Thermally reversible dendronized linear ab step-polymers via "click" chemistry

Article abstract of DOI:10.1021/ma200296t

The synthesis and characterization of thermally labile dendronized linear AB step-polymers is described. First through third generation dendritic AB monomers 14a-c containing both a furan and furan-protected maleimide functionality were prepared by the Cu(I)-catalyzed azide-alkyne cycloaddition reaction followed by polymerization via the thermally reversible furan-maleimide Diels-Alder reaction. The assembly, disassembly, and reassembly behavior of linear dendronized step-polymers 16a-c was studied by GPC.

Full text of DOI:10.1021/ma200296t

ARTICLE
pubs.acs.org/Macromolecules
Thermally Reversible Dendronized Linear AB Step-Polymers
via “Click” Chemistry
Nathan W. Polaske,‡ Dominic V. McGrath,‡,* and James R. McElhanon*,†
‡Contribution from the Department of Chemistry, University of Arizona, Tucson, Arizona 85721, United States
†Organic Materials Department, Sandia National Laboratories, Albuquerque, New Mexico 87185, United States
S Supporting Information
b
ABSTRACT: The synthesis and characterization of thermally labile
dendronized linear AB step-polymers is described. First through third
generation dendritic AB monomers 14aꢀc containing both a furan
and furan-protected maleimide functionality were prepared by the
Cu(I)-catalyzed azideꢀalkyne cycloaddition reaction followed by
polymerization via the thermally reversible furan-maleimide
DielsꢀAlder reaction. The assembly, disassembly, and reassembly
behavior of linear dendronized step-polymers 16aꢀc was studied
by GPC.
’ INTRODUCTION
Complementary to this last work, herein we report the prepara-
tion and characterization of AB17 dendronized step-polymers
derivatized by the Cu(I)-catalyzed azideꢀalkyne cycloaddition
(CuAAC) reaction and polymerized by the furanꢀmaleimide
DA reaction. These materials represent the first examples of
thermally reversible dendronized polymers originating from a
single monomeric species.
Dendronized polymers, also referred to as denpols,1,2 are
linear polymers containing pendant dendrimers3 as side groups
in the polymer repeat unit. The molecular architecture of these
macromolecules can be tailored with respect to dendron size and
generation, providing a high level of control over nanoscopic size,
rigidity, and functionality.2 This tunability has led to potential
applications of denpols including molecular containers,4 catalyst
carriers,5 molecular scaffolds,6 and electrolytes for Li-ion bat-
teries.7 The development of the denpol field is highly dependent
on the ability to synthesize these molecules in a highly efficient
and selective manner. As a result, “click” reactions8 including the
Cu(I)-catalyzed azideꢀalkyne Huisgen 1,3-dipolar9 and the
DielsꢀAlder (DA)10 cycloadditions have been extensively used
in the preparation of dendrimers, denpols, and other polymers.11
Stimuli responsive denpols12,13 exhibit significant and abrupt
physical changes (shape, volume, phase state, material properties,
etc.) in response to small variations in the local environment. The
thermal reversibility of the furanꢀmaleimide DA reaction14
make it an ideal candidate for the production of temperature
responsive systems. The ability to undergo both the forward and
retro furan-maleimide DA reaction at relatively mild tempera-
tures (room temperature to 60 °C for forward DA and
90ꢀ120 °C for retro DA)14 has resulted in a wide variety of
architectures including linear polymers,13,15ꢀ17 alternating
copolymers,18 and cross-linked networks,19 providing useful
applications for nanoscale lithography,20 hydrogels,21 nonlinear
optics,22 interpenetrating networks (IPNs),23 optically active
films,24 and self-healing25 materials. Previously, our group has
demonstrated the preparation of thermally degradable foams26
encapsulants,27 and surfactants,28,29 as well as covalently rever-
sible dendritic macromolecular assemblies30 and AAꢀBB dendro-
nized step-polymers13 utilizing furanꢀmaleimide DA adducts.
’ RESULTS AND DISCUSSION
Synthesis of Dendritic AB Monomers. In designing ther-
mally reversible dendritic AB monomers, we envisioned a tetra-
kis-substituted benzene core containing symmetric opposing
terminal alkyne quadrants, along with asymmetric opposing
terminal furan and maleimide quadrants. Such a molecule would
be capable of both peripheral modification of the alkynes via the
CuAAC reaction and thermally reversible linear polymerization
via the furan-maleimide DA reaction. The synthesis of the
dendritic AB monomers started from commercially available
2,5-dibromohydroquinone (1) (Scheme 1). Reaction with meth-
yl 4-chlorobutanoate (2) under basic conditions at elevated
temperature gave bis-substituted methyl ester 3 in 40% yield.
Sonagashira coupling31 of 3 with TIPS-acetylene realized com-
pound 4 in a yield of 90% (method A). Alternatively, we prepared
compound 4 by a different route. 2,5-Dibromohydroquinone (1)
was treated with p-methoxybenzyl (PMB) chloride under basic
conditions to give bis-PMB-protected compound 5 in 93% yield.
Sonagashira coupling with TIPSꢀacetylene, followed by depro-
tection of the PMB groups with 10% TFA in CH2Cl2 gave the
Received: February 8, 2011
Revised:
March 25, 2011
Published: April 08, 2011
r
2011 American Chemical Society
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Scheme 1. Two Methods for the Synthesis of Diacid 8
Scheme 2. Synthesis of Dendritic AB Monomers 14aꢀc
TIPS-protected acetylenic hydroquinone 7 in 60% yield from 5.
Upon reaction with methyl 4-chlorobutanoate (2) under similar
conditions as described above, 7 was converted to compound 4
in a yield of 86% (method B). With bis-methyl ester 4 in hand,
hydrolysis of the methyl ester with 2 M NaOH gave bis-
carboxylic acid 8 in near quantitative yield.
The TIPS-protected AB furanꢀmaleimide monomer 11 was then
prepared in moderate yield via statistical N,N0-diisopropylcarbodiimide
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Figure 1. GPC chromatograms of dialkynyl AB monomer 12 (green, far
right) and dendritic AB monomers 14a (black, second from right), 14b
(blue, second from left), and 14c (red, far left).
Figure 2. Maleimide deprotection of [G-1] dendritic monomer 14a
followed by the appearance of free maleimide protons in the 1H NMR
(TCE-d2, 1.0 mM) at 110 °C.
Scheme 3. Maleimide Deprotection of 14aꢀc and Subse-
N-(4-hydroxyphenyl)maleimide 10 (Scheme 2). Deprotection
of the TIPS groups with TBAF proved to be a challenge, as the
maleimide ester was found to be extremely sensitive to the
cleavage conditions. Ultimately, it was discovered that stoichio-
metric addition of DIC to the reaction mixture resulted in
selective cleavage of the TIPS groups with TBAF, giving the
bis-acetylenic AB monomer 12 in 56% yield. Finally, dendritic
AB monomers 14aꢀ14c were prepared in good yield via the
CuAAC reaction of 12 with dendritic azides 13aꢀc.13 All new
compounds were fully characterized by 1H and 13C NMR, along
with ESI or MALDI MS and combustion analysis. Elution
volumes of dialkynyl AB monomer 12 and dendritic AB mono-
mers 14aꢀc, determined using GPC (Figure 1), confirmed the
increasing hydrodynamic size with increasing dendrimer genera-
tion.
quent Polymerization
1H NMR Analysis of Maleimide Deprotection. Before the
dendritic AB monomers could be polymerized, suitable condi-
tions for the maleimide deprotection had to be determined in
order to fully “activate” the monomers (Scheme 3). The retro-
DA of furan-maleimide adducts is known to occur at tempera-
tures >60 °C,14 with temperatures between 90 and 120 °C
typically employed.13,29,30,32 We chose to perform the test
reaction on [G-1] AB monomer 14a at 110 °C in 1,10,2,20-
tetrachloroethane-d2 (TCE-d2) so that reaction progress could
1
be monitored by H NMR. Hence, a solution of [G-1] AB
monomer 14a in TCE- d2 (1.0 mM) was prepared for evaluation
by NMR spectroscopy. An NMR tube containing the AB
monomer solution was sealed, heated in an oil bath maintained
at 110 °C, and monitored by 1H NMR at regular time intervals.
The progress of the retro-DA reaction was followed by both the
formation of the free maleimide singlet of 15a at 6.92 ppm and the
disappearance of the bridgehead protons of 14a at 3.07 ppm
(Figures S1ꢀS6, Supporting Information).16 The free maleimide
proton signal was plotted as a percentage of the sum of the free
maleimide and bridgehead proton signals (Figure 2). Initially, no
free maleimide signal was observed, but after only 5 min the
reaction was shown to be 41% complete. Within 20 min, 77%
completion was observed, and after 40 min the bridgehead proton
signals were no longer visible, indicating reaction completion.
GPC Analysis of Polymer Assembly, Disassembly, and
Reassembly. After determining suitable retro-DA conditions,
(DIC) coupling of 8 with furfuryl alcohol (9), followed by DIC
coupling of the crude monofurfuryl product with furan-protected
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Figure 3. Assembly of dendritic AB DA step-polymers 16a (black, left),
16b (blue, middle), and 16c (red, right) at 50 °C in CHCl3. Key: (A) t = 0;
(B) t = 8 h; (C) t = 24 h; (D) t = 48 h; (E) t = 120 h; (F) t = 288 h.
Figure 5. Disassembly of dendritic AB DA polymers 16a (black, left),
16b (blue, middle), and 16c (red, right) at 110 °C in TCE (1.0 mM
based on 14aꢀc from assembly experiments). Key: (A) t = 0; (B) t = 5
min; (C) t = 10 min; (D) t = 20 min; (E) t = 30 min; (F) t = 40 min.
with the formation of 16c showing the most monomer, followed
by 16b and 16a, respectively. The presence of monomer is
consistent with the inefficiency of the DA reaction rather than
retro-DA events, as isolated DA adducts are stable at 50 °C. A
peak in the GPC at greater elution volume than monomer
appeared in the assembly of 16c. It is likely that this peak is
cyclized monomer,33 although definitive evidence is absent. This
peak disappeared upon disassembly and reappeared upon reas-
sembly (vide infra).
Polystyrene equivalent number-average molecular weight
(Mn) values were also determined during the formation of
dendritic polymers 16aꢀc (Figure 4). After 288 h, Mn values
of 7900, 9000, and 9500 g/mol were determined for DA
polymers 16aꢀc, respectively. These Mn values correlated to
degrees of polymerization of 6.2, 4.2, and 2.5 for 16aꢀc,
respectively. An additional time point taken after 456 h showed
no measurable enhancement in polymer growth with respect to
polystyrene equivalent Mn values, suggesting that near-maximal
Mn values had been achieved within 288 h under these condi-
tions.
The crude polymers 16aꢀc from the assembly experiments
were then subjected to disassembly conditions (Figure 5). The
polymer solutions were concentrated under reduced pressure,
followed by dissolving in TCE (1.0 mM based on 14aꢀc from
assembly experiments) and heating in an oil bath set to 110 °C.
Aliquots were removed at regular times intervals and monitored
by GPC. Within 5 min, significant disassembly appeared to take
place, and after 20 min the polymers had been reduced to
monomers 15aꢀc and lower molecular weight oligomers. After
40 min, all disassembly GPC traces appeared nearly identical to
the initial trace from the respective assembly experiments. These
results suggest relatively clean disassembly with minimal decom-
position. In addition, all disassemblies occurred at approximately
the same rate, likely as a result of the first order nature of the
retro-DA reaction.
Figure 4. Polystyrene-equivalent Mn values for the assembly and
reassembly of dendritic AB DA polymers 16aꢀc at 50 °C in CHCl3
for indicated time.
step-polymerizations were performed with dendritic AB mono-
mers 14aꢀc (Scheme 2). Accordingly, 14aꢀc were dissolved in
TCE (1.0 mM) and heated in an oil bath set to 110 °C for 40 min,
at which point TLC suggested complete maleimide deprotection.
The solvent was then removed under reduced pressure, followed
by redissolving in a minimal quantity of CHCl3 and heating to
50 °C. The reaction vessels were left open and a fresh minimal
quantity of CHCl3 was added every 24 h. Aliquots were removed
at regular time intervals and reaction progress was monitored
using GPC.
During the formation of dendritic step-polymers 16aꢀc, a
gradual increase of higher molecular weight compounds was
observed as evidenced by the appearance of peaks at lower GPC
retention volumes (Figure 3). At t = 0 for each reaction, some
degree of progress could be seen, suggesting that the polymer-
ization actually began during the maleimide deprotection of
14aꢀc, most likely while removing the TCE under reduced
pressure. Similarly to our previously published results on AA ꢀ
BB sysems,13 the formation of [G-1] DA polymer 16a appeared
to progress at a faster rate than [G-2] DA polymer 16b, which
was faster still than [G-3] DA polymer 16c. After 288 h,
monomers 15aꢀc were still detected in the reaction mixtures,
The crude disassembled DA polymers were also assembled a
second time to demonstrate the repeatable nature of the ther-
mally reversible system. This reassembly was carried out under
identical conditions as the initial assembly. Not surprisingly, the
reassembly reactions proceeded similarly to the initial assemblies,
with [G-1] DA polymer 16a growing at the fastest rate, followed
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Dimethyl 4,40-((2,5-Dibromo-1,4-phenylene)bisoxy)dib-
utanoate (3). A round-bottom flask was charged with 2,5-dibromohy-
droquinone (1) (5.9 g, 22 mmol), methyl 4-chlorobutanoate (2) (7.5 g,
55 mmol), K2CO3 (15 g, 110 mmol), and DMF (50 mL). The reaction
mixture was then placed in an oil bath and heated to 100 °C for 1 h. After
allowing it to cool to room temperature, the reaction mixture was poured
into brine (500 mL). The suspension was filtered and the resulting solid
purified by flash chromatography (silica gel; CH2Cl2) to give 3 as a white
solid (4.2 g, 40% yield): mp 82ꢀ84 °C. 1H NMR (500 MHz, CDCl3): δ
7.08 (s, 2H), 4.00 (t, J = 6 Hz, 4H), 3.70 (s, 6H), 2.58 (t, J = 8 Hz, 4H),
2.13 (m, 4H). 13C NMR (125 MHz, CDCl3): δ 173.5, 149.9, 118.5,
111.2, 68.9, 51.7, 30.4, 24.5. MS (ESI) m/z 489.2 [M þ Na]þ;
C16H20Br2NaO6 requires 489.1. Anal. Calcd for C16H20Br2O6: C,
41.05; H, 4.31. Found: C, 40.69; H, 4.62.
Dimethyl 4,40-((2,5-Bis((triisopropylsilyl)ethynyl)-1,4-ph-
enylene)bisoxy)dibutanoate (4). (Method A) A round-bottom
flask was charged with 7 (2.50 g, 5.33 mmol), K2CO3 (3.70 g, 26.8
mmol), KI (445 mg, 2.68 mmol), and DMF (50 mL). Methyl 4-chlor-
obutanoate (2) (2.18 g, 16.0 mmol) was then added, followed by heating
in an oil bath set to 100 °C with stirring for 3 h. The reaction mixture was
allowed to cool to room temperature, followed by pouring into brine
(450 mL). The resulting brown precipitate was collected by vacuum
filtration and purified by recrystallization from MeOH to give 4 as off-
white needles (3.03 g, 86% yield). (Method B) A heavy-walled Schlenk
flask was charged with 3 (6.33 g, 13.5 mmol), TIPSꢀacetylene (7.40 g,
40.6 mmol), Pd(PPh3)2Cl2 (950 mg, 1.35 mmol), CuI (260 mg, 1.35
mmol), triphenylphosphine (350 mg, 1.35 mmol), triethylamine
(35 mL), and DMF (70 mL). The flask was sealed, followed by four
cycles of freezeꢀpumpꢀthaw. The sealed vessel was then placed in an
oil bath heated to 60 °C and stirred for 72 h. The reaction mixture was
then allowed to cool to room temperature and was poured on H2O
(500 mL). The suspension was extracted with EtOAc (500 mL),
followed by collecting the EtOAc layer and washing with 1 M HCl
(2 ꢁ 500 mL) and brine (250 mL). The EtOAc layer was collected, dried
over MgSO4, filtered, the filtrate collected and the solvent removed
under reduced pressure. The residue was purified by flash chromatog-
raphy (silica gel; hexanes:CH2Cl2 (1:1)) followed by recrystallization
from MeOH to give 4 as white needles (8.12 g, 90% yield): mp
Figure 6. Reassembly of dendritic AB DA step-polymers 16a
(black, left), 16b (blue, middle), and 16c (red, right) at 50 °C in CHCl3.
Key: (A) t = 0; (B) t = 8 h; (C) t = 24 h; (D) t = 48 h; (E) t = 120 h;
(F) t = 288 h.
by [G-2] DA polymer 16b and finally [G-3] DA polymer 16c
(Figure 6). Polystyrene equivalent Mn values were also determined
during the reassembly of dendritic polymers 16aꢀc (Figure 4).
After 288 h, Mn values of 6600, 8200, and 9400 g/mol were
determined for DA polymers 16aꢀc, respectively. These Mn
values correlated to degrees of polymerization of 5.2, 3.9, and
2.5 for DA polymers 16aꢀc, respectively. During both assembly
and reassembly, qualitative assessment of GPC traces did not
indicate any selectivity of product formation.
’ SUMMARY
The synthesis, characterization, and study of polymerization
behavior of a thermally reversible AB step-polymer utilizing
sequential CuAAC and furanꢀmaleimide DA “click” reactions
has been described. Compound 12, a tetrakis-substituted ben-
zene core containing symmetric opposing terminal alkyne quad-
rants along with asymmetric opposing terminal furan and
maleimide quadrants, was prepared and derivatized via the
CuAAC reaction with first through third generation benzyl aryl
ether azide dendrons 13aꢀc13 to give dendritic AB monomers
14aꢀc. After optimization of the retro-DA conditions using
[G-1] dendritic AB monomer 14a, monomers 14aꢀc were polym-
erized and assembly, disassembly, and reassembly behavior
monitored by GPC. Initial results suggest that polymer forma-
tion and degradation occur in a repeatable manner with minimal
degradation under mild reaction conditions. Methods to increase
polymer size and reaction rates are currently under investigation.
1
102ꢀ104 °C. H NMR (500 MHz, CDCl3): δ 6.84 (s, 2H), 4.03 (t,
J = 6 Hz, 4H), 3.68 (s, 6H), 2.55 (t, J = 8 Hz, 4H), 2.09 (m, 4H), 1.13 (s,
42H). 13C NMR (125 MHz, CDCl3): δ 173.7, 153.8, 117.1, 114.1,
102.7, 96.7, 67.9, 51.5, 30.4, 24.6, 18.7, 11.3. MS (ESI) m/z 671.1 [M þ
H]þ; C38H63O6Si2 requires 671.4. Anal. Calcd for C38H62O6Si2: C,
68.01; H, 9.31. Found: C, 67.67; H, 9.62.
4,40-(((2,5-Dibromo-1,4-phenylene)bis(oxy))bis(methy-
lene))bis(methoxybenzene) (5). A round-bottom flask was
charged with 2,5-dibromohydroquinone (1) (5.00 g, 18.7 mmol),
4-methoxybenzyl chloride (7.31 g, 46.7 mmol), K2CO3 (12.9 g, 93.3
mmol), and DMF (50 mL). The reaction mixture was purged with N2,
followed by heating to 120 °C with stirring for 1 h. After allowing the
reaction mixture to cool to room temperature, the suspension was
poured onto H2O (450 mL). The solution was then placed in the freezer
(ꢀ35 °C) for 30 min, at which point an off-white precipitate was
observed. The precipitate was collected by vacuum filtration and washed
several times with H2O. The solid was air-dried, then recrystallized from
1,2-dichloroethane to give 5 as a white solid (8.85 g, 93% yield): mp
194ꢀ196 °C. 1H NMR (500 MHz, CDCl3): δ 7.37 (d, J = 9 Hz, 4H),
7.17 (s, 2H), 6.92 (d, J = 9 Hz, 4H), 5.00 (s, 4H), 3.82 (s, 6H). 13C NMR
(125 MHz, CDCl3): δ 159.5, 150.0, 129.0, 128.2, 119.4, 114.0, 111.6,
71.9, 55.3. MS (EI) m/z 264.9 [M þ H ꢀ 2PMB]þ; C6H3Br2O2
requires 264.9. Anal. Calcd for C22H20Br2O4: C, 51.99; H, 3.97. Found:
C, 51.60; H, 3.71.
’ EXPERIMENTAL SECTION
Materials and Methods. NMR data was collected on Bruker 500
and 600 MHz Spectrometers running Topspin (Bruker). Chemical
shifts were referenced to the deuterated solvent resonance for 1H (7.26
ppm for CDCl3 and 6.00 ppm for TCE-d2) and 13C NMR (77.0 ppm for
CDCl3). GPC studies were performed using a Waters Alliance 2695
separations module with Jordi DVB columns (500, 1,000, 10 000 Å
columns in series) with THF as the mobile phase at a flow rate of 1 mL/min.
Data was collected with a Waters 2996 photodiode array detector at
330 nm using Empower software (Waters). All chemicals were
purchased from commercial suppliers and used as received unless
otherwise noted.
2,5-Bis(triisopropylsilyl)ethynyl)hydroquinone (7). A heavy-
walled Schlenk flask was charged with 5 (4.00 g, 7.86 mmol),
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TIPSꢀacetylene (4.30 g, 23.6 mmol), Pd(PPh3)2Cl2 (551 mg, 0.786
mmol), CuI (150 mg, 0.786 mmol), triphenylphosphine (206 mg,
0.786 mmol), triethylamine (25 mL), and DMF (50 mL). The flask was
sealed and then degassed by four freezeꢀpumpꢀthaw cycles. The
sealed vessel was then placed in an oil bath maintained at 60 °C and
stirred for 40 h. The reaction mixture was then allowed to cool to room
temperature and was poured on H2O (300 mL). The suspension was
extracted with CH2Cl2 (3 ꢁ 300 mL), followed by addition of Celite
and MgSO4. The suspension was filtered and the filtrate collected.
After removal of solvent under reduced pressure, the solid residue was
redissolved in CH2Cl2 (200 mL), and washed with 1 M HCl (2 ꢁ
200 mL) and brine (1 ꢁ 100 mL). The organic layer was collected,
dried over MgSO4, filtered, and the filtrate concentrated under
reduced pressure to give a yellow solid. The solid was suspended in
CH2Cl2 (50 mL), followed by dropwise addition of TFA (5 mL). The
mixture was stirred for 30 min, followed by dilution with toluene
(50 mL) and removal of solvent under reduced pressure. The resulting
yellow solid was suspended in hexanes and filtered. The filtrate was
concentrated under reduced pressure, followed by purification by flash
chromatography (silica gel; hexanes:CH2Cl2 (9:1)) to give 7 as a white
(3 ꢁ 200 mL). The 1 M NaOH layers were combined, acidified to
pH∼2 with concentrated HCl, and reduced to ∼100 mL under reduced
pressure. The solid was collected by vacuum filtration and washed
several times with H2O to give recovered 8 as a white solid (1.95 g, 39%
yield). The procedure was repeated twice more with the recovered 8.
The CH2Cl2 layers from the three extractions were dried over MgSO4,
filtered, and the filtrate concentrated under reduced pressure to give
crude monofurfuryl coupled product as an off-white low-melting solid
(3.60 g). The solid was dissolved in CH2Cl2 (100 mL), followed by
addition of DMAP (919 mg, 7.5 mmol). DIC was then added dropwise,
followed by portionwise addition of 10 (1.92 g, 7.50 mmol). The
reaction mixture was stirred at room temperature for 4 h. The mixture
was then washed with 1 M HCl (2 ꢁ 100 mL) and brine (100 mL). The
organic layer was collected, dried over MgSO4, filtered, and the filtrate
concentrated under reduced pressure. The residue was purified by flash
chromatography (silica gel; hexanes:EtOAc (3:2)) to give 11 as an off-
white glass that turned into a brittle foam upon drying on high-vac (3.09
g, 41% yield): mp 94ꢀ95 °C. 1H NMR (500 MHz, CDCl3): δ 7.41 (m,
1H), 7.32ꢀ7.30 (m, 2H), 7.19ꢀ7.18 (m, 2H), 6.87 (s, 1H), 6.85 (s, 1H),
6.57 (s, 2H), 6.40ꢀ6.35 (m, 2H), 5.39 (s, 2H), 5.07 (s, 2H), 4.07 (t, J =
6 Hz, 2H), 3.99 (t, J = 6 Hz, 2H), 3.01 (s, 2H), 2.84 (t, J = 8 Hz, 2H), 2.59
(t, J = 8 Hz, 2H), 2.20 (m, 2H), 2.10 (m, 2H), 1.15ꢀ1.11 (m, 42H). 13C
NMR (125 MHz, CDCl3): δ 175.2, 172.9, 171.3, 153.8, 153.7, 150.5,
149.5, 143.2, 136.7, 129.0, 127.5, 122.2, 117.1, 114.1, 114.0, 110.6, 110.5,
102.7, 96.79, 96.72, 81.4, 67.8, 67.7, 58.0, 47.5, 30.8, 30.5, 24.7, 24.6,
18.73, 18.67, 11.35, 11.32. MS (ESI) m/z 984.1 [M þ Na]þ;
C55H71NNaO10Si2 requires 984.5. Anal. Calcd for C55H71NO10Si2: C,
68.65; H, 7.44; N, 1.46. Found: C, 68.61; H, 7.80; N, 1.67.
4,(1,3-Dioxo-3a,4,7,7a-tetrahydro-1H-4,7-epoxyisoindol-
2(3H)-yl)phenyl 4-(2,5-Diethynyl-4-(4-(furan-2-ylmethoxy)-
4-oxobutoxy)phenoxy)butanoate (12). A flask was charged with
11 (2.00 g, 2.08 mmol), DIC (794 mg, 6.23 mmol), and THF (50 mL).
The mixture was cooled in an ice bath, followed by dropwise addition of
TBAF (1 M solution, in THF, 4.36 mL, 4.36 mmol) with stirring. After
stirring for 15 min, 1 M HCl (100 mL) was added, and the resulting
mixture extracted with CH2Cl2 (3 ꢁ 150 mL). The organics were
combined, dried over MgSO4, filtered, and the filtrate concentrated
under reduced pressure. The residue was purified by flash chromatog-
raphy (silica gel; CH2Cl2:acetone (97:3)) to give 12 as a colorless glass
that turned into a brittle foam upon drying under high-vacuum (755 mg,
56% yield): mp 44ꢀ47 °C. 1H NMR (500 MHz, CDCl3): δ 7.42ꢀ7.41
(m, 1H), 7.32ꢀ7.30 (m, 2H), 7.21ꢀ7.19 (m, 2H), 6.97 (s, 1H), 6.95 (s,
1H), 6.57 (s, 2H), 6.40ꢀ6.35 (m, 2H), 5.39 (s, 2H), 5.08 (s, 2H), 4.10
(t, J = 6 Hz, 2H), 4.02 (t, J = 6 Hz, 2H), 3.34 (s, 1H), 3.32 (s, 1H), 3.01
(s, 2H), 2.84 (t, J = 8 Hz, 2H), 2.60 (t, J = 8 Hz, 2H), 2.27ꢀ2.18 (m, 2H),
2.17ꢀ2.08 (m, 2H). 13C NMR (125 MHz, CDCl3): δ 175.1, 172.7,
171.1, 153.8, 153.7, 150.4, 149.4, 143.2, 136.7, 129.0, 127.5, 122.2, 118.0,
117.9, 113.5, 110.6, 110.5, 82.9, 81.4, 79.4, 68.29, 68.24, 58.1, 47.5, 30.7,
30.5, 24.50, 24.4. MS (ESI) m/z 649.9 [M þ H]þ; C37H32NO10
requires 650.2. Anal. Calcd for C37H31NO10: C, 68.41; H, 4.81; N,
2.16. Found: C, 68.80; H, 5.19; N, 2.25.
1
solid (2.20 g, 60% yield): mp 124ꢀ125 °C; H NMR (500 MHz,
CDCl3) δ 6.93 (s, 2H), 5.47 (s, 2H), 1.13 (s, 42H). 13C NMR
(125 MHz, CDCl3): δ 150.4, 116.4, 111.7, 100.7, 100.4, 18.7, 11.2.
MS (ESI) m/z 469.5 [M ꢀ H]ꢀ; C28H45O2Si2 requires 469.3. Anal.
Calcd for C28H46O2Si2: C, 71.43; H, 9.85. Found: C, 71.07; H, 9.47.
4,40-((2,5-Bis((triisopropylsilyl)ethynyl)-1,4-phenylene)-
bisoxy)dibutanoic Acid (8). Compound 4 (7.00 g, 10.5 mmol) was
suspended in a mixture of THF (100 mL), MeOH (50 mL), and 2 M
NaOH (50 mL). The mixture was stirred vigorously at room tempera-
ture for 16 h. The reaction mixture was then acidified with 6 M HCl until
the solution reached pH ∼ 2. The suspension was reduced to ∼75 mL
under reduced pressure, at which point a thick precipitate formed. The
solid was collected by vacuum filtration and was washed several times
with H2O. The resulting solid was dried under high-vac to give 8 as a
white solid (6.45 g, 96% yield): mp 180 °C (dec.). 1H NMR (500 MHz,
DMSO-d6): δ 6.94 (s, 2H), 3.97 (t, J = 5 Hz, 4H), 2.39 (t, J = 6 Hz, 4H),
1.88 (m, 4H), 1.08 (s, 42H). 13C NMR (125 MHz, DMSO-d6): δ 174.3,
153.6, 116.4, 113.3, 102.9, 96.3, 67.9, 30.2, 24.5, 18.6, 10.9. MS (ESI)
m/z 641.2 [M ꢀ H]ꢀ; C36H57O6Si2 requires 641.4. Anal. Calcd for
C36H58O6Si2: C, 67.24; H, 9.09. Found: C, 67.60; H, 8.85.
2,(4-Hydroxyphenyl)-3a,4,7,7a-tetrahydro-1H-4,7-epoxy-
isoindole-1,3(2H)-dione (10). A heavy-walled Schlenk flask was
charged with N-(4-hydroxyphenyl)maleimide34 (20.0 g, 106 mmol),
furan (72.0 g, 1.06 mol), and acetone (50 mL). The flask was sealed and
placed in an oil bath heated to 75 °C. After 30 min, the reaction mixture
formed a thick precipitate. The mixture was allowed to cool to room
temperature and was then placed in the freezer (ꢀ35 °C) for 30 min.
The solid was collected by vacuum filtration and was washed with cold
acetone to give the exo-isomer of 10 as a gray solid (15.5 g, 57% yield):
mp 168 °C (dec.), lit.35 172 °C. 1H NMR (600 MHz, acetone-d6): δ 7.04
(d, J = 9 Hz, 2H), 6.89 (d, J = 9 Hz, 2H), 6.62 (s, 2H), 5.22 (s, 2H), 3.03
(s, 2H), 2.81 (s, 1H). 13C NMR (150 MHz, acetone-d6): δ 176.5, 158.1,
137.5, 135.5, 128.9, 116.1, 82.1, 48.3. MS (ESI) m/z 256.1 [M ꢀ H]ꢀ;
C14H10NO4 requires 256.1. Anal. Calcd for C14H11NO4: C, 65.37; H,
4.31; N, 5.44. Found: C, 65.39; H, 4.51; N, 5.60.
4-(1,3-Dioxo-3a,4,7,7a-tetrahydro-1H-4,7-epoxyisoindol-
2(3H)-yl)phenyl 4-(4-(4-(furan-2-ylmethoxy)-4-oxobutoxy)-
2,5-bis((triisopropylsilyl)ethynyl)phenoxy) Butanoate (11).
A flask was charged with 8 (5.00 g, 7.82 mmol) and DMAP (480 mg,
3.91 mmol), followed by addition of CH2Cl2 (200 mL). DIC (490 mg,
3.91 mmol) was then added dropwise, followed by dropwise addition of
furfuryl alcohol (9) (380 mg, 3.91 mmol). The reaction mixture was
allowed to stir at room temperature for 2 h, at which point the solution
was extracted with 1 M HCl (3 ꢁ 200 mL) followed by 1 M NaOH
4-(1,3-dioxo-3a,4,7,7a-tetrahydro-1H-4,7-epoxyisoindol-
2(3H)-yl)phenyl 4-(2,5-Bis(1-(3,5-bis(benzyloxy)benzyl)-1H-
1,2,3-triazol-4-yl)-4-(4-(furan-2-ylmethoxy)-4-oxobutoxy)-
phenoxy)butanoate (14a). A small vial was charged with 12 (200
mg, 0.308 mmol), [G-1]-azide 13a13 (320 mg, 0.924 mmol), Cu-
(PPh3)3Br (56 mg, 0.0616 mmol), DIPEA (200 mg, 1.54 mmol) and
CH2Cl2 (1.0 mL). The vial was left at room temperature for 16 h, at
which point the reaction mixture was directly purified by flash chroma-
tography (silica gel; CH2Cl2:acetone (92:8)) to give 14a an off-white
glass. The product became a brittle white foam after drying overnight
1
under high-vacuum (340 mg, 82% yield): mp 65 °C (dec). H NMR
(500 MHz, CDCl3): δ 8.07 (s, 1H), 8.05 (s, 1H), 8.01 (s, 1H), 7.96 (s,
1H), 7.39ꢀ7.28 (m, 21H), 7.24ꢀ7.22 (m, 2H), 7.08ꢀ6.92 (m, 2H),
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ARTICLE
6.57ꢀ6.51 (m, 8H), 6.33ꢀ6.29 (m, 2H), 5.50 (s, 2H), 5.38 (s, 2H), 5.36
(s, 2H), 4.99 (s, 6H), 4.97 (s, 4H), 4.27 (t, J = 6 Hz, 2H), 4.19 (t, J = 6
Hz, 2H), 2.99 (s, 2H), 2.68 (t, J = 7 Hz, 2H), 2.45 (t, J = 8 Hz, 2H),
2.30ꢀ2.20 (m, 2H), 2.16ꢀ2.15 (m, 2H). 13C NMR (125 MHz, CDCl3):
δ 175.1, 173.7, 172.6, 171.3, 160.35, 160.30, 150.3, 149.27, 149.23,
149.13, 143.22, 143.19, 143.0, 137.3, 137.1, 136.56, 136.49, 136.46,
134.6, 129.1, 128.55, 128.53, 128.03, 127.99, 127.59, 127.52, 127.4,
123.8, 123.6, 122.3, 122.2, 119.43, 119.38, 111.0, 110.8, 110.7, 110.5,
107.01, 106.97, 102.0, 101.9, 81.4, 79.8, 77.3, 77.0, 76.8, 70.10, 70.07,
68.2, 67.8, 58.1, 54.0, 53.9, 31.4, 30.8, 24.8, 24.7. MS (ESI) m/z 1361.9
[M þ Na]þ; C79H69N7NaO14 requires 1362.6. Anal. Calcd for
C79H69N7O14: C, 70.79; H, 5.19; N, 7.31. Found: C, 70.79; H, 5.42;
N, 7.04.
NMR Studies. To monitor the retro-DA reaction during maleimide
deprotection of [G-1] monomer 14a to form 15a (Scheme 2), [G-1]
monomer 14a (1.3 mg, 1.0 μmol) was dissolved in TCE-d2 (1.0 mL) in
an NMR tube. The tube was sealed, followed by heating in an oil bath set
to 110 °C. A time intervals the reaction was removed from the heating
bath and 1H NMR spectra were collected at ambient temperature.
Reaction progress was determined by plotting the free maleimide proton
(6.92 ppm) signal as a percentage of the sum of the free maleimide and
bridgehead proton (3.07 ppm) signals.16
GPC Studies. To monitor the assembly of dendritic DA polymers
16aꢀc, [G-n] AB monomers 14aꢀc were dissolved in TCE (1.0 mM)
and heated to 110 °C in open flasks. After 1 h, the reaction mixtures were
concentrated under reduced pressure, the residues dissolved in a
minimal quantities of CHCl3 and heated to 50 °C in open vials. Every
24 h, a fresh minimal quantity of CHCl3 was added to each vial. At time
intervals aliquots were taken, dissolved in THF, and analyzed by GPC.
To monitor disassembly, each crude polymer mixture of 16aꢀc was
heated to 110 °C in TCE (1.0 mM based on 14aꢀc from assembly
experiments). Aliquots were taken out at time intervals, diluted with
THF, and analyzed by GPC. To monitor reassembly, the disassembled
polymer samples were concentrated to dryness and dissolved in a
minimal quantity of CHCl3. Every 24 h, a fresh minimal quantity of
CHCl3 was added to each vial. At time intervals aliquots were taken,
dissolved in THF, and analyzed by GPC.
4-(1,3-dioxo-3a,4,7,7a-tetrahydro-1H-4,7-epoxyisoindol-
2(3H)-yl)phenyl 4-(2,5-Bis(1-(3,5-bis((3,5-bis(benzyloxy)-
benzyl)oxy)benzyl)-1H-1,2,3-triazol-4-yl)-4-(4-(furan-2-yl-
methoxy)-4-oxobutoxy)phenoxy)butanoate (14b). A small
vial was charged with 12 (100 mg, 0.154 mmol), [G-2]-azide 13b13
(356 mg, 0.462 mmol), Cu(PPh3)3Br (29 mg, 0.0308 mmol), DIPEA
(99 mg, 0.770 mmol) and CH2Cl2 (1.0 mL). The vial was left at room
temperature for 64 h, at which point the reaction mixture was directly
purified by flash chromatography (silica gel; CH2Cl2:acetone (95:5)
followed by EtOAc:hexanes (3:2)) to give 14b an off-white glass. The
product became a brittle white foam after drying overnight on high-vac
(182 mg, 54% yield): mp 63 °C (dec). 1H NMR (500 MHz, CDCl3): δ
8.07 (s, 1H), 8.05 (s, 1H), 7.97 (s, 1H), 7.93 (s, 1H), 7.41ꢀ7.32 (m,
41H), 7.32ꢀ7.30 (m, 2H), 7.05ꢀ6.95 (m, 2H), 6.65ꢀ6.63 (m, 8H),
6.56ꢀ6.52 (m, 12H), 6.31ꢀ6.26 (m, 2H), 5.50 (s, 2H), 5.34 (s, 2H),
5.33 (s, 2H), 5.03ꢀ4.96 (m, 16H), 4.97 (s, 2H), 4.94 (s, 4H), 4.91 (s,
4H), 4.18 (t, J = 6 Hz, 2H), 4.10 (t, J = 6 Hz, 2H), 2.93 (s, 2H), 2.64 (t,
J = 7 Hz, 2H), 2.41 (t, J = 8 Hz, 2H), 2.27ꢀ2.16 (m, 2H), 2.15ꢀ2.05 (m,
2H). 13C NMR (125 MHz, CDCl3): δ 175.1, 173.7, 172.6, 171.5,
160.33, 160.26, 160.18, 160.16, 150.3, 149.33, 149.25, 149.16, 143.3,
143.1, 139.03, 138.97, 137.4, 137.3, 136.79, 136.77, 136.69, 134.65,
129.1, 128.6, 128.02, 128.01, 127.64, 127.58, 127.4, 123.8, 123.6, 122.3,
119.5, 119.4, 111.0, 110.8, 110.7, 110.6, 107.2, 107.1, 106.4, 102.0, 101.9,
101.6, 81.4, 79.8, 70.12, 70.05, 70.01, 68.2, 67.8, 58.1, 54.1, 53.9, 47.5,
45.9, 31.5, 30.9, 29.8, 24.9, 24.7. MS (MALDI) m/z 2121.4 [M þ H ꢀ
’ ASSOCIATED CONTENT
Supporting Information. 1H NMR spectra of the disas-
S
b
sembly of [G-1] AB monomer 14a. This material is available free
of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*(J.R.M.) E-mail: jrmcelh@sandia.gov. Telephone: 505-844-
9165. (D.V.M.) E-mail: mcgrath@email.arizona.edu. Telephone:
520-626-4690. Fax: 520-621-8407.
furan]þ, C131H114N7O21 requires 2120.8. Anal. Calcd for C135H117
N7O22: C, 74.06; H, 5.39; N, 4.48. Found: C, 73.68; H, 5.74; N, 4.33.
-
’ ACKNOWLEDGMENT
4-(1,3-Dioxo-3a,4,7,7a-tetrahydro-1H-4,7-epoxyisoindol-
2(3H)-yl)phenyl 4-(2,5-Bis(1-(3,5-bis((3,5-bis((3,5-bis(benzy-
loxy)benzyl)oxy)benzyl)oxy)benzyl)-1H-1,2,3-triazol-4-yl)-
4-(4-(furan-2-ylmethoxy)-4-oxobutoxy)phenoxy)butanoate
(14c). A small vial was charged with 12 (50 mg, 0.0770 mmol), [G-3]-
azide 13c13 (374 mg, 0.231 mmol), Cu(PPh3)3Br (14 mg, 0.0154
mmol), DIPEA (50 mg, 0.385 mmol) and CH2Cl2 (1.0 mL). The vial
was left at room temperature for 64 h, at which point the reaction
mixture was directly purified by flash chromatography (silica gel;
CH2Cl2:acetone 97:3) to give 14c an off-white glass. The product
became a brittle white foam after drying overnight on high-vac (230 mg,
77% yield): mp 63 °C (dec). 1H NMR (500 MHz, CDCl3): δ 8.06 (s,
1H), 8.04 (s, 1H), 7.96 (s, 1H), 7.92 (s, 1H), 7.56ꢀ7.22 (m, 81H),
7.18ꢀ7.14 (m, 2H), 6.97ꢀ6.92 (m, 2H), 6.70ꢀ6.42 (m, 44H),
6.28ꢀ6.22 (m, 2H), 5.44 (s, 2H), 5.32ꢀ5.26 (m, 4H), 5.02ꢀ4.85 (m,
58H), 4.12 (t, J = 6 Hz, 2H), 4.06 (t, J = 6 Hz, 2H), 2.87 (s, 2H), 2.60 (t,
J = 7 Hz, 2H), 2.38 (t, J = 8 Hz, 2H), 2.23ꢀ2.13 (m, 2H), 2.12ꢀ2.02 (m,
2H). 13C NMR (125 MHz, CDCl3): δ 175.0, 172.5, 171.4, 160.09,
160.00, 159.98, 143.2, 139.18, 139.16, 138.9, 136.7, 136.6, 128.5, 128.0,
127.5, 122.2, 110.7, 110.5, 107.10, 107.05, 106.4, 101.5, 81.3, 70.0, 69.9,
68.1, 67.7, 58.0, 54.0, 53.8, 47.4, 31.4, 30.8, 24.8, 24.6. MS (ESI) m/z
3817.9 [M þ H]þ; C243H210N7O37 requires 3817.5. Anal. Calcd for
Research supported by the U.S. Department of Energy, Office
of Basic Energy Sciences, Division of Materials Sciences and
Engineering. Sandia National Laboratories is a multiprogram
laboratory managed and operated by Sandia Corporation, a
wholly owned subsidiary of Lockheed Martin Corporation, for
the U.S. Department of Energy’s National Nuclear Security
Administration under Contract DE-AC04-94AL85000. Certain
trade names and company products are identified in order to
specify experimental procedures adequately. In no case does such
identification imply recommendation or endorsement by the
National Institute of Standards and Technology, nor does it
imply that the products are necessarily the best available for the
purpose.
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