Control of molecular weight and polydispersity of hyperbranched polymers using a reactive B3 core: A single-step route to orthogonally functionalizable hyperbranched polymers

Article abstract of DOI:10.1021/ma201817a

Molecular weight and polydispersity are two structural features of hyperbranched polymers that are difficult to control because of the statistical nature of the step-growth polycondensation of AB₂ type monomers; the statistical growth also caus

Full text of DOI:10.1021/ma201817a

ARTICLE
pubs.acs.org/Macromolecules
Control of Molecular Weight and Polydispersity of Hyperbranched
Polymers Using a Reactive B3 Core: A Single-Step Route to
Orthogonally Functionalizable Hyperbranched Polymers
Raj Kumar Roy and S. Ramakrishnan*
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India
S Supporting Information
b
ABSTRACT: Molecular weight and polydispersity are two
structural features of hyperbranched polymers that are difficult
to control because of the statistical nature of the step-growth
polycondensation of AB2 type monomers; the statistical growth
also causes the polydispersity index to increase with percent
conversion (or molecular weight). We demonstrate that using
controlled amounts of a specifically designed B3 core, contain-
ing B-type functionality that are more reactive than those
present in the AB2 monomer, both the molecular weight and
the polydispersity can be readily controlled; the PDI was shown
to improve with increasing mole-fraction of the B3 core while the polymer molecular weight showed an expected decrease.
Incorporation of a “clickable” propargyl group in the B3 core unit permitted the generation of a core-functionalizable hyperbranched
polymer. Importantly, this clickable core, in combination with a recently developed AB2 monomer, wherein the B-type groups are
allyl ethers and A is an hydroxyl group, led to the generation of a hyperbranched polymer carrying orthogonally functionalizable core and
peripheral groups, via a single-step melt polycondensation. Selective functionalization of the core and periphery using two different types
of chromophores was achieved, and the occurrence of fluorescence resonance energy transfer (FRET) between the donor and
acceptor chromophores was demonstrated.
’ INTRODUCTION
theoretical basis for the use of such reactive polyfunctional cores
in regulating the molecular weight and polydispersity of hyper-
branched polymers was described a few years earlier.4 Bharathi
and Moore, on the other hand, developed a solid-phase supported
synthesis to regulate the molecular weight and PDI of hyper-
branched polymers.5 Very recently, Yokozawa and co-workers
extended their interesting pseudochain growth condensation
polymerization approach for the preparation of hyperbranched
polymers with very good control over both molecular weight and
PDI; this approach, however, may be applicable only to certain
classes of polymers.6
Over a decade ago, we developed a novel melt-condensation
technique for the preparation of linear7 and hyperbranched
polyethers8 that proceeds via a trans-etherification process. This
process occurs via the condensation of a benzyl methyl ether
moiety with a hydroxyl group, in the presence of an acid catalyst,
with the liberation of methanol. The equilibrium is driven to
polymer formation by continuous removal of low-boiling metha-
nol, which in turn makes this process a completely reversible one.
Thus, the AB2 monomer (I) (see Figure 1), which is prepared from
Mesitol in three steps,9 self-condenses to generate hyperbranched
Hyperbranched polymers are often prepared by the self-
condensation polymerization of a suitably designed AB2 mono-
mer. Despite the ease of its synthesis, several structural features of
hyperbranched polymers cannot be readily controlled, unlike in
the case of its main rival, namely dendrimers. Among the structural
imperfections present in hyperbranched polymers, the important
ones are as follows: imprecise control over molecular weight,
high polydispersity, absence of a well-defined core, and finally the
presence of a large number of linear defects.1 While the functional
ramifications of these structural imperfections on the properties
and utility of hyperbranched polymers may not be as severe as
was initially perceived, it would still be desirable to develop simple
approaches to minimize these imperfections. Some years ago,
Frey and others demonstrated that use of a polyfunctional core,
in conjunction with slow monomer-addition, can generate hyper-
branched polymers with good control over both the molecular
weight and defect levels.2 This approach works best when the
polymerization rate is reasonably fast, such that most of the
monomer is rapidly consumed as the slow addition is in progress.
An alternate approach was developed by Fossum and co-workers
for the preparation of poly(arylene ether phosphine oxide)s, where-
in a B3 core that bears more reactive B groups was condensed
with an AB2 monomer in a single step to yield polymers with
significantly improved molecular weight and polydispersity;3 the
Received: August 8, 2011
Revised:
September 20, 2011
Published: October 05, 2011
r
2011 American Chemical Society
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polyethers of moderately high molecular weights, although with
high polydispersity. In an effort to regulate the polymerization
process, we have designed a B3 core (II) starting from 1,3,5-
trimethoxybenzene; the presence of the three methoxy groups on
the aromatic ring should render it more susceptible to the acid-
catalyzed trans-etherification process than the AB2 monomer.
In this report, we explore the utility of this specifically designed
B3 core (II) to regulate the molecular weight and polydispersity
of hyperbranched polyethers prepared using monomer I. Further-
more, we designed an alternate core (III), wherein one of the
methoxy groups on the aromatic ring was replaced by a propargyloxy
group; this permitted us to incorporate a single “clickable” unit at
the core of the hyperbranched structure. A similar idea of functio-
nalizing the core of hyperbranched polymers using a comonomer
carrying a labile functionality that could be later transformed as
desired was demonstrated by Twyman and co-workers.10 Finally,
Figure 1. Structures of monomers and core molecules.
Figure 2. Variation of 1H NMR spectra of the reaction mixture (only the relevant region is shown) as a function of time, during the competitive trans-
etherification of the two cores II and IIa with dodecanol. The peaks marked by arrows are due to the methoxy protons (CH3OCH2Ph) belonging to the
two cores, and the numbers above reflect their intensity ratios.
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Figure 3. 1H NMR spectra of the hyperbranched homopolymer, along with those of the copolymers using various mole-fractions of the B3 core II. The
expanded region shows the peaks corresponding to the methoxy protons (CH3OPh) linked directly to the aromatic core.
Table 1. Synthesis, Composition, and Molecular Weights of Core-Containing HBPs
polymer
mol % of core in the feed
mol % of core incorporated
yield (%)
Mn(NMR)
Mn(GPC)
PDI
DPn(GPC)
A
B
0.0
2.5
0.0
2.0
4.1
6.0
89
86
78
65
-
13700
6800
9300
10100
8100
2.78
2.58
2.22
1.46
35
38
30
20
C
D
5.0
10.0
4900
5400
using the core molecule III, along with a recently reported AB2
monomer (IV),11 we synthesized hyperbranched polyethers that
carry a single propargyl group at the core and numerous peripheral
allyl groups; this permitted the orthogonal functionalization of
the core and periphery using the azideÀyne and thiolÀene click
reactions, respectively.
groups in core II, when compared to those in monomer I; this
is because the incipient positive charge that develops at the
benzylic position during the acid-catalyzed polymerization would
be significantly more stabilized by the three methoxy substitu-
tents on the aromatic ring of the core. In order to compare the
relative ease of displacement of the methoxy groups in the core II
versus the AB2 monomer, a model B3 core, namely trimethox-
ymethyl mesitylene, (IIa; see Figure 2) with roughly similar electro-
nic effects as those present in the monomer was prepared from
mesitylene. In a control experiment, equal moles of both these
cores, II and IIa, were reacted with the required amount of
dodecanol (a high boiling alcohol) at 150 °C, in the presence of
pyridinium camphor sulfonate (PCS) as the catalyst; the condi-
tions were chosen to be identical to those during a typical melt
’ RESULTS AND DISCUSSION
Design and Reactivity of the Core. The B3 core was prepared
in two steps starting from 1,3,5-trimethoxybenzene; exhaustive
bromomethylation followed by treatment of the tris-bromo-
methyl product with sodium methoxide in refluxing methanol
yielded the desired product in good yields. The AB2 monomer
(I), bearing a C4 spacer, was selected for this study; this was
prepared using a procedure that was reported earlier.12 On the
basis of the structures of the core molecule II and the monomer I,
it may be expected that the acid-catalyzed trans-etherification
process would occur more rapidly with the methoxymethyl
1
polymerization. A stack plot of the variation of the H NMR
spectra of the reaction mixture as a function of reaction time is
shown in Figure 2. Since the two cores exhibit distinct peaks
corresponding to the methoxy (CH3OCH2Ph) protons (at 3.40
and 3.43 ppm) and the benzylic (CH3OCH2Ph) protons (4.45
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Figure 4. 1H NMR spectra of the hyperbranched homopolymer, along with those of the butoxymethyl core (IIb) core and the copolymer. The
expanded region of the copolymer spectra (top) shows the nearly complete set of the terminal methyl protons (∼0.9 ppm) of the butoxy group of
the core.
and 4.50 ppm), either of these peaks could be used to follow the
relative rates of trans-etherification; in the present study, the former
was the preferred as their intensities decrease due to loss of the
methoxyl group during the reaction. From the variation in their
relative intensities, it is evident that the peak at 3.43 ppm belonging
to core II vanishes at a significantly faster rate compared to that
belonging to IIa, at 3.4 ppm. As the reaction progresses, the
benzylic region of the spectra begins to exhibit two additional
peaks (one each from II and IIa), which correspond to those
benzylic protons after displacement of methoxy group by a dode-
cyloxy one; the presence of multiple peaks in this region makes it
clearly less conducive for following the kinetics. A plot of the
variation of the methoxy proton peak intensities as a function of
time (see Figure S1, Supporting Information) permitted us to
retrieve the relative rate constants for trans-etherification; a roughly
3-fold enhancement in the rate constant was observed for core II
(as evident from the differences in the slopes), confirming that
the electron-rich core can indeed serve effectively to regulate the
condensation process.
Copolymerization and Molecular Weight Regulation. The
copolymerization of the AB2 monomer I was carried out using
three different amounts of the B3 core II: 2.5, 5, and 10 mol %.
The proton NMR spectra of the homopolymer and those of the
three copolymers are presented in Figure 3; although the
benzylic protons due to the core are not clearly discernible due
to overlap, the presence of the aromatic methoxy protons confirms
the incorporation of the core. The extent of incorporation of the
core was estimated from the relative intensities of the core methoxy
protons (CH3OÀAr ∼ 3.95 ppm) compared to those of aromatic
methyl protons (ArÀCH3 ∼ 2.3À2.4 ppm) belonging to the
polymer repeat unit, and the values thus obtained are listed in
Table 1.
The core incorporation level is seen to vary in accordance with
the feed composition, although the incorporation levels esti-
mated by NMR were consistently slightly lower. Use of a B3 core
leads to a stoichiometric imbalance and, therefore, would also
lead to a lowering of the molecular weight. Since the B3 core has
the potential to anchor three directions for growth, it can be
easily shown that the effect of the B3 core on the molecular
weight will be identical to that of a monofunctional impurity
during a simple AÀB type polymerization, and the degree of
polymerization will vary as follows: DP = (1 + r)/(1 + r À p),
where r is the mole ratio of the B3 core to the AB2 monomer and p
is the percent conversion.4 Since, all the polymer samples were
purified by reprecipitation, the measured DP cannot be compared
directly with those calculated based on the above formula; instead,
we compared the Mn values obtained by two different ap-
proaches, one by NMR end-group analysis and the other by
GPC. The Mn values determined by GPC and those estimated by
proton NMR (by direct comparison of the peak intensities of the
ArÀOCH3 belonging to the core with those of the ArÀCH3
peaks belonging to the repeat unit) are listed in Table 1; in the
case of the highest Mn sample the GPC value is a bit lower, while
in the other two cases the GPC value is slightly higher. The may
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Scheme 1. Orthogonally Clickable Hyperbranched Polymera
could not be differentiated in the NMR spectra (the δ values are
almost the same; see Figure S2, Supporting Information) making
it difficult to confirm the completeness of the trans-etherification
of the core. To confirm that all the three locations of the core
have reacted during the copolymerization, we designed an
alternate core (IIb) where the displaceable methoxy groups were
replaced by butoxy ones. The NMR spectra of IIb, along with
those of the homopolymer and the copolymer (using 5 mol % of
this core) are shown in Figure 4. The nearly complete disap-
pearance of the peak due to the terminal methyl protons of the
butoxy group (at ∼0.9 ppm) in the copolymer confirms that
complete trans-etherification of the core unit had indeed oc-
curred. Despite this, the aromatic methoxy protons of the core
unit (at ∼4.0 ppm) appear as two peaks suggesting that the origin
of this could be due to some other reason. As the hyperbranched
polymeric segments (could be called hyperons akin to dendrons)
linked to the core are fairly bulky, they are likely to be located on
either side of the aromatic plane; in other words, two of the
hyperons could be pointing downward while one could be upward;
this in turn would force the aryl methoxy groups to be disposed
similarly. A rough intensity ratio of 1:2 of the two peaks, in all the
three copolymers (see Figure 3), suggests that this could be a
possible reason for the splitting.15 Search of the literature revealed
that a similar conformational effect on the NMR spectra has been
observed in a mesitylene derivative carrying bulky substitutents
on the remaining three locations of the benzene core.16
The use of the butoxylated core IIb also permitted the direct
examination of the relative rates of the trans-etherification of the
B-groups belonging to the core and the monomer I during the
copolymerization, since the terminal methyl protons of the
butoxy group (0.9 ppm) and the methoxy protons (3.4 ppm)
of the monomer could be independently followed. The variation
of the NMR spectra and the relevant peak intensities during the
copolymerization (Figures S3 and S4, Supporting Information)
helped reconfirm the rapid and complete trans-etherification of
the core during the copolymerization; this further validates the
efficacy of the new reactive core to function as an efficient
regulator of the condensation process.
Orthogonal Functionalization of the Core and Periphery.
Having demonstrated that the trialkoxybenzene core works ef-
ficiently to regulate the molecular weight, and consequently is
incorporated within the apparent core of the hyperbranched
polymer, we decided to examine the possibility of incorporating a
functionalizable core unit by suitably modifying the core struc-
ture. In order to do so, we designed a reactive B3 core where one
of the methoxy groups is replaced by a propargyloxy unit; this
when incorporated in the hyperbranched polymer would provide
a clickable unit at the core. Thus, an alternate core, namely III
(see Scheme 1), was prepared by selective demethylation of
1,3,5-trimethoxybenzene17 followed by alkylation with propargyl
bromide; subsequent steps to prepare the core were done as in
case of II. Further, in an effort to generate orthogonally clickable
hyperbranched polymers where the core and the periphery could
be clicked independently, we prepared copolymers using the
azide-yne clickable core III and a recently developed AB2 monomer
IV that carries allyloxymethyl groups instead of the methoxyl-
methyl groups, as depicted in Scheme 1. We had recently showed
that the monomer IV readily undergoes polymerization to yield a
clickable hyperbranched polymer, which can further be clicked
quantitatively using a variety of thiols.11 In the present study, we
chose to click the core first with an azido-chromphore based on
coumarin and subsequently the peripheral allyl groups were
a The core carries an azide-clickable unit while the periphery carries
numerous thiol-clickable units.
be due to the fact that the GPC slightly underestimates the Mn
value at higher molecular weights but the effect is negligible at
low molecular weights.13 The polydispersity index (PDI) of the
sample is also seen to be influenced by the incorporation of the
core; it is seen to decrease from 2.78 in the homopolymer to
about 1.46 at the highest level of core incorporation.14 This
lowering of the PDI is most likely due to the lower molecular
weights of the resulting polymers; this is an expected behavior during
the polycondensation of AB2 type monomers during which the PDI
increases with percent conversion (DPn). The yields of the polymer
are also seen to decrease with increase in core mole fraction; this is
possibly due to the loss of low molecular weight oligomers during
reprecipitation, which expectedly will affect the lower molecular
weight samples more severely. Thus, it is evident that a suitably
designed B3 core bearing more reactive B-groups works effectively to
regulate both the molecular weight as well as the PDI of hyper-
branched polymers; unfortunately, however, the lowering of PDI is
always accompanied by a reduction in DP, which is an intrinsic
limitation of this process.
One intriguing feature of the spectra of the core-containing
copolymers is the appearance of two peaks corresponding to the
methoxy protons (CH3OÀAr) attached to the core unit; one
possible reason for this could be the incomplete trans-etherifica-
tion which would cause a differentiation between the three aryl-
methoxy protons on the core. Unfortunately, the methoxymethyl
protons (CH3OCH2À) belonging to the monomer and the core
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Figure 5. 1H NMR spectra of the hyperbranched copolymer carrying an azide-yne clickable core and numerous thiolÀene clickable periphery, along
with those of core-clicked and core and peripherally clicked polymers.
clicked with a dialkoxynaphthalene chromophore carrying a pen-
dant thiol unit; this pair of chromophores was chosen to facilitate
FRET from the periphery to the core in an effort to mimic the
light-harvesting dendrimers that have been reported earlier.18
The 2-ethylhexyl groups on the naphthalene chromophore was
required to ensure the solubility of the polymer after complete
clicking with the naphthalene units; our first attempt with the
simple methoxynaphthalene units rendered insoluble polymers
after clicking, possibly due to strong tendency of the naphthalene
units to form dimers leading to physical cross-linking. In Figure 5,
the proton NMR spectra of the core containing peripherally
clickable hyperbranched polymer, along with those that have
been clicked with the FRET pair of chromophores, are shown. The
spectrum of the parent polymer clearly showed the presence of a
large number of allyl groups, while the y-expanded region shows a
weak peak due to the incorporation of the propargyl core. After
clicking the core using the Coumarin azide, there is near complete
disappearance of the terminal propargyl triplet. On the other hand,
the thiolÀene click reaction with the dialkoxynaphthalene-bearing
thiol resulted in the transformation of nearly 80% of the allyl groups
leaving behind small residual peaks in the NMR spectrum, as evident
from the expanded depiction of the allyl region.
The fluorescence spectra of the core-clicked and core and
periphery clicked HBPs are shown in Figure 6. Comparison of
the core-clicked HBP with a suitable model chromophore prepared
using the monomer and the coumarin azide showed that the
spectral position matched well, although the spectral shape of the
model was devoid of any fine structure.19 On the other hand, the
fluorescence spectra of the naphthalene clicked HBP and that of a
model matched perfectly, suggesting that anchoring them on the
HBP did not influence their fluorescence spectra. The fluorescence
spectrum of the HBP containing both the Donor and Acceptor
chromophores exhibited peaks both due to the naphthalene
donors and the coumarin acceptor, although the excitation was
done at 295 nm, where the absorption is primarily due to the
naphthalene chromophore. This suggests that FRET has indeed
occurred although it is incomplete; this may be expected as a very
large number of Donors are present in the HBPs while only a
single acceptor is present.20 In an effort to improve the FRET ef-
ficiency, the average distance between the donor and acceptor
could be decreased. One way to achieve this is to desolvate the
polymer chain by adding a suitable nonsolvent; this would cause
the chain to collapse and in turn bring the peripheral donors
closer to the core acceptor unit. To test this hypothesis, a solution
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Ltd. and used directly. Common organic solvents were purified and/or
distilled prior to use. AB2 monomers I and IV and the catalyst PCS were
synthesized using previously reported procedures.12 The clickable
reactive core III was prepared by selective deprotection of 1,3,5-
trimethoxybenzene followed by alkylation with propargybromide; sub-
sequent steps were similar to that used for core II. The coumarin azide
was prepared using a reported procedure21 while the naphthalene thiol
derivative was prepared in four steps from 1,5-naphthalene diol using
some standard methodologies.22
1H NMR spectra was recorded in 400 MHz Bruker spectrometer
using CDCl3 as solvent unless otherwise mentioned. Gel permeation
chromatography (GPC) was carried out using a Viscotek TDA model
300 system, coupled to refractive-index, differential viscometer, and light
scattering detectors, in series. The separation was achieved with two
mixed-bed PLgel columns (5 μm, mixed C) maintained at 35 °C, with
tetrahydrofuran (THF) as the eluent. The molecular weights were deter-
mined using a universal calibration curve based on polystyrene stan-
dards. Fluorescence measurements were carried out using a Jobin Yvon
Fluorolog spectrometer; all measurements were done using μM con-
centration solutions of the model compound or polymer.
Figure 6. Fluorescence spectra of coumarine azide clicked model com-
pound (black, 0.11 μM excited at 335 nm); naphthalene thiol clicked model
compound (blue, 0.08 μM, excited at 295 nm); coumarin azide clicked
HBP 3a (green, 0.035 μM, excited at 335 nm) and coumarin and
naphthalene clicked HBP 3b (red, 0.035 μM, excited at 295 nm).
1,3,5-Tris(bromomethyl)-2,4,6-trimethoxybenzene (1).23 To
a mixture of 1,3,5-trimethoxybenzene (10 g, 60 mmol) and paraformal-
dehyde (6.42 g, 214 mmol) in a pressure tube, glacial acetic acid was
added (30 mL) and the resulting suspension was allowed to stir for 1 h.
HBr (33% in HOAc, 63 mL) was then added and the reaction mixture
was heated to 70 °C for additional 3 h under a tight sealed. After cooling
to room temperature, the reaction mixture was poured into water (250 mL)
and precipitated solids were dissolved in chloroform. The organic layer
was separated and the aqueous solution was extracted with chloroform.
All organics were combined and washed with saturated aqueous NaHCO3,
water, brine; the solution was concentrated to yield orange oil. The
crude products were purified by column chromatography using hex-
anes/ethyl acetate (19:1) as eluting solvent to get the desired product
(7.98 g, 30%, MP = 125 °C) as a white crystalline solid.
of the doubly clicked HBP was taken in THF and water was
added to it. From the normalized spectra (Figure S5, Supporting
Information), it was evident that the efficiency of the FRET does
go up substantially in the presence of water although complete
disappearance of the donor fluorescence was still not observed.
In conclusion, we have demonstrated that the molecular
weight and PDI of hyperbranched polyethers, prepared via the
trans-etherification process, can be controlled by the use of a
more reactive B3 core along with the AB2 monomer. The DP of
the HBP decreased with increase in the relative mole-fraction of
the B3 core; this is in general accordance with earlier theoretical
predications.4 As the design of the core was based on 1,3,5-
trimethoxybenzene, replacement of one of the methoxy groups
with a propargyloxy one, provided an easy access to a clickable
core; this modified core molecule was also shown to function just
as efficiently in regulating the DP, while at the same time it
provided a single functionalizable unit at the core of the HBP. By
using this core in conjunction with a recently developed AB2
monomer, which provided direct access to HBPs carrying thiolÀ
ene clickable peripheral units, we have been able to develop a
single-step approach for the preparation of orthogonally clickable
HBPs; this represents the first demonstration of the direct
synthesis of such orthogonally clickable HBPs. In a test study,
we have also shown that two different chromophores, one bearing
an azide group and the other carrying a thiol unit could be clicked
at the core and periphery, respectively; FRET from the periphery
to the core was shown to occur, the extent of which could be
modulated by varying the solvent composition. This single-step
approach to access orthogonally clickable HBPs could find
several interesting applications wherein selective and differential
functionalization of the core and periphery may be needed, such
as in light harvesting, drug delivery etc.
1H NMR (400 MHz, CDCl3): δ 4.147 (s, 9H, ArOCH3); 4.605
(s, 6H, ArCH2Br).
1,3,5-Tris(methoxymethyl)-2,4,6-trimethoxybenzene (Core II).
Sodium metal (1.85 g, 80.5 mmol) was added to 20 mL of dry methanol
in portions. The solution of 1 (6 g, 13.4 mmol) in dry methanol was
added dropwise to the sodium methoxide solution. The reaction mixture
was refluxed for 18 h. After completion, the reaction mixture was cooled
to room temperature and methanol was removed under reduced pressure.
Then, 10 mL of water was added to the concentrated reaction mixture
and it was extracted in ethyl acetate (2 Â 20 mL). All organics were
combined and washed with brine, passed over sodium sulfate, concen-
trated and distilled in Kugelrohr at 200 °C/6 Torr to get desired product
(3 g, 75%, MP = 50 °C) as a white solid.
1H NMR (400 MHz, CDCl3): δ 3.43 (s, 9H, ArCH2OCH3); 3.91
(s, 9H, ArOCH3); 4.38 (s, 6H, ArCH2OCH3).
1,3,5-Tris(butoxymethyl)-2,4,6-trimethoxybenzene (Core IIb).
Sodium metal (1.85 g, 80.5 mmol) was added to 20 mL of dry butanol in
portions. The solution of 1 (6 g, 13.4 mmol) in dry butanol was added
dropwise to the sodium butoxide solution. The reaction mixture was
refluxed for 18 h. After completion, the reaction mixture was cooled to
room temperature and butanol was removed by using a rotary evapora-
tor. Then 10 mL of water was added to the concentrated reaction mixture,
and it was extracted in ethyl acetate (2 Â 20 mL). All organics were
combined and washed with brine, passed over sodium sulfate, concen-
trated, and distilled in Kugelrohr at 200 °C/6 Torr to get the desired
product (4 g, 70%) as a colorless viscous liquid.
’ EXPERIMENTAL SECTION
Materials and Methods. 2,4,6-Trimethylphenol, 1,3,5-trimethox-
ybenzene, trioxane, camphor sulfonicacid, propargylalcohol, 1,5-dihydroxy
naphthalene, and mesitylene were purchased from Sigma-Aldrich chemical
company and used directly. Metallic sodium, 33% acetic acid solution of
HBr, 49% aqueous solution of HBr were purchased from Spectrochem
1H NMR (400 MHz, CDCl3): δ 0.9 (t, 9H, ArCH2OCH2CH2CH2-
CH3); 1.38 (m, 6H, ArCH2OCH2CH2CH2CH3); 1.57 (m, 6H, ArCH2-
OCH2CH2CH2CH3); 3.54 (t, 6H, ArCH2OCH2CH2CH2CH3); 3.92
(s, 9H, ArOCH3); 4.47 (s, 6H, ArCH2OCH2CH2CH2CH3).
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Typical Copolymerization Procedure. Monomer I (1.5 g, 4.31
mmol) along with required amount of B3 core (II or IIb or III) and 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 polymer-
ization was then carried out at 150 °C under N2 for 1 h with constant
stirring. Subsequently, using a Kugelr€ohr apparatus, the polymerization
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 2 mL of THF, the acid-catalyst was
neutralized with solid NaHCO3 and then the solution was filtered. The
filtrate was concentrated under reduced pressure to a 1 mL of viscous
solution and precipitated in 10 mL methanol. The polymer was purified
once more by dissolution in 1 mL of THF and reprecipitation into 10 mL
of methanol.
Prog. Polym. Sci. 2004, 29, 183. (e) Peleshankoa, S.; Tsukruk, V. V. Prog.
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Polymer 3a (Poly-3a) (Scheme 1) was prepared similarly using
monomer IV and the required amount of core III.
Typical AzideÀYne Click Reaction (Poly-3b). A mixture of the
Poly-3a (200 mg, 0.034 mmol; with respect to propergyl group) and
coumarin azide (22 mg, 0.102 mmol) were taken in 5 mL of THF. To the
reaction mixture, 30 μL aqueous sodium ascorbate (2 mg, 0.01 mmol)
(9) Behera, G. C.; Ramakrishnan., S. J. Polym. Sci., Chem. Ed. 2004,
42, 102.
(10) Gittins, P. J.; Alston, J.; Ge, Y.; Twyman, L. J. Macromolecules
2004, 37, 7428. Gittins, P. J.; Twyman, L. J. J. Am. Chem. Soc. 2005,
127, 1646.
and CuSO4 5H2O (1.27 mg, 0.005 mmol) was added under N2 atmo-
3
sphere. N2 purging was continued for another 10 min to remove dissolved
oxygen from the reaction mixture. The content was heated to 50 °C for 3
days. After completion, the polymer solution was concentrated under
reduced pressure to a viscous solution and precipitated in DMSO; the
polymer was further purified twice by dissolution in THF and pre-
cipitated into DMSO to yield the polymer 3b.
(11) Roy, R. K.; Ramakrishnan, S. J. Polym. Sci., Chem. Ed. 2011,
49, 1735.
(12) Behera, G. C.; Ramakrishnan., S. Macromolecules 2004, 37, 9814.
(13) The effect of generation on the compactness of dendrimers and,
therefore, on the extent to which GPC would underestimate Mn has
been well-established. See: Hawker, C. J.; Malmstrom, E. E.; Frank,
C. W.; Kampf, J. P. J. Am. Chem. Soc. 1997, 119, 9903.
(14) The Mn and PDI values are of purified samples and hence may
be affected by the loss of some of oligomers during reprecipitation
process. To ensure uniformity, all the three copolymers were purified by
reprecipitation under identical conditions using the same quantities of
solvent and nonsolvent. See Supporting Information for experimental
details.
Typical ThiolÀEne Click Reaction (Poly 3c). A mixture of the
Poly-3b (35 mg, 0.11 mmol) and napthalene thiol (130 mg, 0.33 mmol)
were taken in 5 mL of toluene. To the reaction mixture, the photo-
initiator 2,2-dimethoxy 2-phenyl acetophenone (1.12 mg, 0.004 mmol),
was added. The content was irradiated with UV light for 8 h. The
polymer solution was concentrated under reduced pressure to a viscous
solution and precipitated in methanol; the polymer was further purified
twice by dissolution in chloroform and reprecipitation into petroleum
ether to yield the polymer 3c.
1
(15) (a) ) The H-NMR spectra of both the parent methoxy and
butoxy cores (II and IIb), however, exhibited only a single methoxy
proton peak corresponding to all the three methoxy groups; this could
be due to the rapid switching that occurs in the small core molecules,
while this dynamics be inhibited (or considerably slowed down) when
bulky substituents, such as in the case of the polymer, are present.
(16) Kaur, N.; Singh, N.; Cairns, D.; Callan, J. F. Org. Lett. 2009,
11, 2229.
’ ASSOCIATED CONTENT
S
Supporting Information. Detailed experimental schemes
b
and procedures, kinetic plots comparing the relative reactivities
of the two cores, NMR spectral variation during the polymeri-
zation and the corresponding kinetic plots, and fluorescence
spectra. This material is available free of charge via the Internet at
http://pubs.acs.org.
(17) Chae, J. Arch. Pharm. Res. 2008, 31, 305.
(18) (a) Serin, J. M.; Brousmiche, D. W.; Frꢀechet, J. M. J. Chem.
Commun. 2002, 2605. (b) Serin, J. M.; Brousmiche, D. W.; Frꢀechet,
J. M. J. J. Am. Chem. Soc. 2002, 124, 11848. (c) Weil, T.; Reuther, E.;
M€ullen, K. Angew. Chem., Int. Ed. 2002, 41, 1900. (d) Devadoss, C.;
Bharathi, P.; Moore, J. S. J. Am. Chem. Soc. 1996, 118, 9635. (e) Balzani,
V.; Capagna, S.; Denti, G.; Juris, A.; Serroni, S.; Venturi, M. Acc. Chem.
Res. 1998, 31, 26. (f) Gilat, S. L.; Adronov, A.; Frechet, J. M. J. Angew.
Chem. 1999, 111, 1519. Angew. Chem., Int. Ed. Engl. 1999, 38, 1422.
(g) Nantalaksakul, A.; Reddy, D. R.; Bardeen, C. J.; Thayumanavan, S.
Photosynthesis Research 2006, 87, 133.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: raman@ipc.iisc.ernet.in.
’ ACKNOWLEDGMENT
(19) The presence of vibrational fine-structure in the case when the
coumarin is clicked to the core of the HBP may be due to effective
isolation of the fluorophore that precludes interaction with other
coumarin units; which evidently is not the case of the model chromo-
phore. A similar observation of fine-structure was also made in another
study where the coumarin azide was clicked on to a conjugated polymer.
Saptarshi and Ramakrishnan, unpublished work.
S.R. would like to thank the Department of Atomic Energy for
the ORI-Award for the period 2006À2011.
’ REFERENCES
(1) (a) Voit, B. I.; Lederer, A. Chem. Rev. 2009, 109, 5924. (b) Wilms,
D.; Stiriba, S.-E.; Frey, H. Acc. Chem. Res. 2010, 43, 129. (c) Kricheldorf,
H. R. Macromol. Rapid Commun. 2007, 28, 1839. (d) Gao, C.; Yan, D.
(20) Similar observations of incomplete FRET were reported also
in the case of star polymer carrying several donors on the periphery
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Macromolecules
ARTICLE
and a single acceptor at the core. See: Hecht, S.; Vladimirov, N.;
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(22) See Supporting Information for detailed experimental procedures.
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R. K. Org. Lett. 2005, 7, 443.
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