Chain growth mechanism for regioregular nickel-initiated cross-coupling polymerizations

Article abstract of DOI:10.1021/ma0357063

Chain growth mechanism for nickel-initiated regioregular polymerization of alkylthiophenes was analyzed. The 2-bromo-5-chlorozinc-3-hexylthiophene monomer was used for the analysis. It was observed that molecular weight of the polymer could be obtained by the molar ratio of monomer. An increase in the degree of polymerization was also observed with the conversion. The results show that the polymerization proceeds with the selective oxidative addition to the 2-bromopolythiophene.

Full text of DOI:10.1021/ma0357063

3
526
Macromolecules 2004, 37, 3526-3528
Ch a in Gr ow th Mech a n ism for Regior egu la r
Nick el-In itia ted Cr oss-Cou p lin g
P olym er iza tion s
Elen a E. Sh ein a , J in son g Liu ,
Mih a ela Cor in a Iovu , Da r in W. La ir d , a n d
Rich a r d D. McCu llou gh *
Department of Chemistry, Carnegie Mellon University,
Pittsburgh, Pennsylvania 15213
Received November 12, 2003
Revised Manuscript Received April 2, 2004
Poly(3-alkylthiophenes) are conducting polymers that
have good solubility, environmental stability, and proc-
1
essability. The synthesis of regioregular polythiophenes
has produced defect-free, structurally homogeneous,
head-to-tail coupled poly(alkylthiophenes) (HT-PATs)
that have greatly improved electronic and photonic
properties over regiorandom analogues.2 Regioregular
polythiophenes have led to a multitude of important and
novel nano- and microscale electronic materials and
F igu r e 1. Conversion (filled symbols) and logarithm of
monomer concentration (open symbols) vs time plots for
,3
2
-bromo-3-hexylthiophene polymerization at different concen-
tration of Ni(dppp)Cl
L: (9, 0) [M] :[Ni(dppp)Cl ] ) 136:1; (b, O) [M] :[Ni(dppp)Cl ]
) 57:1; (2, 4) [M]
2
initiator (23-25 °C); [M]
0
) 0.075 mol/
0
2
0
2
4
-7
devices.
Very recently, we have developed an end-
0
:[Ni(dppp)Cl ] ) 49:1.
2
group functionalization methodology of HT-PATs that
allows for the synthesis of a plethora of well-defined
block copolymers that form nanowires with high electri-
7
cal conductivity. Here, we have discovered that the
nickel-initiated regioregular polymerization of alkyl-
thiophenes proceeds by a chain growth mechanism and
8
does not occur by the accepted step growth mechanism.
We also observed that the degree of polymerization of
the synthesized poly(alkylthiophenes) increases with the
conversion and can be predicted by the molar ratio of
monomer to nickel initiator. On the basis of our experi-
mental results, we predict that nickel-initiated cross-
coupling polymerization is essentially a living system,
giving PATs with low polydispersities (PDIs).
In a cross-coupling step polymerization catalyzed by
Ni(dppp)Cl2, one would expect a fast disappearance of
the monomer and increase of the polymer molecular
weight toward the end of the polymerization.8
-10
On the
basis of the experimental results, we observed that
relatively high molecular weight polymer forms almost
immediately. As a model reaction, we have also found
that 2 equiv of a variety of aryl dibromides and 1 equiv
of an aryl organometallic (either magnesium or zinc)
always gives a near quantitative yield of the trimeric
F igu r e 2. Dependence of molecular weights and polydisper-
sities on conversion for 2-bromo-3-hexylthiophene polymeri-
zation at different concentrations of Ni(dppp)Cl
25 °C); [M] ) 0.075 mol/L: (9) [M] :[Ni(dppp)Cl
(b) [M] :[Ni(dppp)Cl ] ) 57:1; (2) [M] :[Ni(dppp)Cl ] ) 49:1 (the
dashed line shows the theoretical molecular weight).
2
initiator (23-
0
0
2
] ) 136:1;
0
2
0
2
1
1
aryl and minor amounts (<1%), if any, of the dimer.
These results indicate the very strong preference of the
Ni(0) (see Scheme 1, intermediate 3) to form a nondif-
fusive associated pair, resulting in near 100% formation
of the trimer. All of the results indicate that the
polymerization proceeds with selective oxidative addi-
tion to the growing 2-bromopolythiophene and that
these regioregular polymerizations progress by a chain
growth mechanism rather than a step growth.
3
2
-hexylthiophene monomer (1) generated in situ from
-bromo-3-hexylthiophene reacts with Ni(dppp)Cl2, yield-
ing the organonickel compound (2), is as it has been
described by others.
9,13
We differ in our mechanism in
that reductive elimination of 2 immediately forms an
associated pair [3‚4] of the tail-to-tail aryl halide dimer
(4) and nickel (0) (3). The dimer 4 undergoes fast
oxidative addition to the nickel center generating 5, in
view of the fact that the formation of the complex 3‚4
eliminates potential separation of 4 from 3. Subse-
quently, growth of the polymer chain occurs by insertion
of one monomer at a time as shown in the reaction cycle
Poly(3-hexylthiophene) (HT-PHT) was prepared by
2
the original method that provides a high specificity of
H-T configuration of the repeating units (>98% H-T
coupling).1 The mechanism of the cross-coupling chain-
growth polymerization is outlined in Scheme 1. The first
step in the mechanism, where the 2-bromo-5-chlorozinc-
2
(5 f 6 f [3‚7] f 5), where the Ni(dppp) moiety is
always incorporated into polymer chain as an end group.
Addition of various Grignard reagents (R′MgX) at the
end of polymerization results in end-capping of HT-
*
Corresponding author: e-mail rm5g@andrew.cmu.edu.
1
0.1021/ma0357063 CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/20/2004

Macromolecules, Vol. 37, No. 10, 2004
Communications to the Editor 3527
Sch em e 1. P r op osed Mech a n ism for th e Nick el-In itia ted Cr oss-Cou p lin g P olym er iza tion
PATs with R′ end group,14 which supports the fact that
Ni(dppp)Cl2 acts as an initiator rather than a catalyst.
Furthermore, adding organometallic (e.g., magnesium
or zinc) thiophene bromides to species 5 results in the
formation of block copolymers, providing a strong evi-
ever, indicate the presence of termination reactions,
which could be due to the formation of large supra-
molecular aggregates of polymer species mixed with
unassociated or very weakly associated polythiophene
16
chains. The nature of the termination reactions is
1
5
dence for the living nature of this polymerization.
currently under investigation. A reaction order of ∼1
with respect to Ni(dppp)Cl2 concentration was ob-
tained from the slope of the plot of the logarithm of the
initial rate of polymerization vs the logarithm of the
To support the proposed chain growth mechanism,
several experiments were performed at constant mono-
mer concentration and variable Ni(dppp)Cl2 concentra-
tion. All the reactions were very fast, reaching almost
17
Ni(dppp)Cl2 concentration.
9
0% conversion in less than 2 h, at room temperature.
The molecular weight vs conversion plot (Figure 2)
and the GPC traces (Figure 3) show the increase of
molecular weight with conversion, providing a further
support for the mechanism of the nickel-initiated cross-
coupling polymerization being a chain growth process.
Our results indicate that the molecular weight of
polymer can be predicted by the molar ratio of monomer
to Ni(dppp)Cl2, which means that 1 mol of Ni(dppp)Cl2
initiates one polymer chain. In most of our previ-
ous work, we used relatively low concentrations of
Ni(dppp)Cl2, which leads to higher molecular weights
and broad PDIs as illustrated in Figure 2 (e.g., [M]0:
The reaction rates increased with the increase in the
Ni(dppp)Cl2 concentration (Figure 1). Linear semi-
logarithmic kinetic plots were obtained up to 50%
conversion (e.g., [M]0:[Ni(dppp)Cl2] ) 49:1). The non-
linearity of semilogarithmic kinetic plots would, how-
[Ni(dppp)Cl2] ) 136:1; Mw/Mn ranges from 1.2 to 1.6).
When a lower ratio of monomer to initiator is employed,
however, one finds a good correlation between the
theoretical and observed molecular weights, and narrow
PDIs are obtained (e.g., [M]0:[Ni(dppp)Cl2] ) 49:1, Mw/
1
8
Mn ≈ 1.2-1.3). We have also found that the Grignard
1
9
metathesis method (GRIM) for the synthesis of regio-
regular polythiophenes has similar kinetic behavior as
shown here, indicating that it is a chain growth mech-
2
0
anism as well.
The power of generating polythiophenes with well-
defined molecular weights and narrow PDIs is impor-
tant in optimizing the electrical properties of conducting
polymers. Further work is expected to lead to truly
living conducting polymers.
F igu r e 3. GPC traces for 2-bromo-3-hexylthiophene polymer-
ization (23-25 °C); [M]
7:1.
0
0 2
) 0.075 mol/L; [M] :[Ni(dppp)Cl ] )
5

3
528 Communications to the Editor
Macromolecules, Vol. 37, No. 10, 2004
Ack n ow led gm en t. We gratefully acknowledge the
propriate dibromoaryl compound (e.g., 2,5-dibromo-3-
methylthiophene, 2,5-dibromothiophene, or 1,4-dibromo-
benzene) and 0.05 mmol of [1,3-bis(diphenylphosphino)-
propane]dichloronickel(II) (Ni(dppp)Cl2) via cannula. The
reaction was allowed to proceed for 12 h followed by
quenching in water. The organic layer was extracted with
diethyl ether (Et2O) and subjected to GC-MS analysis to
determine product composition and distribution. The reac-
tion schemes and obtained results are provided in the
Supporting Information.
NSF CHE0107178 for support of the work. We also
thank Genevi e` ve Sauv e´ , Malika J effries-EL, Paul
Ewbank, Lei Zhai, Robert Loewe, Richard Pilston, and
Kris Matyjaszewski for insightful discussions.
Su p p or t in g In for m a t ion Ava ila b le: Mechanism for
model reactions of aryl dibromides and an aryl organometallic
with Ni(dppp)Cl
polymerization vs the logarithm of the Ni(dppp)Cl
2
; plot of the logarithm of the initial rate of
concentra-
(
12) In a typical polymerization experiment, a dry 100 mL three-
neck flask was flashed with dinitrogen (N2) and was charged
with diisopropylamine (0.50 mL, 3.5 mmol) and THF (30
mL); both were added via a syringe. The reaction flask was
cooled to 0 °C, and n-butyllithium (2.0 mL, 3 mmol) was
added dropwise via a syringe. After 20 min of stirring at 0
2
tion. This material is available free of charge via Internet at
http://pubs.acs.org.
Refer en ces a n d Notes
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a previously chilled to -76 °C 0.3 M solution of 2-bromo-3-
hexylthiophene (0.73 g, 3 mmol) in anhydrous THF (10 mL)
was added via cannula. The reaction mixture was stirred
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3.6 mmol) was added in one portion and completely dis-
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RT, at which time 2,2′-bithiophene (0.16 g, 1 mmol) was
added in one portion and used as an internal standard. To
this mixture Ni(dppp)Cl2 (29 mg, 0.053 mmol) was added
in one portion, and the reaction mixture was stirred at RT.
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aliquot a GC sample was prepared in Et2O (2 mL) and was
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11) A typical example is when a 0.1 M solution of thiophene
(17) See Supporting Information for details. In addition, unpub-
lished results in our group showed a value of 1 for the
reaction order with respect to monomer for nickel-initiated
cross-coupling polymerization.
(0.42 g, 5 mmol) or 2-methylthiophene (0.49 g, 5 mmol) in
anhydrous THF (50 mL) cooled to -40 °C (acetonitrile/dry
ice bath) was charged with a dropwise addition of n-
butyllithium (2 mL, 5 mmol) via a syringe. After stirring
the reaction mixture for 40 min at -40 °C, anhydrous ZnCl2
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(
0.7 g, 5 mmol) was added in one portion, and the stirring
(
continued for another 15 min. The cooling bath was re-
moved, and the reaction mixture was allowed to warm to
RT, at which point the reaction mixture was transferred to
a different reaction flask charged with 10 mmol an ap-
(
20) Iovu, M. C.; McCullough, R. D. Manuscript in preparation.
MA0357063