A novel strategy to synthesize well-defined PS brushes on silica particles by combination of lithium-iodine exchange (LIE) and surface-initiated living anionic polymerization (SI-LAP)

Article abstract of DOI:10.1039/c5cc00335k

Core-shell hybrid particles, possessing a hard core of silica particles (SiPs) and a soft shell of brushlike polystyrene (PS), were successfully prepared by the combination of lithium-iodine exchange (LIE) and surface-initiated living anionic polymerization (SI-LAP). The molecular weight, graft density and brush thicknesses of the PS brushes were controllable. This journal is

Full text of DOI:10.1039/c5cc00335k

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A novel strategy to synthesize well-defined PS brushes on silica particles
by combination of lithium-iodine exchange (LIE) and surface-initiated
living anionic polymerization (SI-LAP)
Jiakang Min,a,b Yichao Lin,a,b Jun Zheng*a and Tao Tang*a
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
occur during the radical recombination process, which lead to
crosslinking on the surface of the particles at higher temperatures
50 due to uncontrolled radical reactions.15
Core-shell hybrid particles, possessing a hard core of silica
particles (SiPs) and a soft shell of brushlike polystyrene (PS),
are successfully prepared by combination of lithium-iodine
Besides SI-LRP, there are also others polymerization methods
including cationic polymerization,16 ring–opening metathesis
polymerization (ROMP),17 ring–opening polymerization (ROP),18
and surface-initiated photopolymerization (SIPP)19 and so on.
55 The surface-initiated living anionic polymerization (SI-LAP) is
the best among the polymerization mechanisms mentioned above
in controlling the molecular weights, polydispersities and graft
density, which are important structural parameters for polymer
brushes. Zhou et al. have used the 1,1-diphenylethylene (DPE)
60 derivative as an initiator precursor and subsequent “grafting
from” polymerization of styrene after the ethylene moiety on the
DPE was activated by the addition of n-BuLi.20-22 Unfortunately,
this method shows a poor ability in controlling polydispersity
values of polymer brush (ranging from 1.1 to 2.0220-23). It seemed
65 that densely and tunable polymer brushes could not be fabricated
by SI-LAP. Alternatively, Hubner et al. selected “grafting to” for
10 exchange (LIE) and surface-initiated living anionic
polymerization (SI-LAP). Molecular weights, graft density
and brush thicknesses of PS brushes were controllable.
Increasing attentions have been paid to polymer nanocomposites
since they were first investigated in the 1990s.1-3 Many inherent
15 properties of materials are improved and some new functions are
manufactured simultaneously by just adding a very low volume
fraction of nanoparticles into polymer matrices.4-6 However, the
good wishes are strongly breached by the severe tendency of
particles to aggregate in polymeric matrices. So keeping particles
20 highly and uniformly dispersed in polymer composites is the
crucial factor to solve this problem. To the best of our knowledge,
completely miscible composites can be fabricated by coating
particles with a layer of a matrix-miscible polymer such as a
8
polymer brush.7,
Polymer brush is
a
stretched soft
functionalization
of
silica
particles
using
anionic
25 macromolecular structure which is linked through one end with a
solid surface. Further study has shown that these core-shell
hybrid particles can provide new insights into polymer science
and technology because of their unique structural features9.
However, the graft density, chain length and molecular weight of
30 polymer shell must be tunable, since good interfacial interaction
between polymer shell and matrix is essential.10a
Generally, the polymer brush can be synthesized by covalent
attachment to inorganic surfaces with reactive end groups. Two
synthetic methods are available, that is, “grafting from”10b and
35 “grafting to”. The “grafting from” approach starts grafting from
surfaces by surface-initiated polymerization (SIP), in which the
polymerization initiator needs to be covalently fixed onto the
solid surface and then initiates polymerization with the monomers
under suitable conditions10c. So far, the surface-initiated living
40 radical polymerization (SI-LRP) is one of the most common and
powerful pathways because of its convenience and versatility, in
terms to SIP methodology of building polymer brushes.11,12 Kohji
Ohno and the co-workers successively used atom transfer radical
polymerization (ATRP) for preparing silica particles (SiPs) with a
45 shell layer of a well-defined PMMA brush in 2005,13 and
reversible addition-fragmentation chain transfer (RAFT) for
preparing PS brush on SiPs in 2011.14 Unfortunately, limitation
polymerization.24 This method was based on the reaction of end-
functionalized polymers with reactive sites on a substrate surface,
70 which shows an increase of brush graft density. However,
because of inherent shortcomings for “grafting to” method, the
ability for controlling brush graft density (such as keeping the
same graft density with different brush lengths) is poor due to
steric hindrance, and it is necessary to separate free polymer from
75 the products in order to obtain the pure functionalized particles.
In this work, we demonstrated a novel strategy to synthesize
well-defined PS brushes (as an example) on SiPs through the
combination of lithium-iodine exchange (LIE) and surface-
initiated living anionic polymerization (SI-LAP). Since the LIE
80 was discovered in the late 1930s by Wittig25 and Gilman,26, 27 the
remarkably rapid, reversible metathesis reaction had been widely
employed for replacement of iodine atoms in the substrates with
lithium to prepare relatively stable organometallics. Herein using
this technique, core-shell hybrid particles with inner solid SiPs
85 core surrounded by PS brushes were synthesized successfully.
The molecular weights with tunable graft density and brush
thickness of a series of well-defined PS brushes were under
control. The whole strategy is illustrated in Figure 1a. In the first
step, SiPs with the diameter of 200 nm (Figure 2d) were modified
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with 3-iodopropyltrimethoxysilane (ITS) to obtain SiO2@ITS-
PS brushes on SiPs (SiO2@PSs) were synthesized through the
12h for 12 h or SiO2@ITS-24h for 24 h at 110 oC in toluene. This
process allowed replacing the original surface Si-OH groups by
combination of lithium-iodine exchange (LIE) and surface-
initiated living anionic polymerization (SDI-OLIA: 1P0).1.03IVn9ie/Cwt5hACertCic0lLe0IOE3n3li5nKe
Si-I groups partly or completely (depending on the amount of ITS, 45 process, the alkyl iodide was replaced by alkyllithium through
o
5
with the assumption of 5-6 Si-OH groups per nm2 of silica
surface24). In the second step, the resultant SiO2@ITS was
reacted with enough (2.1-2.2 molar equivalent, relative to Si-I
SiO2@ITS reacting with tert-BuLi at -78 C. Then the anionic
polymerization of styrene was initiated by alkylanion. The
grafted amount of PS brush in the resultant products (nominated
as SiO2@ITS-xh@PS-yh, here “x” is the reaction time between
groups) commercial tert-butyllithium (tert-BuLi, 1.6M) for 6 h at
-78 C in a mixed solvent of n-hexane and diethyl ether.28, 29 50 SiPs and ITS, and “y” is the polymerization time of styrene)
o
10 Subsequently, the mixture was rised to room temperature and
kept for 2 h to remove the residual tert-BuLi which was
consumed by rapid proton abstraction from diethyl ether.30 At last,
PS brushes were immobilized on the surface of silica particles via
anionic polymerization of styrene monomer.
under various conditions was determined by TGA (see Table 1).
Then all the SiO2@PSs hybrid particles (8¯1 g) were put into an
aqueous hydrofluoric acid (HF) solution with 5 vol% HF (a
mixed solution of hydrofluoric acid (HF, 10 ml) with ultrapure
55 water (70 ml)) to perform the cleavage of grafted PS brushes. The
weight-average molecular weights (Mw) and polydispersity
indices (PDI) of the grafted PS brushes (SiO2@ITS-24h@PSs)
were checked by SEC-MALLS (Figure 2b and Table 1). It could
be seen that the molecular weight of PS brush layers scaled
60 linearly with polymerization time (Figure S2 in the SI). On the
basis of the amount of PS brush layers determined by TGA and
Mn derived from SEC-MALLS, the graft density of PS could be
estimated according to eq (1). Figure 2c shows the dependence of
graft density of PS brushes (SiO2@ITS-24h@PSs) on anionic
65 polymerization time. The graft density was nearly constant
despite the polymerization time. And its value approximately
equal to 0.79 chains/nm2 (SiO2@ITS-24h@PSs) and 0.47
chains/nm2 (SiO2@ITS-12h@PSs, see Table 1), respectively. The
detailed information on PS brushes on SiPs is summarized in
70 Table 1.
The morphology of the obtained SiO2@ITS-24h@PS-32h
hybrid particles was observed by HRTEM (Figure 2e). Supposing
that the density of the PS layers was about 1.04 g/cm3, the brush
thickness of SiO2@ITS-24h@PS-32h was calculated to be 23 nm
75 based on the TGA data, which was in good agreement with
HRTEM results. It could be seen that the length of the PS brushes
increased with the Mw of PS in the cases with the same graft
density. In order to investigate the dispersion state of SiO2@PSs
hybrid particles in PS matrix, the raw SiPs and SiO2@ITS-
80 24h@PS-32h hybrid particles were mixed with PS matrix by
solution blending, where the content of SiPs was 5 wt% in the
two cases. As expected, the SiO2@PSs hybrid particles were
highly and uniformly dispersed in PS matrix (Figure 2f), but the
raw SiPs had severe tendency to aggregate in PS matrix (Figure
85 S3).
From the graft density it was inferred that the efficiency of LIE
was around 25% in this work, which was lower than the
previously reported results (∼90%).28 In order to disclose the
reason of this phenomenon, an XPS measurement was used to
90 confirm the content of iodine on the surface of the sample after
the LIE process. According to Figure 3a, the dramatical decrease
in the intensity of iodine peak in XPS profiles after LIE indicated
that most of alkyl iodine was replaced by alkyllithium. Further
study showed that there was the formation of a nonnegligible
95 quantity of hydrocarbon derived from reduction of the alkyl
iodides. As shown in Figure 3b, this side reaction was the result
of rapid reaction of the product RCH2Li with the cogenerated
tert-BuI to give RCH3, isobutylene, and LiI that was inactive for
anionic polymerization.28, 31 In our work, the deactivation effect
15
Figure 1. (a) Synthetic routes to SiO2@ITS and SiO2@PS hybrid
particles; (b) H NMR spectra of ITS and CTS; (c) XPS patterns
of SiO2@ITSs and raw SiPs.
1
20 The ITS was prepared by incorporating dry sodium iodide (NaI)
into commercially-available 3-chloropropyltrimethoxysilane
(CTS, 94%, 1H NMR (CDCl3). Figure 1b shows 1H NMR spectra
of ITS and CTS. The notable change between δ 3.24-3.12 (-CH2I)
and δ 3.54-3.48 (-CH2Cl) indicated that chlorine was successfully
25 replaced by iodine. XPS measurements revealed that the surface
of SiO2@ITSs mainly consisted of C and O, with the covalent
linkage of a small quantity of iodine (Figure 1c). Further study
indicated that the XPS signals of iodine are electrons from 3d5
and 3d3 electronic orbits, whose binding energies were 618 eV
30 and 633 eV, respectively (inset in Figure 1c). The amount of ITS
on the surface of SiPs was determined by TGA analysis. The
o
weight retentions at 800 C were 91.5 wt% for raw SiPs, 90.2
wt% for SiO2@ITS-12h (Figure S1) and 89.4 wt% for
SiO2@ITS-24h (Figure 2a). There was 1.3 wt%, 2.1 wt% mass
35 difference between raw SiPs and SiO2@ITS-12h, raw SiPs and
SiO2@ITS-24h. The weight loss of raw SiPs might be caused by
associated water loss and continued condensation. According to
eq (1) in the supporting information, it could be roughly
estimated that the graft densities of iodine on the particle surface
40 of SiO2@ITS-12h and SiO2@ITS-24h were 1.89 and 3.09
molecules/nm2, respectively.
2
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ChemComm
of the side reaction might be enlarged since the LIE was
SiO2@ITS-
24h@PS-32h
28.3
227
1.05
0.79
22
implemented on the interphase not homogeneous phase within 6
h. Thus, we thought that the final efficiency was the balance
result of the extent of LIE and the side reaction mentioned above.
Grafted amount of PS (wt%), determinDedOIb: 1y0.T10G3VA9ie/Cw(5FACirtgCicu0ler0eO3n3Sli5n1Ke
and Figure 2a). Determined by SEC-MALLS (Figure S4 and
Figure 2b). Determined by eq (1). Determined by HRTEM
20 (Figure S5 and Figure 2e).
a
b
c
d
5
Figure 3. (a) XPS patterns of iodine element on SiO2@ITS: (A)
before, and (B) after LIE; (b) side reaction accompanied with LIE.
Figure 2. (a) TGA curves of A-SiO2, B-SiO2@ITS-24h, (C, D, E,
F)-SiO2@ITS-24h@PSs hybrid particles; (b) SEC-MALLS
curves of grafted PS brushes from SiO2@ITS-24h; (c) Graft
density of grafted PS brushes from SiO2@ITS-24h as a function
Conclusions
25
It was demonstrated that surface-decorated SiPs with alkyl
iodides can transformed into anionic polymerization initiation
sites via lithium-iodine exchange (LIE). Core-shell hybrid
particles were synthesized through surface-initiated living anionic
polymerization (SI-LAP). The length of well-defined PS brushes
10 of anionic polymerization time; (d) TEM images of raw SiPs; (e)
HRTEM images of hybrid particles of SiO2@ITS-24h@PS-32h;
(f) TEM images of PS/SiO2@ITS-24h@PS-32h composite
containing 5 wt% net SiPs.
30 was tunable by control of polymerization time. This method
provides a novel strategy to synthesize core-shell hybrid particles
and polymer nanocomposites.
15 Table 1. Data on Grafted PS Brushes on SiPs by Surface-Initiated
Living Anionic Polymerization.
GA a
Mw
Graft Density
(Ds) c
Brush
Length
L d (nm)
b
Sample
PDIb
(%)
(kg/mol)
(chains/nm2)
We acknowledge the National Natural Science Foundation of
China (Nos. 21374114, 51233005 and 21274152) for support of
35 this work.
SiO2@ITS-
12h@PS-8h
8.4
10.4
13.4
16.0
83
106
142
184
1.03
1.02
1.02
1.05
0.48
0.47
0.47
0.46
4
6
SiO2@ITS-
12h@PS-16h
Notes and references
a State Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun
130022, China. Fax: +86 (0) 431 85262827; Tel: +86 (0) 431 85262004;
40 E-mail: ttang@ciac.ac.cn, zhengjun@ciac.ac.cn
SiO2@ITS-
12h@PS-24h
8
SiO2@ITS-
12h@PS-32h
10
b University of Chinese Academy of Sciences, Beijing 100039, China.
SiO2@ITS-
24h@PS-8h
16.6
110
1.02
0.78
10
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