Multiblock copolymers synthesized by miniemulsion polymerization using multifunctional RAFT agents

Article abstract of DOI:10.1021/ma0485787

The synthesis of multiblock copolymers by miniemulsion polymerization using multifunctional RAFT agents was discussed. It was shown that these multifunctional RAFT agents, depending on the number of RAFT groups they contain, yield triblock or even multibl

Full text of DOI:10.1021/ma0485787

Volume 37, Number 25
December 14, 2004
© Copyright 2004 by the American Chemical Society
Communications to the Editor
Scheme 1. Synthesis of a Multifunctional RAFT
Multiblock Copolymers Synthesized by
Miniemulsion Polymerization Using
Multifunctional RAFT Agents
Agent by Polymerizing a Difunctional
Dithiocarbamate with a Difunctional Chloroformatea
Raf Bussels,† Christianne Bergman-Go1ttgens,†
Jan Meuldijk,‡ and Cor Koning*,†
Laboratory of Polymer Chemistry and Process Development
Group, Eindhoven University of Technology, P.O. Box 513,
5600 MB Eindhoven, The Netherlands
Received July 14, 2004
Revised Manuscript Received October 15, 2004
a Conditions: equimolar quantities of the dithiocarbamate
and the difunctional chloroformate are mixed in THF, and
after addition of 5 equiv of the proton trap TEA, polymerization
is performed for 48 h at -20 °C.
Of the currently known controlled or “living” radical
polymerization techniques, reversible addition fragmen-
tation chain transfer (RAFT) radical polymerization is
the most versatile, the most robust, and accordingly the
most promising mechanism.1,2 This technique allows the
living polymerization of both polar and nonpolar mono-
mers, and polymerization can be performed using a
broad range of reaction conditions, both in solution and
in aqueous dispersions.1-8
Scheme 2.
N,N-Butoxycarbonylmethyldithiocarbamate, the
Difunctional “Butyl-RAFT” Agent Used in This
Papera
The living character of the RAFT polymerization
allows the synthesis of (multi)block copolymers,9 even
in aqueous dispersions.10,11 However, if a multiblock
copolymer consisting of n polar and m nonpolar blocks
is the target molecule, (n + m) separate monomer
additions or polymerization steps are required if a
standard monofunctional RAFT agent is used, even if
superb amphipathic monofunctional macro RAFT agents
are used, as recently described by Pham et al.12
In this paper multifunctional S-tert-alkyl-N,N-alkoxy-
carbonylmethyldithiocarbamate RAFT agents are in-
troduced. If the nitrogen atoms are part of an aromatic
system, or substituted with electron-withdrawing groups,
dithiocarbamates are effective RAFT agents.13-15 In
Scheme 1 the chemistry resulting in a multifunctional
RAFT agent containing 2n RAFT groups is depicted.
a Synthesis conditions: similar to those given with Scheme
1, except a n-butyl chloroformate/dithiocarbamate ratio of 2
is used.
If instead of a difunctional chloroformate N-butyl
chloroformate is used for the coupling reaction with the
dithiocarbamate, (N,N-butoxycarbonylmethyldithiocar-
bamate) is generated, which is the difunctional RAFT
agent used in this paper. With these novel multifunc-
tional RAFT agents, tri- or multiblock copolymers can
be formed in merely two sequential polymerization
steps. We realize that other researchers have published
excellent papers on the concept of monomer insertion
into polymer chains by controlled radical polymeriza-
tion,16-18 but the route described in the present work is
different and novel.
† Laboratory of Polymer Chemistry.
‡ Process Development Group.
* Corresponding author. E-mail: c.e.koning@tue.nl.
10.1021/ma0485787 CCC: $27.50 © 2004 American Chemical Society
Published on Web 11/09/2004

9300 Communications to the Editor
Macromolecules, Vol. 37, No. 25, 2004
For the synthesis of (N,N-butoxycarbonylmethyldithio-
carbamate), thiourea (17.2 g, 0.23 mol) and R,R,R′,R′-
tetramethyl-1,4-benzenedimethanol (20.0 g, 0.10 mol)
were mixed and added slowly with stirring to 48% HBr
(41.7 g, 0.25 mol). The slurry was heated to 50 °C, after
which the slurry solidified. The white solid was cooled,
filtered, washed with 0.1 mol/L aqueous HBr solution,
and dried under vacuum. The resulting white powder
was stirred for 2 h in aqueous NaOH (containing 0.62
mol of NaOH) at 40 °C. The resulting clear and red
solution was filtered, the red filtrate was cooled to 5 °C,
and under an Ar atmosphere a methyl isothiocyanate
(15.8 g, 0.22 mol) solution in methanol was added
dropwise. S-(1,4-Phenylenebis(propane-2,2-diyl)) bis(N-
methyldithiocarbamate) readily precipitated out as a
white solid. After filtration and washing with cold water,
the white solid was recrystallized twice from ethanol
and dried under vacuum. The overall yield was 60%.
Figure 1. Number-average molar mass (Mh n, b) and polydis-
persity index (PDI, ∆) vs conversion of the mimiemulsion
polymerization of n-butyl acrylate (BA), using the “butyl-
RAFT” agent. Drawn line is theoretical Mh n. Conditions: 3 h,
70 °C; organic phase: 8.0 g of BA, 2 wt % hexadecane with
respect to BA, 4.4 × 10-2 mol of “butyl-RAFT” per Lorganic phase
;
aqueous phase: 30 g of water, 2.3 × 10-3 mol of sodium dodecyl
sulfate (SDS) per Lwater, 3.0 × 10-3 mol of potassium peroxo-
disulfate per Lwater
.
1
Purity was confirmed with H and 13C NMR spectros-
copy and with LC-MS (MNa+ experimental: 395.01;
theoretical: 395.07).
measured values for Mh n are somewhat higher than the
calculated values, which might be explained by the fact
that the measured molecular weights are expressed in
polystyrene equivalents.
In another miniemulsion polymerization experiment,
10 wt % of the BA monomer was successfully substi-
tuted by methacrylic acid (MA), leaving the rest of the
variables exactly the same. For this polymerization, the
theoretical and experimental Mh n were 20 100 and 22 500
g/mol, respectively, and the polydispersity index after
92% conversion was 1.48. So, relatively polar blocks with
a controlled length can be synthesized using this new
concept.
n-Butyl chloroformate (3.1 g, 23 mmol) was dissolved
in THF (10 mL) and brought under an Ar atmosphere.
The mixture was cooled to -20 °C. S-(1,4-Phenylenebis-
(propane-2,2-diyl) bis(N-methyldithiocarbamate) (4.0 g,
11 mmol) and triethylamine (5.4 g, 54 mmol) were
dissolved in THF. This solution was added drop-by-drop
to the cold n-butyl chloroformate. The mixture was
stirred for 48 h, after which it was brought back to room
temperature. Triethylamine hydrochloride was filtered
off, and THF was removed under reduced pressure. The
resulting yellow oil was purified by column chromatog-
raphy using dichloromethane as the eluent and yielded
N,N-butoxycarbonylmethyldithiocarbamate as a yellow
The poly(butyl acrylate) latex described above was
applied as a seed latex for the polymerization of the
second monomer isooctyl acrylate (iOA). This seed latex
had a Mh n of 23 500 g/mol after 90% monomer conversion
(see Figure 1). The amount of iOA used was 3.0 g,
whereas 15 g of BA seed latex was used. The amount of
water was 12.0 g, and the (fresh) KPS concentration was
0.003 mol/L. For this seeded emulsion polymerization,
yielding a triblock copolymer, Mh n increases linearly with
iOA conversion.
Although the experimental part only describes the
detailed synthesis of a difunctional butyl-RAFT agent,
a multifunctional RAFT agent containing on average 12
RAFT groups (“poly(decyl-RAFT)” with R ) decyl and
naverage ) 6 in Scheme 1) was synthesized using a
difunctional chloroformate (for conditions, see Scheme
1). The yield was 70%. Figure 2 shows Mh n and PDI vs
BA conversion for this multifunctional RAFT agent.
For this homopolymerization of BA, for which the
conditions were comparable to those applied using the
difunctional butyl-RAFT agent, Mh n increases linearly
with conversion, indicating that also for this multifunc-
tional RAFT agent the radical polymerization occurs in
a controlled way. This polymer shows a good match of
the measured and calculated Mh n value after 85% BA
conversion (Mh n,exp ) 97 000 g/mol, Mh n,calc ) 99 500 g/mol,
PDI ) 3.1, average latex particle size ) 207 nm). On
the other hand, the molar mass distribution is relatively
broad, which is related to the high PDI of the “poly-
(decyl-RAFT)” itself, being 1.59. This high PDI is
inherent in the step-growth polymerization technique,
used to synthesize the multifunctional RAFT agent (see
Scheme 1).
1
liquid. Yield: 85%. Purity was confirmed with H and
13C NMR spectroscopy and with LC-MS (MNa+ experi-
mental: 595.76; theoretical: 595.84).
For the miniemulsion polymerizations, N,N-butoxy-
carbonylmethyldithiocarbamate (the “butyl-RAFT” agent)
was dissolved under stirring in a mixture of monomer
and hexadecane, comprising the organic phase. Sodium
dodecyl sulfate (SDS) was dissolved in water, and the
organic phase was added dropwise to the aqueous phase
under vigorous stirring. The preemulsion was sonicated
for 30 min. After emulsification, the miniemulsion was
transferred into an emulsion reactor under an argon
atmosphere, equipped with a reflux cooler and a ther-
mocouple. The miniemulsion was heated to 70 °C under
stirring, and potassium peroxodisulfate (KPS), dissolved
in water, was added, after which polymerization was
performed for 3 h. At regular time intervals, samples
were taken for gravimetric conversion measurement and
GPC analysis, using PS standards. The seed latexes,
obtained after the described polymerization () forma-
tion first block), were swollen overnight at room tem-
perature with a fresh amount of monomer, after which
the second polymerization step was performed under
argon, as described for the first step, using fresh KPS.
In one miniemulsion polymerization experiment, 8.0
g of n-butyl acrylate (BA) was polymerized, using 2 wt
% hexadecane with respect to the monomer and using
0.044 mol of difunctional butyl-RAFT agent/L. In the
aqueous phase (30 g), 0.0023 mol/L SDS and 0.0030
mol/L KPS were present. Figure 1 shows the linear
increase of Mh n with conversion, which is a good indica-
tion that the addition of the butyl-RAFT agent resulted
in a good control of the radical polymerization. The
The BA latex, obtained in the miniemulsion polym-
erization with the multifunctional RAFT agent de-

Macromolecules, Vol. 37, No. 25, 2004
Communications to the Editor 9301
further raising the RAFT agent/free radical initiator
ratio. This work is in progress.
In summary we have demonstrated that multifunc-
tional RAFT agents can be synthesized in a relatively
easy way. These multifunctional RAFT agents, depend-
ing on the number of RAFT groups they contain, yield
triblock or even multiblock copolymers in merely two
polymerization steps. The number of blocks generated
in the multiblock copolymers is lower than expected in
view of the number of RAFT groups in the multifunc-
tional RAFT agent. This problem can probably be solved
by optimizing the flux of radicals into the latex particles.
Figure 2. Number-average molar mass (Mh n, b) and polydis-
persity index (PDI, ∆) vs conversion for the miniemulsion
polymerization of n-butyl acrylate (BA), using a multifunc-
tional “poly(decyl-RAFT)” agent (with R ) decyl, Mh n,poly(decyl-RAFT)
) 3700 g/mol in PS equivalents, PDIpoly(decyl-RAFT) ) 1.59, on
average 12 RAFT moieties present per molecule, so n ) 6 in
Scheme 1). Drawn line is calculated Mh n. Conditions: 3 h, 70
°C; organic phase: 8.0 g of BA, 2 wt % hexadecane with respect
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