Synthesis and properties of crosslinked chiral nanoparticles via RAFT miniemulsion polymerization

Article abstract of DOI:10.1002/pola.23893

Crosslinked chiral nanoparticles were successfully synthesized via reversible addition-fragmentation chain transfer (RAFT) miniemulsion polymerization of 6-O-p-vinylbenzyl-1,2:3,4di-O-isopropylidene-D-galactopyranose (VBPG) using linear poly(VBPG) as the macro-RAFT agent. The polymerization of VBPG in the absence of crosslinker was first studied and the kinetic results showed that the molecular weights of the obtained poly(VBPG) increased linearly with the monomer conversion and was in good consistency with the corresponding theoretical ones while there remained a relative narrow polydisperslty. The effect of the amount of crosslinker, divlnylbenzene, on the nanoparticle size and chiral separation properties of the obtained nanoparticles were investigated in detail using four racemates ±-3-Amino-1,2propanediol, D,L-arablnose, D,L-tartaric acid, and D,L-mandelic acid.

Full text of DOI:10.1002/pola.23893

Synthesis and Properties of Crosslinked Chiral Nanoparticles
via RAFT Miniemulsion Polymerization
WENLIANG XU, ZHENPING CHENG, LIFEN ZHANG, ZHENGBIAO ZHANG, JIAN ZHU, NIANCHEN ZHOU, XIULIN ZHU
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science,
Soochow University, Suzhou 215123, China
Received 13 August 2009; accepted 13 December 2009
DOI: 10.1002/pola.23893
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Crosslinked chiral nanoparticles were successfully syn-
amount of crosslinker, divinylbenzene, on the nanoparticle size
and chiral separation properties of the obtained nanoparticles
were investigated in detail using four racemates 6-3-Amino-1,2-
propanediol, D,L-arabinose, D,L-tartaric acid, and D,L-mandelic acid.
VC 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48:
thesized via reversible addition-fragmentation chain transfer
(RAFT) miniemulsion polymerization of 6-O-p-vinylbenzyl-1,2:3,4-
di-O-isopropylidene-D-galactopyranose (VBPG) using linear pol-
y(VBPG) as the macro-RAFT agent. The polymerization of VBPG in
the absence of crosslinker was first studied and the kinetic results
showed that the molecular weights of the obtained poly(VBPG)
increased linearly with the monomer conversion and was in good
consistency with the corresponding theoretical ones while there
remained a relative narrow polydispersity. The effect of the
1324–1331, 2010
KEYWORDS: chiral separation; emulsion polymerization; macro-
RAFT agent; nanoparticles; reversible addition-fragmentation
chain transfer (RAFT)
INTRODUCTION Driven by the increasing demand for opti-
permselectivity in surface-modified nanoporous opal films
for chiral separation. All the aforementioned separation
1
,2
11
cally pure enantiomers in the chemical industry, the inter-
est in developing new and efficient methods to produce
enantiomerically pure compounds is critical for the develop-
ment of new materials, particularly pharmaceuticals, agro-
processes are due to enantioselective adsorption on the sub-
1
2
13
strate surfaces, as reported by Gellman and Ernst.
Over the past decade, the controlled/‘‘living’’ radical polymer-
ization (CRP) has been demonstrated to be a potentially useful
and important new approach for producing polymers with pre-
3
chemicals, and flavors. Generally, chiral compounds are pre-
pared either by asymmetric catalysis or by the chiral
resolution of the enantiomers using various separation meth-
ods. Despite impressive advances in stereoselective synthesis,
separation of racemic mixtures still is the most widely used
method of producing single enantiomers in industry. Note-
worthy systematic research of chiral separation techniques
includes various chromatographic methods, such as gas chro-
matograph (GC), supercritical fluid chromatography (SFC),
viously unattainable complex architectures in a controlled man-
14–16
ner.
The CRP is usually conducted in a homophase system.
Compared with polymerization processes in organic solvents
polymerization conducted in environmentally friendly solvent
such as water is the best alternative to obtain polymeric nano-
particles in large-scale providing excellent heat transfer, ease of
mixing, process flexibility, and ease of handling/transporting
the final latex. There are several heterophase processes that
allow the formation of nanoparticles in water, for example,
emulsion, miniemulsion, precipitation, suspension polymeriza-
4
–6
and high performance liquid chromatography (HPLC).
The
HPLC method, the most popular among chromatography,
mainly utilizes chiral stationary phases (CSPs) to carry out
the determinations. Excellent works have been done by Oka-
moto et al. in this field. They developed and applied the
polysaccharide-based CSPs as well as chiral packing materi-
als (CPMs) based on optically active polymers for efficient
1
7,18
tions, and so forth.
The well-known method of miniemul-
sion polymerization process is a very versatile technique for
the formation of a broad range of polymers and structured
materials in confined geometries. In general, miniemulsion con-
sists of small, stable, and narrowly distributed droplets in a
continuous phase. The high stability of the droplets is ensured
by the combination of the surfactant, and the costabilizer, which
is soluble and homogeneously distributed in the droplet phase.
Such small droplets can then act as nanocontainers in which
reactions can take place, either inside the droplets or at the
interface of the droplets, resulting in most of the cases in form-
ing nanoparticles. However, this system has not been
7
–9
separation of enantiomers.
It is noted that publications
covering preparative chiral polymer for separation have con-
tinued to increase in recent years, which solved the problem
of labor and energy intensive of chromatographic methods.
Schlenoff’s group reported optically active polyelectrolyte
1
multilayers as membranes for chiral separations. Goto et al.
extended supported liquid membranes for chiral separa-
1
0
tion. Subsequently, Zharov and coworkers developed chiral
Correspondence to: X. Zhu (E-mail: xlzhu@suda.edu.cn)
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 1324–1331 (2010) V
2010 Wiley Periodicals, Inc.
C
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ARTICLE
2
6
particularly successful due to issues of mass transfer of the mo-
lecular weight control agent. Schork et al. have shown theoreti-
cally and experimentally that oligomers are very effective swel-
ling agents and can cause high swelling of polymer particles
with monomer, whereas polymers with high molecular weight
synthesized as reported elsewhere. Divinylbenzene (DVB)
(50%, mixture of isomers, Shanghai Chemical Reagent Co.)
0
was purified by passing it through the silica gel column. 2,2 -
Azobisisobutyronitrile (AIBN, 99%, from Shanghai Chemical
Reagent, China) was recrystallized twice from ethanol and
dried in vacuum at room temperature overnight. Tetrahydro-
furan (THF) and toluene (analytical reagent, from Shanghai
Chemical Reagent Co., China) was purified by distilling in the
presence of molecular sieves. Hexadecane (HD, Sigma-
Aldrich, 99%), sodium dodecyl benzene sulfonate (SDS),
ethanol, D,L-tartaric acid, D-(ꢀ)-mandelic acid, L-(þ)-mandelic
acid, D-(ꢀ)-tartaric acid, L-(þ)-tartaric acid, and D,L-mandelic
acid (analytical reagent, from Shanghai Chemical Reagent,
China), D,L-arabinose and 6 3-amino-1,2-propanediol (Alfa
Aesar, 97%), and (R)-3-amino-1,2-propanediol, (S)-3-amino-
1,2-propanediol (TCI, 99%), D-(ꢀ)-arabinose and L-(þ)-arabi-
nose (Alfa Aesar, 99%) were used without further purifica-
tion. In all miniemulsion experiments, deionized water
1
9
are much less effective swelling agents. Phase separation may
occur in the resultant particles, possibly resulting in nonspheri-
cal particles and coagulation. Hence, macro-RAFT agent, suita-
ble stabilizer, and costabilizer are the best choice to solve these
2
0
disadvantages.
Sugars are naturally occurring molecules. Derivatives of sug-
ars, especially their oligomers and polymers as well as their
bioconjugates and modifications, are everywhere in the living
systems and particularly abundant on the nanoparticle surfa-
ces. To date, many synthetic glycopolymers have been syn-
thesized via CRP techniques. For example, Yaacoub and co-
workers used RAFT miniemulsion polymerization to prepare
2
1
well-defined polymers carrying saccharide moieties. Lowe
et al. used the same technique to polymerize monomers car-
TM
(ultrapure water from Direct-Q 3 UV Millipore) was used
as a reaction medium. All other solvents and reagents were
purchased from commercial sources and were used as
2
2
rying unprotected saccharide residues, due in part to their
2
3
biomimetic properties. Large amounts of carbohydrates are
commercially available and a substantial part of the surplus
received.
2
4
of their agricultural production may be spent in this way.
1
H NMR for BDB (CDCl
3 2
, 400 MHz, d): 4.60 (s, 2H, CH APh),
.30–7.60 (m, 8H, ArH), 8.02 (m, 2H, ArH).
Chiral recognition has already been established in respect to the
interactions of sugars (glucose, mannose, arabinose, and xylose)
7
25
Characterization
with chiral Pt single crystal surfaces, where electro-oxidation
accompanied adsorption and was the means whereby the enan-
tomeric response was identified. Enlarging the availability of
specific chiral surface area can enhance the efficiency of chiral
separation. From practical and energetically efficient standpoint,
separation of enantiomers through chiral nanoparticales will be
an attractive process to carry on separation at a large scale.
The mean particle sizes and size distributions of the polymer
latexes were determined with a HPP 5001 high-performance
particle size (HPPS) instrument (Malvern) at wavelength of
532 nm at 25 6 0.1 C and dynamic light scattering (DLS)
on a Zetasizer Nano-ZS (Malvern Instrument) at wavelength
ꢁ
ꢁ
of 633 nm at 25 6 0.1 C. Before the measurements, the
original latex samples were diluted with deionized water to
adjust the light strength suitable to the measurement condi-
tion. The cumulant method was chosen for measuring the z-
Our previous work demonstrated that the linear chiral poly(6-
O-p-vinylbenzyl-1,2:3,4-di-O-isopropylidene-D-galactopyranose)
[poly(VBPG)] could be obtained by the RAFT polymerization
z
average hydrodynamic diameter (D ) and size polydispersity
of the monomer, VBPG in solution, and had the ability of
2
6
(denoted size PDI) were automatically analyzed in the cumu-
lant mode by the Malvern Zetasizer software and were
reported as the average of three measurements. The mor-
phology of latex particles was studied by means of transmis-
sion electron microscopy (TEM, Tecnai, G2-20, FEI Company)
with electron microscope operating at 100 kV. The diluted
colloidal solutions were applied to a 400-mesh carbon-
coated copper grid and left to dry for 1 day by air at room
temperature before measurement. The number-average mo-
enantioselective separation. As for particles used for the
enantioselective recognition, there have been some reports
prepared by imprinting techniques. For example, Nilssons’
group firstly prepared molecularly imprinted polymer (MIP)
microparticles by precipitation polymerization, then used for
2
7
enantiomer separations by a partial filling technique. Qua-
glia et al. developed a macroporous silica particles surface-ini-
tiated polymerization technique for the preparation of MIP-
2
8
based CSPs. In this work, the well-defined glycopolymers
and chiral nanoparticles with crosslinked structure were
directly synthesized via RAFT miniemulsion technique, using
chiral VBPG as the monomer and poly(VBPG) as the macro-
RAFT agent. Furthermore, different chiral molecules have
been used to test the ability of enantioselective recognition of
the chiral nanoparticles by checking out the specific rotation
of ethanol solution of the enantioselective separation.
lecular weight (M
) and molecular weight distribution (M /
w
n
M
n
) of the resulting polymers were determined using a
Waters 1515 gel permeation chromatograph (GPC) equipped
with a refractive index detector (Waters 2414), using HR 1
(pore size: 100 Å , 100–5000 Da), HR 2 (pore size: 500 Å,
500–20,000 Da), and HR 4 (pore size 10,000 Å, 50–100,000
2
Da) columns (7.8 ꢂ 300 mm , 5 lm beads size) with molec-
2
5
ular weights ranging from 10 –5 ꢂ 10 g/mol. Tetrahydrofu-
ran (THF) was used as the eluent at a flow rate of 1.0 mL/
EXPERIMENTAL
ꢁ
min and 30 C. The GPC samples were injected using a
Materials
Waters 717 plus autosampler and calibrated with PS stand-
ards from Waters. The specific rotations were measured on
an Autopol IV instrument with k ¼ 589 nm at 20 C. The H
RAFT agent benzyl dithiobenzoate (BDB) and the monomer
VBPG (the chemical structure is shown in Scheme 1) were
ꢁ
1
CROSSLINKED CHIRAL NANOPARTICLES, XU ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Schematic diagram
illustrating the structures of race-
mates: D,L-tartaric acid, D,L-man-
delic acid, D,L-arabinose, and
6
-3-amino-1,2-propanediol; the
monomer VBPG and Macro-
RAFT agent.
NMR spectrum was recorded on an Inova 400-MHz NMR
instrument with CDCl3 as the solvent and tetramethylsilane
and HD, then dissolved in THF and precipitated in 200 mL of
petroleum ether; however, for the crosslinked nanoparticles
prepared in the presence of amount of crosslinker, they were
washed three times with deionized water and THF in this
order to remove SDS, residual monomer, toluene, and HD.
The obtained polymer nanoparticles were dried at room
temperature under vacuum until a constant weight.
(
TMS) as the internal standard.
Preparation of Macro-RAFT Agent
Linear poly(VBPG) (the chemical structure is shown in
Scheme 1) was prepared according to the method described
2
6
previously. A typical procedure was as follows: VBPG (1 g,
2
0
.759 mmol), BDB (22 mg, 0.134 mmol), AIBN (4.8 mg,
.029 mmol), and toluene (4.5 mL) were added to a dried
Chiral Recognition
The chiral recognition ability of the poly(VBPG) nanopar-
ticles was evaluated by an enantiomer-selective adsorption
experiment in ethanol where a racemant was dissolved and
the nanoparticles were suspended. The typical procedure
was as follows: 50 mg of poly(VBPG) nanoparticles was sus-
pended in a solution of a racemate in ethanol (concentration
ampoule. The contents were purged with argon for 10 min
to eliminate the dissolved oxygen. The ampoule was flame-
sealed and then immersed into an oil bath at 75 C. After 5
ꢁ
days, the ampoule was opened, and the contents were dis-
solved in THF and precipitated in 200 mL of petroleum ether
three times. The obtained polymer, poly(VBPG), was dried at
room temperature under vacuum until a constant weight.
Poly(VBPG): Mn,GPC ¼ 6200 g/mol, M /M ¼ 1.12.
ꢀ
3
¼
1 ꢂ 10 g/mL, volume ¼ 5 mL); the mixture was stirred
at room temperature overnight, then filtered, waiting for spe-
cific rotation measure to examine the ability of chiral recog-
nition of the PVBPG nanoparticles in comparison with the
original tartaric acid or mandelic acid solution. The used
nanoparticles after chiral recognition were then washed with
ethanol, dried for the characterization. Considering the effec-
tiveness of reseparation, four separation–abstersion cycles
were performed with various amounts of chiral
nanoparticles.
w
n
Processes of Miniemulsification and Polymerization
The recipe used in preparing the miniemulsions is listed in
Table 1. The aqueous (SDS and DI water) and oil [AIBN, HD,
Macro-RAFT, VBPG, and DVB (if used)] phases were first pre-
pared separately and then mixed via magnetic stirring for 10
min in a 25-mL beaker. The mixture was followed by sonifi-
cation using a Branson sonifier (model 450) at a power out-
put setting of 800 W/L for 10 min with the beaker placed in
an ice bath to absorb the excess heat produced to avoid any
polymerization due to heating of the sample during the
homogenization. The miniemulsion was transferred into a
RESULTS AND DISCUSSION
The Stability of Latex
Before discussing the results in detail, it is mentioned that
phase separation can hardly be eliminated, no matter how to
change the polymerization conditions in our experiments
using BDB as RAFT agent during the RAFT miniemulsion po-
lymerization, and that some coalescence such as floating on
the surface of miniemulsion or stick to magnetic stirrer bar
can easily observed. Thus, the macro-RAFT agent, poly-
(VBPG), was selected in this process to eliminate the second-
ary nucleation of new particles to suppress Ostwald ripen-
15-mL round-bottom flask, degassed by purging with purity
argon. The flask was then immersed into an oil bath at 75
ꢁ
C. The time between miniemulsification and initiation was
minimized to 5 min to reduce the droplet degradation (Ost-
wald ripening) period. After the desired polymerization time,
the flask was opened, and the sample was demulsification by
freezing in a refrigerator. For the nanoparticles prepared
without crosslinker, they were washed three times with
deionized water and ethanol in this order to remove SDS
2
0
ing. In this instance, the macro-RAFT agent was also served
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ARTICLE
TABLE 1 General Recipe for the RAFT Miniemulsion
Polymerization of VBPG at 75 8C
let nucleation at the first 10 h, and when the conversion
exceeded 65%, the polymerization rate slowed down.
Figure 2 shows the evolution of the number-average molecu-
Ingredient
Content
w n
lar weight (Mn,GPC) and PDI (M /M ) of poly(VBPG) on the
Organic Phase (A)
VBPG
0.3 g
conversion for the miniemulsion polymerization of VBPG at
a
ꢁ
Macro-RAFT agent
100 mg
45 mg
75 C. From Figure 2, it is interesting to note that the M /
w
HD
Mn values increased steadily with the evolution of conver-
sion, and even reached 1.50 at the final conversion (93.5%).
DVB
0, 3, 6, and 9 wt %
based on monomer
w
Unlike our pervious report, although the value of M /Mn
increased with the conversion but it remained very low,
AIBN
0.7 mg
0.7 mL
18 mg
5.7 mL
2
6
never exceeded above 1.12. This may be due to the fact
that the superswelling could not be absolutely avoided in
miniemulsion and that the difficulty of monomer transport
into the latex particles from the monomer droplets through
the aqueous phase leads to a disequilibrium distribution of
ratios of RAFT agent to monomer within the particles. This
observation was in agreement with the results reported in
Toluene
SDS
Water Phase (B)
Deionized water
a
Poly(VBPG) : Mn,GPC ¼ 6200 g/mol, M
w
/M
n
¼ 1.12.
for the purpose of controlling the molecular weight and PDI
of the polymer formed within the particle. Then the volume
ratio of monomer to oil-solvent was on our focus. It was
found that 1 g of monomer VBPG dissolved in 2–3 mL of tol-
uene can give good performance. Visual observation and the
change in droplet size as determined by HPPS and DLS were
used to assess the latex stability. It was found that the latex
instabilities were almost solved by reasonable combining
these three important factors.
2
9
the literature. Despite the higher M /Mn values in that
w
case, the Mn,GPC values linearly increase with the monomer
conversion upto 93.5% and were good consistent with their
corresponding theoretical ones, indicating that using macro-
RAFT agent instead of conventional RAFT agent not only
solved the problem of stability of latexes but also resulted in
excellent controllability over the molecular weights of the
obtained polymers in a RAFT miniemulsion polymerization
system.
Macro-RAFT Agent-Mediated RAFT Miniemulsion
Polymerization of VBPG
Effects of the Amounts of DVB on Nanoparticles
The nanoparticle size and size PDI of the obtained poly-
After solving the problem of latex instability, the kinetics of
the miniemulsion polymerization of VBPG was first carried
(
VBPG) nanoparticle latexes were characterized by HPPS and
DLS. The results are summarized in Table 2. From Table 2, it
can be seen that the diameter of the poly(VBPG) nanopar-
ticles first increased and then decreased with the increase of
the amount of crosslinker in the range of 0–9 wt % and that
a same changing trend was observed by both HPPS and DLS.
out using poly(VBPG) (Mn,GPC ¼ 6200 g/mol, M
w
/M
n
¼ 1.12)
as the macro-RAFT agent in the absence of crosslinker DVB.
The general recipe used for the miniemulsion is presented in
Table 1. Figure 1 shows the kinetics of RAFT miniemulsion
polymerization of VBPG. As illustrated by our experimental
results, high reaction rates were obtained by monomer drop-
FIGURE 2 Experimental values of the number-average molecular
w n
weight (Mn,GPC) and the molecular weight distribution (M /M ) as
FIGURE 1 Kinetics of RAFT miniemulsion polymerization of
a function of conversion for the RAFT miniemulsion polymeriza-
tion of VBPG using poly(VBPG) as the macro-RAFT agent and
VBPG using poly(VBPG) as the macro-RAFT agent and AIBN as
the initiator. Polymerization conditions: [VBPG]
0
/[poly(VBPG)]
0
/
AIBN as the initiator. Polymerization conditions: [VBPG]
0
/[poly-
ꢁ
ꢁ
[
AIBN]
0
¼ 200:4:1, temperature ¼ 75 C; poly(VBPG) macro-
(VBPG)]
0
/ [AIBN]
0
¼ 200:4:1, temperature ¼ 75 C; poly(VBPG)
RAFT agent: Mn,GPC ¼ 6200 g/mol, M
w
/M
n
¼ 1.12.
macro-RAFT agent: Mn,GPC ¼ 6200 g/mol, M
/M
w n
¼ 1.12.
CROSSLINKED CHIRAL NANOPARTICLES, XU ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 2 Effect of the Amount of Crosslinker (DVB) on the Diameter and Size PDI of the Poly(VBPG) Nanoparticles
HPPS
DLS
TEM
Amount of
a
b
Crosslinker (wt %)
D
z
(nm)
Size PDI
D
z
(nm)
Size PDI
D
(nm)
0
3
6
9
a
103.9
126.2
116.9
104.5
0.108
0.013
0.098
0.132
101.0
119.8
106.4
100.2
0.188
0.178
0.146
0.142
51.3
130.5
118.0
65.2
Based on monomer.
b
Average diameter from the TEM images.
For 3 wt % of crosslinker, a maximum diameter (ꢃ120 nm)
was obtained in that case. To further study the effect of
crosslinker on the diameter and morphologies of the poly-
mer latexes, we compared TEM images of the chiral nanopar-
ticles prepared with crosslinker of 0, 3, 6, and 9%, as shown
in Figure 3. Relative uniform and surface-smooth
FIGURE 3 TEM images of the poly(VBPG) nanoparticle latexes of (A) without cross-linker, (B) with 3 wt % of DVB, (C) with 6 wt %
of DVB, and (D) with 9 wt % of DVB. These latexes were prepared via RAFT-mediated miniemulsion polymerizations using poly-
(
VBPG) as the macro-RAFT agent and AIBN as the initiator.
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ARTICLE
with prolonging time and then decreased a little. This phe-
2
6
nomenon is consistent with our previous report. However,
2
6
when compared with the results reported previously, the
specific rotation both of D,L-mandelic acid and D,L-tartaric
ꢁ
ꢁ
ꢁ
ꢁ
acid increased from ꢀ18 to ꢀ23 and from ꢀ9 to ꢀ10 ,
respectively, at the same condition, indicating better ability
of chiral discrimination and separation for the chiral nano-
particles than the corresponding linear polymers. This is
contributes to the fact that the crosslinked chiral nanopar-
ticles have larger specific surface area of chiral materials and
better stability in separation process due to the crosslinked
structure of the obtained nanoparticles when compared with
the linear chiral polymers, which can easily agglomerate dur-
ing the separation process. In this study by employing the
chiral nanoparticles as resolving agents, the enantioselective
performance of 6-3-amino-1,2-propanediol and D,L-arabinose
also show good ability of chiral discrimination during sepa-
ꢁ
ꢁ
ration and a maximum specific rotation of ꢀ11 and ꢀ26
FIGURE 4 Kinetics of chiral discrimination and separation for
some racemates: 6-3-amino-1,2-propanediol, D,L-arabinose, D,L-
tartaric acid, and D,L-mandelic acid. Sample: crosslinked chiral
nanoparticles with 3 wt % of crosslinker, 50 mg; Specific rota-
tion: ethanol solution after chiral separation; concentration ¼ 1
was achieved after 15 and 10 h, respectively. To assess the
effectiveness of the chiral separation, the specific rotation of
the corresponding ‘‘pure’’ substances were also measured at
the same conditions, as shown in Table 3. By comparison
with the results from Figure 4, it can be seen that almost
complete separation was achieved for the racemates of both
mg (racemate) /mL, volume ¼ 5 mL; k ¼ 589 nm; temperature
ꢁ
¼
20 C.
6
-3-amino-1,2-propanediol and D,L-tartaric acid with lower
ꢁ
specific rotation (ꢀ14 for ‘‘pure’’ (S)-3-amino-1,2-propane-
nanoparticles were observed in all cases. The diameter (D)
from the TEM images was 51.3, 130.5, 118.0, and 65.2 nm
for the amount of crosslinker of 0, 3, 6, and 9 wt %, respec-
tively, which confirmed the results measured by HPPS and
DLS.
ꢁ
diol and ꢀ11 for ‘‘pure’’ (D)-(ꢀ)-tartaric acid, respectively);
however, lower effectiveness of the chiral separation was
obtained for the racemates of both D,L-arabinose and D,L-man-
ꢁ
delic acid with higher specific rotation (ꢀ86 for ‘‘pure’’ D-
ꢁ
(
ꢀ)-arabinose and ꢀ139 for ‘pure’ (D-(ꢀ)-mandelic acid,
respectively) under same separation conditions.
Kinetics of Chiral Discrimination and Separation
To investigate the enantioselective performance of obtained
chiral polymers on the different kinds of racemates, the 6-3-
amino-1,2-propanediol, D,L-arabinose, D,L-tartaric acid, and
D,L-mandelic acid (their chemical structures are shown in
Scheme 1.) were selected for study, then a specific rotation
character method was employed as reported in the litera-
It is noted that the chiral nanoparticles should be removed
from the ethanol solution before specific rotation analysis to
avoiding the interfering of chiral polymers. To ensure the full
removal of the chiral polymers during analysis process, we
also performed the same experiment just without any race-
mic material. The results of the polarimeter measurements
showed no indication of the presence of chiral polymers in
ethanol solution. In addition, the reason for the observed ki-
netic differences of chiral separation with the four kinds of
racemates in Figure 4 may be contributed to the fact that all
the chiral carbon contained heterochiral pairs of hydrogen
though four different kinds of the racemates with different
structures involved in this paper. In principle, CSP method
was used in this work; therefore, intermolecular hydrogen
2
6
ture. The experiments were performed using chiral nano-
ꢁ
particles with 3 wt % of crosslinker at 20 C. Under all ex-
perimental conditions, we performed time-resolved
polarimeter measurements to examine potential chiral dis-
crimination in all processes of separation. The kinetics is
shown in Figure 4. From the data of Figure 4, it can be seen
that the chiral nanoparticles had a good discrimination for
those racemates, and the specific rotation first increases
a
TABLE 3 Specific Rotations of Pure Substances
Chiral Substance
Specific Rotation
Chiral Substance
Specific Rotation
(
R)-3-Amino-1,2-propanediol
þ16ꢁ
ꢀ86ꢁ
(S)-3-Amino-1,2-propanediol
L-(þ)-Arabinose
ꢀ14ꢁ
þ98ꢁ
D-(ꢀ)-Arabinose
D-(ꢀ)-Mandelic acid
D-(ꢀ)-Tartaric acid
a
ꢁ
ꢁ
ꢀ139
L-(þ)-Mandelic acid
þ136
ꢀ11ꢁ
L-(þ)-Tartaric acid
þ12ꢁ
All the samples were dissolved in ethanol, concentration ¼ 1 ꢂ 10ꢀ3 g/mL, volume ¼ 5 mL; k ¼ 589 nm; temperature ¼ 20 ꢁC.
CROSSLINKED CHIRAL NANOPARTICLES, XU ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
point, the efficiency of the smallest crosslinked nanoparticles
will have the best chiral separation ability. The specific rota-
ꢁ
tion of ethanol solution for D,L-arabinose reached ꢀ29 and
ꢁ
ꢁ
ꢁ
ꢀ
27 after 10 h of separation, ꢀ10 and ꢀ11 for (6)-3-
amino-1,2-propanediol after 15 h of separation, respectively,
corresponding to the samples with 3 and 6 wt % of cross-
ꢁ
ꢁ
linker. However, only ꢀ21 for D,L-arabinose and ꢀ7 for
(
6)-3-amino-1,2-propanediol were reached for the sample
with 9 wt % of crosslinker although the diameter of this
sample was the smallest among the three samples, as shown
in Table 2. This phenomenon was contradictory with the the-
oretical analysis mentioned above. In fact, increasing the
amount of the crosslinker will result in the increase of the
density of crosslinking for the nanoparticles. When too high
density of crosslinking reaches, the diffusion of the racemate
molecules toward the chiral groups wrapped into the nano-
particles will become much difficult. Thus, there is a suitable
amount of crosslinker for the preparation of the crosslinked
nanoparticles, which may result in suitable cavities for the
chiral separation. In that case, less than 6 wt % relative to
chiral monomer is suitable.
FIGURE 5 Effect of the amount of crosslinker on the chiral dis-
crimination and separation of the obtained chiral nanoparticles.
Sample: 50 mg. Specific rotation: ethanol solution after chiral
separation; concentration ¼ 1 mg (racemate) /mL, volume ¼ 5
ꢁ
mL; k ¼ 589 nm; temperature ¼ 20 C; For the racemate D,L-
Reuse of Chiral Nanoparticles
arabinose: separations time ¼ 10 h; For the racemate (6)-3-
amino-1,2-propanediol: separation time ¼ 15 h.
To demonstrate the repetitive ability of chiral separation of
the crosslinked nanoparticles in the surface adsorption pro-
cess, D,L-tartaric acid was selected as model subject for
research mainly because of its biological importance and the
extensive application. The specific rotation of ethanol solu-
tion after separation of D,L-tartaric acid was directly charac-
terized by polarimeter. Briefly, 500 mg of 3% crosslinked
chiral nanoparticles was suspended in a solution of 50 mL of
ethanol with 50 mg of D,L-tartaric acid. The mixture was
bonding interactions was the main force in the selective
adsorption process for enantiomer selection. A maximum of
specific optical rotation can be reached with the increase of
time as shown in Figure 4; and the separation ability
decreased in varying degrees when further extended hours
for D,L-tartaric acid and D,L-mandelic acid. This may be due to
the partial hydrolysis of hydrogen of diastereomer in acidic
solution. However, for 6-3-amino-1,2-propanediol and D,L-
arabinose, when prolonging the hours, the values of specific
optical rotation decreased a little, indicating the separation
reached a balance of maximum stability.
ꢁ
stirred for 7.5 h at 20 C, then filtered. The solution was
Effect of the Amount of Cross-Linker on Chiral
Discrimination and Separation
As many articles reported that enantioselective separation
1
,10–13,25,30
mainly depends on chiral surface.
In consideration
of finding the suitable amount of crosslinker for the separa-
tion ability of the chiral nanoparticles, we employed the
obtained four chiral nanoparticles with different amount of
crosslinker and two racemates D,L-arabinose and (6)-3-
amino-1,2-propanediol for this study. Figure 5 shows the
effect of amount of crosslinker on the chiral discrimination
and separation of these nanoparticles. It can be seen that the
separation abilities for the three crosslinked nanoparticles
(
3, 6, and 9 wt % of crosslinker) were better than the nano-
FIGURE 6 Repetitive ability of the crosslinked nanoparticles
used for the chiral discrimination and separation. Sample:
crosslinked chiral nanoparticles with 3 wt % of crosslinker, 500
mg. Specific rotation: ethanol solution after chiral separation,
ethanol ¼ 50 mL, D,L-tartaric acid ¼ 50 mg, separation time ¼
particles without crosslinking (0 wt % of crosslinker), fur-
ther confirming the advantages of crosslinked nanoparticles
over linear chiral polymers as mentioned above. In addition,
in principle, the smaller the particle size results the greater
specific surface area. The greater specific surface areas
exposed, the more groups of chiral particles exposed to the
surface, and the effect of separation will be better. From this
ꢁ
7.5 h; k ¼ 589 nm; temperature ¼ 20 C. The chiral nanopar-
ticles used for chiral separation washed with ethanol three
times then dried before next separation.
1330
INTERSCIENCE.WILEY.COM/JOURNAL/JPOLA

ARTICLE
applied to specific rotation measurement while the chiral
nanoparticles were washed with ethanol three times before
dried for the next use. Each time after separation, we
deducted the lost nanoparticles out then suspended the
remaining in a corresponding amount of ethanol dissolved
D,L-tartaric acid. In this way four separation–abstersion
cycles were performed. As shown in Figure 6, the nanopar-
ticles can be reused well; however, the separation ability has
somewhat loss after each cycle. This is attributable to the
fact that the crosslinked nanoparticles could not disperse
well in ethanol solution when dried and reused to next cycle,
due to some of the agglomerations or collapsed nanopar-
ticles was occurred while most of the nanoparticles still kept
the same morphology after four times of chiral separation,
which resulted in some loss of the specific area.
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Okamoto, Y. J Polym Sci Part A: Polym Chem 2009, 47,
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Ikai, T.; Okamoto, Y. Chem Rev 2009, 109, 6077–6101.
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A novel way for the synthesis of well-defined chiral poly-
(
VBPG) homopolymers and corresponding crosslinked nano-
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particles with different crosslinking density, via RAFT minie-
mulsion polymerization using linear poly(VBPG) as the
macro-RAFT agent, was demonstrated. The kinetics of RAFT
miniemulsion polymerization of VBPG suggested that the
polymerization proceeded in a living fashion. The amount of
crosslinker had a significant effect on the size and chiral sep-
aration ability of the obtained crosslinked nanoparticles. It
was found that the efficiency of chiral separation was greatly
improved after transferring the crosslinked structure into
the nanoparticles when compared with the chiral linear
chains and that 3–6 wt % of DVB relative to the chiral
monomer VBPG could give the crosslinked chiral nanopar-
ticles better chiral separation ability for the different kinds
of racemates such as D,L-arabinose and (6)-3-amino-1,2-pro-
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