A modular route for the synthesis of ABC miktoarm star terpolymers via a new alkyne-substituted diphenylethylene derivative

Article abstract of DOI:10.1021/ma3017579

We introduce a modular route for the synthesis of well-defined ABC miktoarm star terpolymers. To this aim, the synthesis of a 1,1-diphenylethylene derivative bearing a protected alkyne function (1-[(4-(tert-butyldimethylsilyl) ethynyl)phenyl]-1-phenylethylene) was developed. This compound was for the first time employed in sequential anionic polymerization to readily prepare alkyne mid-functionalized diblock copolymers with polybutadiene as first and a poly(alkyl methacrylate) (poly(tert-butyl methacrylate), poly(N,N- dimethylaminoethyl methacrylate)) as second block. For the third arm controlled radical polymerization methods (polystyrene, poly(tert-butyl methacrylate), poly(N,N-dimethylaminoethyl methacrylate)) and anionic ring-opening polymerization (poly(ethylene oxide)) were used to separately prepare homopolymers with an azido function. Afterward, azide-alkyne Huisgen cycloaddition was successfully employed to synthesize a library of ABC miktoarm star terpolymers with different molecular weights and chemical compositions via modular combination of the functionalized diblock copolymers and homopolymers. The resulting new ABC miktoarm star terpolymers showed narrow, monomodal molecular weight distributions with dispersities typically below 1.10, as determined by size exclusion chromatography.

Full text of DOI:10.1021/ma3017579

Article
pubs.acs.org/Macromolecules
A Modular Route for the Synthesis of ABC Miktoarm Star
Terpolymers via a New Alkyne-Substituted Diphenylethylene
Derivative
Andreas Hanisch, Holger Schmalz, and Axel H. E. Muller*
̈
Makromolekulare Chemie II, Universitat Bayreuth, 95440 Bayreuth, Germany
̈
S
* Supporting Information
ABSTRACT: We introduce a modular route for the synthesis
of well-defined ABC miktoarm star terpolymers. To this aim,
the synthesis of a 1,1-diphenylethylene derivative bearing a
protected alkyne function (1-[(4-(tert-butyldimethylsilyl)-
ethynyl)phenyl]-1-phenylethylene) was developed. This com-
pound was for the first time employed in sequential anionic
polymerization to readily prepare alkyne mid-functionalized
diblock copolymers with polybutadiene as first and a poly(alkyl
methacrylate) (poly(tert-butyl methacrylate), poly(N,N-dime-
thylaminoethyl methacrylate)) as second block. For the third
arm controlled radical polymerization methods (polystyrene, poly(tert-butyl methacrylate), poly(N,N-dimethylaminoethyl
methacrylate)) and anionic ring-opening polymerization (poly(ethylene oxide)) were used to separately prepare homopolymers
with an azido function. Afterward, azide−alkyne Huisgen cycloaddition was successfully employed to synthesize a library of ABC
miktoarm star terpolymers with different molecular weights and chemical compositions via modular combination of the
functionalized diblock copolymers and homopolymers. The resulting new ABC miktoarm star terpolymers showed narrow,
monomodal molecular weight distributions with dispersities typically below 1.10, as determined by size exclusion
chromatography.
strategy. The main difficulties are (1) the exact functionaliza-
tion of a diblock copolymer with a functional group or polymer
block at the junction of the two blocks and (2) the
stoichiometry of the linking reaction. Four different synthetic
strategies have been reported so far for miktoarm star
terpolymers.
INTRODUCTION
■
Polymer architecture and composition is a decisive factor for
controlling structure formation both in bulk and in solution.
Besides the choice of monomer and the number and sequence
of polymeric building blocks the way of molecular conjunction
of these blocks is of outermost importance. Compared to their
linear analogues, miktoarm star terpolymers, where the polymer
chains are connected at one common junction point,1 present a
much more complex system. A variety of unique bulk
morphologies were found,2−4 which are inaccessible by linear
triblock terpolymers. This is a direct consequence of the
confined geometry of the polymer chains, which forces the
common junction points of the three different blocks to be
aligned in a 1-dimensional fashion. These miktoarm star
terpolymer morphologies were also utilized for generating
cylindrical, compartmentalized particles by selective cross-
linking of one block.5 In addition, theoretical studies suggest
that even more complex morphologies are expected under
cylindrical confinement of ABC miktoarm star terpolymers.6
Depending on the length ratio of the respective arms, Hillmyer,
Lodge, and co-workers7,8 obtained different multicompartment
structures in aqueous solution from ABC miktoarm star
terpolymers with a fluorinated segment. These ranged from
hamburger micelles to segmented wormlike micelles and
nanostructured vesicles.
One approach uses the different reactivity of living anionic
polymer chains toward the chlorine−silicon groups of
trichlorosilanes. Using methyltrichlorosilane as linking agent,
Hadjichristidis and co-workers synthesized a miktoarm star
terpolymer consisting of polyisoprene, polystyrene, and
polybutadiene.9 However, the sequence of introduction of
arms is strongly limited to the reactivity and steric demand of
the polymeric anions, and adaption to other monomers
requires a modification of the method.10,11 The second anionic
approach utilizes macromonomers with nonhomopolymeriz-
able end groups which were directly sequentially copolymerized
with two sorts of monomers.2,12−14 In another approach, first
diblock copolymers were synthesized carrying functional
groups at the junction point of the two blocks. Again,
nonhomopolymerizable compounds or special polymerization
strategies have to be employed to ensure that only one
functional group is located exactly in between the two
Received: August 21, 2012
Revised: September 15, 2012
Published: October 12, 2012
The conjunction of three chains at one common linking
point leads to special requirements concerning the synthetic
© 2012 American Chemical Society
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Scheme 1. Synthetic Route for the Synthesis of ABC Miktoarm Star Terpolymers for the Example of a Star Consisting of Arms
of Polybutadiene, Poly(tert-butyl methacrylate), and Polystyrene
polymeric building blocks. Up to now, mainly hydroxyl
functions were used to conjugate the third block, which were
introduced via a 1,1-diphenylethylene (DPE) derivative15,16 or
2-methoxymethyloxymethyloxirane17−19 as end-capping mole-
cule. This hydroxyl function serves as anchoring point for the
third block, which can be “grafted from” by anionic ring-
opening polymerization (AROP)15−17 or controlled radical
polymerization (CRP) after appropriate modification19 or
“grafted to” via direct esterification.18 Hirao and co-workers
recently used in-chain-benzyl bromide-functionalized diblock
copolymers for the miktoarm star terpolymer synthesis
involving specially designed anionic linking reactions.20
The possibility of constructing complex polymeric architec-
tures with controlled radical polymerization methods increased
within the past decade.21,22 This supported the development of
new strategies for the synthesis of ABC miktoarm star
terpolymers where anionic polymerization is not involved. In
addition, click reactions in general, or copper(I)-catalyzed
azide−alkyne cycloaddition (CuAAC) in particular have
become powerful tools for highly efficient linking of polymeric
building blocks.23−25 These novel synthetic routes take
advantage of the orthogonality of the different polymer-
ization/ligation methods, which is a prerequisite for the
synthesis of well-defined ABC miktoarm star terpolymers. By
designing special core molecules with three different function-
alities, the arms can be “grafted onto” or “grafted from” in
different steps.26−31
Herein, we present a more general method by combining
anionic polymerization, employed for the synthesis of readily
alkyne mid-functionalized diblock copolymers, with other
polymerization techniques via azide−alkyne Huisgen cyclo-
addition (Scheme 1). Barner-Kowollik and co-workers already
reported the ligation of alkyne mid-functionalized homo-
polymers with azide-bearing homopolymers.32 However, only
A2B miktoarm star terpolymers were accessible. The synthesis
of an alkyne mid-functionalized diblock copolymer consisting
of polystyrene and poly(tert-butyl acrylate) was also
reported.33,34 Nevertheless, multiple polymer analogous re-
actions had to be conducted and/or switching the polymer-
ization system was necessary. Thus, we synthesized 1-[(4-(tert-
butyldimethylsilyl)ethynyl)phenyl]-1-phenylethylene to di-
rectly prepare midchain alkyne-substituted diblock copolymers
via sequential anionic polymerization. Because of the direct
introduction of the alkyne group into the diblock copolymer by
the use of DPE chemistry, a wide variety of functional diblock
copolymers are accessible, and except for hydrolysis no tedious
transformation reactions have to be conducted. The obtained
diblock copolymers were afterward ligated with different azide-
bearing homopolymers prepared by atom transfer radical
polymerization (ATRP), reversible addition−fragmentation
chain transfer (RAFT) polymerization, and anionic ring-
opening polymerization via azide−alkyne click chemistry.
Different novel ABC miktoarm star terpolymers were
synthesized utilizing this modular combination.
EXPERIMENTAL SECTION
■
Materials. For the purification of monomers, reagents, and solvent
and the synthesis of used compounds see the Supporting Information.
Synthesis of 1-[(4-(tert-Butyldimethylsilyl)ethynyl)phenyl]-
1-phenylethylene (Click-DPE). The starting molecule 1-(4-
bromophenyl)-1-phenylethylene was synthesized as described else-
where.35 The synthetic protocol for the coupling reaction with the
protected alkyne derivative was adopted from the literature.36 First, 1-
(4-bromophenyl)-1-phenylethylene was dissolved in piperidine (∼50
mg/mL) and bis(triphenylphosphine)palladium(II) dichloride (0.03
equiv) and CuI (0.004 equiv) were added under a nitrogen
atmosphere. After degassing with nitrogen for 30 min, tert-
butyldimethylsilylacetylene (1.2 equiv) was added dropwise at 50
°C, and the solution was stirred overnight. The reaction mixture was
filtered, the solvent was evaporated, and the crude product was
dissolved in THF. After addition of water, the product was extracted
with hexane for three times. The organic fractions were dried over
sodium sulfate, and the solvent was evaporated to obtain a brown oil.
This was further purified by column chromatography with hexane as
solvent to obtain a clear oil. Distillation of the product was not
possible even under high vacuum conditions. Before the use in anionic
polymerization, the compound was freeze-dried from benzene solution
on a high vacuum line and subsequently dissolved in dry THF to
obtain a stock solution of low viscosity. sec-BuLi was added dropwise
(typically 1−2 drops) under nitrogen flow until a deeply violet color
was obtained. Besides proving the absence of impurities, the persistent
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Table 1. Molecular Characterization of Alkyne Mid-Functionalized PB-b-P2VP, PB-b-PtBMA, and PB-b-PDMAEMA Diblock
Copolymers
a
b
b
c
d
e
f
sample ID
Mn PB-precursor [kg/mol]
N(PB)
N(2nd block)
Mn diblock [kg/mol]
Đ diblock
falkyne
14.7
14.0
17.2
17.1
12.3
cBV1
cBV2
cBV3
cBT1
cBT2
cBD
(cB40V58
(cB22V76
(cB70V28
(cB76T22
(cB49T49
(cB58D40
)
)
)
)
)
5.9
3.1
109
58
81
101
46
14.7
14.0
17.2
17.1
12.3
17.0
1.02
1.03
1.02
1.03
1.03
1.16
1.20
n.d.
0.93
n.d.
n.d.
12.0
13.0
6.0
223
242
111
181
27
42
17.0
)
9.8
44
1.06
a
b
The subscripts denote the molecular weight fraction and the superscript the overall molecular weight including the click-DPE unit. Determined by
c
MALDI-ToF MS. Calculated by 1H NMR or in the case of PtBMA as second block by the difference in the MALDI-ToF MS spectra of diblock and
d
precursor. The overall molecular weight was determined by a combination of MALDI-ToF MS and 1H NMR in the case of cBV’s and cBD and by
e
MALDI-ToF MS in the case of cBT’s. The molecular weight of the diblock includes the DPE unit. Measured from THF-SEC calibrated with
polystyrene standards; in the case of cBD salt-THF-SEC was used. The degree of alkyne functionalization falkyne was determined by UV−vis
measurements of the perylene-modified diblock copolymers.
f
1
color also served as indication for the purity of the stock solution. H
NMR (300 MHz, CDCl3): δ = 7.50−7.27 (m, 9H, Ar), 5.50 (s, 2H,
CCH2), 1.03 (s, 9H, SiC(CH3)3), 0.22 (s, 6H, SiCH3).
stirred at this temperature for 2 h. After warming up to room
temperature the solution was stirred overnight. The polymer was
precipitated in pure water or isopropanol/water 3/1 (v/v) for PB-b-
P2VP and PB-b-PtBMA, respectively. Afterward, the polymer was
dissolved in THF and dialyzed against THF (MWCO 1000 g/mol) to
remove impurities. Finally, the polymers were freeze-dried from
dioxane. In the case of the diblock copolymer with DMAEMA the
reaction mixture was directly dialyzed against THF prior to freeze-
drying.
Polymerizations. Sequential Living Anionic Polymerization in
THF. All polymerizations were carried out at low temperatures in a
thermostated laboratory autoclave (Buchi) under dry nitrogen
̈
atmosphere using THF as solvent. The day before polymerization,
freshly distilled THF (∼250 mL for 15 g of polymer) was treated with
1 mL of sec-BuLi per 100 mL of solvent at −20 °C, followed by stirring
overnight to form lithium alkoxides. These exhibit stabilizing effects on
the living chain end of polybutadiene and furthermore in the case of
methacrylate-type monomers no addition of LiCl is necessary to
obtain well-defined polymers.37 For the diblock copolymers, first
butadiene was initiated with sec-BuLi at −70 °C and then polymerized
at −50 to −15 °C depending on the block length of the different
polymerizations. To ensure complete consumption of the butadiene
the polymerization was followed by in-line NIR fiber-optic spectros-
copy. The living polybutadienyl anion was end-capped with 1-[(4-
(tert-butyldimethylsilyl)ethynyl)phenyl]-1-phenylethylene (1.1−1.5
equiv) at −50 °C for 2 h. Before and after the addition of the click-
DPE samples for SEC and MALDI-ToF MS were withdrawn. In the
case of poly(2-vinylpyridine) (P2VP) as second block the monomer
was added at −70 °C and polymerized for 5 min. After termination
with degassed methanol the polymer (cBV) was isolated by
precipitation in water. For the diblock copolymers with tert-butyl
methacrylate (tBMA) the monomer addition was conducted at −70
°C, and the polymerization took place at −45 °C for 1 h before it was
terminated with degassed methanol. After the addition of the tBMA
the violet color of the DPE end-capped living anion slowly faded away.
The final polymer (cBT) was precipitated in a mixture of isopropanol/
water 3/1 (v/v). For the third type of diblock the N,N-
dimethylaminoethyl methacrylate (DMAEMA) was injected into the
reactor at −70 °C and polymerized for 1 h. Immediately after addition
of the monomer the color of the DPE end-capped polybutadienyl
anion vanished completely. After termination with degassed methanol,
the resulting polymer (cBD) was dialyzed against THF and finally
freeze-dried from dioxane. The molecular weights of the diblock
copolymers with P2VP or PDMAEMA as second block were
calculated by a combination of MALD-ToF MS and 1H NMR.
Therefore, the Mn of the polybutadiene (PB) precursor polymer was
determined by mass spectrometry. By the relative molar ratios of the
characteristic signals of PB (5.10−5.60 ppm, 2H 1,2-PB; 5.70−5.15
ppm, 1H 1,2-PB and 2H 1,4-PB) and P2VP (8.50−8.00 ppm, 1H) or
PDMAEMA (4.30−3.90 ppm, 2H) the overall molecular weight was
calculated using the PB precursor molecular weight as reference. The
Mn of the PB-b-PtBMA diblock copolymers was directly measured by
MALDI-ToF MS. All the data for the diblock characterization are
summarized in Table 1.
Click Reactions. For the determination of the degree of
functionalization with alkyne-substituted DPE, test click reactions
with N-(1-heptyloctyl)-N′-(hexyl-6′-azido)-perylene-3,4,9,10-tetraca-
boxylic acid bisimide were conducted. In a typical run, 100 mg of
the deprotected diblock copolymer was dissolved in 5 mL of THF in a
screw-cap glass. Then, 2 equiv of the azido-functionalized perylene
bisimide relative to the alkyne function was added. After purging with
nitrogen for 10 min, 1 equiv of CuBr was added, followed by further
degassing for 10 min. By addition of 1 equiv of N,N,N′,N′,N″-
pentamethyldiethylenetriamine (PMDETA) as ligand, the click
reaction was started and stirred at room temperature for 3 days.
After termination by exposure to air, the remaining copper was
removed by filtration over a short silica gel column. Preparative SEC
with THF as eluent was employed to remove excess perylene. An
intense red-colored polymer was obtained after freeze-drying from
dioxane.
The click reactions for the construction of the miktoarm star
terpolymers were conducted in a similar manner. Typically, the molar
ratios of alkyne function:azido function:CuBr:PMDETA was set to
1:1:1:1 (if not stated elsewhere), and the polymer concentration was
∼20 mg/mL in THF. The reactions were conducted at room
temperature and followed by withdrawing samples for SEC measure-
ments. Finally, when no further changes in the SEC eluogramms were
observed, the resulting miktoarm star terpolymer was purified by
passing through a small silica gel column to remove copper. The
polymers were obtained as white powders after freeze-drying from
dioxane.
RESULTS AND DISCUSSION
■
Synthesis of 1-[(4-(tert-Butyldimethylsilyl)ethynyl)-
phenyl]-1-phenylethylene (Click-DPE). The key compound
of this modular route toward ABC miktoarm star terpolymers
(Scheme 1) was directly synthesized by the palladium-catalyzed
Sonogashira coupling reaction between 1-(4-bromophenyl)-1-
phenylethylene and tert-butyldimethylsilyl protected acetylene.
Similar compounds, like 4-(trimethylsilyl)ethynylstyrene36 and
methacrylate derivatives as 3-trimethylsilyl-2-propynyl meth-
acrylate,38 have already been synthesized and used in anionic
polymerization to obtain polymers with predictable molecular
weights and narrow distributions. As known for DPE and its
derivatives homopolymerization of such compounds is not
Alkyne Deprotection. For the deprotection procedure, the
respective polymer was dissolved in THF (∼0.1 g/mL) and degassed
for 30 min. Then, 10 equiv of tetrabutylammonium fluoride (1 M
solution in THF) relative to alkyne functions were added at 0 °C and
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possible due to steric hindrance.39 In contrast to methods
reported in the literature,33,34 this synthetic advantage enables
us to use one sequential polymerization process to selectively
incorporate exactly one alkyne function between the two
polymeric blocks under adequate choice of monomers. The
click-DPE was purified via column chromatography to give a
clear viscous oil. The chemical structure and the purity were
monomers as tert-butyl methacrylate (tBMA) and N,N-
dimethylaminoethyl methacrylate (DMAEMA). The anionic
polymerization of butadiene was conducted in THF at −50 to
−15 °C (depending on the respective block length) using sec-
BuLi as initiator. After complete conversionas followed in
situ with fiber-optics NIR spectroscopy1.1 to 1.5 equiv of the
click-DPE stock solution was added via syringe at −50 °C. The
addition of the diphenylethylene derivative was clearly visible
by the immediate change in color from a slight yellow to
intensive dark violet. To ensure complete end-capping of the
polybutadienyl anions, the solution was stirred at −50 °C for
another 2 h. During this period, no fading or change in color
was detected. Samples of the polybutadiene (PB) precursors for
SEC analysis were withdrawn before and after addition of the
click-DPE, respectively. As the pristine polybutadiene carries no
chromophore, no UV signal was detected at 260 nm. After
reaction with the click-DPE, a significant UV signal was
detected, coinciding with the RI trace. No change in the shape
of the peak was detectable, indicating the absence of unwanted
side reactions. Furthermore, a small shift of the whole peak
toward lower elution volumes took place (Figure S1). The
molecular weight of the polybutadiene precursor was 5910 g/
mol before and 6270 g/mol after the addition of the click-DPE,
as determined by MALDI-ToF MS (Figure S2). This is in good
accordance with the expected increase of 320 g/mol for a click-
DPE monoaddition.
With this new alkyne substituted DPE derivative we prepared
three different types of midchain alkyne-functionalized diblock
copolymers with narrow molecular weight distributions. In the
case of 2-vinylpyridine as second block, the monomer was
added at −70 °C and the polymerization was quenched after 5
min by the addition of degassed methanol. The overall
composition and number-average molecular weight of the
three different diblock copolymers (cBV1, cBV2, and cBV3)
were determined by a combination of MALDI-ToF MS and 1H
NMR. The characterization of the synthesized diblock
copolymers is given in detail in Table 1.
1
confirmed by H NMR spectroscopy (Figure 1a).
Figure 1. 1H NMR spectra of (a) 1-[(4-(tert-butyldimethylsilyl)-
ethynyl)phenyl]-1-phenylethylene (click-DPE), (b) the protected
alkyne-functionalized diblock cBT2, and (c) the corresponding diblock
after hydrolysis of the silyl-protecting group (solvent signals are striked
out).
In addition, we also prepared different diblock copolymers
with PtBMA (cBT1 and cBT2) or PDMAEMA (cBD) as the
second block. The polymerization conditions for polybutadiene
and the end-capping reaction were similar to the diblock
copolymers consisting of butadiene and 2-vinylpyridine
(cBV’s). The tBMA polymerization was started at −70 °C.
As the violet color of the anionic click-DPE chain end did not
disappear the temperature was stepwise raised to −45 °C where
the color slowly faded away. This indicated the start of the
polymerization. In the case of cBD the monomer was also
added at −70 °C; however, the color of the end-capped
polybutadienyl anion disappeared immediately after the
addition of the monomer. DMAEMA polymerized faster and
already at −70 °C, as it shows a higher reactivity than tBMA.
Similar to tBMA no change from colorless to violet was
observed after complete consumption of the monomer. This
substantiates the hypothesis that the click-DPE is only inserted
after the polybutadiene block and not within or at the end of
the methacrylate block. All data for the polybutadiene
precursors and the final diblock copolymers are listed in
Table 1.
Figure 2. UV−vis absorption spectrum of the adduct of 1,1-
diphenylethylene (dotted line) and 1-[(4-(tert-butyldimethylsilyl)-
ethynyl)phenyl]-1-phenylethylene (solid line) with sec-BuLi in THF.
The inset shows the picture of the cuvette containing the living anion
of the unsubstituted DPE (top) and the click-DPE (bottom).
The UV−vis spectrum of the deeply violet solution of the
living anion (Figure 2) generated by reaction of 1-[(4-(tert-
butyldimethylsilyl)ethynyl)phenyl]-1-phenylethylene with sec-
BuLi shows a maximum at 549 nm. In contrast, the living anion
of unsubstituted 1,1-diphenylethylene has a deep red color, and
we determined the absorption maximum at around 500 nm
under the same conditions.40 This clear bathocromic shift
originates from the conjugation of the π-system with the
protected alkyne group. Similarly, Tsuda et al. reported a
brownish-red color in the anionic polymerization of 4-
(trimthylsilyl)ethylenstyrene,36 in contrast to the orange color
of polystyryl anions.41
Theoretically, in combination with sec-BuLi, our click-DPE
can be used to initiate methacrylates for the generation of α-
alkyne-functionalized homopolymers or block copolymers. In
addition ω-alkyne-terminated homo- or block copolymers of
nucleophilic monomers like styrene, butadiene, and their
Synthesis of Alkyne Midfunctionalized Diblock Co-
polymers. Butadiene was chosen for the first block and for the
second block 2-vinylpyridine (2VP) or methacrylate type
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Chart 1. Azido-Functionalized Homopolymers Synthesized via Different Polymerization Methods
derivatives can be prepared by end-capping with this alkyne-
substituted DPE. The alkyne function is directly introduced
into the polymers via DPE-chemistry. Except hydrolysis, no
further polymer-analogous reaction steps are therefore
necessary.
Hydrolysis of the Alkyne Mid-Functionalized Diblock
Copolymers. The tetra-n-butylamonium fluoride-mediated
hydrolysis of the protecting group was monitored via 1H
NMR by the disappearance of the characteristic methyl signals
of the silyl group at 1.03 and 0.22 ppm. The signal at 1.03 is
overlapping with the backbone signals of polybutadiene,
whereas the signal at 0.22 is well separated from other signals.
Therefore, for the latter signal the complete disappearance
indicates the quantitative deprotection (Figure 1b,c).
Supporting Information). Here, one has to note that for the test
reaction the conditions were dissimilar to the diblock synthesis
as a 2-fold excess of click-DPE was used, and the reaction
mixture was stirred for an additional hour after addition of the
click-DPE. In contrast to this, for the synthesis of the diblock
copolymers the excess of click-DPE was already present during
the course of polymerization. Nevertheless, we suppose that the
click-DPE is incorporated after complete consumption of the
2VP monomer. As can be seen from the determined degree of
alkyne-functionalization for the cBV’s the values of 116−120%
were lower than the maximal value of 150% deduced from the
use of 1.5 equiv of click-DPE regarding initiator. Consequently,
even though the P2VP polymerization was terminated
immediately after consumption of all monomer, to a small
extent diblock copolymers were obtained, which partially might
bear an additional click-DPE unit in, or more probably, at the
end of the P2VP block.
Since methacrylates are generally not capable of adding
diphenylethylene, incorporation of only one click-DPE unit at
the block junction is expected for the diblcock copoylmers with
butadiene as first and a methacrylate as second block.39,43 This
is supported by the determined degree of alkyne-functionaliza-
tion of 93% for the perylene modified cBT1 (Figure S3B).
Taking into account the errors of weighing and determination
of the number-average molecular weight, this can be assumed
to be quantitative. Therefore, well-defined diblock copolymers
bearing exactly one alkyne function in between the two blocks
can be synthesized with our approach, if the second block is a
methacrylate. Generally, also well-defined alkyne mid-function-
alized diblock copolymers consisting of poly(2-vinylpyridine) as
first block and a methacrylate type monomer as second block
are accessible utilizing click-DPE.
Synthesis of ω-Azido Homopolymers. In principle,
every homopolymer bearing an azido function can be clicked
to the synthesized alkyne mid-functionlized diblock copoly-
mers. Different polymerization methods are capable of
generating azido-functionalized polymers in situ44,45 or by
appropriate termination reactions46 or postfunctionalization
methods.47 To demonstrate the broad applicability and
modular strategy of our approach, we synthesized azido-
functionalized homopolymers utilizing three different polymer-
izations methods (Chart 1, for details of the corresponding
synthetic procedure see the Experimental Section in the
Supporting Information). Azido-functionalized polystyrene
(PS-N3) as model compound was synthesized via ATRP and
subsequent transformation of the bromine end group with
sodium azide.47 By using an azido-functionalized initiator,
poly(tert-butyl methacrylate) was directly synthesized via
ATRP.45 This can be hydrolyzed to yield poly(acrylic acid).
Similarly poly(N,N-dimethylaminoethyl methacrylate) as func-
tional stimuli-responsive polymer was polymerized via RAFT
employing an azido-functionalized chain transfer agent
As the molecular weights of the diblock copolymers exceeded
10 000 g/mol, the direct determination of the degree of alkyne
substitution from the characteristic signals of the protecting
group was too imprecise. Thus, to determine the degree of
alkyne functionalization, a modification of the diblock
copolymers with an azido-functionalized perylene bisimide42
was conducted via click chemistry at the example of cBV1,
cBV2, and cBT1. After deprotection of the alkyne function and
subsequent click reaction with 2 equiv of N-(1-heptyloctyl)-N′-
(hexyl-6′-azido)perylene-3,4,9,10-tetracarboxylic acid bisimide
(N3-PBI), functionalization of the diblock copolymer was
proven qualitatively by SEC measurements at a wavelength of
458 nm. The nonlabeled diblock copolymers did not show any
significant UV activity at this wavelength, whereas perylene is
UV-active. After complete removal of excess perylene by
preparative SEC (as confirmed by SEC measurements at 458
nm), UV spectra were recorded in dichloromethane at a
polymer concentration of 10−5 mol/L. Thus, by measuring the
optical density and comparing the value of UV absorption at
the maximum of the spectra (λ = 525 nm) with a pure N3-PBI
solution with c = 10−5 mol/L, the degree of alkyne-
functionalization was determined to be ∼116% for cBV1 (see
Figure S3A and Table 1). There might be some minor sources
of error by weighing and by determination of the number-
average molecular weight of the polymer by a combination of
1
MALDI-ToF MS and H NMR. Nevertheless, this value gives
rise to the assumption that there might be incorporation of
more than one click-DPE in the block copolymer even under
the applied conditions. Multiple incorporation of the DPE
derivative in the polybutadiene block can be excluded, as
complete consumption of the monomer was guaranteed by in
situ NIR spectroscopy. Similar results were obtained for the N3-
PBI-labeled cBV2, where also functionalization with more than
one click-DPE took place (concluded from the determined
degree of functionalization of 120%). Test reactions showed
that living 2VP anions can attack click-DPE after extended
reaction times (a more detailed description and the
corresponding MALDI-ToF MS spectra can be found in the
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(CTA).44 Poly(ethylene oxide) (PEO) was achieved via anionic
ring-opening polymerization (AROP) and can serve as water-
soluble block. Therefore, we utilized the possibility of
quenching living polymer anions with azido acid chlorides46
to obtain azido-terminated PEO. In all cases the presence of an
azido function was proven by IR measurements and addition-
1
ally with MALDI-ToF MS or H NMR except for PtBMA-N3
(see characterization of ω-azido homopolymers in the
Supporting Information). The advantage of the applied
“grafting-onto” method is that the third arm is polymerized
and characterized separately. The molecular characterizations of
the different polymers are summarized in Table 2.
Table 2. Molecular Characterization of Azido-Functional
Homopolymers
Figure 3. THF-SEC (RI signal) of alkyne-functionalized diblock cBT1,
azido-functionalized PS (PS1-N3, PS2-N3), and the corresponding
ABC miktoarm star terpolymers (μ-BT1S1 and μ-BT1S2) obtained
after equimolar click reaction for 24 h.
sample ID
Mn [kg/mol]
Đ
a
PS1-N3
3.7
5.9
8.3
6.6
7.6
5.4
1.07
1.10
1.06
1.02
1.17
1.20
a
a
PS2-N3
PS3-N3
b
PEO-N3
S4), resembling the successful generation of the desired
miktoarm star terpolymers. However, despite equimolar
reaction conditions in both cases there was still a minor
amount of unreacted homopolymer left. Further SEC measure-
ments showed that the peak of the PS1-N3 already disappeared
nearly completely after 11.5 h (Figure S5) for the click reaction
with cBT2. Increasing the reaction time did not lead to any
change in the eluogramm. Therefore, we additionally carried
out a reaction between cBT1 and only 0.8 equiv of PS1-N3.
Under the same reaction conditions, again, a comparable
amount of homopolymer was left (results not shown). We
assume that during polymerization and work-up of the
homopolymer, a minor part of the bromine group is eliminated,
as already reported in the literature.51 This leads to a small
amount of non-azido-functionalized homopolymer which
cannot take part in the click reaction (see discussion of
MALDI-TOF spectrum in the Supporting Information). This
was also confirmed by the click reaction of cBT2 with PS3-N3
(Figure S6). Under equimolar reaction conditions no further
ligation took place after 15 h, leading to a non-negligible
amount of unreacted diblock and homopolymer. In contrast to
the click reactions with lower molecular weight polystyrenes the
conversion of the click reaction with PS3-N3 was much lower
after 3.25 h (where the reaction was more or less finished in the
other cases). Further addition of 0.5 equiv of homopolymer
resulted in a complete shift of the diblock due to complete click
efficiency as depicted in Figure S6. Thus, the degree of
unfunctionalized polystyrene increased with increasing molec-
ular weight.
Nevertheless, complete shifts of the molecular weight
distributions of the miktoarm star terpolymers toward lower
elution volumes compared to the pristine diblock copolymers
were detected in all cases. This demonstrates the successful
formation of ABC miktoarm star terpolymers. Additionally, the
overall shifts of the resulting ABC miktoarm star terpolymers
were not too distinct. This is in accordance with theory, where
the hydrodynamic radius of star polymers is expected to be
smaller than the hydrodynamic radius of a linear polymer with
the same composition and molecular weight due to the higher
segmental density of star polymers.52 All synthesized μ-BTS
star terpolymers exhibited symmetrical peaks with a narrow
molecular weight distribution. The theoretical molecular
weights were calculated from the known molecular weights of
c
PtBMA-N3
d
PDMAEMA-N3
a
Molecular weights and dispersities were determined by THF SEC
using polystyrene standards. Molecular weights and dispersities were
determined by THF SEC using poly(ethylene oxide) standards.
Molecular weights and dispersities were determined by THF SEC
using poly(tert-butyl methacrylate) standards. MALDI-ToF MS was
b
c
d
employed for determination of molecular weight and dispersity.
Synthesis of Miktoarm Star Terpolymers via Click
Chemistry. Copper-catalyzed alkyne−azide cycloaddition
(CuAAC) was utilized for the modular synthesis of ABC
miktoarm star terpolymers via combination of alkyne mid-
functionalized diblock copolymers with ω-azido-functionalized
homopolymers of different length and monomer structure
(Chart 1). Here, only the alkyne mid-functionalized diblock
copolymers with a methacrylate as second block (cBT’s and
cBD) were used, as for 2-vinylpyridine partial incorporation of
the click-DPE in or most probably at the end of the P2VP block
could not be excluded completely.
Click Reactions with ω-Azido-Functionalized Polystyrene.
To demonstrate the feasibility of this approach, we first
conducted click reactions of the alkyne-functionalized diblock
copolymers with different azido-functionalized PS homo-
polymers. One prerequisite of our approach is the 100%
efficiency of the click reaction step, so that no laborious
purification steps are needed when the educts are added in
equimolar amounts.48 Even though in literature linear and well-
defined star shaped polymers with molecular weights close to
100 000 g/mol were produced via CuAAC,49,50 we suppose that
in our case the alkyne function at the block junction is sterically
less amenable than an alkyne function in α- or ω-position. Our
first trials with higher molecular weight polymers (exceeding
40 000−50 000 g/mol for the individual compounds) were not
successful. Therefore, we chose moderate molecular weights of
the individual compounds, so that the overall molecular weight
did not exceed 25 000 g/mol and the functional groups were
not too diluted. The course of the click reactions with cBT
diblock copolymers was followed by SEC. The reactions were
conducted under equimolar conditions. After 24 h the
molecular weight distribution of the diblock copolymers
cBT1 and cBT2 shifted completely (see Figure 3 and Figure
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Table 3. Molecular Characterization of ABC Miktoarm Star Terpolymers Obtained from Click Reactions with Alkyne Mid-
Functionalized Diblock Copolymers
a
b
c
d
e
f
f
sample ID
DP(PB)
DP(2nd block)
DP(3rd block)
Mn,th [kg/mol]
Mn,app [kg/mol]
Đ
20.7
22.9
μ-BT1S1
μ-BT1S2
μ-BT1E
μ-BT1D
μ-BT2S1
μ-BT2S2
μ-BT2S3
μ-BT2E
μ-BT2D
(μ-B63T18S18
(μ-B57T17S26
(μ-B55T16E28
(μ-B58T17D24
)
)
242
242
242
242
111
111
111
111
111
181
181
27
27
27
27
42
42
42
42
42
44
44
36
57
20.7
22.9
23.6
22.4
15.9
18.1
20.5
18.8
17.6
24.5
23.5
28.0
30.0
32.7
31.5
20.5
22.4
25.0
25.4
20.8
26.3
32.5
1.03
1.03
1.04
1.10
1.03
1.03
1.03
1.03
1.11
1.08
1.06
23.6
)
150
34
22.4
)
15.9
18.1
20.5
(μ-B38T38S23
(μ-B33T33S33
(μ-B29T29S40
(μ-B32T32E35
(μ-B34T34D31
)
36
)
)
57
g
80
18.8
)
150
34
17.6
)
g
24.5
μ-BDT
(μ-B40D28T31
)
53
g
23.5
μ-BDE
(μ-B42D29E28
)
150
a
The number-average molecular weight of the ABC miktoarm star terpolymer was calculated from the respective values of diblock copolymers and
the homopolymers. Therefore, the superscript denotes the theoretical number-average molecular weight of the ABC miktoarm star terpolymer and
b
the indices the theoretical weight fraction of the respective blocks. Degree of polymerization (DP) determined from MALDI-ToF MS of the PB-
c
d
precursor. Calculated by the difference in Mn determined by the MALDI-ToF MS spectra of diblock and precursor. Calculated from the molecular
e
f
weight of the 3rd block. Theoretical molecular weight. Apparent molecular weight and dispersity determined by SEC with polystyrene calibration.
For the miktoarm star terpolymers containing PDMAEMA THF-SEC with additional 0.25 wt % tetrabutylammonium bromide (TBAB) was used.
g
In these cases the apparent molecular weight and disperisty were determined excluding the separated homopolymer peak.
Figure 4. THF-SEC (A) of alkyne-functionalized diblock copolymers cBT1, cBT2, the azido-functionalized PEO-N3, and the corresponding ABC
miktoarm star terpolymers (μ-BT1E and μ-BT2E) obtained after azide−alkyne click coupling and salt-THF-SEC traces (B) of cBD, PEO-N3, and
the resulting ABC miktoarm star terpolymer (μ-BDE) after dialysis. In all cases the RI signals are shown.
the ligated diblock copolymers. These are shown together with
the apparent molecular weights and the respective dispersities
in Table 3. Furthermore, an exemplarily H NMR of μ-BT1S1
is shown in Figure S7. All characteristic signals of the polymeric
building blocks are present in the obtained miktoarm star
terpolymer.
added to the click reaction containing already 1.5 equiv. As this
did not result in a change compared to the eluogram after
reaction with 1.5 equiv of PEO-N3, the equivalents necessary
for complete ligation were determined to be 1.5 equiv.
1
The reason for the use of such a huge excess of PEO-N3 still
remains unclear. From the amount of coupled product (∼6 wt
% from SEC) only a slight excess of the azido-functionalized
compound would be reasonable. However, maybe partial
elimination of the azido group of PEO or uncomplete
functionalization due to side reaction during the end-capping
reaction could be a feasible explanation (see discussion of
MALDI-ToF spectrum in the Supporting Information). The
respective SEC traces for the click reactions with cBT1, cBT2,
and cBD (where the ratio of cBX:PEO-N3 was minimum 1:1.5)
are shown in Figure 4. In case of the diblock copolymers with
PtBMA as second block the excess PEO-N3 was easily removed
during purification from copper with a short column of silica
due to interactions with the column. Using PEO-N3 in excess
and subsequent removal of unreacted homopolymer guaranteed
the complete conversion of the alkyne function.
Click Reactions with ω-Azido-Functionalized Poly-
(ethylene oxide). These first successful test reactions with
azido-functionalized polystyrene prove the feasibility of our
approach. To prepare amphiphilic miktoarm star terpolymers,
we conducted click reactions with PEO-N3. First attempts
under equimolar reaction conditions led to a non-negligible
amount of unreacted diblock copolymer. Therefore, the click
reactions were carried out with an excess of the functionalized
PEO-N3. By stepwise addition of PEO-N3 to the reaction
mixture, the equivalents necessary for a complete click reaction
were determined (Figure S8). Therefore, in a first step, the
reaction with 1 equiv of PEO-N3 was followed by SEC. As after
24 h no further change in the eluogram was detected, and still a
significant amount of unreacted diblock copolymer was left
over, 0.5 equiv of PEO-N3 was added subsequently. Finally,
after 3.5 h reaction time, a further 0.2 equiv of PEO-N3 was
However, in the case of the diblock cBD containing
DMAEMA a higher amount of PEO homopolymer was left
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Figure 5. Salt-THF-SEC (RI signal) of μ-BTD miktoarm star terpolymers obtained by click coupling of the alkyne-functionalized diblock
copolymers (A) cBT1 and (B) cBT2 with the azido-functionalized PDMAEMA-N3. As in both cases 2 equiv of PDMAEMA was used within the
click reaction, SEC traces of the raw product and the miktoarm star terpolymer after dialysis are shown.
over after work-up. Further purification with a column,
crystallization from cold THF, or dialysis in aqueous media
did not reduce the amount of the undesired PEO
homopolymer. A possible explanation could be that PEO
forms hydrogen bonds with PDMAEMA, which therefore
prevent the complete removal of excess PEO homopolymer.
Here, one has to notice that for further applications, where the
PEO serves as corona in aqueous solutions, this minor amount
of PEO homopolymer is not problematic.
the formation of amine oxide. For the dialysis of μ-BT2D even
under extensive exchange of the solvent, the small residual peak
from unreacted PDMAEMA-N3 could not be removed
completely. However, compared to the product peak this
peak is negligible. The molecular characterization of the μ-BTD
miktoarm star terpolymers is listed in Table 3. In a similar way
a μ-BDT miktoarm star terpolymer consisting of the same
monomer units was synthesized by ligation of cBD with
PtBMA-N3 (see Supporting Information and Table 3).
In contrast to the previous click reactions, distinct shifts of
the molecular weight distributions of the ABC miktoarm star
terpolymers compared to the corresponding diblock precursors
were detected. The absence of a peak from residual diblock also
gives evidence that all diblock copolymer chains carry an alkyne
function. Like for the click reactions with PS-N3 low dispersities
of the resulting ABC miktoarm star terpolymers were detected
(Table 3).
CONCLUSIONS
■
This general modular route for the synthesis of ABC miktoarm
star terpolymers combines anionic polymerization techniques
with CRP methods and anionic ring-opening polymerization
using azide−alkyne Huisgen cycloaddition as the ligation
method. For this purpose, we successfully employed our
alkyne-modified DPE derivative in sequential anionic polymer-
ization. In the case of methacrylate-type monomers well-
defined alkyne midfunctional diblock copolymers consisting of
a first block of PB and a second block of either PtBMA or
PDMAEMA were synthesized. Click reactions with azido-
functionalized perylene bisimide verified the successful
incorporation of only one DPE unit at the block junction. In
contrast to other DPE derivatives, we observed that click-DPE
can copolymerize with 2VP, which offers the advantage of
producing alkyne mid-functionalized diblock copolymers with
P2VP as first block and a methacrylate as second block.
Therefore, 2VP and butadiene and styrene and their derivatives
can be used as first blocks and methacrylate type monomers as
second block to obtain well-defined diblock copolymers bearing
an alkyne function at the block junction. By using a toolbox of
different azido-functionalized homopolymers, we demonstrate
that a variety of well-defined ABC miktorarm star terpolymers
is accessible via this modular approach. These exhibited
monomodal molecular weight distributions with small dis-
peristies close to 1.10 or even lower.
Click Reactions with ω-Azido-Functionalized Poly(N,N-
dimethylaminoethyl methacrylate). To obtain PDMAEMA as
a water-soluble polymer, which responds to both pH and
temperature,53 we used an azido-substituted CTA.44 As the
general applicability of our approach under equimolar
conditions was proven we conducted the click reactions with
a 2-fold excess of PDMAEMA-N3 for 5 days, without
optimizing the reaction time. With this reaction pathway we
were also able to guarantee 100% conversion of the alkyne
compound. The SEC traces of the raw product of the click
reactions of the azido-functionalized PDMAEMA-N3 with the
two cBT diblock copolymers are shown in Figures 5A and 5B,
respectively. In both cases the molecular weight distributions of
the miktoarm star terpolymers shifted completely in the
corresponding SEC traces compared to their diblock copolymer
precursors. The excess PDMAEMA-N3 was removed by dialysis
in a mixture of methanol/isopropanol (2/1 v/v), where the
miktoarm star terpolymer forms micelles with a polybutadiene
core. Therefore, a dialysis membrane with a cutoff (50 000 g/
mol) much higher than the molecular weight of the
homopolymer was used. The SEC traces of the obtained
miktoarm star terpolymers are plotted in Figure 5A and B. In
the case of μ-BT1D the tiny coupling shoulder at lower elution
volume became more pronounced after dialysis. Therefore, we
assume that this shoulder might be the result of some
aggregation effects, which occurred for this reaction. Another
explanation could be the oxidation of the amine, which leads to
The obtained miktoarm star terpolymers are interesting new
polymers regarding their bulk morphologies and solution-based
self-assembly. Results from the self-assembly of these ABC
miktoarm star terpolymers will be reported in subsequent
publications. A further advantage is that due to the use of
anionic polymerization miktoarm star terpolymers with a
polybutadiene block are accessible, which can afterward be
functionalized through thiol−ene click chemistry54 or used for
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selective cross-linking.5,55 Furthermore, the ABC miktoarm star
terpolymer containing arms of polybutadiene, poly(N,N-
dimethylaminoethyl methacrylate) and poly(ethylene oxide)
is a promising candidate in the field of biotechnological
applications like gene delivery.56−58 Our approach enables the
upscaling of the synthesis of ABC miktoarm star terpolymers to
more than 1 g. Both synthetic steps for diblock and
homopolymer synthesis are well-established polymerization
methods, and no further complex synthetic procedures are
necessary, as compared to other methods. Because of the strong
versatility of click chemistry and the increasing amount of
publications dealing with it, we expect our click-DPE to offer a
variety of new possibilities in the design of novel custom
polymer architectures and materials.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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This work was supported by the Deutsche Forschungsgemein-
schaft within SPP 1165 (Mu896/22). We thank Christo B.
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