Controlled/“LiVing” Radical Polymerization
J. Am. Chem. Soc., Vol. 119, No. 4, 1997 679
The preequilibrium constants for both the 1-PEBr/CuBr- and
-PECl/CuCl-initiated ATRPs of styrene at 110 °C were
1.10. The polymerizations exhibited an increase in molecular
weight in direct proportion to the ratio of the monomer
consumed to the initial initiator concentration and also exhibited
internal first-order kinetics with respect to monomer concentra-
tion. The kinetically optimum ratio of ligand-to-copper(I) halide
for these polymerizations was found to be 2:1, which tentatively
indicates that the coordination sphere of the active copper(I)
center contains two bipyridine ligands. The exclusive role for
this copper(I) complex in ATRP is atom transfer, since at typical
1
calculated using eq 9 and kinetic data from polymerizations in
which 10 mol % of CuX2/2L (X ) Br, Cl) was added initially.
In these polymerizations, the amount of Cu(II) generated from
the persistent radical effect would be small compared to the
large added concentration of Cu(II) and therefore [Cu(II)]t ≈
[
Cu(II)]0. Thus, for 50% (v/v) styrene polymerizations per-
formed using diphenyl ether solvent and with 1 mol % initiator
-
7
and 1 mol % Cu(I)/2L, the apparent rate constants were 2.5 ×
concentrations that occur for these polymerizations (≈10 -
-
5
-1
-5 -1
-8
1
0
s
(X ) Br) and 1.4 × 10
s
(X ) Cl), and the
10 M), polymeric radicals were found not to react with the
-8
calculated equilibrium constants were 3.9 × 10 (X ) Br) and
copper(I) center in any manner that enhanced or detracted from
the observed control. ATRP also exhibited first-order kinetics
with respect to both initiator and copper(I) halide concentration;
however, the polymerization kinetics were not simple inverse
first-order with respect to the initial copper(II) halide concentra-
tion. The latter observation was found to be due to the persistent
radical effect, which resulted in an increase in copper(II)
concentration during the initial stages of the polymerization.
This phenomenon also has the effect of regulating the polym-
erization by ensuring that the rate of radical combination and/
or disproportionation is sufficiently less than the rate of
propagation. The temperature dependence of the rate of ATRP
was measured, and the apparent activation enthalpies were found
-
8
2
.1 × 10 (X ) Cl). These values correspond to free energies
-1
of equilibrium (∆G°) at 110 °C of 13.0 kcal mol (X ) Br)
-
1
and 13.5 kcal mol
(X ) Cl). The differences in the
equilibrium constants and free energies of polymerization for
the bromide- and chloride-mediated ATRPs of styrene reflect
the smaller difference in bond strengths between the carbon-
bromine and copper(II)-bromine bonds relative to the carbon-
chlorine and copper(II)-chlorine bonds.
With the free energies and enthalpies of the preequilibrium
thus determined, we calculated the changes of entropy at
equilibrium at 110 °C for both the bromide-mediated, ∆S° )
-
1
-1
-
22 cal mol K , and the chloride-mediated, ∆S° ) -20 cal
-1
-1
q
-1
mol K , ATRPs of styrene. Assuming that these calculations
yielded the correct order of magnitude for ∆S°, we find these
strongly negative entropies of equilibrium quite surprising,
because they are more typical of a 2:1 equilibria involving the
loss of three degrees of translational freedom. A possible
explanation for this large entropy change is the formation of a
caged radical pair (Cu(II) and radical); however, this explanation
is contradicted by the fact that the regio- and stereochemistries
to be ∆H app ) 11.9 kcal mol for the bromide-mediated ATRP
q
-1
of styrene and ∆H app ) 13.4 kcal mol for the chloride-
mediated ATRP of styrene. Estimates of the enthalpies and
entropies of equilibrium for the preequilibrium step were
calculated at ∆H° ) 4.8 and ∆H° ) 6.3 kcal mol for the
ATRP of styrene initiated by 1-PEBr and 1-PECl, respectively,
-
1
-
1
-1
and ∆S° ) -22 and ∆S° ) -20 cal mol
of styrene initiated by 1-PEBr and 1-PECl, respectively.
K
for the ATRP
of polymerization for ATRP are identical to those typically
found for free-radical polymerizations.7
,13
Furthermore, the
Experimental Section
proportion of caged radicals should be small relative to free
radicals if the cage entry and exit rates are diffusion controlled
2
Materials. Styrene was stirred over CaH overnight and vacuum
distilled before use. CuBr (98%, Aldrich) and CuCl (98%, Aldrich)
9
-1 -1
9
-1
(
k1 ) 10 M
s
and k-1 ) 10 s ). In ATRP there is no
27
were purified according to the procedure of Keller and Wycoff. The
physical or chemical reason to expect otherwise. The concen-
tration of growing radicals is approximately 10 M, and the
concentration of Cu(II) species is less than 10 M. So, the
concentration of caged radicals should be less than 10
initiators, 1-phenylethyl bromide and 1-phenylethyl chloride, were
-
8
distilled from CaSO before use. Diphenyl ether (ACROS) was purged
4
-
2
with argon for 15 min before use. Acetonitrile (CH
over anhydrous CuSO for 24 h and then distilled onto P
CH CN was stirred over the P 10 for 24 h, after which time it was
3
CN) was stirred
-
10
M,
4
4
O10. The
3
4
O
which is less than 1% of the free radical concentration.
Therefore, propagation should occur predominantly via free
radicals.
distilled and stored under argon. Tetrahydrofuran (THF) and toluene
were distilled from Na/benzophenone before use. Diisopropylamine
was distilled from CaH before use. dTbipy, 4,4′-di-tert-butyl-2,2′-
2
An alternative explanation is that there is a large loss of
conformational motion about the metal center upon atom
bipyridine, was prepared according to the procedure of Hadda and
28
Bozec. Unless specified, all other reagents were purchased from
+
-
transfer. For the copper(I/II) pair, [(dNbipy)2Cu] PF6 and
[
commercial sources and used without further purification.
General Procedures and Characterizations. Monomer conversion
was determined by GC using THF or diphenyl ether (when present) as
an internal standard. Molecular weights and molecular weight distribu-
tions were measured using a Waters 712 WISP autosampler and the
following Phenogel GPC columns: guard, linear, 1000 Å and 100 Å.
Molecular weights were calibrated using polystyrene standards. Ex-
periments requiring an inert atmosphere were performed using standard
Schlenk and drybox techniques.
(dNbipy)2CuBr]+ PF6 , the bipy ligands in the copper(I)
-
complex show a great range of ligand motion about the metal
center, as evidenced by the large variation in the ligand dihedral
1
7
angle seen in the crystal structures of similar compounds. In
the copper(II) complex, the bromide ligand occupies a large
volume of the ligand sphere,26 and, therefore, the range of
motions available to the bipy ligands is reduced. We are
currently investigating the origin of the equilibrium entropy
magnitude and possibility of variation of structures and activities
of Cu(I) and Cu(II) species with temperature.
4
,4′-Di-n-heptyl-2,2′-bipyridine [dHbipy].29 To a stirred solution
of dry THF (70 mL) and diisopropylamine (7.36 mL, 52.1 mmol) under
Ar at -78 °C was added dropwise n-butyllithium (2.3 M in THF, 21.6
mL, 49.6 mmol). After 15 min, the solution was warmed to 0 °C and
allowed to stir for 15 min. The mixture was cooled to -78 °C again,
and a solution of 4,4′-dimethyl-2,2′-bipyridine (4.00 g, 21.7 mmol) in
dry THF (120 mL) was added slowly via cannula. After 3 h,
Conclusions
The homogeneous atom transfer radical polymerization
(ATRP) of styrene using solubilizing 4,4′-dialkyl substituted
2
,2′-bipyridines yielded well-defined polymers with Mw/Mn e
(27) Keller, R. N.; Wycoff, H. D. Inorg. Synth. 1946, 2, 1.
(28) Hadda, T. B.; Bozec, H. L. Polyhedron 1988, 7, 575.
(
26) For the crystal structure of several halide derivatives, see: Tyagi,
(29) This synthesis is a modification of the following procedure: Kramer,
R.; Lehn, J. M.; Cian, A. D.; Fisher, J. Angew. Chem., Int. Ed. Engl. 1993,
32, 703.
S.; Hathaway, B. J.; Kremer, S.; Stratemeier, H.; Reinen, D. J. Chem. Soc.,
Dalton Trans. 1984, 2087.