Detrimental Ni(0) transfer in Kumada catalyst transfer polycondensation of benzo[2,1-b:3,4-b']dithiophene

Article abstract of DOI:10.1002/pola.28026

This article deals with the Kumada Catalyst Transfer Polycondensation (KCTP) of 4,7-dioctylbenzo[2,1-b:3,4-b']dithiophene (BDP-Oct) using Ni(II) catalyst or In/cat combination. A combination of MALDI MS, GPC, and ³¹P NMR spectroscopy is used to reveal the failure of the KCTP of this particular monomer. Intermolecular transfer reactions to monomer appeared to prevent the formation of polymer. This result is remarkable, since isomeric benzo[1,2-b:4,5-b']dithiophene polymerizes in a controlled way. The presence of a "non-aromatic double bond" in annulated monomers is discussed.

Full text of DOI:10.1002/pola.28026

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Detrimental Ni(0) Transfer in Kumada Catalyst Transfer Polycondensation
of Benzo[2,1-b:3,4-b’]dithiophene
1
2
2
1
Anjan Bedi, Julien De Winter, Pascal Gerbaux, Guy Koeckelberghs
1
Department of Chemistry, Laboratory for Polymer Synthesis, KU Leuven, Celestijnenlaan 200F, B-3001, Heverlee, Belgium
Organic Synthesis and Mass Spectrometry Laboratory, Research Institute for Materials Sciences and Engineering,
2
University of Mons—UMONS, Place Du Parc 23, B-7000, Mons, Belgium
Correspondence to: G. Koeckelberghs (E-mail: guy.koeckelberghs@chem.kuleuven.be)
Received 21 October 2015; accepted 7 December 2015; published online 8 January 2016
DOI: 10.1002/pola.28026
ABSTRACT: This article deals with the Kumada Catalyst Transfer
Polycondensation (KCTP) of 4,7-dioctylbenzo[2,1-b:3,4-b’]dithio-
phene (BDP-Oct) using Ni(II) catalyst or In/cat combination. A
since isomeric benzo[1,2-b:4,5-b’]dithiophene polymerizes in a
controlled way. The presence of a “non-aromatic double bond”
2016 Wiley Periodicals,
in annulated monomers is discussed. VC
3
1
combination of MALDI MS, GPC, and P NMR spectroscopy is
used to reveal the failure of the KCTP of this particular mono-
mer. Intermolecular transfer reactions to monomer appeared to
prevent the formation of polymer. This result is remarkable,
Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 1706–1712
KEYWORDS: conducting polymers; KCTP; living polymerization;
MALDI; metal-polymer complexes
1
5
16
17
INTRODUCTION In the past couple of decades, conjugated
polymers (CPs) having fused aromatic rings has been an
important topic of research due to their interesting optoelec-
tronic properties and ability to furnish a variety of materials
in organic electronics. In this respect, 4,8-disubstituted
benzo[1,2-b:4,5-b’]dithiophene (BDT) (Fig. 1) is a superior
candidate over many others when applied in polymer solar
cell and field effect transistors. This can be attributed to the
large planar conjugated structure of the BDT unit, so that
poly(pyridine)s, poly(pyrrole)s, poly(bithienylmethylene)s,
0
0
18
19
poly(dithieno[3,2-b; 2 ,3 -d]- silole)s, poly(fluorene)s, poly-
cyclopenta[2.1-b:3,4-b ] dithiophene) and many block copoly-
mers. Such a broad diversity in the targeted monomers
clearly reveals the increasing interest in developing controlled
polymerization procedures of CPs, using methods such as the
Kumada catalyst transfer polycondensation (KCTP).
0
20
(
21
The main requirement to steer these organometallic polycon-
densations into the “living” polymerization is to find suitable
conditions for the formation of an associative p-complex
between the growing polymer and the catalytic species gen-
erated after the reductive elimination. In this respect, three
possible side-reactions can hamper the controlled synthesis
of CPs. First, the interaction between the growing polymer
the catalyst can be too weak, resulting in the diffusion of the
catalyst. This is the reason why electron deficient monomers
1
–4
polymers can easily form p–p interactions.
building block, cyclopenta[2,1-b;3,4-b ]dithiophene (CPDT),
that consists of a thiophene dimer bridged in the 3,3 -posi-
Another fused
10,22
0
5
0
tion by a saturated carbon atom, has shown its potential in
6
,7
materials for organic electronics.
A major change in the synthesis of the CPs (from the reported
and vastly used conventional step growth polymerization)
2
3
are notoriously difficult to polymerize in a controlled way.
Second, the monomer can also interact with the catalyst of
the “associative pair” resulting in a transfer reaction. This
8
occurred when the research groups of Yokoyama et al. and
9
Sheina et al. developed their controlled chain growth polymer-
ization in 2004. They used Ni(dppp)Cl2 catalyst (dppp 5 1,3-
bis(diphenylphosphino)propane) in combination with organo-
magnesium or zinc monomers of 3-hexylthiophene. This proto-
col leads to low dispersities, high molar mass polymers and
allows preparing fully conjugated block copolymers and other
24
was observed by Nojima et al. in the case of a monomer
containing a double bond. Third, recently, we observed that
the KCTP of 3,6-dioctylthiothieno [3,2-b]thiophene (TT) is
not possible due to a very strong association of the catalyst
with the “non-aromatic double bond”—a double bond that is
not necessary to have an aromatic structure- in TT, poisoning
10,11
supramolecular structures in one-pot polymerizations.
This
procedure enables the controlled synthesis of poly(3-alkylthio-
25
the catalyst. The peculiar feature of TT is that the differ-
ence in energy of the whole TT cycle and thiophene in
12,13
14
phene)s, poly(3-alkylselenophene)s,
poly(phenylene)s,
Additional Supporting Information may be found in the online version of this article.
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dene]malo-nonitrile (DCTB) as the matrix as already
2
9
1
13
reported elsewhere.
H and C nuclear magnetic reso-
nance (NMR) measurements were carried out with a Bruker
Avance 300 and 600 MHz (some are processed using DELTA
3
1
software from JEOL). The
P NMR chemical shifts are
reported relative to an external, unlocked sample of H PO4
3
(dP 5 0.00 ppm). Always before adding the GRIM monomer
to the catalyst a part of it was quenched with acidified THF
in order to verify complete conversion. All attempts of poly-
merization in this report were always aimed to afford poly-
mers with 20 units of BDP-Oct.
FIGURE 1 Structure of various isomers of benzodithiophene,
TT and CPDT.
Synthesis of the Monomers
conjugation with the additional double bond is only very
small. Interaction of this catalyst with the “double bond”
requires only a small decrease in energy, allowing a strong
interaction. This suggests that the polymerization of annu-
lated rings bearing such a “non-aromatic double bond” is
very difficult, as a very strong complexation with the catalyst
can be expected.
0
4,5-Bis(Octyloxy)Benzo[2,1-B;3,4-B ]Dithiophene 2
A solution of 1 (0.700 g, 3.18 mmol) in dry DMF (20 mL)
was purged with argon before addition of Na S O (0.405 g,
2
2 4
2
.33 mmol). Then, the mixture was vigorously stirred at
room temperature. After 10 min, K CO (1.76 g, 12.8 mmol),
tetrabutylammonium bromide (0.317 g, 0.98 mmol), NaI
0.195 g, 0.98 mmol), and 1-bromooctane (2.465 g, 12.76
2
3
(
An interesting set of annulated rings to test this hypothesis
mmol) were all added to the reaction mixture at once and
stirred at 80 8C for additional 20 h. The reaction mixture
was poured into excess water and the aqueous layer was
extracted with dichloromethane. The combined organic
2
6
is BDT and isomeric benzo[2,1-b:3,4-b’]dithiophene (BDP).
BDP can be considered has as an analogue of CPDT in which
2
the non-conjugated sp C-atoms would be involved into the
“
non-aromatic double bond”. Similarly to BDT, BDP presents
layers were combined, dried over anhydrous MgSO and con-
4
electron rich (thiophene) rings, with BDT lacking the “non-
aromatic double bond”. While a BDT-based monomer was
polymerized in chain-growth manner by Magurudeniya
centrated in vacuo. The obtained crude product was purified
by silica gel column chromatography affording colorless
liquid 2. Yield: 0.354 g (25%).
2
7
et al.,
similar polymerization of BDP has never been
reported. So, we attempt here the KCTP of 4,7-dioctyl-
benzo[2,1-b:3,4-b’]dithiophene (BDP-Oct) using various Ni
based catalysts to investigate the effect of that “non-aromatic
double bond” connecting both the thiophene rings fused at
the either sides of the benzene ring.
IR (KBr): 2922, 2852, 1413, 1378, 1214, 1092, 1041, 950,
21 1
729, 660. cm
3
. H NMR (CDCl , d): 7.50 (d, 2H), 7.35 (d,
2H), 4.19 (m, 4H), 1.88 (q, 4H), 1.52 (m, 4H), 1.32 (br, 16H),
1
3
0.91 (t, J 5 6.81 Hz, 6H). C NMR (CDCl , d): 143.3, 134.2,
3
129.1, 124.0, 121.9, 74.3, 31.8, 29.4, 29.3, 26.1, 22.6, 14.1.
1
MS (EI, m/z): [M] calcd. for C H O S 446.2, found 446.2.
2
6 38 2 2
EXPERIMENTAL
All reagents were purchased from Sigma-Aldrich, Acros
Organics, Merck, or Alfa Aesar. Reagent grade solvents were
dried by a solvent purification system MBRAUN SPS 800
2
,7-Dibromo-4,5-Bis(Octyloxy)Benzo[2,1-B;3,4-
0
B ]Dithiophene 3
0.659 g (0.450 mmol) of BDP-Oct 2 was dissolved in 10 mL
(
1
columns with activated alumina). The dicarbonyl compound
was synthesised from 3-bromothiophene according to pre-
of a 1:1 mixture chloroform and acetic acid. N-bromosuccini-
mide (NBS) (176 mg, 0.98 mmol) was added to the previ-
ously stirred solution of 2 at 0 8C in two portions under
dark. The mixture was then stirred at room temperature for
three additional hours. The solvents were evaporated and
the residue was loaded on short silica gel column and eluted
with hexane to get 0.830 g of pure 3 as a liquid that
2
8
viously reported procedure. Melting points were deter-
mined on a Reichert Thermovar. IR spectra were recorded
from 400 to 4000 cm on a Bruker Alpha Sample compart-
ment RT-DL a T65 ATR platinum diamond with max resolu-
tion 2.00 cm . N,N -dimethylformamide (DMF) was double
distilled over molecular sieve 4 Å. Gel permeation chroma-
tography (GPC) measureme-nts were done using a Shimadzu
2
1
2
1
0
becomes waxy solid on standing. M 5 36 8C. IR (KBr): 2953,
p
2918, 2878, 2848, 1469, 1421, 1378, 1277, 1209, 1167,
10A apparatus with a tunable absorbance detector and a dif-
21
1066, 940, 830, 812, 780 cm
.
ferential refractometer in THF as eluent toward polystyrene
standards with a flow rate of 1 mL/min. Electron ionization
mass spectra (EI-MS) were recorded using an Agilent
HP5989, whereas the MALDI-ToF mass spectra were
recorded using a Waters QToF Premier mass spectrometer
equipped with a Nd-YAG laser (third harmonic, 355 nm)
using trans-2-[3-(4-tert-butyl-phenyl)22-methyl-2-propenyli-
1
H NMR (CDCl , d): 7.44 (s, 2H), 4.14 (t, J 5 6.69 Hz, 4H),
3
1.84 (t, J 5 6.99 Hz, 4H), 1.52 (m, 4H), 1.33 (br, 16H), 0.91
(t, J 5 6.9 Hz, 6H). C NMR (CDCl
124.6, 112.7, 74.3, 74.3, 31.8, 30.3, 29.4, 29.2, 26.0, 22.6,
14.1. MS (EI, m/z): [M] calcd. for C26H36Br O S 604.0,
2 2 2
1
3
, d): 142.7, 134.1, 128.9,
3
1
found 604.1.
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0
-Bromo-4,5-Bis(Octyloxy)Benzo[2,1-B;3,4-B ]Dithiophene 6
2
Method 1. NBS (0.192 g, 1.08 mmol) was added in five por-
tions with regular intervals of 6 h to a solution of 2
(0.482 g, 1.08 mmol) in 10 mL of a THF kept at 0 8C under
dark and then stirred under room temperature for 3 h.
Water was added to the solution and the aqueous layer was
extracted with dichloromethane. The organic layers were
mixed, dried over MgSO and concentrated to get crude light
4
yellow liquid. A long silica gel column chromatography using
hexane as eluent afforded 0.276 g of pure semi solid mass 6
from the crude. Yield: 50%.
Method 2. The same reaction was performed in presence of
two to three drops of HCl and adding NBS in three portions
with regular intervals of 1 h. The reaction finished within
3
h followed by same work up and purification methods as
SCHEME 1 Synthetic strategy to dibromo precursor monomer
above to receive 0.4 g of 6. Yield 5 70%.
and actual monomer.
IR (KBr): 2922, 2852, 1461, 1417, 1374, 1355, 1310, 1285,
2
1
1
1238, 1190, 1091, 1045, 961, 946, 826 cm
.
H NMR
In presence of 2, 3-bromothiophene (0.1 mmol; 16.3 mg)
was mixed to a previously stirred solution of 2 (5 mol %,
(
4
CDCl , d): 7.48 (s, 1H), 7.46 (s, 1H), 4.17 (m, 4H), 1.86 (m,
H), 1.51 (m, 4H), 1.33 (br, 16H), 0.91 (m, 6H). C NMR
3
1
3
3.0 mg) and Ni(dppp)Cl2 (2 mol %; 1.1 mg) in dry Et2O
(CDCl , d): 143.6, 142.4, 134.4, 133.9, 129.9, 128.2, 124.7,
3
(
5 mL) in a flask, kept under argon atmosphere. To this mix-
1
2
C
24.4, 121.9, 112.3, 74.3, 31.8, 30.38, 30.36, 29.4, 29.2,
ture, n-butylmagnesium chloride (60 mL; 2.00 M in THF) was
charged dropwise. After 2 h, an aliquot of the reaction was
washed off the salts with distilled water and the ether layer
1
6.14, 26.11, 22.6, 14.1. . MS (EI, m/z): [M] calcd. for
26 2 2
H37BrO S 526.1, found 526.2.
1
was analyzed with H NMR spectroscopy (Supporting Infor-
2
-Bromo,7-Iodo-4,5-Bis(Octyloxy)Benzo[1,2-B:6,5-
0
mation Figs. S13, S13) of the crude reaction mixture.
B ]Dithiophene 7
Iodine (0.438 g, 1.73 mmol) and iodobenzene diacetate
RESULTS AND DISCUSSION
(
0.724 g, 2.24 mmol) were added successively to a stirred
solution of 6 (1.82 g, 3.453 mmol) in 20 mL of dichlorome-
thane at 0 8C, and the mixture was stirred at room tempera-
ture for 6 h. Then, 10% aqueous Na S O was added for
washing any excess iodine, and the mixture was extracted
with dichloromethane. The washed organic layer was dried
Synthesis of the Monomers
In order to prepare poly(4,7-dioctylbenzo[2,1-b:3,4-b’]dithio-
phene) (PBDP-Oct), two methods have been undertaken.
2
2 3
28
According to a previously reported procedure, the dicar-
bonyl compound 1 was obtained from 3-bromothiophene. In
a first way (Scheme 1), 1 was subjected to consecutive acet-
ylation (vii) and alkylation (viii) reactions to afford 2 in an
overall yield of 83%. As an alternative route, 2 was synthe-
sized in one step from compound 1 simply by reduction
with sodium dithionite in dry dimethylformamide (i) and
quenching the in situ formed dianion with bromooctane
under inert atmosphere with a yield of 25% (Scheme 1).
Next, 2 was symmetrically brominated using a little more
than two equivalents of NBS in chloroform/acetic acid mix-
ture. Given the requirement of extreme purity for the
Grignard metathesis (GRIM) polymerization (Scheme 1), the
dibromo derivative 3 was further purified by silica column
using hexane as the eluent.
over anhydrous MgSO and concentrated in vacuo. The crude
was purified by silica gel column chromatography (eluent:
hexane) to give 7 as 2.02 g of colorless solid. Yield 5 90%.
4
M 5 54 8C. IR (KBr): 2953, 2918, 2870, 2850, 1467, 1417,
p
2
1 1
1
368, 1273, 1209, 1171, 1093, 1012, 971, 820. cm
. H
3
NMR (CDCl , d): 7.63 (s, 1H), 7.43 (s, 1H), 4.14 (m, 4H), 1.84
1
3
(
(
m, 4H), 1.52 (m, 4H), 1.32 (br, 16H), 0.91 (m, 6H). C NMR
CDCl , d): 142.5, 142.3, 135.5, 134.0, 132.7, 121.7, 128.6,
3
1
24.6; 113.0; 74.9; 74.3, 31.8, 30.3, 29.4, 29.2, 26.0, 22.6,
1
14.1. MS (EI, m/z): [M] calcd. for C H BrIO S 650.0,
2
6
36
2 2
found 650.0.
Kumada Cross-Coupling of 3-Bromothiophene and
n-Butylmagnesiumchloride
All attempts to prepare selective monochloromagnesio com-
plex from 3 in several temperature conditions were unsuc-
cessful. The best result of the dibromo monomer conversion
resulted in an equilibrium mixture of 75% monomagnesio,
5% of dimagnesio, and 15% unreacted dibromo monomer
(Table 1). The resulting mixture of organomagnesium salt
In absence of 2, 3-bromothiophene (0.1 mmol; 16.3 mg) and
Ni(dppp)Cl2 (2 mol %; 1.1 mg) were dissolved in dry Et2O
(
5 mL) in a flask, kept under argon atmosphere. To this mix-
ture, n-butylmagnesium chloride (60 mL; 2.00 M in THF) was
charged dropwise. After 2 h, an aliquot of the reaction was
washed off the salts with distilled water and the ether layer
1
was analyzed with H NMR specroscopy (Supporting Infor-
was then tested for KCTP in presence of Ni(dppp)Cl
. Based
2
mation Figs. S12 and S12) of the crude reaction mixture.
on a GPC analysis of the reaction mixture quenched with a
1
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TABLE 1 Outcome of GRIM Formation from 3
iodine atom is more prone to GRIM reaction than Br (Scheme 2).
First, compound 6 was synthesized by slow addition of a stoichi-
Products’ Yield in
ometric amount of NBS in a solution of 2 in THF kept at dark and
a
1
Mixture (%)
0
8C. After 4 days, a 50% yield was determined (by H NMR anal-
ysis of the reaction mixture) alongside 3 (40%) and unreacted 2
10%). The same reaction conducted in presence of a few drops
Temperature (8C)
Time (h)
4
di-GRIM
3
(
2
2
0
5
5
0.5
50
75
50
10
15
5
40
10
45
of HCl resulted in a 70% conversion to 6. The monobromo deriv-
ative 6 was subjected to iodination using iodine and iodobenzene
diacetate, resulting in 2-bromo-5-iodo monomer 7 with 86%
yield. Compound 7 was successfully converted to its monomag-
nesium salt (4) by addition of a quantitative amount of isopropyl-
magnesium chloride (complexed with lithium chloride)
1–24
1–4
a
From NMR.
i
.
(
PrMgCl LiCl) at room temperature within 30 min.
Synthesis of the Polymers Using Ni(II)-Catalysts
The 5-magnesium salt of the 7 in presence of Ni(dppp)Cl
2
underwent KCTP to afford only oligomers of low molar
masses (M ) with the majority of the chloromagnesio salt
n
remaining unconverted (signal at 9.4 min) [Fig. 2(a)] even
after 2 h. This outcome remained unaltered upon extended
polymerization reaction time or upon heating up to reflux.
Test of Poisoning in Ni(Dppp)Cl
2
A first explanation for the failure of these polymerizations is
the possible occurrence of a very strong complex between
2
5
Ni(0) and the BDP monomer such as for TT. This poisoning
effect of the catalysis prevents it from propagating through
the chain. A simple test of a Kumada coupling between 3-
bromothiophene and butyl magnesium chloride in absence
and presence of 2 rules out such a possibility (Fig. 3).
Indeed, under both experimental conditions, the reactions
were observed to similarly proceed in terms of reaction time
to completion, product and degree of conversion. So the
“non-aromatic double bond” in the monomer does not seem
SCHEME 2 Synthesis of the asymmetrically halo-substituted
precursor monomer and the actual monomer.
few drops of HCl, it appeared that only low molar mass
oligomers with a degree of polymerization (DP) of 2 to 3
were prepared in negligible extent, while the major part of
the reaction mixture was constituted by the monomer. This
could be attributed to the inefficient GRIM formation which
in presence of Ni-catalyst undergoes competitive coupling
reactions that inhibit the expected polymerization.
2
5
to interfere with the catalyst as strongly as TT.
Ni(II)Diimine Complex
The above results prompted the introduction of a Ni(II)-
diimine catalyst instead of Ni(dppp)Cl . Indeed, MesAn (Fig.
2
In order to selectively prepare the monochloromagnesio complex,
we incorporated an iodine atom on the monomer unit, as an
4) was shown to allow the polymerization of an electron
deficient benzotriazole monomer in a controlled fashion.
3
0
2
FIGURE 2 GPC analysis of reaction mixture of KCTP using Ni(dppp)Cl (a) and MesAn (b) as catalysts.
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TABLE 2 Outcome of KCTP from Various Catalyst or In/cat
Catalyst System
DP
Ni(dppp)Cl
2
2–3
3–5
MesAn
a
In/cat
3–7
a
1
After Soxhlet. From GPC and H NMR, it was found impure due to
mixture of oligomer with DP of 7 with it (Supporting Information Figs.
S2 and S2).
FIGURE 5 GPC spectra of the reaction mixture of polymeriza-
tion of the GRIM monomer in presence of In/cat. The inset
n
shows the M values corresponding to the peaks.
FIGURE 3 Kumada cross-coupling of 3-bromothiophene and
butylmagnesium chloride in (a) absence and (b) presence of 2.
although the major part of the resulting reaction mixture
remains constituted by the unreacted monomer. Higher Mn
oligomers were observed, without reaching long enough
oligomers to consider the global process as a successful
polymerization [Fig. 2(b)]. This additional data increases the
complexity for understanding KCTP of BDP as polymerization
is unsuccessful either under general conditions for electron
rich building blocks or electron deficient ones.
Poly(thiophene-b-benzotriazole) block co-polymer was also
3
1
successfully prepared with the use of this catalyst. So,
Ni(dppp)Cl was replaced by MesAn keeping all other condi-
2
tions intact. Three reaction temperatures were used, namely
room temperature, 60 8C and reflux. The polymerization was
monitored during 24 h. The use of MesAn seems to improve
the KCTP reaction in terms of monomer consumption,
Synthesis of the Polymers Using External Initiator
All the previous results are gathered in Table 2. In order to
get more information on the intrinsic reasons leading to the
failure of the polymerization reaction, an external initia-
3
2–34
tor
was used, since end-group analysis of the resulting
polymers/oligomers through MALDI MS is likely to help
3
5,36
identifying undesired side-reactions.
This initiator (Fig.
4
) was synthesised by oxidative addition of Ni(PPh to
3 4
)
3
7
FIGURE 4 The three Ni(II)-catalysts/initiator used.
o-tolyl bromide.
The polymerization was carried out at room temperature
and monitored by GPC (Scheme 3, Fig. 5). After 30 min, only
residual monomers and oligomers were detected with negli-
gible overtime evolution. The “polymer” was precipitated
and purified by Soxhlet extraction using methanol and ace-
tone to remove the residual monomers. Then, MALDI-ToF
analysis was performed in order to identify the end-groups.
Although the expected signals for the Tol/H polymers were
FIGURE 6 MALDI-ToF spectrum of the polymer after Soxhlet
SCHEME 3 Synthesis of the polymer with Ni(II) In/cat system.
extraction.
1
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external initiator is fully consumed. The failure of the polymer-
ization can therefore not be attributed to improper initiation.
From the observed features from GPC (Figs. 2 and 5), MALDI
3
1
(
Fig. 6) and P NMR (Fig. 7), we can propose the reactions
that occur during the polymerization (Fig. 8). The first step,
the initiator and growth of the polymer (Process 1) happens
3
1
with excellent conversion as no peak in P-NMR for the ini-
tiator was observed within 10 min of addition of GRIM to it.
However, after reaching a DP of 4-5, the Ni(0) species is
transferred to a fresh monomer (Process 2) and then oligom-
ers without o-tolyl end-group start growing (Process 3). This
is similar to the interchain catalyst transfer reported for the
Pd(0) species when double bonds are present in the mono-
3
1
FIGURE 7 In situ P NMR spectrum of the reaction mixture of
KCTP using In/cat monitored during 3 h scale. Spectra were
3 4
measured in THF and calibrated towards a H PO reference.
detected, the dominant peak distribution was assigned to
polymers containing no o-tolyl group (Fig. 6).
2
4
mer. This process results in the formation of H/Br termi-
nated oligomers of BDP-Oct. Finally, the competitive transfer,
associated to the catalyst instability, leads to no complete
conversion of the GRIM monomer.
So, a side-reaction is responsible for the failure of the KCTP of
BDP-Oct that is neither associated with too strong catalyst
binding (such as in TT), nor the too weak interaction between
the catalyst and the monomer (as in electron deficient mono-
mers). In this mechanism, the GRIM monomer underwent
KCTP to grow further to form a mixture of oligo-BDP-Oct hav-
ing a chain length of 3 to 7 and dominated by the oligomer of
DP 4. This particular DP can be concluded from the fact that
de dominant series of peaks corresponds to oligomers with
DP 5 4. This can only happen if there is a clear catalyst trans-
fer from the growing chain of the o-tolyl end-capped polymer
chain to a new monomer. This is related with the imperfect
catalyst association with the conjugated system as recently
reported by Nojima et al. This could also be attributed to the
intermolecular transfer of Ni(0) from the p-face of the sub-
strate to the C@C of another monomer. It can be speculated
that “non-aromatic double bond” present in BDP can fulfil the
same role as the double bond, resulting in this transfer reac-
tion. The polymers that were initiated by the external tolyl ini-
tiator and of which the catalyst was transferred to a monomer,
result in Br/tolyl capped polymers, which are indeed observed.
The polymers that were initiated by the transfer of the catalyst
to monomer lack a tolyl function; also these are observed.
CONCLUSIONS
3
1
Based on GPC, P NMR spectroscopy and MALDI-ToF analy-
ses, it is revealed that the KCTP reaction fails on BDP, what-
ever the tested catalysts are, because of the occurrence of a
competitive transfer reaction of the catalyst from the grow-
ing chain to the monomer. Since the isomeric BDT monomer
can be polymerized in a controlled polymerization, this can-
not be attributed to the presence of specific groups or to the
electron efficient or deficient nature of the monomer.
Instead, we hypothesize that this is due to the presence of a
24
“
non-aromatic double bond.” This result implies that annu-
lated aromatic rings having such “non-aromatic double bond”
will probably be very challenging to polymerize in a con-
trolled polymerization that relies on catalyst association.
ACKNOWLEDGMENTS
31
The polymerization was further monitored by P NMR spec-
troscopy (Fig. 7). This revealed that even after 10 min, the
The authors are grateful to the special research fund (BOF) (Bij-
zonder Onderzoeksfonds) K.U. Leuven. The Mons laboratory is
grateful to the “Fonds National pour la Recherche Scientifique”
(FRS-FNRS) for financial support in the acquisition of the Waters
QtoF Premier mass spectrometer and for continuing support. A.B.
is grateful to KU Leuven for BOF-funded postdoctoral fellowship.
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