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Amphiphilic Bottlebrush Block Copolymers:
Analysis of Aqueous Self-Assembly by Small Angle
Neutron Scattering and Interfacial Tension
Measurements
Mohammed Alaboalirat1, Luqing Qi2, Kyle J. Arrington1, Shuo Qian3, Jong K. Keum3,4, Hao
Mei2, Kenneth C. Littrell3, Bobby G. Sumpter4,5, Jan-Michael Y. Carrillo4,5, Rafael Verduzco2*
and John Matson1*
1Department of Chemistry and Macromolecules Innovation Institute, Virginia Tech, Blacksburg,
Virginia 24061, United States
2Department of Chemical and Biomolecular Engineering, Rice University, Houston, Texas
77005, United States
3Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831,
United States
4Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge,
Tennessee 37831, United States
5Computational Sciences and Engineering Division, Oak Ridge National Laboratory, Oak Ridge,
Tennessee, 37831, United States
1
E-mail: jbmatson@vt.edu, rafaelv@rice.edu
KEYWORDS
Comb polymers, end group removal, atom-transfer radical polymerization, reversible addition–
fragmentation chain transfer polymerization, reversible-deactivation radical polymerization
ABSTRACT A systematic series of 16 amphiphilic bottlebrush block copolymers (BCPs)
containing polystyrene and poly(N-acryloyl morpholine) (PACMO) side chains was prepared by
a combination of atom-transfer radical polymerization (ATRP), photoiniferter polymerization,
and ring-opening metathesis polymerization (ROMP). The grafting-through method used to
prepare the polymers enabled a high degree of control over backbone and side-chain molar
masses for each block. Interfacial tension measurements on the self-assembled amphiphilic
bottlebrush BCPs in water revealed an ultralow critical micelle concentration (CMC), 1-2 orders
of magnitude lower than linear block copolymer analogs on a molar basis, even for micelles with
>90% PACMO content. Combined with coarse-grained molecular dynamics simulations, fitting
of small-angle neutron scattering traces (SANS) allowed us to evaluate solution conformations
for individual bottlebrush BCPs and micellar nanostructures for self-assembled macromolecules.
Bottlebrush BCPs showed an increase in anisotropy with increasing PACMO content in toluene-
d8, which is a good solvent for both blocks, reflecting an extended conformation for the PACMO
block. SANS traces of bottlebrush BCPs assembled into micelles in D2O, a selective solvent for
PACMO, were fitted to a core-shell-shell model, suggesting the presence of a partially hydrated
inner shell. Results showed an average micelle diameter of 40 nm with combined shell diameters
ranging from 16-18 nm. A general trend of increased stability of micelles (i.e., resistance to
2
precipitation) was observed with increases in PACMO content. These results demonstrate the
stability of bottlebrush polymer micelles, which self-assemble to form spherical micelles with
ultralow (< 70 nmol/L) CMCs across a broad range of compositions.
INTRODUCTION
In recent years, the study of bottlebrush polymers has attracted significant attention due to their
unique topology that results in high rigidity and shape persistence.1-3 Bottlebrush polymers
contain a polymer backbone with densely grafted polymeric side chains, causing the backbone to
have an extended chain conformation.4-5 This topology results in unusual rheological and
mechanical properties, such as lower viscosity compared with linear polymers of similar
molecular weights due to a smaller hydrodynamic radius and lack of chain entanglements.6-7
Moreover, bottlebrush polymers can adopt either spherical8-10 or cylindrical conformations11-13
depending on grafting density and backbone/side chain molecular weights. Thus far, bottlebrush
polymers have been prepared for several potential applications, including drug delivery,14-19
supersoft elastomers,20 pressure-sensitive adhesive,21 antifouling coatings,22-23 photonic crystals,24-
26 lithographic materials,27-29 rheological modifiers,30-31 nanoporous membranes for separations,32
and nano-objects of controlled size and shape.33-36
Due to their large size, bottlebrush polymers exhibit unique self-assembly behavior. In the bulk
or thin films, bottlebrush block copolymers (BCPs) can rapidly assemble to form photonic
structures with large characteristic domains.26 The rapid assembly is a result of the bottlebrush
architecture which results in a very high entanglement molecular weight.31, 37-38 In solution,
3
bottlebrush polymers self-assemble to form large micelles due to their larger size compared with
linear polymers (Figure 1).39 This large size also results in a more stable adsorption to oil-water
interfaces than linear BCPs, improving the stability of Pickering emulsions in the presence of
bottlebrush polymers.30 Finally, work with bottlebrush polymers for drug delivery indicates their
critical micelle concentrations (CMCs) in aqueous solution may be substantially lower than
CMCs of analogous linear polymers, resulting from their much larger size compared with linear
polymers.14 These experimental results were recently corroborated in computational studies by
Jayaraman and coworkers.40
Water
Figure 1. Schematic for the self-assembly of amphiphilic PS-PACMO bottlebrush BCPs in
water, where the red chains represent the hydrophobic PS chains and the blue chains represent
the hydrophilic PACMO chains.
Amphiphilic bottlebrush polymers are potentially relevant as additives for stabilizing
emulsions and for encapsulating hydrophobic agents for delivery to target sites. However, a
detailed understanding of their self-assembly behavior, including CMC and micelle structure in
solution, is not available. This is surprising considering that amphiphilic bottlebrush BCPs may
have undiscovered potential in applications where traditional surfactants are currently used.
Herein, we report the synthesis of a systematic series of amphiphilic bottlebrush BCPs prepared
by ROMP grafting-through. The bottlebrush BCPs are comprised of polystyrene (PS) side-chains
4
and poly(N-acryloyl morpholine) (PACMO) side-chains. We synthesized a comprehensive
library of bottlebrush BCPs and quantified their self-assembly in water through a combination of
interfacial tension measurements (IFT) to quantify their CMC and through small-angle neutron
scattering measurements (SANS) to understand their solution conformation. Our work highlights
the novel self-assembly behavior of bottlebrush BCPs and their extremely low CMC, more than
an order of magnitude lower compared with linear BCPs.
Experimental Section
Materials
All reagents were obtained from commercial vendors and used as received unless otherwise
stated. Solvents were obtained from solvent drying column, and used without further
purification. Styrene, methyl acrylate, and N-acryloyl morpholine (ACMO) were passed through
a small column of basic alumina to remove the radical inhibitor. (H2IMes)(Cl)2(PCy3)Ru=CHPh
(G2) was obtained as a generous gift from Materia (Pasadena, CA.) ROMP catalyst G3 was
prepared from G2 according to literature procedures.41-42
Characterization
NMR spectra were measured on Bruker 500 MHz or Agilent 400 MHz spectrometers. 1H and
13C NMR chemical shifts are reported in ppm relative to internal solvent resonances of CDCl3.
Yields refer to spectroscopically and chromatographically pure compounds unless otherwise
stated. Size exclusion chromatography (SEC) was carried out in tetrahydrofuran (THF)
containing BHT at 1 mL min−1 at 30 °C on two MIXED-B Agilent PLgel 10 µm columns
connected in series with a Wyatt Optilab Rex refractive index detector and a Wyatt Dawn Heleos
2 light scattering detector. No calibration standards were used, and dn/dc values were obtained
5
by assuming 100% mass elution from the columns PS and PACMO macromonomers. The dn/dc
for the bottlebrush BCPs were calculated using a weighted average of each macromonomer.
IFT Measurements
Bottlebrush BCP micelles were prepared by dissolving bottlebrush BCPs in THF at a
concentration of 15 mg in 0.2 mL. The solution was transferred to a dialysis bag, diluted with an
additional 0.2 mL THF and 15 mL water, and dialyzed against DI water for 1 week, exchanging
the DI water every 24 h. The final concentration of bottlebrush BCP in water in the dialysis bag
was 15 mg in 15 mL. The aqueous bottlebrush BCP solution was then further diluted with water
to bring the total solution volume to 50 mL, which corresponds to a polymer concentration of
300 mg/L.
For IFT analysis, the stock solution was further diluted to 3 ppm (3 mg/L) and loaded
into a Kruss K100 sample chamber. The solution was covered and allowed to equilibrate for at
least 12 h prior to measurement using the Wilhelmy plate method with a flat titanium plate. Next,
stock solution was added to the sample chamber to systematically increase the bottlebrush
polymer concentration. Each solution was allowed to equilibrate for at least 12 h prior to
measurement. A series of IFT vs. bottlebrush BCP concentrations in the range of 3 to 45 ppm
was collected for each sample. In a few cases where the PS content was significantly higher than
the PACMO content, aggregates formed in the aqueous solution. These samples were not
analyzed through IFT measurements.
SANS Measurements
In order to prepare samples for SANS analysis, bottlebrush BCPs were either dissolved in
toluene-d8 or D2O at a total concentration of 1 wt %. Samples were allowed to sit for at least 48
6
hours prior to analysis. Samples that did not dissolve were not analyzed by SANS. Samples were
measured at the CG-3 Beamline at the High Flux Isotope Reactor (HFIR) at Oak Ridge National
Laboratory (ORNL). The data were collected with a single instrument configuration covering a
range of 0.0009 < q < 0.24 Å-1 with 6 Å wavelength neutrons (wavelength spread ~15%), a main
position sensitive detector at a 15.5 m sample-to-detector distance (SDD), and a “wing” position
sensitive detector at a 1.13 m SDD. The data were corrected against instrument background,
detector sensitive and geometry, buffer background and then azimuthally averaged and merged
from two detectors to form 1-dimensional SANS profiles.43
Synthesis of Norbornene Alcohol 1
Norbornene alcohol 1 was prepared by adapting previously reported procedures.44-46 Briefly,
cyclopentadiene (70.0 g, 1060 mmol) and methyl acrylate (91.0 g, 1060 mmol) were dissolved in
dichloromethane (DCM) (70 mL) in a round-bottom flask equipped with a stir bar. The reaction
mixture was brought to reflux in an oil bath and allowed to stir for 16 h. The reaction mixture
was then evaporated under low pressure until H1 NMR spectroscopy showed no residual methyl
acrylate, yielding crude 5-norbornene-2-carboxylate (NBCY) (25% exo/75% endo) as a white
solid. Next, potassium tert-butoxide (KOtBu, 134.0 g, 1190 mmol) was dissolved in THF (400
mL) under N2 in a two-neck round bottom flask equipped with a stir bar and an addition funnel.
Crude NBCY (151 g, 992 mmol) was dissolved in THF (200 mL) and added dropwise to the
KOtBu solution via an addition funnel, and the reaction mixture was allowed to stir for 3 h at rt
after complete addition. Next, deionized water (17.9 mL, 990 mmol) in THF (170 mL) was
added dropwise via an addition funnel to the reaction mixture. The reaction mixture was then
7
allowed to stir at rt for 17 h. To complete the hydrolysis reaction, excess water (100 mL) was
added, and the reaction mixture was stirred for an additional 1 h. The reaction mixture was then
concentrated to ~250 mL by rotary evaporation. The reaction mixture was washed 3 times with
diethyl ether to remove residual dicyclopentadiene. To this aqueous solution, concentrated HCl
was added slowly until the pH reached 2. The aqueous solution was extracted 3 times with
diethyl ether (3x100 mL), and then the combined organic layers were dried over Na2SO4, and
concentrated to dryness by rotary evaporation to afford crude 5-norbornene-2-carboxylic acid as
an off-white solid (NBCA) (95.0 g, 690 mmol, 80% exo/ 20% endo).
To isolate the exo isomer, NBCA (80% exo/20% endo) (95.0 g, 690 mmol) and Na2CO3 (30.0
g, 360 mmol) were dissolved in water (450 mL) in a round-bottom flask equipped with a stir bar.
In a separate round bottom flask, I2 (17.5 g, 69 mmol) and KI (22.9 g, 138 mmol), where
equivalency is set to the amount of endo isomer, were dissolved in water (150 mL). The I2/KI
solution was added to the NBCA solution dropwise via an addition funnel until a brown color
persisted. The aqueous solution was extracted with diethyl ether (5x200 mL) to remove the
iodolactone derived from the endo isomer. The aqueous solution was decolorized by adding 10%
Na2S2O3 (90 mL), and the pH was brought to 2 by slow addition of concentrated HCl. The
aqueous solution was then extracted with diethyl ether (4x150 mL). The organic layers were
combined, dried over Na2SO4 and evaporated under reduced pressure to yield exo-NBCA (63.0
g, 460 mmol).
LiAlH4 (17.3 g, 460 mmol) was dissolved in dry THF (400 mL) under an N2 environment in a
two-neck round bottom flask in an ice bath equipped with a stir bar, an adapter for N2, and an
addition funnel. Exo-NBCA (63.0 g, 460 mmol) was dissolved in THF (260 mL) and added
dropwise to the LiAlH4 suspension via addition funnel. The addition funnel was then replaced
8
with a condenser, and the reaction mixture was heated in an oil bath to reflux for 12 h. The
reaction mixture was placed in an ice bath and allowed to cool to 0 ºC; once cool, 1N HCl was
slowly added until no more foaming was observed. Brine was added to the solution until two
separate layers formed. The two layers were transferred to a separatory funnel, and the top
organic layer was collected, dried over Na2SO4, and evaporated under reduced pressure. The
product was purified by distillation under vacuum to yield a colorless viscous liquid (45 g, 360
mmol, 78% yield). 1H NMR (CDCl3): δ 6.07 (m, 2H), 3.70 (m, 1H), 3.54 (m, 1H), 2.83 (s, 1H),
2.74 (s, 1H), 1.64 (m, 1H), 1.31 (m, 3H), 1.12 (m, 1H). 13C NMR (CDCl3): δ 136.9, 136.5, 67.7,
45.1, 43.4, 42.0, 41.6, 29.6.
Synthesis of 2-(Dodecylthiocarbonothioylthio)-2-methylpropanoic Acid
2-(Dodecylthiocarbonothioylthio)-2-methylpropanoic acid (DMPA) was synthesized and
purified according to a literature method.47 Briefly, In a 250 mL round bottom flask, 1-
dodecanethiol (6.0 g, 30 mmol) and tripotassium phosphate (5.0 g, 24 mmol) were dissolved in
20 ml acetone and stirred for 10 min. Carbon disulfide (16.0 g, 210 mmol) was added to the
solution and allowed to stir for 30 min. Next, 2-bromo-2-methylpropanoic acid (5.5 g, 33 mmol)
was added to the solution, and the reaction mixture was allowed to stir for 14 h at rt. The reaction
mixture was dissolved in 200 mL DCM and washed with 1N hydrochloric acid, water, and brine.
The organic layer was dried over Na2SO4, filtered, and concentrated by rotary evaporation. The
crude product (a yellow solid) was purified by silica gel chromatography using DCM as the
mobile phase. The product was recovered as a yellow powder (4.0 g, 11.0 mmol, 37% yield). 1H
NMR (CDCl3): δ 3.28 (t, J = 7.4 Hz, 2H), 1.72 (s, 1H), 1.67 (m, 1H), 1.2-1.4 (m, 1H), 0.88 (t, J =
7.1 Hz, 1H).13C NMR (CDCl3): δ 55.4, 37.1, 31.9, 29.4-28.8, 27.7, 25.2, 22.6, 14.1.
9
Synthesis of ATRP Initiator 2
ATRP initiator 2 was synthesized according to a literature procedure.48 In a two-neck round-
bottom flask, norbornene alcohol 1 (0.75 g, 6.8 mmol) and triethylamine (1.02 g, 10.1 mmol)
were dissolved in degassed THF (40 mL). The reaction mixture was kept in an ice bath and
under flowing N2 as 2-bromo-2-methylpropionyl bromide solution (2.06 g, 9.0 mmol) in a 5 mL
THF was added dropwise. The reaction mixture was allowed to stir and warm up to rt slowly for
12 h. The mixture was filtered, and the solvent was removed by rotary evaporation. The residue
was then dissolved in 50 mL diethyl ether and washed with water three times. The organic layer
was then washed with saturated NaHCO3 and then washed with water. The ether layer was dried
over Na2SO4, filtered, and evaporated. The crude product was loaded onto a silica gel column
and eluted with a mobile phase of 10:1 hexanes/ethyl acetate. The purification yielded a viscous
colorless liquid (1.5 g, 5.8 mmol, 85% yield). 1H NMR (CDCl3): δ 6.16 (m, 1H), 5.95 (m, 2H),
4.72 (m, 1H), 2.92 (s, 1H), 2.87 (s, 2H), 1.90 (s, 3H, CH3), 1.75-1.42 (m. 10H). 13C NMR
(CDCl3): δ 171.6, 141.6, 132.4, 76.9, 56.3, 47.7, 47.1, 46.4, 45.7, 42.3, 40.7, 34.5, 34.4, 30.8,
30.7.
Synthesis of Norbornene-Functionalized Trithiocarbonate 3
Norbornene-functionalized trithiocarbonate 3 was synthesized based on a literature
procedure.49 In a 100 mL two-neck round-bottom flask, norbornene alcohol 1 (0.5 g, 4.0 mmol),
DMPA (1.6 g, 4.4 mmol), and 4-dimethyl-aminopyridine (0.05 g, 0.4 mmol) were dissolved in
dry DCM (20 mL) at rt. N,N′-Dicyclohexylcarbodiimide (DCC) (1.23 g, 6.0 mmol) was added to
the flask under N2 flow, and the reaction mixture was allowed to stir for 13 h. The reaction
mixture was then filtered and washed with 2N hydrochloric acid followed by a brine wash. The
10
organic layer was dried over Na2SO4 and concentrated by rotary evaporation to afford a yellow
oil. The crude product was purified by silica gel chromatography with a mobile phase of 19:1
hexanes/ethyl acetate. Norbornene-functionalized trithiocarbonate 3 was recovered as a yellow
viscous oil (1.15 g, 2.4 mmol, 60% yield ). 1H NMR (CDCl3): δ 6.07 (m, 2H), 4.22 (d, 1H), 3.95
(t, 1H), 3.27 (t, 2H), 2.82 (s, 1H), 2.66 (s, 1H), 1.70 (s, 6H), 1.31 (m, 3H), 1.25 (m, 20H), 0.88 (t,
3H). 13C NMR (CDCl3): δ 221.6, 173.2, 137.0, 136.5, 70.2, 56.2, 45.1, 43.8, 41.8, 37.9, 37.0,
32.1, 29.8, 29.7, 29.6, 29.5, 29.3, 29.1, 28.1, 25.6, 25.6, 22.9, 14.3.
Synthesis of Polystyrene Macromonomers (PS-MM)
A typical styrene polymerization procedure is as follows: ATRP initiator 2 (0.42 g, 1.54
mmol), styrene (30 mL, 260 mmol), and CuBr (57.6 mg, 0.40 mmol) were added to a 100 mL
Schlenk tube equipped with a stir bar. CuBr2 (90 mg, 0.40 mmol) was dissolved in DMF (9 mL)
and added to the Schlenk tube. The mixture in the Schlenk tube was deoxygenated by 3 freeze-
pump-thaw cycles and then backfilled with N2. The reaction mixture was submerged in an oil
bath at 90 ºC, and after approximately 10 min, pentamethyl diethylene triamine (PMDETA)
(0.18 mL, 0.876 mmol) was injected under N2 flow. The reaction mixture was heated in an oil
bath maintained at 90 °C for ca. 8 h. An aliquot was removed via N2-purged syringe and
analyzed via 1H NMR spectroscopy to ensure that ~10% conversion had been reached. At this
point, the reaction was terminated by exposing the content of the Schlenk tube to air. The
resultant PS-MM was purified by four successive precipitations from MeOH. After the last
precipitation, the polymer was recovered via filtration and then dried in a vacuum oven. The
molar ratios of reagents for the ATRP reaction were [styrene]/[II]/[CuBr]/[CuBr2]/[PMDETA] =
170:1:0.5:0.5:0.6 when targeting 2 kg/mol and 360:1:0.5:0.5:0.6 when targeting 4 kg/mol.
11
Synthesis of Poly(N-Acryloylmorpholine) Macromonomers (PACMO-MM)
A typical ACMO polymerization procedure is as follows: Norbornene-functionalized
trithiocarbonate 3 (0.62 g, 1.3 mmol), ACMO (2.8 mL, 22.3 mmol) and THF (15 mL) were
added to a 100 mL Schlenk tube equipped with a stir bar. The mixture in the Schlenk tube was
deoxygenated by 3 freeze-pump-thaw cycles and then backfilled with N2. The Schlenk tube was
placed inside a photoreactor, described in a recent publication,50 on top of a stir plate. The light
was then turned on by plugging the photoreactor into an outlet, and the reaction mixture was
allowed to stir for 8 h. An aliquot was taken, and 1H NMR spectroscopy was run to determine the
extent of conversion had been reached. The reaction was terminated by exposing the content of
the Schlenk tube to air. The resultant PACMO-MM was purified by four successive precipitation
from diethyl ether. The ratios of reagents were [ACMO]/[III] = 14:1 when targeting 2 kg/mol
and 32:1 when targeting 4 kg/mol.
Synthesis of Bottlebrush Block Copolymers (PS-PACMO)
A typical bottlebrush block copolymerization procedure is as follows: PS-MM (76 mg, 2800 g/
mol, 0.027 mmol) was dissolved in DCM (0.75 mL) in a vial equipped with a small stir bar. In a
second vial, G3 (12.6 mg, 0.017 mmol) was dissolved in DCM (0.72 mL) to create a G3 stock
solution. Next, 15 µL of the G3 solution was added to the vial via syringe. During the
polymerization of the first block, a PACMO-MM (60 mg, 2200 g/mol, 0.027 mmol) solution was
prepared in DCM (0.6 mL). After 20 min, an aliquot of PS-MM solution was removed for
analysis and injected into a vial that contained ethyl vinyl ether (0.1 mL). Next, PACMO-MM
was added via a syringe rapidly to the PS-MM solution and allowed to stir for 12 h. To quench
the reaction, ethyl vinyl ether (2 drops) was added to the reaction vial. Aliquots were analyzed by
12
SEC to verify conversion of the first and second blocks to 95% conversion. The ratios of
reagents were [PS-MM]/[PACMO-MM]/[G3] = 75:75:1 when targeting 75 units of PS-MM and
PACMO-MM for each block and 50:100:1 when targeting 50 units of PS-MM and 100 units of
PACMO-MM; 30:120:1 when targeting 30 units of PS-MM and 120 units of PACMO-MM;
15:135:1 when targeting 15 units of PS-MM and 135 units of PACMO-MM.
Synthesis of Linear Block Copolymers (PACMO-PS)
A typical linear block copolymerization procedure is as follows: a 100 mL Schlenk tube was
charged with a stir bar, THF (8.5 mL), ACMO (1.73 mL, 13.7 mmol), and DMPA (25 mg, 0.069
mmol). The solution was then degassed by carrying out three freeze-pump-thaw cycles and
backfilled with N2. The reaction vessel was then placed inside the LED chamber on top of a stir
plate. The light was then turned on by plugging the photoreactor into an outlet, and the reaction
mixture was allowed to stir for 8 h, reaching 80% conversion. The polymer product was
recovered by precipitation from diethyl ether.
A 100 mL Schlenk tube was charged with a stir bar, THF (2 mL), styrene (0.260 mL, 2.27
mmol), and the linear PACMO macroinitiator (100 mg, 4.55 mol). The solution was then
degassed by carrying out three freeze-pump-thaw cycles and backfilled with N2. The reaction
vessel was then placed inside the photoreactor on top of a stir plate. The light was then turned on
by plugging the photoreactor into an outlet, and the reaction mixture was allowed to stir for 70 h,
reaching 42% conversion. The polymer product was recovered by precipitation from diethyl
ether.
RAFT End Group Removal
13
A representative procedure for end group removal was adapted from literature as follows: 51
Bottlebrush BCP with a block ratio of 1:1 PS/PACMO (60 mg, 0.027 mmol) and N-methyl
maleimide (15 mg, 0.14 mmol) were dissolved in 2 mL DCM in a vial equipped with a stir bar
and a rubber septum. The reaction mixture was bubbled with N2 for 10 min. Hexylamine (90 l)
in DCM solution (100 l/mL, 0.067 mmol) was added, and the reaction mixture was allowed to
stir for 14 h. The reaction mixture was precipitated into 1:1 hexanes/diethyl ether, and the
polymer product was recovered by filtration as a white powder.
RESULTS AND DISCUSSION
Macromonomer Synthesis
All MMs were synthesized starting from norbornene alcohol 1 (Scheme 1). Although this
functionalized norbornene is more difficult to synthesize than other related compounds, it was
chosen due to its superior ROMP kinetics.49 An improved synthesis of norbornene alcohol 1 was
adapted in this work instead of a more traditional procedure.44-45 The traditional synthesis of
norbornene alcohol 1 involves an iodolactonization reaction to isolate the pure exo isomer from
an endo-exo mixture of a precursor compound that is typically ~25% exo. We used a recently
reported method, as shown in Scheme 1, to increase the exo component of this precursor to 80%,
thus substantially increasing the overall yield of the reaction sequence.
Scheme 1: Synthesis of exo-5-norbornene-2-methanol (norbornene alcohol 1)a
+
O
O
KOtBu, H2O
O
O
O
OH
iii
25% exo 80% exo
14
I
2
/KI,Na
2
CO
3
iii
O
OH OH
LiAlH
4
iv
aConditions: (i) DCM, rt, 16 h; (ii) THF, rt, 20 h; (iii) H2O, rt, 1 h; (iv) THF, rt, 16 h.
With norborene alcohol 1 in hand, this ROMP-active compound was coupled to two different
reversible-deactivation radical polymerization (RDRP) initiators. First, an α-bromoester was
synthesized by coupling 2-bromo-2-methylpropionyl bromide with norbornene alcohol 1 to form
ATRP initiator 2 (Scheme 2). Next, DMPA and norbornene alcohol 1 were coupled using N,N'-
dicyclohexylcarbodiimide (DCC), which produced norbornene-functionalized trithiocarbonate 3,
a RAFT chain transfer agent (CTA).
Scheme 2: Synthesis of norbornene-functionalized RDRP initiatorsa
O
Br
+Br
O
Br
2
iO
OH NEt3
3
S
S
S
HO
O10
S
S
S
O
10
ii O
+
OH DCC, DMAP
aConditions: (i) THF, rt, 12 h; (ii) DCM, rt, 13 h.
Starting from ATRP initiator 2, polystyrene macromonomers (PS-MMs) were synthesized
under typical ATRP conditions (Cu(I)Br, Cu(II)Br, and PMDETA) as shown in Scheme 3. Two
MMs were produced with molecular weights of 2.8 and 4.2 kg/mol based on size exclusion
chromatography (SEC) analysis and were named S2K and S4K, respectively. Removal of
unreacted styrene monomer is vital as it can terminate the ROMP reaction, so each MM was
succcessively precipitated into CH3OH until no traces of residual monomer could be observed by
15
1H NMR spectroscopy. MM purity can also be verified using ROMP, where a pure MM with
high chain-end fidelity generates a bottlebrush polymer with a MW consistent with the MM/I
ratio and a monomodal peak with low Đ by SEC.35 Figures S6 and S8 show the SEC traces of the
ROMP products of both PS-MMs; integration of the bottlebrush polymer peaks and the residual
MM peaks indicate clean formation of the bottlebrush polymer with <5% residual MM.
Scheme 3: Synthesis of PS-MMs a
O
Br
O
Br
n
O O
i
S
2K
and S
4K
aConditions: (i) Cu(I)Br, Cu(II)Br, PMDETA, DMF, 90 ºC, 8 h.
To prepare bottlebrush BCPs, we chose ACMO as the hydrophilic monomer due to its fast
polymerization kinetics and controlled polymerization. For the synthesis of the poly(ACMO)
macromonomers (PACMO-MM), photoiniferter polymerization with a blue LED centered at 450
nm was utilized based on a recent article from our group, as shown in Scheme 4.50 This
polymerization technique does not require a thermal initiator and is performed at rt. These
advantages enable easy control over the duration of the polymerization reaction by utilizing a
timer without the need for attendance because the polymerization can be stopped and restarted by
switching the light off and on, respectively. We used photoiniferter polymerization of ACMO,
mediated by norbornene-functionalized trithiocarbonate 3, to produce two PACMO MMs with
MWs of 2.2 and 4.2 kg/mol according to SEC analysis (A2K and A4K). Similar to the PS-MMs,
16
both PACMO-MMs underwent clean homopolymization via ROMP to afford bottlebrush
polymers with monomodal SEC chromatograms. It is worth noting that as the molecular weight
of the MM increased, more precipitations from Et2O were required to remove excess unreacted
ACMO to avoid chain transfer reactions in the subsequent ROMP reactions. Table 1 lists the
average molecular weight, DP, and dispersity (Đ), as measured by SEC with absolute molecular
weight determination by light scattering, for each MM as well as for their respective bottlebrush
homopolymers.
Scheme 4: Synthesis of PACMO-MMs a
O
N
O
i
OS
S
S
O
O
N
O
10
v
A
2K
and A
4K
S
S
S
O
10
O
aConditions: (i) THF, rt, 8 h, 450 nm light.
Table 1: Molecular weights by SEC for the MMs and bottlebrush homopolymers
Macromonomers Bottlebrush Homopolymers
Polymers a Mnb (kg/
mol) DP cĐ bMnb
(kg/mol) DPcĐb
%
Conv. to
BBd
S2K 2.8 24 1.04 290 104 1.04 97
S4K 4.2 38 1.04 410 98 1.05 96
A2K 2.2 12 1.04 220 98 1.06 96
A4K 4.2 27 1.04 410 97 1.04 96
aTargeted molecular weight of each macromonomer represented by
XY
where X is the MM
type (S = polystyrene; A = PACMO) and Y is the MM molecular weight. bMeasured by SEC in
THF at 30 °C using light scattering and refractive index detectors. cAverage degree of
polymerization from SEC data using the formula DP = (Mn – MWCTA/Initiator)/MWmonomer.
17
dDetermined from SEC by comparing the integrations of the bottlebrush polymer peak and the
MM peak. All bottlebrush polymerizations were conducted in DCM for 30 min initiated by G3
catalyst.
Bottlebrush Block Copolymer Synthesis
There are four established methods to synthesize bottlebrush polymers: (1) “grafting-from,”
where pendant initiators on a polymeric backbone are utilized to grow side chains from the
backbone; (2) “grafting-to,” where premade polymeric side chains are coupled to a polymer
backbone through highly efficient reactions; (3) “transfer-to,” where a backbone polymer
containing a pendant CTA is synthesized, and side chains detach from the backbone, propagate
freely in solution, and then reattach to the backbone through chain transfer reactions; and (4)
“grafting-through” or “the macromonomer approach,” where polymeric side chains that contain
an orthogonal polymerizable group (macromonomers, MMs) are synthesized, then polymerized
in a second reaction to create the bottlebrush structure.3, 52 The grafting-from, transfer-to, and
grafting-to strategies provide the capability to create macromolecules with a high degree of
polymerization (DP) and overall molecular weight (more than 106 kg/mol). However, these three
strategies lack the control afforded by the grafting-through approach. In grafting-through,
theoretically perfect grafting density results from the presence of a side chain on each repeating
unit on the backbone. In addition, grafting-through incorporates high synthetic versatility in
terms of functional group tolerance when carried out using Grubbs 3rd generation catalyst (G3).3,
49, 53
Using the grafting-through approach, we prepared amphiphilic bottlebrush BCPs with different
block ratios and side chain MWs to investigate the effect of bottlebrush BCP structure on
solution self-assembly. PS-based bottlebrush polymers with similar structures to those described
18
here undergo a morphological transition from spherical to cylindrical at DP=120,8 so we targeted
a DP of 150 for all bottlebrush BCPs in this study in order to maintain a cylindrical morphology.
A total of 16 bottlebrush BCPs were synthesized starting from the two PS-MMs and the two
PACMO-MMs. The nomenclature of these polymers follows the general scheme
Xn
Y
where X is
the MM type (S = polystyrene and A = poly(N-acryloyl morpholine)), Y is the MM molecular
weight, and n is the number of MM repeat units in the bottlebrush BCP. Therefore,
S50
2KA100
2K
represents a DP of 50 of a first block comprised of an S2K MM and a DP of 100 of a second
block comprised of an A2K MM. The MM molecular weights were 2 and 4 kg/mol for both PS
and PACMO, and the DP for the bottlebrush polymers ranged from 15-75 for the PS MM blocks
and 75-135 for the PACMO MM blocks, as shown in Table 2.
Table 2: SEC characterization of bottlebrush BCPs
Macromonomersb Bottlebrush Block Copolymers
Theoretical
DPa
Mn,PS
(kg/mol)
Mn,PACMO
(kg/mol)
Mn BB Sc
(kg/mol)
Mn BB SAd
(kg/mol) Final ĐefPSf (%) Conv. to
BBg (%)
S75
2KA75
2K
2.8 2.2 190 330 1.08 58 96
S50
2KA100
2K
2.8 2.2 129 340 1.06 38 97
S30
2KA120
2K
2.8 2.2 78 341 1.06 23 96
S15
2KA135
2K
2.8 2.2 39 366 1.06 11 95
S75
4KA75
4K
4.2 4.2 303 637 1.06 48 96
S50
4KA100
4K
4.2 4.2 199 657 1.04 30 95
S30
4KA120
4K
4.2 4.2 127 598 1.02 21 95
19
S15
4KA135
4K
4.2 4.2 63 688 1.03 9 95
S75
2KA75
4K
2.8 4.2 205 471 1.04 44 96
S50
2KA100
4K
2.8 4.2 132 560 1.05 24 95
S30
2KA120
4K
2.8 4.2 77 639 1.03 12 95
S15
2KA135
4K
2.8 4.2 36 641 1.03 6 95
S75
4KA75
2K
4.2 2.2 344 492 1.07 70 95
S50
4KA100
2K
4.2 2.2 206 409 1.04 50 96
S30
4KA120
2K
4.2 2.2 124 384 1.03 32 95
S15
4KA135
2K
4.2 2.2 63 359 1.03 18 95
aTargeted bottlebrush BCP structure represented by
Xn
Y
where X is the MM type
(S = polystyrene; A = PACMO), Y is the MM molecular weight and n is the number of MM
repeating units in the bottlebrush BCP. bMM average molecular weight for each block. cMn of the
first block of the bottlebrush BCP as measured by SEC in THF at 30 ºC, determined by removing
an aliquot from the reaction mixture after complete consumption of the first MM. dMn of
bottlebrush BCP as measured by SEC in THF at 30 ºC. eDispersity of bottlebrush BCP. fWeight
fraction of PS in bottlebrush BCP. gDetermined from SEC by comparing the integrations of the
bottlebrush polymer peak and the MM peak.
The ROMP of PS MMs and PACMO MMs was initiated by G3 catalyst and carried out under
typical conditions for ROMP (Scheme 5). Reaction progress for the first block (PS-MM) of each
bottlebrush BCP was monitored by 1H NMR spectroscopy (Figure S5), with full conversion
typically observed in <20 min. In all cases, observed Mn values were close to the targeted values,
and monomodal peaks were observed by SEC (Figures S6, S8, S10 and S12) with <5% residual
MM in each. Upon complete consumption of the PS-MM, the second MM (PA-MM) was added.
Reactions were allowed to proceed for another 2 h before quenching with ethyl vinyl ether.
20
Again, Mn values were close to the targeted values, and monomodal peaks were observed by
SEC (Figures S7, S9, S11 and S13) with <5% residual MM in each.
Scheme 5: Synthesis of bottlebrush BCPs from PS-MMs and PA-MMsa
O
Br
n
O
O
Br
ni
G3
Oii
O
OS
S
S
O
N
O
m
10
O
Br
n
O
O
OS
S
S
O
N
O
m10
aConditions: (i) DCM, rt, 20 min. (ii) DCM, rt, 2 h
Removal of the trithiocarbonate end groups increases the solubility of the hydrophilic side
chains in water, which increases the stability of micelles formed in aqeous solutions, as shown in
previous studies on poly(N-isopropylacrylamide).54 In addition, the labile C-S bond at the end of
trithiocarbonate contain polymers reduces the stability of the polymer once formed, resulting in a
limited shelf life. The labile nature of the trithiocarbonate end group can be exploited in order to
remove it, enabling conjugation of more hydrophilic end groups. Many methods have been
reported for the removal of trithiocarbonate end groups, including removal by radical reactions
and nucleophilic reactions;55 however, thus far most strategies have only been tested on linear
polymers. We aimed to carry out this transformation on a bottlebrush polymer, where radical
reactions on nearby side chains could lead to deliterious side reactions. Therefore, we chose a
recently reported non-radical method involving aminolysis followed by thiol-maleimide coupling
that could allow for quantitative conversion.51
The trithiocarbonate end groups were successfully removed by reacting bottlebrush BCPs with
hexylamine and a 10-fold excess of N-methyl maleimide, as shown in Scheme 6. 1H NMR
21
spectroscopy confirmed trithiocarbonate end group removal and its replacement with N-methyl
maleimide (Figures S14-S29). SEC traces of bottlebrush BCPs remained monomodal with no
shoulders before and after the aminolysis/maleimide reaction but with slightly increased
retention times (Figures S30-S45). Also, the color of the bottlebrush BCPs changed from yellow
to white, which is another indication of the removal of the trithiocarbonate group.
Scheme 6: Trithiocarbonate end group removal by aminolysis/maleimide methoda
O
O
NH
2
N
+
i
O
Br
n
O
O
OS
S
S
O
N
O
m10
O
Br
n
O
O
OS
O
N
O
m
O
O
N
aConditions: (i) DCM, rt, 14 h.
IFT Analysis
Next, the self-assembly behavior of bottlebrush BCPs was analyzed through a
combination of IFT measurements to extract CMC values in water and SANS measurements to
understand the micelle structure. Micelles were prepared through a typical solvent-switch
method by dissolving the bottlebrush BCP in THF, adding water, and dialyzing against water to
form the final micelle solution. While most bottlebrush BCPs formed stable micelles in water at
1 mg/mL, several aggregated and precipitated in solution, as indicated in Table 3. The results
suggest that bottlebrush BCPs with PS blocks that were too large did not form stable aggregates
in water. In general, stable micelles were only formed when the hydrophobic block contained 30
or fewer PS side-chains and the hydrophilic block contained at least 120 PACMO side-chains.
22
This was observed for the series of samples with matched PS and PACMO molecular weights
(either 2 or 4 kg/mol) and for the series with larger PS side-chains (4 and 2 kg/mol for PS and
PACMO, respectively). Bottlebrush BCPs with 4 kg/mol PACMO side-chains and 2 kg/mol PS
side-chains were stable in water across the entire series. This result suggests that micelle stability
varies in bottlebrush BCPs with greater than 30 wt% PS content and depends on the specific
structure. For example, the copolymer
S75
2KA75
4K
formed stable micelles and has an overall PS
content of 44 wt % while both
S50
2KA100
2K
and
S50
4KA100
4K
did not form stable micelles and contained
overall PS contents of 38 and 30 wt %, respectively.
For bottlebrush BCPs that formed stable micelles in solution, IFT measurements confirmed an
ultra-low CMC across the entire bottlebrush BCP series with no apparent compositional
dependence. CMCs for all samples varied between 5 and 19 mg/L, with most samples exhibiting
a CMC in the range of 8 – 16 ppm (see Figures S46-S58 for data and analysis of IFT
measurements). On a mass basis, the CMC is comparable to other measurements of linear
diblock copolymers reported in the literature, both by fluorescence analysis, light scattering, and
Wilhelmy plate methods.14, 56-60 However, on a per molecule basis, the CMC of bottlebrush BCPs
is at least an order of magnitude lower, in the range of 10 – 50 nmol/L as shown in Table 3. This
reflects the strong enthalpic driving force per molecule to form stable micelles even in very
dilute conditions. For comparison, we also measured the CMC for three linear PACMO-b-PS
BCPs
S15 A148 , S37 A148 ,∧S59 A148
, where the subscripts denote the DPs for the PS (‘S’) and
PACMO (‘A’) blocks, respectively. The three linear copolymers had the same length PACMO
block (23 kg/mol, DP = 148) but varying PS blocks, from 1.5 – 6.1 kg/mol, corresponding to
DPs 15 – 59 and overall PS content from 6 to 21 wt. %. On a mass basis, the CMC for the linear
BCPs was similar to that for the bottlebrush BCPs, with the exception of the linear diblock
23
copolymer with the shortest PS block,
S15 A148
. The CMC for
S15 A148
was an order of magnitude
higher than any other linear or bottlebrush BCP studied, reflecting the relatively weak driving
force for self-assembly at low PS contents. By comparison, bottlebrush BCP
S15
2KA135
4K
with the
same overall PS content had a CMC of 9 mg/L, comparable to all other bottlebrush BCPs
studied. This suggests that the CMC for bottlebrush BCPs is much less sensitive to composition
than for linear BCPs and remains low even for low PS weight fractions.
Table 3: Critical micelle concentrations (CMCs) for bottlebrush and linear BCPs in water
determined through IFT measurements.
Bottlebrush
and linear
BCPs
CMC
(mg/L)
CMC
(nmol/L) fPS(%)
S75
2KA75
2K
--a-- a 58
S50
2KA100
2K
-- a -- a 38
S30
2KA120
2K
16 46 23
S15
2KA135
2K
19 53 11
S75
4KA75
4K
-- a -- a 48
S50
4KA100
4K
-- a -- a 30
S30
4KA120
4K
11 88 21
S15
4KA135
4K
16 24 9
S75
2KA75
4K
11 24 44
S50
2KA100
4K
813 24
S30
2KA120
4K
12 18 12
S15
2KA135
4K
916 6
24
S75
4KA75
2K
-- a -- a 70
S50
4KA100
2K
-- a -- a 50
S30
4KA120
2K
11 30 32
S15
4KA135
2K
515 18
S15 A148
128 5205 6
S37 A148
10 372 9
S59 A148
10 343 21
aPrecipitation was observed during dialysis.
SANS Analysis
While a handful of studies have analyzed the self-assembly of bottlebrush BCPs in
solution, few have modeled the scattering curve for amphiphilic bottlebrush BCPs in both
selective and non-selective solvents.29, 34, 39, 61 SANS measurements were conducted on
bottlebrush BCPs in a good solvent for both polymer blocks (toluene-d8) and in a solvent
selective for the PACMO block (D2O). SANS experiments were conducted at the high flux
reactor source at Oak Ridge National Lab with a neutron wavelength of 6 Å and over a q-
range of 0.0009 < q < 0.24 Å-1.
In prior work, we were able to fit the scattering curve from homopolymer bottlebrush
polymers using the Guinier-Porod model to extract the bottlebrush size and shape anisotropy.8, 62
Using the Guinier-Porod model in this work to fit the scattered intensity for the bottlebrush BCPs
in toluene-d8 (Figures S59 – S70), we were able to fit only the low-q region of the scattering
curve for most samples (q < 0.02 Å-1). A deviation in the scattered intensity in the high-q region
reflected the presence of sidechains with distinct scattering length densities and contrast with the
surrounding solvent. The scattering curves for all samples in toluene-d8 along with Guinier-
25
Porod model fits are provided in the Supporting Information (Figures S59 – S70 and Table S1).
Across each series of samples analyzed, the Guinier-Porod model revealed an increasing
anisotropy with increasing PACMO content in the bottlebrush BCPs, reflecting an extended
conformation of the PACMO block in toluene-d8.
To further analyze the scattered intensity, we carried out implicit solvent coarse-grained
molecular dynamics simulations of single bottlebrush BCPs using the LAMMPS software
package.63-64 Here we varied the solvent quality of the PS and PACMO blocks by tuning the
interactions between sidechain beads and then calculated the single chain form factor, P(q).
Details of the simulations are described in the supporting information. These simulations
predicted a scattered intensity that is qualitatively consistent with the experimental results. The
simulations showed a plateau in the scattered intensity at low-q and a broad form factor fringe or
a shift in the Porod exponent at high-q due to the presence of sidechains (see arrows in Figure
S82). The structure of the bottlebrush BCP can be viewed as a flexible cylinder with a blob
scattering term (see Figure S82), where the backbone behaves like a semiflexible chain with a
Porod exponent (1.2) approaching that of a rod (Porod exponent for a thin rod is 1), and less than
that of a linear chain in a good solvent (Porod exponent = 5/3).65-67
In Figure 2, we interpret the structure of the bottlebrush BCPs based on the SANS
scattering traces by observing the trend of the q value where the Porod exponent shifts (see
arrows in Figure 2). Both experiments and simulations show that the q value where the shift
occurs increases as the amount of PACMO block in the bottlebrush BCP increases. This
represents a decrease in the fuzziness of the structure, because the PACMO side chains are less
soluble in toluene-d8 than the PS side chains, and this is consistent with the Guinier-Porod
26
analysis showing a more extended conformation of the PACMO side chains with the increase of
wt. % PACMO in the bottlebrush BCP.
Figure 2. SANS analysis of bottlebrush BCPs
S
y
4K
A
150−y
4K
in toluene-d8 (left) and coarse-grained
molecular dynamics simulation predictions for scattered intensity along with snapshot of
molecular conformation from coarse-grained simulations (right). Arrows indicate the q values
where the shift in the Porod exponent occurs. The images are snapshots of the simulation with
cyan S beads, orange backbone beads, and magenta A beads. Scattering plots are shifted
vertically for clarity.
Next, we analyzed amphiphilic bottlebrush BCP micelles formed in D2O for samples that
formed stable micelles in water. The scattered intensity from the micelles presented very
different features from that of these polymers in a good solvent, as expected. Most notably, most
samples exhibited a more clearly discerned form factor fringe in the scattered intensity at q ~
27
0.05 Å -1. This is consistent with a sharper interface between the hydrophobic PS domains and
the hydrophilic PACMO domains swollen with D2O, i.e., a better-defined core-shell structure.
Additionally, a downturn in the scattered intensity was observed at low-q for several samples,
down to the lowest q analyzed (0.003 Å-1). This suggests possible intermicellar repulsion, while
a low-q upturn appearing in the rest of samples could be due to the attraction of micelles in
solution.68 A representative example of the scattered intensity from amphiphilic polymer
S50
2KA100
4K
in D2O is shown in Figure 3. A clearly discerned form factor fringe in the scattered intensity at q
= 0.05 Å-1 and a downturn in the scattered intensity at low-q was observed for this particular
sample. All SANS curves of bottlebrush BCPs in D2O are provided in the Supporting
Information (Figures S71 – S80) as are results from model fitting (Table S2).
1 E - 3 0 . 0 1 0 . 1
0 . 1
1
1 0
1 0 0
1 0 0 0
I n t e n s i t y , c m
- 1
q ( Å
- 1
)
Core
Shell 1
Shell 2
28
Figure 3. SANS analysis of
S50
2KA100
4K
in D2O along with model fit using core-shell-shell model
(top) and schematic for core-shell-shell model with a more highly solvent-swollen shell in the
micellar periphery (bottom).
The scattered intensity from amphiphilic bottlebrush BCP micelles in D2O was modeled using
a core-shell-shell model, which produced a suitable fit to the scattered intensity for most of the
stable micellar samples analyzed. Intuitively, this model reflects a micellar structure with a
solvent-depleted PS core and two shells of PACMO, one in the micelle interior and partially
swollen with solvent and one in the micellar periphery and more highly swollen with D2O.69 In
applying this model to the scattered intensity, we held the scattering length density for the core
fixed to the value for pure polystyrene (PS) and used a constant scattering length density for the
solvent. The model was therefore able to estimate sizes for the micellar core and shells and
scattering length densities for each shell.
Using the same simulation methodology as was used for SANS in toluene-d8, we predicted the
scattering features of the bottlebrush BCPs in D2O at the dilute limit. For this case, the PS block
is in poor solvent conditions, while the PACMO block is in good solvent conditions (see Figure
S83). The structure of a single bottlebrush BCP was predicted to be a combination of a globule
and a fuzzy flexible cylinder, where a trend of increasing wt.% PACMO showed scattering
curves with a smaller proportion of the globular property (see bumps in the spectra at high-q in
S25
13 A25
9
S25
13 A25
9
and
S
18
13
A
32
9
S
18
13
A
32
9
in Figure S83) to only a shift in the Porod exponent, similar to a
fuzzy flexible cylinder (see
S
5
13
A
45
9
S
5
13
A
45
9
in Figure S83). These features closely match the
features of the experimentally determined scattering traces.
29
Results from SANS Model Fitting
The series of bottlebrush BCPs with 2K PS side-chains and 4K PACMO side-chains were
successfully fitted to the core-shell-shell model, and results from model analysis are provided in
Table 4. Across the series, the core radius decreased slightly with increasing PACMO content,
from a radius near 3 nm to approximately 2 nm. Across the same series, the model revealed a
decrease in the size of the interior PACMO shell along with an increase in the size of the more
highly swollen exterior PACMO shell. The total radii of the shells increased slightly, from
approximately 16 to 18 nm. The average diameters for the micelles estimated from these fits
were approximately 40 nm, which is similar to diameters reported for other amphiphilic
bottlebrush BCPs studied through electron microscopy39, 61, 70-71 and slightly less than double the
size of bottlebrush BCPs in toluene-d8, consistent with solvent exclusion from the core and with
the schematic for self-assembly shown in Figure 1. The SANS analysis suggests that across this
series of amphiphilic bottlebrush BCPs, the micelles retained a spherical, core-shell-shell
conformation.
Table 4. Fitting Parameters from Core-Shell-Shell Model for selected Amphiphilic Bottlebrush
BCPs in D2O.a
Bottlebrush
BCP Scale Background
(cm-1)
Core
Radius
(Å)
Shell 1
SLD
(10-6 Å-2)
Shell 1
Thickness
(Å)
Shell 2
SLD
(10-6 Å-2)
Shell 2
Thickness
(Å)
S75
2KA75
4K
0.172 0.084 27.6 5.5 138 5.4 29.7
S50
2KA100
4K
0.355 0.079 29.6 5.7 131 6.2 33.6
S30
2KA120
4K
1.59 0.13 21.4 5.9 93.4 6.3 124
S15
2KA135
4K
1.57 0.05 23.2 5.7 44.9 6.3 136
30
aThe scattering length densities (SLDs) of the core (polystyrene) and solvent (D2O) were
specified to be 1.453x10-6 Å-2 and 6.4x10-6 Å-2, respectively.
CONCLUSION
In summary, an ATRP initiator and a trithiocarbonate photoinferter were coupled to a
norbornene to create dual-functional units. This allowed for polymerization of PS by ATRP and
PACMO by photoinferter polymerization, creating hydrophobic and hydrophilic MMs,
respectively. The norbornene group allowed for an orthogonal ROMP reaction via the grafting-
through strategy, which resulted in a high conversion to bottlebrush BCPs with controllable
molecular weights and low dispersities. Trithiocarbonate end group removal was necessary to
afford stable micelles and a water soluble PACMO block. In a toluene-d8, a good solvent for
both blocks, analysis of SANS curves through model fitting and simulations showed an
increasing shape anisotropy with increasing PACMO content, reflecting an extended PACMO
block conformation. The amphiphilic bottlebrush BCPs were then analyzed in D2O, a selective
solvent for the PACMO block. IFT measurements revealed ultralow CMC values for these
micellar aggregates, and analysis of SANS scattering curves using a core-shell-shell model
showed an increase in Dh with increasing PACMO content. This work highlights the complex
internal structures of self-assembled amphiphilic bottlebrush BCPs, along with their strong
driving force for assembly. The ultralow CMCs, even for highly hydrophilic bottlebrush BCPs,
are particularly strinking compared with linear BCPs, which have CMCs of 1-2 orders of
magnitude larger on a molar basis. This apparent insensitivity of CMC to block ratios in
bottlebrush BCPs may be useful in preparing micelles with a wide range of sizes and
hydrophobic/hydrophilic ratios without affecting the CMC.
31
ASSOCIATED CONTENT
Supporting Information. Experimental details, SEC traces and NMR spectra of amphiphilic
bottlebrush BCPs, IFT data and analysis, SANS data and fittings, summary of SANS fitting
parameters for micelles, details on coarse-grained molecular dynamics simulations, and predicted
SANS traces for various bottlebrush BCPs. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*jbmatson@vt.edu, *rafaelv@rice.edu
Author Contributions
All authors have given approval to the final version of the manuscript.
Funding Sources
This work was supported by Saudi Aramco (fellowship to MA), the American Chemical
Society Petroleum Research Fund (54884-DNI7), and the National Science Foundation (CMMI –
1563008). A portion of this work was supported by the NSF Nanosystems Engineering Research
Center for Nanotechnology-Enabled Water Treatment (EEC-1449500). Portions of this research
were conducted at the Center for Nanophase Materials Sciences (CNMS) and at the CG-2 GP-
SANS beamline, High Flux Isotope Reactor (HFIR), which are sponsored at Oak Ridge National
Laboratory by User Facilities Division of the Office of Basic Energy Sciences, U.S. Department
of Energy (DOE). The Bio-SANS (CG3) at HFIR is sponsored by the Office of Biological and
32
Environment Research, U.S. DOE. This research also used resources of the Oak Ridge
Leadership Computing Facility, which is a DOE Office of Science User Facility supported under
Contract DE-AC05-00OR22725. This work benefited from the use of the SasView application,
originally developed under NSF Award DMR-0520547. SasView also contains code developed
with funding from the EU Horizon 2020 program under the SINE2020 project Grant No 654000.
ACKNOWLEDGMENT
We thank Materia for catalyst, as well as Dr. Scott Radzinski and Dr. Jeffrey Foster for helpful
discussions.
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