Rapid metal-free macromolecular coupling via in Situ nitrile oxide-activated alkene cycloaddition

Article abstract of DOI:10.1002/pola.27371

Nitrile oxide 1,3 dipolar cycloaddition is a simple and powerful coupling methodology. However, the self-dimerization of nitrile oxides has prevented the widespread use of this strategy for macromolecular coupling. By combining an in situ nitrile oxide ge

Full text of DOI:10.1002/pola.27371

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Rapid Metal-Free Macromolecular Coupling via In Situ Nitrile
Oxide-Activated Alkene Cycloaddition
Michael J. Isaacman,1,2 Weibin Cui,1,2 Luke S. Theogarajan2,3
1Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106
2California Nanosystems Institute at UC Santa Barbara, University of California, Santa Barbara, California 93106
3Department of Electrical and Computer Engineering, University of California, Santa Barbara, California 93106
Correspondence to: L. S. Theogarajan (E-mail: ltheogar@ece.ucsb.edu)
Received 10 July 2014; accepted 3 August 2014; published online 25 August 2014
DOI: 10.1002/pola.27371
ABSTRACT: Nitrile oxide 1,3 dipolar cycloaddition is a simple
and powerful coupling methodology. However, the self-
dimerization of nitrile oxides has prevented the widespread
use of this strategy for macromolecular coupling. By combin-
ing an in situ nitrile oxide generation with a highly reactive
activated dipolarophile, we have overcome these obstacles and
present a metal-free macromolecular coupling strategy for the
modular synthesis of several ABA triblock copolymers. Nitrile
oxides were generated in situ from chloroxime terminated pol-
y(dimethylsiloxane) B-blocks and coupled with several distinct
hydrophilic (poly(2-methyloxazoline) and poly(ethylene gly-
col)), and poly(N-isopropylacrylamide) or hydrophobic (poly(L-
lactide) A-blocks terminated in activated dipolarophiles in a
rapid fashion with high yield. This methodology overcomes
many drawbacks of previously reported metal-free methods
due to its rapid kinetics, versatility, scalability, and ease of
introduction of necessary functionality. Nitrile oxide cycloaddi-
tion should find use as an attractive macromolecular coupling
strategy for the synthesis of biocompatible polymeric nano-
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KEYWORDS: 1,3-dipolar cycloaddition; nitrile oxide; poly(2-meth-
yloxazoline); poly(dimethylsiloxane); poly(ethylene glycol);
poly(L-lactide);
copolymers
poly(N-isopropylacrylamide);
triblock
Click chemistry has emerged as a powerful tool for the syn-
thesis of polymeric architectures via polymer-polymer cou-
pling.13–16 Polymeric blocks may be clicked in a modular
fashion, often in near-quantitative yields using molar equiva-
lents of reactants resulting in minimal purification. By allow-
ing for the synthesis and characterization of individual
blocks prior to coupling, well-defined polymeric architec-
tures can be designed. Furthermore, the click approach
allows for the systematic investigation into the effects of sin-
gle parameter variations, such as block length or identity, on
the resulting nanostructure morphology or biocompatibility.
Although the click approach is an exceptional method for
block copolymer synthesis, not all click reactions meet the
stringent requirements for use in biomedical applications.
INTRODUCTION Polymeric amphiphiles capable of self-
assembling into higher-ordered structures have emerged as
an important class of soft materials in the biomedical field.
Their amphiphilic nature leads to self-assembly upon hydra-
tion, allowing for the compartmentalization of therapeutics
or other biomediscally useful agents.1–4 These are often
block copolymers that can self-assemble into a wide variety
of morphologies, including micelles, vesicles, and tubular
structures, under appropriate conditions.5–7 Vesicular struc-
tures are particularly attractive due to their shell-like mem-
brane surrounding an aqueous core, which can encapsulate
therapeutics.8,9 It is critical to generate nanostructures with
well-defined physical parameters such as size, morphology,
and shape, since these features influence their interactions
with the complement system determining their immune
response.10 The molecular structure of block copolymers dic-
tates the physical properties of resulting nanostructures,
with molecular weight and hydrophobic/hydrophilic ratios
being of utmost importance.5,6,11,12 Therefore a precise and
well-controlled synthesis is essential, which is often difficult
to achieve with common methods such as macroinitiation.
The optimal polymer-polymer coupling strategy should be
clickable, scalable, and versatile in reaction scope, and pro-
vide biocompatible nanostructures. We have previously dem-
onstrated the synthesis of amphiphilic ABA triblock
copolymers containing a hydrophobic poly(dimethylsiloxane)
(PDMS) B-block flanked by hydrophilic A-blocks using two
distinct click methods. Our initial efforts used the prevalent
Additional Supporting Information may be found in the online version of this article.
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readily cleaved at low pH, and though useful for stimuli
responsiveness, the cleavable linker is undesirable for other
applications. Inverse electron demand Diels-Alder reactions
between tetrazine and transcyclooctene display rapid
kinetics, but the reactants require cumbersome synthetic
techniques, rapidly decompose in water, and have low nucle-
ophilic group tolerance.24 Furthermore, there are no reports
of macromolecular coupling using this methodology.
The 1,3-dipolar cycloaddition between nitrile oxide dipoles and
dipolarophiles offers an appealing strategy for the synthesis of
block copolymers via metal-free click coupling. The nitrile oxide
cycloaddition (NOC) couples nitrile oxides with activated-
alkenes to provide isoxazoline heterocycles. Due to the inherent
instability of nitrile oxides, they are derived in situ from chlor-
oxime precursors using an organic base. These highly reactive
dipoles cyclize with a variety of activated-alkene dipolarophiles
with attractive reaction kinetics, are easily installed moieties,
and lead to highly regioselective products.25–27 They have found
use as metal-free click reactions for polymer end group modifi-
cation,28,29 biomolecular conjugation, and as monomers for pol-
ymerizations via polycycloaddition,30–32 but their utility as
macromolecular coupling agents has not yet been established.
Herein, we demonstrate the first report of using NOC as a
metal-free macromolecular coupling strategy for the modular
synthesis of several ABA triblock copolymers (Scheme 1). How-
ever, the difficulty of obtaining the precise molecular weight of
the polysiloxane B-block used in this work prevents the equi-
molar addition of the compounds. Thus, we refrain from refer-
ring to this as a click coupling strategy. However, it should be
stressed this is due to the choice of B-block and not a drawback
of the coupling methodology.
SCHEME 1
1,3-dipolar cycloaddition between polymeric blocks
terminated in nitrile oxides and activated-alkene end-groups to
provide ABA triblock copolymers. Due to their inherent instability,
nitrile oxides are formed in situ from chloroximes. [Color figure
can be viewed in the online issue, which is available at wileyonline-
library.com.]
copper-catalyzed azide-alkyne cycloaddition (CuAAC) to syn-
thesize poly(oxazoline)-poly(siloxane)-poly(oxazoline) (Ox-
ABA) and poly(ethylene glycol)-poly(siloxane)-poly(ethylene
glycol) (P-ABA) triblock copolymers.17 Although CuAAC dem-
onstrated scalability and is known to be wide in scope, trace
amounts of residual copper diminished pivotal in vitro
stealth and biocompatibility attributes of polymer vesicles
(polymersomes) assembled from this method. This led to the
development of a metal-free coupling strategy using the
strain-promoted azide-alkyne cycloaddition (SPAAC) for the
synthesis of P-ABA and Ox-ABA triblocks.18 Polymersomes
assembled via SPAAC demonstrated superior in vitro biocom-
patibility and stealth properties compared with their copper-
catalyzed counterparts, confirming the requirement for a
metal-free click methodology for these structures. However,
the cyclooctyne component is cumbersome to synthesize and
commercially expensive to purchase, therefore SPAAC does
not lend itself to scale-up.
EXPERIMENTAL
Materials
p-Toluenesulfonyl chloride (TsCl; Alfa Aesar, 98%), 4-(dime-
thylamino)pyridine (DMAP; Fluka, 99%), sodium hydroxide
(NaOH; Fisher Chemical), sodium sulfate anhydrous (Na2SO4;
Fisher Chemical, 99%), potassium carbonate anhydrous
(K2CO3; Fisher Chemical), 1,3-bis(4-hydroxybutyl)tetrame-
thyldisiloxane (Gelest, 95%), sulfuric acid (H2SO4; Fisher
Chemical, 98%), triethylamine (TEA; Fisher Chemical), di-
tert-butyl dicarbonate [(Boc)2O] (Aldrich, 99%), acryloyl
chloride (Aldrich, 97%), 4-formylbenzoic acid (Aldrich,
97%), hydroxylamine hydrochloride (Sigma-Aldrich, 99%),
pyridine (Fisher Chemical, 99%), N-chlorosuccinimide (NCS;
Aldrich, 98%), N,N0-dicyclohexylcarbodiimide (DCC; Aldrich,
99%), poly(N-isopropylacrylamide), maleimide terminated
(NIPAM-maleimide; Aldrich, Mn 2.00 k), and poly(L-lactide),
2-hydroxyethyl methacrylate terminated (PLA-methacrylate;
Aldrich, Mn 5 2.00 k, PDI ꢀ 1.1) were used as received.
Poly( ethylene glycol) methyl ether was a gift from Prof. Dr.
Craig Hawker. PEG-acrylate (Mn 5 2.00 k) was prepared from
poly(ethylene glycol) methyl ether and acryloyl chloride fol-
lowing a previously reported protocol.33 Acrylic acid was
distilled over the radical quencher p-methoxyphenol and dis-
tilled under reduced pressure. Silica gel (Silicycle, 230–400
Although several clickable metal-free polymer-polymer cou-
pling methods have been reported, each has drawbacks that
hinder their scope as an ideal and universally applicable
methodology. A conventional Diels-Alder reaction has been
used for macromolecular coupling, however this reversible
cycloaddition required harsh conditions and nonequimolar
reactant ratios.19 The hetero-Diels-Alder reaction has found
use for coupling macromolecules synthesized via reversible-
addition-fragmentation transfer polymerization,20 but intro-
duction of the activated dithioester dienophile moiety does
not lend itself to other polymerization techniques such as
cationic ring-opening polymerization (CROP). Tetrazine-
norbornene polymer-polymer coupling has demonstrated
versatility in synthesizing several diblock copolymers. How-
ever, tetrazine synthesis is low yielding and cumbersome,
thus undesirable for scale-up.21 Despite its popularity, thiol-
ene metal-free click coupling was found to be an unsuitable
method for block copolymer coupling due to poor conver-
sion.22 Aldehyde-hydrazine coupling was achieved via a
Schiff base linker to form diblock copolymers under harsh
reaction conditions.23 The resulting acylhydrazone linker is
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mesh) was deactivated by spinning in an appropriate amount
of TEA:Hexanes v/v 5:95 solution for 10 min followed by fil-
tration and several washes with hexanes. 2-Methyl-2-
oxazoline (Aldrich, 98%) was purified by stirring in calcium
hydride (CaH2) overnight then fractionally distilling through
a 10 cm vigreux column under argon. Octamethylcyclotetra-
siloxane (D4; Gelest, 95%) was distilled from CaH2 under
reduced pressure. Tetrahydrofuran (THF), dichloromethane
(DCM), toluene (PhMe), acetonitrile (MeCN), and N,N0-dime-
thylformamide (DMF) were obtained from a PureSolv solvent
purification system.
7.18 k, PDI 1.50. 1H NMR (500 MHz, CDCl3, d):10.1 (s, 2H,
CHO), 8.19 (d, J 5 8.5 Hz, 4H, ArH), 7.94 (d, J 5 8.5 Hz, 4H,
ArH), 4.36 (t, J 5 6.5 Hz, 4H, CH2), 1.82–1.87 (m, 4H, CH2),
1.51 (m, 4H, CH2), 0.62 (t, 4H, CH2), 0.070 (s, 612H, CH3).
Synthesis of Bis-Aldoxime PDMS (5)
Bis-benzaldehyde PDMS (4) (3.45 g, 0.434 mmol, 1.0 equiv)
was dissolved in THF (20 mL) then pyridine (0.20 mL, 2.60
mmol, 6.0 equiv) was added. A solution of hydroxylamine
hydrochloride (0.12 g, 1.74 mmol, 4.0 equiv) in MeOH
(10 mL) was added dropwise at room temperature. The
solution was stirred overnight, after which 1H NMR demon-
strated complete conversion to the bis-aldoxime. The reac-
tion was diluted with hexanes, washed with MeOH (3 times),
dried over anhydrous Na2SO4 and concentrated to give clear
oil. Yield: 3.14 g (91%). Mn,1HNMR 8.58 k. Mn, GPC 7.44 k, PDI
1.52. 1H NMR (500 MHz, CDCl3, d):8.16 (s, 2H, CNOH), 8.04
(d, J 5 8.5 Hz, 4H, ArH), 7.64 (d, J 5 8.5 Hz, 4H, ArH), 4.33 (t,
J 5 6.5 Hz, 4H, CH2), 1.80–1.85 (m, 4H, CH2), 1.51 (m, 4H,
CH2), 0.61 (t, 4H, CH2), 0.070 (s, 660H, CH3).
Instrumentation
1H NMR and 13C NMR spectra were recorded on a Bruker
Avance DMX500MHz SB NMR Spectrometer or Varian
VNMRS 600MHz SB NMR Spectrometer using solutions of
samples in deuterated chloroform. Gel permeation chroma-
tography (GPC) was performed in chloroform (0.25% TEA)
on a Waters equipped with a refractive index detector.
Molecular weights of polymers were calculated relative to
linear polystyrene standards.
Synthesis of Bis-Chloroxime PDMS (6)
Synthesis of Bis-Benzaldehyde Disiloxane End-Blocker (3)
A solution of 1,3-bis(4-hydroxybutyl)tetramethyldisiloxane
(5.40 mL, 18.0 mmol, 1.0 equiv) and 4-formylbenzoic acid
(6.74 g, 450 mmol, 2.5 equiv) in DCM (90 mL) was cooled to 0
ꢁC. DCC (10.2 g, 50.0 mmol, 2.8 equiv) was then added portion
wise followed by DMAP (44 mg, 0.36 mmol, 0.02 equiv). The
mixture was allowed to warm to room temperature and
stirred overnight after which time TLC showed consumption
of the starting material. The slurry was diluted with Et2O, fil-
tered through celite, and concentrated to give pale yellow oil.
This crude oil was purified by column chromatography on
silica gel (EtOAc:Hexanes, 15/85) to give 3 as a colorless oily
To a solution of bis-aldoxime PDMS (5) (2.14 g, 0.250 mmol,
1 equiv) in THF (12 mL) was added DMF (4 mL). The solu-
tion was cooled to 0 ꢁC then NCS (0.080 g, 0.60 mmol, 2.4
equiv) was added. The solution was allowed to warm to
1
room temperature and stirred overnight after which H NMR
demonstrated complete conversion to the bis-chloroxime.
The reaction was diluted with hexanes, transferred to a sepa-
ratory funnel and a small amount of MeOH (about 1 mL)
was added to promote phase separation. The DMF/MeOH
layer was removed and the hexanes layer was washed with
MeOH (6 times), dried over anhydrous Na2SO4 and concen-
1
trated to give clear oil. Yield: 2.00 g (93.4%). Mn,
1HNMR
solid. Yield: 6.54 g (67%). H NMR (500 MHz, CDCl3, d):10.1
9.54 k.1H NMR (500 MHz, CDCl3, d):8.06 (d, J 5 8.5 Hz, 4H,
ArH), 7.92 (d, J 5 8.5 Hz, 4H, ArH), 4.34 (t, J 5 6.5 Hz, 4H,
CH2), 1.78–1.83 (m, 4H, CH2), 1.51 (m, 4H, CH2), 0.61 (t, 4H,
CH2), 0.070 (s, 732H, CH3).
(s, 2H, CHO), 8.18 (d, J 5 8.0 Hz, 4H, ArH), 7.94 (d, J 5 8.5 Hz,
4H, ArH), 4.36 (t, J 5 6.75 Hz, 4H, CH2), 1.81 (m, 4H, CH2), 1.49
(m, 4H. CH2), 0.59 (t, 4H, CH2), 0.057 (s, 12H, CH3).13C NMR
(600 MHz, CDCl3, d):191.8, 165.8, 139.2, 135.6, 130.3, 129.6,
65.5, 32.2, 20.0, 18.1, 0.486.
Synthesis of 3-Boc-Aminobenzyl Alcohol (8)
Synthesis of Bis-Benzaldehyde PDMS (4)
To a solution of 3-aminobenzyl alcohol (12.9 g, 105 mmol,
1.0 equiv) in THF (112 mL) was added 2M NaOH aq.
(66 mL, 132 mmol, 1.2 equiv) at room temperature. The sus-
pension was cooled to 0 ꢁC and solution of Boc2O (25.7 g,
117 mmol, 1.1 equiv) in THF (112 mL) was added dropwise.
The reaction was allowed to warm to room temperature and
stirred overnight, after which TLC showed consumption of
the starting material. The slurry was filtered through celite,
diluted with ethyl acetate then placed in a separatory funnel
and the aqueous layer was removed. The organic layer was
washed with H2O, brine, dried over anhydrous Na2SO4, and
concentrated. The resulting crude oil was purified by column
chromatography on deactivated silica gel (EtOAc:Hexanes,
30/70) to give 8 as white solid. Yield: 13.9 g (59%). 1H
NMR (500 MHz, CDCl3, d):7.45 (s, 1H, ArH), 7.28 (t, J 5 7.8
Hz, 1H, ArH), 7.21 (t, J 5 7.2 Hz, 1H, ArH), 7.04 (d, J 5 6.6
Hz, 1H, ArH), 6.47 (s, 1H, NH), 4.67 (m, 2H, CH2), 1.63 (t,
Bis-benzaldehyde end-blocker (3) (0.700 g, 1.29 mmol, 1.0
equiv) was added to an oven-dried round bottom flask with a
stir bar and stirred under reduced pressure for 30 min at
room temperature to remove trace water. The end-blocker
was first dissolved in PhMe (1 mL), then freshly distilled D4
(8.00 mL, 25.8 mmol, 20 equiv) was added, and the mixture
ꢁ
was heated to 80 C to fully solubilize the reagents. Concen-
trated H2SO4 (0.030 mL) was added dropwise, at which time
the solution became viscous. After 21 h an aliquot was taken
for 1H NMR, which indicated complete monomer consumption.
The reaction was cooled to room temperature, diluted with
hexanes, quenched with Et3N (1 mL), and stirred for 30 min.
The solvent was concentrated in vacuo. The polymer was dis-
solved in hexanes, washed with MeOH (3 times), and dried
over anhydrous Na2SO4 and concentrated to give pale yellow
viscous oil. Yield: 7.00 g (80%), Mn,1HNMR 7.96 k. Mn,
GPC
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1H, OH), 1.52 (s, 9H, CH3). 13C NMR (600 Hz, CDCl3,
d):152.9, 142.1, 138.7, 129.3, 121.6, 117.9, 117.1, 65.3, 28.5.
0.0209 mmol, 1 equiv) in CHCl3 (2 mL) was added Et3N
(0.030 mL, 0.209 mmol, 10 equiv). The solution was stirred
for 1 h at room temperature. On removal of the solvent,
diethyl ether (20 mL) was added. The solution was filtered
and filtrate was concentrated. The residue was dissolved in
THF, filtered to remove the TEA salt, and concentrated to
yield 11 as a colorless solid, yield 91.2%. 1H NMR (600
MHz, CDCl3, d):8.30–6.95 (m, 6H, ArH), 8.08 (d, 4H, ArH),
7.72 (d, 4H, ArH), 6.83 (m 2H, ArH), 5.24 (m, 2H, CH), 4.48
(t, 4H, CH2), 4.33 (m, 8H, CH2), 3.45-3.49 (br. m, 192H, CH2),
2.15 (br. m, 144H, CH3), 1.80 (m, 4H, CH2), 1.50 (s, 18H,
CH3) 0.61 (t, 4H, CH2), 0.066 (s, 732H, CH3). GPC Mn 21.2 k;
PDI 1.2.
Synthesis of 3-Boc-Aminobenzyl Tosylate (9)
To a solution of 3-Boc-aminobenzyl alcohol (8)ꢁ(4.05 g, 18.1
mmol, 1.0 equiv) and DCM (90 mL) cooled to 0 C was added
TsCl (3.62 g, 19.0 mmol, 1.05 equiv) and DMAP (0.22 g, 1.81
mmol, 0.1 equiv) followed by Et3N (3.80 mL, 27.2 mmol, 1.5
ꢁ
equiv) dropwise. The solution was stirred at 0 C, and after 1
hr TLC showed consumption of the staring material. The reac-
tion was quenched with saturated NaHCO3 aq. and the organic
layer was washed with H2O, brine, dried over anhydrous
Na2SO4, and concentrated. The resulting gummy solid was
purified by column chromatography on deactivated silica gel
(EtOAc:Hexanes, 30/70) to give 9 as white solid. Yield: 3.24 g
(47%). 1H NMR (500 MHz, CDCl3, d):7.79 (d, J 5 10.2, 2H,
ArH), 7.33–7.27 (m, 4H, ArH), 7.22 (t, J 5 9.3 Hz, 1H, ArH),
6.93 (d, J 5 9.0 Hz, 1H, ArH), 6.44 (s, 1H, ArH), 5.02 (s, 2H,
CH2), 2.44 (s, 3H, CH3), 1.52 (s, 9H, CH3). 13C NMR (600 MHz,
CDCl3): d 5 152.7, 144.9, 138.8, 134.4, 133.4, 130.0, 129.5,
128.1, 123.0, 119.0, 118.4, 28.5, 21.8.
Synthesis of PLA-PDMS-PLA (12)
Same procedures were followed as preparation of 11 except
the reaction was stirred for 2 h at room temperature. PLA-
PDMS-PLA (12) was obtained as a white solid, yield 90.9%.
1H NMR (500 MHz, CDCl3, d): 8.06 (d, J 5 8.5 Hz, 4H, ArH),
7.71 (d, J 5 8.0 Hz, 4H, ArH), 5.16–6.12 (m, 68 H, CH), 4.32–
4.46 (m, 14H), 1.96 (m, 4H, CH2), 1.72 (s, 6H, CH3), 1.48–
1.60 (m, 208H, CH3), 0.61 (t, 4H, CH2), 0.070 (s, 732H, CH3).
GPC Mn 5 15.4 k; PDI 1.26.
Synthesis of PMOXA-Acrylate (10)
3-Boc-aminobenzyl tosylate (9) (1.62 g, 4.29 mmol, 1.0 equiv)
was added to an oven-dried round bottom flask with a stir
bar and stirred at room temperature under reduced pressure
overnight to remove trace water. MeCN (5.5 mL) was added
Synthesis of PEG-PDMS-PEG (13)
Same procedures were followed as preparation of 11. PEG-
PDMS-PEG (13) was obtained as a colorless solid, yield
1
91.5%. H NMR (500 MHz, CDCl3, d): 8.07 (d, J 5 8.5 Hz, 4H,
ꢁ
and the reaction was heated to 45 C to solubilize the initia-
ArH), 7.74 (d, J 5 8.5 Hz, 4H, ArH), 5.23–5.26 (m, 2H, CH),
4.32–4.38 (m, 8H, CH2), 3.63–3.79 (m, 324H, CH2), 3.38 (s,
6H, CH3), 1.81–1,85 (m, 4H, CH2), 1.48–1.52 (m, 4H, CH2),
0.61 (m, 4H, CH2), 0.070 (s, 732H, CH3). GPC, Mn 21.5 k; PDI
1.13.
tor, at which time freshly distilled 2-methyl-2-oxazoline
(5.5 mL, 64.4 mmol,ꢁ15 equiv) was added. The temperature
was increased to 80 C and the reaction was stirred for 21 h
at which time 1H NMR demonstrated full initiation and com-
plete monomer consumption. Additional MeCN (6 mL) was
added to decrease the reactions viscosity. The reaction was
cooled to 0 ꢁC and freshly distilled acrylic acid was added
(0.42 mL, 6.14 mmol, 1.4 equiv) followed by Et3N (1.2 mL,
Synthesis of PNIPAM-PDMS-PNIPAM (14)
Same procedures were followed as preparation of 11 except
ꢁ
TEA was added at 0 C and the reaction was stirred for 1 h
ꢁ
8.37 mmol, 1.95 equiv). The reaction was heated to 60 C and
at 0 ꢁC and another hour at room temperature. PNIPAM-
PDMS-PNIPAM (14) was obtained as a light yellow solid,
yield 93%. 1H NMR (600 MHz, CDCl3, d): 8.02–8.08 (m,
8H,ArH), 7.56 (m, 2H, CH), 6.34 (br, 36H, CH), 4.31–4.33 (m,
4H, CH2), 4.00 (m, 36H, CH), 3.58–3.69 (m, 4H, CH2), 1.32–
2.18 (m, 348H, CH3), 0.069 (s, 732H, CH3). GPC Mn 14.2 k;
PDI 1.30.
1
stirred overnight, after which H NMR demonstrated complete
termination. The reaction was cooled to room temperature,
precipitated into cold Et2O, filtered, and the solid was washed
with Et2O then collected with DCM and concentrated. The
solid was dissolved in acetone, K2CO3 (10 g) was added, and
the mixture was stirred at room temperature overnight. The
solution was then concentrated, dissolved in DCM, filtered
through celite, and the filtrate was concentrated. The solid
was redissolved in a minimal amount of DCM and precipitated
into cold Et2O, filtered, and the solid was washed with Et2O
then collected with DCM and concentrated to yield PMOXA-
acrylate (10) as a white solid in quantitative yield. Mn, 1H NMR
Synthesis of High Mn PMOXA-PDMS-PMOXA (19)
Same procedures were followed as preparation of 11. PMOXA-
1
PDMS-PMOXA (19) was obtained as a colorless solid. H NMR
(600 MHz, CDCl3, d): 8.30–6.95 (m, 6H, ArH), 8.08 (d, 4H, ArH),
7.72 (d, 4H, ArH), 6.83 (m, 2H, ArH), 5.24 (m, 2H, CH), 4.48 (t,
4H, CH2), 4.33 (m, 8H,CH2), 3.45 (br. m, 140H, CH2), 2.15 (br. m,
102H, CH3), 1.50–1.66 (s, 18H, CH3), 0.59 (t, 4H, CH2), 0.060 (s,
2160H, CH3). GPC, Mn 34.6 k; PDI 1.96.
2.32 k, Mn,
1.73 k, PDI 1.42.1H NMR (500 MHz, CDCl3,
GPC
d):6.95–7.73 (m, 3H, ArH), 6.85 (m, 1H, ArH), 6.38–6.46 (m,
1H, Acrylate), 6.10–6.16 (m, 1H, Acrylate), 5.88–5.95 (m, 1H),
4.49 (t, 2H, CH2), 4.29 (s, 2H, CH2), 3.46-3.50 (br. m, 96H,
CH2), 2.15 (br. m, 72H, CH3), 1.52 (s, 9H, CH3).
RESULTS AND DISCUSSION
Synthesis of PMOXA-PDMS-PMOXA (11)
Synthesis of Bis-Chloroxime PDMS
To a solution of PMOXA-acrylate (10) (0.106 g, 0.0460
mmol, 2.2 equiv) and bis-chloroxime PDMS (6) (0.200 g,
Due to their inherent instability, nitrile oxides were prepared
from chloroximes in situ, which were readily installed as
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precursor bis-chloroxime PDMS (6) was readily synthesized in
two steps using standard procedures. Condensation of bis-
aldehyde PDMS (4) was performed with hydroxylamine in the
presence of pyridine to generate bis-aldoxime PDMS (5) in
good yield (Mn, 1H NMR 8.58 k; Mn,GPC, 7.44 k, PDI 1.52). Chlori-
nation of the bis-aldoxime with NCS led to quantitative conver-
sion to bis-chloroxime PDMS (6), as demonstrated by 1H NMR
with Mn,
9.54 k. The inability to obtain the precise
1H NMR
molecular weight of the PDMS B-block, using GPC, prevents
the equimolar addition of polymers during the coupling step
and a 2.2 equivalents (1.1 for each side) of the A block was
used. For all subsequent coupling reactions, the molecular
weight of the B-block was carefully estimated by increasing
the relaxation times in the 1H NMR.
Synthesis of Acrylate-Functionalized Poly(oxazoline)
The synthesis of bi-functional acrylate-terminated poly(me-
thyloxazoline) (PMOXA) A-block (10) was achieved via CROP
of 2-methyl-2-oxazoline using an amine-protected benzyl tos-
ylate initiator, followed by termination with an clickable
acrylate group (Scheme 3). The acrylate end-group may be
clicked with nitrile oxides, while a free amine can be used as
a functional handle to affix additional chemical moieties. Tos-
ylate initiator (9) was synthesized in two steps starting from
3-aminobenzyl alcohol (7). The Boc group is stable to most
bases and has high nucleophilic tolerance, but is readily
cleaved under mild acidic conditions.35 Selective protection
of the amine with Boc was achieved by using (Boc)2O in a
biphasic solution of 2 M NaOH aq./THF to give 3-Boc-
aminobenzyl alcohol (8) using a modified literature proce-
dure.36 Alcohol tosylation with Et3N and DMAP provided 3-
Boc-amino benzyl tosylate (9). CROP of 2-methyl-2-oxazoline
initiated by tosylate (9) was performed in MeCN at 80 ꢁC,
SCHEME 2
Synthesis of the nitrile oxide precursor bis-chlor-
1
and after 21 h H NMR demonstrated full initiation and com-
oxime PDMS (6) via CROP of cyclic siloxane monomer using
plete monomer consumption. Using a modified termination
procedure, the solution was first cooled to 0 ꢁC and diluted
with MeCN to decrease the viscosity of the solution.37,38
Acrylic acid was added dropwise followed by Et3N, the reac-
tion was heated to 60 ꢁC and stirred overnight to terminate
the polymerization. 1H NMR demonstrated complete termi-
nation, and the polymer was isolated by precipitation into
cold Et2O. The resulting acrylate salt was deprotonated by
stirring with K2CO3 in acetone. PMOXA-acrylate (10) was
the end-blocker method.
end-groups on a PDMS B-block (Scheme 2). By initially synthe-
sizing a bis-benzaldehyde PDMS B-block, we envisioned the
desired bis-chloroximes could be constructed via postpolyme-
rization functional group modification. The synthesis of bis-
aldehyde PDMS was accomplished via CROP of cyclic siloxane
monomers using the end-blocker method.34 This short-chain
disiloxane, referred to as the end-blocker, serves as the end-
groups of the poly(siloxane) polymer while terminating the
polymerization. Bis-aldehyde end-blocker (3) was synthesized
by coupling 1,3-bis(4-hydroxybutyl)tetramethyldisiloxane (1)
with 4-formylbenzoic acid (2) in the presence of N,N-dichclo-
hexylcarbodiimide (DCC) and 4-dimethylaminopyridine
(DMAP). Using benzaldehyde end-blocker (3) and octamethyl-
cyclotetrasiloxane (D4) as the cyclic siloxane monomer, bifunc-
tional bis-aldehyde PDMS (4) was synthesized via CROP using
H2SO4 as the catalyst. After workup, the PDMS B-block was
provided in good yield with Mn 7.96 k as determined by 1H
NMR using end-group analysis and Mn 7.19 k, PDI 1.50 from
GPC. By means of end-group modification, the nitrile oxide
1
provided with Mn 2.32 k, as demonstrated by H NMR using
SCHEME 3
Synthesis of acrylate-functionalized poly(methy-
loxazoline) was achieved via CROP of 2-methyl-2-oxazoline
using Boc-protected tosylate initiator 9, followed by termina-
tion with acrylic acid.
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SCHEME 4
Synthesis of Ox-ABA triblock copolymer (11) from PMOXA-acrylate (10) and bis-chloroxime PDMS (6) using Et3N for
the in situ generation of the nitrile oxides. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.
com.]
end-group analysis and Mn 1.73 k (PDI 1.42) as determined
by GPC.
tions, by observing the appearance of isoxazoline peaks con-
current with the consumption of dipolarophile peaks. Since
GPC columns used typically contain TEA, monitoring the pro-
gress of the reaction using GPC is infeasible since the chlor-
oxime is converted to the nitrile oxide in the column leading
to dimerization.
ABA Triblock Copolymers via NOC
Nitrile oxide dimerization is a competing reaction with the
desired 1,3-dipolar cycloaddition, thus dipolarophiles with
high cycloaddition rates must be used.25 We therefore chose
the activated dipolarophiles acrylate, methacrylate or malei-
mide, whose electron withdrawing properties enhance the
reactivity of the dipolarophiles by lowering the energy gap
of the interacting frontier molecular orbitals.39 1H NMR was
utilized to monitor the progress of all polymer-coupling reac-
We initially investigated the synthesis of PMOXA-PDMS-
PMOXA (Ox-ABA) triblock copolymers from bis-chloroxime
PDMS (6) and PMOXA-acrylate (10) with Et3N as an
organic base, which converts the chloroxime into the reac-
tive nitrile oxide in situ. The cycloaddition proceeded
FIGURE 1
1H NMR spectra of bis-chloroxime PDMS (6) (a), PMOXA-acrylate (10) (b), and NOC product Ox-ABA (11) (c). Con-
sumption of the acrylate peaks in conjunction with the appearance of isoxazoline peaks demonstrates polymer-polymer coupling.
[Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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TABLE 1 ABA Triblock Copolymers Synthesized via the NOC Coupling Reaction
A-Block
B-Block
Condition
ABA polymers
Et3N, CH3Cl, rt, 1 h,
Yield: 91.2%
Et3N, CH3Cl, rt, 2 h,
Yield: 90.0%
Et3N, CH3Cl, rt, 1 h,
Yield: 91.5%
Et3N, CH3Cl,
0
ꢁC-rt, 2 h,
Yield: 93%
rapidly in a solution of chloroform at room temperature
(Scheme 4). Samples were taken every hour, 1H NMR
demonstrated almost no change of the intensity PMOXA-
acrylate (10) peaks at 6.36, 6.13, and 5.88 ppm and iso-
xazoline peak of Ox-ABA (11) at 5.24 ppm after 1 h and
no side products indicative of nitrile oxide dimerization
(Fig. 1).
temperature to provide PLA-PDMS-PLA (PLA-ABA) (12),
which can be ascribed to the steric hindrance of methyl
group of the methacrylate. When the coupling was attempted
with PNIPAM-maleimide at room temperature, some amount
of nitrile oxide dimerization product was observed via 1H
NMR. When the reaction temperature was cooled to 0 ꢁC,
PNIPAM-PDMS-PNIPAM (N-ABA) (14) was synthesized in 2 h
with no nitrile oxide dimerization observed. This indicates
that the decreased temperature did not hinder the rapid
cycloaddition kinetics of the madimide dipolarophile. The
NOC reaction of much higher molecular weight (Mn 27.0 k)
PDMS B-blocks with PMOXA-acrylate (10) was also investi-
gated. Bis-chloroxime PDMS (18) (Mn 27.0 k) was prepared
by means of end-group modification, similar to the proce-
dure used for the low molecular weight PDMS (6), starting
from commercial available bis(3-aminopropyl) terminated
PDMS (Supporting Information Scheme S1). The coupling of
high molecular weight bis-chloroxime PDMS (18) with
PMOXA-acrylate (10) via NOC was efficient under standard
reaction conditions demonstrating the reactions utility for
coupling high molecular weight polymers.
The scope of the NOC was investigated by synthesizing
several ABA triblock copolymers with various block identi-
ties, molecular weights, and dipolarophiles. We investigated
the NOC with hydrophilic A-blocks poly(ethylene glycol)
(PEG) and poly(N-isopropylacrylamide) (PNIPAM) or hydro-
phobic poly(L-lactide) (PLA) terminated with acrylate, mal-
eimide or methacrylate dipolarophiles, respectively (Table
1). PEG and PMOXA are known to impart stealth like
properties,18 while PNIPAM block copolymers are known
to be thermoresponsive40 and PLA-PDMS-PLA block
copolymers are known to self-assemble into nano dots.41
In addition, a high molecular weight bis-chloroxime PDMS
(18) B-block (Mn 27.0 k) was used to demonstrate the
feasibility of high molecular weight polymer-polymer cou-
pling using the NOC.
CONCLUSIONS
In all the cases, the corresponding triblock copolymers were
obtained successfully with TEA as base at room temperature
using the standard procedure. However, due to the differen-
ces in reactivity of end-group dipolarophiles and solvation of
polymer blocks, reaction time and temperature required
optimization with certain substrates. In the case of PLA-
metharylate, conversion was accomplished in 2 h at room
We have established the NOC as a robust, rapid, and versatile
metal-free polymer-polymer coupling methodology for the
modular synthesis of amphiphilic ABA triblock copolymers.
Nitrile oxide precursors and various dipolarophiles were read-
ily introduced as end-groups into polymeric blocks, which
were then coupled rapidly using mild reaction conditions. We
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16 B. S. Sumerlin, A. P. Vogt, Macromolecules, 2010, 43, 1–13.
have successfully clicked nitrile oxide terminated PDMS B-
blocks with several distinct hydrophilic (PMOXA, PEG, and
PNIPAM) or hydrophobic (PLA) A-blocks terminated in acti-
vated dipolarophiles in a straightforward, mild, and rapid fash-
ion. This method overcomes many drawbacks of previous
metal-free methods due to its rapid kinetics, versatility, scal-
ability, and ease of introduction of necessary functionality.
These results confirm NOC as versatile coupling strategy for
the synthesis amphiphilic ABA triblock copolymers. This
method should find use as an attractive macromolecular cou-
pling strategy for the synthesis of biocompatible polymeric
nanostructures. Efforts to expand this methodology and
explore the utilization of polymeric amphiphiles as drug deliv-
ery vehicles are currently being undertaken in our laboratory.
17 M. J. Isaacman, K. a Barron, L. S. Theogarajan, J. Polym.
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The authors wish to thank Jerry Hu. The authors gratefully
acknowledge support from the National Institutes of Health,
through the NIH Director’s New Innovator Award Program (1-
DP2-OD007472-01) and the National Science Foundation
under Award No. DMR-1121053.
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