Synthesis and self-assembly of amphiphilic chiral poly(amino acid) star polymers

Article abstract of DOI:10.1021/ma101019g

Reversible addition-fragmentation chain transfer (RAFT) polymerization techniques were utilized to polymerize N-acryloyl-l-phenylalanine methyl ester (l-Phe-OMe) and tetrahydropyranyl acrylate to afford amphiphilic star polymers (with 4 and 6 arms). These

Full text of DOI:10.1021/ma101019g

Macromolecules 2010, 43, 5949–5955 5949
DOI: 10.1021/ma101019g
Synthesis and Self-Assembly of Amphiphilic Chiral Poly(amino acid)
Star Polymers
Jared Skey,† Helen Willcock,† Mieke Lammens,‡ Filip Du Prez,‡ and Rachel K. O’Reilly*,†
†Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, U.K., and
‡Department of Organic Chemistry, Polymer Chemistry Research Group, Ghent University,
Krijgslaan 281 S4-bis, B-9000 Ghent, Belgium
Received May 7, 2010; Revised Manuscript Received June 23, 2010
ABSTRACT: Reversible addition-fragmentation chain transfer (RAFT) polymerization techniques were
utilized to polymerize N-acryloyl-L-phenylalanine methyl ester (L-Phe-OMe) and tetrahydropyranyl acrylate
to afford amphiphilic star polymers (with 4 and 6 arms). These copolymer stars have a chiral hydrophobic
core domain and can undergo further solution self-assembly to form well-defined nanostructures, which have
been characterized using DLS, TEM, and cryo-TEM analysis. The characterization and properties of these
novel chiral assemblies and their potential in racemic resolution are reported.
Introduction
An R-approach CTA also promotes bimolecular terminations
although the products are different than in the Z group approach;
Branched polymers with architectures varying from stars and
dendrimers to gels and hyperbranched systems have generated
considerable interest over recent years. This is because, compared
to linear polymers of the same molecular weight, they have
considerably lower bulk and solution viscosities.1,2 They also pot-
entially show novel behavior in selective solvents leading to com-
plex self-assembled morphologies. About a decade before the
term “star” was used to describe branched polymers, Morton re-
ported the synthesis of branchedpolystyrenes by quenching living
styryllithium with silicon tetrachloride.3 However, due to the avail-
able synthetic techniques there was always a compromise between
control and functionality within the polymers. It was not until the
advent of controlled radical polymerizations (CRP) that func-
tional starlike architectures became readily accessible using con-
siderably more facile and robust chemistries.4-12
Three different pathways are used to synthesize star polymers:
(a) “arm first”, (b) “coupling onto”, and (c) “core first” approach.
Using the “arm first” method, monofunctional living linear macro-
molecules are initially synthesized. Star formation then occurs by
adding a difunctional comonomer, which provides cross-linking
through propagation. In the “coupling onto” method, monofunc-
tional living linear polymer chains are coupled onto a multifunc-
tional coupling agent.13,14 Using the “core first” method, the arms
grow from multifunctional initiatorsorprecursor molecules.15-18
Unique to RAFT polymerization processes is the possibility of
advancing the “core first” method using two different ap-
proaches: the Z-approach19-23 and the R-approach,24-27 where
the chain transfer agent (CTA) is covalently bound via the Z-
group and R-group to the core, respectively. It has to be men-
tioned that the choice of the approach has a significant impact on
the outcome of the polymerization. Using the Z-approach CTA,
if the number of arms attached to the core is high and the mono-
mer is bulky, then bimolecular termination in the form of
arm-arm coupling is often observed.28,29 It is thought that this
happens in most polymerizations using a Z-approach CTA,
although to what extent depends on the number of arms, bulki-
ness of the monomer, monomer concentration, and conversion.
what is seen predominantly are star-star coupled products where
the propagating radicals residing on the end of arms attached to
the core of the star have coupled. There are two advantages to
using the Z-approach: first, the coupling products are not seen as
early on in the polymerization as they are for the R-approach,
and second, if a chain extension is to be undertaken, the linear
arm-arm coupled termination products of the Z-approach are
no longer active RAFT centers (unlike in the R-approach).4
In 1999, Chong and the CSIRO team reported the synthesis of
two multifunctional dithioester initiators and their use as mediat-
ing CTAs in the synthesis of four- and six-armed polystyrene
stars.30 Since then, many reports have been made on the synthesis
of multifunctional initiators and the use of these species for
the polymerization of various monomers. The CAMD team in
Australia has been largely instrumental in defining some of the
fundamental aspects of star polymer synthesis using RAFT.31-33
While studies into the synthesis and application of amphiphilic
stars have been extensively reported by Tenhu and others,34-43
the synthesis of star polymers (regardless of lyophilicity) contain-
ing chiral monomers has not been widely reported to date.44 The
application of CRP techniques for the synthesis of chiral poly-
mers is of great recent interest, and of particular note is the work
of Endo and Sanda in the area of poly(amino acids).45,46 It has
been shown that N-acryloyl amino acid monomers can be readily
polymerized in a controlled manner to afford functional and
responsive copolymers. Of further interest is the ability of amino
acids, in particular phenylalanine microbeads, to facilitate the
separation of racemates.47 Indeed, we have recently demon-
strated that a heterointeraction between the D- and L-isomers of
leucine can occur within a micelle core to stabilize the resultant
structure, thus indicating that chirally pure amino acid nanos-
tructures may allow for selective entrapment of the opposite
isomer.48
Hence, the preparations of chiral polymer stars, which are
capable of undergoing self-assembly to form nanostructures, are
of interest as they may find application in asymmetric organo-
catalysis, either directly or through providing confined chiral
environments that could encapsulate achiral catalysts. They also
*Corresponding author. E-mail: r.k.o-reilly@warwick.ac.uk.
r 2010 American Chemical Society
Published on Web 07/01/2010
pubs.acs.org/Macromolecules

5950 Macromolecules, Vol. 43, No. 14, 2010
Skey et al.
have potential application in separation science as they may
potentially behave in a similar fashion to cross-linked beads that
are used to make stationary phases for column chromatography.
In this work we explore the preparation of two amphiphilic star
copolymers with a chiral core domain, their self-assembly, and
potential in racemic separations.
chromatography on silica eluting with 20% EtOAc in hexane to
yield a viscous yellow oil.
CTA (1): Yield = 62%. C49H52O8S12 required C: 51.01%, H:
4.54%, found C: 51.52%, H: 4.98%. 1H NMR (400 MHz,
CDCl3): δH = 7.24 (5 Â Ar-H, m), 4.51 (CH2-Ph, s), 4.09
(O-CH2, s), 3.53 (S-CH2-CH2, t), 2.70 (S-CH2-CH2, t); δC
(100 MHz, CDCl3): 222.2, 178.3, 136.0, 129.7, 129.4, 128.6, 66.2,
42.4, 34.8, 32.9. IR ν (cm-1): 2968, 2909, 1736, 1496, 1456, 1394,
1351, 1233, 1151, 1059, 803, 698.
Experimental Details
CTA (2): Yield = 72%. C74H78O13S18 required C: 50.71%,
H: 4.49%, found C: 50.83%, H: 4.62%. H NMR (400 MHz,
AIBN was recrystallized twice from methanol and stored in the
dark at 4 °C. L-Phe-OMe, THPA, pentaerythritoltetrakis(3-(S-
benzyltrithiocarbonyl)propionate), CTA (1), and pentaerythrito-
lhexakis(3-(S-benzyltrithiocarbonyl)propionate), CTA (2), were
synthesized using modified literature preparations.19,49-52 All
other reagents were purchased from Sigma-Aldrich and were
used without further purification.
1
CDCl3): δH = 7.24 (5 Â Ar-H, m), 4.18 (CH2-Ph, s), 4.09
(O-CH2, s), 3.53 (S-CH2-CH2, t), 2.70 (S-CH2-CH2, t); δC
(100 MHz, CDCl3): 221.5, 179.8, 135.7.0, 129.9, 128.8, 128.2,
66.9, 42.2, 35.1, 33.2. IR ν (cm-1): 2969, 2909, 1736, 1497, 1455,
1394, 1233, 1151, 1059, 803, 698.
1H and 13C NMR spectra were obtained with a Bruker DPX-
300/DPX-400 spectrometer using CDCl3 or d6-DMSO. Chemical
shifts are reported in ppm (δ) relative to CHCl3 (7.26 ppm for 1H
Polymerization of Tetrahydropyran Acrylate (THPA) Using
CTA 1 and 2. A typical polymerization procedure is described
as follows: CTA (1), (0.0132 g, 0.012 mmol, THPA (1.498 g,
9.57 mmol), AIBN (0.0008 g, 0.0048 mmol), and dioxane (1:1 v/v
with THPA) (2:1 v/v with THPA used with CTA (2)) were
placed in a dry glass ampule equipped with magnetic stirrer bar.
After degassing the solution using three freeze-evacuate-thaw
cycles, the vessel was sealed and held at 60 °C for 2 h. Conver-
sions were calculated using 1H NMR spectroscopy by compar-
ing the vinyl signals of the monomer (δ = 6.45) with the back-
bone signals and heterocycle signals from the monomer and
polymer signals (δ ca. 2.52-1.32). The reaction was quenched
with liquid N2, diluted with THF, and precipitated twice into
cold diethyl ether. Polymer 3 was isolated as a glassy yellow
solid. 1H NMR (400 MHz, CDCl3): δ = 5.92-5.74 (O-CH-O,
br), 3.88-3.43 (O-CH2, br; O-CH3, br), 2.43-2.19 (CH back-
bone, br), 2.13-1.23 (CH2 backbone, br; O-CH2(CH2)3-CH).
IR ν (cm-1): 2927, 2854, 1733, 1445, 1252, 1159, 1115, 1020, 897,
863, 820. MnNMR (CDCl3) = 52.9 kDa, MnGPC (CHCl3) = 37.2
kDa, Mw/Mn = 1.12.
1
and 77.2 ppm for 13C) or d6-DMSO (2.50 ppm for H) as an
internal reference. Gel permeation chromatography (GPC) data
were obtained on a PL GPC 50 integrated GPC system with a PL-
AS RT autosampler and a Varian 390-LC detector suite with
DRI and viscosimetry. Three mixed D, 5 μm columns (one guard
and two main) were used at 50 °C with a dimethylformamide
(DMF) flow rate of 1.0 mL min-1. All molecular weights are
relative to PMMA standards. Specific rotation measurements for
the polymers were recorded in THF or CHCl3 on a Perkin-Elmer
241 polarimeter using a sodium source (λ = 589 nm). Infrared
(IR) spectra were obtained on Perkin-Elmer Spectrum 100 ATR
FT-IR spectrometer. Critical aggregation concentration determi-
nations were performed using fluorescence spectroscopy on a
Cary Eclipse single-beam fluorimeter. Elemental analysis was
performed in duplicate by Warwick Analytical services. Hydro-
dynamic diameters (Dh) and size distributions of the stars in
aqueous solution were determined by dynamic light scattering
(DLS). HPLCs were performed on a Varian 920-LC integrated
liquid chromatography system. The DLS measurements were
taken ona MalvernNanoSZetasizer NanoSeries instrument. All
determinations were made in triplicate (with 12 runs recorded).
Transmission electron microscopy (TEM) samples were prepared
by drop deposition onto copper/carbon grids that had been
treated with oxygen plasma to increase the surface hydrophilicity.
The particles were stained using a dilute 5% solution of uranyl
acetate. Micrographs were collected at magnifications varying
from 12K to25K and calibrated digitally. Histograms ofnumber-
average particle diameters (Dav) and standard deviations were
generated from the analysis of a minimum of 100particles from at
least three different micrographs. Cryo-TEM was performed on a
JEOL 2100 TEM with GATAN 924 cryoholder, and a CP3
cryoplunge from GATAN, with the imaging performed on a
GATAN 4K camera.
NMR
Polymer 4 was isolated as a glassy yellow solid. Mn
(CDCl3) = 79.4 kDa, MnGPC (CHCl3) = 65.6 kDa, Mw/Mn =1.14.
Chain Extension of 3 with L-Phe-OMe To Afford [P(THPA)85-
b-P(L-Phe-OMe)47]4, 5. A typical polymerization procedure is
described as follows: macro-CTA (3) [0.2 g, 3.69 Â 10-3 mmol,
synthesized from THPA using CTA (1), (0.0132 g, 0.012 mmol,
THPA (1.498 g, 9.57 mmol), AIBN (0.0008 g, 0.0048 mmol) and
dioxane (1:1 v/v with THPA)], L-Phe-OMe (0.689 g, 2.96 mmol),
AIBN (2.4 Â 10-4 g, 1.47 Â 10-3 mmol), and dioxane ([M] =
0.82 mol dm-3) were placed in a dry glass ampule equipped with
a magnetic stirrer. The solution was degassed using three freeze-
evacuate-thaw cycles, and the vessel was then sealed and held at
70 °C for 0.5 h. After this time the ampule was cooled in liquid
N2, and the reaction mixture was diluted with THF. The poly-
mer was precipitated into stirred, cold diethyl ether and allowed
to stand, and then the solvent was decanted off. The resulting
polymer, 5, was freeze-dried from dioxane and isolated as a very
pale yellow solid. 1H NMR (400 MHz, CDCl3): δ = 7.91-7.33
(N-H, br), 7.29-6.85 (5 Â Ar-H), 5.92-5.74 (O-CH-O, br),
4.79-4.27 (CH-CO2, br), 3.88-3.43 (O-CH2, br; CO2CH3,
br), 3.2-2.7 (Ar-CH2, br), 2.5-2.2 (CH backbone, br), 2.2-1.3
(CH2 backbone, br). IR νmax (cm-1): 3358, 2952, 2884, 1735,
1678, 1531, 1444, 1208, 1160, 1119, 1022, 898, 870, 747, 702.
MnNMR (CDCl3) = 97.9 kDa, MnGPC (THF) = 72.7 kDa, Mw/
Mn = 1.38.
The homopolymerizations detailed in the Experimental Det-
ails section were explored using both CTA (1) and CTA (2); for
reasons of clarity each polymerization will only be detailed using
CTA (1). It can be assumed that molar equivalents of reagents per
arm on the CTA, reaction conditions, and work-up are the same
for both CTAs. Likewise, only the preparation of CTA (1) is out-
lined; however, an identical procedure was carried out for CTA
(2) with the same equivalents of base, CS2, and benzyl bromide
per thiol functionality in the starting material.
Synthesis of CTA (1) and CTA (2). Pentaerythritoltetrakis-
(3-(S-benzyltrithiocarbonyl)propionate), CTA (1), was prepared by
adding pentaerythritol (3-mercaptopropionate) (1 g, 2.05 mmol) to
a stirred suspension of K3PO4 (2.60 g, 12.27 mmol) in CS2 (1.87 g,
24.60 mmol) and THF (10 mL). Stirring was maintained for
30 min, after which time benzyl bromide (2.10 g, 12.27 mmol)
was added, and stirring continued for a further 24 h. After
filtration to remove the inorganic salts, the solvent was removed
in vacuo. The resulting yellow oil was purified using column
Chain Extension of 4 with L-Phe-OMe To Afford [P(THPA)83-
b-P(L-Phe-OMe)27]6, 6. A typical polymerization procedure is
described as follows: macro-CTA (4) [0.191 g, 2.41 Â 10-3
mmol, synthesized from THPA using CTA (2) (0.0132 g, 0.012
mmol), THPA (1.498 g, 9.57 mmol), AIBN (0.0008 g, 0.0048
mmol), and dioxane (1:1 v/v with THPA)], L-Phe-OMe (0.673 g,
2.89 mmol), AIBN (1.57 Â 10-4 g, 9.62 Â 10-3 mmol), and
dioxane ([M] = 0.82 mol dm-3) were placed in a dry glass
ampule equipped with a magnetic stirrer. The solution was

Article
Macromolecules, Vol. 43, No. 14, 2010 5951
Scheme 1. Structures of 4- and 6-Armed Star CTAs 1 and 2
Scheme 2. Polymerization of THPA Using CTA 1 and 2
degassed using three freeze-evacuate-thaw cycles, and the
vessel was then sealed and held at 70 °C for 0.5 h. After this
time the ampule was cooled in liquid N2, and the reaction
mixture was diluted with THF as necessary. The polymer was
precipitated into stirred, cold diethyl ether and allowed to stand,
and then the solvent was decanted off. The resulting polymer, 6,
was freeze-dried from dioxane and isolated as a very pale yellow
solid. 1H NMR (400 MHz, CDCl3): δ = 7.91-7.33 (N-H, br),
7.29-6.85 (5 Â Ar-H), 5.92-5.74 (O-CH-O, br), 4.79-4.27
(CH-CO2, br), 3.88-3.43 (O-CH2, br; CO2CH3, br), 3.2-2.7
(Ar-CH2, br), 2.5-2.2 (CH backbone, br), 2.2-1.3 (CH2 back-
bone, br). IR νmax (cm-1): 3482 (broad), 2963, 2861, 1731, 1669,
1455, 1255, 1176, 1119, 1082, 1048, 870, 832. MnNMR(CDCl3) =
117.1 kDa, MnGPC (THF) = 66.1 kDa, Mw/Mn = 1.48.
Deprotection of 5 and 6. P(THPA)-b-P(L-Phe-OMe) (5) or (6)
was stirred in dioxane (0.1 g mL-1) for 24 h at 100 °C. The
solution was freeze-dried to remove solvent and dihydropyran
to yield an off-white solid. 1H NMR (400 MHz, d6-DMSO): δ =
12.45-12.19 (COOH, br), 7.99-7.51 (N-H, br), 7.38-7.03 (5 Â
Ar-H), 4.76-4.24 (CH-CO2, br), 3.88-3.43 (CO2CH3, br),
3.22-2.79 (Ar-CH2, br), 2.51-2.00 (CH backbone, br),
2.00-0.76 (CH2 backbone, br). IR ν(cm-1): 3500, 2966, 2861,
Figure 1. Pseudo-first-order rate plot for the solution polymerization
of THPA in 2:1 v/v dioxane (with respect to THPA) at 60 °C using CTA
(2). M:CTA (2):AIBN = 1200:1:0.6. kp = 0.0041 min-1
.
synthesized from modified literature preparations using THF as
solvent and K3PO4 as base.50 At the core of both of these CTAs is
a thiol-functionalized pentaerythritol or bispentaerythritol build-
ing block.19 Both CTAs were bright yellow and very viscous and
were characterized using infrared, elemental analysis, and 13C
and 1H NMR spectroscopy.
1730, 1712, 1668, 1455, 1255, 1118, 1083, 890, 870. Tg = 84 and
NMR
95 °C. (7) [P(AA)85-b-P(L-Phe-OMe)47]4: Mn
(d6-DMSO) =
69.4 kDa. (8) [P(AA)83-b-P(L-Phe-OMe)27]6:MnNMR (d6-DMSO) =
75.3 kDa.
As the end goal was to synthesize chiral amphiphilic star block
copolymers that would undergo self-assembly to form core-
shell nanostructures, the hydrophilic block needs to be furthest
away from the core of the star, which impacts the polymer
synthesis. To synthesize a blocky star polymer that will self-
assemble in aqueous solution from a Z-approach CTA, it is
necessary to form the hydrophilic shell precursor block first; the
opposite is true for an R-approach CTA. We chose tetrahydro-
pyranyl acrylate given its readily deprotectable nature to afford
hydrophilic poly(acrylic acid) (PAA) bymaintaining a solution of
the polymer in dioxane (0.1 g mL-1) at 100 °C for 24 h. This
strategy could only be employed ifthe trithiocarbonate was stable
to the deprotection conditions. Indeed, if it was degraded during
the deprotection, the arms could be cleaved from the central core
of the star. In order to assess this, two solutions, each contain-
ing one of the CTAs, were stirred at 100 °C for 24 h. After
removal of the dioxane, 1H NMR spectroscopic analysis of
Micellization of Star Copolymers 7 and 8. An example is as
follows: A round-bottom flask equipped with a stirrer bar was
charged with polymer (7 or 8) (0.1 g), DMSO (100 mL) was
added, and the solution was allowed to stir at RT for 30 min to
ensure the mixture was homogeneous. Nanopure water (100
mL) was added to the rapidly agitated solution via a peristaltic
pump at the rate of 15 mL h-1. After all of the water had been
added, the bluish/opaque micelle solution was transferred to
dialysis tubing (MWCO ca. 12-14 kDa) and dialyzed exhaus-
tively against nanopure water to remove all of the residual
(DLS)
DMSO. (9): Dh
= 83 nm (PDI = 0.06); Dav(TEM): 50 (
10 nm. (10):Dh(DLS) = 122 nm (PDI = 0.04); Dav(TEM):84(6nm.
Results and Discussion
Two CTAs were used in this study: one with four arms, CTA
(1), the other with six, CTA (2) (Scheme 1). They were both

5952 Macromolecules, Vol. 43, No. 14, 2010
Skey et al.
the resulting material showed that no degradation of the CTA
occurred.
to calculate the molecular weights by comparing the relative inte-
grals of the signals in the aromatic region (Figure 3), correspond-
ing to fiveprotons from the CTA, with the signal (δ = 5.92-5.81)
corresponding to one proton from PTHPA. Inspection of the 1H
NMR spectra as well as IR spectroscopy confirmed the absence
of any deprotected PAA signals.
The polymerization of N-acryloylphenylalanine methyl ester
has been reported previously using RAFT polymerization tech-
niques and has been shown to proceed in a controlled manner
when the polymerizations were run at relatively high dilution and
stopped at relatively low conversion. Using multifunctional CTA
1, the polymerization of N-acryloylphenylalanine methyl ester
was initially explored under a range of conditions (see Table S1
from 60 to 90 °C) and as previously reported for the monofunc-
tional CTAs the monomer concentration played a key role in
ensuringpolymerizationcontrol.53 Atlow concentrations reason-
able polymerization control (Mw/Mn ca. 1.4) could be obtained in
around 3 h at 70 °C.
Hence, the polymerization of THPA using CTA 1 and 2 was
explored using well-established polymerization conditions for the
trithiocarbonate-mediated polymerization of acrylates (Scheme 2).
Under these conditions (60 °C, dioxane, AIBN 0.1 equiv relative
to trithiocarbonate group, THPA 200 equiv relative to trithio-
carbonate group) well-defined polymers (Mw/Mn ca. 1.2) were
obtained in 2 h using both CTAs. The polymerization kinetics of
both CTAs were explored and were found to display a linear
pseudo-first-order rate as expected for a controlled radical poly-
merization (Figure 1 and also see Supporting Information). In
addition, the molecular weight (MnGPC) was observed to increase
linearly with conversion (Figure 2) but was always lower that the
theoretical Mn. This is often observed in the GPC molecular
weight analysis of star polymers given their smaller hydrody-
namic radius compared tothe linear analogue. We also useda UV
detector on the GPC (at 309 nm) to confirm the presence of the
trithiocarbonate functionality in both the 4- and 6-armed star
copolymers (see SI).
Using the macroinitiators 3 and 4, the preparation of chiral
diblock copolymers stars was explored (Scheme 3). During the
star chain extension process some deprotection of the PTHPA
was observed (ca. 5% in the 4 arm and 10% in the 6 arm) as
PTHPA macroinitiators were synthesized using the optimized
polymerization conditions as detailed above for CTA (1) and
CTA (2). These were synthesized to be chain extended withL-Phe-
1
confirmed by the presence of a signal (ca. δ = 12.2) in the H
NMR spectrum relating to the PAA proton. It appears that
that the conditions required for the chain extension lead to some
thermal cleavage of the protecting group; however, the THPA
group of the stars is stable after preparation of the diblock. This
OMe in order to make amphiphilic star block precursors. All
polymerizations proceeded in a controlled fashion and yielded
polymers with good control over the molecular weight and
polydispersity. The characterization data are summarized in
Table 1. Given the difficulty in obtaining accurate molecular
weight data from GPC analysis, 1H NMR spectroscopy was used
Figure 3. 1H NMR spectrum in d6-DMSO of 4-armed THPA macro-
CTA, 3 (MnNMR = 42.3 kDa, Mw/Mn = 1.19). The expanded region of
the spectrum shows the end-group signals used to calculate the DP and
Mn of all the macroinitiators used in this study.
Figure 2. Evolution of molecular weight and polydispersity with con-
version for thepolymerizationofTHPA in 1:1 v/v dioxanewithCTA (2)
at 60 °C. M:CTA (1):I 800:1:0.4 with the trend line Mn,th
.
Table 1. Polymerization Data for THPA Macroinitiators Synthesized from either CTA (1) or (2)
Mn,th (kDa)a
MnNMR (kDa)b
MnGPC (kDa)c
Mw/Mn
c
polymer
CTA
conv (%)
time (h)
(3)
(4)
(1)
(2)
44
45
2.75
3.5
56.0
85.9
52.9
79.4
37.2
65.6
1.12
1.14
a Calculated from 1H NMR spectroscopy. b Calculated from end-group analysis by 1H NMR spectroscopy. c Determined using DMF GPC calibrated
with PMMA.
Scheme 3. Conditions for Chain Extension of 4- and 6-Armed PTHPA Stars (3 and 4) with L-Phe-OMe

Article
Macromolecules, Vol. 43, No. 14, 2010 5953
along with the interaction of the phenylalanine block with the
column material leads to a broadening of the molecular weight
trace by GPC analysis. Hence, given the difficulty in obtaining
accurate molecular weight data by GPC analysis, 1H NMR
analysis was used (see Supporting Information). For both poly-
mers 5 and 6, the relative integral of the characteristic THPA
signal at 5.8 ppm was compared to that for the signal at 3.0 ppm
for the P(L-Phe-OMe). The Mn values obtained by NMR
analysis compare well with those calculated based on the
conversion.
The deprotection of the 4- and 6-armed stars, (5) and (6), was
carried out at 100 °C in dioxane to yield two amphiphilic diblock
star copolymers, (7) and (8). The deprotection was observed to
1
have gone to >95% in both cases and was confirmed by H
NMR spectroscopy in d6-DMSO by the loss of the signal (ca. δ =
5.8) associated withPTHPAandthe appearanceofthe carboxylic
acid signal (ca. δ = 12.2). The deprotection was further confir-
med using IR by the presence of a large broad signal related to the
acid stretch centered around 3416 cm-1. The expected shift of the
carbonyl stretching frequency from 1739 cm-1 (associated with
the THPA ester) to ca. 1720 cm-1 (associated with the free car-
boxylic acid) is difficult to observe due to the presence of the ester
Table 2. Polymerization Data for the Chain Extension of 3 and 4 with
L-Phe-OMe in Dioxane ([M] = 0.82 mol dm-3) for 2 h at 70 °C
(M:macroCTA:I for 3: 800:1:0.4; for 4: 1200:1:0.6)
signal from the L-Phe-OMe; however, a slight broadening of the
signal was observed, and the presence of a small shoulder was
also apparent. The amphiphilic stars 7 and 8 were explored by
DLS analysis at low concentration in DMF and were found to
have hydrodynamic diameters of 4.8 and 5.6 nm, respectively.
This is in the range of sizes reported for related PS-PAA star
copolymers.38
conv (%) Mn,th (kDa)a MnNMR (kDa)b MnGPC (kDa)c Mw/Mn
c
(5)
(6)
28
15
106.3
121.3
97.9
117.1
72.7
66.1
1.38
1.48
a Calculated from 1H NMR spectroscopy. b Calculated from end-
group analysis by 1H NMR spectroscopy. c Determined using DMF
GPC calibrated with PMMA.
The self-assembly of amphiphilic diblock copolymers is widely
studied and is often achieved by the dissolution of the polymer in
a good organic solvent for both blocks, followed by a slow,
controlled solvent swap and dialysis to yield an aqueous solution.
The same methodology can be readily applied to the self-
assembly of the chiral amphiphilic star-shaped block copolymers
7 and 8. Hence, the polymers 7 and 8 were dissolved in DMSO at
a concentration of 1 mg/mL, and an equal volume of nanopure
water was added using a peristaltic pump. After the water
addition, the solutions were transferred to membrane bags
(MWCO = 3 kDa) and dialyzed exhaustively to remove all
small molecule contaminants. The resulting structures 9 and 10
Table 3. TEM and DLS Characterization Data Obtained
after Sample Filtration (0.45 μm Nylon Syringe Filter) for the
Nanostructures 9 and 10
DLS: Dha (nm)
conc (g L-1
)
vol
num
int
PDI
TEM: Davb (nm)
(9)
0.42
0.33
95
135
83
122
105
139
0.06
0.04
50 ( 10
84 ( 6
(10)
a Hydrodynamic diameters in aqueous solution by dynamic light
scattering. b Average diameters were measured by TEM, calculated from
the values for 100 particles.
Figure 4. Representative TEM pictures obtained by drop-depositing solutions onto copper/carbon grids and staining with 5% uranyl acetate solution
along with histograms to show size distribution of particles made from (a) 9 and (b) 10 containing amphiphilic diblock star copolymers. Scale bar = 200 nm.
Inset image in TEM shows a magnified particle.

5954 Macromolecules, Vol. 43, No. 14, 2010
Skey et al.
0.42) were incubated overnight with
D/L-phenylalanine. After this
time the micelle solution was transferred to an ultrafiltration cell
(MWCO 3.5 kDa), and ca. 0.5 mL of solute was collected.
Alongside the experimental for 9 a control experiment was
performed using a PMA micelle of similar size and block ratio
([PAA83-b-PMA20]4, Dh = 93 nm). The resulting solutions were
analyzed by polarimetry, and the control reaction had a specific
rotation that was close to zero, which corresponds to the original
racemic solution. However, the solution isolated from 9 had a
specific rotation [R]D of -12.3, indicating that upon incubation
with the chiral micelle 9 the surrounding solute was depleted in
the
D-isomer. This indicated the selective interaction and seques-
tration potential of the
L
-phenylalanine micelle core with the
Figure 5. Representative cryo-TEM images of nanostructures 9 (LHS)
and 10 (RHS), where the scale bar = 100 nm.
opposite enantiomer, thus allowing for racemic enhancement of
the solution. Further work is currently exploring the extent and
scope of this enhancement.
were characterized using DLS, TEM, and cryo-TEM techniques
(Table 3).
Conclusions
DLS analysis of the assemblies 9 and 10 confirmed that well-
defined nanostructures were obtained with narrow polydisper-
sities and good correlation data. Interestingly, the size of the
6-armed star assembly is larger than the 4-armed despite their
similar block lengths. This is in agreement with reports by
Monteiro from a 4-armed PAA-PS star system that the larger
the hydrophobic domain, the larger the resultant particle.38 It is
expected that the number of arms influences the interaction
between the star polymers through the higher steric repulsion
of the arms, and previous work has indicated that the greater
number of arms lowers the aggregation number.39,54 In these
systems we were unable to obtain convincing static light scatter-
ing data due to changes in the micelle size upon changes in
concentration; hence, the aggregation number of these particles
was not determined. Further attempts to determine the size and
shape of all the samples were made using TEM. The TEM images
with uranyl acetate staining (Figure 4) of the filtered samples 9
and 10 showed them to be spherical, monodisperse, and, as
expected, somewhat smaller than the size of the fully solvated
particles observed in solution by DLS. The frequency distribu-
tions of the particles seen in each sample show the narrow
polydispersity of the samples (Figure 4). From these stained
TEM images the exact morphology of the particles was unclear.
Upon examination of the hydrophobic:hydrophilic block ratio
(by mass) polymer 7 has a ratio of around 1:1.8, whereas the ratio
in 8 is around 1:1.3. This would suggest that spherical micelle
morphologies may be formed (although the weight fraction for
polymer 8, in analogue to work by Bates et al. on poly(ethylene
glycol)-b-polybutadiene, may suggest wormlike structures).55,56
The formation of solely spherical micelles was confirmed upon
further analysis of the particles by cryo-TEM for both 9 and 10
(Figure 5).
To further explore the self-assembly properties of these chiral
star polymers, the critical aggregation concentration (cac) was
determined using well-established pyrene excitation techniques
(see Supporting Information).57,58 Using this technique, the cac
for 9 and 10 was found to be similar to that for the linear pre-
cursor reported previously; 4.8 Â 10-3 and 2.5 Â 10-3 respec-
tively versus 9.1 Â 10-3 M for the linear block.53 The values of cac
for 9 and 10 are similar to the values previously reported for 3-
and 4-armed stars by Kim and also 6-armed stars by Tenhu.35,59
In addition, the chiral properties of the assembled stars were
confirmed by CD analysis (Figure S9).
Having demonstrated that the 4- and 6-armed amphiphilic star
copolymers form well-defined micelle aggregates, we were inter-
ested in exploiting the chiral nature of the core domain. Hence,
using the procedures described by Zhu in 2007, we explored the
chiral recognition of our 4-armed star polymers.60 Experimen-
tally, the nanostructures 9 (2 mL of solution of concentration
Using RAFT polymerization, two well-defined amphiphilic
star block copolymers which have an
L-phenylalanine core
domain have been prepared. The self-assembly of these 4- and
6-armed amphiphilic copolymers has been explored using a range
of characterization techniques. These polymers were found to
form spherical micelle morphologies with relatively narrow dis-
persities. The solution properties of these structures were ex-
plored, and the critical aggregation concentration was deter-
mined. Overall, the micelle aggregates formed in thiswork behave
in a similar manner to those previously reported. However, one
unique feature of these nanostructures is the presence of chirality
within the core domain which has been utilized in this work for
enhancement of a racemic phenylalanine solution. This demon-
strates the potential application of these star materials as two
phase chiral separation media and also as selective nanoreactors.
Current work is exploring further the potential of these materials
and their enhancement ability compared to existing techniques.
Acknowledgment. J.S., H.W., and R.O.R. acknowledge the
Royal Society, ESPRC, The Leverhulme Trust, and the Univer-
sity of Warwick for funding. Some of the equipment used in this
research was obtained through Birmingham Science City with
support from Advantage West Midlands (AWM) and part funded
by the European Regional Development Fund (ERDF). M.L.
thanks the Institute for the Promotion of Innovation through
Science and Technology in Flanders (IWT-Vlaanderen) for a PhD
scholarship. The European Science Foundation (ESF) Scientific
Programme (STIPOMAT) and the Belgian Program on Inter-
university Attraction Poles initiated by the Belgian State, Prime
Minister’s office (Program P6/27), are acknowledged for financial
support. Professor Alison Rodger is thanked for assistance with
circular dichroism experiments. Mr Joseph Patterson is thanked
for DSC analysis. Dr Steve Furzeland from Unilever is thanked
for cryo-TEM analysis.
Supporting Information Available: Further characterization
data for the star polymers along with cac data for the micelles.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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