Single-step synthesis of internally functionalizable hyperbranched polyethers

Article abstract of DOI:10.1002/pola.26821

Radical catalyzed thiol-ene reaction has become a useful alternative to the Hüisgen-type azide-yne click reaction as it helps expand the variability in reaction conditions as well as the range of clickable entities. In this study, the direct generation of a hyperbranched polyether (HBPE) having decyl units at the periphery and a pendant allyl group on every repeat unit of the polymer backbone is described; the allyl groups serve as a reactive handle for postpolymerization modifications and permits the generation of a variety of internally functionalized HBPEs. In this design, the AB₂ monomer carries two decylbenzyl ether units (B-functionality), an aliphatic -OH (A-functionality) and a pendant allyl group within the spacer segment; polymerization of the monomer readily occurs at 150 °C via melt transetherification process by continuous removal of 1-decanol under reduced pressure. The resulting HBPE has a hydrophobic periphery due to the presence of numerous decyl chains, while the allyl groups that remain unaffected during the melt polymerization provides an opportunity to install a variety of functional groups within the interior; thiol-ene click reaction with two different thiols, namely 3-mercaptopropionic acid and mercaptosuccinic acid, generated interesting amphiphilic structures. Preliminary field emission scanning electron microscope (FESEM) and Atomic Force Microscopy (AFM) imaging studies reveal the formation of fairly uniform spherical aggregates in water with sizes ranging from 200 to 400 nm; this suggests that these amphiphilic HBPs is able to reconfigure to generate jellyfish-like conformations that subsequently aggregate in an alkaline medium. The internal allyl functional groups were also used to generate intramolecularly core-crosslinked HBPEs, by the use of dithiol crosslinkers; gel permeation chromatography traces provided clear evidence for reduction in the size after crosslinking. In summary, we have developed a simple route to prepare core-clickable HBPEs and have demonstrated the quantitative reaction of the allyl groups present within the interior of the polymers; such HB polymeric systems that carry numerous functional groups within the core could have interesting applications in analyte sequestration and possibly sensing, especially from organic media.

Full text of DOI:10.1002/pola.26821

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Single-Step Synthesis of Internally Functionalizable Hyperbranched
Polyethers
Raj Kumar Roy, S. Ramakrishnan
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India
Correspondence to: S. Ramakrishnan (E-mail: raman@ipc.iisc.ernet.in)
Received 23 March 2013; accepted 3 June 2013; published online 3 July 2013
DOI: 10.1002/pola.26821
ABSTRACT: Radical catalyzed thiol-ene reaction has become a
scanning electron microscope (FESEM) and Atomic Force
Microscopy (AFM) imaging studies reveal the formation of fairly
uniform spherical aggregates in water with sizes ranging from
200 to 400 nm; this suggests that these amphiphilic HBPs is able
to reconfigure to generate jellyfish-like conformations that subse-
quently aggregate in an alkaline medium. The internal allyl func-
tional groups were also used to generate intramolecularly core-
crosslinked HBPEs, by the use of dithiol crosslinkers; gel permea-
tion chromatography traces provided clear evidence for reduc-
tion in the size after crosslinking. In summary, we have
developed a simple route to prepare core-clickable HBPEs and
have demonstrated the quantitative reaction of the allyl groups
present within the interior of the polymers; such HB polymeric
systems that carry numerous functional groups within the core
€
useful alternative to the Huisgen-type azide-yne click reaction as
it helps expand the variability in reaction conditions as well as
the range of clickable entities. In this study, the direct generation
of a hyperbranched polyether (HBPE) having decyl units at the
periphery and a pendant allyl group on every repeat unit of the
polymer backbone is described; the allyl groups serve as a reac-
tive handle for postpolymerization modifications and permits the
generation of a variety of internally functionalized HBPEs. In this
design, the AB2 monomer carries two decylbenzyl ether units (B-
functionality), an aliphatic AOH (A-functionality) and a pendant
allyl group within the spacer segment; polymerization of the
monomer readily occurs at 150 ꢀC via melt transetherification
process by continuous removal of 1-decanol under reduced pres-
sure. The resulting HBPE has a hydrophobic periphery due to
the presence of numerous decyl chains, while the allyl groups
that remain unaffected during the melt polymerization provides
an opportunity to install a variety of functional groups within the
interior; thiol-ene click reaction with two different thiols, namely
3-mercaptopropionic acid and mercaptosuccinic acid, generated
interesting amphiphilic structures. Preliminary field emission
could have interesting applications in analyte sequestration and
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possibly sensing, especially from organic media.
2013 Wiley
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KEYWORDS: amphiphiles; core–shell; crosslinking; hyper-
branched; nanoparticles; polyethers
and so forth. Owing to the step-wise synthesis of den-
drimers, functional groups can also be selectively incorpo-
rated at various locations, such as the core, periphery or
even within specific internal layers of the Dendrimer. Con-
versely, hyperbranched polymers (HBPs) that are prepared
by a single-step methodology do not have a distinct core or
periphery; therefore, it is a challenging task to design suita-
ble monomers for internal functionalization. Although HBPs
contain numerous linear defects, amphiphilic HBPs have
been shown to reconfigure to create distinct core and shell
domains.9 Therefore, exploring methodologies to install spe-
cific functional groups within the core of reconfigured HBPs
could create interesting application potentials.
INTRODUCTION Dendrimers are highly branched, symmetric,
monodisperse macromolecules with several unique features
that are uncharacteristic of conventional macromolecules.1
These unusual features have attracted researchers to explore
their potential applications in biotechnology, nanotechnology,
and so forth.2 The unique architecture of dendrimers is very
useful for mimicking some of the nature’s complex functions;
for example, site-isolation,3 light-harvesting,4 catalysis,5 and
so forth. Several applications of dendrimers rely on the com-
partmentalization of the core and shell.6 Recently, research-
ers have also begun to explore site-specific functionalization
of the core region to develop newer applications.2(h) Several
interesting properties of internally functionalized den-
drimers, in conjunction with peripheral functionalization,
have been developed; for instance, energy cascading from
periphery to the core of the dendrimer,7 catalytic process,8
Some years ago, Haag et al.10 exploited the presence of two
types of hydroxyl groups in HB polyglycerol, one an isolated
Additional Supporting Information may be found in the online version of this article.
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SCHEME 1 Synthesis of the internally clickable HBP and its derivatization.
AOH groups in the linear defect sites and the other the vici-
nal diols at the terminal units, for selective functionalization.
They used the acetal or ketal forming reaction to discrimi-
nate between the linear and terminal units; this enabled
them to create core–shell architectures in the hyperbranched
polymer. Ambade and Kumar11 prepared a potentially func-
and its wide range of applicability, it would be useful to
develop simple single-step strategies to synthesize hyper-
branched polymers that carry numerous thiol-ene clickable
allyl groups within the interior. We have recently demon-
strated that HBPs carrying numerous allyl groups at their
periphery can be readily synthesized in a single step and
subsequently used to access a wide range of peripherally
functionalized structures using the thiol-ene click process.14
In this report, we describe a single-step preparation of a
peripherally hydrophobic hyperbranched polyether (HBPE)
with clickable internal functionality, using a suitably design
AB2 monomer. Further, we demonstrate that these HBPEs
can be readily transformed to amphiphilic systems by reac-
tion with polar thiols that renders the core hydrophilic; we
have then examined the solution behavior and aggregation
properties of these interesting class of HBPs, in addition to
exploring intramolecular core-crosslinking.
tionalizable hyperbranched polyurea from
a
suitably
designed carbonyl azide-based AB2 monomer; here N-allyl
groups were present in every repeat unit of the hyper-
branched polymer. Later, using a suitable capping agent, they
decorated the periphery with long chain hydrocarbon units.
The N-allyl groups within the HBP could, in principle, have
been transformed to a variety of interesting functional
groups making this polymer one of the early examples of
potentially functionalizable HBPs; however, the authors do
not appear to have explored this possibility.
In an effort to prepare internally functionalized hyper-
branched polymers, we selected the thiol-ene click reaction
to confer the interior of the HBP with variety of functional
groups. Although the thiol-ene reaction is over a century-old
reaction, interest in this reaction has been revived recently
because of the mild reaction conditions, quantitative yield,
absence of side-products, and so forth.12 More importantly,
comparison of this reaction with the Cu-catalyzed azide-yne
click reaction reveals some distinct advantages: (a) the thiol-
ene reaction can be done in the absence of a metal ion, and
(b) it can be initiated photochemically in the absence of a
radical initiator.13 Given the merits of the thiol-ene reaction
RESULTS AND DISCUSSIONS
Over a decade ago, we developed a simple strategy for pre-
paring HBPs via a novel melt transetherification process,
wherein the AB2 monomer carried two methoxybenzyl
groups (B type) and an aliphatic AOH group (A type).15 The
polymerization was performed under mildly acidic condi-
tions using pyridinium camphorsulfonate (PCS) as the cata-
lyst, at 150 ꢀC. As the transetherification is a reversible
reaction similar to transesterification, removal of low boiling
methanol drives the equilibrium in the forward direction to
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SCHEME 3 Synthesis of the allyl-containing spacer (C).
zylic protons (peak c) changes from 1.5:1, in the monomer,
to nearly 0.75:1, in the polymer. This implies that one equiv-
alent of the decyl group has been lost as decanol and con-
firms that high conversion has indeed been achieved.
Further, the benzylic methylene peak splits in two due to the
presence of both backbone and terminal benzylic units. Most
importantly, it is also evident that the pendant allyl groups
remain unaffected during the high temperature melt poly-
merization process; the relative intensities of the peaks due
to the allyl units match well with the expected value (a:l
is 1:3). The molecular weight of the polymer was determined
using gel permeation chromatography (GPC), which was
coupled to a triple detector system, and Mn was found to be
around 12,000 based on the Universal calibration method.
Although the degree of branching was difficult to establish in
this case, it could be assumed to be similar to those contain-
ing a simple alkylene spacer,17 which was found to be
around 0.5, along expected lines.
SCHEME 2 Synthesis of the AB2 (4) monomer carrying pend-
ant allyl group.
generate the HBPE. Subsequently, we had also shown that
the polymerization process is not only limited to methoxy-
benzyl group but is equally applicable to propargyloxyben-
zyl16 and allyloxybenzyl groups.14 Thus, it became evident
that the transetherification process can be readily exploited
for polymer synthesis as long as the removal of the conden-
sate, that is, the alcohol, is feasible under the polymerization
conditions. Based on these earlier studies, we designed a
novel AB2 monomer (Scheme 1) that carries two decyloxy-
benzyl groups and an aliphatic AOH group; the unique fea-
ture of this design is inclusion of a pendant allyl group in
the spacers segment. It may be expected that this monomer
will undergo melt condensation under standard acid-
catalyzed transetherification conditions at 150 ꢀC; the equi-
librium would be driven to polymer formation by the contin-
uous removal of 1-decanol under reduced pressure. The AB2
monomer in turn was prepared as outlined in Scheme 2; the
bis(methoxymethyl) Mesitol derivative (2) was transformed
to the corresponding decyloxy analogue (3) by transetherifi-
cation in the presence of a very effective Lewis acid catalyst,
namely yitturbium triflate; an excess of 1-decanol was used
to prevent the possibility of premature polymerization. Com-
pound 3 was then coupled with the allyl-carrying spacer
(synthesized as per Scheme 3) to give the required AB2
monomer (4). The monomer was then polymerized under
standard melt transetherification conditions.15
One of the virtues of generating HBPs that carry internal
allyl groups is that it provides an opportunity to generate a
wide range of polymers with distinct properties by simply
clicking with different organic thiols. Of the various
approaches to carry out thiol-ene reactions, the photochemi-
cally initiated approach, using 2,2-dimethoxy 2-phenyl aceto-
phenone as the radical-initiator, is often the method of
choice. Hence, in this study, a solution of the parent polymer
HBP-Allyl and the required thiol was taken along with the
initiator in chloroform and irradiated using a Hg-vapor lamp
1
for about 4 h. The isolated polymers were analyzed using H
NMR and GPC.
Two different thiols (3-mercaptopropionic acid [PA] and
mercaptosuccinic acid [SA]) were clicked onto the HBPE in
this study; and the samples thus obtained were labeled
HBP-PA and HBP-SA, respectively. In both cases, our objec-
tive was to prepare an amphiphilic hyperbranched polymer
having hydrophobic shell and hydrophilic core. The expected
structures of these clicked hyperbranched polymers are
The proton NMR spectra of the monomer 4 and the parent
HBPE (HBP-Allyl) are shown in Figure 1, along with the
peak assignments. One of the salient differences in the poly-
mer spectra is the decrease in the relative intensity of the
peaks belonging to the decyloxy group; the ratio of the ter-
minal methyl protons of the decyl units (peak l) to the ben-
1
shown in Scheme 1. The H NMR spectra of the parent poly-
mer, along with the two clicked polymers, are shown in
Figure 2; the region where the allyl proton signals is
expected has been expanded to reveal the absence of resid-
ual allyl groups in the polymer, thereby confirming that the
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FIGURE 1 1H NMR spectra of monomer 4 and the HBP, HBP-Allyl.
click reaction has proceeded to near completion in both the
cases. In addition to the disappearance of the allyl proton
signals, the relative intensities of the signals arising from the
clicked moiety also matched well with the expected values
for quantitative conversion. These observations clearly sug-
gest that HBPs bearing allyl groups within the core can
indeed serve as compact hyperscaffolds, within the core of
which a variety of function-performing units could be ligated
under mild conditions.
Thus, it is clear that the presence of internal functional groups,
especially those that can interact strongly, could modify the
thermal properties in fairly unexpected ways to generate
amorphous system.
When compared with dendrimers, hyperbranched polymers
contain numerous structural defects and are typically highly
polydisperse; thus formation of uniform aggregated struc-
tures appears would be difficult. However, recent studies
suggest that the HBPs also self-assemble to form fairly uni-
form and interesting structures; for instance, micelles,
vesicles, microscopic tubes, fibers, and so forth, have been
reported using amphiphilic hyperbranched polymers.19 The
interest in self-assembled structures based on HBPs is pri-
marily because of the relatively straightforward methodology
used for their synthesis, which makes them more readily
accessible.
It is well-known that the thermal properties of HBPs depend
strongly on the nature of the peripheral functional groups.18
However, little is known about the dependence of Tg on the
nature of functional groups within the core of HBPs. The DSC
thermograms of the two clicked HBPEs, along with that of the
parent polymer HBP-Allyl, are shown in Supporting Informa-
tion Figure S1. The parent hyperbranched polymer exhibited a
Very recently, we showed that hyperbranched polymers, ran-
domly functionalized with PEG (polyethylene glycol) and
docosanol (C-22) at their periphery, adopt a Janus struc-
ture;20 the immiscibility of docosyl and PEG segments and
the crystallization of docosyl units into a paraffinic lattice
were shown to be the driving motivation for the formation
of self-segregated Janus structures. In this study, the HBPEs
carrying long chain hydrocarbon (decyl unit) at the periph-
ery and either mercaptopropionic acid (HBP-PA) or SA
(HBP-SA) units within the interior were examined for their
aggregation behavior.
ꢀ
ꢀ
sharp peak at 245 C and a broad one at 215 C; these were
very weak and could reflect the melting (and some associated
transition) of the segregated decyl chains, although unequivo-
cal interpretation of this is not possible without further char-
acterization. However, both the internally functionalized
HBPEs were found to be completely amorphous and exhibited
only a glass transition temperature; the Tg of ꢀHBP-SA (210
ꢀC) was slightly lower than that of HBP-PA (0 C). The amor-
phous nature of these derivatives is intriguing; this may be
due to the intramolecular H-bonding within the core region
that renders a single chain nanoparticle-like nature to these
polymers and consequently the improper segregation of the
peripheral decyl chains could hinder their crystallization.
All the polymers were readily soluble in a wide range of
solvents, such as chloroform, THF, hexane, and so forth.
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FIGURE 2 1H NMR spectral stack plot of parent polyether HBP-Allyl (bottom), and those clicked with 3-mercaptopropanoic acid
HBP-PA (middle) and mercaptosuccinic acid HBP-SA (top). The expanded region shows the near completion disappearance of the
allyl protons.
HBP-PA was completely insoluble in water, whereas HBP-
SA was partially soluble; however, both of them were com-
pletely soluble in aqueous alkali due to the ionization of
the internal carboxylic acid groups. The solubility in aque-
ous alkaline medium indicates that the internally function-
alized HBPEs are able to invert so that the carboxylate
groups are solvated in water; such inverted amphiphilic
structures could then self-assemble to form variety of
aggregates. To examine the nature of aggregates formed,
ꢁ1 mg of the polymer was taken in 2 mL of water con-
taining the required amount of NaHCO3; the solution was
sonicated until a clear solution is formed, filtered through
a 0.47 mm filter and a few drops of this solution was cast
on a substrate and allowed to dry. Visualization under
field emission scanning electron microscope (FESEM)
clearly showed the formation of spherical aggregates (Fig.
3) with reasonably uniform sizes (ꢁ200–400 nm). To
ascertain the sizes of these self-assembled structures in
solution, we carried the dynamic light scattering (DLS)
measurements, which showed that the average size of the
FIGURE 3 FESEM image of HBP-SA.
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FIGURE 4 1H NMR spectral stack plot of parent polymer HBP-Allyl (bottom); partially clicked HBPE (middle); and core-crosslinked
HBPE (top). Peaks marked with an * are due to OH from ASCH2CH2OH and those with an ** due to residual MeOH (used for repre-
cipitation of polymer).
aggregates were about ꢁ240 nm (Supporting Information
conditions.22 To devise a simple method to preclude cross-
linking, we considered an alternate strategy wherein the
dithiol crosslinker would have an intrinsic preference to
reside within the core-region of the amphiphilic HBPE,
thereby reducing the probability of interchain reactions. To
achieve this, we conceived of a two-step process (Scheme 4);
in the first step, we transformed the parent polymer HBP-
Allyl to an amphiphilic core–shell system by installing numer-
ous hydroxyl groups within the core by reacting the parent
polymer with 2-mercaptoethanol; however, care was taken to
ensure that a reasonable fraction of the allyl groups were left
unreacted. The resulting polymer carried a very hydrophobic
periphery due to the dodecyl chains and a hydrophilic core
due to the presence of a large number of hydroxyl groups
within the core. In the second step, the core-modified polymer
was dissolved in a nonpolar solvent, like hexane, and a rela-
tively polar dithiol crosslinker, namely diethylene dithioglycol
(Scheme 4), was introduced; under these conditions, it may be
expected that the crosslinker will preferentially partition
within the polar core of the HBPE, wherein the residual allyl
groups would also be present. The photo-initiated crosslinking
resulted in the generation of the core-crosslinked HBPE;
despite this precaution, a small amount of insoluble product
was formed due to some crosslinking, which was removed by
filtration using a 0.45 mm filter.
Fig. S2), which is in fair agreement with the FESEM data.
Atomic Force Microscopy (AFM) images of the same sample
also confirmed the formation of spherically aggregates struc-
tures (Supporting Information Fig. S2); however, the concen-
tration of the aggregates was far higher and hence substantial
particle overlap was seen. The line scan in the AFM image
reveals that the average size of these spherical aggregates is
around 200 nm. Given the fairly large size of these aggregates,
they could be either vesicular or a multimicellar aggregate. In
aqueous alkaline medium it might be expected that the inter-
nally carboxylated HBPs would turn inside-out and generate a
jelly-fish like structure with alkyl groups sticking out; such
reconfigured entities can then aggregate to form micelles. It is
clear that simple micelles formed by such inverted molecular
entities cannot exceed 10 nm in size; hence, the large aggre-
gates must be built of several such micelles that aggregate
together to for these giant structures.21
Intramolecular Core Crosslinking
One of the other objectives in this study was to use the
internal allyl groups to generate core-crosslinked hyper-
branched polymers; this could be achieved by performing
the thiol-ene click reaction in the presence of a dithiol. How-
ever, our first attempts to do so under normal conditions led
to the formation of some insoluble cross-linked products;
this clearly suggested that intermolecular process could not
be completely precluded. Core and shell-crosslinked den-
drimers have been previously reported, wherein the cross-
linking reactions were performed under very dilute
The 1H NMR spectra of the partially clicked amphiphilic
HBPE along with core-crosslinked HBPE are shown in Figure
4; the extent of mercaptoethanol incorporation was readily
estimated by comparing the signal intensity of the allyl peak
(a) with that of the benzylic peak (c), and it was established
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SCHEME 4 Schematic representation of the formation of core-crosslinked HBP; the first step renders hydrophilicity to the core
and the second step crosslinks the core.
that about ꢁ70% of the allyl groups was transformed in the
first step, leaving behind about 30% for the second cross-
linking step. The spectrum of the polymer after the second
intramolecular crosslinking step revealed a near complete
disappearance of allyl proton signals, thereby confirming
that the thiol-ene crosslinking process within the core had
occurred effectively. Furthermore, in the case of the core-
crosslinked polymer, a considerable broadening of the peaks
associated with the backbone segments also possibly reflects
the decreased mobility of the core segments due to cross-
linking. Finally, comparison of the GPC traces (Fig. 5) of the
partially clicked amphiphilic HBPE with that of the sample
after core-crosslinking clearly reveals a substantial reduction
in the hydrodynamic volume; the apparent molecular
weights retrieved from this run were Mw 5 72,000 and
Mw 5 50,000, respectively. Although one does not expect a
change in the PDI, as per its definition, a slight decrease
after crosslinking reflects the lowering of the dispersity in
the hydrodynamic sizes upon crosslinking, which is the
parameter that GPC measures. This crosslinking studies
reveal an interesting strategy to access core-crosslinked
hyperbranched polymers utilizing a preferential partitioning
of the crosslinker within the core of a suitably funtionalized
HBP; one important feature of this process is that the polar
functionality installed in the first step could be chosen to
perform other specific functions, such as specific analyte
sequestration, and possibly sensing.
CONCLUSIONS
In conclusion, a novel AB2 monomer carrying a pendant allyl
group in the spacer segment was designed; the monomer car-
ried two decyloxybenzyl groups and a single hydroxyl group at
the far end of the spacer, such that the melt condensation
polymerization under standard acid-catalyzed transetherifica-
tion conditions directly yielded a HBPE that bore numerous
decyl chains on the periphery and a number of allyl groups
within the core. The allyl groups were readily clicked using
photo-initiated thiol-ene reaction to install a large number of
carboxylic acid groups within the core-region of the HBP; two
thiols, namely PA and SA, were used for the click reaction, and
in both cases nearly complete transformation of the allyl
groups was achieved. The clicked HBPs represent an interest-
ing category of polymers where the periphery is hydrophobic
while the core is hydrophilic; the flexibility of the HBP back-
bone confers the macromolecule the adaptability to generate
segregated amphiphilic entities that could self-assemble, like
normal amphiphiles. DLS, AFM, and FESEM studies clearly
demonstrate the formation of fairly uniform spherical aggre-
gates, which are believed to be multimicellar aggregates based
on their large dimensions of ꢁ200 nm. The numerous core
allyl groups were also used to generate core-crosslinked HBPs
(CCHBPs); to achieve this with minimum formation of inter-
chain crosslinked products, a novel strategy was developed
wherein partial transformation of the allyl groups rendered
core–shell amphiphilicity to the system, and, subsequently,
preferential partitioning of a relative polar crosslinker into the
core of the HBP dissolved in a nonpolar solvent substantially
enhanced the intrachain crosslinking process yielding soluble
core-crosslinked HBPs. One novel feature of this process is
generation of single-chain core–shell nanoparticle-like entities,
FIGURE 5 GPC traces of partially clicked parent hyperbranched
polymer; core crosslinked hyperbranched polymer.
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with the potential for varying the nature of the internal func-
tionality for specific applications. We are presently examining
the possibility of using this strategy for creating single-chain
core-crosslinked polymeric nanoparticles for analyte sequestra-
tion and sensing.
(4H, q, ACO2CH2CH3); 5.08 (2H, m, CH2CHCH2CHA); 5.76
(1H, m, CH2CHCH2CHA).
2-Allylpropane-1,3-diol (B)
6.84 g (180 mmol) of LiAlH4 was added to the dry THF in
the ice-cold condition. To this, 18 g (90 mmol) of A
(Scheme 3) was added in dropwise manner and the reaction
mixture was refluxed for 10 h. The reaction was then
quenched very carefully by adding 10% aqueous NaOH. The
dense-white slurry was filtered under suction, the filtrate
was collected, concentrated, and subjeꢀct to distillation in
Kugelrohr under reduced pressure (100 C at 2 torr) to yield
the required product. Yield 5 55%.
EXPERIMENTAL
2,4,6-Trimethylphenol, 1-decanol, 2-mecaptoethanol, trioxane,
p-toluene sulfonyl chloride, lithium aluminum hydride,
sodium hydride, and camphor sulfonic acid were purchased
from Sigma-Aldrich Chemical Company and used directly.
Allyl bromide, diethyl malonate, metallic sodium, AgNO3, and
33% acetic acid solution of HBr were purchased from Spec-
trochem and used directly. 3,5-Bismethoxymethyl-2,4,6-tri-
methyl phenol was synthesized from Mesitol using
previously reported procedure.23 Common organic solvents
were purchased locally and distilled prior to use. 1H NMR
spectra were recorded using a 400 MHz Bruker spectrome-
ter using CDCl3 as solvent, unless mentioned otherwise. DSC
measurements were performed on a Mettler Toledo instru-
ment at a heating rate of 10ꢀ/min; the samples were first
heated to melt, cooled, and the subsequent heating and cool-
ing runs were recorded. For FESEM measurement, the poly-
mer samples are drop-casted on mica substrate; imaging
was performed using FESEM, FEI Nova-Nano SEM-600, The
Netherlands. AFM measurements of the polymer samples
were performed using Nanoscope IVA multimode AFM (Digi-
tal Instrument, Santa Barbara, CA). All the images presented
are tapping mode height images, recorded using a tip of
force constant 2.8 N/m and resonance frequency of 75 kHz.
Image analysis was performed using the software provided
along with the Nanoscope IVA. GPC was performed using a
Viscotek TDA model 300 system, which is coupled to
refractive-index, differential viscometer, and light scattering
detectors in series. The separation was achieved using two
mixed-bed PL gel columns (5 mm, mixed C) maintained at 35
ꢀC; tetrahydrofuran (THF) was used as the eluent. The
molecular weights were estimated using a universal calibra-
tion curve based on polystyrene standards.
1H NMR (d, ppm, CDCl3): 1.91 (1H, m, CH2CHCH2CHA); 2.12
(2H, m, OHCH2CHA); 2.12 (2H, m, CH2CHCH2CHA); 3.66–
3.83 (4H, m, OHCH2CHA); 5.06 (2H, m, CH2CHCH2CHA);
5.81 (1H, m, CH2CHCH2CHA).
2-(Hydroxymethyl)pent-4-enyl 4-Methylbenzenesulfonate
(C)
6.8 g (58.6 mmol) of B along with 20.4 g (88 mmol) of
freshly prepared Ag2O and a catalytic amount of KI were
taken in 150 mL of DCM. To the ice-water bath cooled
reaction mixture, 12.3 g (64.5 mmol) p-toluene sulfonyl
chloride was slowly added using a solid addition funnel;
the progress of the reaction was monitored by TLC and
the reaction was stopped when the ditosylated product
begins to form. After completion, the reaction mixture
was filtered through a celite-pad and the filtrate was con-
centrated under reduced pressure. The monotosylated
product was purified by column chromatography using
petroleum ether and ethyl acetate (60:40) as the eluent.
Yield 5 58%.
1H NMR (d, ppm, CDCl3): 1.91 (1H, m, CH2CHCH2CHA); 2.12
(1H, m, OHCH2CHA); 2.12 (2H, m, CH2CHCH2CHA); 3.61
(2H, m, OHCH2CHA); 4.09 (2H, m, ACHCH2OTs); 5.03 (2H,
m, CH2CHCH2CHA); 5.72 (1H, m, CH2CHCH2CHA); 7.35 (2H,
d, ArCHA); 7.80 (2H, d, Ar’CH).
3,5-Bisdecyloxymethyl-2,4,6-trimethyl phenol (3)24
Synthesis of Monomers
3 g (17.85 mmol, 1 eq.) of monomer 2 (Scheme 4) was taken
in 5 mL of 1-decanol; to this, 0.11 g (0.178 mmol, 0.01 eq.) of
Yb(OTf)3 was added and the system was purged ꢀwith dry N2
for 15 min. The solution was then heated to 100 C with con-
tinuous N2 purging for 4 h. After removal of excess decanol
under reduced pressure, ꢁ15 mL of water was added and the
mixture was extracted with 50 mL of CHCl3. The chloroform
layer was dried using anhydrous sodium sulfate, filtered, and
concentrated under reduced pressure; distillation using a
Kugelrohr apparatus yielded the pure product. Yield5 79%.
Diethyl, 2-Allylmalonate (A)
To a suspension of NaH (9.6 g, 60%, 240 mmol) in THF,
diethyl malonate (76 g, 480 mmol) was slowly added in the
ice-cold condition; excess malonate was used to minimize
the formation of the diallyl product. After 1 h, allyl bromide
(29.1 g, 240 mmol) was added to the reaction mixture and
the mixture was stirred for 12 h at room temperature. The
reaction was quenched by addition of saturated solution of
ammonium chloride and extracted in ethyl acetate. The ethyl
acetate layer was washed with brine, dried with anhydrous
sodium sulfate, and concentrated under reduced pressure.
Fractional distillation yielded the pure product as the second
fraction (yield 5 80% with respect to allyl bromide).
1H NMR (d, ppm, CDCl3): 0.88 (6H, t, ArCH2-
OCH2CH2(CH2)7CH3); 1.26 (28H, m, ArCH2OCH2CH2(CH2)7
CH3); 1.59 (4H, m, ArCH2OCH2CH2(CH2)7CH3); 2.31 (6H, s,
Ar(CH3)2); 2.35 (s, 3H, ArCH3); 3.48 (t, 4H, ArCH2OCH2
CH2(CH2)7CH3); 4.47 (s, 4H, ArCH2OCH2CH2(CH2)7CH3); 4.72
(s, 1H, ArOH).
1H NMR (d, ppm, CDCl3): 1.27 (6H, t, ACO2CH2CH3); 2.66
(2H, m, CH2CHCH2CHA); 3.42 (1H, t, CH2CHCH2CHA); 4.21
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2-((3,5-Bis((decyloxy)methyl)22,4,6-
trimethylphenoxy)methyl)pent-4-en-1-ol (4)
nol; the polymer was further purified by dissolution in
CHCl3 and reprecipitation into methanol to yield the HBP-PA
(yield 5 64%; molecular weight: Mn 5 13,400).
NaH (0.18 g, 60%, 4.41 mmol) was taken in 60 mL of THF,
the solution was purged with dry N2 for a few minutes and
then the monomer 3 (2.1 g, 4.41 mmol) was slowly added to
it. 1.90 g (7.1 mmol, 1.6 eq.) of C was added dropwise to the
reaction mixture and the contents were refluxed for 72 h
under N2 atm. The reaction was quenched by adding 50 mL of
water and then extracted in diethyl ether. To remove unreacted
3, the ether layer was washed with 10% NaOH solution. The
ether layer was dried using anhydrous sodium sulfate and con-
centrated under reduced pressure. The product was purified
by the column chromatography using petroleum ether and
ethyl acetate (75:25) as the eluent. Yield5 47%.
1H NMR (d, ppm, CDCl3): 0.87 (3H, t, ArCH2-
OCH2CH2(CH2)7CH3); 1.25 (14H, m, ArCH2OCH2CH2(CH2)7
CH3); 1.62 (2H, m, ArCH2OCH2CH2(CH2)7CH3); 1.91 (1H, m,
ASCH2CH2CH2CHA); 2.01 (2H, m, ASCH2CH2CH2CHA); 2.12
(2H, m, ASCH2CH2CH2CHA); 2.28 (6H, s, Ar(CH3)2); 2.35 (s,
3H, ArCH3); 2.57 (t, 2H, CO2HCH2CH2SCH2A); 2.57 (t, 2H,
CO2HCH2CH2SCH2A); 2.75 (m, 2H, CO2HCH2CH2SCH2A);
3.48 (t, 2H, ArCH2OCH2CH2(CH2)7CH3); 3.63 (2H, m,
AOCH2CHA); 3.74 (m, 2H, ArOCH2CHA); 4.5 (s, 4H,
ArCH2OCH2CH2(CH2)7CH3).
1H NMR (d, ppm, CDCl3): 0.88 (6H, t, ArCH2-
OCH2CH2(CH2)7CH3); 1.26 (28H, m, ArCH2OCH2CH2(CH2)7
CH3); 1.59 (4H, m, ArCH2OCH2CH2(CH2)7CH3); 1.91 (1H, m,
CH2CHCH2CHA); 2.12 (2H, m, CH2CHCH2CHA); 2.31 (6H, s,
Ar(CH3)2); 2.35 (s, 3H, ArCH3); 3.48 (t, 4H, ArCH2-
OCH2CH2(CH2)7CH3); 3.72 (2H, m, OHCH2CHA); 3.86 (m, 2H,
ArOCH2CHA); 4.47 (s, 4H, ArCH2OCH2CH2(CH2)7CH3); 5.03
(2H, m, CH2CHCH2CHA); 5.72 (1H, m, CH2CHCH2CHA).
Thiol-ene Reaction with SA (HBP-SA)
A mixture of HBP-Allyl (100 mg, 0.23 mmol) and 2-SA (104
mg, 0.69 mmol) were taken in 5 mL CHCl3, along with the
photo-initiator, 2,2-dimethoxy 2-phenyl acetophenone (2 mg,
0.008 mmol). The contents were irradiated using a 150 W
Hg-vapor lamp for 4 h. The polymer solution was then con-
centrated under reduced pressure and precipitated in diethyl
ether; the polymer was further purified twice by dissolution
in CHCl3 and reprecipitation into diethyl ether to yield HBP-
SA (yield 5 73%; molecular weight Mn 5 12,700).
Synthesis of Polymers
HBP-allyl: Monomer 4 (1.1 g, 1.916 mmol) along with 2 mol
% of pyridinium camphorsulfonate (PCS) was taken in a
test-tube shaped polymerization vessel. The mixture was
degassed for 10 min and maintained at a temperature of
110 ꢀC under continuous N2 purge, to ensure homogeneous
mixing of catalyst and monomers. The polymerization was
then performed at 150 ꢀC under N2 for 2 h with constant
1H NMR (d, ppm, CDCl3): 0.87 (3H, t, ArCH2-
OCH2CH2(CH2)7CH3); 1.25 (14H, m, ArCH2OCH2CH2(CH2)7
CH3); 1.62 (2H, m, ArCH2OCH2CH2(CH2)7CH3); 1.91 (1H, m,
ASCH2CH2CH2CHA); 2.01 (2H, m, ASCH2CH2CH2CHA); 2.12
(2H, m, ASCH2CH2CH2CHA); 2.31 (6H, s, Ar(CH3)2); 2.35 (s,
3H, ArCH3); 2.65 (q, 2H, CO2HCH2CH(CO2H)SCH2A); 2.65 (t,
2H, CO2HCH2CH(CO2H)SCH2A); 2.95 (m, 1H, CO2HCH2CH
(CO2H)SCH2A); 3.48 (t, 2H, ArCH2OCH2CH2(CH2)7CH3); 3.63
(2H, m, AOCH2CHA); 3.74 (m, 2H, ArOCH2CHA); 4.5 (s, 4H,
ArCH2OCH2CH2(CH2)7CH3).
€
stirring. Subsequently, using a Kugelrohr apparatus, the poly-
merization was continued for an additional period of 45 min
at 150 ꢀC under reduced pressure (2 torr), with continuous
mixing of the melt by rotation. The resultant polymer was
dissolved in THF, the acid-catalyst was neutralized with solid
NaHCO3, and then the solution was filtered. The filtrate was
concentrated under reduced pressure to a viscous solution
and precipitated in methanol. The polymer was further puri-
fied twice by dissolution in THF and reprecipitation into
methanol (yield 5 70%; molecular weight Mn 5 12,000).
Diethylene Glycol Ditosylate
Diethylene glycol (5 g, 47.15 mmol) was taken in 20 mL of
THF and the contents were cooled to 0 ꢀC. To this solution,
11.32 g (283 mmol) of KOH in 20 mL of water was added
and the contents were stirred for 30 min. 27 g (141.5 mmol)
of tosyl chloride was taken in 50 mL of THF and added
dropwise to the cold reaction mixture; the contents were
stirred overnight. After completion, the organic layer was
separated, dried, and concentrated under reduced pressure.
CHCl3 (50 mL) was added to the concentrated reaction mix-
ture and the precipitated salt was filtered. The concentrated
filtrate was used in the next step without any further purifi-
cation. Crude yield 5 90%.
1H NMR (d, ppm, CDCl3): 0.87 (3H, t, ArCH2-
OCH2CH2(CH2)7CH3); 1.25 (14H, m, ArCH2OCH2CH2(CH2)7
CH3); 1.59 (2H, m, ArCH2OCH2CH2(CH2)7CH3); 1.91 (1H, m,
CH2CHCH2CHA); 2.12 (2H, m, CH2CHCH2CHA); 2.31 (6H, s,
Ar(CH3)2); 2.35 (s, 3H, ArCH3); 3.48 (t, 2H, ArCH2-
OCH2CH2(CH2)7CH3); 3.63 (2H, m, AOCH2CHA); 3.74 (m,
2H, ArOCH2CHA); 4.47 (s, 4H, ArCH2OCH2CH2(CH2)7 CH3);
5.03 (2H, m, CH2CHCH2CHA); 5.72 (1H, m, CH2CHCH2 CHA).
1H NMR (d, ppm, CDCl3): 3.56 (2H, t, AOCH2CH2OSOOAr);
3.70 (2H, t, AOCH2CH2OSOOAr); 7.46 (2H, d, AOCH2CH2OSO
OAr-H); 7.75 (2H, d, AOCH2CH2OSOOAr-H).
Thiol-ene Reaction with PA (HBP-PA)
A mixture of HBP-Allyl (100 mg, 0.23 mmol) and PA (74
mg, 0.69 mmol) were taken in 5 mL CHCl3, along with the
photo-initiator, 2,2-dimethoxy 2-phenyl acetophenone (2 mg,
0.008 mmol). The contents were irradiated using a 150 W
Hg-vapor lamp for 4 h. The polymer solution was then con-
centrated under reduced pressure and precipitated in metha-
Diethylene Dithioglycol
Twenty grams (48.3 mmol) of the above ditosylate along
with 11 g (145 mmol) of thiourea was taken in 100 mL of
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Mater. 1998, 10, 286–296; (d) K. Inoue, Prog. Polym. Sci.
2000, 25, 453–571; (e) R. Duncan, Polym. Mater. Sci. Eng.
2001, 84, 214; (f) J. L. Hedrick, T. Magbitang, E. F. Connor,
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ethanol and the contents were refluxed for 6 h. After cooling
the solution to room temperature, 4.38 g (120 mmol) of
KOH in 20 mL of water was added. The reaction mixture
was further refluxed for 6 h under N2 atmosphere. After
completion, the reaction mixture was neutralized with dilute
HCl and concentrated under reduced pressure. Twenty milli-
liters of water was added and the contents were extracted
twice with 50 mL of ethyl acetate. The combined organic
layer was dried using anhydrous sodium sulfate and concen-
trated under reduced pressure to yield the pure product in
75% yield.
ꢀ
3 S. Hecht, J. M. J. Frechet, Angew. Chem. Int. Ed. 2001, 40,
74–91.
ꢀ
4 A. Adronov, J. M. J. Frechet, Chem. Commun. 2000, 1701–
1710.
5 (a) R. Kreiter, A. W. Kleij, R. J. M. Klein Gebbink, G. van
Koten, Top. Curr. Chem. 2001, 217, 163–199; (b) G. E.
Oosterom, J. N. H. Reek, P. C. J. Kamer, P. W. N. M. Van
Leeuwen, Angew. Chem. Int. Ed. 2001, 40, 1828–1849; (c) L. J.
Twyman, A. S. H. King, I. K. Martin, Chem. Soc. Rev. 2002, 31,
69–82.
1H NMR (d, ppm, CDCl3): 1.7 (1H, t, SHCH2CH2OA); 2.73
(2H, q, SHCH2CH2OA); 3.67 (2h, t, SHCH2CH2OA).
Thiol-ene Reaction with Mercaptoethanol (HBP-ME)
A mixture of HBP-Allyl (200 mg, 0.46 mmol) and mercapto-
ethanol (32 mg, 0.41 mmol) were taken in 5 mL CHCl3, along
with the photo-initiator, 2,2-dimethoxy 2-phenyl acetophe-
none (4 mg, 0.015 mmol). The contents were irradiated
using a 150 W Hg-vapor lamp for 4 h. The polymer solution
was then concentrated under reduced pressure and precipi-
tated in methanol; the polymer was further purified twice by
dissolution in CHCl3 and reprecipitation into methanol to
yield the partially clicked HBP-ME. Yield 5 70% (molecular
weight: Mw 5 72,000).
6 (a) J. F. G. A. Jansen, E. M. M. de Brabander-van den Berg,
E. W. Meijer, Science 1994, 266, 1226–1229; (b) J.-P. Majoral,
A.-M. Caminade, Chem. Rev. 1999, 99, 845–880; (c) G. R.
Newkome, E. He, C. N. Moorefield, Chem. Rev. 1999, 99, 1689–
1746.
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7 (a) J. M. Serin, D. W. Brousmiche, J. M. J. Frechet, Chem.
Commun. 2002, 2605; (b) J. M. Serin, D. W. Brousmiche, J. M.
ꢀ
J. Frechet, J. Am. Chem. Soc. 2002, 124, 11848–11849; (c) T.
Weil, E. Reuther, K. Mullen, Angew. Chem. Int. Ed. 2002, 41,
1900–1904; (d) S. Thayumanavan, Photosynth. Res. 2006, 87,
133–150.
8 M. E. Piotti, F. J. Rivera, R. Bond, C. J. Hawker, J. M. J.
1H NMR (d, ppm, CDCl3): 0.87 (3H, t, ArCH2-
OCH2CH2(CH2)7CH3); 1.25 (14H, m, ArCH2OCH2CH2(CH2)7
CH3); 1.62 (2H, m, ArCH2OCH2CH2(CH2)7CH3); 1.91 (1H, m,
ASCH2CH2CH2CHA); 2.01 (2H, m, ASCH2CH2CH2CHA); 2.12
(2H, m, ASCH2CH2CH2CHA); 2.25–2.38 (m, 9H, Ar-CH3);
2.59 (t, 2H, ACH2CH2SCH2CH2OH); 2.65 (t, 2H, CH2CH2SCH2
CH2OH); 3.59–3.64 (m, 8H, ArOCH2CH2CH2CH2OH, ArCH2-
OCH2A, ArSCH2CH2OH); 4.49 (m, 4H, Ar-CH2OA); 5.034
(2H, m, CH2CHCH2CHA); 5.72 (1H, m, CH2CHCH2CHA).
ꢀ
Frechet, J. Am. Chem. Soc. 1999, 121, 9471–9472.
9 (a) S. E. Stiriba, H. Kautz, H. Frey, J. Am. Chem. Soc. 2002,
124, 9698–9699; (b) E. Burakowska, J. R. Quinn, S. C.
Zimmerman, R. Haag, J. Am. Chem. Soc. 2009, 131, 10574–
10580; (c) M. Kramer, J. F. Stumbe, H. Turk, S. Krause, A.
Komp, L. Delineau, S. Prokhorova, H. Kautz, R. Haag, Angew.
Chem. Int. Ed. Engl. 2002, 41, 4252–4256.
ꢀ
10 R. Haag, J.-F. Stumbe, A. Sunder, H. Frey, A. Hebel, Macro-
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11 A. V. Ambade, A. Kumar, J. Polym. Sci., Part A: Polym.
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Core-crosslinking Using Diethylene Dithioglycol
12 C. E. Hoyle, C. N. Bowman, Angew. Chem. Int. Ed. 2010, 49,
A mixture of the partially clicked polymer HBP-ME (100 mg)
along with the photo-initiator, 2,2-dimethoxy 2-phenyl aceto-
phenone (4 mg), was taken in 5 mL of hexane. To this reac-
tion mixture, a stoichiometric amount of the dithiol (with
respect to residual allyl groups) was added slowly. The con-
tents were irradiated using a 150 W Hg-vapor lamp for 4 h.
The polymer solution was then concentrated under reduced
pressure and precipitated in methanol; the polymer was fur-
ther purified by dissolution in CHCl3 and reprecipitation into
methanol to yield the core-crosslinked hyperbranched poly-
mer. Yield 5 56% (molecular weight Mw 5 50,000).
1540–1573.
13 M. J. Kade, D. J. Burke, C. J. Hawker, J. Polym. Sci., Part A:
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