Quasi-block copolymers: Design, synthesis, and evidence for their formation in solution and in the melt

Article abstract of DOI:10.1021/ma201395h

A family of block copolymers featuring dynamically controlled compositions is presented. These copolymers, termed "quasi-block copolymers" (q-BCP), consist of a supramolecular polymer as one of the blocks. A conventional polymer end-capped with a functionality that is complementary to the supramolecular monomer is used to terminate the supramolecular block, giving rise to a block copolymer architecture. In this work we have utilized N,N′-2,4-bis((2-ethylhexyl)ureido)toluene (EHUT) as the supramolecular monomer and employed two types of modified 2,4-bis(ureido)toluene polystyrenes as the end-functionalized conventional polymer. Solution viscosity measurements with different solvent compositions and DSC analysis of poly(EHUT)/ functionalized-PS blends provide compelling evidence for the formation of self-assembled q-BCP structures both in solution and in the melt. The qualitative role of chain stoppers on the molecular weight distribution is studied by simulations.

Full text of DOI:10.1021/ma201395h

ARTICLE
pubs.acs.org/Macromolecules
Quasi-Block Copolymers: Design, Synthesis, and Evidence for Their
Formation in Solution and in the Melt
Ester Weiss,† Kostas Ch. Daoulas,‡ Marcus M€uller,§ and Roy Shenhar*,†
†Institute of Chemistry and the Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem,
Edmond J. Safra Campus, Givat Ram, Jerusalem 91904, Israel
‡Max-Planck-Institute f€ur Polymerforschung, 55128 Mainz, Germany, and Innovation Lab GMBH, 69115 Heidelberg, Germany
§Institut f€ur Theoretische Physik, Georg-August-Universit€at, 37077 G€ottingen, Germany
S Supporting Information
b
ABSTRACT: A family of block copolymers featuring dynami-
cally controlled compositions is presented. These copolymers,
termed “quasi-block copolymers” (q-BCP), consist of a supra-
molecular polymer as one of the blocks. A conventional polymer
end-capped with a functionality that is complementary to the
supramolecular monomer is used to terminate the supramolecular
block, giving rise to a block copolymer architecture. In this
work we have utilized N,N0-2,4-bis((2-ethylhexyl)ureido)toluene
(EHUT) as the supramolecular monomer and employed two types of modified 2,4-bis(ureido)toluene polystyrenes as the end-
functionalized conventional polymer. Solution viscosity measurements with different solvent compositions and DSC analysis of
poly(EHUT)/functionalized-PS blends provide compelling evidence for the formation of self-assembled q-BCP structures both in
solution and in the melt. The qualitative role of chain stoppers on the molecular weight distribution is studied by simulations.
’ INTRODUCTION
supramolecular polymers highly sensitive to different factors that
affect the bonding such as temperature, type of solvent, and
presence of competitive molecules. Supramolecular polymers
thus give rise to environmentally responsive materials that can be
easily assembled and dismantled under mild conditions, a feature
that strongly affects their material properties.6ꢀ15 The combina-
tion between supramolecular polymers and block copolymers
should thus afford new materials featuring responsiveness to environ-
mental conditions as well as nanoscale, phase-separated structures.
The simplest design of a block copolymer consisting of a
supramolecular block includes two components: a conventional
(i.e., covalently bonded) polymer terminated with a supra-
molecular unit and self-complementary, bifunctional molecules
(“supramolecular monomers”), which form the supramolecular
block and also feature high affinity to the end group capping the
conventional polymer (Figure 1). These copolymers, termed
“quasi-block copolymers” (q-BCPs),16 give rise to diblock and
triblock copolymers depending on the symmetry of the supra-
molecular monomer and coexist in the bulk with homopolymers
of the supramolecular molecules.
Nanostructured materials represent a major research thrust in
materials science today because the nanometer scale is relevant to
various fields, ranging from microelectronics1 to catalysis2 and
medicine.3 Another focus of interest is the so-called “smart”
materials,4 which respond to changes in environmental param-
eters such as pH, temperature, and ionic strength. Responsive-
ness is usually obtained via molecular functionalities that can be
reversibly switched between stable states by the application of
external stimuli or a change in environmental conditions. In
general, polymers can provide solutions to both challenges
because they can be synthesized in different architectures, with
molecular sizes in the range of nanometers, and tailored with a
variety of functionalities that lead to the desired material
properties.
Block copolymers consist of two or more sequences of
different repeat units and give rise to materials, which phase
separate into a variety of useful morphologies on the scale of
10ꢀ100 nm. The self-assembly is controlled by the interplay
between incompatibility and chain stretching.5 Supramolecular
polymers, on the other hand, consist of bifunctional molecules
(“monomers”) that are held together by reversible and highly
directional noncovalent interactions (e.g., multiple hydrogen bonding,
metalꢀligand coordination, aromatic stacking, etc.).6ꢀ15 Similar to
traditional polymers, these materials exhibit strong concentration-
dependent viscosities and obey the predictions of the well-
established theories of polymer physics. The reversibility of
the bonding between the monomeric units, however, makes
The number-average degree of polymerization ÆNæ of supra-
molecular polymers is dictated by the association constant K and
the concentration of the monomer c. For sufficiently high values
of K and c, ÆNæ ≈ 2(Kc)1/2 17
.
While large values of K are a
Received: June 20, 2011
Revised:
November 2, 2011
Published: November 18, 2011
r
2011 American Chemical Society
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short q-BCPs segregate to the phase rich with the conventional
polymer blocks while long q-BCPs localize at the interfaces.23
Recent simulations performed on q-BCP systems on patterned
substrates have shown that the dynamic nature lends great
versatility to q-BCP systems in the context of pattern replication
of nonregular features.24
In this paper we describe an experimental demonstration for
the formation of a model q-BCP system based on capped
polystyrene (PS) as the conventional block and on N,N0-2,4-
bis((2-ethylhexyl)ureido)toluene (EHUT)12 as the supramolec-
ular monomer. In chloroform (CHCl3), EHUT forms thin, one-
dimensional supramolecular filaments with one molecule in the
cross section.12h In toluene, EHUT forms long, tubular self-
assembled structures, giving rise to very high viscosities.12d,f,h In
this tubular structure, the cross section contains three EHUT
molecules,12f where the 2-ethylhexyl groups point away from the
tube, the core aromatic rings are roughly perpendicular to the
tube axis, and the cavity (about 7 Å in diameter) accommodates
solvent molecules.12c The tubular form is stable at high concen-
trations and low temperatures with nonpolar solvents that fit
the dimensions of the cavity and undergoes a rather sharp transi-
tion with a decrease in concentration (within only 20%) or an
increase in temperature (within only 5 ꢀC) to the much shorter,
thin filaments.12h Several effective chain stoppers were developed
for EHUT;12a,b,g in our case, the chain stopper is modified with a
polymer.
Figure 1. Schematic illustration of the formation of a quasi-block
copolymer (q-BCP): bifunctional supramolecular monomers first create
long supramolecular chains; addition of end-functionalized conventional
polymers, acting as chain stoppers, leads to the formation of q-BCPs.
prerequisite for obtaining long polymer chains from supramole-
cular monomers, they also cause the polydispersity index (PDI)
to approach the theoretical value of 2 (in analogy to step
polymerization). Better control over molecular weight and PDI
can be achieved with the addition of complementary monofunc-
tional units, termed “chain stoppers”, which cap the chain ends,
block their growth, and additionally suppress the formation of
supramolecular macrocycles.12g,17,18 When the mole fraction of
chain stoppers, x, and the monomer concentration exceed the
’ RESULTS AND DISCUSSION
Synthesis and Characterization of the End-Functionalized
Block. Polystyrene (PS) was polymerized by atom transfer
radical polymerization (ATRP) from modified initiators21 to
ensure that every chain is capped with the end-functionality.
Figure 2 shows the synthesis of two types of end-functionalized
polystyrenes, containing either two or three butyl substituents on
the urea groups (denoted as PS-DiBUT and PS-TriBUT, re-
spectively), and Table 1 summarizes their data, including that of a
nonfunctionalized PS used for comparison throughout this
study. As these capped polymers serve as chain stoppers for
poly(EHUT), the comparison between these end-functionalities
was meant to provide insights into the mode of interaction and
the effectiveness of chain stopper action on the poly(EHUT)
chains.
Figure 3 shows the GPC traces of PS-DiBUT polymers
obtained after different time intervals. The linear dependence
between the number-average molecular weight (Mn) and the
conversion as well as the relatively narrow molecular weight
distributions attest to the controlled nature of the ATRP
polymerization using the DiBUT-modified initiator. Nonethe-
less, polymers obtained under extended polymerization times
exhibit bimodal molecular weight distributions with an increasing
fraction of a high molecular weight component (from 13% for the
2k polymer to 21% in the 7k polymer), featuring molecular
weights that are approximately double the molecular weight of
the majority fraction. We attribute these higher molecular weight
fractions to undesired coupling reactions, which terminate the
growing chains and double their lengths. It should be noted that
such a termination reaction is not detrimental for the purpose of
preparing a q-BCP, since it gives rise to chains that are capped at
both ends with the supramolecular unit (i.e., of the structure
DiBUT-PS-DiBUT). Such polymers should still act as chain
stoppers for poly(EHUT), giving rise to multiblock q-BCPs with
critical value of (xc1/2
)
= n/(2K1/2), ÆNæ becomes indepen-
crit
dent of K and c and is totally dictated by x: ÆNæ ≈ n/x (where n is
the number of chain stoppers that have to be added to the
solution to produce one new chain).17 Hence, in the context of
q-BCPs, the end-functionalized conventional polymer acts as a
chain stopper to the growing supramolecular chain, which both
regulates its length (to some extent) and adds the ability to create
micro- and macrophase-separated structures both in solution
(e.g., as micelles) and in the melt.
Despite the extensive activity in the field of supramolecular
polymers, both experimentally6ꢀ12 and theoretically,13,14 pub-
lished reports on the formation of nanoscale structures from such
polymers are rare19,20 and usually deal with block copolymers
featuring conventional blocks that are linked by noncovalent
interactions (i.e., a fundamentally different concept).21,22 So far,
only a few papers have reported on systems consisting of supra-
molecular polymer and conventional polymer components.20
These, however, were either structured as a statistical graft
copolymer19 (i.e., the functionalized conventional polymer did
not act as a chain stopper) or involved supramolecular mono-
mers that associate with an exceedingly high association constant,
which reduces their dynamic nature.20
Simulations, which we have performed on the behavior of
q-BCP systems in the bulk, have revealed a variety of morphol-
ogies (bicontinuous networks, cylinders, lamellae, and macro-
phase separation) that can be obtained by varying the association
constant and the level of incompatibility between the conven-
tional and supramolecular blocks.23 The simulations gave additional
insights into the distribution of compositions of the formed chains
(among supramolecular homopolymers and di- and triblock
q-BCPs) as well as their spatial distributions in the micro-
phase-separated morphology. For example, it was found that
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Figure 2. Synthesis scheme for the formation of DiBUT and TriBUT-capped PS.
Table 1. Polymerization Data of the Covalent (PS-Based)
Polymers
polymerization
time (min)
conv
(%)
Mn
(kDa)a
notation
PDI
PS-DiBUT 2k
PS-DiBUT 4k
PS-DiBUT 6k
PS-DiBUT 7k
PS-TriBUT 4k
PS 4k
15
30
45
60
30
3.5
22
2.5
3.9
6.0
7.0
3.9
4.2
1.12
1.14
1.18
1.24
1.22
1.38
48
49
22
a According to SEC with PS standards.
Figure 3. GPC traces of PS-DiBUT polymers obtained after different
polymerization times. Inset shows the dependence between the resulting
Mn and the conversion.
the same compositions as the intended diblock q-BCPs. Never-
theless, to strike a compromise between the desire to avoid this
complication and the need to use as long a polymer as possible
(to reduce the influence of the end-functionality on the general
nature of the polymer),21 we used PS-DiBUT and PS-TriBUT
obtained after 30 min of polymerization, featuring Mn of ca.
4 kDa.
The PS-DiBUT and PS-TriBUT were characterized by infra-
red (IR) and NMR spectroscopy. Figure 4 shows a comparison
between the IR spectra of the PS-DiBUT, the DiBUT-modified
initiator (denoted as DiBUT for brevity), and a mixture of the
latter with non-end-functionalized PS (denoted as PS/DiBUT).
While the NꢀH stretching modes of the pure initiator and the
PS/DiBUT appear to be identical, there is a noticeable shift in the
NH vibration peaks in the case of the end-functionalized polymer
(PS-DiBUT), indicating that the DiBUT functionality is indeed
attached to the PS chain.
Further support for the presence of the DiBUT functionality
terminating the PS chains was obtained from NMR measure-
ments performed on the PS-DiBUT (Figure 5). Evidently, the
methylene proton peaks in the region of 3ꢀ4.5 ppm, which are
indicative of the DiBUT functionality, appear also in the spec-
trum of the PS-DiBUT. A similar spectrum was obtained for the
analogous PS-TriBUT. The line broadening observed for these
peaks, as well as the failure to obtain polymers in polymerization
attempts of styrene performed under the same conditions in the
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Figure 4. NꢀH stretching region in the IR spectra of the DiBUT
initiator, PS/DiBUT mixture, and the end-functionalized polymer PS-
DiBUT 4k. Curves are offset for clarity.
Figure 5. 1H NMR of the DiBUT initiator and PS-DiBUT 4k. Peaks
indicative of the DiBUT functionality are marked with asterisks.
absence of the DiBUT initiator, precludes the option of a mixture
of unreacted initiator and self-polymerized PS.
Formation of the q-BCP in Solution. Compelling evidence
for the formation of q-BCPs in solution was obtained from
viscosity measurements performed on solutions containing
varying amounts of EHUT and employing different solvent
compositions. In these experiments, the measured viscosities of
solutions containing EHUT with either PS-DiBUT or PS-
TriBUT (denoted as PS-DiBUT/EHUT and PS-TriBUT/EHUT,
respectively) were compared to solutions containing EHUT and
nonfunctionalized PS (denoted as PS/EHUT) and were normalized
to the solvent viscosity. This comparison enabled us to study the
functioning and efficiency of the supramolecular end-functionality
as a chain stopper for poly(EHUT).
Figure 6a shows the relative viscosities measured for the three
types of systems as a function of EHUT concentration in 3:7
toluene:chloroform solutions at a constant concentration of the
PS-derivative (i.e., the PS, PS-TriBUT, or PS-DiBUT in the
respective system). All three systems exhibit rather low and con-
stant relative viscosities up to EHUT concentration of 4.0 mg/mL.
This suggests that in this solvent composition at low EHUT
concentrations the poly(EHUT) chains formed are rather short
and hence do not contribute to a considerable increase in viscosity
above that of the solvent. This situation remains the same for
the solutions containing the end-functionalized PS-DiBUT and
PS-TriBUT polymers also at higher EHUT concentrations. In
contrast, the viscosities of the PS/EHUT system with EHUT
Figure 6. Relative viscosities measured for the PS/EHUT, PS-DiBUT/
EHUT, and PS-TriBUT/EHUT systems at constant PS-derivative
concentration (7.4 mg/mL) and different EHUT concentrations dis-
solved in different solvent compositions: (a) 7:3 CHCl3:toluene, (b) 5:5
CHCl3:toluene, (c) 3:7 CHCl3:toluene. The dashed lines represent
reaching the maximum detection limit in the viscosity measurements of
the next concentration.
concentration exceeding 8 mg/mL become remarkably high
(beyond the measurement range of the viscometer), possibly
indicating a transition from the short filament structure to the
long tubular structure.12d,f,h Since nonfunctionalized PS cannot
serve as a chain stopper for poly(EHUT), this indicates that at
EHUT concentrations exceeding 8 mg/mL the unperturbed
poly(EHUT) chains grow to substantial lengths. The sharp
contrast between the behavior of the systems containing PS-
DiBUT and PS-TriBUT and the behavior of the control system
employing nonfunctionalized PS indicates an effective chain
stopper effect displayed by the PS-DiBUT and PS-TriBUT,
which result in shorter poly(EHUT) chains (which possibly
results also a larger fraction of the filament form). The inevitable
consequence of this effective chain stopper functioning of PS-
DiBUT and PS-TriBUT is the formation of q-BCPs architectures.
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Figure 7. Ratios of IR absorbances at 3344 and 3300 cmꢀ1 as a function
of EHUT:PS or EHUT:PS-DiBUT ratio for the 3:7 CHCl3:toluene
solutions corresponding to Figure 6c. The ratios for solutions containing
only EHUT at the same concentrations as the q-BCP mixtures are given
as reference.
Figure 8. DSC curves of blends of (a) PS and EHUT at different
EHUT:PS ratios (x) and (b) PS-DiBUT 4k and EHUT at different
EHUT:PS-DiBUT ratios. For comparison, the curves of the neat PS and
PS-DiBUT 4k are provided on top (green), and the neat EHUT curve is
provided on the bottom (blue). Curves are offset for clarity.
Increasing the toluene fraction in the solvent composition was
expected to result in longer poly(EHUT) chains (with an
additional change in morphology from short filaments to long
tubular structures).12d,f,h Figure 6b,c indeed shows this effect by
the general trend of increasing relative viscosities already at lower
EHUT concentrations for all systems. Remarkably, under these
conditions we observe a difference between the DiBUT and the
TriBUT end-functionalities, where it becomes obvious that the
DiBUT functionality serves as a much better chain stopper for
poly(EHUT) than the TriBUT functionality. Considering the
chemical differences between the DiBUT and TriBUT function-
alities, this provides evidence that with these macromolecular
chain stoppers the chain stopping effect is achieved mostly
through the NH groups, acting as hydrogen bond donors, rather
than through the carbonyl groups. This finding seemingly stands
in contrast to recent findings on molecular chain stoppers for
EHUT, where it was shown that the carbonyl groups are
responsible to the chain stopping effect and the involvement of
existing NH groups may only reduce this effect by making the
chain stopper act as a supramolecular comonomer.12a,g More
information is required to elucidate the reason for this behavior;
we hypothesize that the PS chain tethered to the chain stopper
functionality contributes to its ability to serve as a chain stopper,
especially in the case of the tubular form of poly(EHUT), where
it may disrupt the supramolecular assembly.
CHCl3:toluene solutions (see also Supporting Information).25
The high A3344/A3300 ratios (in the range of 1.26ꢀ1.29)
measured for the PS/EHUT solutions, which are close to the
values measured for pure EHUT at the same concentrations
(1.28ꢀ1.29), indicate a tubular morphology of the poly(EHUT)
in this system, which is only slightly affected by the presence of
the PS chains at low EHUT concentrations. This result is in
accord with the high viscosities measured for these solutions
(Figure 6c) and corroborates the lack of chain stopping effect in
the PS/EHUT system. In contrast, the PS-DiBUT/EHUT
solutions display much lower ratios (in the range of 1.17ꢀ1.22),
which align with the much lower viscosities measured for these
solutions (Figure 6c) and agree with a filamentous morphology of
the supramolecular block within the q-BCP architecture. This
finding means that supramolecular chain shortening caused by the
DiBUT functionality is also accompanied by a transition from a
tubular morphology with three EHUT molecules in the cross
section into a filamentous morphology containing only one
EHUT molecule in the cross section. A possible explanation for
this behavior isthe steric hindrance for the formation ofthetubular
form by the polymeric chain stopper. It should be noted, however,
that the A3344/A3300 ratios increase with increasing EHUT content
for the q-BCP structure, indicating the existence of an increasing
fraction of tubular poly(EHUT) assemblies. These assemblies
could be either tubular homopolymers coexisting with q-BCPs in
solution23 or tubular portions of the blocks in the q-BCP. At this
point we cannot differentiate between these two options, but
the strong cooperativity effect observed for the filmant-tube
transition12d,h tends to favor the first.
Quantitative analysis of the IR spectra at the NꢀH stretching
modes provides insights into the molecular structure of the
supramolecular blocks in the q-BCP architecture. Bouteiller
and co-workers have identified that the transition from filamen-
tous to tubular morphology of the supramolecular polymer,
noted as a change in the linear density calculated from fitting
small-angle neutron scattering (SANS) curves, is accompanied
by a similar change in the absorption pattern of the hydrogen-
Formation of the q-BCP in the Melt. Differential scanning
calorimetry (DSC) measurements performed on the control
system, PS/EHUT, reveals no significant change in the Tg of
the PS caused by the increasing amounts of EHUT added to
the blend (Figure 8a). In addition, two melting transitions
attributed to EHUT appear already from EHUT:PS mole
ratio of 2. The low-temperature melting transition occurs at
nearly the same temperature as the corresponding transition in
pure EHUT regardless of the EHUT concentration in the blend.
bonded NꢀH vibration band at 3344 cm
.
ꢀ1 12h Hence, when the
intensity of this absorbance is normalized to the intensity of the
absorbance at 3300 cmꢀ1, ratios between 1.12 and 1.19 are
attributed to the filament form, and ratios between 1.30 and
1.36 are exhibited by the tubular form. Figure 7 shows a marked
difference in the measured ratios between analogous PS/EHUT
and PS-DiBUT/EHUT systems for high EHUT contents in 3:7
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the PS-DiBUT in the poly(EHUT) assemblies and thus an
enhanced formation of the q-BCP structures in the melt.
Effect of Functionalized PS Chain Stoppers on the Molec-
ular Weight of Poly(EHUT): Coarse-Grained Simulations. To
illustrate qualitatively the effect of the functionalized PS (i.e., the
PS-DiBUT or the PS-TriBUT) as chain stoppers on the molec-
ular weight distribution, we considered a simple model system
with similar properties as the experimental system using hybrid
density functional theory (DFT)/particle-based simulations.
This mesoscopic modeling strategy combines concepts from
classical DFT of liquids with standard particle-based simulation
techniques (e.g., Monte Carlo simulations). It has been exten-
sively discussed in earlier publications28,29 and has been applied
to supramolecular polymers.23,24 Here we consider nPS PS-
homopolymer chains dissolved in nEHUT EHUT molecules so
that nEHUT/nPS = 19, which corresponds to the limit of high
EHUT concentrations in this study. Each PS homopolymer is
described through a beadꢀspring model with NPS = 13 coarse-
grained segments, while each EHUT molecule is represented by
a single coarse-grained bead. In the case of nonfunctionalized PS,
the EHUT monomers can reversibly bind only between them-
selves, forming a polydisperse melt of linear poly(EHUT)
homopolymers (h-EHUT) mixed with PS homopolymers. When
modeling functionalized PS, we allow the EHUT monomers to
associate with one of the two termini of the PS-chains as well.
Thus, in addition to the h-EHUT polymers, the system contains
PS-EHUT diblock and PS-EHUT-PS triblock copolymers.
The energy due to the connectivity of the chains is given by
Figure 9. Ratios of IR absorbances at 3344 and 3300 cmꢀ1 as a function
of EHUT:PS, EHUT:PS-TriBUT, and EHUT:PS-DiBUT ratios mea-
sured for films cast from CHCl3 solutions corresponding to the same
compositions as in Figures 6 and 8. The ratio measured for a pure EHUT
film cast from CHCl3 solution is given as reference.
The high-temperature melting transition in the blends occurs at
lower temperatures than the corresponding transition in pure
EHUT, but their temperatures increase steadily with the EHUT:
PS ratio in the direction of the Tm of the corresponding transition
in pure EHUT. These observations indicate a small influence of
the EHUT phase on the PS phase and vice versa, leading to the
conclusion that these control blends are phase separated on a
macroscopic scale, where the shifts in the high Tm transitions in
these blends is attributed to the increasing size of pure EHUT
domains with EHUT:PS ratios. In sharp contrast to these
findings, PS-DiBUT/EHUT blends exhibit a strong decrease of
the Tg of the PS-DiBUT with increasing EHUT:PS-DiBUT
ratios (Figure 8b). Additionally, only the low Tm transition of
EHUT appears and only at high EHUT:PS-DiBUT mole ratios.
These observations demonstrate strong mutual influence be-
tween PS-DiBUT and EHUT, resulting from an interaction
between EHUT and the DiBUT end group and, consequently,
the formation of a q-BCP. The reduction in Tg of the PS
domains26 and the absence of the high temperature melting
transition suggest the formation of rather small PS and poly-
(EHUT) domains, respectively, which are consistent with a
microphase-separated structure typical of a block copolymer.
In analogy to the solution phase study, insights into the
molecular structure of poly(EHUT) in the q-BCP structure in
the melt were obtained by measuring the ratios of A3344/A3300 of
the IR intensities (Figure 9; see also Supporting Information). Here
again, the PS/EHUT exhibited ratios exceeding 1.30, only slightly
lower than of pure EHUT, indicating the formation of tubular
poly(EHUT) assemblies also in the melt.27 The PS-TriBUT/
EHUT and PS-DiBUT/EHUT systems demonstrated much
lower ratios, which increased with increasing EHUT content in
the film. Similar to the situation in solution, the stronger chain
stopping effect of the DiBUT functionality compared to the
TriBUT functionality is manifested by lower ratios measured for
the former, indicating the existence of a smaller fraction of
tubular poly(EHUT) assemblies. If the assumption made above
regarding the identity of tubular poly(EHUT) assemblies as
homopolymers coexisting with q-BCP structures is correct,23
then the lower fraction of such homopolymers in the PS-
DiBUT/EHUT melts supports a pronounced intervention of
"
#
2
3kBTjΔrj
2b2
Hb ¼
Eb þ
ð1Þ
∑
bonds
where the summation is performed over all the bonds in the system
(covalent and supramolecular). Δr denotes the distance of the
neighbors forming the considered bond, b sets the characteristic
length scale of the segments (PS and EHUT), and Eb represents the
bonding strength of the supramolecular association. The systematic
mapping of the strength of the EHUT/EHUT, EHUT/DiBUT,
and the EHUT/TriBUT bonding on the phenomenological Eb
parameter is out of the scope of the simulations performed within
the current work. Here, we used Eb = ꢀ6kBT, which corresponds to
strong supramolecular association23 and is representative of the
situation encountered in the experimental systems. The nonbonded
interactions are described through the functional
Z
Hnb
k0
2
2
¼
dr ½j^ ðrÞ þ j^EHUTðrÞ ꢀ 1ꢁ
ð2Þ
PS
F0kBT
where j^PS(r) and j^EHUT(r) are the local volume fractions of the
PS and the EHUT monomers calculated directly from the coordi-
nates of the corresponding monomers with the help of a grid.28 The
quantity F0 = (NPSnPS + nEHUT)/V ≈ 23.73bꢀ3 stands for the
average number density of the monomers while k0 controls the
compressibility of the liquid. Here, we employ k0 ≈ 1.56, which is
sufficiently large to suppress fluctuations of the total density on the
mesoscale.28 A cubic system of size of 27.83b with periodic
boundary conditions is considered in the following simulations.
The polymer conformations in the supramolecular system are
sampled using “smart Monte Carlo” monomer displacements,30
while the chain connectivity is altered with the help of
supramolecular Monte Carlo moves.14 Figure 10a shows a semi-
logarithmic plot of the molecular weight distribution (MWD)
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of the supramolecular poly(EHUT)-containing chains in systems
involving either nonfunctionalized (solid symbols) or function-
alized (open symbols) PS. In the “no-stoppers” case involving
only nonfunctionalized PS chains, the supramolecular assemblies
include only h-EHUT macromolecules, which are markedly
longer than the h-EHUT homopolymers, PS-EHUT diblocks,
and PS-EHUT-PS triblocks observed in the presence of func-
tionalized-PS. This is manifested by the significant difference in
the slope of the decay of the MWD in the two cases. The solid red
line corresponds to a simple theoretical prediction for the
MWD in the limit where Eb is sufficiently high so that each
supramolecular chain has two stoppers (i.e., when only PS-
EHUT-PS triblocks are formed). In this case one can assume an
exponential MWD of the supramolecular EHUT part:17 P(N) =
ÆNæꢀ1 ꢀN/ÆNæ
. Taking into account that in these simulations we
e
have an average of 19 EHUT molecules per functionalized PS
chain stopper (Nav), we obtain ÆNæ = 2Nav = 38. This simple
analytical argument follows the simulation results closely; the
minor discrepancy stems from the fact that for Eb = ꢀ6kBT not all
supramolecular chains are terminated by two stoppers; i.e.,
h-EHUT chains and PS-EHUT diblocks are still present in the
system. Our result nicely agrees with a similar argument de-
scribed in ref 17. To highlight the influence of the strength of the
supramolecular association on the effect of chain stoppers on the
MWD, Figure 10b illustrates the MWD for an analogous system
with Eb = ꢀ2kBT. In this case the effect of the functionalized PS
on the stoichiometry of the supramolecular system is much less
pronounced. The system contains more PS-EHUT diblocks than
PS-EHUT-PS triblocks (manifested by the relative position of
the plots of the corresponding MWD), emphasizing the fact that
there are many supramolecular chains having only one stopper
and one “dangling” active end. The discrepancy between the
analytical prediction and the simulation results shows that the
simple argument17 used to derive the MWD in the case of strong
association (i.e., assuming that both ends of a poly(EHUT) chain
are capped by functionalized PS chain stoppers) is no longer valid
in the weak association case.
Figure 10. Semilogarithmic plots depicting the molecular weight dis-
tributions of the supramolecular EHUT-containing chains as obtained
by hybrid DFT/particle-based simulations. Systems contain either end-
functionalized PS (open symbols) or nonfunctionalized PS (solid
symbols). In the former case, the system is comprised of h-EHUT
homopolymers, PS-EHUT diblocks, and PS-EHUT-PS triblocks, while
in the latter only h-EHUT homopolymers can be created. The red solid
line marks a simple theoretical prediction for the molecular weight
distribution in the case of end-functionalized PS, derived under the
assumption that only PS-EHUT-PS triblocks are formed (see main
text for details). The energy of bond formation is (a) Eb = ꢀ6kBT and
(b) Eb = ꢀ2kBT.
’ CONCLUSIONS
The design and synthesis of a model q-BCP based on a
hydrogen-bonded supramolecular monomer and the corre-
sponding end-functionalized conventional polymer were
described. Evidence for the formation of the q-BCP, both in
solution and in the melt, was obtained by viscosity measure-
ments and DSC analyses, respectively. The crucial effect of
the end-functionalized polymer on the average length and
molecular weight distribution of the EHUT chains was
described by simulations. We anticipate that the formation
of these dynamic block copolymer architectures may lead to
new opportunities in the field of environmentally responsive
materials, with possible applications ranging from tunable
micelles to “smart” coatings and self-assembled, nanostruc-
tured resists.24
the case of a solution cell). Solution and film measurements were taken
using either 2 or 4 cmꢀ1 resolution and averaging 64 scans. Gel
permeation chromatography (GPC) was performed using a Thermo
SpectraSYSTEM HPLC equipped with a P200 pump, SDV linear M
column (5 μm particle size, 8 ꢂ 300 mm; Polymer Standards Service),
and a SpectraSYSTEM RI-150 detector. The column was calibrated
against PS standards (PSS ReadyCal kit), and samples were analyzed at a
flow rate of 1 mL/min using 5% triethylamine in THF as the mobile
phase. Glass transition temperatures were determined using differential
scanning calorimetry (DSC), performed on 5ꢀ12 mg samples under
continuous nitrogen purge (50 mL/min) on a Mettler Toledo DSC822
(Greifensee, Switzerland). Data were collected in the range of ꢀ50 to
200 ꢀC at a heating scan rate of 10 ꢀC/min and represent the second
’ EXPERIMENTAL SECTION
Instrumentation. NMR measurements were performed on a 400
MHz Bruker spectrometer. Infrared (IR) measurements were performed at
room temperature on Nicolet FT-IR spectrometers (models 380 or
6700, Thermo Scientific, Boston, MA) using either KBr disks (1 mm
thickess) or CaF2 disks (2 mm diameter; including a 100 μm spacer in
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ARTICLE
heating cycle. Viscosity measurements were performed at room tem-
perature on a DV-II+ viscometer (Brookfield, Middleboro, MA) at a
3.33 (t, J = 7.8 Hz, 2 H, NꢀCH2), 3.23 (t, J = 7.6 Hz, 4 H, NꢀCH2), 2.20
(s, 3H, PhꢀCH3), 1.82 (d, J = 6.9 Hz, 3 H, CH3CHꢀBr), 1.63 (m, 6 H,
NꢀCH2CH2), 1.37 (m, 6 H, CH2CH3), 0.96 (m, 9 H, CH2CH3).
Polymerization. CuBr (0.048 mmol), PMDETA (0.096), styrene
(19.14 mmol), and DiBUT or TriBUT (0.048 mmol) dissolved in
0.5 mL of DMF were inserted into a pressure tube. The tube was
connected to a high vacuum line, and degassing using three cycles of
freezeꢀpumpꢀthaw was performed. Purified nitrogen was then added
to the frozen mixture, and the tube was closed under slightly less than
1 atm. The tube was inserted into an oil bath preheated to 110 ꢀC for
different time intervals (from 15 to 60 min), after which the resulting
polymer was cooled to ambient temperature, opened to the atmosphere,
diluted with dichloromethane, and precipitated in methanol. The
precipitation procedure was repeated until the polymer obtained
was white.
Sample Preparation for Viscosity, DSC, and IR Measure-
ments. Mother solutions of EHUT and PS, PS-DiBUT, and PS-
TriBUT polymer were prepared in dry CHCl3, stirred for 3 h, and
filtered. Portions of the EHUT solution and the desired PS-derivative
solution were mixed in chloroform to obtain 7 mL solutions of 0.5 wt %
(7.4 mg/mL) concentration of the PS derivative and different mole
ratios between EHUT and the PS derivative (x; 0 < x < 20). The
mixtures were stirred for 16 h. Prior to the viscosity measurements and
solution IR measurements, the chloroform was evaporated and the
mixture was further dried under high vacuum for 2 days, dissolved in
7 mL of CHCl3:toluene at the desired volume ratio (either 3:7, 5:5, or
7:3), and stirred for 2.5 h. Prior to DSC measurements, solution
mixtures were evaporated and dried for 3 days under high vacuum,
weighed into standard aluminum pans, and sealed. Films for IR
measurements were created by drop-casting chloroform solutions onto
CaF2 disks and drying in air.
shear rate of 264 sꢀ1
.
Synthesis. All reagents were purchased from either Acros or Sigma-
Aldrich and were used without further purification, apart from styrene,
which was dried for 2 days on calcium hydride and distilled before use.
1: Dibutylamine (0.957 mmol) was dissolved in 30 mL of dry CHCl3
and slowly added under a nitrogen atmosphere to toluene-2,4-diiso-
cyante (11.48 mmol) dissolved in 40 mL of dry CHCl3. The solution was
let to stir overnight. Column chromatography (SiO2, 4:1 hexane:ethyl
acetate, Rf = 0.48) provided 1.5 g (50%) of the titled compound as a
yellow-white solid; mp 64ꢀ66 ꢀC. The isomer resulting from the
modification of the other isocyanate group was obtained in minor
quantities;31 structural verification of the major isomer as 1 was obtained
from NOE spectroscopy. 1H NMR (CDCl3): δ 7.29 (d, J = 2 Hz, 1 H,
OCNꢀCdCH), 7.05 (d, J = 8.4 Hz, 1 H, MeꢀCdCH), 7.00 (dd, J =
8.2, 2.2 Hz, 1 H, MeꢀCdCHꢀCH), 6.21 (br s, 1 H, NꢀH), 3.27 (t, J =
7.6 Hz, 4 H, NꢀCH2), 2.25 (s, 3 H, PhꢀCH3), 1.60 (m, 4 H,
NꢀCH2CH2), 1.36(m, 4H, CH2CH3), 0.96(t, J= 7.4 Hz, 6 H, CH2CH3).
Compounds 2a and 2b. 2-Aminoethanol or 2-butylaminoethanol
(7.42 mmol) was added to a solution of 1 (4.94 mmol) in 40 mL of dry
CHCl3 to obtain compound 2a or 2b, respectively. The mixture was
stirred for 4 h under nitrogen. After solvent evaporation the product was
dissolved in dichloromethane, washed with water, NaHCO3, and brine,
and dried over sodium sulfate. Evaporation of the solvent provided a
yellow-white solid product of either 2a (1.35 g, 75%, mp 85 ꢀC) or 2b
(1.55 g, 75%, mp 65 ꢀC).
2a: 1H NMR (CDCl3): δ 7.67 (d, J = 2.0 Hz, 1 H, NHꢀCdCHꢀ
CꢀNH), 7.02 (d, J = 8.0 Hz, 1 H, MeꢀCdCH), 6.68 (dd, J = 8.0, 2.2
Hz, 1 H, MeꢀCdCHꢀCH), 6.31 (br s, 1 H, NH), 6.24 (br s, 1 H, NH),
5.93 (br s, 1 H, NHꢀCH2), 4.20 (t, J = 6.3 Hz, 1 H, OH), 3.74 (dd, J =
9.7, 5.9 Hz, 2 H, HOꢀCH2), 3.40 (dd, J = 9.6, 5.5 Hz, 2 H,
HOꢀCH2CH2), 3.28 (t, J = 7.6 Hz, 4 H, NꢀCH2), 2.06 (s, 3 H,
PhꢀCH3), 1.60 (m, 4 H, NꢀCH2CH2), 1.38 (m, 4 H, CH2CH3), 0.97
(t, J = 7.3 Hz, 6 H, CH2CH3).
’ ASSOCIATED CONTENT
S
Supporting Information. NMR spectra of all synthesized
b
2b: 1H NMR (CDCl3): δ 7.49 (s, 1 H, NHꢀCdCHꢀCꢀNH), 7.43
(br s, 1 H, NH), 7.21 (d, J = 8.8 Hz, 1 H, MeꢀCdCHꢀCH), 6.99 (d, J =
8.0 Hz, 1 H, MeꢀCdCH), 6.33 (br s, 1 H, NH), 3.75 (m, 2 H,
HOꢀCH2), 3.43 (m, 2 H, HOꢀCH2CH2), 3.25 (m, 6 H, NꢀCH2),
2.08 (s, 3 H, PhꢀCH3), 1.58 (m, 6 H, NꢀCH2CH2), 1.34 (m, 6 H,
CH2CH3), 0.96 (m, 9 H, CH2CH3).
compounds (Figures S1ꢀS5) and IR spectra of the different
systems in 3:7 CHCl3:toluene solutions and in the melt (Figures
S6 and S7). This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
DiBUT and TriBUT. To a 40 mL THF solution of either 2a or 2b
(3.56 mmol), triethylamine (5.35 mmol) and pyridine (1.16 mmol)
were added under a nitrogen atmosphere to obtain DiBUT or TriBUT,
respectively. After 10 min of stirring the flask was cooled using an ice
bath, and stirring was continued for an additional 10 min. 2-Bromopro-
pionyl bromide (5.35 mmol) was added, and the resulting cloudy
solution was stirred overnight. The crude product was dried, dissolved
in dichloromethane, washed with water, NaHCO3, and brine, dried over
sodium sulfate, and purified by flash chromatography (SiO2, 1:2 hexane:
ethyl acetate, Rf = 0.25) to yield as a white solid (DiBUT: 400 mg, 20%,
mp 135ꢀ139 ꢀC; TriBUT: 450 mg, 25%, mp 66ꢀ69 ꢀC).
Corresponding Author
*E-mail: roys@chem.ch.huji.ac.il.
’ ACKNOWLEDGMENT
We thank Prof. Shlomo Magdassi and Dr. Yelena Vitnizky for
assistance with the viscosity measurements, Dr. Meital Reches
and Inbal Davidi for assistance with the IR measurements, and
Zohar Pode and Ronit Dor for assistance with the synthesis.
Financial support from the Julius Oppenheimer Endowment
Fund and the US-Israel Binational Science Foundation (Grant
2010046) is gratefully acknowledged.
DiBUT: 1H NMR (CDCl3): δ 7.30 (d, J = 2 Hz, 1 H, NHꢀCdCHꢀ
CꢀNH), 6.96 (br s, 1 H, NH), 6.86 (d, J = 8.4 Hz, 1 H, MeꢀCdCH),
6.77 (dd, J = 8.0, 2.0 Hz, 1 H, MeꢀCdCHꢀCH), 6.37 (br s, 1 H, NH),
6.21 (t, J = 5.6 Hz, 1 H, NHꢀCH2), 4.36 (q, J = 6.9 Hz, 1 H, BrꢀCH),
4.19 (t, J = 5.8 Hz, 2 H, OꢀCH2), 3.40 (m, 2 H, OꢀCH2CH2), 3.26
(t, J = 7.6 Hz, 4 H, NꢀCH2) 1.85 (s, 3 H, PhꢀCH3), 1.80 (d, J = 7.2 Hz,
3 H, CH3CHꢀBr), 1.57 (m, 4 H, NꢀCH2CH2), 1.34 (m, 4 H,
CH2CH3), 0.96 (t, J = 7.2 Hz, 6 H, CH2CH3).
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