Ambient temperature reversible addition-fragmentation chain transfer polymerisation

Article abstract of DOI:10.1039/b101794m

Reversible addition fragmentation chain transfer was performed at ambient temperature for the first time.

Full text of DOI:10.1039/b101794m

Ambient temperature reversible addition–fragmentation chain
transfer polymerisation†
John F. Quinn,a Ezio Rizzardo*b and Thomas P. Davis*a
a Centre for Advanced Macromolecular Design, School of Chemical Engineering and Industrial
Chemistry, University of New South Wales, Sydney, New South Wales, Australia.
E-mail: t.davis@unsw.edu.au
b CSIRO Molecular Science, Clayton South, Victoria, Australia, 3169
Received (in Cambridge, UK) 26th February 2001, Accepted 24th April 2001
First published as an Advance Article on the web 16th May 2001
Reversible addition fragmentation chain transfer was per-
formed at ambient temperature for the first time.
ethyl group to re-initiate polymerization at these low tem-
peratures. To counter the first problem, a simple variation was
made to the Z group of the RAFT agent, in order to give a less
stable macroradical intermediate, thereby increasing the rate of
fragmentation. In this study, by changing the Z group from a
phenyl to a benzyl group, the radical in the RAFT intermediate
is changed from being in a disulfur benzylic position to a less
stable disulfur alkyl position. This should increase the rate of
fragmentation and result in faster establishment of the RAFT
equilibrium. Therefore, using such a RAFT agent it could be
anticipated to observe living behaviour in the polymerization of
alkyl acrylates at lower temperatures, rather than the demon-
strated retardation caused by 1-phenylethyl dithiobenzoate. The
RAFT agent synthesised for this study was 1-phenylethyl
phenyldithioacetate (2).‡
Methyl acrylate was polymerised in septa capped ampoules
in a water bath at rt using AIBN as the initiator and
1-phenylethyl phenyldithioacetate (1-PEPDTA) as the RAFT
agent. Given that AIBN has a much longer half-life at 25 °C
than it does at 60 °C, a higher than normal concentration was
used (36.1 3 1023 molL21). A significant exotherm was
observed in the control experiments (methyl acrylate, AIBN
only), with the resultant runaway reaction causing the monomer
to boil and the septa to break in the first ten minutes of the
reaction. Conversely, the polymerization of methyl acrylate
with added 1-PEPDTA proceeded without autoacceleration via
pseudo first order kinetics, as shown in Fig. 1.§
Reversible Addition Fragmentation Chain Transfer Polymer-
isation (the RAFT process) is a powerful technique for
synthesising well-defined polymer architectures with low
polydispersity.1–3 The technique employs a transfer agent of the
general formula:
which reacts with growing polymer chains via Scheme 1 shown
below. Reaction then proceeds with growing polymer chains
reacting alternately with the two sulfur atoms in the polymeric
RAFT agent. In this system, R must be a good leaving group
that is able to re-initiate polymerisation and the Z group strongly
influences the stability of the intermediate disulfur macroradical
species.4,5
In order to synthesise certain novel materials, it is desirable to
develop a RAFT agent that can be used at rt and with initiation
techniques such as UV or gamma radiation. However, prelimi-
nary studies by the authors have shown that at low temperatures
( ~ 25 °C), the presence of a common RAFT agent (1-phenyl-
ethyl dithiobenzoate, 1) strongly retards the polymerisation of
Further, use of 1-PEPDTA in polymerisation of methyl
acrylate at 25 °C gives low polydispersity poly(methyl acrylate)
(Mn = 241,417, PDI = 1.19) that shows living behaviour (i.e.
the molecular weight increases linearly with conversion). This
is demonstrated in Fig. 2.
It should be noted that the molecular weights obtained
experimentally adhere closely to those predicted from theory
(Fig. 2, unbroken line). The exception to this is the final data
point, where the theoretical molecular weight is considerably
lower than the experimental one. An explanation for this
deviation is the use of Mark Houwink Sakurada coefficients for
alkyl acrylates. This is consistent with previous work from the
CSIRO group, which has shown that variation of the Z group
from phenyl to methyl reduces retardation in the polymerization
of n-butyl acrylate at 80 °C.4 This retardation may be due to
either a low rate of fragmentation of the intermediate (lessening
the number of propagating radicals), or failure of the phenyl-
Scheme 1
Fig. 1 Pseudo first order rate plot for the bulk polymerisation of methyl
acrylate mediated with 1-PEPDTA at 25 °C. Error determined from
duplicates.
† Electronic supplementary information (ESI) available: data used for the
graphs in Figs. 1 and 2. See http://www.rsc.org/suppdata/cc/b1/b101794m/
1044
Chem. Commun., 2001, 1044–1045
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b101794m

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of magnesium turnings (3.75 g) in dry diethyl ether (100 ml). Following the
vigorous initial reaction, the solution was refluxed for 3 h to ensure
complete reaction. The mixture was then chilled and carbon disulfide (12.0
g) was added dropwise over 30 min, and then the mixture stirred at 0 °C for
the following 2 h. The mixture was then poured onto ice-water (300 ml) and
the aqueous portion collected following three washes with diethyl ether. A
final layer of diethyl ether was added, and the mixture acidified using 30%
aqueous HCl. Phenyldithioacetic acid ( ~ 7 g) was collected via rotary
evaporation of the ether. The acid was then reacted with styrene (9.0 g), with
a small amount of acid catalyst (toluene-p-sulfonic acid) in CCl4 (10 g). The
product was then precipitated in cold methanol and recrystallised from
methanol as fine yellow crystals (3.2 g). 1H NMR: d = 1.7 d 3H, d = 4.2
s 2H, d = 5.1 q 1H, d = 7.3 m 10H. 13C NMR (CDCl3): 20.5, 49.9, 57.9,
127.2, 127.6, 127.7, 128.5, 128.6, 129.1, 136.9, 141.0 and 233.6. IR:
Aromatic C–H stretch, 3062, 3028 cm21; Aliphatic C–H stretch, 2966, 2925
cm21; Overtone indicative of monosubstituted aromatic 2000–1650 cm21
;
Fig. 2 Evolution of molecular weight and polydispersity index with
conversion for 1-PEPDTA mediated polymerisation of methyl acrylate at
25 °C.
Aromatic ring stretch, 1601, 1494, 1453 cm21; Thiocarbonyl CNS stretch,
1219, 1125, 1028 cm21; Out of plane aromatic C–H bend 764, 697 cm21
Weak sulfide C–S stretch, 646, 591 cm21. Melting point = 35 °C.
;
§ Polymerization: A solution of methyl acrylate with an initial 1-PEPDTA
concentration of 3.9 3 1023 mol L21 and an AIBN concentration of 36.1 3
1023 mol L21 was prepared. The stock solution was divided into five
individual ampoules and deoxygenated by purging with nitrogen for
approximately 15 min. The ampoules were then placed in a constant
temperature water bath at 25 °C, and an ampoule was removed after 60, 100,
150, 220 and 280 min. The reactions were stopped by cooling the solutions
in an ice bath followed by the addition of hydroquinone. The polymer was
isolated by evaporating off the residual methyl acrylate, initially in a fume
cupboard to remove the bulk of the liquid, and then in a vacuum oven at 25
°C. Final conversions were measured by gravimetry, and the molecular
weight distribution measured using gel permeation chromatography. Each
experiment was performed in duplicate. Blank solutions, containing only
methyl acrylate and AIBN were also prepared and polymerised using the
same methods.
polystyrene in the molecular weight determination of the
polymer. When analysing a polyacrylate, this would be
expected to give an error anywhere between 10 and 100% (with
larger error at higher molecular weight), and therefore may
account for the observed discrepancy between the actual and
theoretical values.
These results demonstrate that by adjusting the structure of
the Z group in the RAFT agent living polymerization at rt is
possible. The structural adjustment is required to decrease the
stability of the intermediate radical, therefore causing the
equilibrium (Scheme 1) to shift to increase the concentration of
propagating polymer chains. In this study this was achieved by
changing the RAFT agent from 1-phenylethyl dithiobenzoate,
which gives rise to a disulfur benzylic radical intermediate, to
1-phenylethyl phenyldithioacetate, which yields a less stable
disulfur alkyl radical intermediate.
1 Y. K. Chong, T. P. T. Le, G. Moad, E. Rizzardo and S. H. Thang,
Macromolecules, 1999, 32, 2071.
2 H. De Brouwer, M. A. J. Schellekens, B. Klumpermann, M. J. Monteiro
and A. L. German, J. Pol. Sci. Part A, 2000, 38, 19, 3596.
3 E. Rizzardo, J. Chiefari, R. T. A. Mayadunne, G. Moad and S. H. Thang,
‘Synthesis of Defined Polymers by Reversible Addition Fragmentation
Chain Transfer: The RAFT Process’, in ACS Symposium Series 768,
2000.
The authors wish to acknowledge the support of the Co-
operative Research Centre for Polymers and Dr J. P. A. Heuts
and A/Professor R. P. Chaplin for helpful discussions.
4 G. Moad, J. Chiefari, Y. K. Chong, J. Krstina, R. T. A. Mayadunne, A.
Postma, E. Rizzardo and S. H. Thang, Polym Int., 2000, 49, 993.
5 C. Barner-Kowollik, J. F. Quinn, D. R. Morsley and T. P. Davis, J. Pol.
Sci. Part A: Chem., 2001, 39, 1353.
Notes and references
‡ Synthesis: 1-Phenylethyl phenyldithioacetate was synthesised using the
following method. Benzyl chloride (20 g) was added dropwise to a mixture
Chem. Commun., 2001, 1044–1045
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