Controlled Pd(0)/ t -Bu3P-catalyzed suzuki cross-coupling polymerization of AB-type monomers with PhPd(t -Bu3P)I or Pd 2(dba)3/ t -Bu3P/ArI as the initiator

Article abstract of DOI:10.1021/ja302745t

Controlled Pd(0)/t-Bu₃P-catalyzed Suzuki cross-coupling polymerizations of AB-type monomers via the chain-growth mechanism with an ArPd(t-Bu₃P)I complex as the initiator are described. ArPd(t-Bu ₃P)I complexes, either prepurified or generated in situ from Pd ₂(dba)₃/t-Bu₃P/ArI (dba = dibenzylideneacetone) without separation/purification, were found to be efficient initiators in general for the controlled Suzuki cross-coupling polymerization, with narrow polydispersity indexes (PDIs) of 1.13-1.35 being observed. The Pd ₂(dba)₃/t-Bu₃P/p-BrC₆H₄I combination was identified as a highly robust initiator system, with PDIs of ≤1.20 in general and as low as 1.13 being obtained. Higher number-average molecular weights (M_n) were achieved without a significant increase in the PDI (from 1.14 for a polymer with a M_n = 9500 to 1.20 for a polymer with M_n = 31 400) by using a smaller amount of the Pd ₂(dba)₃/t-Bu₃P/p-BrC₆H₄I initiator in the polymerization.

Full text of DOI:10.1021/ja302745t

Communication
pubs.acs.org/JACS
Controlled Pd(0)/t‑Bu3P‑Catalyzed Suzuki Cross-Coupling
Polymerization of AB-Type Monomers with PhPd(t‑Bu3P)I or
Pd2(dba)3/t‑Bu3P/ArI as the Initiator
Hong-Hai Zhang,† Chun-Hui Xing,† and Qiao-Sheng Hu*
Department of Chemistry, College of Staten Island and the Graduate Center of the City University of New York, Staten Island, New
York 10314, United States
S
* Supporting Information
ABSTRACT: Controlled Pd(0)/t-Bu3P-catalyzed Suzuki
cross-coupling polymerizations of AB-type monomers via
the chain-growth mechanism with an ArPd(t-Bu3P)I
complex as the initiator are described. ArPd(t-Bu3P)I
complexes, either prepurified or generated in situ from
Pd2(dba)3/t-Bu3P/ArI (dba = dibenzylideneacetone) with-
out separation/purification, were found to be efficient
initiators in general for the controlled Suzuki cross-
coupling polymerization, with narrow polydispersity
indexes (PDIs) of 1.13−1.35 being observed. The
Pd2(dba)3/t-Bu3P/p-BrC6H4I combination was identified
as a highly robust initiator system, with PDIs of ≤1.20 in
general and as low as 1.13 being obtained. Higher number-
average molecular weights (Mn) were achieved without a
significant increase in the PDI (from 1.14 for a polymer
with a Mn = 9500 to 1.20 for a polymer with Mn = 31 400)
by using a smaller amount of the Pd2(dba)3/t-Bu3P/p-
BrC6H4I initiator in the polymerization.
Figure 1. Pd(0)/t-Bu3P-catalyzed Suzuki cross-coupling polymer-
ization of 7-bromo-9,9-dihexylfluoren-2-ylboronic acid pinacol ester
with PhPd(t-Bu3P)I as the initiator ([monomer]0 = 15.4 mmol/L,
[PhPd(t-Bu3P)I]0 = 0.46 mmol/L, 2 M K3PO4(aq), THF, 0 °C; see
the SI for details).
controlled Pd(0)-catalyzed Suzuki cross-coupling polymer-
izations via the chain-growth mechanism can only be regarded
to be in an infant stage to date.3−7 In 2005, we hypothesized
that the key to achieving Pd(0)-catalyzed Suzuki cross-coupling
polymerization in a controlled fashion would be to achieve the
preferential oxidative addition of the regenerated Pd(0) catalyst
with the newly formed coupling product from the same
catalytic cycle, a concept that was essential for changing the
Pd(0)-catalyzed Suzuki cross-coupling polymerization from a
step-growth process into a chain-growth process.3 We
established that Pd(0)/t-Bu3P is an efficient catalyst to achieve
such a preferential oxidative addition,3 paving the road for the
development of controlled Pd(0)/t-Bu3P-catalyzed Suzuki
cross-coupling reactions.4 Since the disclosure of our results,
Yokozawa reported Pd(0)/t-Bu3P-catalyzed Suzuki cross-
coupling polymerizations of AB-type monomers with PhPd(t-
Bu3P)Br as the initiator,5,6 in which a polydispersity index (PDI
= Mw/Mn) of ≥1.33 was observed with aqueous Na2CO3 as the
base5a and a narrower PDI (≥1.18) but lower number-average
molecular weight (Mn = 6400) were observed with CsF/18-
crown-6 as the base.5b,c Huck has reported Pd(0)/t-Bu3P-
catalyzed Suzuki cross-coupling polymerizations of n-type
fluorene-containing AB-type monomers with ArPd(t-Bu3P)Br
as the initiator,7 with narrow PDIs (1.15−1.16) at low
lthough Pd(0)-catalyzed Suzuki cross-coupling polymer-
Aizations via the step-growth mechanism have been
established as powerful tools for the synthesis of a wide
range of conjugated polymers over the past decades,1,2
Table 1. Pd(0)/t-Bu3P-Catalyzed Suzuki Cross-Coupling
Polymerization of 7-Bromo-9,9-dihexylfluoren-2-ylboronic
a
Acid Pinacol Ester
b
c
Entry
Catalyst
Base
Yield (%)
Mn (PDI)
de
,
1
2
3
4
5
PhPd(t-Bu3P)I
PhPd(t-Bu3P)I
PhPd(t-Bu3P)I
Pd2(dba)3/t-Bu3P
PhPd(t-Bu3P)Br
K3PO4(s)
K3PO4(s)
K3PO4(aq)
K3PO4(aq)
K3PO4(aq)
−
900 (1.32)
8600 (1.82)
14800 (1.33)
47300 (1.95)
14500 (1.35)
f
83
76
80
76
g
g
g
a
Polymerization conditions: monomer (1 equiv), Pd catalyst (6 mol
b
c
%), base (10 equiv), THF, 0 °C, 30 min. Isolated yields. Determined
by gel-permeation chromatography (GPC) [polystyrene (PS) stand-
d
e
ards, THF, 40 °C]. Reaction time 6 h. No precipitation was
f
g
observed upon the addition of MeOH. Reaction time 36 h. 2 M
concentration.
Received: March 21, 2012
Published: August 3, 2012
© 2012 American Chemical Society
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Communication
a
Table 2. Pd(0)/t-Bu3P-Catalyzed Suzuki Cross-Coupling Polymerization with Pd2(dba)3/t-Bu3P/ArI as the Initiator
b
c
Entry
Pd(0)/t-Bu3P
R
X
Base
T (°C)
Yield (%)
Mn (PDI)
1
2
Pd(t-Bu3P)2
H
H
H
Cl
Br
F
I
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K2HPO4
Na2CO3
K2CO3
K3PO4
0
0
55
70
87
80
80
77
70
60
80
83
70
87
99
90
18
70
23500 (1.60)
9700 (1.29)
25900 (1.42)
9100 (1.25)
9500 (1.14)
11200 (1.21)
7700 (1.25)
9500 (1.35)
7800 (1.26)
10400 (1.20)
10300 (1.17)
9100 (1.17)
4700 (1.24)
5300 (1.30)
2500 (1.78)
11100 (1.20)
Pd2(dba)3/t-Bu3P
Pd2(dba)3/t-Bu3P
Pd2(dba)3/t-Bu3P
Pd2(dba)3/t-Bu3P
Pd2(dba)3/t-Bu3P
Pd2(dba)3/t-Bu3P
Pd2(dba)3/t-Bu3P
Pd2(dba)3/t-Bu3P
Pd2(dba)3/t-Bu3P
Pd2(dba)3/t-Bu3P
Pd2(dba)3/t-Bu3P
Pd2(dba)3/t-Bu3P
Pd2(dba)3/t-Bu3P
Pd2(dba)3/t-Bu3P
Pd2(dba)3/t-Bu3P
I
3
Br
I
0
4
0
5
I
0
6
I
0
7
I
I
0
8
OMe
CO2Et
Br
I
0
9
I
0
d
e
10
11
12
13
14
15
16
I
0
Br
I
0
Br
I
rt
rt
rt
rt
−10
Br
I
Br
I
Br
I
Br
I
a
Polymerization conditions: monomer (1 equiv), “Pd(0)/t-Bu3P” [either Pd(t-Bu3P)2 (6 mol %) or Pd2(dba)3 (6 mol %)/t-Bu3P (24 mol %)], p-
b
c
d
RC6H4X (10 mol %), base (10 equiv), THF, rt, 0.5 h. Isolated yields. As determined by GPC (PS standards, THF, 40 °C). 18 mol % t-Bu3P was
used. 30 mol % t-Bu3P was used.
e
ArPd(L)X would significantly increase the PDI and that
PhPd(t-Bu3P)I has been demonstrated to undergo reductive
elimination more reluctantly than PhPd(t-Bu3P)Br,9 we
envisioned that ArPd(t-Bu3P)I complexes could be efficient
initiators for controlled Suzuki cross-coupling polymerization.
In addition, because it has been established that aryl iodides
readily undergo oxidative addition with Pd(0)/t-Bu3P to form
ArPd(t-Bu3P)I complexes,10 we further envisioned that a
combination of Pd(0)/t-Bu3P and ArI, which could generate
an ArPd(t-Bu3P)I complex in situ, might be used as an initiator
system for the controlled polymerization. Importantly, the
direct use of Pd(0)/t-Bu3P and ArI as the initiator system,
which would eliminate the isolation and purification steps for
Figure 2. Pd(0)/t-Bu3P-catalyzed Suzuki cross-coupling polymer-
the ArPd(t-Bu3P)I complex and thus could minimize the
ization of 7-bromo-9,9-dihexylfluoren-2-ylboronic acid pinacol ester
decomposition of the ArPd(t-Bu3P)I complex due to the
with Pd2(dba)3/t-Bu3P/p-BrC6H4I as the initiator ([monomer]0
=
handling process, might further narrow the PDI for the
controlled polymerization. In this communication, we report
our results on establishing such possibilities, specifically, the use
of ArPd(t-Bu3P)I complexes, including ones generated in situ
from Pd2(dba)3/t-Bu3P/ArI, as a class of highly efficient
initiators for controlled Pd(0)/t-Bu3P-catalyzed Suzuki cross-
coupling polymerization of AB-type monomers, with PDIs of
≤1.20 in general and as low as 1.13 being achieved.
Our study began with the polymerization of 7-bromo-9,9-
dihexylfluoren-2-ylboronic acid pinacol ester using PhPd(t-
Bu3P)I10a as the initiator. We found that with solid K3PO4 as
the base,3 the polymerization occurred very slowly, affording a
polymer with a PDI of 1.82 after 36 h (Table 1, entries 1 and
2). The slow polymerization likely was due to the insolubility of
K3PO4 in tetrahydrofuran (THF), which resulted in slow
transmetalation processes, including the initiation step, and
thus slow polymerization and a high PDI for the generated
polymer. Such speculation was confirmed by the use of aqueous
K3PO4 as the base,1 with which the polymerization occurred
significantly faster and, more importantly, afforded the polymer
with narrower PDI of 1.33 (Table 1, entry 3). For comparison
purposes, the polymerization with Pd2(dba)3/t-Bu3P as the
15.6 mmol/L, [Pd2(dba)3]0 = 0.95 mmol/L, [p-BrC6H4I]0 = 1.6
mmol/L, 2 M K3PO4(aq), THF, 0 °C; see the SI for details).
molecular weights (Mn = 3300) and increased PDIs (1.27−
1.29) at slightly higher molecular weights (Mn = 7700) being
observed.6 In addition, the use of AB2-type monomers for the
preparation of hyperbranched polymers with Pd2(dba)3/t-Bu3P
(dba = dibenzylideneacetone) as the catalyst was reported by
Bo.8
In our laboratory, after establishing the preferential oxidative
addition concept,3 we naturally turned our attention to the
controlled Suzuki cross-coupling polymerization of AB-type
monomers, as hypothesized in our previous report.3 Because
our study of the preferential oxidative addition addressed the
propagation step for the polymerization via a chain-growth
mechanism, we reasoned that the identification of appropriate
initiators for an efficient initiation process could likely be the
last obstacle for us to achieve controlled Pd(0)/t-Bu3P-
catalyzed Suzuki cross-coupling polymerizations with narrow
PDIs (≤1.20). On the basis of the considerations that the
decomposition of even a very small amount of the initiator
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Journal of the American Chemical Society
Communication
Bu3P species generated from Pd2(dba)3/t-Bu3P are more active
than those from Pd(t-Bu3P)2,10c we examined the combination
of Pd2(dba)3/t-Bu3P and PhI for the in situ generation of
PhPd(t-Bu3P)I and found it to be an efficient initiator for the
controlled polymerization, affording the polymer with a PDI of
1.29 (Table 2, entry 2). For comparison purposes, we also
examined the combination of Pd2(dba)3/t-Bu3P with PhBr for
in situ generation of the initiator and found this system to be a
less efficient initiator for the controlled polymerization, as
evidenced by the PDI of 1.42 for the generated polymer (Table
2, entry 3). Because the oxidative addition reactivity of
iodobenzenes would be influenced by the substitutent on the
phenyl ring, we thus examined several para-substituted
iodobenzenes in the Pd2(dba)3/t-Bu3P/ArI initiator system.
We found these Pd2(dba)3/t-Bu3P/ArI combinations in general
to be efficient initiators that gave narrow PDIs (1.14−1.35)
(Table 2, entries 4−9). The Pd2(dba)3/t-Bu3P/p-BrC6H4I
combination was found to be the best initiator for the
controlled polymerization, affording a polymer with a PDI of
1.14 (Table 2, entry 5). Further studies showed that a 2:1 ratio
of t-Bu3P to Pd yielded the best initiator system (Table 2,
entries 5, 10, and 11). A brief screen of the base and
temperature showed K3PO4 to be the best base and 0 °C to be
the best temperature for the controlled polymerization (Table
2, entries 5 and 12−16).11
Table 3. Pd(0)/t-Bu3P-Catalyzed Suzuki Cross-Coupling
Polymerization with Pd2(dba)3/t-Bu3P/p-BrC6H4I as the
Initiator
a
To support the chain-growth nature of the Pd2(dba)3/t-
Bu3P/p-BrC6H4I-initiated polymerization process, we examined
the relationship between the monomer conversion and the
molecular weight of the generated polymer. We found a linear
relationship between them, with almost no change in PDI
(1.13−1.16) for polymers obtained at different conversions
(Figure 2), a characteristic of chain-growth polymerization.
These results suggested that the polymerization with
Pd2(dba)3/t-Bu3P/p-BrC6H4I as the initiator likely occurs via
a chain-growth process.12
With Pd2(dba)3/t-Bu3P/p-BrC6H4I as the initiator, several
bromoarylboronic acids/acid esters were examined for the
controlled polymerization (Table 3). We found the Pd2(dba)3/
t-Bu3P/p-BrC6H4I system to be an excellent initiator for these
AB-type monomers, with very narrow PDIs (1.13−1.19) being
obtained for the generated polymers (Table 3, entries 1−4).
We further examined the polymerization using different
amounts of the initiator. We found higher molecular weights
could be achieved by using smaller amounts of the initiator, and
importantly, the PDIs increased only slightly [Table 3, entry 3
vs 5 and entry 1 vs 6−8; also see the Supporting Information
(SI)]. These results compared favorably with those for
controlled polymerizations using ArPd(t-Bu3P)Br complexes
as the initiators, in which the PDI increased from 1.33 to 1.51
as Mn increased from 17 700 to 20 700 with Na2CO3(aq) as the
base5a and from 1.15 to 1.27 as Mn increased from 3300 to
7700.6 Our results show that the Pd2(dba)3/t-Bu3P/p-BrC6H4I
combination is a highly robust initiator for controlled Suzuki
cross-coupling polymerization of AB-type monomers.13
a
Polymerization conditions: monomer (1 equiv), initiator [6 mol %
Pd2(dba)3 + 24 mol % t-Bu3P + 10 mol % p-BrC6H4I], K3PO4 (10
equiv), THF, 0 °C, 30 min. Isolated yields. Determined by GPC (PS
b
c
d
standards, THF, 40 °C). Formation of the minor isomers having Br−
e
Ar−p-C6H4−[Ar−]nBr or H termini was also observed. Initiator
loading: 1.5 mol % Pd2(dba)3/6 mol % t-Bu3P/2.5 mol % p-BrC6H4I.
f
Initiator loading: 3 mol % Pd2(dba)3/12 mol % t-Bu3P/5 mol % p-
g
BrC6H4I. Initiator loading: 4 mol % Pd2(dba)3/16 mol % t-Bu3P/6.7
mol % p-BrC6H4I. Initiator loading: 2 mol % Pd2(dba)3/8 mol % t-
Bu3P/3.3 mol % p-BrC6H4I.
h
catalyst system was also carried out and yielded a polymer with
a PDI of 1.95 (Table 1, entry 4), a typical PDI for a polymer
generated via the step-growth polymerization mechanism. The
much narrower PDI (1.33) for PhPd(t-Bu3P)I-initiated
polymerization suggested that PhPd(t-Bu3P)I-initiated poly-
merizations likely did not occur via step-growth polymerization
but rather proceeded via the chain-growth polymerization
mechanism. This chain-growth nature was confirmed by the
linear relationship between the monomer conversion and the
molecular weight of the generated polymer (Figure 1).11 We
also employed PhPd(t-Bu3P)Br10a as the initiator for the
polymerization, and a slightly higher PDI of 1.35 was observed
for the generated polymer (Table 1, entry 5). These results
showed that PhPd(t-Bu3P)I complex is an efficient initiator for
Suzuki cross-coupling polymerization via a chain-growth
mechanism.
We next turned our attention to the use of in situ-generated
ArPd(t-Bu3P)I complexes for the controlled polymerization.
We first employed Pd(t-Bu3P)2 and PhI to generate the
initiator, and the polymerization afforded a polymer with Mn =
23 500 and PDI = 1.60 (Table 2, entry 1). We speculated that
the increased PDI might be due to slow oxidative addition of
Pd(t-Bu3P)2 with PhI, which might leave some Pd(t-Bu3P)2 in
the initiator system. On the basis of Fu’s results that Pd(0)/t-
In summary, we have demonstrated for the first time that
ArPd(t-Bu3P)I complexes are a class of efficient initiators for
the controlled Suzuki cross-coupling polymerization of
bromoarylboronic acids/acid esters (AB-type monomers) via
the chain-growth mechanism. ArPd(t-Bu3P)I complexes, either
prepurified or generated in situ from Pd2(dba)3/t-Bu3P/ArI
without separation/purification, were found to be efficient
initiators in general, with narrow PDIs being observed (1.13−
1.35). The Pd2(dba)3/t-Bu3P/p-BrC6H4I combination was
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Journal of the American Chemical Society
Communication
(6) Also see: Beryozkina, T.; Boyko, K.; Khanduyeva, N.;
Senkovskyy, V.; Horecha, M.; Oertel, U.; Simon, F.; Stamm, M.;
Kiriy, A. Angew. Chem., Int. Ed. 2009, 48, 2695−2698.
(7) Elmalem, E.; Kiriy, A.; Huck, W. T. S. Macromolecules 2011, 44,
9057−9061.
found to be a highly robust initiator for the controlled Suzuki
cross-coupling polymerization, with PDIs of ≤1.20 in general
and as low as 1.13 being observed. Importantly, higher
molecular weights could be achieved by using smaller amounts
of the Pd2(dba)3/t-Bu3P/p-BrC6H4I initiator with only a slight
increase in the PDI (from 1.14 for the polymer with Mn = 9500
to 1.20 for the polymer with Mn = 31400). Our study has
provided a class of readily available, robust initiators and a
general initiator generation strategy for the controlled Pd(0)/t-
Bu3P-catalyzed Suzuki cross-coupling polymerization of AB-
type monomers. The Pd2(dba)3/t-Bu3P/p-BrC6H4I initiator
system may find application in other controlled Pd(0)-catalyzed
cross-coupling reactions.14 The in situ initiator generation
strategy described herein has also paved the road for us to
explore the use of other Pd(0) sources and other ArX (X = Br,
OTf, etc.) for the in situ generation of initiators for controlled
cross-coupling polymerizations, including Suzuki cross-coupling
polymerization. Our future work in these directions will be
reported in due course.
(8) Huang, W.; Su, L.; Bo, Z. J. Am. Chem. Soc. 2009, 131, 10348−
10349.
(9) Roy, A. H.; Hartwig, J. F. J. Am. Chem. Soc. 2003, 125, 13944−
13945.
(10) (a) Stambuli, J. P.; Incarvito, C. D.; Buhl, M.; Hartwig, J. F. J.
̈
Am. Chem. Soc. 2004, 126, 1184−1194. (b) Stambuli, J. P.; Buhl, M.;
̈
Hartwig, J. F. J. Am. Chem. Soc. 2002, 124, 9346−9347. (c) Littke, A.
F.; Dai, C.; Fu, G. C. J. Am. Chem. Soc. 2000, 122, 4020−4028.
(11) The end groups of the polyfluorene polymer, as analyzed by
matrix-assisted laser desorption ionization−time of flight (MALDI−
TOF) mass spectrometry, were found to be mainly Br/Br and Br/H
(see the SI for details).
(12) To understand the polymerization propagation direction after
the initial reductive elimination, which would generate a product with
two Br groups as possible propagation sites, the reaction of 7-bromo-
9,9-dihexylfluoren-2-ylboronic acid pinacol ester with the Pd2(dba)3/t-
Bu3P/p-BrC6H4I initiator system in a 1:1 ratio was carried out. GC−
MS analysis showed that 2-(4-bromophenyl)-9,9-dihexylfluorene and
2-bromo-7-phenyl-9,9-dihexylfluorene were formed in a 3:1 ratio,
suggesting that either Br group could serve as the polymerization
propagation site (see the SI for details).
(13) Other bromoiodoarenes instead of 1-bromo-4-iodobenzene
could be used in the initiator system. For example, the use of
Pd2(dba)3/t-Bu3P/2-bromo-9,9-dihexyl-7-iodofluorene as the initiator
for the polymerization of 7-bromo-9,9-dihexylfluoren-2-ylboronic acid
pinacol ester, which could avoid the structural defect caused by the use
of 1-bromo-4-iodobenzene in the initiator system, yielded a
polyfluorene polymer in 80% yield with Mn = 15 300 and PDI =
1.19 as determined by GPC with PS standards (see the SI for details).
(14) Examples of other cross-coupling polymerizations: (a) Suzuki−
Heck polymerization: Grisorio, R.; Suranna, G. P.; Mastrorilli, P.
Chem.Eur. J. 2010, 16, 8054−8061. (b) Negishi coupling
polymerization: Verswyvel, M.; Verstappen, P.; De Cremer, L.;
Verbiest, T.; Koeckelberghs, G. J. Polym. Sci., Part A: Polym. Chem.
2011, 49, 5339−5349.
ASSOCIATED CONTENT
* Supporting Information
Experimental details and characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
Qiaosheng.hu@csi.cuny.edu
■
Author Contributions
†H.-H.Z. and C.-H.X. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the NSF (CHE0911533) for funding. Partial support
from PSC−CUNY Research Award Programs is also gratefully
acknowledged.
REFERENCES
■
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