Discovery of an anionic polymerization mechanism for high molecular weight PPV derivatives via the sulfinyl precursor route

Article abstract of DOI:10.1021/ma201453s

The polymerization of PPV via the sulfinyl precursor route has been investigated with respect to its mechanism. When polymerized in sec-butanol, a purely radical polymerization mechanism is observed as in most precursor polymerization routes. Accordingly,

Full text of DOI:10.1021/ma201453s

ARTICLE
pubs.acs.org/Macromolecules
Discovery of an Anionic Polymerization Mechanism for High
Molecular Weight PPV Derivatives via the Sulfinyl Precursor Route
Inge Cosemans,† Lieve Hontis,† David Van Den Berghe,† Arne Palmaerts,† Jimmy Wouters,†
Thomas J. Cleij,† Laurence Lutsen,‡ Wouter Maes,† Thomas Junkers,† and Dirk J. M. Vanderzande*,†,‡
†Institute for Materials Research (IMO), Hasselt University, Universitaire Campus, Building D, B-3590 Diepenbeek, Belgium
‡Division IMOMEC, IMEC, Wetenschapspark 1, B-3590 Diepenbeek, Belgium
ABSTRACT: The polymerization of PPV via the sulfinyl precursor route has been
investigated with respect to its mechanism. When polymerized in sec-butanol, a purely
radical polymerization mechanism is observed as in most precursor polymerization routes.
Accordingly, an increase in the reaction temperature induced an increase in the overall
yield alongside with a reduction of the average molecular weight of the polymer. Upon
changing the monomer concentration in solution before addition of the base NatBuO, an
increase in molecular weight is observed, signifying that the polymerization is faster than
the mixing of the two reaction components. When changing the solvent to NMP, a
competition of anionic and radical polymerization has been established while in THF an
anionic polymerization mechanism occurs exclusively. To prevent termination reactions,
LDA and LHMDS were introduced as base whereby LHMDS shows less propensity to initiate anionic chain growth due to higher
steric hindrance. With polymerizations in presence of the radical quencher TEMPO, the anionic polymerization mechanism could
unambiguously be proven.
’ INTRODUCTION
route was studied in our group in N-methylpyrrolidone (NMP)
as the solvent and sodium tert-butoxide (NatBuO) was applied as
the required base. Thereby bimodal polymer product distribu-
tions were observed, and indication was given that a competition
between a radical (resulting in high molecular weight polymer)
and an anionic (resulting in low molecular weight material) poly-
merization mechanism was responsible for the bimodality.19À22
In this paper, we now report on a polymerization of a mono-
substituted p-quinodimethane system that purely proceeds via an
anionic polymerization mechanism, which is achieved via careful
selection of reaction conditions and type of base employed to
form the monomer.
During the past decades, conjugated polymers have gained a
lot of interest because of their excellent characteristics for
applications in electronic devices such as organic light-emitting
diodes (OLEDs), organic field-effect transistors (OFETs),
and generally organic photovoltaics (OPVs).1 Poly(p-pheny-
lenevinylene) (PPV) and its derivatives in particular are con-
jugated polymers with interesting electrical and optoelectronic
properties. To introduce these materials in all kinds of devices, it
is of utmost importance that the solubility and processability of
these polymers are guaranteed.2 PPVs with suitable physical
properties are accessible via so-called precursor approaches
based on in situ formation of the active monomer that yields
nonconjugated polymers that can, however, easily be trans-
formed into the final PPV via elimination reactions. There are
many known precursor routes toward PPV materials, notably
the Gilch,3 Wessling,4,5 xanthate,6 sulfinyl,7 and the dithiocarba-
mate8,9 route. All these routes have in common that they proceed
via the in situ formation of a p-quinodimethane system, which is
formed through a base-induced elimination reaction (Scheme 1).
For all these routes, except for the xanthate route, a self-initiating
radical mechanism has been proposed.10À16 The sulfinyl route
should be distinguished from all other precursor routes since
it starts from a nonsymmetrical monomer, which allows for
highly efficient formation of the p-quinodimethane system which
was confirmed by in situ UVÀvis experiments and theoretical
calculations.17 At the same time the sulfinyl functional group
prevents the formation of head-to-head and tail-to-tail additions.
In this way, polymers with a low intrinsic chemical defect level
can hence be synthesized.18 Some years ago the sulfinyl precursor
’ EXPERIMENTAL SECTION
Materials. All solvents and reagents were purchased from Acros or
Aldrich and were used without further purification. Tetrahydrofuran
(THF) was dried by distillation from Na/benzophenone. Analytical size
exclusion chromatography (SEC) was performed using a Spectra Series
P100 (Spectra Physics) pump equipped with two mixed-B columns
(10 μm, 2 cm  30 cm, Polymer Laboratories) and a refractive index
detector (Shodex) at 70 °C. THF was used as the eluent at a flow rate of
1.0 mL/min. Molecular weight distributions were determined relative to
polystyrene standards.
Synthesis of Monomer 4 (1-(Chloromethyl)-4[(octylsul-
finyl)methyl]benzene). Monomer 4 was synthesized according to
a known procedure.23À25 In the final step a few drops of concentrated
Received: June 26, 2011
Revised:
August 21, 2011
Published: September 13, 2011
r
2011 American Chemical Society
7610
dx.doi.org/10.1021/ma201453s Macromolecules 2011, 44, 7610–7616
|

Macromolecules
ARTICLE
Scheme 1. General Reaction Scheme for the Formation of p-Quinodimethane Systems and Subsequent Conversion to PPVs
Scheme 2. Synthesis of Monomer 4
HCl (37%) were added to catalyze the reaction. Premonomer purity is of
utmost importance for the polymerization outcome, so special attention
was devoted to the purification of the monomer 4. Monomer 4 was
purified using column chromatography (silica, eluent CHCl3) and
subsequently recrystallized twice from a hexane/chloroform mixture.
Polymerization Procedure. Standard radical polymerization
procedure with sudden base addition: All glassware was dried overnight
in a drying oven at 110 °C and flamed under vacuum prior to use. A
solution of premonomer 4 (0.6 g, 2 mmol) in sec-butanol (10 mL) and a
solution of NatBuO (0.5 g, 5 mmol) in sec-butanol (25 mL) were flushed
with N2. The base solution was then added in one go to the monomer
solution at a given temperature and was stirred for 1 h. The reaction
mixture was poured in water (50 mL), neutralized with 1.0 M HCl, and
extracted with CH2Cl2. The solvent of the combined organic layers was
removed under reduced pressure, and the prepolymer was analyzed
without further purification. The prepolymer was then dissolved in
toluene (10 mL) and heated at 110 °C for 3 h. After cooling down, the
polymer was precipitated in ice cold methanol (100 mL) and filtered on
a Teflon filter. The polymer was obtained as a red powder. For the test
using reversed addition the premonomer solution was added as fast as
possible to the base solution.
Test the Anionic Nature of the Polymerization with
TEMPO. The procedure was similar to the standard polymerization
procedure, but 0.5 equiv of TEMPO was added to the premonomer
solution.
’ RESULTS AND DISCUSSION
In order to find pathways toward a purely anionic PPV
polymerization, detailed studies into the exact reaction mechan-
ism also for the radical pathway are important. Only if the latter
process is understood, then a differentiation between the both
reaction modes can be made on the basis of the resulting polymer
distributions. Scheme 3 summarizes the reactions that can take
place in each case. In the anionic pathway, the base that is added
to form the quinodimethane from the premonomer also can act
as an anionic chain initiator as in classical anionic polymerization.
Subsequently, the chains are growing until either all monomer
is consumed or until the reaction is quenched dedicatedly with
an end-capper molecule. In the radical polymerization mode,
a biradical species is formed from the monomer (see also
Scheme 3), whereby a dimerization of the monomer was
proposed to self-initiate the reaction. The so-obtained biradicals
can subsequently undergo chain propagation, growing on both
sides of the chain. Upon termination of the reaction, again a
biradical is formed (alongside a chain defect in the final polymer
due to head-to-head coupling), thus reducing the overall radical
concentration but not terminating chain growth. It is for this
reason that the reaction can also be referred to as a “pseudo”
termination.
Influence of Temperature. The general procedure for fast
addition of the base to monomer was followed. The different polymer-
ization temperatures were obtained as follows: 0 °C with an ice/water
mixture; À64 °C with a CHCl3/liquid nitrogen mixture. The polymer-
izations at 30, 50, and 75 °C were performed in a thermostatic flask.
Anionic Polymerization. The general procedure for fast base
addition was followed. In this case 1.2 equiv of base (LDA (2 M in THF/
n-heptane) or LHMDS (1 M in THF)) was used during polymerization.
7611
dx.doi.org/10.1021/ma201453s |Macromolecules 2011, 44, 7610–7616

Macromolecules
ARTICLE
From the fact that high molecular weight material is typically
obtained with a low defect level in the product (stemming
from head-to-head couplings) the propagation rate constant
(kp) has to be much larger than the rate constant for initiation
(ki).26 Self-termination reactions that would indeed lead to a
stop in chain growth, like cyclization, are unlikely to occur
once the formation of high molecular weight material is reached.
For smaller (bi)radical species, such reaction was however
observed.27 Also, termination by disproportionation, which
would eliminate one of the radical centers per chain is not
possible due to the absence of β-hydrogen atoms, so recombina-
tion is the only mode of termination.
For most quinodimethane polymerizations, the radical po-
lymerization pathway is postulated.28 Thus, in the next section
the radical polymerization is described with respect to the
outcome of the polymerization upon variation of certain reaction
conditions.
Radical Polymerization. To assess if there was a possibility to
achieve a purely anionic PPV polymerization, more information
on the radical mechanism and particularly on the influence of
certain reaction parameters on the outcome of the polymeriza-
tion is required. It was demonstrated earlier that if sec-butanol
was used as the solvent, polymerization proceeds exclusively
along a radical pathway.29 So far, however, it was not yet
systematically studied how the outcome of these polymerizations
is changed when the reaction conditions are varied. In order to
identify conditions for anionic polymerizations, it is of high
importance to understand these influences as only then a truly
anionic PPV polymerization can be characterized and discerned
from a “conventional” radical polymerization.
Scheme 3. Anionic Polymerization Compared to the Radical
Polymerization Reactions in p-Quinodimethane
Polymerization
As a proof-of-concept we studied the polymerization of
1-chloromethyl-4-[(n-octylsulfinyl)methyl]benzene (4) as the
premonomer in sec-butanol as the solvent employing NatBuO
as the base, affording plain-PPV which is insoluble when elimi-
nated (Scheme 4). The full three-step synthesis and characteriza-
tion of the sulfinyl premonomer 4 are described elsewhere.23À25
To start, the effect of temperature on the radical polymeriza-
tion was investigated. Results from polymerizations performed
at temperatures between 0 and 75 °C are collated in Table 1.
All reactions have been carried out twice to ensure reproduci-
bility. All polymerizations, except stated otherwise, were started
by sudden addition of the entire base solution to the monomer
solution. All reported molecular weights (based on polystyrene
calibration of the SEC) are given for the noneliminated PPV
material 5 because of the insolubility of the conjugated plain-
PPV 6.
The obtained data show good reproducibility. The obtained
yields increase with the reaction temperature. This can be
interpreted as a result of an increased propagation rate compared
to the initiation or termination rate with increasing temperature.
At the same time, the average degree of polymerization increases
with decreasing temperature which may as well be attributed to a
lower initiation rate toward lower temperatures. Analysis of the
filtrate confirms that all monomer is consumed and that only
paracyclophanes are present as a side product from the initiating
biradical.27 This species effectively limits the achievable yield and
hence an increase in initiation (as expected with increasing
temperature) must not necessary be followed by a higher yield.
In addition to the change in temperature, also the effect of
varying the initial base concentration [B]i and premonomer [M]i
(resulting however in constant total concentrations in the
mixture) upon the outcome of the reaction was investigated at
30 and 0 °C. Also here, a good reproducibility is given. It should
be noted that both sets of experiments represent solutions that
Scheme 4. Synthesis of Plain-PPV via the Sulfinyl Precursor Route
7612
dx.doi.org/10.1021/ma201453s |Macromolecules 2011, 44, 7610–7616

Macromolecules
ARTICLE
Table 1. Effect of Temperature on the Polymerization of
Premonomera in sec-Butanol
Table 3. Effect of Sequentail Polymerizations of Premono-
mer 4 in sec-Butanol
polymerization conc premonomer/
initial solvent
volume/mLb
10À3Mw/g
molÀ1 PDI
procedure 1b
procedure 2c
step
mmol LÀ1 a
10À3Mw/
g molÀ1
10À3Mw/
g molÀ1
1
2
3
4
5
80
15
35
55
75
95
394
319
270
242
209
3.69
3.40
3.37
3.04
3.13
T (°C) yield (%)
PDI yield (%)
PDI
44.4
30.8
23.5
19.1
75
50
30
0
90
89
88
83
78
80
73
65
150
145
370
450
535
560
685
680
1.80
1.95
2.27
2.85
a Concentration premonomer 4 of the complete reaction mixture after
addition of base solution. b Total solvent volume of given reaction step,
initial base concentration [B]i = 210 mM; T = 30 °C; all experiments
were performed in duplo, and results are given as averages.
2.98
2.66
2.45
2.96
83
78
63
65
620
590
780
810
2.80
2.95
2.49
2.47
a Polymerizations with 1-chloromethyl-4-[(n-butylsulfinyl)methyl]benzene
as premonomer. b Premonomer dissolved in sec-butanol (14 mL), initial
premonomer concentration [M]i = 143 mM; base (NatBuO) dissolved
in sec-butanol (6 mL), initial base concentration [B]i = 434 mM;
[B]i/[M]i = 3.03. c Premonomer dissolved in sec-butanol (10 mL), initial
premonomer concentration [M]i = 200 mM; base (NatBuO) dissolved
in sec-butanol (10 mL), initial base concentration [B]i = 260 mM;
[B]i/[M]i = 1.30.
decreasing the premonomer concentration can be clearly ob-
served. Higher premonomer concentrations afford higher con-
centrations of the intermediate p-quinodimethane system, thus
accelerating the propagation reaction and consequently allowing
the chains to grow to higher molecular weights. All these results
are indicative of a radical chain growth mechanism.
In the next step, it was tested if a termination reaction is
operational. Whether termination occurs or not can be probed by
sequential polymerization reactions. If no termination occurs,
then addition of a new batch of premonomer to a already poly-
merized solution should afford for polymers of higher molecular
weight due to chain extension reactions. Hence, premonomer 4
(2 mmol in 15 mL of sec-butanol) and base solutions (2.1 mmol
of NatBuO in 10 mL of sec-butanol) were added to fully poly-
merized reaction mixtures (after 40 min of the initial reaction).
This process was repeated five times. Before adding the mono-
mer and base solution, 5 mL of the reaction mixture was
quenched to provide a sample for comparison. The results from
the experiments are given in Table 3.
The obtained results reveal that the molecular weight drops
after each sequential addition of monomer. Clearly no chain
extension takes place, and thus it can be concluded that a
termination process is active during polymerization. When the
results of the molecular weights in function of the premonomer
concentration of the last two sets of experiments are compared,
it is evident that the slope for the sequential polymerization
experiment is less steep than the slope for the premonomer
concentration experiment (Figure 1). This difference can be
explained by the fact that every time a sample is taken out of the
reaction medium for GPC analysis the GPC sample will also
contain polymeric material obtained in the previous polymeri-
zation step, and an accumulation of several individual polymer-
izations is seen. It should be noted that the above experiments
demonstrate that a chain-terminating reaction takes clearly place;
the nature of this reaction remains however unclear. Transfer
reactions could in principle also be responsible, but also termina-
tion related to traces of oxygen in the reaction mixture. Anyhow,
no living polymerization is observable under the chosen reaction
conditions, and the polymerization behavior is in line with a
radical chain growth mechanism.
Table 2. Effect of Premonomer Concentration on the
Polymerization of Premonomer 4 in sec-Butanol
conc premonomer/ initial solvent
mmol LÀ1 a
volume (mL)b yield (%) 10À3Mw/g molÀ1 PDI
80
15
20
56
66
73
68
57
61
399
328
265
213
145
119
3.44
3.82
3.26
3.23
2.89
2.93
66.7
50
30
40
40
25
70
18.2
100
a Concentration premonomer 4 after addition of base solution. b 2 mmol
of premonomer 4 dissolved in sec-butanol; base dissolved in sec-butanol
(10 mL), initial base concentration [B]i = 210 mM; T = 30 °C; all
experiments were performed in duplo, and results are given as averages.
result in identical base and monomer concentrations after
mixing. Still, a moderate increase of ∼15% in molecular weight
can be observed when [B]i/[M]i is decreased. Thus, it may be
concluded that the polymerization must proceed with a very high
overall rate of polymerization; otherwise, the initial concentra-
tions of both components could not have an influence on the
outcome of the polymerization. It should be noted that such
effect complicates the kinetic analysis tremendously since mixing
effects must be considered as a rate-limiting step, a task that is not
easily done.
To clarify unambiguously the effect of concentration of base
and premonomer on the molecular weight, further experiments
were performed whereby the same amount of premonomer 4
(2 mmol) was systematically dissolved in an increasing amount
of solvent. The NatBuO base (2.1 mmol in 10 mL of solvent) was
added without delay. The results from these polymerizations are
collated in Table 2.
Anionic Polymerization. To the best of our knowledge, there
are no literature examples in which unambiguously an anionic
polymerization mechanism was observed for p-quinodimethane
systems, except for the procedure in which the sulfinyl route
is performed in a solvent like dry NMP19À22 (see Figure 2).
In this case, bimodal behavior was observed and attributed to
The results indicate that the initial premonomer concentration
has only a minor effect on the overall yield of the polymerization;
however, a monotonous decrease of the molecular weight with
7613
dx.doi.org/10.1021/ma201453s |Macromolecules 2011, 44, 7610–7616

Macromolecules
ARTICLE
simultaneous occurrence of an anionic and a radical polymeri-
zation pathway.
premonomer and base concentration was investigated. All ex-
periments were performed in duplo, and results are given as
averages.
To describe the extent of competition between the radical and
the anionic polymerization mechanism, this reaction was subse-
quently further investigated in NMP as the solvent. The relative
integrated surface in the GPC chromatogram of the polymeric
versus the oligomeric fraction was calculated, and the sum of both
surfaces was taken as 100%. This practical approach may not
be fully correct since the distributions overlap with each other.
Nevertheless, for a qualitative comparison such evaluation is
sufficient. In Table 4 the results are illustrated for a series of
experiments for which the dependence of the competition
between anionic and radical polymerization on the initial
It is clear that there is a high sensitivity for the competition
between the two polymerization mechanisms and that already
small changes in the reaction preparation have a profound
influence on the transition in mechanism. In the analysis of the
results, the relative peak areas for the oligomeric (OM) and
polymeric (PM) material is used to assess the predominance of
each mechanism. The OM material is assigned (as previously
done) to material stemming from anionic polymerization, while
PM is the result of the radical growth mode as described in the
previous section. It should hereby be noted that the peak areas
from SEC analysis do not represent true ratios of concentration
of polymers due to the weighting of the molecular weight
distributions. Transformation into the number distribution is
required for that purpose, which however requires absolute
molecular weight detection. Thus, with the numbers in Table 4,
the amount of oligomers is underestimated. Also, the given
molecular weights are only crude estimates since the selection
of integration limits are only arbitrary. A full deconvolution of the
distributions is not easily done, but the presented data are good
enough to clearly show the underpinning trends. Small changes
in initial concentrations of base and premonomer have pro-
nounced effects on the molecular weight and the relative amount
of the polymeric fraction. In comparison with Table 2 (radical
polymerization only), changes of initial base or premonomer
concentration do not give rise to strong variations in the outcome
of the polymerization reaction. However, the more competitive
the anionic mechanism becomes, the lower the molecular weight
of the polymeric fraction is. A higher amount of the oligomeric
material seems to go along with a high ratio of initial base
concentration versus initial premonomer concentration. In an
anionic polymerization, a sensitivity for the initial base versus
premonomer concentration can be expected because the base
concentration directly influences the rate of initiation (compared
to the radical pathway where the rate of initiation is dependent on
the p-quinodimethane concentration and hence only indirectly
coupled with the amount of base). An experiment that demon-
strates this hypothesis unambiguously is an experiment in which
the effect of a reversed addition (addition of premonomer 4
solution to the base solution) is studied (Table 5).
Figure 1. Radical propagation and termination reactions: (squares)
dilution experiment (see Table 2) and (triangles) sequential polymer-
izations (see Table 3).
Clearly, reversed addition has a tremendous effect on the
molecular weight and on the relative contribution of the poly-
meric fraction. When mixed, the monomer faces a comparatively
Table 5. Addition of Base versus Reversed Additiona
10À3Mw (PM)/
10À3Mw (OM)/
addition
yield (%)
g molÀ1
% PM
g molÀ1
normal
61
41
170
87
36
3.5
2.9
reversed
a T = room temperature.
10.4
Figure 2. Typical GPC chromatogram for the polymerization in NMP
(OM = oligomeric material, PM = polymeric material).
Table 4. Overview of Polymeric (PM) and Oligomeric (OM) Fractions for the Various Premonomer 4 and Base Concentrationsa
ratio mL(M)b/mL(B)c
total yield (%)
[B]i/mM
[M]i/mM
[B]i/[M]i
10À3Mw (PM)/g molÀ1
% PM
10À3Mw (OM)/g molÀ1
14/6
14/8
10/10
33
44
65
0.37
0.28
0.22
0.14
0.14
0.20
2.6
2.0
1.1
73
117
176
67
80
89
3.1
3.1
3.3
a T = room temperature. b Amount of solvent (NMP) in which the premonomer is dissolved; [M]i = initial premonomer concentration. c Amount of
solvent (NMP) in which the base is dissolved; [B]i = initial base concentration.
7614
dx.doi.org/10.1021/ma201453s |Macromolecules 2011, 44, 7610–7616

Macromolecules
ARTICLE
Table 6. Effect of Temperature on the Amount of the
Oligomeric (OM) Fraction
Table 7. Results for Polymerization of Premonomer 4 in
THF with LDA and LHMDSa
10À3Mw (PM)/
10À3Mw (OM)/
base
T (°C)
10À3Mw/g molÀ1
PDI
T (°C)
yield (%)
g molÀ1
% PM
g molÀ1
LDA
À64
0
1.8
3.6
1.3
1.9
2.1
3.2
RT
0
68
43
182
54
90
52
3.0
3.3
LDA
LHMDS
À64
0
46.1
LHMDS
64.0
a T = room temperature; [M]i (initial monomer concentration) =
high base concentration, which apparently favors the anionic
initiation over the radical pathway. An alternative experiment
that demonstrates that the reaction can be directed toward one
specific mechanism is to perform the polymerization at low
temperature (see Table 6 for results). Anionic initiation typically
is associated with a low activation energy; thus, decreasing the
temperature should increase the amount of oligomeric material.
As can be seen from the data in Table 6, this hypothesis can be
confirmed.
To identify reaction conditions that allow for a purely anionic
polymerization mechanism, all radical initiation events must be
suppressed and termination reactions of an anionic polymeriza-
tion eliminated. There are two possible (anionic) termination
reactions that can potentially occur, i.e., quenching from depro-
tonation of the solvent or reaction with the protonated base,
which is formed during the p-quinodimethane formation. This
reasoning implies that strong bases (which will be less likely to
terminate an anionic chain growth in its protonated form) and an
aprotic solvent are essential conditions for a living anionic
polymerization.
0.05 M.
Table 8. Results for the Verification of the Anionic Nature of
the Polymerization of Premonomer 4 with LHMDS as a Base
in THFa
base
solvent
additive 10À3Mw/g molÀ1 PDI yield (%)
NatBuO sec-butanol none
208.4
9.8
4.0
1.4
6.9
2.7
2.5
3.5
52
<1
79
21
84
82
NatBuO sec-butanol TEMPO
NatBuO THF
NatBuO THF
LHMDS THF
LHMDS THF
none
1324.1
111.8
43.3
TEMPO
none
TEMPO
49.4
a T = room temperature; [M]i (initial monomer concentration) =
0.05 M.
Consequently, lithium diisopropylamide (LDA) was tested as
a base in dry THF. In such reaction, only low molecular weight
oligomers were formed, indicating that anionic polymerization
occurs exclusively. LDA has been reported in the literature as
being an initiator for the anionic polymerization of methacrylate
monomers.30 For this reason LDA was exchanged for a more
sterically hindered base, i.e., lithium hexamethyldisilazide (LHMDS),
and used as the base during the polymerization. For the poly-
merization of premonomer 4, plain-PPV polymers with 10 times
higher molecular weight were obtained with LHMDS compared
to LDA as a base in dry THF (Table 7). This increase in molecular
weight is indicative of a largely reduced anionic initiation rate with
the sterically more hindered base. In consequence, less chains are
initiated, and thus each individual chain can grow to higher degree
of polymerization due to the changed ratio of initiated species over
the total monomer concentration.
To verify the anionic character of the polymerization proce-
dure using LHMDS in THF, the effect of 2,2,6,6-tetramethyl-
piperidin-1-oxyl (TEMPO) on the molecular weight was inves-
tigated and compared to a classical radical polymerization of PPV
(NatBuO in sec-butanol) (Table 8 and Figure 3) since a stable
nitroxide should inhibit polymerization if a radical mechanism is
operational.
For the polymerizations conducted with LHMDS no major
changes were observed, nor on the molecular weight, nor in the
yields of the reactions. This nicely verifies that the polymers were
indeed obtained through an anionic polymerization mechanism.
To further illustrate that the base is responsible for the change to
an anionic mechanism, the effect of TEMPO was also studied
on the polymerization in THF using NatBuO as a base. Again
high molecular weight material was obtained in the absence
of the radical inhibitor, and a drastic decrease of both yield
and molecular weight was observed in the presence of TEMPO.
Figure 3. Verification of anionic nature of the polymerization of
premonomer 4 with LHMDS as a base in THF.
It was shown earlier on that 1 equiv of TEMPO is needed to
stop the radical polymerization completely.31 This explains the
observed reduced molecular weight but still the presence of
oligomers when 0.5 equiv of TEMPO is used.
It can hence be concluded that the polymerization of PPVs in
THF proceeds entirely via an anionic polymerization mechanism
if a strong base such as LDA or LHMDS is used for the formation
of the active p-quinodimethane monomer. The base acts at the
same time as reagent to form the monomer but also to some
extent initiates the polymerization.
’ CONCLUSION
The polymerization mechanism and the change in the out-
come of the polymerizations upon variation of the reaction
7615
dx.doi.org/10.1021/ma201453s |Macromolecules 2011, 44, 7610–7616

Macromolecules
ARTICLE
conditions are demonstrated for the sulfinyl precursor route.
When polymerized in sec-butanol with a base such as NatBuO,
a purely radical polymerization mechanism is observed. In this
case, the polymer molecular weight can be systematically varied
by changing the reaction temperature or via changing the initial
monomer concentration in solution before mixing with the base.
A purely anionic polymerization mechanism for PPVs can be
obtained when the reaction conditions are chosen carefully to
exclude radical initiation and quenching reactions of an anionic
polymerization. This is achievable when the sulfinyl precursor
route is performed in dry THF as the solvent and LHMDS is used
as the base. LDA also leads to an exclusive anionic polymerization
but does result in overall lower molecular weights due to an
increased propensity to initiate the polymerization compared to
the sterically more hindered LHMDS.
(15) Hontis, L.; Lutsen, L.; Vanderzande, D.; Gelan, J. Synth. Met.
2001, 119, 135–136.
(16) Vandenbergh, J.; Wouters, J.; Adriaensens, P.; Mens, R.; Cleij,
T.; Lutsen, L.; Vanderzande, D. Macromolecules 2009, 42, 3661–3668.
(17) Hermosilla, L.; Catak, S.; Van Speybroeck, V.; Waroquier, M.;
Vandenbergh, J.; Motmans, F.; Adriaensens, P.; Lutsen, L.; Cleij, T.;
Vanderzande, D. Macromolecules 2010, 43, 7424–7433.
(18) Roux, H.; Adriaensens, P.; Vanderzande, D.; Gelan, J. Macro-
molecules 2003, 36, 5613–5622.
(19) Louwet, F.; Vanderzande, D.; Gelan, J. Synth. Met. 1995, 69,
509–510.
(20) Hontis, L.; Van Der Borght, M.; Vanderzande, D.; Gelan, J.
Polymer 1999, 40, 6615–6617.
(21) Van Der Borght, M.; Adriaensens, P.; Vanderzande, D.; Gelan,
J. Polymer 2000, 41, 2743–2753.
(22) Adriaensens, P.; Van Der Borght, M.; Hontis, L.; Issaris, A.; van
Breemen, A.; de Kok, M.; Vanderzande, D.; Gelan, J. Polymer 2000,
41, 7003–7009.
(23) van Breemen, A.; Vanderzande, D.; Adriaensens, P.; Gelan, J.
J. Org. Chem. 1999, 64, 3106–3112.
(24) Van Der Borght, M.; Vanderzande, D.; Adriaensens, P.; Gelan,
J. J. Org. Chem. 2000, 65, 284–289.
(25) Lutsen, L.; van Breemen, A.; Kreuder, W.; Vanderzande, D.;
Gelan, J. Helv. Chim. Acta 2000, 83, 3113–3121.
(26) Becker, H.; Spreitzer, H.; Ibrom, K.; Kreuder, W. Macromole-
cules 1999, 32, 4925–4932.
(27) Wiesecke, J.; Rehahn, M. Angew. Chem., Int. Ed. 2003, 42, 567–
570.
(28) Schwalm, T.; Wiesecke, J.; Immel, S.; Rehahn, M. Macromol.
Rapid Commun. 2009, 30, 1295–1322.
(29) van Breemen, A.; Issaris, A.; de Kok, M.; Van Der Borght, M.;
Adriaensens, P.; Gelan, J.; Vanderzande, D. Macromolecules 1999, 32,
5728–5735.
’ AUTHOR INFORMATION
Corresponding Author
*Ph + 32 (0)11 26 83 21; Fax + 32 (0)11 26 83 01; e-mail dirk.
vanderzande@uhasselt.be.
’ ACKNOWLEDGMENT
The authors gratefully acknowledge the IWT (Institute for
the Promotion of Innovation by Science and Technology in
Flanders) for the financial support via the SBO-project 060843
“PolySpec”. We also thank BELSPO in the frame of the IAP P6/27
network and the FWO (Fund for Scientific Research-Flanders) via
the project G.0091.07N for the financial support and for the PhD
grant for A.P. The BOF (Bijzonder OnderzoeksFonds) of the
UHasselt is acknowledged for the PhD grants for J.W., D.V.D.B.,
and I.C.
(30) Antoun, S.; Teyssiꢀe, P.; Jꢀer^ome, R. J. Polym. Sci., Part A 1997,
35, 3637–3644.
(31) Wiesecke, J.; Rehahn, M. Macromol. Rapid Commun. 2007,
28, 78–83.
’ REFERENCES
(1) Skotheim, T. A.; Reynolds, J. R. In Handbook of Conducting
Polymers: Conjugated Polymers Processing and Applications, 3rd ed.; CRC
Press: New York, 2007; pp 5-3, 7-1, 10-1.
(2) Burroughes, J.; Bradley, D.; Brown, A.; Marks, R.; Mackay, K.;
Friend, R.; Burns, P.; Holmes, A. Nature 1990, 347, 539–541.
(3) Gilch, H. G.; Wheelwright, W. L. J. Polym. Chem. Ed. 1966, 4,
1337–1349.
(4) Wessling, R. A.; Zimmerman, R. G. US Patent 3401152, 1968.
(5) Wessling, R. A.; Zimmerman, R. G. US Patent 3706677, 1972.
(6) Son, S.; Dodabalapur, A.; Lovinger, A. J.; Galvin, M. E. Science
1995, 269, 376–378.
(7) Louwet, F.; Vanderzande, D.; Gelan, J.; Mullens, J. Macromole-
cules 1995, 28, 1330–1331.
(8) Henckens, A.; Duyssens, I.; Lutsen, L.; Vanderzande, D.; Cleij,
T. Polymer 2006, 47, 123–131.
(9) Henckens, A.; Lutsen, L.; Vanderzande, D.; Knipper, M.; Manca,
J.; Arnouts, T.; Poortman, J. Proc. SPIE Int. Soc. Opt. Eng. 2004,
5464, 52–59.
(10) Hontis, L.; Vrindts, V.; Lutsen, L.; Vanderzande, D.; Gelan, J.
Polymer 2001, 42, 5793–5796.
(11) Schwalm, T.; Wiesecke, J.; Immel, S.; Rehahn, M. Macromole-
cules 2007, 40, 8842–8854.
(12) Cho, B.; Kim, Y.; Han, M. Macromolecules 1998, 31, 2098–
2106.
(13) Issaris, A.; Vanderzande, D.; Gelan, J. Polymer 1997, 38, 2571–
2574.
(14) Issaris, A.; Vanderzande, D.; Adriaensens, P.; Gelan, J. Macro-
molecules 1998, 31, 4426–4431.
7616
dx.doi.org/10.1021/ma201453s |Macromolecules 2011, 44, 7610–7616