmol;
styrene: 51 mmol; ratio 2: (1-bromoethyl)benzene = 1:1; ratio
2: monomer = 1:700)
Fig. 4. GPC curves (chloroform as eluent) for a few samples
obtained during polymerization of styrene in the presence of 2
([(CH 3 ) 2 CHO] 3 Al: 0.292 mmol; styrene: 51 mmol; ratio 2: (1-
bromoethyl)benzene = 1:1; ratio 2: monomer = 1:700); (a)
19.4% conversion, (b) 42.2% conversion, (c) 49.1% conversion)
of the halogen-carbon bond. This assumption is supported
by the observation that a controlled radical polymerization
at 758C is not possible without aluminium isopropoxide
(see also similar results for the polymerization of
MMA with RuCl 2 (PPh 3 ) 3 systems 23) ). Experiments utilizing
higher polymerization temperatures are currently in
progress. Fig. 2 shows the linear dependence of the
monomer conversion as function of time using 2. A complete
monomer conversion can be obtained after approximately
65 h. The dependence of the number average
molecular weights and the corresponding polydispersities
of polystyrene as function of conversion are plotted in
Fig. 3 showing a linear relationship between number
average molecular weight of the polymers and monomer
conversion. The observed polydispersities are rather
broad at low conversions. However, they narrow above
10% conversion and reach a level typical for living systems
(M — w/M — n between 1.1 and 1.4, see Tab. 1). The results
of the GPC investigations for three samples taken at different
times are shown in Fig. 4 ((a): t = 665 min, C =
19.4%; (b): t = 1585 min, C = 42.2%; (c): t = 1905 min,
C = 49.1%). Experiments with much lower complex to
monomer ratio (1:4000) resulted again in a controlled
polymerization with narrow molecular weight distributions.
A very promising result was observed concerning the
color of the polymers and the copper contents. After precipitation
in methanol the obtained polymer samples
were completely colorless. Furthermore, using UV/VIS
spectroscopy the typical p-p* transition band of the complexes
could not be found (Fig. 5, curve a). The absorption
behavior of the polystyrene were similar to polystyrene
obtained by autopolymerization (Fig. 5, curve b).
However, addition of only 20 lg complex to 1 mg poly-

354 U. S. Schubert, G. Hochwimmer, Ch. E. Spindler, O. Nuyken
Fig. 5. UV/VIS spectra: curve (a): polystyrene obtained in the presence of 2; curve (b): polystyrene
obtained from autopolymerization; curve (c) 20 lg 2 added to 1 mg of polystyrene (in
CHCl 3 :CH 3 CN = 5:1). Concentrations: (a) 0.50 mg PS/mL; (b) and (c) 0.46 mg PS/mL
Fig. 6. Dependence of monomer conversion as function of
time in the polymerization of styrene at 758C in the presence of
4 ([(CH 3 ) 2 CHO] 3 Al: 0.292 mmol; styrene: 51 mmol; ratio 4: (1-
bromoethyl)benzene = 1:2; ratio 4: monomer = 1:700)
Fig. 7. Dependence of molecular weights (f) and polydispersities
(9) as function of monomer conversion in the polymerization
of styrene at 758C in the presence of 4 ([(CH 3 ) 2 CHO] 3 Al:
0.292 mmol; styrene: 51 mmol; ratio 4: (1-bromoethyl)benzene
= 1:2; ratio 4: monomer = 1:700)
styrene obtained from autopolymerization gave a significant
p-p* transition band (Fig. 5, curve c). Therefore it
can be concluded, that the content of copper in the sample
is less than 150 ppm. First experiments utilizing AAS
gave a copper content of 210 ppm (compared to 150 ppm
of a polystyrene obtained from autopolymerization). The
aluminium content is lower than 2.5 ppm (flame AAS).
Polymerization of styrene utilizing bipyridine/metal salt
mixtures gave polymers with a copper content of 2000 to
2500 ppm.
Similar linear relationships between molecular weights
of the polystyrenes on conversion and conversion on time
could be observed when 2 was replaced by 5,59-dimethyl-
2,29-bipyridine copper(II) complex 4 in the reaction mixture
(Fig. 6 and 7). However, using 4 the polymerization
rate drops to the half (e.g. 20% conversion after 30 h for
4 compared to 40% conversion after 30 h for 2). The
observed polydispersities vary between 1.2 and 1.4
(Tab. 2). However, in this case narrow molecular weight
distributions could be found already at low conversions.
Further investigations using 6,69-dimethyl-2,29-bipyridine
copper complexes are in progress. First results showed a
very fast polymerization up to 10% conversion with quite
broad polydispersities.

Design of effective systems for controlled radical polymerization of styrene: ... 355
Tab. 2. Molecular weight data for the investigated polystyrenes
in the presence of 4 (aluminium isopropoxide: 0.292 mmol; styrene:
51 mmol; ratio 4: (1-bromoethyl)benzene = 1:2; ratio 4:
monomer = 1:700)
Time
in min
Conclusions
Conversion
in %
M n …GPC†
g=mol
M w …GPC†
g=mol
M — w/M — n
260 4.1 1090 1310 1.22 a)
1170 9.6 2120 2760 1.35 a)
1750 17.1 4210 5890 1.46
2045 23.1 5410 7580 1.45
2690 28.9 5820 7560 1.35
a)
A second small peak at higher molecular weights was
referred to uncontrolled polymerization of styrene and not
used in the calculation.
It could be shown that a mixture of 2,29-bipyridine copper(II)
complexes in conjunction with aluminium isopropoxide
and (1-bromoethyl)benzene is able to polymerize
styrene. The polymerization rate depends strongly on
the substitution pattern of the bipyridine ligands. The
described systems allow a good control of molecular
weight and molecular weight distribution of polystyrene.
However, the mechanism of this reaction and the fundamental
steps are not yet well-understood and therefore
further investigations are necessary. The difference of our
systems over established bipyridine/copper(I)/alkyl halide
systems are the completely colorless products with very
low contents of copper as well as the low reaction temperature.
These findings opens new avenues to develop
effective and useful systems for controlled radical polymerization.
Acknowledgement: The research was supported partly by the
Bayerisches Staatsministerium für Unterricht, Kultus, Wissenschaft
und Kunst and the Fonds der Chemischen Industrie.
We thank Reilly Tar & Chem. Corp. for contributing 1.
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