Synthesis and aqueous solution
properties of a well-defined thermoresponsive Schizophrenic diblock
copolymer 1
J. V. M. Weaver and S. P. Armes
School of Chemistry, Physics and Environmental Science
University of Sussex, BN1 9QJ, UK
1
J. V. M. Weaver et al., Chem. Commun., 18, 2122, 2002
A brief history of “Schizophrenic” micelles
First Generation: Salt and pH responsive
V. Butun et al, J. Am. Chem. Soc., 120, 11818, 1998
1.0 M Na2SO4
pH 6.7
Dialysis or
pH < 6.7
pH 8
Second Generation: Purely thermo-responsive
PPO-core micelles at
40 °C, pH 6.5,
Nagg= 760
pH < 7
MEMA-DEA
S.Liu et al, Angew. Chem., 40, 2328, 2001
PPO-DEA
unimers at 5 °C,
pH 6.5
Third Generation: Purely pH-responsive
DEA-core micelles at
5 °C, pH 8.5,
Nagg= 650
S.Liu et al, Angew. Chem., 41, 1413, 2002
Thermo-responsive, schizophrenic diblock
copolymers
Laschewsky and co-workers have reported the labour-intensive RAFT
synthesis of a sulfobetaine-block-N-isopropylacrylamide copolymer 2
Methacrylamide
sulfobetaine
(UCST)
O
_
O 3S
+
N
NH
O
NIPAM
(LCST)
HN
Conversion of the first block was allowed to reach only 27 % for efficient
blocking and purification was tedious. Mw / Mn = 1.35
BUT
This was the first example of an entirely thermo-responsive schizophrenic
(forms micelle and inverted-micelles) diblock copolymer.
2
M. Arotcarena et al, J. Am. Chem. Soc., 124, 3737, 2002,
Synthesis of the “schizophrenic” diblock
copolymers
DMA
m
O
n
O
O
N
O
MEMA
Synthesised by GTP
Mw/Mn = 1.08 - 1.10
N
DMA content = 35 or 51 mol %
O
1,3 propane sultone, THF,
20 oC, 3 days
SBMA
_
O3S
m
O
n
O
+
N
O
O
MEMA
N
Selective betainisation
of DMA residues give
the thermo-responsive
diblock copolymers 3
O
3
V. Butun et al, J. Mater. Chem., 7, 1693, 1997.
Aqueous solution properties of
poly(SBMA)
Poly sulfobetaines often show Upper Consolute Solution Temperatures
(UCST) in aqueous solution
At low temperature, in the absence of electrolyte, attractive inter-chain
electrostatic interactions prevent dissolution of the polymer in water
At elevated temperatures, the
thermal energy becomes enough to
break the relatively weak inter-chain
ionic interactions. The polymer
becomes water-soluble. This is a
fully reversible process.
In the presence of salt, polymer
chains preferentially complex the
electrolyte and reduce the attractive
inter-chain interaction. The polymer
becomes water-soluble. This is a
reversible process (requires dialysis
against pure water).
R
N+
SO3 _
_
SO3
+N
R
UCST behaviour of
poly(sulfobetaines)
Aqueous solution properties of poly(MEMA)
Poly (2-(N-morpholino)ethyl methacrylate) is a weak polybase, which is watersoluble at both neutral and acidic pH at ambient temperature
Poly MEMA precipitates from neutral or basic aqueous solutions at elevated
temperatures (32-53 °C, depending on molecular weight) 4
P(MEMA)
H O
At elevated temperatures, the
thermal energy breaks the extensive
hydrogen bonding network with the
water. The polymer phase-separates
from water. This is a fully reversible
process.
O
O
H O
H
H
O H
H
N
O
H O
Addition of as little as 0.2 M M2SO4
salts out the polymer under neutral
or basic conditions
H
H O
H
P(MEMA)
LCST behaviour of poly(MEMA)
4
V. Butun et al., Polymer, 42, 5993, 2001
DLS for SBMA-MEMA diblock
copolymer
Intensity / kcps
16
16
50 nm
14
14
42 nm
12
12
10
10
88
µ2 /Γ2 = 0.45
µ2 /Γ2 = 0.09
66
44
00
10
10
20
20
30
30
40
40
50
50
60
60
70
Temperature / oC
< 20 oC Loose,
SBMA-core
micelles
20-50 oC
Molecularly
dissolved
> 50 oC
Compact,
MEMA-core
micelles
1.0 w/v % aqueous solution
1H
NMR spectra of the thermo-responsive,
‘schizophrenic’ SBMA-MEMA diblock
copolymer
1H
NMR in D 2O at pH 6.5
Mn = 44,000 g mol -1
Mw/Mn = 1.10
SBMA content = 35 mol %
⇐ Molecularly dissolved,
SBMA-MEMA unimers
⇐ hydrated SBMA-core
micelles
⇐ dehydrated MEMA-core
micelles
Conclusions
1. Near-monodisperse diblock copolymer precursors are readily
synthesised in high yield by GTP
2. Molecular dissolution occurs at around 20-50 °C
3. Hydrated, polydisperse SBMA-core micelles at below 20 °C
4. Relatively dehydrated, monodisperse MEMA-core micelles at above
50 °C
5. This is only the second example of a purely thermo-responsive,
schizophrenic diblock copolymer
6. Continuing studies by SANS and SLS in collaboration with Alice
Gast’s group (Stanford University/MIT)
Acknowledgements
Prof. Steven P. Armes (academic supervisor)
Dr Peter McKenna (industrial supervisor)
Dr Vural Butun
University of Sussex Polymer Group