Macromolecules
1995,28,
7901-7910
7901
Controlled/"Living" Radical Polymerization. Halogen Atom
Transfer Radical Polymerization Promoted
by
a Cu(I)/Cu(
11)
Redox Process
Jin-Shan
Wang
and Krzysztof Matyjaszewski*
Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue,
Pittsburgh, Pennsylvania 15213
Received May 2, 1995; Revised Manuscript Received August 14, 1995@
ABSTRACT:
An
extension
of
atom transfer radical addition, ATRA, to atom transfer radical polymeri-
zation, ATRP, provided
a
new and efficient way
to
conduct controlled/"living" radical polymerization. By
using a simple alkyl halide, R-X (X
=
C1 and Br), as an initiator and
a
transition metal species complexed
by suitable ligand(s),
Mtfl/LX,
e.g., CuX/2,2'-bipyridine, as a catalyst, ATRP of vinyl monomers such as
styrenes and (meth)acrylates proceeded in a living fashion, yielding polymers with degrees of polymer-
ization predetermined by
A[Ml/[Ilo
up to
M,
RZ
lo5
and low polydispersities,
1.1
<
MJM,
<
1.5. The
participation of free radical intermediates was supported by analysis
of
the end groups and the
stereochemistry of the polymerization. The general principle and the mechanism of ATRP are elucidated.
Various factors affecting the ATRP process are discussed.
Introduction
Atom transfer radical addition, ATRA, is a well-
known method for carbon-carbon bond formation in
organic synthesis.
Two
types of atom transfer methods
have been developed. One of them is also called atom
abstraction or homolytic substitution,2 in which a uni-
valent atom, typically a halogen, or a group, such as
SAr,
SeAr, is transferred from a neutral molecule to a
radical to form a new o-bond and a new radical. In this
respect, the use
of
an iodine atom or a SePh group
was successful, due
to
the presence of weak C-I and
C-SePh bonds toward the reactive radicah2 Indeed,
we have recently demonstrated that alkyl iodides may
also induce a degenerative transfer process in radical
polymerization, leading
to
a controlled radical polym-
erization of several alkenes. This is consistent with the
fact that alkyl iodides with groups stabilizing radicals
can undergo a fast transfer in the initiation step and
degenerative
transfer in the propagation step.3
Another atom transfer method is promoted by a
transition metal
specie^.^-^
In these reactions, the
catalytic amount
of
transition metal compound acts as
a carrier
of
the halogen atom in a redox process, Scheme
1.
Initially, the transition metal species, Mtn, abstracts
the halogen atom X from the organic halide, R-X,
to
form the oxidized species, Mtn+lX, and the carbon-
centered radical
R.
In the subsequent step, the radical,
R',
reacts with alkene,
M,
with the formation
of
the
intermediate radical species, R-M'. The reaction be-
tween
Mtn+lX
and R-M results in the target product,
R-M-X, and regenerates the reduced transition metal
species,
Mtn,
which further reacts with R-X and pro-
motes a new redox cycle.
The high efficiency of the transition metal catalyzed
atom transfer reaction in producing the target product,
R-M-X, in good to excellent yields, often
suggests that the presence
of
such a Mtn/Mtn+l redox
process can effectively induce a low concentration
of
free
radicals, resulting in less significant termination reac-
tions between radica1s.l If polymeric halides, R-Mi-
X,
are reactive enough toward
Mtn
and the monomer is
in excess, a number of atom transfer radical additions,
@
Abstract published in
Advance
ACS
Abstracts,
October
15,
1995.
0024-9297/95/2228-790 1$09.00/0
Scheme
1.
Atom Transfer Radical Addition, ATRA6@
I
R'
k.
Scheme
2.
From ATRAs.e to Atom Transfer Radical
Polymerization, ATRPlO
i.e., a possible "living"/controlled radical polymerization,
may occur, Scheme
2.
In a preliminary communication,1° we reported that,
using 1-phenylethyl chloride, l-(PE)Cl, as an initiator,
CuCl as a catalyst, and 2,2'-bipyridine, bpy, as a com-
plexing ligand, a living radical bulk polymerization
of
styrene at 130
"C
yielded polymers with molecular
weights predetermined by the ratio A[Ml/[Ilo up to
Mn
x
lo5
and with molecular weight distribution,
Mw/Mn
<
1.5, narrower than in conventional radical systems,
which at high conversion are
M,IM,
>
2.
By analogy
with ATRA, we called this process
atom transfer radical
polymerization,
ATRP,1° which describes
the
involve-
ment of the
atom transfer
pathway and the
radical
intermediates.
In this paper, we report that a number of com-
mercially available alkyl halides, R-X, combined with
Cu'Xhpy,
X
=
C1
and Br, can be used as eacient
initiating systems for the ATRP
of
styrene and (methl-
acrylates. The effects of various parameters on ATRP
are also discussed.
0
1995 American Chemical Society
7902
Wang and Matyjaszewski
Table
1.
ATRP
of
Styrene
and
Various
(Meth)acrylates
Initiated
with
RX/CuX/bpy"
Conv.
Macromolecules,
Vol. 28,
No.
23,
1995
St
1IPE)CVCuCl
130
120000
110500
1.45
l-(PE)CVCuCl
100
97000
93300
1.50
l-(PE)Br/CuBr
80
8300
8000
1.25
l-(PE)Br/CuBr
110
8500
8750
1.10
a,a'-DBr-xylene/CuBr
110
12
500
12
000
1.12
MA
2-(EPN)CVCuCl
130
30500
31000
1.40
2-(EPN)Br/CuBr
80
19100 21500
1.25
2-(MPN)Br/CuBr
100
27500 29000
1.15
a,a'-DBr-xylene/CuBr
100
29
500
31
000
1.25
BuAd
2-(MPN)Br/CuBr
130
15000
13500
1.50
MMAd
2-(EiB)Br/CuBr
100
10000
9800
1.40
a
Molar
ratio
of
WCUXmpy,
UU3,
monomer
conversions,
85-
100%.
Abbreviations:
l-(PE)Cl, l-phenylethyl
chloride;
l-(PE)Br,
l-phenylethyl
bromide;
2-(EPN)C1,2-ethyl
chloropropionate;
2-(EP-
N)Br,
2-ethyl
bromopropionate;
2-(MPN)Br, 2-methyl
bromopro-
pionate;
a,a'-DBr-xylene,
a,a'-dibromoxylene;
2-(EiB)Br,
2-ethyl
bromoisobutyrate.
Calculated
according
to
eq
1.
In
EAc
solution,
50%
in
volume.
0.8
0.6
i
0
u
0.4
0.2
0
0
20
40
60
80
100
120
1,
rnln.
Figure
1.
Kinetics
of
the
bulk polymerization
of
methyl
acrylate at 130
"C:
[MA10
=
11.1
M;
[l-(PE)Cl]o
=
[CuClIo
=
0.1
M;
bpylo
=
0.3
M.
Results
Atom Transfer Radical Polymerization of Sty-
rene and (Methlacrylates Initiated with Alkyl
Halide,
R-X,
and in the Presence
of
CuX
Com-
plexed by 2,8'-Bipyridine.
As reported previously,
lo
using 1-PEC1 as an initiator,
1
molar equiv of Cu'C1
as
a catalyst, and
3
molar equiv of 2,2'-bipyridine, bpy, as
a ligand (both relative to l-(PECl), the ATRP of styrene,
St,
provides controlled polymerization at 130 "C. More-
over, the desired block copolymers of PSt-b- PMA have
also been successfully prepared using the same tech-
nique.1°
Similarly, using various R-WCuXmpy
(l/l/3)
initiat-
ing systems, the atom transfer radical polymerization
of styrene and various (methlacrylates
at
different
temperatures also afforded the polymers with the pre-
determined molecular weights up to
Mn
=Z
lo5
and
polydispersities
as
low
as
1.10, Table
1.
Figure
1
presents the kinetics of the bulk polymeri-
zation of methyl acrylate,
MA,
at
130 "C initiated by
l-(PE)Cl in the presence
of
CulC1
(1
equiv) and bpy
(3
equiv). The straight semilogarithmic kinetic plot of ln-
([Mld[MI) vs time,
t,
indicates that the concentration
of
growing radicals is constant.
Moreover, the experimental molecular weight,
M*,sEc,
increases with monomer conversion, Figure
2,
and
matches the theoretical one,
Mn,th,
Figure 3, calculated
from eq 1, where A[Ml, [R-Xlo, and
(MW)o
represent
Mn
=
(A[Ml/[R-Xlo)(MW)o
(1)
the concentration of consumed monomer
MA,
the initial
90%
/,/
\
':.Eo
n
I
I
I
I
I
1
10'
10'
10'
mn
Figure
2.
Evolution
of
molecular weight,
M,,
and molecular
weight distribution,
MJM,,,
with monomer conversion
for
the
bull; polymerization
of
methyl acrylate at
130
"C:
[MAlO
=
11.1
M;
[l-(PE)Cl]o
=
[C~Cllo
=
0.038
M;
[bpylo
=
0.11
M.
2.5
10'
2
[
2
10'
1.5
10'
1 10'
3=
5000
1
.a
1.6
s=
-
.=
1.4
1.2
0
0.2
0.4
0.6
0.8
1
Figure
3.
Molecular weight,
M,,
and
molecular weight
distribution,
MJM,,
dependence
on
monomer
conversion
for
the
bulk
polymerization
of
methyl
acrylate
at
130
"C:
[MA10
concentration of l-(PE)Cl, and the molecular weight of
MA,
respectively. This provides evidence that l-(PE)-
C1 acts as an efficient initiator and the number of active
chains remains constant during the polymerization.
Both of these results suggest a living process of ATRP
of
MA
with fast initiation and negligible irreversible
transfer and termination reactions.
Furthermore,
a
series of bulk ATRP of
MA
have been
carried out
at
130 "C, using various monomer/initiator
molar ratios, [MAld[l-(PE)Cllo, and a constant com-
plexing
ligandcatalystlinitiator
molar ratio of 3/1/1.
Figure 4 shows the correlation of the experimental
molecular weights,
Mn,s~c,
with the theoretical ones,
Mn,th.,
calculated by eq
1.
A
linear plot
is
obtained in a
molecular weight range from 1.5
x
lo3
to 1.35
x
lo5.
The slope of the straight line is
0.95,
indicating a high
initiator efficiency and small contribution of irreversible
transfer, supporting a living process
of
MA
polymeri-
zation initiated with l-(PE)CVCuChpy.
End
Group
of
Polymers Obtained by Atom
Transfer Radical Polymerization.
The structure of
the chain ends of low molecular weight poly (methyl
acrylate),
PMA,
synthesized by the ATRP technique was
analyzed by
lH
NMR spectroscopy. Figure
5
presents
the
'H
NMR spectra of PMA which was prepared
at
130
"C
using l-phenylethyl chloride as an initiator and in
the presence of
1
molar equiv of CuCl and 3 molar equiv
of bpy. The broad triplet at ca. 4.2 ppm is assigned to
the -CH(COOMe)Cl end group
e.
Another two broad
bands at 7.1 and 7.4 ppm in Figure
5
represent the end
group
a.
Methyl group fresonates at ca.
1.15
ppm.
conv.
=
11.1
M;
[l-(PE)Cl
IO
=
[C~Cllo
=
0.038
M;
[bpylo
=
0.11
M.
Macromolecules,
Vol.
28,
No.
23,
1995
12 10'
1
10'
Controlled/"Living" Radical Polymerization
7903
Figure 6 compares the
13C
NMR spectra of the C=O
group and the quaternary carbon group
of
PMMA
prepared at 100
"C
using methyl 2-bromoisobutyrate/
CuBrhpy (1/1/3) as the initiating system, Figure 6A,
and a classic radical initiator, AIBN, Figure 6B, respec-
tively. Both spectra are almost identical. Indeed, up
to
the pentad sequence, PMMA prepared using AIBN
or BPO and various ATRP initiator systems have the
same composition within experimental error, Table 2.
The stereochemistry for ATRP of MMA also appears
to
be consistent with a Bernoullian process as indicated
by
p
-
1.
These results indicate the presence
of
the
same type of active species in the CuIX catalyzed
polymerization and the classic free radical polymeriza-
tion. The similarities in stereochemistry and regio-
chemistry in the AIBNBu3SnH mediated radical
cyclizations and Cu(1) catalyzed chlorine transfer
cy-
clizations have already been reported by several groups
as evidence for radical
intermediate^.^,^
Effect of the Structure of Alkyl Halide,
R-X,
on
Atom Transfer Radical Polymerization.
Table
3
reports the data for the ATRP of styrene at 130
"C
using
various commercially available alkyl chlorides, R-C1,
Cu'C1
(1
molar equiv), and bpy
(3
molar equiv) as
initiator, catalyst, and ligand, respectively. Alkyl chlo-
rides with either inductive or resonance stabilizing
substituents are efficient mono-
or
bifunctional initiators
and lead to polymers with narrow molecular weight
distribution, Le.,
M,IMn
-
1.25-1.5.
In contrast, such simple alkyl chlorides as butyl
chloride, C4HgC1, and dichloromethane, CHzClz, are less
efficient, giving uncontrolled polymers with much higher
molecular weights than expected and broader molecular
weight distributions. These results are very similar to
that obtained without any alkyl chlorides under similar
conditions, Table
3.
This indicates very poor efficiency
of
C4H&1 and CH2Clz at the initiating step
of
ATRP
of
St.
The results shown in Table
3
may be correlated with
the bond strength of carbon-halide, i.e., bond dissocia-
tion energy, BDE, in alkyl chlorides. For alkyl chlorides
with a high BDE, such as in the case
of
C4HgC1 and CHz-
C12,14
the chloride atom transfer from R-C1
to
CUT1
-1
AZ2
2
1.4 10'1
I
24
8
810'
Y
6
10'
4
10'
2
10'
1.8
sz
.
1.6
2'
1.4
1.2
11
0
2
lo4
4
10'
6
10'
8
10'
1
lo5
1.2
10' 1.4
10'
%In
Figure
4.
Comparison
of
theoretical molecular weight,
Mn,t,,,
calculated
on
the basis
of
eq
1
and experimental molecular
weight,
Mn,~~~,
determined
by
SEC
for
the bulk polymerization
of
methyl acrylate at 130
"C:
[MA10
=
11.1
M;
[l-(PE)Cl]o:
Comparison of the integration values for two end
group resonances in the spectrum, Figure 5, shows a
511 ratio of a and e. This suggests that the
MA
polymerization was initiated with l-phenylethyl radicals
and efficiently deactivated with an equimolar amount
of chlorine atom (relative to l-phenylethyl group).ll
Comparison of the integration of the end groups with
the methoxy group,
d,
at ca. 3.5 ppm, or other groups
in backbone,
b
and
c,
at 1.2-2.6 ppm, in the
PMA
chain
provides a molecular weight similar
to
the one obtained
from SEC, i.e.,
Mn,NMR
X
1450 against
Mn,SEC
X
1500,
indicating a quantitative initiation by l-phenylethyl
chloride,
Stereochemistry of Atom Transfer Radical
Po-
lymerization.
To better understand the mechanism
of ATRP, the stereochemistry
of
MMA polymerization
was investigated.
The tacticity
of
PMMA was calculated from the 13C
NMR
of
the C=O group and the quaternary carbon
group andlor the
'H
NMR of the a-methyl group. The
13C
NMR
of
the C=O group and the quaternary carbon
group resonate in the regions 175-179 and 44-46.5
ppm, respectively.
The assignment of the
13C
signals
was performed according
to
Peat and Reynold~.'~J~
[C~Cllo:[bpy]o
=
1:1:3.
COOCH,
d
d
1%).
, ,
,
1,.
.
,
.
,
. .
, .
.
,
.
. . .
,
,
.,,.
t,,
,
I,
,.
T
7.5
6.0
5.0
4.0
35
2.0 1.0
PFll
Figure
5.
'H
NMR
of
ATR
PMA initiated with l-(PE)CYCuCl/bpy
(l/l/3)
at
130
"C:
*,
resonances
of
residual
bpy.
7904
Wang and Matyjaszewski
Macromolecules,
Vol.
28,
No.
23,
1995
Table
2.
Comparison
of
Fractions
of
Pentads, Tetrads, Triads, and Diads in Poly(methy1 methacrylate) (PMMA)
Prepared Using BPO,
AIBN,
and
Various
ATRP
Initiator Systems
rmrm+ rmrr+
T,"C Initiator system mmmm mmmr rmmr mmrm mmrr mrrm mrrr
rrrr
mm mr rr m
Pa
130 l-(PE)CVCuCl/bpy6 0.06 0.38 0.56 0.25 0.75 0.99
BPO' 0.06 0.37
0.55
0.245
0.755
1.00
100 (EiB)Br/CuBrhpyd
0.01
0.03 0.04 0.11
0.26
0.06 0.22 0.27
0.05
0.36 0.59 0.23 0.77 1.04
AIBNe 0.02 0.03 0.04 0.11 0.27 0.04
0.21
0.28 0.06 0.34 0.60 0.23 0.77 1.04
60 (EiB)Br/CuBr/bpyd 0.04 0.33 0.63 0.205 0.795 0.99
AIBNf 0.03 0.35 0.62 0.205 0.795 0.94
a
The persistence ratio,
p
=
2(mKr)/(rnr).
*
Polymerization conditions: [MMA],
=
9.36 M; [l-(PE)Cl],
=
0.11
M;
[l-(PE)Clld[CuClld
[bpyl,
=
1/1/3.
Polymerization conditions:
[MMAI,
=
9.36 M; [BPO]
=
0.1 M. [(EiB)Brl,
=
0.055 M;
[l-(PE)Cll~[CuClld[bpylo
=
1/1/3.
e
Polymerization conditions:
[MMAI,
=
9.36 M; [AIBN]
=
0.1 M. fHatada,
K.;
et al.
Polym.
J.
1987,
19,
413.
c=o
I
I
-c-
Figure
6.
Comparison
of
13C
NMR
of
PMMA
prepared
at
100
"C with
the
2-methyl
2-bromoisobutyrate/CuBr/bpy
(l/l/3)
initiating system,
A,
and
a
conventional radical initiator,
AIBN, B,
respectively.
Table
3.
Styrene ATRP Using
Various
R-Cl as Initiator
in the Presence
of
CuCl
(1
molar equiv) and bpy
(3
molar
equiv)=
R-Cl [R-Cllo mom
Mn.thb
Mn.SEC
MdMn
CaHsCl
134700 1.95
0.082 10000 111500 1.75
CHzClz
0.085 9 700
CHC13
0.040 20500
cc14
0.047 17 600
CH&H(Cl)CN 0.037 22300
CHsCH(C1)CN 0.35 2 280
CH&H(Cl)COOCzHj 0.038 21 500
CH&H(Cl)COOCzHj 0.65 1210
C6H&H&1 0.075
11
000
ClCHzCsH4CHzCl
0.12
6890
129
000
21
900
15
500
22 400
2 100
20
000
1290
10
600
6 600
2.20
1.45
1.30
1.35
1.25
1.45
1.35
1.45
1.45
Conversion
of
the polymerization: 90-100%.
Mn,a
=
M,(AM/
with the formation
of
R'
and CuIIC1 is very difficult
because
of
the strong carbon-chlorine bond. Introduc-
tion
of
the inductive
or
resonance stabilizing substituent
[R-ClI,).
0
50
100
150
200
250
300
350
400
I,
min
Figure
7.
Kinetics
of
the bulk polymerization
of
methyl
acrylate, styrene,
and
methyl methacrylate
at
130 "C:
[l-(PE)-
into the R group reduces the BDE
of
the R-C1 bond,14
and the generation of initiating radicals
by
chlorine
atom transfer becomes more resulting in higher
initiator efficiency and narrower MWD.
Effect of the Structure of Monomer on Atom
Transfer Radical Polymerization.
Figure
7
illus-
trates the kinetic plots
of
ATRP
of
three typical mono-
mers, St, MA, and MMA, using the same initiator
system, l-(PE)Cl/CuCl/bpy (l/l/3), and under the same
experimental conditions, in bulk, at 130
"C.
The slopes
of
the straight kinetic plots in Figure
7
allow for the calculation
of
the apparent propagation
rate constants
of
ATRP
of
St,
MA, and MMA,
kpaPP.
Assuming propagation occurs via "normal" free radicals,
the stationary concentration
of
radicals,
[PIst,
can be
estimated from the ratio
of
the apparent rate constant,
kpaPP,
and the rate constant
of
radical propagation
available,
k;.
Cllo
=
[C~Cllo
=
0.038
M;
bpylo
=
0.11
M.
[PI,
=
kpaPP/kp*
(4)
Table
4
shows the
kinetic
data and estimated
con-
centrations of growing radicals in bulk ATRP
of
St,
MMA, and MA initiated with 1-(PE)CYCuCl/bpy (l/l/
3) at 130 "C. The concentration
of
growing radicals
decreases in the order
[Pi,MMA]
=.
[P;,,,]
=
[P;,MA].
Effect of the Leaving Group,
X,
on Atom Trans-
fer Radical Polymerization.
Since the atom transfer
process reflects the strength of the bond breaking and
forming in
M,-X,'
it is expected that the leaving group,
X,
will also strongly affect the atom transfer radical
polymerization.
From Table
5,
it
can be noted that ATRP with
bromine as a ligand is faster than with chlorine. This
can be explained by the enhanced contribution
of
Macromolecules,
Vol.
28,
No.
23,
1995
Table
4.
Kinetic Data and Estimated Concentration
of
Growing radicals,
[PI,
for
Bulk
ATRP
of
Monomers St,
MA,
and
MMA
Initiated with l-(PE)Cl/CuCl/Bpy
(1/1/3)
at
130
Oca
MA
MMA
St
[MI,,
mom
11.1
9.36
8.7
kp',
(130
"C),
lo3
M-'
s-l
14.
lb
3.17c
2.3d
kp
10-4
s-1
3.14
5.83
1.35
[PI
10-7~
0.22
1.90
0.58
a
[l-(PE)Cl],
=
0.038
mol&.
In(k,,u)
=
18.42-3574T;
see:
Odian,
G.
Principles
of
Polymerization;
Wiley-Interscience:
New
York,
1991.
ln(kp,Mm)
=
14.685
-
2669/T,
see:
Hutchinson,
R.
A.;
Aronson,
M.
T.;
Richards,
J.
R.
Macromolecules
1993,26,6410.
Value
extrapolated
from
the
30
to
90
"C
range
to
130
"C;
see:
Hutchinson,
R.
A.;
Aronson,
M.
T.;
Richards,
J.
R.
Macromolecules
1993,26,
6410.
Table
5.
Effect
of
the Leaving Group,
X,
on
the Kinetics
of
ATRP at Different Temperatures"
Controlled/"Living" Radical Polymerization
7905
kpaPP,
kP*,
P.1,
monomer
T,
"C
ATRP
s-l
lo3
M-'
s-l
mom
MMA
80
ClATRP
Br
ATRP
MA
80
ClATRP
Br
ATRP
100
ClATRP
Br
ATRP
St
80
ClATRP
Br
ATRP
-1.71
-3.52
b
-1.28
1.45
3.41
b
-1.45
1.24
13.8
1.24
28.4
4.01
4.01
3.19
6.89
2.10
6.89
5.02
0.64
0.64
22.6
l-(PE)Cl
and
l-(PE)Br
were
used
as
initiators
for
C1
and
Br
ATRP, respectively: [l-PEXI,
=
0.1
M,
and
[l-PEXlJ[CuXld[Bpyl,
=
1/1/3.
No
polymer can be
detected
in
ca.
40
h.
25000
20000
15000
M"
10000
5000
0
09
0.4
0.6
0.8
1
Cow.
Figure
8.
Evolution
of
molecular weight,
M,,
and molecular
weight distribution,
MJM,,
with
monomer conversion
for
the
bulk
ATRP
of
methyl acrylate at
100
"C:
[MA10
=
11.1
M;
[Pethyl chloropropionatel~
=
[CuClIo
=
0.040
M;
bpylo
=
0.12
M.
growing radicals in the former polymerization process
than in the latter one, Table
5.
The effect of the leaving group,
X,
on the degree of
the control of polymerization is significant. For in-
stance, in the cases of
MA
polymerization
at
100 "C
using the same molar ratio of R-X/CuX/bpy of 1/1/3 and
the same initiating radical, ethyl propionate,
the
mo-
lecular weight is better controlled in Br ATRP than in
C1 ATRP, Figures
8
and
9.
Polydispersities of resulting
polymers obtained with Br are lower than those with
C1, Figures
8
and
9,
e.g.,
Mw/Mn
=
1.15-1.35 against
1.30-1.55.
Effect of the Concentrations of the Components
in the Initiator System, R-X/CuX/Bpy, on Atom
Transfer Radical Polymerization.
In order to gain
a
better understanding of
the
ATRP mechanism, we
have studied
the
effect of the ratio of the components
in the initiating system on the kinetics and the level of
the control of polymerization.
As
discussed in the previous sections, the slope of the
kinetic semilogarithmic anamorphoses allows the cal-
l
.6
15
1
4
p
.
1.3
''
12
1.1
0
0.2
0.4
0.6
0.8
1
Conv.
Figure
9.
Evolution
of
molecular weight,
M,,
and molecular
weight distribution,
MwIM,,,
with monomer conversion for the
bulk
ATRP
of
methyl acrylate at 100
"C:
[MA10
=
11.1
M;
[a-ethyl bromopropionate10
=
[CuBrIo
=
0.040
M;
[bpylo
=
0.12
M.
culation of kpaPP, and thus the external orders in
initiator, catalyst, and ligand, can be determined:
kpaPP
=
d(ln[Ml
)/dt
=
k
[RXI
0"
[CuXl:
bpyl
0'
ln(k,aPP)
=
ln(k)
+
x
ln([RXl,)
+
y
ln([CuXl,)
+
z
ln([bpyI,)
the
(5)
(6)
The plots of ln(kpapp)
us
ln([l-(PE)ClI,), ln(kpaPP)
us
ln-
([CuClIo), and ln(kpapp)
us
ln(bpy10) for
St
ATRP in bulk
at 130 "C are given in Figure 1OA-C.
The
fractional
orders observed in these graphs are approximately
1,
0.4,
and
0.6
for [l-(PE)Cl]o, [CuClIo, and bpyl,, respec-
tively. The
first
order of
kpaPP
in initiator, tl-(PE)ClIo,
is as expected. However, since the systems which we
studied were not completely homogenous,
it
is difficult
to explain the precise physical meanings for
0.4
and
0.6
orders in [CuClIo and [bpylo, respectively.
The effect of the compositions of the components in
the
initiator system on the degree of control of ATRP of
St
reveals several important features. As seen from
Figure 11, there appears to be no significant effect of
[CuC1lo on the initiator efficiency and the molecular
weight distribution. Indeed, even in the presence of 0.3
molar equiv of cucl relative to l-(PE)Cl,
Mn,SEC
still
increases linearly with monomer conversion and
is
close
to
kfn,th,,
obtained by means of eq
1.
The similar results
were also found for the ATRP of
MA,
Figures
3
and 12.
These findings suggest that, in the ATRP, the CuX acts
as
a catalyst and the addition
of
a
catalytic amount of
CuX complexed by bpy
is
sufficient for promoting
a
controlled ATRP, even in these heterogeneous systems.
Discussion
Transition Metal Catalyzed Atom Transfer Radi-
cal Addition and Transition Metal Catalyzed Atom
Transfer Radical Polymerization.
As
'described in
the Introduction ATRP can be considered
as
a sequence
of consecutive AT&. The prerequisite for
a
successful
transformation
of
transition metal catalyzed ATRA to
transition metal catalyzed ATRP
is
that the macromo-
lecular halides, R-M,-X, can be effectively activated
by
Mtn,
Scheme
2.
The present work demonstrates that
the Cu(I)/Cu(II) based redox process in
the
presence of
bpy can achieve that goal.
Indeed, to prevent possible polymerization and to
obtain the monomeric adduct, R-M-X, in good to
excellent yields in
the
ATRA process, organic chemists
oRen use the activated organic halides
as
radical sources
and the alkenes without resonance stabilizing sub-
stitute~.~-~ Under such conditions, the further genera-