Organometallic-polypeptide block copolymers: Synthesis and self-assembly of poly(ferrocenyldimethylsilane)-b-poly(ε-benzyloxycarbonyl-L-lysine)

Article abstract of DOI:10.1002/chem.200800762

A new type of metallopolymer-polypeptide block copolymer poly(ferrocenyldimethylsilane)-b-poly (e-benzyloxycarbonyl-L-lysine) was synthesized by ring-opening polymerization of ε-benzyloxycarbonyl-L-lysine N-carboxyanhydride initiated by using a primary am

Full text of DOI:10.1002/chem.200800762

DOI: 10.1002/chem.200800762
Organometallic–Polypeptide Block Copolymers: Synthesis and Self-Assembly
of Poly(ferrocenyldimethylsilane)-b-Poly(e-benzyloxycarbonyl-l-Lysine)
Yishan Wang,[a] Shan Zou,[b] Kyoung Taek Kim,[a] Ian Manners,*[a, c] and
Mitchell A. Winnik*[a]
Abstract: A new type of metallopoly-
mer-polypeptide block copolymer poly-
bulk state and in solution were per-
formed. In the bulk state, a cylindrical-
in-lamellar structure was observed in a
compositionally asymmetric sample.
Rod-like micelles with a polyferroce-
nylsilane core formed in a polypeptide-
selective solvent alone or in the pres-
ence of a common solvent. Significant-
ly, an additional small quantity of the
common solvent assisted the formation
of longer micelles and micelles with
better shape-regularity. This is attribut-
ed to a decrease in the number of nu-
cleation events and PFS core reorgani-
zation effects.
A
(e-benzyloxycarbonyl-l-lysine) was syn-
thesized by ring-opening polymeri-
zation of e-benzyloxycarbonyl-l-lysine
N-carboxyanhydride initiated by using
a primary amino-terminated poly(fer-
rocenyldimethylsilane). Studies on the
self-organization behavior of this poly-
peptide block copolymer in both the
Keywords: block
micelles polyferrocenylsilane
polypeptides · rod-coil
copolymers
·
·
·
Introduction
nanotechnology, drug delivery, tissue engineering, biominer-
alization and bioanalysis.[3] The incorporation of metallopo-
lymer segments into block copolymer structures results in
materials with novel chemical and physical characteristics.[4]
For example, block copolymers with a polyferrocenylsilane
(PFS) block have shown interesting redox, preceramic, etch-
resistant, and catalytic properties.[5]
Research on the bulk state behavior of polypeptide block
polymers has been focused mainly on rod-coil type copoly-
mers with a rod-like rigid polypeptide segment such as
poly(g-benzyl-l-glutamate) (PBLG) or poly(e-benzyloxycar-
bonyl-l-lysine) (PZLys).[6] The self-assembly of coil–coil
block copolymers is controlled primarily by two parameters,
the volume fraction F of each block and the Flory–Huggins
interaction parameter cN.[1b] The behavior of rod-coil block
copolymers is complicated by the liquid crystalline ordering
of the rod blocks and the topological disparity between the
rod and the coil blocks. To describe their influence on self-
assembly, two additional parameters have been introduced,
the Maier–Saupe parameter mN expressing the rod-rod
aligning interactions, and a geometrical parameter n that
characterizes the relative block size.[7]
Self-assembly of block copolymers occurs in the bulk state
or in a selective solvent, driven by the immiscibility of the
constituent segments.[1] This approach is a powerful way to
obtain nanostructured materials with well-defined shapes
and functions.[2] Polypeptide hybrid-block copolymers have
attracted considerable attention as interesting building
blocks for novel supramolecular materials. Molecular design
by combining the structural and functional control of poly-
peptides with the versatility of synthetic polymers has given
access to materials with applications in areas as diverse as
[a] Y. Wang, Dr. K. T. Kim, Prof. I. Manners, Prof. M. A. Winnik
Department of Chemistry, University of Toronto
80 St. George Street, Toronto, Ontario M5S 3H6 (Canada)
Fax : (+1)416-978-0541
E-mail: Ian.Manners@bristol.ac.uk
mwinnik@chem.utoronto.ca
[b] Dr. S. Zou
Steacie Institute for Molecular Sciences
National Research Council Canada,
100 Sussex Drive, Ottawa, Ontario K1A 0R6 (Canada)
[c] Prof. I. Manners
The self-assembly of block copolymers in solution gener-
ally results in three basic types of structures whose shapes
are, in order of decreasing curvature, spherical micelles, cy-
lindrical micelles, and vesicles/lamellar platelets.[8] The size
and shape of the micelles is normally determined by the
School of Chemistry, University of Bristol
Cantockꢀs Close, Bristol, BS8 1TS (UK)
Fax : (+44)117-929-0509
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200800762.
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FULL PAPER
volume fractions of the constituent blocks and environmen-
tal factors such as the solvent and ionic strength.[9] More
complex superstructures can form when specific factors,
such as chirality and secondary structure effects,[10] as well as
the crystallization of the core-forming block,[11,12] are in-
volved. In most of cases, block copolymers with soluble,
corona-forming polypeptide blocks form spherical micelles
or vesicles.[10b] Non-spherical aggregates generated from
polystyrene-b-poly(l-lysine) copolymers were reported by
G
Klok and coworkers based on light scattering and small-
angle neutron scattering data.[10c] Recently, Savin reported
elongated aggregates from polybutadiene-b-poly
block
copolymers.[10d]
A
ACHTREUNG
Scheme 1. Synthesis of PFS-b-PZLys block copolymers.
The micellization of asymmetric block copolymers with a
core-forming PFS block as the minority component have
been studied by our group and also by other researchers.[12]
For example, well-defined rod-like micelles have been ob-
tained with various PFS block copolymers and we have in-
ferred that the crystallization of the core-forming PFS block
can act as the driving force for their formation. We have
also shown that the formation of rod-like micelles behaves
as a living supramolecular polymerization allowed micelle
length control and the formation of block co-micelle archi-
tectures.[12g,h] With this background in mind, we anticipated
that PFS-b-polypeptide copolymers could couple the ability
of PFS block copolymer to form well-defined rod-like mi-
celles and the advantageous features of polypeptides, such
as secondary structure formation, diverse functionality and
biocompatibility. Previously, our group reported the synthe-
sis and self-assembly of a PFS-b-PBLG diblock copoly-
mer.[13] Spherical micelles in water were prepared from the
deprotected polymer, PFS-b-poly(l-glutamic acid). In this
paper we present the synthesis and characterization of PFS-
b-PZLys diblock copolymers, as well as an examination of
the self-assembly of these materials in bulk and in solvents
selective for the PZLys block.
mary amino-terminated PFS homopolymer was successfully
separated from unfunctionalized PFS using flash silica
column chromatography in a 50–60% yield. Aliquots of H-
terminated polymer were analyzed by GPC and yielded
values of Mn =11000, corresponding to 45 repeat units. The
molecular weight distribution of the PFS homopolymer was
very narrow, with a polydispersity index (PDI) of 1.03.
The quenching of a living anionic polymer with an alkyl
halide is often accompanied with a side reaction that in-
volves polymer dimerization, resulting from a Wurtz-type
coupling reaction between two living polymer chains.[15,16]
Formation of dimers in our system was confirmed by GPC
as shown previously by Kim et al,[13] which may account for
the relatively low yield here. The susceptibility of an alkyl
halide toward the Wurtz-type reaction is in the order RI>
RBr>RCl. To avoid Wurtz-type coupling, other researchers
have used 1-(3-chloropropyl)-2,2,5,5-tetramethyl-1-aza-2,5-
disilacyclo-pentane as a quencher for anionic chains to pre-
pare primary amino-terminated polystyrene,[15,17] polyiso-
prene[18] and polybutadiene.[10d,19] Therefore, we also tested
the suitability of this alkyl chloride as a quencher of anionic
PFS. The quenching processes were carried out at T=À78,
0, 25, and 508C, respectively. Unfortunately, the yield of the
amino functionality on the PFS was always lower than 10%.
We assume that this low yield was caused by the relatively
low nucleophilicity of the PFS carbanion. The conditions de-
scribed above using the 3-bromopropyl derivative at À788C
represent our optimum coupling conditions.
In the second synthetic step, the primary amino-terminat-
ed PFS homopolymer was used as macroinitiator to initiate
the ring-opening polymerization (ROP) of e-benzyloxycar-
bonyl-l-lysine N-carboxyanhydride (Z-Lys NCA) in order
to obtain diblock copolymers. This ROPof Z-Lys NCA was
carried out in a THF/DMF mixed solvent, because the PFS
macroinitiators we used here are insoluble in DMF, which is
generally used as the solvent for the ROPof N-carboxyan-
hydrides. The reaction was allowed to proceed over 5 days
under a purified nitrogen atmosphere in a dry box, after
which the resulting viscous amber solution was precipitated
into methanol. After vacuum-drying, the amber block co-
polymers were obtained as glassy solids. All block copoly-
mers were easily soluble in common organic solvents such
Results and Discussion
Synthesis of PFS-b-PZLys block copolymers: The block co-
polymers were synthesized in a two-step reaction as shown
in Scheme 1. The first step involved the synthesis of PFS ter-
minated with a primary amine group by using a method sim-
ilar to that reported by Kim et al.[13] Dimethyl[1]silaferroce-
nophane was polymerized anionically at room temperature
in THF using n-butyllithium as an initiator.[14] An aliquot of
the solution was removed 40 minutes after the initiation and
quenched with degassed methanol to obtain an H-terminat-
ed PFS sample for GPC analysis. Subsequently, the living
anionic PFS chains were quenched with 1-(3-bromopropyl)-
2,2,5,5-tetramethyl-1-aza-2,5-disilacyclopentane at low tem-
perature. The primary amino groups were deprotected by
simply precipitating the reaction solution into methanol.
The tetramethyl-1-aza-2,5-disilacyclopentane protective
group is very labile toward methanol; thus complete depro-
tection was achieved during this precipitation step. The pri-
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I. Manners, M. A. Winnik et al.
Table 1. Molecular characteristics of PFS macroinitiator and PFS-b-
PZLys block copolymers.
as THF, CH2Cl2, and chloroform. The block copolymers
were separated from any macroinitiator residues by repeat-
ed precipitations of CH2Cl2 solutions of block copolymers
into warm cyclohexane until PFS homopolymer was not de-
tectable with a UV-vis detector (l=450 nm) on GPC analy-
sis. Figure 1 shows the GPC profiles of PFS-b-PZLys 1
[c]
polymer
DPn (PFS)[a] DPn (PZLys)[b] Mn
PDI (Mw/Mn)
PFS macroinitiator 45
–
75
180
11000 1.03[d]
PFS-b-PZLys 1
PFS-b-PZLys 2
45
45
30500 1.25[e]
58800 1.16[e]
[a] Degree of polymerization of PFS obtained by gel permeation chroma-
tography (GPC) with a triple detector and THF as the eluant. [b] Degree
of polymerization of PZLys obtained by 1H NMR. [c] Number average
molecular weight obtained by calculation based on the degree of poly-
merization. [d] Polydispersity index (PDI) of the PFS macroinitiator ob-
tained by GPC with a triple detector and THF as eluant. [e] PDI of PFS-
b-PZLys obtained by GPC with a triple detector and THF (0.003m tetra-
butyl ammonium bromide) as the eluant.
removed from the oven and quenched in liquid nitrogen to
suppress PFS crystallization.
The films were microtomed at room temperature to
obtain block copolymer sections with a thickness of ꢀ70 nm
for TEM measurements. Because of the presence of iron
and silicon as heavy atoms in the PFS domains, no staining
was needed to obtain sufficient contrast in the TEM images.
TEM studies of PFS-b-PZLys 1 revealed a lamellar structure
(see Figure S1 in the Supporting Information). Babin et al.
observed that the a-helix to b-sheet transition temperature
for PZLys blocks in polyisoprene-b-PZLys block copolymers
increases with increasing the degree of polymerization (DP)
of PZLys blocks.[18] The transition temperature for a PZLys
block with a DPn of 75 is ꢀ1558C,[18] which is close to the
temperature (1608C) at which we annealed our PFS-b-
PZLys block copolymer samples. Therefore, our study was
focused on block copolymer PFS-b-PZLys 2, in which the
DPn of PZLys block is 180, corresponding to a transition
temperature higher than 2008C.[18]
Figure 1. Gel permeation chromatograms (UV-vis detector, l=450 nm)
of PFS-b-PZLys 1 in THF (0.003m tetrabutylammonium bromide) at
258C (A) before and (B) after purification. The shoulder (arrow) in pro-
file A is due to the PFS macroinitiator residue.
before and after purification. The GPC traces showed
narrow molecular weight distributions for the block poly-
1
mers with PDI values of 1.1 to 1.3. The H NMR spectrum
of PFS-b-PZLys 2 recorded in CD2Cl2 with the peak assign-
ments is shown in Figure 2. The block ratios for the block
copolymers were deduced from the integration of the signals
from PFS and PZLys. The molecular characteristics of the
PFS-b-PZLys samples are summarized in Table 1.
The bright field TEM images of PFS-b-PZLys 2 in
Figure 3 revealed a lamellar structure. The dark domains
correspond to the electron-rich PFS phase, and the brighter
domains represent the PZLys domains. This assignment was
confirmed by EDX, as shown in Figure 3C. The small angle
X-ray scattering (SAXS) pattern in Figure 4A shows peaks
at approximate q ratios of 1:2:3:4, indicating a lamellar
structure. The spacing calculated from the SAXS pattern
A
45 nm). The peaks in the SAXS pattern are quite broad.
The reason for the observation of broad SAXS peaks in po-
lypeptide block copolymers has been discussed by Schlaad
et al.[20] It is caused by fluctuations in the thickness of poly-
peptide layers, as a consequence of the packing of helices
with different lengths. A fluctuation in the thickness of
PZLys layers can be seen in Figure 3B and Figure 3C.
Figure 2. 1H NMR spectrum of PFS-b-PZLys 2 in CD2Cl2 at 258C. (? sol-
vent residue)
Figure 4B shows the wide angle X-ray scattering (WAXS)
pattern of a PFS-b-PZLys 2 film. The pattern displays a set
À1
of three Bragg peaks at q=0.46, 0.79 and 0.92 Š and in a
ratio of 1:31= :2, corresponding to hexagonal packing of
2
Self-assembly in the bulk state: PFS-b-PZLys block copoly-
mer films were cast from THF solution onto glass slides.
The films were annealed in a vacuum oven first at 508C for
one hour, then at 1608C for 3 days, before they were rapidly
PZLys a-helices with a spacing of 13.7 Š. The spacing can
be assigned to the distance between neighboring PZLys
chains.[21] The bulk state morphology of a series of polypep-
tide-b-polyvinyl rod-coil diblock copolymers have been stud-
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Figure 3. Bright field TEM images of PFS-b-PZLys 2 section. A) Under
low magnification; Scale bar: 300 nm. The bright spots correspond to
very thin areas or holes in the section. B) Under high magnification;
Scale bar: 100 nm. C) Under high magnification with EDX analysis pro-
files (top: iron; bottom: silicon); Scale bar: 100 nm.
ied by other researchers.[6] For most of the block copolymer
samples studied, regardless of the volume fraction of the
two components, only a lamellar morphology was observed.
This organization has been termed a hexagonal-in-lamellar
(HL) morphology, referring to the local hexagonal packing
of the a-helical polypeptide chains within phase-segregated
lamellas. The lack of more complex phases for rod-coil
block copolymers originates from the anisotropic liquid crys-
talline behavior of the rod blocks.[7] Therefore, it is not sur-
prising that PFS-b-PZLys 2 presents a lamellar structure, al-
though the volume fraction of PFS is as low as 0.19, calculat-
ed using a PFS density of 1.26 gmLÀ1[22] and a PZLys density
of 1.26 gmLÀ1.[23] A schematic representation of HL mor-
phology for PFS-b-PZLys 2 is given in Figure 4C.
Poly(ferrocenyldimethylsilane) is a semicrystalline poly-
mer with a melting point of ꢀ130–1458C.[24] The WAXS pat-
tern of the as-cast PFS-b-PZLys 2 film indicated the pres-
ence of PFS crystallites. The purpose of thermal annealing
was to eliminate the influence of crystallization of PFS on
the self-assembly of the block copolymer. The necessity for
a thermal annealing step makes it difficult to study this
block copolymer systematically. Future work will involve
more detailed and systematic studies on PFS-b-PZLys block
copolymers, in which the PFS block is amorphous such as
poly(ferrocenylethylmethylsilane) or poly(ferrocenylmethyl-
phenyl-silane).[25]
Figure 4. A) SAXS and B) WAXS Patterns for a PFS-b-PZLys 2 film.
C) A schematic representation of HL morphology for PFS-b-PZLys 2
(not to scale). Helices are presented as rods.
micelles with a PFS core and a PZLys corona was expected.
Micelles were prepared from PFS-b-PZLys 1 in this way.
Two days after the preparation, a micelle sample was col-
lected on a carbon-coated grid for TEM imaging. Figure 5A
and Figure 5B are TEM images showing the presence of
rod-like micelles. Owing to the high electron density of the
PFS core, the PZLys corona is invisible in the images. The
lengths of the micelles ranged from 40 to 300 nm and the
widths of the PFS core varied from 10 to 20 nm. The mi-
celles have somewhat irregular shapes, characterized by the
rough surface profiles of the PFS cores.
By TEM, the morphology of the micelles exhibited no ob-
vious change after a two-week aging period at room temper-
ature in DMF. The micelle solution was then divided into
two portions. To one portion a small amount of THF was
added. THF is a good solvent for both the PFS and the
PZLys blocks. Figure 5C and Figure 5D show the TEM
images of micelles after aging in the DMF/THF mixed sol-
vent for 2 weeks. The shape of micelles became more uni-
form. Most of the micelles have a smooth surface. The mor-
Self-assembly in solution: We attempted to prepare PFS-b-
PZLys block copolymer micelles by two methods. In the
first method, the block copolymer was dissolved directly in
DMF at room temperature with stirring. DMF is a good sol-
vent only for the PZLys block. Therefore, the formation of
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I. Manners, M. A. Winnik et al.
filaments at the ends. After 7 days, longer micelles with one
or two thin filaments at the ends formed. After 9 days, all
the micelles evolved into well-defined rod-like objects with
lengths up to 1 mm. The widths of the micelle cores are
quite uniform and are estimated to be 12–14 nm, significant-
ly smaller than the irregular structures formed initially and
more similar to the values for the filament structures appar-
ent in Figure 6A,B.[26]
Micelles of PFS-b-PZLys 2 were prepared by method 2.
The TEM image in Figure 7A shows the presence of
Figure 5. Bright field TEM images at low (A,C) and higher magnification
(B,D) of the micelles prepared from PFS-b-PZLys 1 by method 1. A) and
B) 2 days after preparation; C) and D) after aging 2 weeks in DMF and
another 2 weeks in DMF/THF (9/1 by volume); Scale bar: 100 nm.
phology of the micelles in the other portion, without the
presence of THF, showed no apparent change.
In the second method, a sample of PFS-b-PZLys block co-
polymer was first dissolved in THF. Then DMF was added
dropwise to reach a 90% volume fraction of DMF. In this
way, longer micelles with a well-defined rod-like shape were
obtained. Figure 6 shows the evolution of micelle morpholo-
gy over time. The micelles were prepared from PFS-b-
PZLys 1. Irregular elongated structures with a core diameter
of 25 to 40 nm were observed 3 days after sample prepara-
tion. A small portion of the micelles appear to possess thin
Figure 7. Morphology of PFS-b-PZLys 2 micelles 2 weeks after prepara-
tion. A) Bright field TEM image; Scale bar: 1000 nm. B) AFM image in
height mode and line profile.
rod-like micelles with lengths up to a few mm. The widths of
the micelle cores are similar to those of PFS-b-PZLys 1 mi-
celles. An AFM image of the micelles in Figure 7B reveals a
uniform height of ꢀ11 nm and an apparent width of
ꢀ60 nm. The latter distance presumably reflects the width
of both the core and corona of the micelle.
Micelle films for WAXS measurements were cast from
PFS-b-PZLys 2 micelle solutions 5 and 14 days after the mi-
celle preparation. The presence of rod-like micelles in the
micelle solution 5 days after the preparation was confirmed
by TEM (see Figure S3). Both WAXS patterns in Figure 8
show a strong peak at 2q=13.78 corresponding to a lattice
spacing of 6.4 Š, which can be assigned to the distance
between adjacent planes containing planar zigzag PFS
chains.[24a,b] The WAXS pattern of the film cast 5 days after
the micelle preparation also shows a set of three Bragg
peaks with q ratio of 1:31= :2, characterizing the hexagonal
2
packing of PZLys a-helices. This may originate from the
presence of free PFS-b-PZLys polymer chains in the micelle
solution. The self-assembly of the free chains upon casting
Figure 6. Bright field TEM images of micelles prepared from PFS-b-
PZLys 1 by method 2. A) 3 days, B) 7 days, C) 9 days after preparation;
Scale bar: 300 nm.
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Organometallic-Polypeptide Block Copolymers
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more soluble, leading to the formation of longer micelles.
Again, the irregular shape of the micelles shown in Fig-
ure 6A may be caused by kinetic factors in the early stage
of micellization. Further growth of the micelles was less ki-
netically controlled. Therefore, the addition of more PFS
chains to the micelles resulted in the thin filaments with uni-
form width as shown in Figure 6B. The growth of the mi-
celles accompanied with the removal of some defects
formed in the early stages eventually led to micelles with
uniform shape as shown in Figure 6C.
Figure 9 summarizes the micellizaton process for PFS-b-
PZLys block copolymer in DMF alone or in the presence of
a small amount of the common solvent THF. A detailed
light scattering study of the kinetic process of micellization
would be useful to probe the exact role of THF in this
Figure 8. WAXS patterns of PFS-b-PZLys 2 micelle films cast A) 5 and
B) 14 days after the preparation of micelle solution.
may then have induced the hexagonal packing of PZLys a-
helices. By comparing the WAXS pattern of the film cast 14
days after the micelle preparation to that of the film cast 5
days after the micelle preparation, a decrease in the intensi-
ty at angles associated with the hexagonal packing of PZLys
a-helices relative to that at 13.78 can be observed. This is
due to the decrease of free polymer in the micelle solution
upon aging.[27] Similar WAXS results were obtained from
PFS-b-PZLys 1. (see Figure S5 in the Supporting Informa-
tion)
The crystallization of PFS appears to be the dominating
factor for the formation of long rod-like micelles from most
asymmetric PFS block copolymers.[12a,b] In this context, we
assume the effect of THF on the shape regularity and length
of the PFS-b-PZLys micelles is related to its ability to in-
crease the block copolymer solubility and to influence the
crystallization of PFS. The crystallization process consists of
two major events: nucleation and crystal growth. In the nu-
cleation stage, some of the PFS chains gather and arrange
themselves to form nuclei. In the crystal growth stage, more
PFS chains add to the nuclei and then to the growing crys-
tal, which results in the formation of larger/longer micelles.
We observed that the common solvent THF can improve
the shape regularity of the micelles when we tried to pre-
pare micelles using method 1. Using this method, short mi-
celles with an irregular shape formed upon dissolving the
PFS-b-PZLys block copolymer directly in DMF. The irregu-
lar shape of the micelles may be caused by kinetic factors
that leave defects in the semi-crystalline PFS core. The PFS
chains at the defect points would be less stable thermody-
namically than in crystalline domains of the PFS core, but
they might be “frozen” because of the low mobility of the
PFS. The addition of THF may plasticize regions of the core
and increase the mobility of PFS chains at the defect point.
The PFS-b-PZLys chains could then reorganize to eliminate
many of the defects, and yield micelles with a more uniform
shape.
Figure 9. A mechanism for the self-assembly of PFS-b-PZLys diblock co-
polymer in a selective solvent DMF alone or in the presence of common
solvent THF. The shape of the micelles is shown as cylindrical for simpli-
fication.
system.
Summary
A new type of metallopolymer-polypeptide block copolymer
poly(ferrocenyldimethylsilane)-b-poly(e-benzyloxycarbonyl-
l-lysine) was synthesized by ring-opening polymerization of
e-benzyloxycarbonyl-l-lysine N-carboxyanhydride initiated
with primary amino-terminated poly(ferrocenyldimethylsi-
lane). Studies on the self-organization behavior of this poly-
peptide block copolymer in both the bulk state and in solu-
tion were carried out. In the bulk state, a cylindrical-in-la-
mellar structure was observed in a compositionally asym-
metric sample. Micelles with a rod-like shape were obtained
in a selective solvent alone or in the presence of a common
solvent. The presence of a small amount of the common sol-
vent resulted in the formation of longer micelles or micelles
with improved structural uniformity. We suggest that this is
a result of a decrease in the number of nucleation events
and the facilitation of PFS core chain reorganization.
If the micelles were prepared by the second method, THF
was introduced into the system before the formation of mi-
celles and longer micelles formed. We believe that fewer
PFS nuclei formed in the mixed solvent, as the PFS block is
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I. Manners, M. A. Winnik et al.
(50 mL) until PFS homopolymer was not detectable by a UV-vis detector
Experimental Section
(450 nm) by GPC. 1H NMR (400 MHz, CD2Cl2, 258C): d=7.90–8.40
A
Materials and instrumentation: All reagents were purchased from Al-
drich. THF was distilled over Na/benzophenone and redistilled over
n-butyllithium under vacuum. DMF was vacuum distilled over CaH2. 1-
3.98 (a-CH), 2.93–3.25 (a-CH-(CH2)3-CH2-), 0.90–2.17 (a-CH-(CH2)3),
0.48 ppm (CH3-Si).
Preparation of PFS-b-PZLys films: The films were cast from THF solu-
tion of diblock copolymer samples (ꢀ20 mg/mL) onto a glass slide. The
films were allowed to dry slowly in a half-sealed 200 mL vial loaded with
100 mL of THF. The films were annealed in a vacuum oven first at 508C
for one hour, then at 1608C for 3 days before they were rapidly removed
from the oven and quenched in liquid nitrogen. The films were scratched
off from the glass slide by a razor blade.
(3-bromopropyl)-2,2,5,5-tetramethyl-1-aza-2,5-disilacyclopentane
was
vacuum distilled over CaH2. Distilled reagents were used immediately.
[1]Dimethylsilaferrocenophane,[14] 1-(3-chloropropyl)-2,2,5,5-tetramethyl-
1-aza-2,5-disilacyclopentane[15] and e-benzyloxycarbonyl-l-lysine N-car-
boxyanhydride (Z-Lys NCA)[28] were synthesized according to the meth-
ods reported in the literature.
All of the polymerizations were carried out in an M-Braun dry box
Preparation of PFS-b-PZLys micelles: Two methods were employed to
prepare PFS-b-PZLys micelles. Method 1: a sample of block copolymer
(1.0 mg) was dissolved directly in dry DMF (1.0 mL) with stirring.
Method 2: a sample of block copolymer (1.0 mg) was dissolved first in
THF (0.1 mL) with stirring, followed by the dropwise addition of dry
DMF (0.9 mL).
1
under a purified N2 atmosphere. H NMR spectra were obtained by using
a Varian 400 spectrometer with CD2Cl2 as solvent. Molecular weights of
the polymers were measured by using a Viscotek GPC max system (VE
2001 GPC solvent/sample module and TriSEC Model 302 triple detector
array) or a Viscotek GPC max liquid system equipped with a UV-Vis de-
tector (model 2501) with THF or THF (0.003m tetrabutylammonium bro-
mide) as the eluant. Small angle X-ray scattering (SAXS) measurement
was performed by using a Nanostar SAXS system (CuKa radiation, l=
1.54 Š) from Bruker AXS GmbH. The sample-to-detector distance was
set to 0.6 m. The SAXS pattern was smoothed in order to obtain a better
resolution of the peaks. Wide angle X-ray scattering (WAXS) measure-
ments were performed in the reflection mode by means of a Bruker AXS
D8 Discovery Diffraction System (CuKa radiation, l=1.54 Š). TEM
measurements were made by using a Hitachi H-600 instrument at an ac-
celeration voltage of 75 kV or a Hitachi S-5200 instrument equipped with
an Oxford Instruments Inca EDX system at an accelerating voltage of
30 kV. EDX measurements were performed in the line-scan mode. Di-
block copolymer thin sections were prepared by microtoming the block
copolymer film by using a Leica UCT ultramicrotome. TEM specimens
were prepared on a 200 mesh carbon-coated copper grid. AFM imaging
was carried out in air using the tapping mode feature of a Nanoscope
IIIa Dimension 5000 microscope (Veeco Digital Instruments). The silicon
probe cantilevers (MikroMasch, resonance frequencies in the range of
135–190 kHz, free amplitude: 20–25 nm) were used with nominal spring
constants of between 3.5 and 12.5 NmÀ1. In the AFM experiments, mi-
celles were deposited onto a freshly cleaved mica surface.
Preparation of PFS-b-PZLys micelle films: Micelle films were prepared
by casting micelle solutions onto glass slides. The films were dried in air
at room temperature for 1 day and then dried under vacuum for 1 day.
Acknowledgements
M.A.W. and I.M. thank the NSERC Canada for funding. I.M. thanks the
European Union for a Marie Curie Chair and the Royal Society for a
Wolfson Research Merit Award. The authors also thank Dr. Srebri
Petrov for his assistance with X-ray scattering measurements.
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Received: April 22, 2008
Published online: July 30, 2008
Chem. Eur. J. 2008, 14, 8624 – 8631
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8631