(
a)
(b)
Table 2 Radical polymerisation of MMA (1.2 m) with AIBN (2 wt%) as
2
initiator in scCO with and without block copolymers as surfactant
Surfactant
(1 wt%)
a
b/1023
b
Entry
Yield (%)
M
n
w n
M /M
1
2
3
4
—
4B
6A
22
56
69
12.0
38.0
77.4
81.9
2.0
2.2
3.0
2.3
4
6
8
10
12
c
6B
72
t / min
Fig. 1 GPC trace (polystyrene standards, CHCl
3
) of a sample of (a) 5B (M
1.1) and (b) the PMMA block before addition of 2b (M
1.3)
n
a
Yields quoted after two reprecipitations in hexane. b GPC analysis (CHCl
3
,
2
9
18000, M
w n
/M
n
n
polystyrene standards) confirmed by multiple angle light scattering analysis
in THF. Yield quoted without reprecipitation; free-flowing powder
300, M /M
w
c
obtained.
of the insolubility of the propagating fluorinated block. In the
case of the copolymer 4A with a shorter perfluoroalkyl side
chain, this solubility problem was less apparent. Good yields of
these block copolymers, according to specific requirements, is
an advantage of the synthetic route described. Moreover, the
improved stabilising efficiency of block copolymer 6B, com-
pared with block copolymer 4B, in the dispersion polymer-
4A were obtained, and the polydispersity was relatively narrow,
even for samples with a high proportion of PFMA in relation to
the PMMA block (entry 1). The solubility problem could be
overcome by the use 1,3-bis(trifluoromethyl)benzene as a co-
solvent with toluene, which enabled the preparation of block
copolymer 5B with a high proportion of PFMA block (entry 4).
The GPC chromatogram of copolymer 5B is shown in Fig. 1.
The chromatogram of a sample of PMMA obtained before
addition of 2b is also shown. Both unimodal traces with narrow
polydispersity reveal that a controlled polymerisation is main-
tained throughout the reaction.†
2
isation of MMA in scCO demonstrates the applicability of this
synthetic methodology. We shall describe the power of the
tunability of the surfactant in a forthcoming publication.
We thank EPSRC for support of this work and provision of
the Swansea Mass Spectroscopy Service, ICI for a Strategic
Research Fund award (T. M. Y.), the UK Committee of Vice-
Chancellors and Principals for an ORS award, the Cambridge
Commonwealth Trust for a bursary, Clare College for a
Research Fellowship (T. M. Y.) and Zeneca Agrochemicals for
a CASE award (W. P. H.). We acknowledge with gratitude the
extensive advice received from Professor M. Poliakoff and Dr
S. Howdle (Nottingham).
Block copolymers can exhibit micelle-like aggregation if
they are dissolved in a solvent which is more selective towards
one of the blocks.6 Fluorinated homopolymers have been
7
known to be very soluble in both liquid CO
case of the AB block copolymers described above, the PFMA
block will have a much higher solubility in scCO compared
2 2
and scCO . In the
Footnotes and References
2
with the PMMA block, and may thus be employed as a stabiliser
or surfactant in a dispersion polymerisation. In preliminary
studies we have polymerised MMA under free radical condi-
*
E-mail: abh1@cus.cam.ac.uk
7
† It is possible that the molecular weight obtained for 4, 5 and 6 is
overestimated, owing to the rigidity of the PFMA block compared with the
flexible polystyrene reference standards used.
tions in scCO
2
2
using copolymers 4 and 6 as surfactants (Scheme
). These results demonstrate that these surfactants play an
important r oˆ le in stabilising a dispersion of the growing PMMA
chain in scCO . In the presence of 4B (Table 2, entry 2), both the
yield and molecular weight (M ) of the polymer are sig-
8
1
H. F. Mark, N. M. Bikales and C. G. Overberger, Encyclopaedia of
Polymer Science, ed. J. I. Kroschwitz, Wiley, New York, 2nd edn., 1987,
vol. 7, p. 256.
P. E. Cassidy, T. M. Aminabhavi and J. M. Faraday, J. Macromol. Sci.
Rev. Macromol. Chem. Phys., 1989, C29, 365.
2
n
2
3
nificantly higher than observed in the control experiment
without surfactant (Table 2, entry 1). However, elongation of
the fluorinated side-chain improved these results even more
Contact Angle, Wettability and Adhesion, Advances in Chemistry Series,
ed. W. A. Zisman, ACS Symposium, Washington, 1964, vol. 43.
(
Table 2, entry 3). The effect of the molecular weight of the
4 D. G. H. Ballard, R. J. Bowles, D. M. Haddleton, S. N. Richards,
R. Sellens and D. L. Twose, Macromolecules, 1992, 25, 590.
5 For the use of fluorinated homopolymers as surfactants, see Z. Guan and
J. M. DeSimone, Macromolecules, 1994, 27, 5527; J. M. DeSimone,
E. E. Maury, Y. Z. Menceloglu, J. B. McClain, T. J. Romack and
J. R. Combes, Science, 1994, 265, 356.
surfactant, where there is an increase of the length of the
different blocks, is illustrated by comparing the surfactant 6A
with 6B. In the latter case (Table 2, entry 4) we were able to
obtain PMMA as a free flowing powder together with further
increase in molar mass.
6
L. J. Vagberg, K. A. Cogan and A. P. Gast, Macromolecules, 1991, 24,
670 and references cited therein.
In summary, we have developed a highly controlled synthesis
of fluorinated AB block copolymers. The ability to vary the side
chain as well as to ‘tune’ the length of the different blocks in
1
7
8
J. M. DeSimone, Z. Guan and C. S. Elsbernd, Science, 1992, 257, 945.
During the course of our study the use of fluorinated block copolymers in
the dispersion polymerisation of styrene (see D. A. Canelas, D. E. Betts,
J. M. DeSimone, Macromolecules, 1996, 29, 2818) as well as the use of
fluorinated graft copolymers in the dispersion polymerisation of MMA in
Me
Me CO2Me
i
scCO
cules, 1997, 30, 745).
2
was reported (see C. Lepilleur and E. J. Beckman, Macromole-
CO2Me
n
Scheme 2 Reagents and conditions: i, AIBN, scCO
or 6
2
, 285 bar, 70 °C, 5 h, 4
Received in Cambridge, UK. 6th May 1997; 7/05399A
1812
Chem. Commun., 1997