Synthesis of mixed poly(ε-caprolactone)/polystyrene brushes from Y-initiator-functionalized silica particles by surface-initiated ring-opening polymerization and nitroxide-mediated radical polymerization

Article abstract of DOI:10.1039/c4ra02429j

This article reports on the synthesis of mixed homopolymer brushes grafted on silica particles composed of poly(ε-caprolactone) (PCL), a chemically and biologically degradable polymer, and polystyrene (PS), a vinyl polymer, by sequential surface-initiated ring-opening polymerization (ROP) of ε-caprolactone (εCL) at 75 °C and nitroxide-mediated radical polymerization (NMRP) of styrene at 120 °C. The silica particles were functionalized with an asymmetric difunctional initiator (Y-initiator) bearing a hydroxyl group, an initiator for ROP, and an alkoxyamine moiety, an initiator for NMRP. We found that for ROP, PCL with M_n,SEC values of ～25 kDa and narrow polydispersity indexes (≤1.20) relative to PS standards can be reproducibly synthesized at 75 °C using tin(ii) 2-ethylhexanoate as catalyst after the Y-initiator-functionalized silica particles and the catalyst were thoroughly dried. Both surface-initiated ROP and NMRP polymerizations were carried out in the presence of a free initiator. The polymer brushes were cleaved from silica particles using hydrofluoric acid, and the SEC analysis showed that the molecular weights of the grafted polymers were close to those of the free polymers formed from the free initiators. The overall grafting density of mixed PCL/PS brushes can be tuned from 1.24 to 0.40 chains per nm ² by varying the mass ratio of Y-initiator-terminated triethoxysilane to bare silica particles in the initiator immobilization step, though the grafting density of PS was always higher than that of PCL.

Full text of DOI:10.1039/c4ra02429j

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Synthesis of mixed poly(3-caprolactone)/
polystyrene brushes from Y-initiator-
functionalized silica particles by surface-initiated
ring-opening polymerization and nitroxide-
mediated radical polymerization†
Cite this: RSC Adv., 2014, 4, 18772
*
Weikun Li,‡ Chunhui Bao,‡ Roger A. E. Wright and Bin Zhao
This article reports on the synthesis of mixed homopolymer brushes grafted on silica particles composed of
poly(3-caprolactone) (PCL), a chemically and biologically degradable polymer, and polystyrene (PS), a vinyl
polymer, by sequential surface-initiated ring-opening polymerization (ROP) of 3-caprolactone (3CL) at
75 ꢀC and nitroxide-mediated radical polymerization (NMRP) of styrene at 120 ꢀC. The silica particles
were functionalized with an asymmetric difunctional initiator (Y-initiator) bearing a hydroxyl group, an
initiator for ROP, and an alkoxyamine moiety, an initiator for NMRP. We found that for ROP, PCL with
Mn,SEC values of ꢁ25 kDa and narrow polydispersity indexes (#1.20) relative to PS standards can be
reproducibly synthesized at 75 ꢀC using tin(II) 2-ethylhexanoate as catalyst after the Y-initiator-
functionalized silica particles and the catalyst were thoroughly dried. Both surface-initiated ROP and
NMRP polymerizations were carried out in the presence of a free initiator. The polymer brushes were
cleaved from silica particles using hydrofluoric acid, and the SEC analysis showed that the molecular
weights of the grafted polymers were close to those of the free polymers formed from the free initiators.
The overall grafting density of mixed PCL/PS brushes can be tuned from 1.24 to 0.40 chains per nm2 by
varying the mass ratio of Y-initiator-terminated triethoxysilane to bare silica particles in the initiator
immobilization step, though the grafting density of PS was always higher than that of PCL.
Received 8th February 2014
Accepted 1st April 2014
DOI: 10.1039/c4ra02429j
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stimulated tremendous research activities on mixed brushes.1–14
The effects of various molecular parameters on the self-
Introduction
assembly behavior of mixed brushes were elucidated by theo-
retical and computer simulation studies, and a rich variety of
nanostructures was predicted.1–8
Binary mixed brushes, composed of two chemically distinct
homopolymers randomly or alternately graed by one end on a
solid substrate, are an intriguing class of surface-responsive
materials.1–5 These multicomponent brushes can undergo
spontaneous self-reorganization in response to environmental
changes, exhibiting different nanostructures and properties
under different conditions. The intriguing self-assembly
behavior of binary mixed brushes has attracted considerable
interest in both theoretical and experimental studies in the past
years.1–5 Marko and Witten were the rst to study symmetric
mixed homopolymer brushes and predicted that they phase
separate laterally in melt and the feature size of the formed
“ripple” nanopattern is on the order of the polymer chain root-
mean-square end-to-end distance.1 Their seminal work
In general, binary mixed homopolymer brushes can be made
by using three methods: (i) “graing from”, where two different
polymers are grown from the surface by surface-initiated poly-
merization,9–11 (ii) “graing to”, where two preformed, end-
functionalized polymers are graed onto a solid substrate via
the reaction with surface functional groups,12,13 and (iii)
combination of “graing to” and “graing from”, where one
polymer is graed onto the substrate while another polymer is
grown from the surface.14 Sidorenko et al. reported the synthesis
of rst binary mixed brushes of polystyrene (PS)/poly(2-vinyl-
pyridine) (PVP) by a “graing from” method.9a An azo free
radical initiator was immobilized on silicon wafers, followed by
a two-step conventional free radical polymerization process to
grow two polymers; the second polymer was grown from the
residual surface azo initiator that did not decompose in the
synthesis of the rst polymer.9a The obtained mixed brushes
exhibited switching properties in response to environmental
changes. We previously reported the synthesis of well-dened
Department of Chemistry, University of Tennessee, Knoxville, TN 37996, USA. E-mail:
zhao@ion.chem.utk.edu
† Electronic supplementary information (ESI) available: TGA data of other Y
initiator-functionalized silica particles and corresponding hairy particles. See
DOI: 10.1039/c4ra02429j
‡ Contributed equally to the work.
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binary mixed brushes by sequential atom transfer radical shown that mixed brushes can phase separate into a variety of
polymerization (ATRP) and nitroxide-mediated radical poly- morphologies ranging from isolated, nearly spherical nano-
merization (NMRP) from asymmetric difunctional initiator domains in the matrix of another polymer, to short cylindrical
(Y-initiator)-functionalized silica substrates.10 The use of Y- nanodomains, to bicontinuous wormlike nanopatterns, and
initiators ensures that the two types of initiators are well mixed, two-layered structures.10h,i If one polymer is etchable under mild
in fact alternately distributed, on the silica surface and thus it is conditions, many interesting porous nanostructures can be
possible to make well-mixed binary polymer brushes. The fabricated from such multicomponent hairy particles, and the
molecular weights and molecular weight distributions of two obtained nanomaterials may nd applications, e.g., in separa-
tethered polymers can be well controlled, allowing the study of tion or catalysis. Therefore, we embarked on an effort to
microphase separation of mixed brushes by transmission elec- synthesize mixed brushes on silica particles composed of one
tron microscopy and differential scanning calorimetry.10f–l
The “graing to” method is straightforward and allows the
degradable polymer and one vinyl polymer.
In the present work, we synthesized mixed poly(3-caprolactone)
use of polymers with controlled molecular weights, well-dened (PCL)/PS brushes from silica particles that were functionalized
end groups, and predetermined architectures for the prepara- with an asymmetric difunctional initiator (Y-initiator) bearing a
tion of binary mixed brushes.12,13 For example, Minko et al. hydroxyl group, an initiator for ring-opening polymerization
reported the synthesis of mixed PS/PVP brushes by sequentially (ROP),15 and an NMRP initiator by combining metal-catalyzed
graing carboxyl-terminated PS and PVP onto silica wafers that ROP of 3-caprolactone (3CL) and NMRP of styrene (Scheme 1). The
were functionalized with 3-glycidoxypropyltrimethoxysilane.12a,b triethoxysilane-terminated Y-initiator was immobilized onto the
Julthongpiput et al. prepared Y-shaped brushes by graing surface of bare silica particle via an ammonia-catalyzed hydrolysis
diblock copolymers to silicon wafers via the reaction between and condensation process.10i–k,16 The surface-initiated ROP of 3CL
ꢀ
the carboxylic acid group at the junction point of the two blocks was carried out at 75 C in the presence of a free initiator with
and the silica surface.13a ABC triblock copolymers with a reac- tin(II) 2-ethylhexanoate (Sn(Oct)2) as catalyst, followed by the
tive, short central B block were also employed to make mixed NMRP of styrene at 120 ꢀC. We also showed that the overall
brushes by reacting the B block with the surface of a solid graing density of mixed PCL/PS brushes can be tuned by
substrate, either at or curved.13b–e Although “graing to” is changing the mass ratio of the triethoxysilane-terminated Y-
simple, the thickness of the obtained brushes tends to be low initiator to bare silica particles in the initiator immobilization
because of the steric hindrance presented by the graed poly- step. The method described in this article can be extended to the
mer to the incoming polymer chains.
synthesis of mixed brushes containing other degradable poly-
Duan et al. reported a method for the synthesis of binary mers, which could open up new opportunities for applications of
mixed brushes graed on gold nanoparticles by combining mixed brush-graed particles. Note that the preparation of block
“graing to” and “graing from” techniques. A thiol-terminated copolymers by combining ROP and NMRP from difunctional
poly(ethylene glycol) (PEG) with a molecular weight of 5 kDa initiators in solution has been previously demonstrated.17
and a dithiol-terminated ATRP initiator were co-assembled onto
gold nanoparticles via the formation of Au–S bonds, followed by
surface-initiated ATRP to grow the second polymer from gold
nanoparticles, yielding well-dened binary mixed brushes.14
Experimental
Materials
The experimental studies of mixed brushes thus far mainly
focus on the brush systems composed of two vinyl polymers;
there are only a few examples of mixed brushes containing
nonvinyl polymers or chemically or biologically degradable
polymers in the literature.9h,14 Recent experimental studies have
Tetraethyl orthosilicate (or tetraethoxysilane, TEOS, 98%),
ammonium hydroxide (25% in water), triethoxysilane
(HSi(OC2H5)3, 95%), and 2,2,6,6-tetramethylpiperidinooxy
(TEMPO, 98%) were purchased from Acros Organics and
used as received. The platinum–divinyltetramethyldisiloxane
Scheme 1 Synthesis of mixed poly(3-caprolactone) (PCL)/polystyrene (PS) brushes on silica particles functionalized with a Y-initiator bearing a
hydroxyl group and a TEMPO moiety by combining Sn(Oct)2-catalyzed ring-opening polymerization (ROP) of 3-caprolactone (3CL) and nitro-
xide-mediated radical polymerization (NMRP) of styrene.
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complex in xylene (2.1–2.4% Pt concentration in xylene) was
Transmission electron microscopy (TEM) was employed to
obtained from Gelest, Inc. 2-[4-(But-3-enyl)phenyl]-2-(20,20,60,60- study PCL brush-graed and mixed PCL/PS brush-graed silica
tetramethyl-10-piperidinyloxy)ethanol (Y-silane precursor) was particles. Chloroform, a good solvent for PCL and PS, was used
prepared according to a procedure described in a previous to prepare hairy particle dispersions. For each sample, 1–2 mg
publication.10d Styrene (99%, Aldrich) and 3-caprolactone (3CL, of particles was dispersed in 1 mL of chloroform in a small vial
99%, Acros Organics) were dried with CaH2 and distilled under by ultrasonication. The particle dispersion was drop cast onto a
reduced pressure. The puried monomers were stored in a carbon-coated, copper TEM grid using a glass pipette and was
refrigerator prior to use. Tin(II) 2-ethylhexanoate (Sn(Oct)2, allowed to dry at ambient condition. The samples were solvent
ꢁ95%) was obtained from Sigma-Aldrich. u-Undecylenyl annealed with CHCl3 vapor in a closed container at room
alcohol (98%) was purchased from Aldrich and used as the free temperature for >3 h. Transmission electron microscopy (TEM)
initiator for ROP in the synthesis of PCL brushes on silica images were acquired using a Zeiss Libra 200 MC transmission
particles. 1-Phenyl-1-(20,20,60,60-tetramethyl-10-piperidinyloxy)- electron microscope equipped with
ethane (STEMPO), an initiator for NMRP, was prepared by US1000XP CCD camera.
following a procedure from the literature.10k,18 Toluene and
a Gatan UltraScan
tetrahydrofuran (THF) were dried by reuxing over sodium and
benzophenone under nitrogen atmosphere until the color
Synthesis
changed to purple, and then distilled and used immediately.
Synthesis of 2-[4-(1-triethoxysilylbutyl)phenyl]-2-(20,20,60,60-
Ethanol (200 proof) was obtained from Decon Laboratories, Inc. tetramethyl-10-piperidinyloxy)ethanol (Y-silane). 2-[4-(But-3-
All other chemical reagents were purchased from either Aldrich enyl)phenyl]-2-(20,20,60,60-tetramethyl-10-piperidinyloxy)ethanol
or Fisher and used without further purication.
(Y-silane precursor, 0.465 g, 1.404 mmol) was added into a 25
mL two-necked ask and dried under vacuum at room
temperature for 1 h. Triethoxysilane (HSi(OC2H5)3, 2.50 mL,
13.5 mmol) was injected into the ask via a syringe under N2
Instrumentation
Size exclusion chromatography (SEC) was carried out at atmosphere, followed by addition of the platinum–divinylte-
ambient temperature using PL-GPC 20 (an integrated SEC tramethyldisiloxane complex in xylene (80 mL). The mixture was
system from Polymer Laboratories, Inc.) with a differential stirred at 45 ꢀC under nitrogen and the hydrosilylation reaction
refractive index detector, one PLgel 5 mm guard column (50 ꢂ was monitored by 1H NMR spectroscopy analysis. Once the
7.5 mm), and two PLgel 5 mm mixed-C columns (each 300 ꢂ 7.5 reaction was complete, excess triethoxysilane was removed
mm, linear range of molecular weight from 200 to 2 000 000 under high vacuum at 45 ꢀC, and the product was further
according to Polymer Laboratories, Inc.). THF was used as the puried by silica gel column chromatography using methylene
carrier solvent at a ow rate of 1.0 mL minꢃ1. Polystyrene chloride as eluent (yield: 64.3%). 1H NMR d (ppm, CDCl3,
standards (Polymer Laboratories, Inc.) were employed for cali- tetramethylsilane (TMS) as internal standard): 0.59 (t,
bration. The data were processed using Cirrus GPC/SEC so- 2H, SiCH2CH2–), 1.08, 1.18, 1.27, 1.43 (each s, 12H,
ware (Polymer Laboratories, Inc.). PCL free polymers, formed –C(CH3)2), 1.14 (t, 9H, –OCH2CH3), 1.30–1.66 (m, 10H,
from the free initiator in the synthesis of PCL brushes, were also –C(CH3)2CH2CH2CH2C(CH3)2– and –SiCH2CH2CH2CH2–), 2.53
analyzed at 40 ꢀC by PL-GPC 120 equipped with a Precision (t, 2H, –ArCH2CH2–), 3.62 (dd, 1H, HOCHH–), 3.73 (q, 6H,
Detector PD2040 (a two angle static light scattering detector, 15ꢀ –SiOCH2CH3), 4.15 (dd, 1H, HOCHH–), 5.20 (dd, 1H, HOCH2-
and 90ꢀ), Viscotek 220 differential viscometer, and a Polymer CHO–), 5.83 (br, 1H, –OH), 7.13 (dd, 4H, ArH). 13C NMR d (ppm,
Laboratories differential refractometer. The system consists of TMS as internal standard): 9.23, 16.13, 17.29, 21.44, 28.69,
four Polymer Laboratories PLgel columns (300 ꢂ 7.5, 10 mm 33.71, 34.23, 39.18, 39.39, 57.30, 68.73, 82.42, 125.65, 127.34,
3
5
6
particles with pore sizes of 500, 10 , 10 , and 10 A, respectively). 135.04, 141.41. Mass spectrum: [M + H]+ at m/z 496.343.
˚
The calibration range is 600 to 7 500 000 Da using either PS or Synthesis of bare silica particles. Ammonium hydroxide
PMMA standards. A dn/dc value of 0.075 mL gꢃ1 for PCL, which (25% in water, 14.181 g) and tetraethoxysilane (TEOS, 7.101 g)
was from a literature paper,19 was employed in the measure- were each mixed with ethanol (5 mL) to form homogeneous
ments of PCL absolute molecular weights by PL-GPC 120 with a solutions. The two solutions were added into a 500 mL ask that
two-angle light scattering detector.1H NMR spectra were recor- contained 190 mL of ethanol under stirring. The concentrations
ded on a Varian Mercury 300 MHz NMR spectrometer with of NH3, TEOS, and water in the solution were 0.45, 0.15, and
tetramethylsilane (TMS) as the internal standard in 1H NMR 3.10 M, respectively. Aer the mixture was stirred vigorously at
spectra. Thermogravimetrꢃic1 analysis (TGA) was performed at a room temperature for 12 h, the silica particles were isolated by
ꢀ
ꢀ
heating rate of 20 C min from room temperature to 800 C centrifugation (Eppendorf 5804 centrifuge, 6000 rpm), redis-
using either TA Q-series Q50 in air or a SII Nanotechnology TG/ persed in ethanol, and centrifugated again. This washing
DTA 320 in pure oxygen. The hairy particle samples were dried process was repeated with ethanol one more time, water three
ꢀ
at 55 C in vacuum for at least 6 h prior to TGA analysis. Mass times, and ethanol again. Particles were then dried with a
spectrometric analysis was performed using a QSTAR Elite stream of air ow (1.936 g).
quadrupole time-of-ight (QTOF) mass spectrometer equipped
Synthesis of Y-initiator-functionalized silica particles. The
with an ESI ion source and an integrated syringe pump (AB triethoxysilane-terminated Y-initiator (Y-silane, Scheme 1) was
Sciex, Foster City, CA, USA).
immobilized onto the surface of silica particles via an
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ammonia-catalyzed hydrolysis/condensation process.10i–k,16 injected via a dry syringe into the ask that contained the
Seven batches of Y-initiator-functionalized silica particles (YI- initiator particles under the N2 atmosphere. The mixture was
Particles) were prepared using different mass ratios of Y-silane ultrasonicated to form a homogeneous stable dispersion, which
to bare silica particles. The following is the procedure for the was then transferred into the catalyst ask using a syringe. The
preparation of YI-P-3 (Table 1) using a Y-silane-to-silica particle mixture was stirred at 40 ꢀC under N2 for 30 min. Aer the ask
mass ratio of 87.5%. Other initiator particles were made using was cooled to room temperature, u-undecylenyl alcohol (54.7
the same procedure except different mass ratios of Y-silane to mg, 0.321 mmol) and 3-caprolactone (16.575 g, 145.2 mmol)
bare silica particles.
were injected using syringes while stirring. The polymerization
Bare silica particles (149.2 mg) were dispersed in ethanol (10 mixture was degassed by three freeze–pump–thaw cycles, and
mL) in a ask by ultrasonication to form a homogeneous, stable then the ask was placed in an oil bath with a preset temper-
dispersion. A solution of ammonium hydroxide (25% in water, ature of 75 ꢀC to start the polymerization. SEC was employed to
1.10 g) in ethanol (5.0 mL) was added dropwise into the particle monitor the reaction. Aer 13 h, the ask was removed from the
dispersion. The mixture was stirred at 40 ꢀC for 2 h, followed by oil bath and opened to air. THF (20 mL) was added into the ask
the addition of a solution of Y-silane in ethanol with a to dilute the mixture and a small amount of acetic acid was
concentration of 3.21 wt% (4.065 g). Aer the immobilization added to quench the polymerization. The particles were sepa-
ꢀ
reaction proceeded at 40 C for 19 h, the Y-initiator-function- rated by centrifugation and redispersed in THF. This washing
alized silica particles were isolated by centrifugation and re- process was repeated with THF for a total of ve times, followed
dispersed in THF. This washing process was repeated for a total by drying the particles with a stream of air ow. The Mn,SEC and
of ve times. The obtained initiator particles, YI-P-3, were then polydispersity index (PDI) of the free PCL formed from the free
dried with a stream of air ow (127.9 mg) and used for the initiator in the polymerization were 24.3 kDa and 1.20, respec-
preparation of mixed PCL/PS brush-graed silica particles.
tively, measured by SEC analysis using PS standards for
Synthesis of PCL brushes from Y-initiator-functionalized calibration.
silica particles by surface-initiated ring opening polymerization
Synthesis of mixed PCL/PS brushes from PCL brush-graed
of 3-caprolactone (3CL). The synthesis of PCL brushes from YI- silica particles by surface-initiated nitroxide-mediated radical
P-3 by surface-initiated ROP of 3CL is described in the following. polymerization. The PCL brush-graed silica particles (made
Similar procedures were used to grow PCL brushes from other from YI-P-3, PCL Mn,SEC ¼ 24.3 kDa, 0.0695 g) were dispersed in
initiator particles.
anisole (8.019 g) in a 25 mL two-necked ask using an ultrasonic
Y-initiator functionalized silica particles (YI-P-3, 116.9 mg) water bath. The particle dispersion was then transferred into a
and dry toluene (2.0 mL) were added into a 25 mL two-necked 50 mL two-necked ask that contained 1-phenyl-1-(20,20,60,60-
ask. The mixture was ultrasonicated in an ultrasonic water tetramethyl-10-piperidinyloxy)ethane (STEMPO, 46.6 mg, 0.178
bath to form a dispersion. The toluene was then distilled under mmol) and 2,2,6,6-tetramethylpiperidinooxy (TEMPO, 1.0 mg,
vacuum to remove the trace amount of water in the ask. The 0.0064 mmol), followed by the addition of styrene (10.165 g,
azeotropic distillation with dry toluene was repeated one more 97.6 mmol). Aer the mixture was degassed by three freeze–
ꢀ
time, and the initiator particles were further dried at 55 C in pump–thaw cycles, the polymerization was started by placing
vacuum for 8 h. The catalyst for ROP, tin(II) 2-ethylhexanoate the ask in a 120 ꢀC oil bath and monitored by SEC. Three
(Sn(Oct)2, 343.3 mg, 0.847 mmol), was weighed into a 50 mL samples were taken from the reaction mixture at 5.0, 8.3, and
two-necked ask and dried rst at room temperature via azeo- 13.0 h when the molecular weight of the free polystyrene
tropic distillation with dry toluene and then at 100 ꢀC in vacuum reached the desired values. The polymerization was stopped
for 8 h. The asks were removed from the oil bath and allowed aer 19.5 h. The mixed PCL/PS brush-graed particles were
to cool to room temperature. Freshly dried THF (4.0 mL) was isolated by centrifugation, redispersed in THF, and
Table 1 Characterization data for mixed PCL/PS brush-grafted silica particles synthesized by sequential surface-initiated ROP and NMRP from
Y-initiator-functionalized particles prepared by using different mass ratios of Y-silane to bare silica particles in the initiator-immobilization step
d
Sample
no.
Initiator
particles
Mass ratio of Y-silane to
bare particlesa
PCL Mn,SEC
PS Mn,SEC
sPCL, sPS
soverall
(chains per nm2)
and PDIb
PCL Mn,abs
and PDIb
(chains per nm2)
c
MB-1
MB-2
MB-3-4
MB-4
MB-5
MB-6
MB-7
YI-P-1
YI-P-2
YI-P-3
YI-P-4
YI-P-5
YI-P-6
YI-P-7
153.4%
127.4%
87.5%
64.3%
29.0%
18.7%
14.4%
25.8 kDa, 1.14
25.4 kDa, 1.14
24.3 kDa, 1.20
24.0 kDa, 1.20
25.6 kDa, 1.17
27.1 kDa, 1.19
24.2 kDa, 1.14
17.0 kDa
16.7 kDa
16.0 kDa
16.0 kDa
16.8 kDa
18.7 kDa
16.1 kDa
24.5 kDa, 1.16
25.2 kDa, 1.17
24.6 kDa, 1.18
24.0 kDa, 1.15
24.3 kDa, 1.16
24.2 kDa, 1.16
23.8 kDa, 1.14
0.37, 0.69
0.38, 0.86
0.40, 0.83
0.25, 0.53
0.14, 0.53
0.19, 0.45
0.15, 0.25
1.06
1.24
1.23
0.78
0.67
0.64
0.40
a
The mass ratio of the triethoxysilane-terminated Y-initiator (Y-silane) to the bare silica particles in the initiator immobilization step. b Molecular
c
weight (Mn,SEC) and polydispersity index (PDI) were determined by using SEC relative to polystyrene standards. PCL absolute molecular weight
(Mn,abs) determined by SEC with a multi-angle light scattering detector. The graing densities were calculated using the size of bare silica
d
particles (157 nm), TGA data, and the molecular weights of free polymers (Mn,abs for PCL and Mn,SEC for PS), and assuming that the density of
silica particles was identical to that of bulk SiO2 (2.07 g cmꢃ3) and the molecular weights of the graed PCL and the free PCL are the same.
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centrifugated again. This washing process was repeated four tetramethyl-10-piperidinyloxy)ethanol (Y-silane precursor) with
times to remove the physically adsorbed free polymer. The hairy HSi(OC2H5)3 (Scheme 2). The combination of two different
particle samples were then dried with a stream of air ow.
initiators into one molecule ensures that they are alternately
distributed in the initiator layer on the surface of silica parti-
cles, allowing the preparation of well-mixed PCL/PS brushes.
The obtained Y-silane, 2-[4-(1-triethoxysilylbutyl)phenyl]-2-
(20,20,60,60-tetramethyl-10-piperidinyloxy)ethanol, was puried by
silica gel column chromatography using methylene chloride as
Cleavage of PCL brushes from silica particles
PCL brush-graed silica particles made from YI-P-3 (18.1 mg,
PCL Mn,SEC ¼ 24.3 kDa and PDI ¼ 1.20) were dispersed in toluene
(2.5 mL) in a Teon centrifuge tube by ultrasonication, followed
by the addition of tetraoctylammonium bromide (21.3 mg),
hydrouoric acid (HF, 48–51% aqueous solution, 0.3 mL), and
water (1.5 mL). Aer the mixture was stirred at room temperature
for 2.5 h, ammonium hydroxide (25% in water) was added to
neutralize the hydrouoric acid (until the mixture was neutral,
tested by pH paper). The mixture was extracted with toluene twice
(10 mL ꢂ 2), and the organic layer was separated. The organic
extracts were combined and washed with water twice. Aer the
solvent was removed using a rotary evaporator, the polymer was
dried under high vacuum and characterized by SEC. The values of
Mn,SEC and PDI were 18.8 kDa and 1.39, respectively.
1
eluent, and the molecular structure was conrmed by H and
13C NMR spectroscopy and mass spectrometry analysis. Fig. 1
shows the 1H NMR spectrum of Y-silane with the characteristic
peaks assigned and labelled.
The bare silica particles used in the present work were made
by the Stober process.20 The average diameter of the bare silica
¨
particles was 157 nm, measured from transmission electron
microscopy (TEM) micrographs (a typical TEM image is shown
in Fig. 2a). The Y-initiator was immobilized onto the surface of
silica particles via an ammonia-catalyzed hydrolysis and
condensation process in ethanol, similar to those reported in
the literature for other triethoxysilane-terminated ini-
tiators.10i–k,16 As shown in a previous work, the initiator density
and hence the graing density of polymer brushes can be
systematically tuned by varying the mass ratio of the triethoxy-
silane-terminated initiator to bare silica particles in the initiator
immobilization process.10k Using this method, we made seven
Molecular weight and molecular weight distribution of
graed polystyrene in mixed PCL/PS brush-graed silica
particles (MB-3-4)
To determine the molecular weight and molecular weight
distribution of the graed polystyrene in the mixed brushes
synthesized by surface-initiated NMRP from PCL brush-graed
silica particles, we cleaved the mixed PCL/PS brushes off the
silica particles (MB-3-4) using HF, hydrolyzed the obtained
polymer mixture, and then extracted polystyrene using cyclo-
hexane. The detailed procedure is presented in the following.
Mixed PCL/PS brush-graed silica particles (MB-3-4, Mn,PCL
¼
24.3 kDa, Mn,PS ¼ 24.6 kDa, 20.4 mg) were dispersed in toluene (5
mL) in a Teon centrifuge tube by ultrasonication, followed by
addition of tetraoctylammonium bromide (24.6 mg) and hydro-
uoric acid (HF, 48–51% aqueous solution, 1.0 mL). Aer the
mixture was stirred at room temperature overnight, ammonium
hydroxide (25% in water) was used to neutralize the mixture. The
mixture was extracted with toluene (10 mL ꢂ 2) and the organic
layer was separated. The organic extracts were combined, washed
with water twice, and dried under high vacuum. A 30 wt% solu-
tion of sodium methoxide in methanol (52.3 mg) was added into
the ask that contained the polymer mixture cleaved from MB-3-
4 along with THF (2 mL) and methanol (0.15 mL). The reaction
mixture was stirred at 70 ꢀC for 6 days. Aer the removal of
solvents, the residue was extracted with cyclohexane and the
obtained polymer was characterized by 1H NMR spectroscopy
Scheme 2 Synthesis of Y-silane by platinum-catalyzed hydrosilylation
of Y-silane precursor with triethoxysilane (HSi(OC2H5)3).
and SEC. SEC results (relative to polystyrene standards): Mn,SEC
25.9 kDa and Mw,SEC ¼ 31.9 kDa.
¼
Results and discussion
Synthesis of Y-initiator-functionalized silica particles
The triethoxysilane-terminated Y-initiator (Y-silane) bearing a
hydroxyl group, an initiator for ROP of cyclic esters,15 and an
NMRP initiator was synthesized by platinum-catalyzed hydro-
Fig.
(20,20,60,60-tetramethyl-10-piperidinyloxy)ethanol (Y-silane). CDCl3 was
1
1H NMR spectrum of 2-[4-(1-triethoxysilylbutyl)phenyl]-2-
silylation reaction of 2-[4-(but-3-enyl)phenyl]-2-(20,20,60,60- used as solvent.
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Fig. 3 Thermogravimetric analysis (TGA) of (a) bare silica particles, (b)
Y-initiator-functionalized silica particles (YI-P-3), (c) PCL brush-graf-
ted silica particles with PCL Mn,SEC of 24.3 kDa synthesized from YI-P-
3, (d) mixed PCL/PS brush-grafted silica particles with PCL Mn,SEC of
24.3 kDa and PS Mn,SEC of 7.1 kDa, (e) mixed PCL/PS brush-grafted
silica particles with PCL Mn,SEC of 24.3 kDa and PS Mn,SEC of 11.4 kDa, (f)
mixed PCL/PS brush-grafted silica particles with PCL Mn,SEC of 24.3
kDa and PS Mn,SEC of 17.3 kDa, and (g) mixed PCL/PS brush-grafted
particles with PCL Mn,SEC of 24.3 kDa and PS Mn,SEC of 24.6 kDa. TGA
was performed in air (cuꢃrv1es a–f) or in pure oxygen (curve g) at a
heating rate of 20 ꢀC min from room temperature to 800 ꢀC.
Fig. 2 Transmission electron microscopy micrographs of (a) bare
silica particles, (b) Y-initiator-functionalized silica particles prepared by
using the mass ratio of Y-silane to bare silica particles of 87.5% in the
using Sn(Oct)2 as catalyst and u-undecylenyl alcohol as the free
initiator immobilization step (YI-P-3), (c) PCL brush-grafted silica initiator produced relatively low molecular weights even aer
particles made from YI-P-3 with PCL Mn,SEC of 24.3 kDa, and (d) mixed
PCL/PS brush-grafted silica particles prepared from YI-P-3 with PCL
Mn,SEC of 24.3 kDa and PS Mn,SEC of 24.6 kDa.
prolonged polymerization time and broad molecular weight
distributions. The reproducibility was also poor. Recognizing
that the moisture in the reaction system could affect the ROP,
we azeotropically distilled freshly dried toluene twice from YI-
particles to remove water and further dried the particles under
batches of Y-initiator-functionalized silica particles (YI-parti-
cles) with the mass ratio of Y-silane to silica particles ranging
high vacuum at 55 ꢀC for 8 h. The catalyst for ROP of 3CL, tin(II)
2-ethylhexanoate, was also dried by azeotropic distillation with
from 153.4% (the obtained initiator-functionalized particles
denoted as YI-P-1), to 127.4% (YI-P-2), to 87.5% (YI-P-3), to
64.3% (YI-P-4), to 29.0% (YI-P-5), to 18.7% (YI-P-6), and 14.4%
ꢀ
toluene and then under high vacuum at 100 C for 8 h. More-
over, we increased the amount of the catalyst with respect to the
free initiator from a molar ratio of ꢁ1 : 1 to ꢁ2.5 : 1.21 We found
that this condition reproducibly yielded PCL polymers with
Mn,SEC of ꢁ25 kDa (relative to polystyrene standards) and
polydispersity indexes of #1.20.
ꢀ
(YI-P-7). Aer the immobilization reactions proceeded at 40 C
for 19 h, the Y-initiator-functionalized silica particles were iso-
lated by centrifugation and repeatedly washed with THF. The
YI-Particles were then characterized by thermogravimetric
analysis (TGA). Fig. 3a and b shows the TGA curves of bare silica
particles and YI-P-3 with the weight retentions at 800 ꢀC of
90.7%, and 88.5%, respectively, which are similar to our
previous observations for similarly sized bare silica particles
and corresponding silica particles functionalized with a Y-
initiator containing ATRP and NMRP initiators.10k A typical TEM
micrograph of YI-P-3 is shown in Fig. 2b.
Using this condition, we carried out a surface-initiated ROP
ꢀ
of 3CL to grow PCL brushes from YI-P-3 in THF at 75 C in the
presence of a free initiator, u-undecylenyl alcohol. The molar
ratios of 3CL : u-undecylenyl alcohol : Sn(Oct)2 ¼ 452 : 1.0 : 2.6.
Aer the polymerization proceeded for 13 h, the viscosity of the
reaction mixture was high and the stir bar stopped moving. The
ask was removed from the oil bath and the mixture was diluted
with THF, and the particles were isolated by centrifugation, re-
dispersed in THF, and centrifugated again. This washing
process was repeated for a total of ve times to remove the
physically adsorbed PCL. The Mn,SEC and PDI of the free PCL
Synthesis of PCL brushes from Y-initiator-functionalized
silica particles
Ring-opening polymerization of 3CL catalyzed by Sn(Oct)2 is formed from u-undecylenyl alcohol was 24.3 kDa and 1.20
usually carried out at relatively high temperatures, typically (Fig. 4a), relative to polystyrene standards. The narrow PDI
around 120 ꢀC.15 This condition cannot be applied to the indicated that the polymerization was controlled. The actual
synthesis of PCL brushes from Y-initiator-functionalized silica molecular weight Mn,abs of the free PCL determined by SEC
particles because of the thermally induced decomposition of using a multi-angle light scattering detector was 16.0 kDa, using
the NMRP initiator. Therefore, we focused on the growth of PCL a dn/dc value of 0.0750 for PCL in THF.19 Thermogravimetric
brushes by ROP of 3CL at 75 ꢀC, at which the TEMPO moiety has analysis showed that the weight retention of the obtained PCL
been proven to be stable.10a–d Our initial attempt in the synthesis brush-graed silica particles at 800 C was 75.6%, much lower
ꢀ
ꢀ
of PCL brushes from Y-initiator particles in dry THF at 75 C than that of YI-P-3. From the TEM micrograph (Fig. 2c), the PCL
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Fig. 4 Size exclusion chromatography analysis of (a) the free PCL,
formed from the free initiator in the synthesis of PCL brush-grafted
silica particles from YI-P-3, and the free polystyrene formed from the
free initiator STEMPO in the synthesis of mixed PCL/PS brush-grafted Fig. 5 The amount of the grafted polymers relative to the residual
silica at 800 ꢀC versus the PS molecular weight determined by SEC.
silica particles from PCL brush-grafted silica particles by NMRP of
styrene at 5.0 h (b), 8.3 h (c), 13.0 h (d), and 19.5 h (e).
The relative amount of the grafted polymers was calculated using the
equation [(1 ꢃ WHP)/WHP ꢃ (1 ꢃ WYI-P-3)/WYI-P-3] ꢂ 100, where WHP
and WYI-P-3 represent the percent weight retention at 800 ꢀC of hairy
particles and Y-initiator particles (YI-P-3), respectively, corrected for
the weight retention difference at 100 ꢀC, which was presumably from
the adsorbed water or other volatiles.
brushes can be clearly seen. By using the size of silica particles
(157 nm), TGA data, and the Mn,abs of PCL, and assuming that
the density of silica particles was identical to that of bulk SiO2
(2.07 g cmꢃ3
)
and the molecular weights of the graed PCL
10k
and the free PCL are the same, the graing density of PCL (sPCL
)
was calculated to be 0.40 chains per nm2.22
corresponding PCL brush-graed particles (Fig. 2c), the inter-
particle distance became larger, consistent with the increased
polymer content in the hairy particles.
By using the bare silica particle size, TGA data, and the
molecular weights of free PS, and assuming that the density of
Synthesis of mixed PCL/PS brushes from PCL brush-graed
silica particles by surface-initiated NMRP
The PCL brush-graed silica particles made from YI-P-3 with silica particles was identical to that of bulk SiO2 (2.07 g cmꢃ3) and
PCL Mn,SEC of 24.3 kDa were then used for the synthesis of the values of Mn of the graed PS and the free PS are the same,
mixed PCL/PS brushes by surface-initiated NMRP of styrene. the graing densities of PS in four mixed brush samples were
The polymerization was carried out in anisole at 120 ꢀC with calculated.22 They were 0.71, 0.74, 0.90, and 0.83 chains/nm2 for
addition of a free initiator, 1-phenyl-1-(20,20,60,60-tetramethyl-10- MB-3-1, MB-3-2, MB-3-3, and MB-3-4, respectively. The graing
piperidinyloxy)ethane (STEMPO), and a small amount of free density increased slightly with the polymerization time, similar
radical TEMPO (3.6 mol% with respect to STEMPO), and was to our previous observation in the study of the effect of the chain
monitored by SEC. Three samples were taken at 5.0 h, 8.3 h, and length disparity on microphase separation of mixed homopol-
13.0 h before the reaction was stopped at 19.5 h. The hairy ymer brushes graed on silica particles.10h,i This is likely because
particles were separated by centrifuge and repeatedly washed; that not all alkoxyamine moieties on the surface of silica particles
the free PS polymers were analyzed by SEC.
successfully initiated the polymerization at the same time,
Fig. 4b–e shows the GPC traces of four free PS polymers, leading to a lower initiator efficiency at the beginning of the
respectively. With the increase of the polymerization time from polymerization. We previously observed in the kinetics study of
5.0 to 8.3, to 13.0, and 19.5 h, the peak gradually shied to the the TEMPO-mediated NMRP of styrene that the initial polymer-
high molecular weight side. The Mn,SEC of PS increased from 7.1 ization was slower than expected.10d Note that the graing
kDa (PS-1; the corresponding mixed PCL/PS brush-graed densities of PS in these mixed brush samples are signicantly
particle sample designated as MB-3-1), to 11.4 kDa (PS-2; the higher than that of PCL (0.40 chains per nm2). This is likely
particles designated as MB-3-2), to 17.3 kDa (PS-3; the particles because the initiating radicals in NMRP are generated by thermal
designated as MB-3-3), and 24.6 kDa (PS-4; the particles desig- decomposition of the TEMPO moiety, which is a unimolecular
nated as MB-3-4). The PDI decreased from 1.32 (PS-1) to 1.23 process,23 while the initiation in ROP involves the formation of
(PS-2) to 1.18 (PS-3 and PS-4), indicating that the polymerization Sn-alkoxide on the surface and the insertion of monomer mole-
was controlled. The mixed PCL/PS brush-graed silica particles cules.15 Consequently, the surface-initiated ROP is more sensitive
were characterized by TGA. As can be seen from Fig. 3d–g, the than NMRP to the steric hindrance presented by the conned
weight retention of mixed PCL/PS brush-graed silica particles geometry on the surface of silica particles and the inherent
at 800 ꢀC decreased with the increase of PS Mn,SEC, from 69.5% heterogeneity of the particle surface and the initiator layer.
for MB-3-1, to 64.5% for MB-3-2, to 56.0% for MB-3-3, and 51.2%
for MB-3-4. A further analysis showed that the amount of the
graed polymers increased nearly linearly with the PS molec-
ular weight (Fig. 5), suggesting that the surface-initiated NMRP
is also controlled. Fig. 2d shows a typical TEM micrograph of
Tuning overall graing density of mixed PCL/PS brushes –
effect of different mass ratios of Y-silane to bare silica
particles used in the initiator immobilization step
MB-3-4. Clearly, the interstitial space among silica particles was We previously reported that the overall graing density of mixed
lled with the graed polymers; compared with the homopolymer brushes synthesized by sequential ATRP and
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NMRP can be tuned by changing the mass ratio of the initiator cleaved PCL overlapped to a great extent with that of the free
to bare silica particles in the initiator immobilization process.10k PCL, but it was broader. The Mn,SEC and PDI (relative to poly-
In the present work, we prepared a series of Y-initiator-func- styrene standards) were 18.8 kDa and 1.39, respectively, in
tionalized silica particles from the same batch of bare silica contrast to those of the free PCL (Mn,SEC ¼ 24.3 kDa and PDI ¼
particles using different mass ratios of Y-silane to bare particles: 1.20). It is unclear why the molecular weight distribution of the
153.4% (YI-P-1), 127.4% (YI-P-2), 87.5% (YI-P-3), 64.3% (YI-P-4), graed PCL is slightly broader. We suspect that it might be
29.0% (YI-P-5), 18.7% (YI-P-6), and 14.4% (YI-P-7). Using the caused by the metal-catalyzed transesterication reaction
same procedure for the synthesis of mixed PCL/PS brushes from within the polymer brush layer because of the high concentra-
YI-P-3, we carried out surface-initiated ROP of 3-CL and NMRP tion of the ester bonds. The undesired transesterication
of styrene sequentially from other initiator-functionalized silica reaction among the free growing PCL chains in the bulk solu-
particles. The values of Mn,SEC of PCL and PS (relative to poly- tion appeared to be suppressed as indicated by the narrower
styrene standards) were targeted at ꢁ25 kDa. The obtained PCL PDI. Because we did not do the cleavage experiments for other
brush- and mixed PCL/PS brush-graed silica particles were PCL brush-graed silica particles, to be consistent, we still used
characterized by TGA, and the absolute molecular weights of the molecular weights of free polymers in the calculation of PCL
free PCL were measured by SEC with a multiangle light scat- graing densities as shown in Table 1.
tering detector. The data are summarized in Table 1. Using the
same assumptions that we made in the calculation of graing
Molecular weight and molecular weight distribution of the
graed polystyrene cleaved from the mixed PCL/PS brush-
graed silica particles
densities of polymer brushes prepared from YI-P-3, we esti-
mated the graing densities of PCL (sPCL) and PS (sPS) in each
mixed brush particle sample (Table 1) as well as the overall
graing density (soverall ¼ sPCL + sPS). For comparison, the data
for MB-3-4 are also included in Table 1.
We also compared the molecular weight and molecular weight
distribution of the graed PS in the mixed PCL/PS brushes
with those of the free PS formed from the free initiator. The
mixed PCL/PS brush-graed silica particles with PCL Mn,SEC of
24.3 kDa and PS Mn of 24.6 kDa (MB-3-4) were etched with HF
in a aqueous/toluene biphasic system. The collected polymer
mixture was treated under a basic condition in a mixed solvent
of THF and methanol to hydrolyze the PCL. The volatiles were
removed and the PS was extracted from the solid residue with
cyclohexane. Fig. 7 shows the SEC traces of the cleaved PS and
the corresponding free PS; the Mn,SEC of the cleaved PS was
25.9 kDa, reasonably close to that of the free polymer (Mn,SEC
¼ 24.6 kDa). This conrmed that the surface-initiated NMRP
was controlled. However, the two curves did not superimpose
each other entirely, though the two samples were run on the
same SEC system back to back; there was a small shi,
reected by the slightly higher Mn,SEC of the cleaved PS. 1H
NMR spectroscopy analysis indicated the absence of ester
bonds of PCL.
As can be seen from Table 1, there is a general trend that the
overall graing density decreased with the decrease of the mass
ratio of Y-silane to bare silica particles in the initiator immo-
bilization step, from above 1.0 chains per nm2 in MB-1, -2, and
-3 to 0.40 chains per nm2 for MB-7, though the soverall of MB-1
(1.06 chains per nm2) was slightly lower than those of MB-2
(1.24 chains per nm2) and MB-3 (1.23 chains per nm2). The
individual graing densities of PCL and PS also exhibited a
similar trend, but there were some irregularities, particularly
sPCL, which is likely because ROP is more sensitive to the
polymerization conditions and the possible impurities in the
reaction systems. Similar to MB-3-4, the PCL graing density
was noticeably smaller than that of the corresponding PS; in
most cases, sPCL was only about half of sPS or even smaller.
Overall, Table 1 showed that the overall graing density can be
tuned by changing the mass ratio of Y-silane to bare silica
particles.
Molecular weight and molecular weight distribution of the
graed PCL on silica particles
Many researchers have reported that the molecular weights and
molecular weight distributions of polymer brushes graed on
(nano)particles synthesized by surface-initiated “living”/
controlled (radical) polymerization were essentially the same as
those of the free polymers formed from the free initiators.10d,16,24
To measure the molecular weight and molecular weight distri-
bution of the graed PCL, we cleaved the PCL brushes from
silica particles (made from YI-P-3) using hydrouoric acid (HF)
to etch the silica core in an aqueous/toluene biphasic system in
the presence of a phase transfer catalyst, tetraoctylammonium
bromide. The reaction was allowed to proceed at room
temperature for 2.5 h, and then the mixture was neutralized
with 25% ammonium hydroxide in water. The cleaved polymer
Fig. 6 Size exclusion chromatography traces of (a) the free PCL
(Mn,SEC of 24.3 kDa and PDI of 1.20) formed from the free initiator in the
synthesis of PCL brushes from YI-P-3 (solid line) and (b) the grafted
PCL cleaved from the corresponding PCL brush-grafted silica
was collected and characterized by SEC (Fig. 6). The peak of the particles.
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Acknowledgements
This work was supported by the U.S. National Science Foun-
dation (DMR-1007986 and DMR-1206385).
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