Synthesis of a diblock copolymer with pendent luminescent and charge transport units through nitroxide-mediated free radical polymerization

Article abstract of DOI:10.1021/ma0115850

We present the efficient polymerization of a vinyl monomer bearing electron-transporting units, 2-[4-(4′-vinylbiphenylyl)]-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (vPBD), by using a TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-mediated radical polymerizat

Full text of DOI:10.1021/ma0115850

Macromolecules 2002, 35, 1543-1548
1543
Synthesis of a Diblock Copolymer with Pendent Luminescent and
Charge Transport Units through Nitroxide-Mediated Free Radical
Polymerization
La u r en t Boitea u , Ma r c Mor on i, Ala in Hilber er , Mich el Wer ts, Ber t d e Boer , a n d
Geor ges Ha d ziioa n n ou *
Department of Polymer Chemistry, Materials Science Centre, University of Groningen, Nijenborgh 4,
9747 AG Groningen, The Netherlands
Received September 7, 2001; Revised Manuscript Received November 26, 2001
ABSTRACT: We present the efficient polymerization of a vinyl monomer bearing electron-transporting
units, 2-[4-(4′-vinylbiphenylyl)]-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (vPBD), by using a TEMPO (2,2,6,6-
tetramethylpiperidine-1-oxyl)-mediated radical polymerization method. A kinetic study using this monomer
showed a quasi-living free radical character. The corresponding homopolymer was obtained in high yield
with a very narrow polydispersity. Further reaction of the isolated polymer with another vinyl monomer
bearing luminescent groups, 4-tert-butyl-4′-(4-vinylstyryl)-trans-stilbene (v3PV), gave a diblock copolymer
containing both functional groups.
Sch em e 1. Syn th esis of Mon om er 2-[4-(4′-Vin yl-
bip h en ylyl)]-5-(4-ter t-bu tylp h en yl)-1,3,4-oxa d ia zole
(vP BD)
In tr od u ction
Among the various techniques to polymerize vinyl
monomers, ranging from ionic to group transfer, the
most convenient remains free radical polymerization.
Its mild reaction conditions and the wide range of
polymerizable monomers make it suitable for both
industry and research laboratories. However, the main
disadvantage of classical free radical polymerization is
that it does not allow architecture and structure control.
The obtained polymers are generally polydisperse, the
molecular weights are difficult to control, and there is
no possibility to make block copolymers. These features
are indeed accessible through living anionic or cationic
processes, but these techniques require rigorous condi-
tions and delicate procedures. In the past decade there
have been many efforts toward the development of
“living” free radical polymerization processes. Three
successful and commonly used approaches were intro-
duced, namely iniferters1-3 (initiator-chain transfer-
terminator), atom transfer radical polymerization
(ATRP),4 and nitroxide-mediated free radical polymer-
ization (NMRP).5-8 These techniques employ the prin-
ciple of an equilibrium between a low concentration of
active radicals and a rather large number of dormant
species. This suppresses bimolecular side reactions such
as recombination or disproportionation to such a degree
that the overall polymerization process shows “living”
characteristics, e.g., the increase of the molecular weight
with conversion, and the possibility to synthesize block
copolymers and other complex macromolecular archi-
tectures.8-10 NMRP utilizes stable free radicals, such
as nitroxides, as reversible terminating agents to control
the polymerization. The superiority of this method,
when compared to other living processes, comes from
the stability of the end-capping groups, which enables
isolation of a polymer with “dormant” chain ends that
can later be reactivated. The use of this technique for
the synthesis of copolymers of styrene and acrylonitrile11
or N-cyclohexylmaleimide12 was recently reported. With
the development of novel nitroxides,13 a wide variety of
monomers, e.g., acrylates, acrylonitrile, 1,4-butadiene,
and derivatives thereof, became available to the chemist
for the use in NMRP. However, only very few monomers
have so far been shown to be reactive toward nitroxide-
mediated free radical polymerization based on 2,2,6,6-
tetramethylpiperidine-1-oxyl (TEMPO). In our previous
work we demonstrated the functionalization of a poly-
(p-phenylenevinylene) (PPV) block with a TEMPO
initiator and its use in the synthesis of rod-coil block
copolymers. The controlled radical polymerization of the
coil (polystyrene) block leaves the PPV moiety un-
touched.14 Furthermore, we incorporated an electron-
accepting moiety (C60) in the coil block, resulting in a
donor-acceptor block copolymer.14a,15
We have also demonstrated that the NMRP tech-
nique can be used in dilute solution to polymerize a
large functional monomer.16 From the conjugated mono-
mer 4-tert-butyl-4′-(4-vinylstyryl)-trans-stilbene (v3PV,
Scheme 2), we obtained a well-defined polymer, with
narrow polydispersity, having blue-light-emitting side
chains. This oligo(phenylenevinylene) side chain can be
* To whom correspondence should be addressed. e-mail: hadzii@
chem.rug.nl.
10.1021/ma0115850 CCC: $22.00 © 2002 American Chemical Society
Published on Web 02/01/2002

1544 Boiteau et al.
Macromolecules, Vol. 35, No. 5, 2002
Sch em e 2. Syn th esis of P P BD Hom op olym er a n d Su bsequ en t Exten sion to P (P BD-b-3P V) Diblock Cop olym er . Y
) P h COO- for BP O/TEMP O a n d Y ) H for (I)
The monomer vPBD was prepared in three steps starting
with the commercially available PBD, by adapting a method
previously reported in the literature (Scheme 1).26 The uni-
molecular initiator (I) used in later experiments was synthe-
sized according to literature method by reacting sodium-
TEMPO salt on (1-bromoethyl)benzene.10 The synthesis of
v3PV monomer was described previously.16
2-[4-(4′-Vin ylbip h en ylyl)]-5-(4-ter t-bu tylp h en yl)-1,3,4-
oxa d ia zole (vP BD). 2-[4-(4′-Acetylbiphenylyl)-5-(4-tert-butyl-
phenyl)]-1,3,4-oxadiazole (1): The tert-BuPBD (15 g, 42 mmol)
was dissolved under argon in 380 mL of dry methylene
chloride. Freshly distilled acetyl chloride (7.6 mL, 107 mmol)
was added while stirring. Aluminum chloride (48 g, 360 mmol)
was then slowly added, and the mixture was heated to reflux
for 5 h. The cooled mixture was poured on 300 g of ice. The
organic phase was washed twice with 200 mL of water and
dried over Na2SO4. The solvent was evaporated to obtain 16 g
(40.35 mmol) of crude product, which could be used without
further purification.1H NMR (CDCl3): δ (ppm) ) 8.21 (d, 2H,
ΦH), 8.06 (2d, 4H, ΦH), 7.79 (2t, 4H, ΦH), 7.58 (d, 2H, ΦH),
2.64 (s, 3H, -COCH3), 1.37 (s, 3H, -Φ-CH3).
2-[4-(4′-(1-H yd r oxyet h yl)b ip h en ylyl)]-5-(4-ter t-b u t yl-
p h en yl)-1,3,4-oxa d ia zole (2). To a stirred solution of the
ketone 1 (3.01 g, 7.6 mmol) in 70 mL of ethanol was slowly
added NaBH4 (0.68 g, 18 mmol). The mixture was stirred for
5 h at room temperature. 20 mL of 2% HCl solution was added,
and the precipitate was filtered off and washed with water to
give 2.99 g (7.5 mmol) of product 2 that could be used without
further purification. 1H NMR (CDCl3): δ (ppm) ) 8.19 (d, 2H,
ΦH), 8.07 (d, 2H, ΦH), 7.77 (d, 2H, ΦH), 7.61 (d, 2H, ΦH),
7.54 (dd, 4H, ΦH), 4.96 (qu, 1H, -CHOH-), 1.70 (s, 1H, -OH),
1.58 (d, 3H, -CHOH-CH3), 1.36 (s, 9H, Φ-CH3).
readily fine-tuned over a wide range by the addition of
functional groups17 or by increasing its length. Cacialli
et al. used a similar approach to incorporate side chains
of distyrylbenzene and 2-(4-biphenylyl)-5-phenyl-1,3,4-
oxadiazole (PBD), a molecule well-known for its electron
transport properties, into poly(methacrylate) copolymer
analogues.18 Similarly, J iang et al. synthesized statisti-
cal copolymers with side-chain hole- and electron-
transporting groups that demonstrated good efficiencies
in dye-doped devices.19
The incorporation of oxadiazole-based side chains into
PPVs was found to improve the electroluminescent
properties of PPVs, due to the improved electron-
transporting properties of the conjugated polymer.20-23
Peng et al. introduced oxadiazole repeat units in the
backbone of a conjugated polymer, creating polymers
of alternating phenylenevinylene and oxadiazole moi-
eties.24,25
In this paper we extend the work on PBD-based
monomers. The molecule was functionalized with a
terminal vinyl group to give 2-[4-(4′-vinylbiphenylyl)]-
5-(4-tert-butylphenyl)-1,3,4-oxadiazole (vPBD, Scheme
1), and its polymerization was studied with a 2,2,6,6-
tetramethylpiperidine-1-oxyl (TEMPO)/dibenzoyl per-
oxide (BPO) mixture or a TEMPO-ethylbenzene de-
rivative (I) as initiator (Scheme 2). Further reaction of
the isolated homopolymer (PPBD) with the v3PV mono-
mer16 enabled the preparation of a diblock copolymer,
P(PBD-b-3PV), as shown in Scheme 2.
2-[4-(4′-Vin ylbip h en ylyl)]-5-(4-ter t-bu tylp h en yl)-1,3,4-
oxa d ia zole (vP BD). A solution of 2 (2.31 g, 5.80 mmol) and
pTSA (115 mg; 0.61 mmol) in 80 mL of dry toluene was heated
to reflux in a Dean-Stark apparatus. The distilled solvent was
continuously removed, while same amounts of dry toluene
were added to the reaction. The reaction was followed by TLC
and continued until complete conversion (ca. 18-24 h). The
solution was then allowed to concentrate to ca. half the volume,
and the cooled mixture was mixed with 30 mL of chloroform
and washed twice with 30 mL of 1 N aqueous Na2CO3 and
twice with 40 mL of water. The organic layer was dried over
Na2SO4, evaporated, and then purified by column chromatog-
Exp er im en ta l Section
Ma ter ia ls. Aluminum chloride, acetyl chloride, 2-(4-biphen-
ylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (tert-BuPBD), so-
dium borohydride, p-toluenesulfonic acid monohydrate (pTSA),
(1-bromoethyl)benzene, 10-camphorsulfonic acid (CSA), silica
gel, deuterated chloroform, and 2,2,6,6-tetramethylpiperidine-
1-oxyl (TEMPO) were obtained from Aldrich. Dibenzoyl per-
oxide (BPO), sodium, hydrochloric acid, ethanol, methanol,
tetrahydrofuran, methylene chloride, toluene, and xylene were
obtained from Merck. BPO was recrystallized from chloroform/
methanol before use.

Macromolecules, Vol. 35, No. 5, 2002
Synthesis of a Diblock Copolymer 1545
d), in xylene. Kinetic studies were performed using both
initiating systems (PPBDb,d). A blank polymerization experi-
ment (PPBDe) without initiator was also performed. To
prepare the diblock P(PBD-b-3PV), the isolated PPBDa poly-
mer and the v3PV monomer were refluxed together in xylene
(Scheme 2).
P P BDa . The monomer vPBD (1.52 g, 4 mmol), BPO (2.4
mg, 0.01 mmol), the TEMPO radical (3.8 mg, 0.024 mmol), CSA
(20 mg, 0.086 mmol), and 5 mL of xylene were charged under
argon in a vessel, stirred at 95 °C for 1 h, and then refluxed
for 16 h. The product was directly precipitated in 50 mL of
methanol. The precipitate was collected, dissolved in 5 mL of
warm toluene, and reprecipitated in 50 mL of methanol. The
precipitate was collected by filtration and dried under vacuum
to obtain 0.89 g of a white powder PPBDa. Yield: 59%. 13C
NMR (CDCl3): δ (ppm) ) 163-164 (2 peaks, C7-C8), 155 (C3),
144-143 (C12-C16), 137 (C13), 128-125 (6 peaks, C4-C5-
C10-C11-C14-C15), 121 (C6), 123 (C9), 41 (C17-C18), 36
(C2), 32 (C1).
Anal. Calcd. %: C, 82.07; H, 6.36; N, 7.36; Found: C, 81.42;
H, 6.50; N, 7.11.
P P BDb. The same procedure as above was followed with
half the quantities of reagents (monomer, initiator) in 3 mL
of xylene. After 1, 2.5, and 16 h, 0.5 mL aliquots were taken
and precipitated in 10 mL of methanol. After 48 h, the
remaining solution was precipitated in 20 mL of methanol.
All the precipitates were dissolved in methylene chloride and
reprecipitated in methanol.
P P BDc. Same procedure as above, with initiator I (9.9 mg,
0.038 mmol) and vPBD (1.4 g, 3.68 mmol) in 5 mL of xylene,
refluxing for 15 h. The hot reaction mixture was precipitated
in 20 mL of methanol. The precipitate was collected, dissolved
in 5 mL of hot toluene, and reprecipitated in 20 mL of
methanol to afford 850 mg of polymer.
F igu r e 1. 13C NMR spectrum of polymer PPBDa in CDCl3.
P P BDd . Same procedure as for PPBDc, with I (4.7 mg,
0.018 mmol) and vPBD (940 mg, 2.47 mmol) in 4 mL of xylene,
refluxing for 48 h. At different times, 0.5 mL aliquots were
taken, precipitated once in 10 mL of methanol, filtered, and
dried in a vacuum. The precipitates were dried and weighed,
raphy (silica gel, CHCl3) to obtain 1.70 g (4.47 mmol) of
monomer vPBD as a white powder. Overall yield: 70%. 1H
NMR (CDCl3): δ (ppm) ) 8.2 (d, 2H, ΦH), 8.09 (d, 2H, ΦH),
7.76 (d, 2H, ΦH), 7.58 (m, 6H, ΦH), 6.76 (qu, 1H, Φ-CHd
CH2), 5.82 (d, 1H, -CHdCH2), 5.33 (d, 2H, -CHdCH2), 1.37
(s, 9H, Φ-CH3). 13C NMR (CDCl3): δ (ppm) ) 164 (2 peaks,
C7-C8), 155 (C3), 144 (C12), 139 (C16), 137 (C13), 136 (C17),
128-125 (6 peaks, C4-C5-C10-C11-C14-C15), 121 (C6),
123 (C9), 114, (C18), 36 (C2), 32 (C1).
Anal. Calcd. %: C, 82.07; H, 6.36; N, 7.36; O, 4.20; Found:
C, 81.61; H, 6.44; N, 7.19; O, 4.52. The carbon atom numbering
in the monomer exactly refers to that of the monomer unit in
the polymer structure (Figure 1).
Syn t h esis of t h e Un im olecu la r In it ia t or (1-TE MP O-
eth yl)ben zen e (I). Prepared according to Catala et al.28 To a
solution of 600 mg (3.8 mmol) of TEMPO in 40 mL of dry THF
was added 180 mg (7.8 mmol) of finely cut sodium. The
suspension was submitted to ultrasound while cooling with a
water bath, until it completely decolorized (ca. 1 h). After 5
min standing, the clear solution was transferred to a solution
of (1-bromoethyl)benzene (3.66 mmol, 0.5 mL) in 30 mL of dry
THF. The suspension was stirred overnight at room temper-
ature and then filtered. After solvent evaporation, the crude
product was purified by chromatography (eluent hexane/
dichloromethane 4/1) to afford 560 mg (2.15 mmol, 58%) of I,
whose spectral data were identical to those found in the
literature.28
P olym er iza tion s. The polymers described were synthe-
sized through a TEMPO-mediated “living” free radical process
(Scheme 2). This method, which was initially described for
simple monomers like styrene, is usually conducted in the
bulk. However, the high melting points of our monomers make
it necessary to run the polymerization in solution. Hot xylene
is a good solvent for the monomers, its boiling point (140 °C)
being well adapted to TEMPO-mediated polymerization. This
route was previously used with v3PV, which showed no
significant thermal polymerization under these reaction condi-
tions.16 We first prepared the PPBD homopolymers by reflux-
ing the monomer and the initiator, either a BPO/TEMPO
mixture (PPBDa-b) or the unimolecular initiator (I) (PPBDc-
1
and the polymer/monomer ratio was determined by H NMR.
P P BDe (Self-In itia ted ). vPBD (50 mg, 0.13 mmol) only
in 0.2 mL of xylene, refluxing for 15 h. The mixture was diluted
to 1 mL in toluene and precipitated in 5 mL of methanol. The
precipitate was collected, dissolved in 5 mL of hot toluene, and
reprecipitated in 20 mL of methanol.
Diblock Cop olym er P (P BD-b-3P V). The polymer PPBDa
(300 mg, 0.79 mmol of repeating units), the v3PV monomer
(350 mg, 0.96 mmol), CSA (10 mg, 0.043 mmol), and 5 mL of
xylene were charged under argon in a vessel. The mixture was
stirred and refluxed for 3 days. The product was then
precipitated in 30 mL of methanol, and the precipitate was
collected by filtration to obtain 480 mg of a light yellow-beige
2
powder. Anal. Calcd for a polymer containing /3 of PBD and
1/3 of 3PV (according to SEC values), %: C, 85.38; H, 6.81; N,
4.98; Found: C, 84.53; H, 7.04: N, 4.72.
Mea su r em en ts. All the polymers are white or pale beige
solids. PPBD polymers are well soluble in solvents such as
THF, CH2Cl2, CHCl3, toluene, and xylene. The P(PBD-b-3PV)
diblock exhibits poorer solubility. Hot toluene and xylene may
be considered to be the best solvents.
NMR Sp ectr oscop y. 1H and 13C NMR spectra were col-
lected at room temperature on a Varian (VXR 300) spectrom-
eter with chloroform-d as solvent and internal standard. The
spectra of the polymers show typical broad resonances. The
1H NMR spectra show no peaks in the region of the vinyl
hydrogen resonances. The 13C NMR spectrum of PPBDa is
given in Figure 1. The peaks of the vinyl carbons of the
monomer at 136 and 114 ppm are not present anymore. The
peak of the carbon C16, in R position of the polymer chain
(formerly in R position of the double bond in the monomer), is
shifted from 139 to 144 ppm, and a broad resonance appears
at about 41 ppm, related to the carbon atoms of the polymer
main chain. The six peaks corresponding to the signals of the
sp2 carbon atoms of the three phenyl rings (twice six different
carbons) of the PBD group appear clearly on the PPBDa

1546 Boiteau et al.
Macromolecules, Vol. 35, No. 5, 2002
Ta ble 1. Resu lts on VP BD P olym er iza tion in Reflu xin g
Xylen e, Usin g TEMP O/BP O (P P BDa -b) or I (P P BDc-d )
a s In itia tor a
reaction conv
Mw
(g/mol)
poly-
disp
theor
Mw
run
initiator
time (h)
(%)
PPBDa BPO/TEMPO
PPBDb BPO/TEMPO
16
1
59
38 000 1.21 45 000
43 000 1.34
2.5
16
48
15
0.8
2
54 000 1.46
63 000 1.29
65 000 1.31
26 000 1.3
16 000 1.13
20 000 1.12 12 000
24 000 1.14 22 000
31 500 1.29 29 000
32 000 1.25 31 000
120 000 2.6
PPBDc
PPBDd
I
I
61
18
23
42
56
60
80
23 000
9 400
4
17
50
15
PPBDe
a
Molecular weight, polydispersity, and conversion (PPBDb: not
F igu r e 3. Comparison of TEMPO-controlled (PPBDc: con-
tinuous line) and self-initiated (PPBDe: dashed line) polym-
erization of vPBD. Size exclusion chromatograms of polymers
eluted in THF.
measured) vs reaction time. Theoretical Mw ) conversion
×
([monomer]/[initiator]); theoretical maximum Mw ) 76 000 (PPB-
Db) and 52 000 (PPBDd).
F igu r e 4. Size exclusion chromatograms of PPBDa (dashed
line) and of diblock copolymer P(PBDa-b-3PV) (continuous line)
eluted in THF.
F igu r e 2. Kinetic study on PPBDd: molecular weight (Mn,
open triangles; Mw, open circles), theoretical molecular weight
(Mw, filled circles), and polydispersity (filled squares) as a
function of monomer conversion.
Ta ble 2. P olym er Ch a r a cter iza tion a n d Sp ectr oscop ic
Da ta in THF Solu tion s of th e P 3P V, P P BDa , a n d
P (P BD-b-3P V) P olym er s
spectra between 124 and 128 ppm. The P(PBD-b-3PV) block
copolymer was not soluble enough to get a well-defined 13C
NMR spectrum. However, the spectrum of the diblock presents
the same resonances as that of PPBDa, but with a poor
resolution, and additional resonance peaks appear around 126
ppm which are attributed to the sp2 carbon atoms of the three
phenyl rings and the two double bonds of the 3PV part.
SEC. The molecular weights were obtained by size exclusion
chromatography (SEC) in eluent THF with the coupled detec-
tion of refractive index and light scattering (Spectra Physics
1000 system equipped with a Shodex RI-71 refractive index
detector, white light, coupled to a Dawn light scattering
apparatus). The refractive increments were measured on a
LDC Analytical KMX-16 laser differential refractometer at 630
nm. The dn/dc of PPBDa was measured in THF at 25 °C. A
value of 0.253 mL/g was found. Data on vPBD polymerization
are given in Table 1. We can see the molecular weight increase
for PPBDb and PPBDd, especially during the first hours of
polymerization. After 16 h, the molecular weight did not
change significantly (ca. 1-3%). Kinetic results on PPBDd
show an increase in molecular weight following the conversion,
however not completely linearly (Figure 2). The self-initiated
polymerization of vPBD in refluxing xylene led to high
molecular weight, highly polydisperse PPBDe (Figure 3).
Data for the diblock P(PBD-b-3PV) and related homopoly-
mers PPBDa and P3PV are summarized in Table 2; the
chromatograms are given in Figure 4. A molecular weight Mw
of 3.8 × 104 was detected for PPBDa, with a low polydispersity
of about 1.21. The molecular weight Mw of the diblock P(PBD-
b-3PV) was found to be 5.6 × 104, with a polydispersity of 1.53.
On the basis of the raw SEC data and neglecting the presence
P3PV
PPBDa
P(PBD-b-3PV)
isolated yield (%)
Mw (g/mol)
polydispersity
76
37000
1.5
361
11200
420
85
59
51
38000
1.21
311
50500
387
56000
1.53
309
34400
401
λmax absorption (nm)
ꢀ (M-1 cm-1
)
λmax emission (nm)
fluorescence yield (%)
50
60
of homopolymer, one calculates a PBD to 3PV monomer unit
ratio of 2:1 for the block copolymer.
Th er m a l P r op er ties. Thermogravimetric analyses of PPB-
Da and P(PBD-b-3PV) show a good thermal stability, with less
than 1% weight loss at 300 °C and less than 10% weight loss
at 400 °C.
Op tica l P r op er ties. The ultraviolet absorption spectra
were measured in THF solution with a SLM Aminco 3000
Array spectrophotometer. Fluorescence measurements were
performed on a SLM Aminco SPF-500 spectrofluorometer. The
fluorescence quantum yields in THF solution were determined
relative to quinine sulfate in sulfuric acid (φ ) 0.55).
Discu ssion
Previous studies on the v3PV monomer16 showed that
the TEMPO-mediated free-radical polymerization could
be applied successfully to large functional monomers.
The results obtained were much better than in the case
of classical free-radical polymerization. But the low

Macromolecules, Vol. 35, No. 5, 2002
Synthesis of a Diblock Copolymer 1547
reactivity of the monomer, due to the large delocaliza-
tion of the radicals, induced longer reaction times
(several days) and slightly higher polydispersity (1.5)
than what is observed for simple monomers such as
styrene.
Polymerization of vPBD under the same conditions
appears to be very efficient. Homopolymer PPBDa
exhibited a low polydispersity (1.21) and reasonably
high molecular weight (38 000 g/mol). PPBDb, prepared
under slightly different conditions (smaller quantities,
increased dilution), showed a slightly broader distribu-
tion (ca. 1.3) and a higher molecular weight (63 000
g/mol). Using the unimolecular initiator I led to similar
results in terms of conversion and polydispersity, while
lower molecular weights were obtained by increasing
the [initiator]/[monomer] ratio. The final conversions
(50-60%) are higher than those observed in the case of
vPBD classical (AIBN-initiated) free-radical polymeri-
zation.26 These results are comparable with the values
observed for polystyrene obtained via a similar route.27,28
The kinetic studies carried out on PPBDb,d (Table 1,
Figure 2) show a correlated increase of conversion and
molecular weight vs reaction time, thus pleading for a
quasi-living character of the polymerization. Low poly-
dispersities (down to 1.15) can be obtained by limiting
the reaction time (and therefore conversion); a major
drawback of this strategy being the difficult removal of
unreacted monomer by many successive reprecipita-
tions. The deviation of the molecular weight measured
for PPBDd from the theoretical molecular weight (as-
suming 100% initiation efficiency) is indicative of a slow
initiation mechanism, which can also be an explanation
for the increased polydispersity.
F igu r e 5. UV-vis absorption and photoluminescence spectra
of PPBDa (- - -), P3PV (‚ ‚ ‚), and P(PBDa-b-3PV) (s) in THF.
showing the presence of chains of the same relatively
low molecular weight. The percentage of dead polymer
chains is very difficult to quantify. However, it is
noteworthy that this kind of copolymer can probably not
be obtained by other polymerization techniques: these
conjugated monomers are not suitable for anionic
processes, due to the presence of reactive double bonds
(v3PV) or acidic aromatic hydrogens (vPBD) that would
give side reactions. Using classical free radical polym-
erization, no block copolymers can be obtained, and
random copolymers would be unlikely to form due to
the quite different reactivities of the monomers.
Figure 5 shows the UV-vis absorption spectra of the
PPBDa, P3PV, and P(PBD-b-3PV) polymers in THF
solution. The polymers present an absorption band in
the near-UV, which is due to the conjugated phenyle-
nevinylene and/or PBD parts. The maximum of absorp-
tion of the PBD part is at about 309 nm in both the
homopolymer and the diblock copolymer. The λmax of the
3PV part (367 nm) is at higher wavelengths, corre-
sponding to a larger conjugation. This shift is consistent
with a better electron delocalization for the phenyle-
nevinylene system of the 3PV part compared to the
biphenyl group of the PBD part.
The reactivity of vPBD monomer is much better than
that of v3PV: the polymerization appears to be almost
complete after 15-16 h. This higher reactivity can be
explained by the fact that the vPBD monomer is based
on a biphenyl unit in which the delocalization should
be less than in a distyrylbenzene unit. Whereas the
enhanced reactivity allows shorter reaction times and
narrower polydispersities in the TEMPO-mediated po-
lymerization, the drawback of this reactivity is that
vPBD self-polymerizes in refluxing xylene, then leading
to a high molecular weight, highly polydisperse material
(PPBDe, Figure 3). However, under the conditions we
used (whether BPO/TEMPO or I is used as initiator),
the TEMPO-mediated polymerization seems to effi-
ciently compete with such a self-initiated polymeriza-
tion, as seen on the molecular weight distribution (Table
1).
Figure 5 gives also the fluorescence spectra (excitation
wavelength at the absorption maximum) in THF solu-
tion of the polymers PPBDa, P3PV, and the diblock
copolymer P(PBD-b-3PV). The emission peak of the PBD
part is mainly in the near-UV, whereas the 3PV-part
emission band is in the blue range of the visible
spectrum. The fluorescence quantum yields of the
polymers were measured at different excitation wave-
lengths. The values obtained for P3PV were around 85%
(see Table 2). PPBDa presents a lower yield of 50%. A
value of 60% was measured on the diblock P(PBD-b-
3PV). The optical properties of the diblock copolymer
in solution are a sum of the optical properties of the
two homopolymers. The absorption spectrum (Figure 5)
can be deduced by adding the spectra of the two
separate polymers, normalized to the percentage of each
block in the copolymer, and the emission spectrum
(Figure 5) is very similar to that obtained from a
mixture of the two homopolymers. Therefore, the block
copolymer P(PBD-b-3PV) shows both a very strong
absorption in the UV and an efficient fluorescence in
the blue part of the visible spectrum.
By reacting the isolated PPBDa polymer with v3PV
monomer, we obtained a diblock copolymer P(PBD-b-
3PV) (Scheme 2). Figure 4 presents the SEC diagrams
of PPBDa and P(PBD-b-3PV). It clearly shows the
increase of the molecular weight after adding the second
monomer v3PV and results in an extension of the
polymer chains by approximately half the length of the
PBD block. The polydispersity of the diblock copolymer
P(PBD-b-3PV) (1.53) is somewhat higher than that of
the PPBD and does not compare so favorably with the
values obtained on other “TEMPO diblocks”, but it is
consistent with the low reactivity of the second mono-
mer. A partial inactivation of the PPBD chains (dead
ends) cannot be neglected, which might be the reason
for the increased polydispersity of the diblock copolymer.
It can also explain the shape of the SEC diagrams of
PPBDa and of P(PBD-b-3PV) at high elution volume,

1548 Boiteau et al.
Macromolecules, Vol. 35, No. 5, 2002
H. K.; Werts, M. P. L.; van der Vegte, E. W.; Hadziioannou,
G. Macromolecules 2000, 33, 349-356.
Con clu sion
In this work we have confirmed that the TEMPO-
mediated free radical polymerization route offers a great
potential for the preparation of side-chain functional
polymers and diblock copolymers, the latter being
impossible to obtain via classical polymerization tech-
niques.
(4) Patten, T. E.; Matyjaszewski, K. Adv. Mater. 1998, 10, 901-
915.
(5) Georges, M. K.; Veregin, R. P. N.; Kazmaier, P. M.; Hamer,
G. K. Macromolecules 1993, 26, 2987-2988.
(6) Hawker, C. J . J . Am. Chem. Soc. 1994, 116, 11185-11186.
(7) Malmstro¨m, E. E.; Hawker, C. J . Macromol. Chem. Phys.
1998, 199, 923-935.
The vPBD monomer showed a very high reactivity
with this polymerization technique and gave results
comparable to what is observed for classical monomers
like styrene. In particular, the isolated TEMPO-end-
capped polymer showed a narrow polydispersity and
was still able to initiate the polymerization of another
monomer, making it possible to prepare diblock copoly-
mers containing two different functional side chains.
The combination of two (or more) functionalities into
a single polymer is a very promising approach to making
advanced materials for polymer-based electronic de-
vices, since it can greatly simplify the processing steps.
Tailor-made block copolymers offer the advantage of
ultimately controlling the morphology of the materials
by varying the size of the different blocks. The diblock
copolymer we described here is a typical example of such
a material. It consists of a block with conjugated
chromophores as side groups and a block bearing
electron-transport units. These two functionalities are
essential for adjusting the properties of semiconducting
devices. Variation of the size of the different blocks and
evaluation of the corresponding morphologies are now
of prime importance for optimizing the applicability of
this new material.
(8) Hawker, C. J . Trends Polym. Sci. 1996, 4, 183-188.
(9) Hawker, C. J . Angew. Chem. 1995, 34, 1456-1459.
(10) Oulad Hammouch, S.; Catala, J . M. Macromol. Rapid Com-
mun. 1996, 17, 149-154.
(11) Fukuda, T.; Terauchi, T.; Goto, A.; Tsujii, Y.; Miyamoto, T.
Macromolecules 1996, 29, 3050-3052.
(12) Schmidt-Naake, G.; Butz, S. Macromol. Rapid Commun.
1996, 17, 661-665.
(13) (a) Benoit, D.; Chaplinski, V.; Braslau, R.; Hawker, C. J . J .
Am. Chem. Soc. 1999, 121, 3904-3920. (b) Benoit, D.; Harth,
E.; Fox, P.; Waymoth, R. M.; Hawker, C. J . Macromolecules
2000, 33, 363-370.
(14) (a) Stalmach, U.; de Boer, B.; Videlot, C.; van Hutten, P. F.;
Hadziioannou, G. J . Am. Chem. Soc. 2000, 122, 5464-5472.
(b) Stalmach, U.; de Boer, B.; Post, A.; van Hutten, P. F.;
Hadziioannou, G. Angew. Chem., Int. Ed. 2001, 40, 428-430.
(15) de Boer, B.; Stalmach, U.; van Hutten, P. F.; Melzer, C.;
Krasnikov, V. V.; Hadziioannou, G. Polymer 2001, 42, 9097-
9109.
(16) Moroni, M.; Hilberer, A.; Hadziioannou, G. Macromol. Rapid
Commun. 1996, 17, 693-702.
(17) Kraft, A, Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int.
Ed. 1998, 37, 402-428.
(18) Cacialli, F.; Li, X.-C.; Friend, R. H.; Moratti, S. C.; Holmes,
A. B. Synth. Met. 1995, 75, 161-168.
(19) J iang, X.; Register, R. A.; Killeen, K. A.; Thompson, M. E.;
Pschenitzka, F.; Sturm, J . C. Chem. Mater. 2000, 12, 2542-
2549.
(20) Chen, Z.-K.; Meng, H.; Huang, W. Macromolecules 1999, 32,
4351-4358.
(21) Bao, Z.; Peng, Z.; Galvin, M. E.; Chandross, E. A. Chem.
Mater. 1998, 10, 1201-1204.
(22) Chung, S.-J .; Kwon, K.-Y.; Lee, S.-W.; J in, J .-I.; Lee, C. H.;
Lee, C. E.; Park, Y. Adv. Mater. 1998, 10, 1112-1116.
(23) Lee, D. W.; Kwon, K.-Y.; J in, J .-I.; Park, Y.; Kim, Y.-R.;
Hwang, I.-W. Chem. Mater. 2001, 13, 565-574.
(24) Peng, Z.; Bao, Z.; Galvin, M. E. Chem. Mater. 1998, 10, 2086-
2090.
Ackn owledgm en t. The Dutch Organization of Tech-
nology (SON/STW), the Commission of the European
Communities (Human Capital and Mobility program),
and the Netherlands Organization for Scientific Re-
search, Physics Division (NWO-FOM), financially sup-
ported this work. P. F. van Hutten and S. C. Veenstra
are gratefully acknowledged for their assistance.
(25) Peng, Z.; Bao, Z.; Galvin, M. E. Adv. Mater. 1998, 10, 680-
Refer en ces a n d Notes
684.
(1) Reghunadhan Nair, C. P.; Chaumont, P.; Clouet, G. In
Macromolecular Design: Concept and Practice; Mishra, M.
K., Ed.; Polymer Frontiers International Inc.: Hopewell J ct,
NY, 1994; p 431.
(2) Otsu, T.; Matsumoto, A. In Advances in Polymer Science;
Springer-Verlag: Berlin, 1998; Vol. 136, p 75.
(26) Anderson, R. A.; Davidson, K.; Soutar, I. Eur. Polym. J . 1989,
25, 745-751.
(27) Georges, M. K.; Veregin, R. P. N.; Kazmaier, P. M.; Hamer,
G. K.; Saban, M. Macromolecules 1994, 27, 7228-7229.
(28) Catala, J . M.; Bubel, F.; Oulad Hammouch, S. Macromol-
ecules 1995, 28, 8441-8443.
(3) (a) Kroeze, E.; de Boer, B.; ten Brinke, G.; Hadziioannou, G.
Macromolecules 1996, 29, 8599-8605. (b) de Boer, B.; Simon,
MA0115850