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Journal ArticleDOI

The first example of a poly(ethylene oxide)–poly(methylphenylsilane) amphiphilic block copolymer: vesicle formation in water

01 Jan 1998-Chemical Communications (Royal soc chemistry)-Vol. 1998, Iss: 14, pp 1445-1446

Abstract: A new amphiphilic multiblock copolymer of polymethylphenylsilane and poly(ethylene oxide) has been synthesised and demonstrated to form well-defined aggregates in water.
Topics: Ethylene oxide (57%), Copolymer (54%), Oxide (52%), Amphiphile (50%)

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X
Si
Me
X HO CH
2
CH
2
O
MeO
Si
Me
O CH
2
CH
2
O
Si
Me
O CH
2
CH
2
O Si
Me
OMe
+
n
m
m
n
n
(i) pyridine/THF/toluene
(ii) MeOH/pentane
(X = Cl or Br)
PMPS-PEO
m m
H
The first example of a poly(ethylene oxide)–poly(methylphenylsilane)
amphiphilic block copolymer: vesicle formation in water
Simon J. Holder,*
a
Roger C. Hiorns,
b
Nico A. J. M. Sommerdijk,
a
Stuart J. Williams,
a
Richard G. Jones
b
and
Roeland J. M. Nolte
a
a
Department of Organic Chemistry, NSR Centre, University of Nijmegen, Toernooiveld, Nijmegen 6525 ED, The Netherlands
b
Centre for Materials Research, School of Physical Sciences, University of Kent, Canterbury, Kent, UK CT2 7NH
A new amphiphilic multiblock copolymer of polymethylphe-
nylsilane and poly(ethylene oxide) has been synthesised and
demonstrated to form well-defined aggregates in water.
Polysilanes have been the subject of extensive research over the
past decade due to their remarkable electronic properties that
allow for a number of potential applications as conductive,
electroluminescent, non-linear optical and lithograpic mate-
rials.
1
Many of these properties can be expected to be more fully
exploited through the incorporation of polysilanes into copoly-
mer systems
2
which would allow for greater processability but
also allow for the manipulation of the macroscopic order of the
materials by supramolecular assembly (e.g. by microphase
separation or aggregation in aqueous dispersions). The majority
of amphiphilic block copolymers, none of which are derived
from polysilanes, form spherical micellar structures in aqueous
dispersions.
3
Recently, further morphologies have been ob-
served in aqueous dispersons such as rodlike, lamellar and
vesicular aggregates.
4
However the formation of vesicles by
block copolymers remains rare and is confined to copolymers
where the component blocks are monodisperse. We report here
the formation of vesicles from the multiblock poly(methylphe-
nylsilane)-co-poly(ethylene oxide) (PMPS-PEO), in which the
PMPS blocks have a normal size distribution. We believe that
this is the first example of vesicle formation by a multiblock
copolymer and the first defined aqueous aggregate formed by a
polysilane.
The PMPS-PEO copolymer was synthesised according to
Scheme 1 utilising Schlenk techniques under a dry argon
atmosphere. A solution of 4.78 g of poly(ethylene oxide) (M
n
=
7000, 6.8 3 10
24
mol, M
n
/M
w
= 1.03) in toluene was added to
a solution of a,w-dihalopoly(methylphenylsilane)
5
(M
n
=
4400, 6.8 3 10
24
mol, M
n
/M
w
= 2.00) in THF and toluene.
Subsequently pyridine (3 ml, 3.7 3 10
22
mol) was added and
the reaction solution was stirred for 30 min. This solution was
then added dropwise to MeOH (300 ml) and pentane (400 ml)
was slowly added to the resultant mixture. PMPS-PEO was
obtained as a yellowish white powder (60% yield, M
n
= 27 000,
M
w
/M
n
= 1.6) after filtration and vacuum drying for 72 h. The
molecular weights quoted are based upon size-exclusion
chromatography (SEC) measurements of THF solutions relative
to polystyrene standards using a refractive index (RI) detector.
The block structure of the copolymer was confirmed by
1
H,
13
C
and
29
Si NMR spectroscopy and also by analysis of the
molecular weight determinations of the copolymer and its
precursors using both UV and RI detectors in the SEC
experiment.
6
Although the molecular weights have been determined
relative to polystyrene standards it can be assumed that the
hydrodynamic properties of the parent homopolymers and the
segments in the copolymer structure are very similar. Thus the
M
n
value of 27 000 for the copolymer corresponds closely to the
structure shown in Scheme 1, [PMPS-PEO]
2
-PMPS, with a
degree of polymerisation (DP) of 2.5.‡ This is merely the most
abundant structure and comprises up to 30% of the overall
distribution which ranges from PMPS-PEO to (PMPS-
PEO)
16
.
Direct addition of the copolymer to water (1 mg PMPS-PEO/
1 ml H
2
O) gave poor quality dispersions with most of the
PMPS-PEO remaining as bulk solid. However transmission
electron microscopic (TEM) analysis of samples of the aqueous
portion of the mixture revealed vesicles [Fig. 1(a)].
Homogenous dispersions of the copolymer in water could be
prepared in two ways. The copolymer was dissolved in THF
(100 mg/10 ml THF) and water (3.5 ml) was added dropwise to
the stirred solution (the solution became turbid after the addition
of ca. 1.5 ml water). Subsequently the mixture was subjected to
ultrafiltration with continuous concentration and water dilution
(3 3 10 ml water). A homogenous white opaque dispersion
resulted (concentration = 1.2 g l
21
). TEM analysis of freeze
fractured samples of this dispersion showed that the copolymer
existed as vesicles with remarkably little micellar material [Fig.
1(b)]. Both convex and concave hemispheres are clearly visible.
The diameters of the vesicles ranged from ca. 100 nm to ca. 180
nm.
Scheme 1
Fig. 1 Transmission electron micrographs of vesicles observed for
copolymer dispersions obtained by (a) direct addition to water (negative
staining), (b) ultrafiltration/dilution method (freeze-fracture). Bars repre-
sent 300 nm.
Chem. Commun., 1998 1445

2 6 10 14 30
10
20
30
40
50
60
p / mN m
–1
A
/ nm
2
macromolecule
–1
a
b
Vesicle dispersions could also be prepared by the dialysis
procedure reported by Zhang and Eisenberg.
7
The copolymer
dispersion in water–THF was prepared in a similar manner to
the aforementioned procedure (3 ml THF, 1 ml water) and
placed in a dialysis bag (exclusion limit = 20 000 Da) and
dialysed against pure water (500 ml) for 72 h. EM analysis of
negatively stained and platinum shadowed samples of the
dispersion after this time again showed vesicles to be the
predominant aggregate structure.
To confirm that the observed structures were vesicles an
encapsulation experiment was performed utilising the water
soluble fluorescent dye 5-carboxyfluorescein in the dialysis
procedure.
8
After 72 h the dispersion was eluted (in water)
through a Sephadex column (G150, mesh size 40–120 m). The
elution volume of the encapsulated dye (30–110 ml, emission at
519.5 nm) coincided with that of the copolymer (emission 355
nm) indicating that closed vesicles are formed. The elution
volume of the free dye was substantially larger (160–190 ml).
To investigate the orientation of the copolymer chains in the
vesicle walls surface pressure–area isotherms were recorded for
a monolayer of PMS-PEO at the air/water interface.§ The
isotherm revealed a very large lift-off area of ca. 30 nm
2
molecule
21
which would correspond to the approximate area
for three PMPS chains orientated parallel to the water surface.
The observed plateau from 30 to 5 nm
2
molecule
21
(Fig. 2) and
the transition to a state characterised by a macromolecular area
of 4.7 nm
2
molecule
21
indicates a pseudo-first-order transition
to a phase in which the PMPS chains become orientated
perpendicular to the air/water interface. The collapse point is
reached at a pressure of ca. 27 mN m
21
where the area per
polymer chain is 3.7 nm
2
, which is in remarkably good
agreement with the estimated cross-sectional area for three
PMPS rods (ca. 3.6 nm
2
based upon models). When PMPS-
PEO was spread upon a subphase containing 0.1 m NaCl the
monolayer behaviour was essentially the same, the only
substantial exception being the higher collapse pressure. This
tends to confirm the above model where the limiting area is
defined by the PMPS segments.
UV spectroscopic analysis of the dispersion prepared by
ultrafiltration revealed a very weak absorption, due to the ss*
transition, with a maximum at 342 nm (l
max
in THF = 339 nm)
superimposed upon the scattering background. This is tenta-
tively attributed to chain straightening and an extension of the
effective conjugation length.
The freeze fracture electron micrographs, monolayer studies,
structural considerations and UV data support a tentative model
of the packing in the vesicle walls as shown in Fig. 3.¶ It is
apparent from our results that well-defined and very low
polydispersity systems need not necessarily be requirements for
the self-assembly of well-defined aggregates. Further work is in
progress to study the structures adopted by this remarkable
copolymer system.
Notes and References
† E-mail: sjh4@sci.kun.nl
‡ A polydispersity of 1.6 (recorded by SEC) corresponds to an extent of
reaction, p, of 0.6 in terms of the kinetics of the linear step-reaction
polymerisation involved in the block copolymer formation. This in turn
specifies a number average degree of polymerisation (DP) of 2.5 in
excellent agreement with the expected Flory distribution.
9
§ Monolayer experiments were carried out at 20.0 ± 0.1 °C using a double
barrier R&K trough of dimensions 6 3 25 cm with a compression speed of
8.8 cm
2
min
21
. The copolymer was spread from a solution of CHCl
3
.
¶ The lengths of the PMPS segments were calculated from literature data;
10
light-scattering experiments have shown a strong correlation between the
molecular weights obtained for polystyrene and PMPS in THF solutions by
SEC.
11
The lengths of the PEO chains were based upon the molecular
weight characteristics supplied by Aldrich; the width of the PEO coronae
were calculated assuming repeated folding of the chain parallel to the
orientation of the PMPS chains and the value given is therefore the
minimum width expected.
1 R. D. Miller and J. Michl, Chem. Rev., 1989, 89, 1359; Inorganic
Polymers, ed. J. E. Mark, H. R. Allcock and R. West, Prentice-Hall,
New Jersey, 1992, ch. 5, p. 186.
2 K. Sakamoto, K. Obata, H. Hirata, M. Nakajima and H. Sakurai, J. Am.
Chem. Soc., 1989, 111, 7641; S. Demoustier-Champagne, A.-F. de
Mahieu, J. Devaux, R. Fayt and P. J. Teyssie, J. Polm. Sci., Polym.
Chem., 1993, 31, 2009; E. Fossum, K. Matyjaszewski, S. S. Sheiko and
M. M¨oller, Macromolecules, 1997, 30, 1765.
3 C. Price, in Developments in block copolymers, ed. I. Goodman,
Applied Science Publishers, London, 1982, vol. 1, p. 39; J. Selb and Y.
Gallot, in Developments in block copolymers, ed. I. Goodman, Applied
Science Publishers, London, 1985, vol. 2, p. 327; Z. Tuzar and P.
Kratochvil, in Surface and Colloid Science, ed. E. Matijevic, Plenum
Press, New York, 1993, vol. 15, p. 1.
4 L. Zhang and A. Eisenberg, Science, 1995, 268, 1728.
5 R. C. Hiorns, R. G. Jones and F. Schue, unpublished results.
6 R. G. Jones and S. J. Holder, Macromol. Chem. Phys., 1997, 198,
3571.
7 L. Zhang and A. Eisenberg, J. Am. Chem. Soc., 1996, 118, 3168.
8 J. M. Fendler, Membrane Mimetic Chemistry, Wiley, New York,
1982.
9 P. J. Flory, Principles of Polymer Chemistry, Cornell University Press,
Ithaca, New York, 1953, ch. 8.
10 W. J. Welsh, J. R. Damewood, Jr. and R. C. West, 1989, 22, 2947.
11 C. Strazielle, A.-F. de Mahieu, D. Daoust and J. Devaux, Polymer, 1992,
33, 4171.
Received in Cambridge, UK, 29th April 1998; 8/03250E
Fig. 2 pA isotherms of PMPS-PEO at (a) pure water interface and (b) 100
mm NaCl
(aq)
interface
Fig. 3 Proposed model for copolymer organisation in the vesicle walls
1446 Chem. Commun., 1998
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