Well-defined biocompatible block copolymers via atom transfer radical polymerization of 2-methacryloyloxyethyl phosphorylcholine in protic media

Article abstract of DOI:10.1021/ma021762c

2-Methacryloyloxyethyl phosphorylcholine (MPC) is commonly used to prepare biocompatible copolymers that have delivered clinically proven benefits in various biomedical applications. Recently, we reported that MPC could be homopolymerized to high conversions with good control via atom transfer radical polymerization (ATRP) in protic media. In the present study we describe the synthesis of a wide range of well-defined MPC-based block copolymers using either near-monodisperse macroinitiators or sequential monomer addition. With the former approach, the macroinitiators were based on either poly-(alkylene oxides) or poly(dimethylsiloxane). With the latter approach, suitable comonomers included a wide range of methacrylic and other monomers, including 2-(dimethylamino)ethyl methacrylate (DMA) and its quaternized derivatives, 2-(diethylamino)ethyl methacrylate (DEA), 2-(diisopropylamino)ethyl methacrylate (DPA), methyl methacrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, and glycerol monomethacrylate. Polymerization of MPC using the three macroinitiators yielded novel PEO-MPC, PPO-MPC, and PDMS-MPC diblock copolymers. The PPO-MPC diblock copolymer proved to be thermoresponsive: molecular dissolution occurred in cold water, with colloidal aggregates being formed reversibly at elevated temperatures due to the inverse temperature solubility behavior of the PPO block. For the sequential monomer addition syntheses, the MPC monomer was generally polymerized first under optimized conditions, followed by the second monomer. High conversions were obtained for both stages of polymerization, and where applicable, aqueous GPC analyses indicated reasonably low polydispersities and good blocking efficiencies. Above pH 8, the MPC-DMA diblock copolymers also exhibited thermoresponsive behavior, forming DMA-core aggregates at elevated temperature. Spontaneous dissociation occurred on cooling to ambient temperature as the hydrophobic DMA block became hydrophilic again. The MPC-DMA, MPC-DEA, and MPC-DPA diblock copolymers proved to be pH-responsive polymeric surfactants at ambient temperature: molecular dissolution occurred in dilute acidic solution with well-defined, near-monodisperse micelles being formed at around neutral pH. In each case, the MPC block formed the biocompatible micelle coronas and the tertiary amine methacrylate block formed the hydrophobic micelle cores. In the case of the MPC-DPA diblock copolymer, the pyrene partition constant for the DPA-core micelles at pH 9 was similar to that reported previously for polystyrene-core micelles. These new MPC-based diblock copolymers are being evaluated as new nonviral vectors for DNA condensation and stealthy nanocapsules for the delivery of hydrophobic drugs and also for the synthesis of biocompatible shell cross-linked micelles.

Full text of DOI:10.1021/ma021762c

Macromolecules 2003, 36, 3475-3484
3475
Well-Defined Biocompatible Block Copolymers via Atom Transfer
Radical Polymerization of 2-Methacryloyloxyethyl Phosphorylcholine in
Protic Media
Yin gh u a Ma , Yiqin g Ta n g, Nor m a n C. Billin gh a m , a n d Steven P . Ar m es*
School of Chemistry, Physics and Environmental Science, University of Sussex,
Falmer, Brighton BN1 9QJ , UK
An d r ew L. Lew is
Biocompatibles, Chapman House, Farnham Business Park, Weydon Lane,
Farnham, Surrey GU9 8QL, UK
An d r ew W. Lloyd a n d J on a th a n P . Sa lva ge
School of Pharmacy and Biomolecular Sciences, University of Brighton,
Moulescoomb, Brighton, BN2 4GJ , East Sussex, UK
Received December 16, 2002; Revised Manuscript Received March 19, 2003
ABSTRACT: 2-Methacryloyloxyethyl phosphorylcholine (MPC) is commonly used to prepare biocompatible
copolymers that have delivered clinically proven benefits in various biomedical applications. Recently,
we reported that MPC could be homopolymerized to high conversions with good control via atom transfer
radical polymerization (ATRP) in protic media. In the present study we describe the synthesis of a wide
range of well-defined MPC-based block copolymers using either near-monodisperse macroinitiators or
sequential monomer addition. With the former approach, the macroinitiators were based on either poly-
(alkylene oxides) or poly(dimethylsiloxane). With the latter approach, suitable comonomers included a
wide range of methacrylic and other monomers, including 2-(dimethylamino)ethyl methacrylate (DMA)
and its quaternized derivatives, 2-(diethylamino)ethyl methacrylate (DEA), 2-(diisopropylamino)ethyl
methacrylate (DPA), methyl methacrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate,
and glycerol monomethacrylate. Polymerization of MPC using the three macroinitiators yielded novel
PEO-MPC, PPO-MPC, and PDMS-MPC diblock copolymers. The PPO-MPC diblock copolymer proved
to be thermoresponsive: molecular dissolution occurred in cold water, with colloidal aggregates being
formed reversibly at elevated temperatures due to the inverse temperature solubility behavior of the
PPO block. For the sequential monomer addition syntheses, the MPC monomer was generally polymerized
first under optimized conditions, followed by the second monomer. High conversions were obtained for
both stages of polymerization, and where applicable, aqueous GPC analyses indicated reasonably low
polydispersities and good blocking efficiencies. Above pH 8, the MPC-DMA diblock copolymers also
exhibited thermoresponsive behavior, forming DMA-core aggregates at elevated temperature. Spontaneous
dissociation occurred on cooling to ambient temperature as the hydrophobic DMA block became hydrophilic
again. The MPC-DMA, MPC-DEA, and MPC-DPA diblock copolymers proved to be pH-responsive
polymeric surfactants at ambient temperature: molecular dissolution occurred in dilute acidic solution
with well-defined, near-monodisperse micelles being formed at around neutral pH. In each case, the MPC
block formed the biocompatible micelle coronas and the tertiary amine methacrylate block formed the
hydrophobic micelle cores. In the case of the MPC-DPA diblock copolymer, the pyrene partition constant
for the DPA-core micelles at pH 9 was similar to that reported previously for polystyrene-core micelles.
These new MPC-based diblock copolymers are being evaluated as new nonviral vectors for DNA
condensation and “stealthy” nanocapsules for the delivery of hydrophobic drugs and also for the synthesis
of biocompatible shell cross-linked micelles.
In tr od u ction
ethyl phosphorylcholine (MPC) and rely on conventional
free radical polymerization chemistry to produce either
The phosphorylcholine motif is an important compo-
nent of cell membranes, and it is well-known that
synthetic phosphorylcholine-based polymers can be used
to produce surface coatings that are remarkably re-
sistant to protein adsorption and bacterial/cellular
3
-6
statistical copolymers or macromonomers.
Leading
players in this field include Ishihara’s group,4 who
have suggested on the basis of differential scanning
calorimetry studies that the biocompatibility perfor-
mance of MPC-based coatings is related to the highly
-6
adhesion.1-8 On the basis of this ‘bioinspired’ approach,
5
b
hydrophilic nature of the MPC monomer.
various biomedical devices and implants with clinically
proven enhanced biocompatibility have been developed
over the past decade, including high-performance coro-
nary stents and “low irritation” soft contact lenses.
Usually such coatings incorporate phosphorylcholine-
containing vinyl monomers such as 2-methacryloyloxy-
Although conceptually simple, this approach allows
only very limited control over the target molecular
weight and the molecular weight distribution, and the
synthesis of well-defined diblock copolymer architec-
tures is not possible. As far as we are aware, there have
been no reports of the controlled polymerization of
2-methacryloyloxyethyl phosphorylcholine (MPC). One
3
*
To whom correspondence should be addressed.
1
0.1021/ma021762c CCC: $25.00 © 2003 American Chemical Society
Published on Web 04/24/2003

3
476 Ma et al.
Macromolecules, Vol. 36, No. 10, 2003
protic media such as water and/or lower alcohols under
1
5-19
mild conditions.
In view of these results, and given
that MPC is insoluble in many organic solvents, we
investigated the feasibility of polymerizing MPC via
aqueous or methanolic ATRP. Our initial results re-
garding the controlled homopolymerization of MPC were
2
0
reported in a recent communication, and a follow-up
2
1
paper has been recently published; herein we describe
the block copolymerization of MPC. Two approaches
have been investigated: the macroinitiator route (see
Figure 1) and the sequential monomer addition route
(
see Figure 2). By use of the latter route, the successful
block copolymerization of MPC with a wide range of
methacrylic monomers has been achieved.
Exp er im en ta l Section
Ma ter ia ls. The MPC monomer (99.5% purity) was obtained
from Biocompatibles UK Ltd. The Cu(I)Br and 2,2′-bipyridine
(bpy) were both purchased from Aldrich and used as received.
The silica used for removal of the ATRP copper catalyst was
column chromatography grade silica gel 60 (0.063-0.200 mm)
purchased from E. Merck (Darmstadt, Germany). Methanol
and 2-propanol were purchased from Aldrich. The water used
in all experiments was deionized and doubly distilled prior to
F igu r e 1. Reaction scheme for the block copolymerization of
MPC via ATRP in protic media at 20 °C using the macro-
initiator route.
problem is that such betaine monomers are normally
insoluble in the organic solvents (e.g., toluene, THF)
typically used for conventional ionic living polymeriza-
tions, which is the traditional method for producing
controlled-structure, near-monodisperse copolymers. In
use. The OEGBr initiator was synthesized and purified ac-
cording to a literature procedure.1
5-17,22
The two monohydroxy-
capped poly(alkylene oxide) precursors were donated by Laporte
Performance Chemicals (Hythe, UK). Poly(ethylene oxide)
(
PEO-OH) had an M
Poly(propylene oxide) (PPO-OH) had an M
and an M /M of 1.06. These two precursors were each
n
of 2000 (Dp ) 45) and an M
w n
/M of 1.10.
of 1940 (Dp ) 33)
n
1
995, atom transfer radical polymerization (ATRP) was
w
n
developed independently by Wang and Matyjaszewski
and Sawamoto’s group.9-12 Reversible chain-end cap-
ping using halogen atoms ensures that the instanta-
neous polymer radical concentration is lower than in
conventional radical polymerization, which leads to the
suppression of termination relative to propagation and
hence enables relatively narrow molecular weight dis-
tributions to be achieved. Efficient block copolymer
syntheses can be achieved under optimized conditions,
and this pseudo-living free radical chemistry has gained
a deserved reputation for its excellent tolerance of both
monomer functionality and protic sources.1
converted into the corresponding ATRP macroinitiators (PEO-
Br and PPO-Br) by reaction with 2-bromoisobutyryl bromide
using previously reported protocols.22 Monocarbinol-terminated
and dicarbinol-terminated poly(dimethylsiloxane) (PDMS),
each with a nominal molecular weight of 1000, were purchased
1
from Gelest Inc., Germany. H NMR studies in CDCl
3
indicated
a number-average molecular weight of 1250 for the mono-
carbinol PDMS (corresponding to a Dp of 13) and a number-
average molecular weight of 1330 for the dicarbinol PDMS
(also corresponding to a Dp of 13). GPC analyses gave
polydispersities of 1.16 and 1.25 for the monocarbinol- and
dicarbinol-terminated PDMS, respectively.
3,14
[2-(Methacryloyloxy)ethyl]dimethyl(3-sulfopropyl)ammoni-
We recently reported that ATRP is particularly ef-
fective for a wide range of hydrophilic monomers in
um hydroxide (SBMA), methyl methacrylate (MMA), 2-(di-
methylamino)ethyl methacrylate (DMA), and 2-(diethylamino)-
F igu r e 2. Reaction scheme for the block copolymerization of MPC via ATRP in protic media at 20 °C using the sequential monomer
addition route.

Macromolecules, Vol. 36, No. 10, 2003
ATRP of MPC in Protic Media 3477
ethyl methacrylate (DEA) were purchased from Aldrich.
Glycerol monomethacrylate (GMA) was a gift from R o¨ hm
monomer conversion of 99%. The reaction solution was passed
through a silica gel column to remove the spent catalyst. After
solvent evaporation, the copolymer was washed with excess
THF to remove residual HEMA monomer and then dried in a
vacuum oven at room temperature to yield a colorless solid.
(Germany). The methyl chloride and benzyl chloride quater-
nized derivatives of DMA (designated MeDMA and BzDMA,
respectively) were kindly provided as 75 wt % aqueous solu-
tions by Elf Atochem, Germany. 2-Hydroxyethyl methacrylate
MP C-SBMA Diblock Cop olym er . MPC (4.00 g, 1.35 ×
(HEMA) and monomethoxy-capped oligo(ethylene glycol) meth-
-2
1
0
mol) was polymerized first in water (10 mL) using [MPC]:
acrylate (OEGMA) were donated by Laporte Performance
Chemicals (Hythe, UK). 2-(Diisopropylamino)ethyl methacry-
late (DPA) was purchased from Scientific Polymer Products.
The carboxybetaine methacrylate (CBMA) was synthesized by
reacting DMA with sodium iodoacetate in water at 20 °C for
several days at pH 8, followed by precipitation into either THF
or acetone to remove the sodium iodide byproduct.
Gen er a l P olym er iza tion P r otocols. 1. P olym er iza tion
of MP C Usin g Ma cr oin itia tor s. Typical protocols for the
controlled polymerization of MPC using the PEO-Br, PPO-Br,
and PDMS-Br macroinitiators in methanol, 2-propanol, or a
[
OEGBr]:[CuBr]:[bpy} ) 20:1:1:2. After 2 h, the MPC conver-
sion was greater than 99%, and the homopolymer obtained
had an M of 6200 and an M /M of 1.20. SBMA (3.80 g, 1.35
10 mol, Dp ) 20) was then added to this reaction solution
1
n
w
n
-2
×
as a solid. After 21 h, H NMR studies indicated that both
monomers had been consumed. The MPC-SBMA diblock
copolymer had an M of 12 000 and an M /M of 1.27, as
n w n
determined by aqueous GPC. The reaction solution was then
passed through a silica gel column to remove the spent
catalyst. After solvent evaporation, the purified copolymer was
dried in a vacuum oven at 20 °C for at least 24 h to yield a
colorless solid. This purification protocol resulted in the loss
of up to 15% MPC-SBMA diblock copolymer due to adsorption
onto the silica gel.
4
:1 methanol/2-propanol mixture were as follows. PEO-Br (1.01
g, 0.505 mmol, 1 equiv) was dissolved in methanol (10 mL).
After purging with nitrogen for 30 min, the Cu(I)Br catalyst
(72.0 mg, 0.505 mmol, 1 equiv) and bpy ligand (158 mg, 1.01
MP C-MeDMA Diblock Cop olym er . MPC (6.00 g, 2.02
mmol, 2 equiv) were added to the stirred solution under
nitrogen. MPC (6.00 g, 20.2 mmol) was then added as a solid
to the reaction mixture under nitrogen. The reaction mixture
immediately became dark brown and progressively more
viscous. In aqueous-based reaction solutions, exotherms of 2-4
-
2
×
10 mol) was homopolymerized first in a 4:1 methanol/
water solvent mixture using [MPC]:[OEG-Br]:[CuBr]:[bipy] )
0:1:1:2. After 1 h, over 99% conversion was achieved and the
isolated homopolymer had an M /M of 1.19. A 75 wt %
3
w
n
-2
aqueous solution of MeDMA monomer (5.50 g, 2.02 × 10 mol,
target Dp ) 30) was then added to the polymerizing solution.
After 46 h, an MPC-MeDMA diblock copolymer was obtained
at an overall conversion of more than 99%, as indicated by H
NMR spectroscopy (no residual vinyl double bonds at δ 5.5-
6
gel column to remove the copper catalyst, and the diblock
copolymer was freeze-dried from water overnight. This puri-
fication protocol resulted in the loss of up to 20% diblock
copolymer due to adsorption. The diblock copolymer had an
n w n
M of 16 000 and an M /M of 1.27 as determined by aqueous
GPC using poly(2-vinylpyridine) standards.
°
C were typically observed, indicating the onset of polymeri-
1
zation. After approximately 24 h, H NMR analysis indicated
that more than 99% of the MPC had been polymerized
1
(disappearance of vinyl signals at δ 5.5-6.0). On exposure to
air, the reaction solution turned blue, indicating aerial oxida-
tion of the Cu(I) catalyst. The resulting PEO-MPC diblock
copolymer was precipitated into THF, then redissolved in
water, and passed through a silica column to remove the spent
ATRP catalyst. The diblock copolymer was dried by freeze-
.0). The reaction solution was then passed through a silica
drying for 24 h. Aqueous GPC analysis indicated an M
n
of
of
1
1
w
2 000 (vs poly(ethylene oxide) standards) and an M /M
n
.19.
-
2
MP C-DP A Diblock Cop olym er . MPC was polymerized
MPC (6.24 g, 2.1 × 10 mol) was block copolymerized in
-
2
first (6.00 g, 2.02 × 10 mol) in 10 mL of methanol, using
[MPC]:[OEGBr]:[CuBr]:[bpy] ) 30:1:1:2 under a nitrogen
atmosphere at 20 °C. After 2.0 h, the MPC conversion was
greater than 99%, and the MPC homopolymer obtained had a
low polydispersity (M
(
1
2
0 mL of methanol using the PPO-Br macroinitiator (0.45 g,
-
4
.1 × 10 mol). The relative molar ratio of [MPC]:[PPO-Br]:
[
CuBr]:[bipy] was 100:1:1:2. After 24 h, an MPC-PPO diblock
copolymer was obtained with an MPC conversion of almost
00%, as indicated by 1H NMR spectroscopy. The reaction
w
/M
n
) 1.09 with M ) 10 000 vs poly-
n
1
ethylene oxide) standards). Then DPA monomer (8.65 g, 4.04
10 mol, target Dp ) 60) was added to this reaction
1
solution was then passed through a silica gel column to remove
the spent ATRP catalyst. After solvent evaporation, the
remaining copolymer was dried in a vacuum oven at room
temperature to yield a colorless solid.
-
2
×
solution. After 40 h, H NMR studies indicated that both
monomers had been consumed. The reaction solution was
passed through a silica gel column to remove the spent ATRP
catalyst, which resulted in the loss of around 10% copolymer
due to adsorption onto the silica. After solvent evaporation,
the solid copolymer was washed with excess n-hexane to
remove any traces of residual DPA monomer, redissolved in
water, and then freeze-dried overnight. The resulting colorless
Finally, block copolymerization of MPC using PDMS-Br was
conducted in 10 mL of a 4:1 methanol/2-propanol solvent
mixture under the following conditions: MPC (4.00 g, 1.35 ×
-
2
1
2
0
mol); [MPC]:[PDMS-Br]:[CuBr]:[bipy] ) 100:1:1:2. After
1
5 h, H NMR spectroscopy indicated no residual vinyl double
bonds at δ 5.5-6.0. The blue alcoholic solution of the MPC-
PDMS diblock copolymer was then passed through a silica gel
column to remove the spent catalyst. After solvent evaporation,
the remaining copolymer was dried in a vacuum oven at 20
MPC30-DPA60 diblock copolymer had an M
/M of 1.27, as determined by aqueous GPC using poly(2-
vinylpyridine) standards.
n
of 21 000 and an
M
w
n
°
C for at least 16 h to yield a colorless solid. A similar protocol
was employed for the bifunctional Br-PDMS-Br macroinitiator.
. Block Cop olym er iza tion of MP C w ith Oth er Meth -
Cop olym er Ch a r a ct er iza t ion . GP C P r ot ocols. The
molecular weights and molecular weight distributions of
selected MPC homopolymers and diblock copolymers were
determined by aqueous gel permeation chromatography (GPC).
The standard GPC protocol involved using a Pharmacia
Biotech “Superose 6” column connected to a Polymer Labs
ERC-7517A refractive index detector. The eluent was 0.20 M
2
a cr ylic Mon om er s. The following four examples are illustra-
tive of the general synthetic protocols employed.
MP C-HEMA Diblock Cop olym er . MPC was polymerized
-
2
first (6.00 g, 1.35 × 10 mol) in 10 mL of methanol, using
[
MPC]:[OEGBr]:[CuBr]:[bpy] ) 30:1:1:2. After 2.5 h, the
monomer conversion was greater than 99%, and the MPC
homopolymer obtained had a low polydispersity (M /M
.19) with M ) 9000 (vs poly(ethylene oxide) standards).
3
NaNO solution with 50 mM Trizma buffer at pH 8-10, and
poly(ethylene oxide) standards were used for calibration. For
MPC-based diblocks based on cationic monomers (DMA, DEA,
DPA, MeDMA, BzDMA) an alternative protocol was employed.
This involved using two ViscoGEL columns (G5000 PWXL and
G2500 PWXL) connected to a Polymer Labs ERC-7517A
refractive index detector. The eluent was an aqueous solution
w
n
)
1
n
-
2
HEMA monomer (4.39 g, 3.37 × 10 mol, target Dp ) 50)
was then added to this reaction solution. The reaction mixture
was maintained under a dry nitrogen purge for the duration
of the polymerization. On exposure to air after 24 h, the
reaction solution turned blue, indicating aerial oxidation of
2 4
comprising 0.50 M acetic acid and 0.30 M Na SO at pH 2;
poly(2-vinylpyridine) standards (PSS, Germany) were used for
calibration.
1
the ATRP catalyst. H NMR studies indicated a HEMA

3
478 Ma et al.
Macromolecules, Vol. 36, No. 10, 2003
1
1
H NMR Sp ectr oscop y. All H NMR spectra were recorded
argon plasma, and the Cu emission at 327.396 nm was
measured and compared to that found for stock solutions
containing 0.100, 0.050, and 0.020 ppm Cu; a blank solution
containing 5% (v/v) nitric acid in distilled water was used as
a fourth calibration solution. For sample solutions that
exceeded 0.100 ppm of Cu, a second analysis was performed
using calibration solutions containing 1.000 and 0.100 ppm
together with the blank solution. The manufacturer’s esti-
mated detection limit is around 0.0097 ppm of Cu for the above
emission wavelength. This ICP-AES technique indicated re-
sidual Cu levels in the range of 1-3 ppm for the purified
diblock polymers analyzed.
using a 300 MHz Bruker Avance DPX300 spectrometer. The
kinetics of polymerization were determined for reactions
carried out in either D O or CD OD by comparing the peak
2 3
integrals due to the monomer vinyl signals at δ 5.5 and 5.9 to
those of the methacrylate backbone at δ 0.5-1.1. If appropri-
ate, the macroinitiator was used as an “end group” to deter-
mine the degree of polymerization of the MPC block. In these
calculations it was assumed that the macroinitiator efficiency
was 100%, that chain transfer was negligible, and that every
polymer chain contained a macroinitiator end group.
Dyn a m ic Ligh t Sca tter in g Stu d ies. The MPC-DEA and
MPC-DPA diblock copolymers were molecularly dissolved in
water at pH 2 at different concentrations, and the solution
pH was adjusted to pH 8-9 to induce micelle formation.
Dynamic light scattering (DLS) studies were performed at 25
C on a Brookhaven Instruments Corp. BI-200SM goniometer
equipped with a BI-9000AT digital correlator using a solid-
state laser (125 mW, λ ) 532 nm) at a fixed scattering angle
of 90°. The intensity-average hydrodynamic diameter, 〈D 〉, and
h
polydispersity, µ
lants analysis of the experimental correlation function.
polydispersity, µ
width of the size distribution, which is equal to the normalized
second moment from a cumulant analysis of the scattered light
autocorrelation function. For relatively narrow size distribu-
tions the polydispersity may be equated to the variance of the
distribution.
F lu or escen ce Stu d ies of Micelliza tion . Critical Micel-
lization pH Studies. The MPC30-DMA60, MPC30-DEA60, and
MPC30-DPA60 diblocks were molecularly dissolved in turn in
doubly distilled water at pH 2. Pyrene/copolymer solutions
were prepared by adding acetone solutions of pyrene into dry
Resu lts a n d Discu ssion
Recently, we reported a detailed optimization study
of the homopolymerization of MPC via ATRP.2
0,21
Excel-
°
lent yields, first-order monomer kinetics, linear Mn vs
conversion plots, and relatively narrow polydispersities
(Mw/Mn ) 1.15-1.35) were obtained in both aqueous and
2
2
/Γ , of the micelles were obtained by cumu-
The
/Γ , is a model-independent estimate of the
alcoholic media at 20 °C. However, slower polymeriza-
tions and narrower polydispersities were always ob-
tained in alcoholic solution, and chain extension experi-
ments indicated significantly greater living character
(i.e., greater self-blocking efficiencies) under these con-
18,19
2
2
2
1
ditions. ATRP kinetics were significantly slower in
2-propanol (IPA) than in methanol due to the reduced
polarity of the former solvent. However, acceptable rates
of polymerization and reasonable control were obtained
at elevated temperature in IPA. Alternatively, the
addition of a relatively small amount of water to the
IPA led to a significantly faster polymerization at
1
0.0 mL volumetric flasks. After evaporation of the acetone,
diluted copolymer stock solutions were added so as to obtain
a final copolymer concentration of 0.50 g L in each case; the
2
3
ambient temperature. The effect of varying the ligand
type and target Dp was also investigated, with optimal
results being obtained for 2,2′-bipyridine and target Dp’s
of 20-200. Herein we have extended this initial homo-
polymerization study in order to prepare a wide range
of controlled-structure MPC-based block copolymers. In
view of the good living character obtained for the
homopolymerization of MPC, we elected to polymerize
this monomer first in all of the “one-pot” block copolymer
syntheses involving sequential monomer addition. Al-
ternatively, well-defined diblock and triblock copolymers
were obtained using macroinitiators derived from hy-
droxy-capped polymers for the homopolymerization of
MPC. These two approaches are discussed separately
below.
-
1
-7
-3
final concentration of pyrene was fixed at 6.0 × 10 mol dm
.
NaOH solution was added to each copolymer solution to
increase the solution pH; this pH adjustment caused micelles
to be formed in situ. Each copolymer solution was left for 30
min at room temperature to ensure equilibration between the
pyrene in the micelles and that in the aqueous solution. Pyrene
emission spectra were recorded at 25 °C on a Cary Eclipse
fluorescence spectrometer using an excitation wavelength of
3
2
33 nm. The emission and excitation slit widths were set at
.5 and 10 nm, respectively. Spectra were accumulated with
a signal-to-noise ratio of 1000.
Pyrene Partition Experiments. The MPC30-DMA60, MPC30
-
DEA60, and MPC30-DPA60 diblocks were molecularly dissolved
in turn in doubly distilled water at pH 2. Pyrene/copolymer
solutions were prepared by adding acetone solutions of pyrene
into dry 10.0 mL volumetric flasks. After evaporation of the
acetone, diluted copolymer stock solutions were added so as
P r ep a r a tion of MP C-Ba sed Diblock Cop olym er s
Usin g Ma cr oin itia tor s. Three types of macroinitiators
based on poly(ethylene oxide) (PEO), poly(propylene
oxide) (PPO), and poly(dimethylsiloxane) (PDMS) were
employed in this study. Molecular weight and poly-
dispersity data for the near-monodisperse hydroxy-
functional PEO, PPO, and PDMS precursors prior to
esterification using 2-bromoisobutyryl bromide are given
in the Experimental Section and also summarized in
Figure 1.
The evolution of conversion vs time and the corre-
sponding semilogarithmic plot are shown together in
Figure 3 for the homopolymerization of MPC using the
PEO-Br macroinitiator. The linear semilogarithmic plot
indicates that the polymerization is first order with
respect to MPC and implies that the polymer radical
concentration remains constant on the time scale of the
polymerization. The corresponding aqueous GPC curves
for the PEO macroinitiator and the final PEO-MPC
diblock copolymer are shown in Figure 4a. There is a
clear shift to higher molecular weight for the PEO-
MPC diblock compared to the PEO-Br macroinitiator,
-4
to obtain final copolymer concentrations ranging from 1 × 10
-
1
to 5.0 g L ; the final concentration of pyrene was fixed at 6.0
-
7
-3
×
10 mol dm . A drop of 2 M NaOH solution was added to
each copolymer solution to obtain pH 9; this pH adjustment
caused micelles to be formed in situ. Each copolymer solution
was left for 3 days at room temperature to ensure equilibration
between the pyrene in the micelles and that in the aqueous
solution. Pyrene excitation spectra were recorded at 25 °C on
the same fluorescence spectrometer as that described above
using an emission wavelength of 373 nm. The emission and
excitation slit widths were set at 10 and 2.5 nm, respectively.
Spectra were accumulated with a signal-to-noise ratio of 1000.
Deter m in a tion of th e Resid u a l ATRP Ca ta lyst Level.
After silica or alumina treatment to remove the spent ATRP
catalyst, aqueous solutions of selected, purified MPC-based
diblock polymers were analyzed for their Cu contents using a
Perkin-Elmer Plasma 400 inductively coupled plasma atomic
emission spectrometer (ICP-AES) using the following protocol.
Approximately 50 mg of polymer was accurately weighed,
dissolved in distilled water containing 5.0% (v/v) “trace
analysis” grade nitric acid, and made up to 5.00 mL. The
unknown aqueous polymer solution was introduced into the

Macromolecules, Vol. 36, No. 10, 2003
ATRP of MPC in Protic Media 3479
F igu r e 3. Conversion vs time curve and the corresponding
semilogarithmic plot for the homopolymerization of MPC in
methanol using a PEO45-Br macroinitiator at 20 °C. Condi-
-
2
tions: MPC (6.0 g, 2.02 × 10 mol), PEO45-Br (1.01 g, 5.05 ×
-4
1
0
mol), the relative molar ratios of MPC:PEO45-Br:Cu(I)Br:
bpy were 40:1:1:2.
F igu r e 5. Variable temperature 1H NMR studies of the
thermoresponsive MPC50-PPO33 diblock copolymer. Note the
attenuation of the NMR signals assigned to the PPO block.
The semilogarithmic plot of monomer concentration
vs time for the polymerization of MPC (target Dp ) 50)
using the PPO macroinitiator was linear up to 80%
conversion (not shown). Unfortunately, the PPO block
is not sufficiently hydrophilic to allow aqueous GPC
analysis, and the MPC block is insoluble in THF and
other common GPC solvents such as DMF, which
precludes conventional nonaqueous GPC analysis. At-
tempts to use methanol/water mixtures were unsuc-
cessful, since this led to adsorption of the PEO stand-
ards on the column. Thus, GPC analysis of the PPO-
MPC diblock copolymers was not possible. However, the
GPC data obtained for the PEO-MPC diblock copoly-
mer and also our earlier results on related PPO-based
diblock copolymers indicate that these PPO-MPC diblock
1
6c,17-19
copolymers should be near-monodisperse.
Fur-
thermore, attempts to remove residual PPO homopoly-
1
mer by n-hexane extraction were unsuccessful: H NMR
analysis of the MPC-PPO diblock copolymer before and
after n-hexane treatment indicated no significant change
in block composition, which suggested that a high
macroinitiator efficiency had been achieved during MPC
homopolymerization. By varying the MPC/PPO-Br mo-
lar ratio, the target degree of polymerization of the MPC
block could be varied (see Table 1). It is well-known that
the PPO block is thermoresponsive, with an LCST of
1
8,19
around 15-20 °C.
Thus, we studied the dilute
aqueous solution behavior of the PPO33-MPC50 diblock
copolymer by dynamic light scattering at various tem-
peratures. As expected, molecular dissolution was
achieved at 5 °C (i.e., below the cloud point of the PPO
block), but micellization occurred at elevated tempera-
tures. The intensity-average micelle diameter was
around 65 nm at 60 °C. The micelles were somewhat
polydisperse, but the addition of salt reduced the width
of the size distribution considerably. Variable temper-
F igu r e 4. (a) Aqueous GPC data for the PEO45-Br macro-
initiator and the corresponding PEO45-MPC40 diblock copoly-
n w n
mer. (b) Evolution of M and M /M with conversion for the
same homopolymerization. See Figure 3 caption for the reac-
tion conditions.
and there is essentially zero contamination, indicating
very high macroinitiator efficiency. The evolution of Mn
is linear with conversion, and polydispersities remain
low throughout the polymerization (see Figure 4b), as
expected for MPC polymerizations conducted in metha-
nol.2
1
ature H NMR spectra were recorded for this PPO -
33
MPC diblock copolymer (see Figure 5). The signal
5
0
labeled “a” due to the PPO backbone at δ 1.2 becomes
broadened and attenuated relative to the methacrylate
0,21

3
480 Ma et al.
Macromolecules, Vol. 36, No. 10, 2003
Ta ble 1. Su m m a r y of th e Con ver sion a n d Molecu la r Weigh t Da ta for MP C-Ba sed Diblock Cop olym er s P r ep a r ed by
Alcoh olic ATRP a t 20 °C Usin g th e Ma cr oin itia tor Rou tea
target block
copolymer composition
[MPC]
(mol × 10 )
[macroinitiator]
(mol × 10 )
solvent
composition
reaction
time (h)
MPC
conversion (%)
residual Cu by
ICP-AES (ppm)
2
4
b
c
PEO45-MPC40
2.02
1.35
2.0
2.02
2.10
2.10
1.35
2.0
2.02
2.02
1.22
2.70
5.05
13.5
10.0
6.73
4.20
2.10
1.35
4.0
6.73
2.02
2.02
1.35
MeOH
MeOH
MeOH
MeOH
IPA
MeOH
4:1 MeOH:IPA
4:1 MeOH:IPA
4:1 MeOH:IPA
MeOH
24
3
3
100
100
100
100
100
>99
>99
100
100
>99
>99
>99
2.0
1.5
2.5
3.1
2.9
3.2
2.7
1.5
1.8
2.4
1.3
2.4
PEO45-MPC10
PPO33-MPC20
PPO33-MPC30
20
23
24
25
23
23
21
20
18
PPO33-MPC50
PPO33-MPC100
PDMS13-MPC100
PDMS13-MPC50
PDMS13-MPC30
MPC50-PDMS16-MPC50
MPC30-PDMS16-MPC30
MPC10-PDMS16-MPC10
MeOH
MeOH
a
The relative molar ratios of macroinitiator:Cu(I)Br:bpy were 1:1:2, and the total solvent volume was 10 mL for all syntheses.
b
1
c
Conversions determined by H NMR spectroscopy. Cu levels determined by inductively coupled plasma atomic emission spectroscopy.
Ta ble 2. Su m m a r y of th e Con ver sion a n d Molecu la r Weigh t Da ta for MP C-Ba sed Diblock Cop olym er s P r ep a r ed by ATRP
in P r otic Med ia a t 20 °C Usin g th e Sequ en tia l Mon om er Ad d ition Rou tea
time for >95% conversionb
Mn (GPC)c
Mw/Mn c
target
composition
residual Cu by
d
expt no.
solvent
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
H2O
H2O
MeOH
4:1 MeOH:H2O
4:1 MeOH:H2O
homo (h)
diblock (h)
homo
diblock homo diblock ICP-AES/ppm
1
2
3
4
5
6
7
8
9
MPC30-HEMA50
MPC50-HEMA40
MPC50-GMA40
2.5
1.6
1.5
1.5
3.0
3.0
0.5
1.0
2.0
1.0
1.0
24
24
10
24
20
22
21
20
21
48
48
9 000
13 000
12 000
12 000
28 000
28 000
6 000
19 000
21 000
20 000
27 000
1.19
1.16
1.17
1.17
1.15
1.12
1.12
1.21
1.19
1.19
1.18
1.29
1.45
1.21
1.28
2.0
1.9
1.7
1.9
1.6
1.9
2.1
1.8
1.4
2.0
1.9
MPC50-GMA100
MPC100-MMA30
MPC100-CBMA100
MPC20-SBMA20
MPC20-OEGMA20
MPC30-HPMA60
MPC30-MeDMA30
MPC30-BzDMA30
50 000
12 000
16 000
1.20
1.27
1.34
6 000
9 000
1
1
0
1
10 000
11 000
16 000
16 000
1.27
1.32
a
The relative molar ratios of OEG-Br initiator:Cu(I)Br:bpy were 1:1:2. As determined by H NMR spectroscopy. c As determined by
b
1
aqueous GPC using poly(2-vinylpyridine) calibration standards. d As determined by inductively coupled plasma atomic emission spectroscopy
ICP-AES].
[
Ta ble 3. Su m m a r y of th e Con ver sion a n d Molecu la r Weigh t Da ta for MP C-Ter tia r y Am in e Meth a cr yla te Diblock
a
Cop olym er s P r ep a r ed by Meth a n olic ATRP a t 20 °C Usin g th e Sequ en tia l Mon om er Ad d ition Rou te
time for >95% conversionb
Mn (GPC)c
Mw/Mnc
diblock
target
composition
d
exp no.
homo (h)
diblock (h)
homo
diblock
homo
Cu /ppm
1
2
3
4
5
6
7
8
9
MPC30-DMA30
MPC30-DMA60
MPC30-DMA100
MPC30-DEA30
MPC30-DEA60
MPC30-DEA100
MPC30-DPA30
MPC30-DPA60
MPC30-DPA100
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
15
24
48
20
24
48
20
24
40
10 000
10 000
10 000
9 000
10 000
10 000
10 000
10 000
10 000
22 000
34 000
46 000
21 000
31 000
43 000
14 000
21 000
30 000
1.19
1.15
1.17
1.15
1.19
1.17
1.08
1.09
1.12
1.27
1.29
1.32
1.30
1.29
1.28
1.20
1.27
1.27
2.0
1.5
1.7
1.6
1.6
1.9
1.6
2.3
1.9
a
The relative molar ratios of OEG-Br initiator:Cu(I)Br:bpy were 1:1:2. As determined by H NMR spectroscopy. c As determined by
b
1
aqueous GPC using poly(2-vinylpyridine) calibration standards. d As determined by inductively coupled plasma atomic emission spectroscopy
ICP-AES].
[
backbone signal “b” at elevated temperature, indicating
reduced mobility and solvation of the PPO block as it
MPC can be polymerized efficiently using a highly
hydrophobic PDMS-based macroinitiator in protic me-
dia.
1
6c
forms the partially dehydrated micelle cores.
As
expected, this temperature-induced micellization is fully
reversible: on cooling to room temperature the original
NMR spectrum is obtained. Thus, this new PPO-MPC
polymeric surfactant is an interesting biocompatible
analogue of the well-known PPO-PEO “Pluronic” or
P r ep a r a tion of MP C-Ba sed Diblock Cop olym er s
Usin g Sequ en tia l Mon om er Ad d ition . A wide range
of diblock copolymers were prepared by sequential
monomer addition via ATRP at 20 °C using MPC in
combination with various methacrylic comonomers (see
Tables 2 and 3). In all cases the MPC was polymerized
first, followed by the second monomer. In most cases
the solvent used was methanol, and the conditions were
selected to provide optimal living character and hence
high blocking efficiencies. In Table 2, 9 of the 11 entries
are so-called “double-hydrophilic” diblock copolymers.
In contrast, entries 5 and 9 are examples of hydro-
“Synperonic” series.
A series of PDMS-MPC diblocks and MPC-PDMS-
MPC triblocks were also synthesized, and the results
are summarized in Table 1. Unfortunately, the very
different solubility behavior of the MPC and PDMS
blocks again precluded GPC analysis. However, it is
noteworthy that a highly hydrophilic monomer such as

Macromolecules, Vol. 36, No. 10, 2003
ATRP of MPC in Protic Media 3481
Like the MPC-PPO diblocks described above, the
MPC-HEMA diblock copolymers also proved to be
thermoresponsive: as the solution temperature was
increased above the cloud point of the HEMA block,
near-monodisperse micelles of 63 nm were formed at
1
5
0 °C, and H NMR studies (not shown) indicated that
these micelle cores were relatively hydrated.
A summary of the various MPC-based diblock copoly-
mers prepared by sequential monomer addition of the
three tertiary amine methacrylates is provided in Table
3
. The MPC-DMA diblock copolymers exhibited ther-
moresponsive micellization behavior at neutral pH and
elevated temperature. Variable temperature H NMR
1
spectra of a MPC30-DMA60 diblock copolymer recorded
at 20, 40, and 60 °C are shown in Figure 7. All the
signals due to the DMA residues are visible at 20 °C,
with signals “c” and “d” being particularly prominent.
At elevated temperature signal “d” is almost completely
attenuated, and signal “c” is strongly attenuated com-
pared to signals “h” and “i” due to the MPC residues.
This is consistent with the DMA blocks forming dehy-
drated micelle cores, with the hydrophilic MPC blocks
forming the micelle coronas. DLS studies of the same
diblock copolymer at 50 °C indicated an intensity-
average micelle diameter of approximately 26 nm.
The pH-induced micellization behavior of an MPC-
DEA diblock copolymer was briefly reported in our
2
0
earlier communication so in the present study we focus
instead on the behavior of the MPC-DPA diblock
copolymers. These copolymers can be molecularly dis-
solved in dilute acidic solution since the DPA block is
protonated and hence hydrophilic under these condi-
tions. On adjusting the copolymer solution to around
pH 6-7, the DPA becomes deprotonated and hence
F igu r e 6. (a) Aqueous GPC data for the MPC50-GMA100
-
2
diblock copolymer. Conditions: MPC (6.0 g, 2.02 × 10 mol),
-
4
OEGBr (0.20 g, 4.04 × 10 mol), the relative molar ratios of
MPC:OEGBr:Cu(I)Br:bpy were 50:1:1:2, with GMA (6.47 g,
-
2
4
.04 × 10 mol) being added as the second monomer. (b)
Evolution of M and M /M with conversion during the
n
w
n
hydrophobic, leading to the formation of micelles with
synthesis of the MPC-GMA diblock copolymer [same condi-
tions as in (a) above].
dehydrated DPA cores and MPC coronas. 1H NMR
spectroscopy studies of an MPC30-DPA60 diblock co-
polymer (see Figure 8) confirmed that the protonated
DPA residues are fully solvated at pH 2.3 (see signal
philic-hydrophobic diblock copolymers, since the HPMA
and MMA blocks remain permanently water-insoluble
regardless of the solution pH or temperature. Unfortu-
nately, this precluded aqueous GPC analysis of these
MPC-HPMA and MPC-MMA diblock copolymers; only
the extracted MPC homopolymers were suitable for
analysis by aqueous GPC (see Table 2). In contrast,
aqueous GPC could be used to assess the living char-
acter (blocking efficiency) of selected “double-hydro-
philic” diblock copolymers. For example, in Figure 6a
the aqueous GPC curves for the initial MPC homopoly-
mer and the final MPC-GMA diblock copolymer are
shown. There is a shift of the whole distribution curve
to higher molecular weight as expected for the diblock
copolymer, with minimal MPC homopolymer contami-
nation. Similar results were obtained with the MPC-
HEMA, MPC-SBMA, and MPC-CBMA diblock copoly-
mers (not shown). The evolution of Mn and Mw/Mn with
conversion for the MPC-GMA diblock is shown in
Figure 6b. The GMA was added after approximately
“c” ). At pH 7.1 the DPA residues are only partially
protonated and hence signal “c” is attenuated, while at
pH 10.8 the fully deprotonated DPA block is now
hydrophobic and signal “c” disappears completely. For
a fixed Dp of 30 for the MPC block, the intensity-average
diameter of the micelles formed at around pH 7.1-7.3
depends on the DPA block length and increases from
27 nm (Dp ) 30) to 44 nm (Dp ) 60) to 55 nm (Dp )
100). In each case the micelles obtained between pH 7
and pH 8 were near-monodisperse, with polydispersities
ranging from 0.08 to 0.10. Similar results were obtained
for the MPC -DEA diblocks, where n ) 30, 60, or 100.
3
0
n
These observations are in good agreement with several
other reports that the length of the core-forming block
dictates the size and aggregation number of the mi-
2
4
celles.
Pyrene fluorescence is a very sensitive technique for
detecting the formation of block copolymer micelles.
Pyrene is highly hydrophobic and has very low solubility
in water so it migrates preferentially into the hydro-
phobic micelle cores. Thus, a red shift is observed in the
pyrene fluorescence spectra, and there are also changes
in relative peak intensities in the vibrational fine
9
5% conversion of the MPC, and the Mn vs conversion
plots were linear for both stages of the polymerization.
It is noteworthy that the Mw/Mn decreases with conver-
sion for the MPC polymerization but increases gradually
for the GMA polymerization. This suggests that the
GMA polymerization has less living character than the
MPC polymerization, although further work is required
to confirm this hypothesis. The final polydispersity of
the MPC-GMA diblock copolymer was 1.28.
2
5,26
structure.
In the present work the protocol based
on the analysis of pyrene emission spectra reported by
2
4
Gast and co-workers is employed. Figure 9 shows the
variation in the intensity ratio (I1/I3) vs solution pH for
three MPC-based diblock copolymers. As the solution

3
482 Ma et al.
Macromolecules, Vol. 36, No. 10, 2003
1
F igu r e 7. Variable temperature H NMR studies of the thermoresponsive MPC30-DMA60 diblock copolymer. Note the attenuation
of the NMR signals assigned to the DMA residues.
F igu r e 8. 1H NMR study of the pH-induced micellization behavior of the MPC30-DPA60 diblock copolymer. Note the disappearance
of the NMR signals assigned to the DPA residues.
pH is increased, the tertiary amine methacrylate resi-
dues become progressively deprotonated. The critical pH
for micellization was estimated from the reduced I1/I3
ratio, which indicates a more hydrophobic (micellar)
environment for the pyrene probe. (In the case of the
MPC30-DMA60 diblock copolymer, there was very little
reduction in the I1/I3 ratio; hence, either no micelles are
formed under these conditions or the micelles are not

Macromolecules, Vol. 36, No. 10, 2003
ATRP of MPC in Protic Media 3483
condensation. Preliminary control experiments are en-
couraging: MPC homopolymer does not form inter-
polyelectrolyte complexes with synthetic anionic poly-
electrolytes so the MPC component of the diblock
copolymer should not interfere with the electrostatic
complexation required to achieve DNA condensation.
One potentially significant advantage of the MPC block
over the PEO block is that the ATRP chemistry used to
prepare the former should allow the use of functional
ATRP initiators for the polymerization of MPC. This
should allow the preparation of biocompatible micelles
with appropriate functional groups (e.g., primary amines)
decorating the outside of the micelles, which is a
prerequisite for the exploitation of cell-targeting strate-
3
1
gies.
Finally, it is noteworthy that simply treating aqueous
solutions of these MPC-based block copolymers with
silica enables the residual ATRP catalyst to be reduced
to just a few ppm in most cases (see final columns in
Tables 1-3). Although even lower catalyst levels are
probably desirable for biomedical applications, we be-
lieve that these new biocompatible diblock copolymers
show great promise for a range of biomedical applica-
F igu r e 9. Determination of the critical pH for micellization
of the MPC30-DMA60, MPC30-DEA60, and MPC30-DPA60
diblock copolymers at 25 °C from pyrene emission spectra
(
excitation wavelength λex ) 333 nm; the diblock copolymer
-1
concentration was 0.50 g L in each case).
hydrophobic enough to ensure efficient pyrene uptake.)
The critical micellization pH values estimated for the
MPC30-DPA60 and MPC30-DEA60 diblock copolymers
using this method are pH 5.6 and pH 6.9, respectively.
These values correlate quite well with the known pKa’s
of 6.0 and 7.3 for DPA and DEA homopolymer reported
3
2
tions.
Con clu sion s
In summary, we have synthesized a wide range of
novel, well-defined, biocompatible MPC-based diblock
copolymers using ATRP in protic media. Macroinitiators
were employed to prepare PEO-MPC, PPO-MPC, and
PDMS-MPC diblocks, and the sequential monomer
addition route was exploited for the block copolymeri-
zation of MPC with 12 different methacrylic comono-
mers. Generally high conversions were achieved under
mild conditions, and polydispersities were relatively low
2
7
by B u¨ t u¨ n et al. It is also noteworthy that the plateau
value for the I1/I3 ratio observed at high pH is indicative
of the relative hydrophobicity of the micelle cores. The
plateau value of approximately 1.15 obtained for the
MPC30-DPA60 diblock copolymer micelles is comparable
to that observed by Wilhelm et al. for polystyrene-core
micelles and suggests that highly hydrophobic micelle
cores are formed.25 In contrast, the micelles formed by
(Mw/Mn ) 1.1-1.3). The PPO-MPC diblocks exhibited
the MPC30-DEA60 diblock copolymer clearly have sig-
nificantly less hydrophobic character. This hypothesis
was supported by further fluorescence studies in order
to determine the degree of pyrene partitioning within
thermoresponsive behavior in dilute aqueous solution:
molecular dissolution was achieved at low temperature,
with large, hydrated PPO-core micelles being formed at
elevated temperature. The MPC-DMA diblock copoly-
mer also proved to be thermoresponsive as judged by
the micelles. Here the protocol reported by Eisenberg
and co-workers28 was adopted. Assuming that the DEA
1
and DPA micelle core densities are around 1.0 g cm-
3
H NMR spectroscopy and dynamic light scattering
,
studies. The MPC-DEA and MPC-DPA diblocks were
pH-responsive: molecular dissolution was achieved at
low pH due to protonation of the tertiary amine groups,
and micellization occurred between pH 7 and 9, as the
DEA or DPA blocks became deprotonated and hence
hydrophobic. In the case of the MPC-DPA diblock, the
pyrene partition constant for the DPA-core micelles was
within a factor of 3 of that reported previously for
polystyrene-core micelles. This augurs well for the
efficient loading of MPC-DPA micelles with hydropho-
bic drugs. In all cases micellization of these biocompat-
ible stimuli-responsive diblock copolymers was fully
reversible: on reverting to the original solution tem-
perature or pH the micelles dissociate rapidly to give
individual copolymer chains. Analyses for residual
ATRP catalyst using ICP-AES indicated typical Cu
levels of 1-2 ppm in the final purified diblock copoly-
mers (see Tables 1-3).
the pyrene partition coefficients for the MPC30-DEA60
and MPC30-DPA60 diblock copolymer micelles were
4
5
calculated to be 3.2 × 10 and 1.1 × 10 , respectively.
Thus, the pyrene partition coefficient for the DPA-core
5
micelles is close to the value of (1.9-2.4) × 10 reported
2
8
for highly hydrophobic polystyrene micelle cores. On
the basis of our fluorescence studies we anticipate that,
compared to the MPC-DEA diblock copolymers, the
MPC-DPA diblocks should form more stable micelles
with higher loading capacities for the encapsulation of
various “actives” such as hydrophobic drugs. We are now
2
9
examining this prediction in our laboratories.
In collaboration with Stolnik and co-workers, we have
recently shown that PEO-DMA diblock copolymers are
very effective synthetic vectors for the efficient conden-
sation of calf thymus DNA.30 Here the cationic DMA
residues interact electrostatically with the anionic
phosphate residues on the DNA backbone, leading to
efficient condensation with the PEO chains, providing
an effective steric barrier to further particle aggregation.
The result is colloidally stable DNA-loaded micelles with
dimensions of less than 100 nm. Clearly, it will be of
interest to see whether the MPC-DMA diblock co-
polymers can also act as synthetic vectors for DNA
Selected MPC-based diblock copolymers are now
being evaluated in various biomedical applications in
our laboratory. These include novel synthetic vectors for
gene therapy, stimuli-responsive biocompatible micelles
for hydrophobic drugs, and precursors for the prepara-
tion of new biocompatible shell cross-linked micelles.

3
484 Ma et al.
Macromolecules, Vol. 36, No. 10, 2003
Ack n ow led gm en t. BBSRC is acknowledged for
(14) Haddleton, D. M.; Kukulj, D.; Duncalf, D. J .; Heming, A. M.;
Shooter, A. J . Macromolecules 1998, 31, 5201.
financial support for IYM and J S (85/E11450). Biocom-
patibles are thanked for donation of the MPC monomer
and for permission to publish this work.
(
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