Steric effects of the initiator substituent position on the externally initiated polymerization of 2-bromo-5-iodo-3-hexylthiophene

Article abstract of DOI:10.1021/ma1024494

Externally initiated polymerization of 2-bromo-3-hexyl-5-iodothiophene was attempted from four aryl and thiophene based small molecule initiators functionalized with a phosphonate moiety. Initiated poly(3-hexylthiophene) product was obtained in various yields depending on the nature of the initiating molecule. Reaction intermediates for the oxidative addition and the ligand exchange steps were analyzed utilizing both experimental and theoretical methods. It was observed that an ortho substituent plays a crucial role in the outcome of the polymerization mechanism and that aryl based initiators are generally more stable than thiophene based initiators. Density functional theory (DFT) calculations revealed the importance of the steric effects on the success of the externally initiated chain growth polymerization mechanism.

Full text of DOI:10.1021/ma1024494

512 Macromolecules 2011, 44, 512–520
DOI: 10.1021/ma1024494
Steric Effects of the Initiator Substituent Position on the Externally
Initiated Polymerization of 2-Bromo-5-iodo-3-hexylthiophene
Natalia Doubina,† Sergio A. Paniagua,‡ Alexandra V. Soldatova,† Alex K. Y. Jen,†,§
Seth R. Marder,‡ and Christine K. Luscombe*,§
†Department of Chemistry, University of Washington, Seattle, Washington 98195, United States, ‡School of
Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States, and
§Department of Material Science and Engineering, University of Washington, Seattle, Washington 98195-2120,
United States
Received October 27, 2010; Revised Manuscript Received December 17, 2010
ABSTRACT: Externally initiated polymerization of 2-bromo-3-hexyl-5-iodothiophene was attempted
from four aryl and thiophene based small molecule initiators functionalized with a phosphonate moiety.
Initiated poly(3-hexylthiophene) product was obtained in various yields depending on the nature of the
initiating molecule. Reaction intermediates for the oxidative addition and the ligand exchange steps were
analyzed utilizing both experimental and theoretical methods. It was observed that an ortho substituent plays
a crucial role in the outcome of the polymerization mechanism and that aryl based initiators are generally
more stable than thiophene based initiators. Density functional theory (DFT) calculations revealed the
importance of the steric effects on the success of the externally initiated chain growth polymerization
mechanism.
Introduction
catalyst.44 Our work, as well as by others mentioned above,
indicates that oxidative addition of the aryl halide bond of the
Conjugated polymers, among these polythiophenes, are an
important class of materials in current research field of organic
electronics and photovoltaics. Poly(3-hexylthiophene) (P3HT) in
particular has attracted much interest in photovoltaic applica-
tions due to its desirable electronic and physical properties such
as high thermal and environmental stability, high charge-carrier
mobility, low optical gap and high solubility.1-12 Regiocon-
trolled synthesis of poly(3-alkylthiophenes) was first developed
by McCullough13-15 and Rieke.16-18 Yokozawa19,20 and McCul-
lough21 later pioneered the Kumada catalyst transfer polymeri-
zation (KCTP) and the Grignard metathesis (GRIM) technique
respectively, which yields highly regioregular polymers with con-
trolled molecular weights, narrow polydispersity index and uni-
form end groups. The catalytic cycle of the KCTP mechanism
begins with the bithiophene initiator which is generated through
the transmetalation reaction between two molecules of the grig-
nard functionalized thiophene monomer and NiII(dppp)Cl2 cata-
lyst.19,21 The polymerization control is thought to originate from
either intramolecular transfer of the Ni catalyst or an asso-
ciated pair formed by the growing polymer chain and the Ni
catalyst. There has been substantial evidence for the propensity of
the Ni catalyst to undergo selective intramolecular “ring walk-
ing” along conjugated and nonconjugated systems that leads to
the desired “one-by-one” addition of monomers to a growing end
of the polymer chain.22-25 The KCTP technique has also been
applied to the synthesis of other polymers such as polythiophene-
containing block copolymers26-33 as well as poly(p-phenylenes)
and polypyrroles.34-36 A few research groups including ours have
succeeded in synthesizing poly(3-hexyl)thiophene (P3HT) di-
rectly from external aryl halide initiators both in solution and
on the surface.37-43 Senkovskyy et al. have successfully managed
to externally initiate a P3HT polymerization from both small
molecules and polymeric supports with the use of a Ni(PPh3)4
initiator to the Ni(0) catalyst (or catalyst precursor) is strongly
dependent on the position and the type of functionality on the
aryl group. In our previously published work, we observed that
the presence of another substituent on the aryl halide initiator
dramatically influenced the percent headgroup incorporation of
the polymerization. The initiation decreases by at least 50% if
such a substituent is an electron-withdrawing group and falls to
almost 0% when the functionality is electron donating.39 We
hypothesized that this effect could be attributed to the para
position of the substituent which renders the Ni initiator/catalyst
complex less stable than its unsubstituted counterpart. Work
published by Uchida et al.45 indicated that in formation of
aryl-Ni(II) complexes, ortho-substitution of the aromatic ligand
resulted in a dramatic increase in the stability of the Ni complex.
It is believed that the interactions of the ortho-substituent with the
Ni atom increases the stability of the complex by lowering the
energy of the highest occupied molecular orbital (HOMO)
through bonding of the dxy-orbitals of Ni with π-orbitals of the
aromatic ring. We observed that while the initiation of ortho-
chlorotoluene proceeds in almost quantitative manner, its para-
substituted isomer fails to initiate at all, which provides support
for this hypothesis.40 Further experiments performed in our
laboratory have indicated that the stabilization of the propagat-
ing Ni(II) catalyst-capped polymer by alkyl chains located ortho
to the terminal Br is a factor in promoting chain-growth polym-
erization character.46 Most of the studies on the external initiator
performed by our group as well as others, have focused on the
thermodynamic effect of the ortho-stabilization, however the
investigation of the kinetic effects as a result of steric hindrance
has not been well-documented. It has been shown that a methyl
substituent ortho to the halide group on the aryl initiator
stabilizes the complex, however the effect of other bulkier sub-
stituents in that same position has not been investigated. In fact,
much about the intermediate Ni catalyst/initiator complex remains
*Corresponding author. E-mail: luscombe@u.washington.edu.
pubs.acs.org/Macromolecules
Published on Web 01/12/2011
r 2011 American Chemical Society

Article
Macromolecules, Vol. 44, No. 3, 2011 513
Figure 1. Molecular structures of the initiators, their Ni complexes with PPh3 and dppp ligands, and the final polymer products.
Scheme 1. Synthesis of Phosphonate Initiators: Diethyl 4-Chloro-3-methylbenzylphosphonate, In1; Diethyl 2-Chlorobenzylphosphonate, In2; Diethyl
(5-Bromo-4-hexylthiophen-2-yl)methylphosphonate, In3; and Diethyl (5-Bromothiophen-2-yl)methylphosphonate, In4
speculative as many have not been isolated and thoroughly
studied due to their air-sensitive nature. We synthesized several
initiators (Figure 1, Scheme 1) in order to study the electronic vs
steric interactions and their effect on the chain-growth polymer-
ization mechanism.
insight on the geometry of the intermediate complexes. Geome-
trical optimizations of the two starting initiators (Scheme 1), In1
and In2 and their corresponding nickel catalyst/initiator com-
plexes were performed using Gaussian 09 program. Additionally,
the relative energies of the starting initiator (In1 and In2) to their
oxidative addition product (In1PPh3 and In2PPh3) were calcu-
lated. These theoretical studies combined with experimental
results allowed us to elucidate main factors determining the
initiator efficiency in the chain-growth polymerization reaction.
The phosphonate functionality on the initiator was chosen for
its many potential applications such as the ability to be utilized in
Horner-Wadsworth-Emmons reactions and the possibility of
deprotection to the phosphonic acid moiety known to form
robust monolayers on metal oxides which could possibly be
utilized as an anchoring layer for polymer brush synthesis.47
Therefore, we sought to verify whether we could incorporate such
a functionality at the beginning of the polymer chain. It is
worthwhile noting that when we have previously attempted
P3HT synthesis from an aryl halide inititator with the phospho-
nate group para to the halide, we observed complete degradation
of the Ni catalyst/initiator complex and it remained to be
investigated whether or not the ortho substituent would provide
sufficient stabilization to change the polymerization outcome. In
order to fully understand which effects are significant in the
formation of the nickel catalyst/initiator complex, it is important
to know the geometry of this transiently formed complex. Since
these materials are air-sensitive and thus difficult to structurally
characterize experimentally, DFT modeling was utilized to get
Experimental Section
Instrumentation and Materials. 1H NMR spectra were ob-
tained on Bruker AV300, Varian Mercuries Vx 300 and 400, and
AV-500 spectrometers using CDCl3 as a solvent (peak position
δ1H = 7.27 ppm) and internal standard of 1H NMR spectra was
tetramethylsilane (0.00 ppm). 31P NMR spectra were obtained
on Bruker AV300, Varian Mercury Vx400, and AV499 spectro-
meters with phosphoric acid as the standard. MALDI spectra
were recorded on Bruker Autoflex II spectrometer using terthio-
phene as the sample matrix. Samples were prepared by dissol-
ving 0.5 mg in 100 μL of matrix solution in chloroform and
approximately 1.5 μL of this solution was deposited on the plate.
The MALDI experiments were performed in the reflecton mode.
The % headgroup incorporation was calculated by analyzing

514 Macromolecules, Vol. 44, No. 3, 2011
Doubina et al.
1
the MALDI-TOF and the NMR spectra. The area under the
peaks on the MALDI spectra corresponding to the mass of the
polymer with the attached initiator group were divided by the
area under all the polymer peaks present and multiplied by 100
to obtain % headgroup incorporation. Molecular weights of
polymers were determined using a Waters-1515 gel permeation
chromatography (GPC) coupled with UV and RI detectors, in
reference of polystyrene standards with THF as the eluent with a
flow rate equal to 1 mL/min. GC-MS analyses were performed
using a 60 m, 0.32 mm ID, 1.0 μm HP-1 column or a 30 m, 0.32
mm ID, 0.25 μm HP-5MS column, programmed at 70 ꢀC for
2 min, to 300 or 340 at 15 ꢀC/min holding for 20 min. All elemen-
tal analyses were done at Atlantic Microlab (Norcross, GA).
All chemicals were purchased from Aldrich, Alfa-Aesar, or
TCI and used as received unless otherwise stated. Tetrahydro-
furan (THF) and toluene were dried using a solvent drying
system from Innovative Technologies. All polymerizations were
performed under N2.
eluent to afford In3 as deep orange oil. H NMR (300 MHz,
CDCl3) δ 6.63 (d, J = 3.6, 1H), 4.10-3.98 (m, 4H), 3.18 (dd,
JH-P = 21.7, JH-H = 0.8 Hz, 2H), 2.45 (t, J = 7.6 Hz, 2H),
1.55-1.43 (m, 2H), 1.29-1.21 (m, 12H), 0.84 (t, J = 6.4 Hz,
3H). 13C NMR (75 MHz, CDCl3): δ 142.2, 132.3, 128.7, 122.4,
62.7, 30.8 (d, JC-P = 150 Hz), 29.6, 29.5, 29.0, 27.7, 22.8, 16.6,
14.3. 31P NMR (162 MHz, CDCl3): δ 24.6. HRMS: calculated,
396.0524; found, 396.0507. Anal. Calcd: C, 45.35; H, 6.60; Br,
20.11; S, 8.07. Found: C, 45.52; H, 6.77; Br, 20.19; S, 8.33.
Synthesis of Nickel Catalyst/Initiator Complexes. Phospho-
nate initiators (15 equiv) were allowed to react with Ni(PPh3)4 (1
equiv) in 1 mL of toluene overnight in a glovebox environment
to form In1-PPh3 (100%), In2-PPh3 (39%), In3-PPh3 (31%)
(Figure 1). This step was followed by ligand exchange with dppp
(1.5 equiv) for 2-3 h without purification to form In1-dppp
(100%), In2-dppp (21%), In3-dppp (26%) also done in the glove-
box environment. The complex yields were obtained by analyz-
1
ing the H NMR spectra of the crude reaction mixture. Upon
Synthesis of Diethyl 4-Chloro-3-methylbenzylphosphonate
(In1). The starting carboxylic acid was reduced to the alcohol
by a procedure reported for a similar compound.48 A mixture of
4-chloro-3-methylbenzoic acid in distilled THF was stirred at
0 ꢀC under N2 atmosphere. A solution of 1 M borane in THF
(1.3 equiv) was added dropwise via an addition funnel and the
mixture stirred for 4.5 h. The excess hydride was quenched by
addition of water/THF (1:1) mixture, and basification with
carbonate/bicarbonate followed. After extraction, 1b was re-
covered as orange-colored crystals from toluene (19%). Con-
version to 1c was done by mixing 1b in dry toluene with
phosphorus tribromide under nitrogen at 40 ꢀC, followed by
heating to 100 ꢀC for 10 min, then letting cool to room tempe-
rature (80% yield). After extraction, conversion to phosphonate
proceeded through neat reaction of 1c with triethyl phosphite at
150 ꢀC under nitrogen (quantitative crude yield). Pure product
was obtained in 73% yield after purification through silica plug
reaction of In4 with Ni(PPh3)4 and subsequent ligand exchange
with dppp, the Ni/initiator complex underwent rapid decom-
position, thus the reaction yields could not be obtained. After
the reaction, the complex mixtures were transferred into sealed
round-bottom flasks and taken out of the glovebox.
General Polymerization Protocol. The monomer, 2-bromo-3-
hexyl-5-iodothiophene, was synthesized according to previously
published procedures.8 Addition of reagents into the reaction
flask was carried out via a syringe under a stream of nitrogen.
Polymerization was initiated by addition of the crude complex
mixture to a solution of 2-bromo-5-chloromagnesio-3-hexyl-
thiophene (30 equiv) which was prepared by the reaction of
i
2-bromo-3-hexyl-5-iodothiophene with 0.95 equiv of PrMgCl
at 0 ꢀC in THF for 1 h. After the addition of the complexes, the
reaction mixtures were allowed to polymerize for 1 h. After
quenching, the solutions were precipitated into methanol and
poly(3-hexyl)thiophene products, polyIn1 (90%), polyIn2 (78%),
and polyIn3 (69%) were collected via vacuum filtration and
analyzed without further purification.
DFT Calculations. Density functional theory (DFT; B3LYP)
calculations on the two chosen initiators, In1 and In2 as well as
two initiator/catalyst complexes In1-PPh3 and In2-PPh3 were
performed using the Gaussian 09 program.52 The standard
6-31G* basis set was used for all the atoms except Cl, and Ni,
for which 6-31G** and Ahlrichs’ valence triple-ζ (VTZ) basis
sets were chosen, respectively.53 Geometry optimization was
carried out using tight convergence criteria and an ultrafine inte-
gration grid. All calculations were performed assuming a singlet
ground state. The optimized structures were verified to be true
minima by frequency calculations. The nickel-catalyst binding
energies to initiator complexes, ΔENi-initiator, were calculated via
1
with ethyl acetate as the eluent. H NMR (300 MHz, CDCl3)
δ 7.21 (d, J = 8.2 Hz, 1H), 7.12 (dd, J = 2.4, 2.2 Hz, 1H), 7.01
(ddd, J = 8.2, 2.4, 2.2, 1H), 3.98 (dq, JH-P = 8.4, JH-H = 7.1
Hz, 4H), 3.03 (d, JH-P = 21.6 Hz, 2H), 2.30 (s, 3H), 1.21 (t, J =
7.1 Hz, 6H). 13C NMR (75 MHz, CDCl3): δ 136.3, 133.2, 132.5,
130.3, 129.3, 128.6, 62.4, 33.2 (d, JC-P = 138.6 Hz), 20.2, 16.6.
31P NMR (162 MHz, CDCl3): δ 26.6. HRMS: calculated,
276.0682; found, 276.0686. Anal. Calcd: C, 52.09; H, 6.56; Cl,
12.81. Found: C, 51.85; H, 6.67; Cl, 12.58.
Synthesis of Diethyl 2-Chlorobenzylphosphonate (In2). Neat
reaction of bromide 2a with triethyl phosphite (3 equiv) at 150 ꢀC
under nitrogen atmosphere overnight was followed by pump off
at 60 ꢀC under reduced pressure which resulted in In2, a color-
less oil in 96% yield which did not require further purification.49
1H NMR (300 MHz, CDCl3) δ 7.46-7.38 (m, 1H), 7.37-7.32
(m, 1H), 7.24-7.12 (m, 2H), 4.02 (dq, JH-P = 8.0, JH-H = 7.1
Hz, 4H), 3.34 (d, JH-P = 22.0 Hz, 2H), 1.25 (t, J = 7.1 Hz, 6H).
13C NMR (75 MHz, CDCl3): δ 134.4, 131.9, 130.1, 129.8, 128.5,
127.0, 62.4, 30.9 (d, JC-P = 133 Hz), 16.6. 31P NMR (162 MHz,
CDCl3): δ 26.3. HRMS: calculated, 262.0526; found, 262.0518.
Anal. Calcd: C, 50.30; H, 6.14; Cl, 13.50. Found: C, 50.16; H,
6.41; Cl, 13.23.
ΔENi - initiator ¼ ENi - initiator complex - ðEinitiator þ ENi catalyst
Þ
where ENi-initiator complex is the energy of the catalyst/initiator
complex, Einitiator is the energy of the initiator compound, and
ENi
is the energy of Ni(II) diphosphine catalyst. To
catalyst
facilitate the calculations, the ethyl substituents on the phos-
phonate groups were replaced by methyl groups.
Synthesis of (5-Bromo-4-hexylthiophen-2-yl)methylphospho-
nate (In3) and (5-Bromothiophen-2-yl)methylphosphonate (In4).
The procedure was based on previous reports for the synthesis of
In4.50,51 Pure product of In4 was obtained as lightly yellow
colored oil. 1H NMR (400 MHz, CDCl3): δ 6.85 (d, J = 3.7 Hz,
1H), 6.69 (ddt, J = 3.7, 3.7, 0.9 Hz, 1H), 4.05 (dq, J = 7.1, 7.1
Hz, 4H), 3.23 (dd, JH-P = 20.7, JH-H = 0.8 Hz, 2H), 1.26
(t, J = 7.1 Hz, 6H).
Similar conditions were used to obtain In3 (not previously
reported in the literature). The crude phosphonate (a dark
brown oil, 58% pure product by GC-MS) was subjected to
silica column chromatography with 2% ethanol in DCM, follo-
wed by a subsequent column with 1:1 hexane:ethyl acetate as the
Results and Discussion
KCTP Reaction Pathway. If the reaction proceeds via a
chain-growth polymerization mechanism, the P3HT product
will have one set of terminal groups, a low PDI, a molecular
weight that can easily be tuned by manipulating catalyst/
monomer feed ratio and almost exclusively head to tail
regioregularity (∼98%).54 During the “ideal” catalyst trans-
fer polymerization, where one Ni catalyst molecule propa-
gates through one polymer chain with addition of monomer
one at a time, after the reaction is quenched with acid, only
initiator/H end groups are observed (Scheme 2). However,

Article
Macromolecules, Vol. 44, No. 3, 2011 515
Scheme 2. Polymerization Reaction Pathway
undesired chain termination or chain-transfer reactions lead
to a variety of end groups such as initiator/Br if the catalyst
reductively eliminates from the polymer chain before com-
plete conversion of the monomer is reached or H/Br, H/H,
and Br/Br groups which result from the step-growth polym-
erization catalyzed by the various Ni species in the solution.38
The externally initiated P3HT synthesis pathway begins with
the oxidative addition of the aryl halide initiator to the
Ni(PPh3)4 catalyst where the halide group is either chlorine
or bromine. The oxidative addition step is followed by a
ligand exchange with dppp: we have previously shown this
ligand exchange procedure to yield fully regioregular P3HT
from an externally added o-chlorotoluene initiator with
controlled molecular weights and extremely narrow poly-
dispersities (Scheme 2). The final step is the addition of the
Grignard-functionalized 2-bromo-3-hexylthiophene mono-
mer that initiates the polymerization.
Figure 2. 31P spectra of nickel(II) complexes in toluene: (top) In1-dppp;
(bottom) In1-PPh3.
31P NMR Studies. The intermediates in the KCTP reac-
tion of the four initiators were monitored via 31P NMR
spectroscopy. After the addition of Ni(PPh3)4, the 31P NMR
spectrum (Figure 2) shows a sharp peak at 19 ppm, consistent
with trans-chloro-(3-methylbenzylphosphonate)bis(triphenyl-
phosphine)nickel(II) complex (In1-PPh3).55 The peak at 24
ppm corresponds to the phosphonate group on the starting
In1 which was present in a 15 equivalent excess in the re-
action mixture. The remaining peak at 26 ppm is attributed
to the phosphonate group in In1-PPh3 complex. Addition of
1.5 equivalents of dppp resulted in rapid ligand subsitution
to form the thermodynamically favored cis-chloro(3-methyl-
benzylphosphonate)(dppp)nickel(II)complex, (In1-dppp). The
formation and the geometry of this complex were confirmed
by the appearance of two sets of doublets at 18 ppm and -6
ppm (which is obscured by the peak of liberated PPh3). A
peak corresponding to Ni(dppp)2 was also observed at 11
ppm. The 1H NMR spectra of In1-PPh3 and In1-dppp show
that the oxidative addition and ligand exchange reactions
proceed to completion resulting in 100% yield of desired
complexes.
dppp. Analysis of the 1H NMR spectrum of In2-PPh3 reveals
that the oxidative addition reaction did not proceed to com-
pletion as seen by two sets of peaks corresponding to the
protons nearest to the reaction center. The yield of the oxidative
addition adduct, which is obtained through integration of both
peaks, is calculated to be 39%. After the addition of dppp, 1H
NMR data confirms only partial ligand exchange as evidenced
by a set of three peaks. The analysis of the final mixture after
oxidative addition and ligand exchange steps reveals the
following species containing the initiator molecule: In2
(58%), In2-PPh3 (21%), and In2-dppp (21%). Ni(dppp)2 is
1
present as well, however it is difficult to quantify with H
NMR. The 31P NMR spectrum of In3-PPh3 (Figure S3, Sup-
porting Information) indicates the presence of the trans-
bromo(4-hexylthiophen-2-yl methyl phosphonate)nickel(II)
complex. The phosphine peak of the complex appears in the
region of 19 ppm while the phosphonate groups of In3-PPh3
and In3 are present at 25 and 23 ppm respectively. An
additional peak is observed at 21 ppm that could correlate
to the phosphine peak where the thiophene ring is coordi-
nating to the Ni complex. After the addition of dppp,
the characteristic set of doublets can be seen at þ16 and
-5 ppm. Additional peaks are present between the phos-
phonate groups of In3 and In3-PPh3 (22-24 ppm) that are
attributed to the nickel species where the thiophene ring is
The 31P NMR spectrum of In2-PPh3 (Figure S2, Support-
ing Information), also confirms the successful formation of
the trans-chloro(diethylbenzylphosphonate)nickel(II) com-
plex via the presence of the PPh3 peak which is slightly shifted
possibly due to the coordination of the nickel with the nearby
oxygen atom. The phosphonate groups of In2-PPh3 and of
excess In2 are present at 26 and 24 ppm respectively. After
the addition of dppp, the two sets of doublets corresponding
to the cis complex In2-dppp are observed at þ19 and -7 ppm.
The PPh3 peak of In2-PPh3 is still present, which indicates
that the ligand reaction did not reach completion in the
allotted reaction time frame. The peak for Ni(dppp)2 is also
larger suggesting partial dissociation of the Ni catalyst from
initiator/catalyst complex and its subsequent chelation by
1
coordinating to the metal center. H NMR spectra reveals
incomplete conversion of the original initiator into the
desired oxidative adduct with the complex yield of In3-
PPh3 determined to be 31%. However, the ligand exchange
goes almost to completion evidenced by the lack of In3-
PPh3 peak. The In3-dppp complex yield was calculated to
be 26% over the two steps. In4 has proved to be the least

516 Macromolecules, Vol. 44, No. 3, 2011
Doubina et al.
Table 1. Overview of Results from the Polymerization Reactions of
polyIn1, polyIn2, and polyIn3
stable of the initiators. Upon addition of the Ni(PPh3)4
to the initiator, a white precipitate, likely being a mixture of
Ni(II) halide salts and other decomposition products, was
observed in the reaction mixture after a short period of time.
The 31P NMR analysis revealed the presence of only the starting
material In4 which was in excess. Since the desired In4-PPh3 and
In4-dppp initiating complexes and consequently polyIn4 are not
formed with this thiophene molecule, this initiator was not
included in further studies. The disproportionation reaction
likely arises from the lack of a substituent in the ortho position.
Since the aromatic stabilization of thiophene is already weak
(less than half of benzene),56 we speculate that in the absence of
favorable interactions between the thiophene π system and the
nickel d-orbitals the reacting species decompose rapidly.
Polymerization Results. The results of varying initiators on
the polymer end groups and molecular weight are summar-
ized in Table 1.
Figure 3. MALDI-TOF spectra of polyIn1 (top) and polyIn2
(bottom).
The use of In1 has afforded a polymer with a good %
headgroup incorporation, a molecular weight that is close to
the calculated Mn and narrow PDI all of which indicate the
predominance of a chain growth polymerization mechanism.
Various end groups are observed such as initiator/H, initi-
ator/iPr, and H/H. Other extremely small peaks in the
MALDI-TOF spectrum could not be identified and are
likely polymers with a fragmented initiator group. The
initiator/iPr groups are likely to arise from a minor mismatch
yields, polyIn2 shows higher % headgroup incorporation
than polyIn3 (45% vs 27%). The end group distribution of
polyIn2 is similar to those for polyIn1 albeit with a higher
percentage of H/H (Figure 3). As previously mentioned, the
final mixture composition before the monomer addition
contained In2, In2-PPh3, and In2-dppp with the total percen-
tage of catalytically active species equal to 41%. Both In2-
PPh3 and In2-dppp are able to initiate the polymerization,
however the major difference between the two species is how
long the Ni catalyst can propagate through the polymer
chain. As evidenced in our previous work, Ni with PPh3
ligand tends to reductively eliminate much faster than the
catalyst with the dppp ligand.38-40 Once the Ni catalyst
leaves the polymer chain, it begins linking monomer units
in a random step growth fashion, without the intramolecular
reinsertion into the same polymer chain that is inherent to
the KCTP mechanism, which results in the H/H terminated
P3HT product. Thus, the % headgroup incorporation is very
similar to the total amount of complexed initiator/catalyst
species (45% vs 41%), however, the tendency of Ni(PPh3)2
species to quickly undergo reductive elimination results in a
higher fraction of H/H polymer.
i
between the relative amounts of m1 and PrMgCl solution
because of difficulties in precisely measuring very small volu-
mes of iPrMgCl solution and its high sensitivity to the mois-
ture and air. Despite a reaction time of only 1 h in order to
avoid precipitation of P3HT at high conversion, the molec-
ular weight determined by SEC (5200 Da) is very close to the
calculated molecular weight (5224 Da) of the 30 unit 3-hexyl
thiophene chain. The molecular weight calculated via NMR
agrees quite well with the molecular weight determined by
SEC showing a polymer with 30 monomer repeat units is
indeed obtained in such a short reaction time. The PDI value
of 1.2 is much lower than the PDI values for initiated P3HT
using other ligands,39 which shows the more controlled poly-
merization mechanism with the dppp ligand. As previously
mentioned, ealier attempts of initiating a polymerization
from a para-substituted benzylphosphonate molecule have
failed and were attributed to the destabilization of the Ni
catalyst by the phosphonate moiety. These polymerization
results illustrate both the importance of the ortho-substituent
and the possibility of introducing a phosphonate function-
ality with numerous potential applications onto the polymer
chain. In agreement to the expected trend based on the 31P
The major end groups of polyIn3 such as H/Br and
H/H indicate significant occurrence of chain-termination
and chain-transfer processes that are common to a non-
initiated step-growth polymerization mechanism. Lower
molecular weights (3900 and 2900 Da) as well as higher PDIs
(1.6 and 2.6) reflect the decreasing chain-growth character
and increasing step-growth character of the polymerizations
from In2 and In3. The lowest % headgroup incorporation
1
NMR and H NMR spectra as well as calculated complex

Article
Macromolecules, Vol. 44, No. 3, 2011 517
1
from In3 is supported by H NMR data which reveals a
low yield of the desired catalytic complex (26%). Thiophene
is known to bond to metals in several different configura-
tions in organometallic complexes.57,58 It can bond through
the aromatic π-system of the thiophene ring (η5 config-
uration) or through the S atom as a weak Lewis base.
Alternatively, the thiophene ring may pucker and the bond-
ing can occur through the four carbon atoms (η4 config-
uration) with the sulfur out of plane of the C4 system. It was
also previously demonstrated that thiophene initially coor-
dinates to a platinium bisalkylphosphine fragment through
the C-C double bond (η2 configuration) through the
π-backbonding from the metal dxy-orbital to a thiophene C-C
π*-orbital.59 Other d8 metals such as nickel and palladium
are known to form thiophene metallocycle complexes which
exist in equilibrium with thiophenic bridged dinuclear com-
plexes and free thiophene.60 In that case, the bonding inter-
actions result from the donation of electron density from the
HOMO of the Ni(dmpe) fragment into the LUMO of the
thiophene fragment and back-donation from the HOMO of
the thiophene into the LUMO of the metal fragment. Since
donation of electron density from the metal to the thiophene
ligand is the main factor in this coordination mechanism, it is
reasonable to speculate that the strongly electron-withd-
rawing phosphonate group leaves the thiophene π-system
electron deficient thus promoting the formation of the
coordinated complex. We believe that the coordination of
the nickel to the thiophene ring prevents the formation of
In3-PPh3, which decreases headgroup incorporation into the
polymer chain.
Figure 4. DFT-optimized structures and atom labeling for In1 (left)
and In2 (right). Mulliken charges for the selected atoms are indicated in
blue.
rather similar as well. Both catalyst-initiator complexes In1-
PPh3 and In2-PPh3 have slightly distorted square planar
geometries especially for In2-PPh3 where the angles slightly
deviate from the expected 90ꢀ. In terms of bond length, the
Ni(II) coordination site is also similar in both complexes.
However, in In2-PPh3, the oxygen atom of the ortho-phos-
phonate functionality on the aryl ligand comes to close
proximity to the Ni(II) fifth coordination site. This structural
feature might play a determining role in polymerization
outcome, as outlined below.
There are at least two possible explanations how steric
effects could inhibit the % headgroup incorporation for In2-
PPh3, one of which is the possible coordination of the oxygen
atom of the phosphonate group to the Ni metal center,
inhibiting reaction with an incoming ligand. There have been
numerous reports of the PdO group coordinating to the
metal center via the oxygen atom62-75 particularly when the
two are in close proximity to each other such as the case with
Molecular Modeling and Binding Energies. In order to
investigate the effects that may have played a role in the
decrease in the initiation of In2 as compared to In1, the
geometries of both compounds as well as their corresponding
oxidative adducts In1-PPh3 and In2-PPh3 were modeled
with the Gaussian 09 program. Initially, the potential en-
ergies of the optimized structures of In1-PPh3 and In2-PPh3
were calculated and compared (for this purpose, a methyl
substituent was added to the para position of In2-PPh3 in
order to keep the equivalent number of atoms). It is inter-
esting to note that changing the substituent at the ortho
position of the initiator from a methyl group to the phos-
phonate group has no effect on the potential energy value of
the molecule leading to the conclusion that both substituents
offer similar degree of electronic stability to the nickel(II)
complex. Therefore, it is likely that kinetic effects are re-
sponsible for the decrease in initiation. To test this hypoth-
esis, the energies for the formation of In1-PPh3 and In2-PPh3
from the starting reaction components of In1 and In2 and
Ni(PPh3)2 were calculated. If electronic effects played a
determining role in the reaction mechanism, the binding
energy of the complex with the more favorable substituent
would be more exothermic. The energy of complexation of
In1 with Ni(PPh3)2, ΔE1 was calculated to be -36.27 kcal/
mol while the energy of the complexation of In2 with the Ni
catalyst, ΔE2 afforded a very similar value with a negligible
difference of less than 1 kcal. These values are similar to other
reported values for the energy of formation for nickel
complexes.61 This shows that both reactions are equally
spontaneous in agreement with the hypothesis that the
effects on the decreased % headgroup incorporation are
more likely due to kinetic effects rather than electronic
effects. Optimized molecular structures of the starting com-
pounds and the complexes are shown in Figures 4 and 5,
respectively. Selected bond distances and angles are given in
Tables 2 and 3. Compounds In1 and In2 have very similar
˚
In2-PPh3 where the Ni O distance is 3.443 A. Such bind-
ing would inhibit the dppp ligand exhange reaction as was
3 3 3
observed in the 31P NMR and H NMR, where there was
1
significant amount of In2-PPh3 remaining after the addition
of 1.5 equiv of dppp. Since the dppp and PPh3 ligand
exchange occurs via an associative mechanism, as do a vast
majority of ligand exchanges for the d10 metals,76,77 a five-
coordinate Ni complex would lack an available site that
would facilitate the ligand exchange. Thus, a significant
fraction of In2-PPh3 complex remains behind leading to a
higher yield of step-growth polymer. It should be noted,
however, that the DFT modeling of the In2-PPh3 complex
showed that the phosphonate functionality can accept dif-
ferent orientations around the C3-P1 bond. The geometry
optimization initiated with different starting geometries for
the In2-PPh3 complex, led to stable structures with different
phosphonate group orientations, where the oxygen atom
was either in close proximity to the Ni center as shown in
Figure 5, or turned away from the Ni. These structures were
found to be less than 0.5 kcal/mol from each other. Thus, it is
plausible that in solution, an average ensemble of these
conformations exist allowing ligand exchange to take place,
which was observed experimentally.
A second possible explanation is that a bulky phosphonate
group ortho to the initiation site may affect the kinetics of the
oxidative addition reaction. Kochi and others have shown
that the mechanism of oxidative addition reaction of nickel-
(0) complexes with aromatic halides is quite complex and can
involve multiple Ni(I) and Ni(II) species depending on the
nature of the aryl halide and it was found that the mechanism
involves several steps.78,79 The first step is the equilibrium
formation of the coordinatively unsaturated NiL3 from NiL4
(eq 1), with the ligand studied in this case being L = PEt3.
˚
C1-Cl bond distances of ca. 1.763 A. The bond angles are

518 Macromolecules, Vol. 44, No. 3, 2011
Doubina et al.
Figure 5. Top-view of the DFT-optimized structures of In1-PPh3 (left) and In2-PPh3 (right) showing Ni(II) coordination site (hydrogen atoms have
been omitted for clarity). Mulliken charges for the selected atoms are indicated in blue.
Table 2. Selected Bond Lengths, Atom Distances, and Bond Angles
for In1 and In2
Table 3. Selected Bond Lengths, Atom Distances, and Bond Angles
for the Ni(II) Complexes In1-PPh3 and In2-PPh3
˚
bond length/atom distance (A)
˚
bond length/atom distance (A)
bond of interest
In1
In2
bond of interest
In1-PPh3
In2-PPh3
C1-Cl
C2-C3
P-O1
1.764
1.508
1.487
3.094
8.513
1.763
1.510
1.485
3.139
3.943
Ni-Cl
Ni-C1
Ni-P2
Ni-P3
C2-C3
P1-O1
2.278
1.895
2.296
2.294
1.512
1.488
3.295
8.738
2.279
1.897
2.283
2.342
1.519
1.492
3.395
3.443
C3 Cl
3 3 3
O1 Cl
3 3 3
bond angles (deg)
C3 Ni
3 3 3
O1 Ni
3 3 3
angle of interest
In1
In2
bond angles (deg)
—ClC1C2
—C1C2C3
119.8
122.3
120.7
123.1
angle of interest
In1-PPh3
In2-PPh3
This step is followed by the slow rate-limiting electron tran-
sfer in eq 2 to afford an ion pair indicated in brackets. Cage
collapse affords oxidative adduct in eq 3.
—C1NiCl
—P2NiP3
—ClNiC1C2
170.1
179.6
176.3
172.9
174.1
164.6
NiL4 / NiL3 þ L ðK1Þ
ð1Þ
ð2Þ
ð3Þ
phosphonate functional group with varying % headgroup in-
corporation. Attempts of polymerization from a para-phospho-
nate substituted aryl halide initiator were made in our previous
work and proved unsuccessful.39 These current findings further
emphasize the importance of a stabilizing ortho-substituent on
the success of the initiation of a KCTP reaction. Thiophene based
initiator, In4, lacking a hexyl chain in the 3-position failed to form
the desired oxidative adduct upon its reaction with the nickel
catalyst, suggesting rapid decomposition of the reactant likely
due to its weak aromatic stabilization and the lack of a stabilizing
group. 31P NMR studies of initiators In1-In3 showed peaks
attributed to desired products of oxidative addition and ligand
exchange steps in addition to a few side product peaks. In1
provided the best results, with the highest % headgroup incor-
poration of 84%, a molecular weight close to the calculated Mn
and a narrow PDI which all pointed to a chain-growth mechan-
ism and is considered the most promising initiator in this study.
In2 and In3 resulted in a mixture of chain-growth and step-
growth polymerization products. In2 performed better than In3
with % incorporation of 45% and 27% respectively. The poor
yield of initiated polymer from the In3 initiator was attributed to
the nickel metal center coordinating to the thiophene ring and
inhibitingits abilityto undergo the crucial intermolecular transfer
step. The difference in initiation efficiencies between In1 and In2
was suggested to originate primarily from the steric effects. The
proximity of the phosphonate group to the oxidative addition site
was speculated to promote oxygen atom coordination to the
nickel preventing it from undergoing the dppp ligand exchange
Ni0L3 þ ArX f ½NiIL3ArX-
ꢀ ðk2Þ
½NiIL3ArX- ꢀ f ArNiIIXL2 þ L
They showed that the triphenylphosphine ligand is more
highly dissociated and is less readily oxidized than the trie-
thylphosphine ligand. Thus under comparable conditions,
the reaction of aryl halides with Ni(PPh3)3 is significantly
slower than that with Ni(PEt3)3.80 Also, Kochi et al. have
carried out a study of the steric effects from ortho-methyl
substitution on the reaction of nickel(0) with bromobenzene
in toluene solution and found that addional steric bulk from
the substituents results in the decrease of the rate constant for
the rate determining step (RDS).79 Taking all of the above
information into the account, it is reasonable to draw a con-
clusion that the phosphonate substituent in such close prox-
imity to the reaction site, decreases the rate of the reaction.
Since the two complexes are allowed to react for exactly the
same time, the decreased rate constant for the In2-PPh3 reac-
tion results in lower yield of the oxidative addition product as
1
evidenced by H NMR which would result in decreased %
headgroup incorporation.
Conclusions
In this work, poly(3-hexylthiophene) was externally initiated
from aryl and thiophene halide molecules with an attached

Article
Macromolecules, Vol. 44, No. 3, 2011 519
and also to slow down kinetics of the reaction both contributing
to decreased % headgroup incorporation.
(31) Zhang, Y.; Tajima, K.; Hirota, K.; Hashimoto, K. J. Am. Chem.
Soc. 2008, 130, 7812.
(32) Tu, G.; Li, H.; Forster, M.; Heiderhoff, R.; Balk, L. J.; Sigel, R.;
Scherf, U. Small 2007, 3, 1001.
Acknowledgment. This work was supported by the NSF (STC-
MDITR DMR 0120967, MRSEC-GEMSEC DMR 0520567, CA-
REER Award DMR 0747489), and the Office of Science DOE
Energy Frontier Research Center for Interface Science: Solar Electric
Materials (DE-SC0001084).
(33) Van den Bergh, K.; Huybrechts, J.; Verbiest, T.; Koeckelberghs, G.
Chem.;Eur. J. 2008, 14, 9122.
(34) Adachi, I.; Miyakoshi, R.; Yokoyama, A.; Yokozawa, T. Macro-
molecules 2006, 39, 7793.
(35) Huang, L.; Wu, S. P.; Qu, Y.; Geng, Y. H.; Wang, F. S. Macro-
molecules 2008, 41, 8944.
(36) Stefan, M. C.; Javier, A. E.; Osaka, I.; McCullough, R. D. Macro-
Supporting Information Available: MALDI-TOF spectra
of polyIn1, polyIn2, and polyIn3, 31P NMR spectra of In1-PPh3,
In2-PPh3, In3-PPh3, In1-dppp, In2-dppp, and In3-dppp, and
Cartesian coordinates of the DFT models. This material is
available free of charge via the Internet at http://pubs.acs.org
molecules 2009, 42, 30.
(37) 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.
(38) Bronstein, H.; Luscombe, C. K. J. Am. Chem. Soc. 2009, 131,
12894.
(39) Doubina, N.; Ho, A.; Jen, A. K. Y.; Luscombe, C. K. Macro-
molecules 2009, 42, 7670.
References and Notes
(40) Doubina, N.; Stoddard, M.; Bronstein, H.; Jen, A. K. Y.; Luscombe,
C. K. Macromol. Chem. Phys. 2009, 210, 1966.
(41) Khanduyeva, N.; Senkovskyy, V.; Beryozkina, T.; Bocharova, V.;
Simon, F.; Nitschke, M.; Stamm, M.; Grotzschel, R.; Kiriy, A.
Macromolecules 2008, 41, 7383.
(42) Khanduyeva, N.; Senkovskyy, V.; Beryozkina, T.; Horecha, M.;
Stamm, M.; Uhrich, C.; Riede, M.; Leo, K.; Kiriy, A. J. Am. Chem.
Soc. 2009, 131, 153.
(1) Bao, Z. N.; Lovinger, A. J. Chem. Mater. 1999, 11, 2607.
(2) Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007, 107,
1324.
(3) Iovu, M. C.; Jeffries-El, M.; Sheina, E. E.; Cooper, J. R.; McCullough,
R. D. Polymer 2005, 46, 8582.
(4) Loewe, R. S.; Khersonsky, S. M.; McCullough, R. D. Adv. Mater.
1999, 11, 250.
(5) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. Adv. Funct.
(43) Smeets, A.; Van den Bergh, K.; De Winter, J.; Gerbaux, P.;
Verbiest, T.; Koeckelberghs, G. Macromolecules 2009, 42, 7638.
(44) Senkovskyy, V.; Khanduyeva, N.; Komber, H.; Oertel, U.; Stamm,
M.; Kuckling, D.; Kiriy, A. J. Am. Chem. Soc. 2007, 129, 6626.
(45) Hidai, M.; Kashiwag., T; Ikeuchi, T.; Uchida, Y. J. Organomet.
Chem. 1971, 30, 279.
Mater. 2005, 15, 1617.
(6) McCullough, R. D. Adv. Mater. 1998, 10, 93.
(7) McCullough, R. D.; Lowe, R. D. J. Chem. Soc., Chem. Commun.
1992, 70.
(8) Miyakoshi, R.; Yokoyama, A.; Yokozawa, T. J. Am. Chem. Soc.
2005, 127, 17542.
(46) Boyd, S.; Jen, A. K. Y.; Luscombe, C. K. Macromolecules 2009, 42,
9387.
(9) Reyes-Reyes, M.; Kim, K.; Carroll, D. L. Appl. Phys. Lett. 2005,
87.
(47) Paramonov, P. B.; Paniagua, S. A.; Hotchkiss, P. J.; Jones, S. C.;
Armstrong, N. R.; Marder, S. R.; Bredas, J. L. Chem. Mater. 2008,
20, 5131.
(48) Kelley, J. L.; Linn, J. A.; Selway, J. W. T. J. Med. Chem. 1989, 32,
1757.
(49) Franke, A.; Frickel, F. F.; Schlecker, R.; Thieme, P. C. Synthesis
(10) Sacco, A.; Mastrorilli, P. J. Chem. Soc., Dalton Trans. 1994,
2761.
(11) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.;
Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.;
Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; De Leeuw, D. M.
Nature 1999, 401, 685.
1979, 712.
(12) Sontag, S. K.; Marshall, N.; Locklin, J. Chem. Commun. 2009,
3354.
(13) Loewe, R. S.; Ewbank, P. C.; Liu, J. S.; Zhai, L.; McCullough,
R. D. Macromolecules 2001, 34, 4324.
(14) McCullough, R. D.; Lowe, R. D.; Jayaraman, M.; Anderson, D. L.
(50) Jiang, X.; Yang, X. C.; Zhao, C. Z.; Jin, K.; Sun, L. C. J. Phys.
Chem. C 2007, 111, 9595.
(51) Mamane, V.; Ledoux-Rak, I.; Deveau, S.; Zyss, J.; Riant, O.
Synthesis 2003, 455.
(52) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.;
Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian,
H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.;
Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.;
Vreven, T.; Montgomery, J., J., A.; Peralta, J. E.; Ogliaro, F.;
Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov,
V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell,
A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.;
Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin,
R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador,
P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09,
Revision A.02, Gaussian, Inc.: Wallingford, CT, 2009.
(53) Bauernschmitt, R.; Ahlrichs, R. Chem. Phys. Lett. 1996, 256,
454.
J. Org. Chem. 1993, 58, 904.
(15) Sheina, E. E.; Liu, J. S.; Iovu, M. C.; Laird, D. W.; McCullough,
R. D. Macromolecules 2004, 37, 3526.
(16) Chen, T. A.; Obrien, R. A.; Rieke, R. D. Macromolecules 1993, 26,
3462.
(17) Chen, T. A.; Rieke, R. D. J. Am. Chem. Soc. 1992, 114, 10087.
(18) Chen, T. A.; Wu, X. M.; Rieke, R. D. J. Am. Chem. Soc. 1995, 117,
233.
(19) Miyakoshi, R.; Yokoyama, A.; Yokozawa, T. J. Polym. Sci., Part
A 2008, 46, 753.
(20) Yokoyama, A.; Miyakoshi, R.; Yokozawa, T. Macromolecules
2004, 37, 1169.
(21) Iovu, M. C.; Sheina, E. E.; Gil, R. R.; McCullough, R. D. Macro-
molecules 2005, 38, 8649.
(22) Beryozkina, T.; Senkovskyy, V.; Kaul, E.; Kiriy, A. Macromole-
cules 2008, 41, 7817.
(23) Zenkina, O. V.; Karton, A.; Freeman, D.; Shimon, L. J. W.;
Martin, J. M. L.; van der Boom, M. E. Inorg. Chem. 2008, 47,
5114.
(24) Lanni, E. L.; McNeil, A. J. J. Am. Chem. Soc. 2009, 131, 16573.
(25) Lanni, E. L.; McNeil, A. J. Macromolecules 2010, 43, 8039.
(26) Iovu, M. C.; Craley, C. R.; Jeffries-El, M.; Krankowski, A. B.;
Zhang, R.; Kowalewski, T.; McCullough, R. D. Macromolecules
2007, 40, 4733.
(54) Yokoyama, A.; Yokozawa, T. Macromolecules 2007, 40, 4093.
(55) Zeller, A.; Herdtweck, E.; Strassner, T. Eur. J. Inorg. Chem. 2003,
1802.
(56) Huntley, D. R.; Mullins, D. R.; Wingeier, M. P. J. Phys. Chem.
1996, 100, 19620.
(57) Angelici, R. J. Coordin. Chem. Rev. 1990, 105, 61.
(58) Rauchfuss, T. B. Prog. Inorg. Chem. 1991, 39, 259.
(59) Atesin, T. A.; Jones, W. D. Organometallics 2008, 27, 53.
(60) Atesin, T. A.; Oster, S. S.; Skugrud, K.; Jones, W. D. Inorg. Chim.
Acta 2006, 359, 2798.
(27) Jeffries-El, M.; Sauve, G.; McCullough, R. D. Adv. Mater. 2004,
16, 1017.
(28) Ohshimizu, K.; Ueda, M. Macromolecules 2008, 41, 5289.
(29) Richard, F.; Brochon, C.; Leclerc, N.; Eckhardt, D.; Heiser, T.;
Hadziioannou, G. Macromol. Rapid Commun. 2008, 29, 885.
(30) Urien, M.; Erothu, H.; Cloutet, E.; Hiorns, R. C.; Vignau, L.;
Cramail, H. Macromolecules 2008, 41, 7033.
(61) Craciun, R.; Vincent, A. J.; Shaughnessy, K. H.; Dixon, D. A.
Inorg. Chem. 2010, 49, 5546.

520 Macromolecules, Vol. 44, No. 3, 2011
Doubina et al.
(62) Chandrasekhar, V.; Azhakar, R.; Senapati, T.; Thilagar, P.;
Ghosh, S.; Verma, S.; Boomishankar, R.; Steiner, A.; Kogerler,
P. K. Dalton Trans. 2008, 1150.
(63) Gahagan, M.; Mackie, R. K.; Colehamilton, D. J.; Cupertino,
D. C.; Harman, M.; Hursthouse, M. B. J. Chem. Soc., Dalton
Trans. 1990, 2195.
(64) Grushin, V. V. Chem. Rev. 2004, 104, 1629.
(65) Heinicke, J.; Musina, E.; Peulecke, N.; Karasik, A. A.; Kindermann,
M. K.; Dobrynin, A. B.; Litvinov, I. A. Z. Anorg. Allg. Chem. 2007,
633, 1995.
(71) Popa, A.; Davidescu, C. M.; Negrea, P.; Ilia, G.; Katsaros, A.;
Demadis, K. D. Ind. Eng. Chem. 2008, 47, 2010.
(72) Reisinger, C. M.; Nowack, R. J.; Volkmer, D.; Rieger, B. Dalton
Trans. 2007, 272.
(73) Sgarbossa, P.; Pizzo, E.; Scarso, A.; Sbovata, S. M.; Michelin,
R. A.; Mozzon, M.; Strukul, G.; Benetollo, F. J. Organomet. Chem.
2006, 691, 3659.
(74) Weis, K.; Rombach, M.; Vahrenkamp, H. Inorg. Chem. 1998, 37,
2470.
(75) Cadierno, V.; Diez, J.; Garcia-Alvarez, J.; Gimeno, J.; Rubio-
(66) Konar, S.; Clearfield, A. Inorg. Chem. 2008, 47, 5573.
(67) Kong, D. Y.; Medvedev, D. G.; Clearfield, A. Inorg. Chem. 2004,
43, 7308.
Garcia, J. Dalton Trans. 2008, 5737.
(76) Crabtree, R. H. The organometallic chemistry of the transition
metals; Wiley-Interscience: NJ, 2005.
(68) Morise, X.; Braunstein, P.; Welter, R. CR. Chim. 2003, 6, 91.
(69) Morise, X.; Braunstein, P.; Welter, R. Inorg. Chem. 2003, 42, 7752.
(70) Oberbeckmann-Winter, N.; Morise, X.; Braunstein, P.; Welter, R.
Inorg. Chem. 2005, 44, 1391.
(77) Kalek, M.; Stawinski, J. Organometallics 2008, 27, 5876.
(78) Stille, J. K.; Lau, K. S. Y. Acc. Chem. Res. 1977, 10, 434.
(79) Tsou, T. T.; Kochi, J. K. J. Am. Chem. Soc. 1979, 101, 6319.
(80) Parshall, G. W. J. Am. Chem. Soc. 1974, 96, 2360.