Bifunctional 2-(alkoxycarbonothioylthio)acetic acids for the synthesis of TiO2-poly(vinyl acetate) nanocomposites via RAFT polymerization

Article abstract of DOI:10.1002/pola.27043

Reversible addition-fragmentation chain-transfer (RAFT) polymerization has been known as a convenient method for the synthesis of polymers of designed molecular structures. Of particular interest are bifunctional or multifunctional chain-transfer agents (CTAs) which could be employed in the development of advanced materials via RAFT polymerization. In the present study, four bifunctional 2-(alkoxycarbonothioylthio) RAFT CTAs with i￡ COOH functionalities containing methoxy, ethoxy, isopropoxy, and octyloxy groups, respectively, were synthesized and characterized by FTIR and NMR spectroscopy. Polymerizations of vinyl acetate using these CTAs exhibited increased molecular weight with consumption of monomer and relatively narrow dispersities, indicative of living polymerization behavior. The effect of the concentration of 2-(ethoxycarbonothioylthio) acetic acid on the polymerization was examined, revealing that higher concentration of CTA led to lower molecular weight and narrower dispersity. As an example of the application of the synthesized bifunctional CTAs, TiO₂-poly(vinyl acetate) (PVAc) nanocomposites were synthesized via a one-pot process and characterized by TGA, DSC, TEM, and affinity test, suggesting attachment of PVAc onto the nano-TiO₂ particles. Copyright

Full text of DOI:10.1002/pola.27043

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Bifunctional 2-(Alkoxycarbonothioylthio)acetic Acids for the Synthesis of
TiO2-Poly(vinyl acetate) Nanocomposites via RAFT Polymerization
Shude Xiao, William Z. Xu, Paul A. Charpentier
Department of Chemical and Biochemical Engineering, Western University, London, Ontario N6A 5B9, Canada
Correspondence to: P. A. Charpentier (E-mail: pcharpentier@eng.uwo.ca)
Received 4 September 2013; accepted 24 November 2013; published online 14 January 2014
DOI: 10.1002/pola.27043
ABSTRACT: Reversible addition-fragmentation chain-transfer
(RAFT) polymerization has been known as a convenient method
for the synthesis of polymers of designed molecular structures.
Of particular interest are bifunctional or multifunctional chain-
transfer agents (CTAs) which could be employed in the develop-
ment of advanced materials via RAFT polymerization. In the
present study, four bifunctional 2-(alkoxycarbonothioylthio)
RAFT CTAs with ACOOH functionalities containing methoxy,
ethoxy, isopropoxy, and octyloxy groups, respectively, were
synthesized and characterized by FTIR and NMR spectroscopy.
Polymerizations of vinyl acetate using these CTAs exhibited
increased molecular weight with consumption of monomer and
relatively narrow dispersities, indicative of living polymerization
behavior. The effect of the concentration of 2-(ethoxycarbono-
thioylthio) acetic acid on the polymerization was examined,
revealing that higher concentration of CTA led to lower molecu-
lar weight and narrower dispersity. As an example of the
application of the synthesized bifunctional CTAs, TiO2-poly(vinyl
acetate) (PVAc) nanocomposites were synthesized via a one-pot
process and characterized by TGA, DSC, TEM, and affinity
test, suggesting attachment of PVAc onto the nano-TiO2
C
V
particles.
2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A:
Polym. Chem. 2014, 52, 606–618
KEYWORDS: chain transfer agents; living polymerization; nano-
TiO2; nanocomposite; poly(vinyl acetate); reversible addition-
fragmentation chain transfer (RAFT); xanthates
fragmentation, the xanthate CTAs reduce the addition rate to
the propagating radicals, and facilitate fragmentation of the
intermediate radicals.4 Stenzel et al. synthesized several
xanthates and found that four compounds, methyl (4-methox-
yphenoxy)carbonothioylsulfanylacetate, methyl (methoxycarbo-
nothioyl)sulfanyl acetate, methyl (ethoxycarbonothioyl)sulfanyl
acetate, and methyl (isopropoxycarbonothioyl) sulfanyl acetate
were suitable MADIX/RAFT CTAs for polymerization of VAc.6
Coote et al. studied the effects of Z-groups and the leaving
groups (R-group) by ab initio computation.7,8 Research was
conducted on RAFT polymerization of VAc using these CTAs in
bulk and emulsion polymerizations,9–12 and additional CTAs
having similar structures were synthesized to prepare poly
(VAc) (PVAc) branches, stars, combs, or nanoparticles.13–18
INTRODUCTION Reversible addition-fragmentation chain
transfer (RAFT) polymerization is a type of living radical
polymerization termed degenerate-transfer radical polymer-
ization (DTRP). In DTRP, the deactivation of radicals involves
degenerative transfer of the carbonothioylthio group from
the RAFT chain transfer agent (CTA).1 Different from
nitroxide-mediated polymerization and atom transfer radical
polymerization, RAFT polymerization requires specific CTAs
for various monomers to be polymerized. Some common
vinyl monomers such as styrene and methacrylates are easy
to polymerize via RAFT polymerization, with a variety of
dithioesters, dithiocarbamates and trithiocarbonates being
suitable CTAs.2 However, vinyl esters are rather challenging
monomers to polymerize by RAFT polymerization, with xan-
thates normally used as the RAFT CTAs. This type of poly-
merization is also termed as macromolecular design via
interexchange of xanthate (MADIX).3 In O-alkyl xanthates, the
conjugation of the O lone pair electrons and the C@S bond is
favorable to the zwitterionic canonical forms, and contrib-
utes to the low reactivity in RAFT polymerization.4,5 How-
ever, when reacting with unstable radicals, such as vinyl
acetate (VAc) radicals that undergo fast addition and slow
Grafting polymers onto the surface of a solid, such as poly-
mers or nanoparticles, has found applications in many
research areas.19–21 However, lack of functionality limits the
applications of the aforementioned RAFT CTAs such as 2-
(alkoxycarbonothioylthio)acetates in the synthesis of poly-
mer nanocomposites. Various efforts have been made to
attach polymer chains to the surface of solid particles.
Additional Supporting Information may be found in the online version of this article.
C
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Nguyen et al. synthesized a surface-immobilized CTA via the
reaction of 1,10-thiocarbonyl diimidazole with a thiol, fol-
lowed by reaction with the hydroxyl groups of a Wang resin,
and grafted PVAc chains onto the solid surface.22 Some
attempts were made to immobilize RAFT CTAs onto the solid
chemical shifts were referenced to tetramethylsilane (0.0
ppm). Fourier transform infrared (FTIR) spectra were
obtained from a Thermo Scientific Nicolet 6700 FT-IR spec-
trometer with Smart iTR (diamond ATR) and transmittance
modules. Differential scanning calorimetry (DSC) and ther-
mogravimetric analysis (TGA) were measured on a TA instru-
ments DSC Q200 and a SDT Q600, respectively. Thermal
analyses were performed at a heating rate of 10 ꢁC/min
under N2 and air for DSC and TGA, respectively. Transmis-
sion electron microscopy (TEM) images were recorded using
a Philips CM10 Transmission Electron Microscope operated
at 80kV. The samples were dispersed in THF by sonication
for 30 min before being placed on a copper grid covered
particles, such as SiO2,23 carbon black,24 and TiO2 nanopar-
20
ticles, followed by RAFT polymerization yielding hybrid
nanocomposites. As metal carboxylate coordination bonding
can be formed between the carboxyl group and several metal
oxide nanoparticles such as TiO2,25,26 the grafted polymer
chains from the nanoparticles would greatly improve the
compatibility of the nanoparticles to both organic solvents
and other matrix polymers of similar miscibility.27–31
-
with carbon film. Molecular weight (Mn) and dispersity (D)
In this study, four xanthate CTAs containing a carboxyl group
were synthesized and tested in the RAFT polymerization of
VAc. Nano-sized titanium oxides have shown exceptional
properties and potential applications in photocatalysis, solar
energy conversion, ultraviolet screening, and electronic data
storage medium.32,33 Chemical linkage to the polymer chains
can enhance the properties of the metal oxide, and make
them more melt processible,34 as well as reduce their envi-
ronmental exposure from leaching.35 To demonstrate the
application of the xanthate CTAs in the synthesis of polymer
nanocomposites, TiO2-PVAc nanocomposites were synthe-
sized using a one-pot process comprising RAFT polymeriza-
tion of VAc and simultaneous bonding of the polymer chains
to the surface of nano-TiO2. This approach could be applied
in the synthesis of a variety of novel polymer nanocompo-
sites using RAFT polymerization.
of the polymers were measured on a Viscotek TDAmax gel
permeation chromatography (GPC) System equipped with a
TDA 302 Triple Detector Array, a VE2001 GPC solvent/sam-
ple module and two Polyanalytik PAS 103 M-UL GPC col-
umns. THF eluent flow rate was 1 mL/min, and universal
calibration was used with Polyanalytik Triple Detection Cali-
bration & Verification Kit and Varian EasiVial PS-L Polysty-
rene Calibration Kit.
Synthesis of RAFT Chain Transfer Agents
Synthesis of 2-(methoxy carbonothioylthio)
acetic acid (MeXAcOH)
KOH (5.6 g, 50 mmol) was dissolved in 30 mL of methanol
at room temperature. The solution was cooled with an ice
bath followed by addition of 20 mL of CS2 into the flask
dropwise. The reaction took place at room temperature for
24 h at which point 20 mL of 2-BrAcOH solution (6.9 g, 50
mmol in methanol) was added into the flask dropwise in an
ice bath. After 48 h reaction at room temperature, the pre-
cipitates were filtered off and the solvent was removed with
a rotary evaporator, resulting in a crude product of light yel-
low oil. The crude product was run through a silica gel col-
umn using ethyl ether/hexanes (5:1) mixture. Colorless
crystals of 2-(methoxy carbonothioylthio)acetic acid were
obtained from crystallization in hexanes/cyclohexane (4:1).
Note: it showed light yellow when decomposing or contain-
ing impurities. Yield: 29.0%. Melting point (DSC): 112.6 ꢁC.
1H NMR (CDCl3, 600 MHz) d (ppm): 9.13 (br. s, 1H), 4.21(s,
3H), 4.01(s, 2H). 13C NMR (CDCl3, 150.8 MHz) d (ppm):
213.0 (OACSS), 173.7 (COOH), 60.8(CH3AO), 37.8
(SACH2ACOOH). FTIR (cm21): 2954 (tCH3(as)), 2926
(tCH2(as)), 2854 (tCH2(s)), 1720 (tC@O), 1458 (dCH2), 1377
(dCH3(s)), 1286 (VCAO), 1234 (tCAOAC(as)), 1155
(tCAOAC(s)), 1065 (tC@S).
EXPERIMENTAL
Materials
Potassium O-ethyl dithiocarbonate (98.0%), 2-propanol (iso-
propanol, IPA, 99.5%), carbon disulfide (CS2, 99.0%), 2-
bromoacetic acid (BrAcOH, 99.0%), tetrahydrofuran (THF,
99.9%, inhibitor free), 2,6-di-tert-butyl-4-methylphenol (BHT,
99%), hydrochloric acid (HCl, 37%), tetrapropylammonium
bromide (TPAB, 98%), and titanium(IV) oxide nanopowder
(anatase, <25 nm, 997%) were obtained from Sigma-Aldrich
(Oakville, ON, Canada). Methanol (99.8%, Emanuel Merck,
Darmstadt, EMD), hexanes (ꢀ60 % n-hexane, ꢀ98.5 % mix-
ture of hexane isomers, British Drug Houses, BDH), acetone
(99.5%, BDH), ethyl ether (98.0%, EMD), 1-octanol (99.0%,
Alfa Aesar), and VAc (stabilized, 99.0%, Alfa Aesar) were
supplied by VWR International (Mississauga, ON, Canada).
Potassium hydroxide (KOH, 85%), cylcohexane (99.5%), and
silica gel 60 (>99%) were purchased from Caledon (George-
0
R
V
town, ON, Canada). Vazo 64 (2,2 -azobis(2-methylpropioni-
Synthesis of 2-(ethoxycarbonothioylthio)acetic acid
(EtXAcOH)
trile), AIBN) was provided by DuPont (Wilmington, DE). VAc
was distilled under argon to remove the inhibitor before
polymerization, and other chemicals were used as received.
Potassium O-ethyl dithiocarbonate (13.6 g, 55 mmol) was
dissolved in 50 mL of THF, and mixed with 20 mL of 2-
BrAcOH solution (6.9 g, 50 mmol in THF). The reaction took
place at room temperature for 48 h, followed by precipita-
tion into a large amount of hexanes. Subsequently, the pre-
cipitates were filtered off and the solvent was removed with
a rotary evaporator. The resulted light yellow oil was further
Characterization
1H and 13C nuclear magnetic resonance (NMR) spectra were
recorded on a Varian INOVA 600 or a Varian INOVA 400
NMR spectrometer. Samples were dissolved in CDCl3, and
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purified by extraction and recrystallization with hexanes to
afford colorless crystals of 2-(ethoxycarbonothioylthio)acetic
acid. Yield: 30.4%. Melting point (DSC): 56.4 ꢁC. 1H NMR
(CDCl3, 400 MHz) d (ppm): 11.79 (br. s, 1H), 4.65 (q,
J 5 7.28 Hz, 2H), 3.97 (s, 2H), 1.42 (t, J 5 7.21 Hz, 3H). 13C
NMR (CDCl3, 100.5 MHz) d (ppm): 211.9 (OACSS), 174.5
(COOH), 70.8 (CH3ACH2AO), 37.5 (SACH2ACOOH), 13.6
(CH3ACH2AO). FTIR (cm21): 2956 (tCH3(as)), 2926
(tCH2(as)), 2854 (tCH2(s)), 1705 (tC@O), 1464 (dCH2), 1377
(dCH3(s)), 1365, 1311, 1246 (tCAOAC(as)), 1188, 1115
(tCAOAC(s)), 1049 (tC@S).
tic acid as a colorless liquid. Yield: 28.8%. Melting point
1
ꢁ
(DSC): 3.7 C. H NMR (CDCl3, 600 MHz) d (ppm): 10.16 (br.
s, 1H), 4.58 (t, J 5 6.74 Hz, 2H), 3.97 (s, 2H), 1.79 (quin,
J 5 7.18 Hz, 2H), 1.40 (m, 2H), 1.29 (m, 8H), 0.88 (t,
J 5 7.03Hz, 3H). 13C NMR (CDCl3, 150.8 MHz) d (ppm): 212.0
(OACSS), 174.0 (COOH), 75.1 (CH2ACH2ACH2ACH2AO),
37.5 (SACH2ACOOH), 31.7 (CH3ACH2ACH2ACH2), 29.1
(CH3ACH2ACH2ACH2), 29.0 (CH2ACH2ACH2ACH2AO), 28.1
(CH2ACH2ACH2ACH2AO), 25.8 (CH2ACH2ACH2ACH2AO),
22.6 (CH3ACH2ACH2ACH2), 14.0 (CH3ACH2ACH2ACH2).
FTIR (cm21): 2954 (tCH3(as)), 2927 (tCH2(as)), 2856
(tCH2(s)), 1718 (tC@O), 1466 (dCH2), 1416 (ACOOH dimer,
combination band due to tCAO and dOAH), 1377 (dCH3(s)),
1282 (tCAO), 1232 (tCAOAC(as)), 1174 (tCAOAC(s)), 1057
(tC@S).
Synthesis of 2-(isopropoxycarbonothioylthio) acetic acid
(PrXAcOH)
KOH (5.6 g, 50 mmol) was dissolved in 100 mL of isopropa-
nol, to which 20 mL of CS2 was added dropwise at room
temperature. Precipitation occurred instantly, and the reac-
tion mixture was stirred at 40 ꢁC for 24 h. After removing
the solvent and residual CS2 with a rotary evaporator, a light
yellow powder was obtained. Subsequently, the powder was
dissolved in 20 mL of methanol, to which 20 mL of 2-
BrAcOH solution (6.9 g, 50 mmol in methanol) was added.
Synthesis of PVAc and TiO2-PVAc Nanocomposite with
the Synthesized Xanthate CTAs
Typically, distilled VAc (8.6 g, 100 mmol), xanthate CTA
(varying type and amount), and AIBN (0.016 g, 0.5 mmol)
were mixed with 15 mL of THF in a Schlenk flask, followed
by purging with argon gas at room temperaꢁture for 30 min.
The reaction mixture was then heated to 60 C in an oil bath
under stirring. About 1 g of reaction mixture was taken at
different time intervals, to which about 0.05 g of BHT was
added to quench the reaction. After removing the solvent
and residual VAc with a rotary evaporator, the conversion
was estimated using the following equation:
ꢁ
The reaction mixture was stirred at 40 C for 20 h, followed
by filtering off the precipitates and removing the solvent
with a rotary evaporator. The obtained oil was washed with
excess diluted HCl and extracted with ethyl ether. Anhydrous
sodium sulfate was used to dry the extract before evaporat-
ing ethyl ether. The product was purified by running through
a silica gel column with a solvent mixture of ethyl ether/hex-
anes (1:2). Colorless crystals of 2-(isopropoxycarbono-
thioylthio)acetic acid were obtained from recrystallization in
Conversion ð%Þ
h
ꢀ
ꢁi
o
VAc 1WToHF
1003 W22WBHT 2ðW12WBHT Þ3 12 W
WMo ixture
(1)
1
ꢁ
5
WVoAc
hexanes. Yield: 29.1%. Melting point (DSC): 44.3 C. H NMR
(CDCl3, 600 MHz) d (ppm): 10.71 (br. s, 1H), 5.72 (spt,
J 5 6.25 Hz, 1H), 3.93 (s, 2H), 1.39 (d, J 5 6.45 Hz, 6H). 13C
NMR (CDCl3, 150.8 MHz) d (ppm): 211.0 (OACSS), 174.4
(COOH), 79.1 (CH3ACH), 37.3 (SACH2ACOOH), 21.1
(CH3ACH). FTIR (cm21): 2954 (tCH3(as)), 2924 (tCH2(as)),
2854 (tCH2(s)), 1716 (tC@O), 1456 (dCH2), 1375 (dCH3(s)),
1362, 1277 (tCAO), 1244 (tCAOAC(as)), 1173 (tCAOAC(s)),
1092, 1047 (tC@S).
ðW12WBHT Þ3
WMo ixture
where WBHT is the weight of BHT added, W1 is the weight of
sample solution after adding BHT, W2 is the weight of the
dried polymer containing BHT, WVoAc , WToHF and WMo ixture are
the initial weights of VAc, THF, and total reaction mixture in
the flask, respectively.
To synthesize TiO2-PVAc polymer nanocomposites, titania
nanopowder (0.5 g) was dispersed in 7.5 mL of THF with
the aid of ultrasonic mixing for 30 min. Distilled VAc (4.3 g,
50 mmol), xanthate CTA (0.5 mmol), and AIBN (0.041 g,
0.025 mmol) were then charged into the above dispersion of
titania in THF. After being purged with argon for 30 min, the
Synthesis of 2-(octyloxycarbonothioylthio) acetic acid
(OctXAcOH)
To a solution of KOH (5.8 g, 110 mmol) in 20 mL of water
was added 10 mL of acetone and 1-octanol (13.1 g, 100
mmol), followed by addition of TPAB (0.3 g, 1 mmol) and 15
mL of CS2. A dark brown homogeneous mixture was formed
after 5 h reaction at room temperature. A solution of 2-
BrAcOH (13.8 g, 100 mmol) in 20 mL of acetone was then
charged into the reaction mixture. After stirring for 17 h at
room temperature, the solvents were removed from the HCl
(0.5 mL) treated mixture. Subsequently, a large amount of
hexanes was added, followed by filtering off the precipitates
and removing the solvent with a rotary evaporator. The
formed yellow oil was further purified by running through a
silica gel column with a solvent mixture of hexanes/ethyl
ether (3:1), followed by recrystallization in hexanes at 210
ꢁC, to afford the product of 2-(octyloxycarbonothioylthio)ace-
ꢁ
reaction mixture was heated to 60 C under stirring for 24 h
at which point a small amount of BHT was added to quench
the polymerization. The nanocomposite was separated from
the solution by centrifugation, washed with THF, and then,
dried in vacuo.
RESULTS AND DISCUSSION
Synthesis of Xanthate CTAs
The synthesis of bifunctional xanthate CTAs consisted of two
main steps: (1) formation of xanthanic acid salts; and (2)
conversion of the xanthanic acid salts to the corresponding
xanthate esters. A schematic diagram for the synthesis of the
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SCHEME 1 Schematic diagram for the synthesis of xanthate CTAs. [Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
four prepared xanthate esters is displayed in Scheme 1.
Methanol, isopropanol, and 1-octanol were reacted with CS2
in the presence of KOH to form the corresponding xanthanic
acid salts. These xanthanic acid salts and the commercial
potassium O-ethyl dithiocarbonate were reacted with BrA-
cOH to form the corresponding xanthate CTAs. In the synthe-
sis of potassium O-octyl dithiocarbonate, a phase transfer
catalyst, TPAB, and a cosolvent acetone were additionally
used to facilitate the reaction.36 The purified xanthate CTAs
were soluble in polar solvents and their solubilities were
sensitive to the polarity difference in nonpolar solvents such
as hexanes and cyclohexane. The moieties of the correspond-
ing alcohols were utilized as the recrystallization solvents.
MeXAcOH was found soluble in hot cyclohexane, while the
other xanthate CTAs can dissolve in hot hexanes.
The synthesized xanthate CTAs were characterized with FTIR
and NMR. FTIR measurement of the solid CTAs (MeXAcOH,
EtXAcOH and PrXAcOH) was made with nujol mull, while the
liquid OctXAcOH was tested directly on a KBr window. An
FTIR spectrum of the typical CTA, EtXAcOH, is displayed in
Figure 1. As expected, a weak sharp band of the carboxylic
acid monomers near 3540 cm21 was not observed for the
xanthate CTAs in the solid or liquid phase; instead, a broad
band appeared at 3300–2500 cm21, attributable to the OAH
stretching mode in the presence of hydrogen bonding. This
broad band was also overlapped with the absorption band of
alkyl CAH stretching vibrations at 3000–2800 cm21 37
The
.
absorption of C@O stretching mode of the dimeric carboxylic
acid occurred at 1705–1720 cm21, while the peaks at 1232–
1246 and 1115–1174 cm21 are ascribed to the asymmetric
FIGURE 1 FTIR spectrum of the synthesized EtXAcOH. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
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FIGURE 2 1H NMR spectra of the synthesized xanthate CTAs (in CDCl3). [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
and symmetric CAOAC stretching vibrations, respectively.
Together with these FTIR peaks, the presence of the asym-
metric CH2 bending vibration at 1456–1466 cm21, the sym-
metric CH3 bending vibration at 1375–1377 cm21, the
stretching mode of C@S at 1047–1065 cm21 indicated suc-
cessful synthesis of these xanthate CTAs.
NMR spectroscopy was utilized to determine the molecular
structure of novel bifunctioanl compounds. 1H and 13C NMR
spectra of the xanthate CTAs are displayed in Figures 2 and
3, respectively. In the 1H NMR spectra of all four xanthate
CTAs (Fig. 2), the methylene protons in the SACH2ACOOH
group show a singlet peak in the region of 3.93–4.01 ppm. A
FIGURE 3 13C NMR spectra of the synthesized xanthate CTAs (in CDCl3). [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
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FIGURE 4 (a) 1H and (b) 13C NMR spectra of the synthesized poly(vinyl acetate) using PrXAcOH (in CDCl3). [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.com.]
singlet peak appears at 4.21 ppm in the 1H NMR spectrum
of MeXAcOH, attributable to the methyl protons in the
methoxy group (CH3AO). A quartet at 4.65 ppm and a triplet
at 1.42 ppm in the 1H NMR spectrum of EtXAcOH are
assigned to the methylene and methyl protons of the ethoxy
group (CH3ACH2AO), respectively. In the 1H NMR spectrum
of PrXAcOH, a septet at 5.72 ppm is attributed to the
methine proton (CH3ACH), while a doublet at 1.39 ppm is
attributed to the methyl protons (CH3ACH). In the 1H NMR
spectrum of OctXAcOH, the methylene protons in the octy-
loxy group [CH3A(CH2)4ACH2ACH2ACH2AO] show a triplet
at 4.58 ppm [CH3A (CH2)4ACH2ACH2ACH2AO], a quintet at
1.79 ppm [CH3A (CH2)4ACH2ACH2ACH2AO], multiplets at
1.40 [CH3A(CH2)4ACH2ACH2ACH2AO] and 1.29 [CH3A
(CH2)4ACH2ACH2ACH2AO] ppm, while the end methyl
protons show
a triplet at 0.88 ppm [CH3A(CH2)4A
CH2ACH2ACH2AO]. In addition, four very small broad peaks
(visible when zooming in) appearing at 9.13, 11.79, 10.71,
and 10.16 ppm in the 1H NMR spectra of MeXAcOH, EtXA-
cOH, PrXAcOH, and OctXAcOH, respectively, are ascribed to
the carboxylic proton (COOH) of the corresponding xanthate
CTAs. In the 13C NMR spectra of all four xanthate CTAs
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FIGURE 5 Plots of (a) number-average molecular weight and (b) dispersity of the synthesized polymers versus conversion of
monomer with different amount of EtXAcOH in the polymerization of vinyl acetate. (Reaction conditions: VAc 100 mmol; EtXAcOH
as noted in the legend; AIBN 0.5 mmol; THF 15 mL; T 5 60 ꢁC; t 5 0–24 h). [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
(Fig. 3), the thiocarbonyl (S@C) and carbonyl (C@O) carbons
show characteristic chemical shifts at 211.0–213.0 and
173.7–174.5 ppm, respectively, while a peak at 37.3–37.8
ppm is ascribed to the methylene carbon in the SACH2A
COOH group.
end of polymer chains were observed due to the low content
in the polymer chains and overlapping the peaks of the
repeating unit.
Livingness of the Polymerization
In free radical polymerization without adding CTAs, poly-
mers of the highest molecular weights are formed at the
early stage of the polymerization, resulting in decreased
molecular weight with increasing conversion of monomer. In
contrast, a linear increase in molecular weight with conver-
sion of monomer and narrow molecular weight distribution
are typically associated with a controlled polymerization. In
order to verify the livingness of the RAFT polymerization,
one of the synthesized CTAs, EtXAcOH, of different concen-
trations were added in the polymerization of VAc. Figure 5
displays the effect of the concentration of EtXAcOH on the
radical polymerization of VAc. In comparison with the
decreased molecular weight with increasing conversion in
the polymerization without adding CTAs, by introducing the
RAFT CTA EtXAcOH to the reaction mixture, the molecular
weight increased with increasing conversion of monomer up
to ca 57% (Figure 5a). The increased molecular weight with
increasing conversion of monomer can be clearly visualized
by the peaks shifting to smaller retention times in the GPC
chromatogram (see supporting information). In addition,
compared to the high dispersities between 2.03 and 2.45 in
the conventional free radical polymerization where no CTA
was used, the application of the xanthate CTAs resulted in
lower dispersities between 1.22 and 1.92 (Figure 5b), with
higher amount of CTAs resulting in lower dispersities. The
low dispersities were attributed to the CTAs facilitating the
fragmentation in the well studied RAFT mechanism.1 The
results of polymerization of VAc under different reaction con-
ditions are summarized in (Table 1). Due to the fast addition
and slow fragmentation of the unstable VAc radicals, the dis-
persity of the formed PVAc is higher than that from RAFT
polymerization of other monomers. The relatively high dis-
persity and increased dispersity with conversion are in good
agreement with those reported in the literature for the RAFT
polymerization of VAc.6,10 Our lower Mns than those
The methoxy, ethoxy, isopropoxy, and octyloxy peaks are all
evident in the corresponding 13C NMR spectra, as shown in
Fig. 3. In addition, by recrystallization, three single crystals
(MeXAcOH, EtXAcOH, and PrXAcOH) were obtained at room
temperature from hexanes (or hexanes/cyclohexane), and
OctXAcOH was purified at a lower temperature using this
method. Investigation of X-ray crystallography on the single
crystals of MeXAcOH, EtXAcOH, and PrXAcOH indicates the
monoclinic symmetry (P21/c, P21/n, and P21/n, respectively)
and intermolecular hydrogen bonding within the crystals.38–40
Together with the FTIR and NMR results, the X-ray crystallog-
raphy result confirmed successful synthesis and purification
of the xanthate CTAs.
RAFT Polymerization of VAc using the Synthesized
Xanthate CTAs
Characterization of the synthesized polymer
With the four xanthate CTAs being synthesized, these CTAs
were then tested in the polymerization of VAc monomer via
RAFT polymerization. The synthesized poly(vinyl acetate)
1
was examined by NMR, with the H and 13C NMR spectra of
the synthesized poly(vinyl acetate) by using PrXAcOH as the
1
RAFT CTA being shown in Figure 4. In the H NMR spectrum
(Figure 4a), the broad peak at 1.80–1.90 ppm is attributed
to the methyl protons (CH3AC@O) in the pendant acetyl
group while the peaks at 1.50–1.75 ppm and the board peak
at 4.65–4.90 ppm are assigned to the backbone methylene
protons (ACH2ACHA) and methine proton (ACH2ACHA),
respectively. In the 13C NMR spectrum (Figure 4b), the pend-
ant acetyl group shows two carbon peaks at 169.9
(CH3AC@O) and 20.6 ppm (CH3AC@O) while the backbone
methine (ACH2ACHA) and methylene (ACH2ACHA) car-
bons show two peaks at 66-68 and 38-39 ppm, respectively.
1
Neither H nor 13C peaks of the fragmented PrXAcOH at the
612
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TABLE 1 Reaction Conditions for the Polymerization of VAc
-
Entry
1
CTA (mmol)
0
AIBN (mmol)
0.5
Time (h)
Conversion (%)
Mn (Da)
D
aMntheo(Da)
0.5
1
28
67
74
88
100
4
6,220
5,670
4,640
3,880
3,660
750
2.17
2.03
2.21
2.42
2.45
1.28
1.32
1.36
1.47
1.50
1.55
1.60
1.32
1.42
1.60
1.69
1.48
1.63
1.83
1.90
1.92
1.32
1.34
1.45
1.60
1.64
1.70
1.73
1.32
1.44
1.61
1.76
1.82
1.87
1.22
1.25
1.34
1.50
1.55
1.29
1.34
1.39
1.55
1.61
1.65
1.70
2
4
6
2
1 (MeXAcOH)
0.1
0.5
1
510
8.4
20
29
35
45
77
11
40
81
100
28
39
57
92
100
4
1,140
2,080
2,520
2,720
2,710
2,810
1,430
2,790
3,180
3,400
1,830
2,530
2,920
2,860
2,870
1,150
1,880
2,610
3,140
3,250
3,060
3,070
1,740
2,490
2,900
2,980
2,920
2,610
820
890
2
1890
2660
3180
4040
6800
1110
3610
7140
8780
5000
6900
9990
16020
17400
530
4
6
10
24
0.5
1
3
4
1 (MeXAcOH)
0.5 (EtXAcOH)
0.5
0.5
2
4
0.5
1
2
4
6
5
1 (EtXAcOH)
0.1
0.5
1
20
40
65
75
87
100
28
45
57
74
85
100
20
40
54
96
100
3.5
17
23
45
62
75
100
1900
3620
5780
6640
7670
8790
2590
4050
5090
6550
7500
8790
1040
1900
2500
4310
4480
500
2
4
6
10
24
0.5
1
6
1 (EtXAcOH)
0.5
2
4
6
10
0.5
1
7
8
2 (EtXAcOH)
1 (PrXAcOH)
0.5
0.1
1,190
1,730
2,050
2,250
950
2
4
6
0.5
1
1,560
2,300
3,030
3,080
3,150
3,160
1660
2170
4070
5530
6650
8800
2
4
6
10
24
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TABLE 1. Continued
-
Entry
9
CTA (mmol)
AIBN (mmol)
0.5
Time (h)
Conversion (%)
Mn (Da)
D
aMntheo(Da)
1 (PrXAcOH)
0.5
1
12
1,610
2,660
3,080
3,130
1,140
2,230
2,700
3,030
2,960
2,950
2,080
3,030
3,420
3,500
1.32
1.42
1.60
1.74
1.35
1.39
1.46
1.61
1.69
1.72
1.32
1.44
1.60
1.69
1230
3550
6560
8800
1210
1900
4140
6980
7750
8870
1730
5080
8790
8870
39
2
74
4
100
11
10
1 (OctXAcOH)
0.1
0.5
1
19
2
45
4
78
6
87
10
0.5
1
100
17
11
1 (OctXAcOH)
0.5
56
2
99
4
100
Note: Additionally, VAc 100 mmol, THF 15 mL, and reaction temperature 60 ꢁC for all entries.
a
Mntheo is theoretical molecular weight calculated according to Equation 2.
reported in the literature is due to the lower molar ratios of
monomer to CTAs, which will be further discussed below.
trend to, but slightly higher than those obtained experimen-
tally (Fig. 5a), suggesting a living free radical polymerization
when adding EtXAcOH to the reaction mixture.
The molecular weight of polymer in RAFT polymerization can
be theoretically estimated using the following equation41:
Effect of concentration of initiator and type of CTAs
In addition to the initial molar ratio of monomer to CTA, the
concentration of initiator and the type of RAFT CTAs also
plays a role in RAFT polymerization. The effect of the con-
centration of initiator, AIBN, on the polymerization by using
the four synthesized xanthate CTAs was examined. By com-
paring the curves of conversion versus reaction time for the
two different concentrations of AIBN, it is clearly seen that a
complete conversion of monomer was achieved within 4 -
10h when employing 0.5 mmol of initiator (AIBN) while it
took at least 24h when employing 0.1 mmol of initiator (see
supporting information). The quicker conversion of monomer
when employing the higher concentration of initiator is due
to the higher concentration of active sites generated by the
initiator.42 Due to a sufficient concentration of CTA being
used, the livingness of the RAFT polymerization was not
affected significantly by the increased concentration of initia-
tor, as evidenced by the similar curves of molecular weights
and dispersities versus conversion of monomer (see Sup-
porting Information).
½Mꢂ0
ꢀ
M
n;theo 5
ꢃ x ꢃ MU1MX
(2)
½Xꢂ0
ꢀ
where Mn;theo is the theoretical number-average molecular
weight, ½Mꢂ0 and ½Xꢂ0are the initial concentrations of the
monomer and the xanthate CTA, respectively, MUand MX are
molecular weights of the monomer unit and the xanthate
CTA, respectively, and x is the conversion rate of the mono-
mer. According to eq 2, the initial molar ratio of the mono-
mer to CTA (½Mꢂ0=½Xꢂ0) is a major parameter affecting the
molecular weight of the polymer formed in RAFT polymer-
ization. Although the molecular weight did not change much
when increasing the amount of EtXAcOH from 0.5 to 1
mmol, further doubling the amount from 1 to 2 mmol
resulted in almost halved molecular weights up to a conver-
sion of 57%. In addition, based on eq 2, a high concentration
of CTA would lead to the formation of polymers with low
molecular weight and the molecular weight would increase
with increasing conversion. The theoretically calculated
molecular weights for 2 mmol of EtXAcOH showed a similar
In RAFT polymerization, the Z-group is the functional group
that remains attached to the RAFT agent throughout the
polymerization, and acts to stabilize the radical intermedi-
ates. By varying the Z-group structure, this group can help
activate the C@S bond toward radical addition, and then,
impart minimal stabilization of the adduct radical formed.
This is due to the interaction between O or N lone pairs and
the C@S double bond, resulting in delocalization of this
bond; the phenomenon in which electrons in some molecules
SCHEME
2 Canonical
form
of
xanthates
and
dithiocarbamates.44
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TABLE 2 Number-Average Molecular Weight and Polydisper-
sity of the Unattached poly(vinyl acetate) PVAc formed from
the Synthesis of TiO2-PVAc Nanocomposites
Entry
CTA
Mn (Da)
aMntheo(Da)
-
D
1
2
3
4
MeXAcOH
EtXAcOH
PrXAcOH
OctXAcOH
3,024
2,616
2,855
3,134
1.94
2.05
1.95
1.87
8780
8790
8800
8870
Note: Reaction conditions: nano-TiO2 0.500g; VAc 50 mmol; xanthate
CTA 0.5 mmol; AIBN 0.25 mmol; THF 7.5 mL; T 5 60 ꢁC; t 5 24 h. aTheor-
etical molecular weight Mntheo was calculated using Equation 2 with an
assumption of 100% conversion.
FIGURE 7 DSC plots of (a) nano-TiO2, (b) PVAc (Mn 5 3160 Da,
-
D=1.70), and (c-d) the synthesized TiO2-PVAc nanocomposites.
[Reaction conditions: nano-TiO2 0.500g; VAc 50 mmol; (c)
EtXAcOH or (d) PrXAcOH 0.5 mmol; AIBN 0.25 mmol; THF 7.5
mL; T 5 60 ꢁC; t 5 24 h]. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
are not fixed to specific atoms or bonds but are spread out
over several atoms or bonds (Scheme 2).43
By comparing the four different O-substituent xanthate CTAs
with the application of 0.1 mmol of AIBN, there was no sig-
became insignificant when employing a higher amount of
AIBN (0.5 mmol) (see supporting information).
-
nificant difference in the profiles of Mn and D (see support-
ing information). However, the use of MeXAcOH resulted in
the slowest rate of conversion while the use of OctXAcOH
led to the highest rate of conversion. The lowest rate of con-
version by using MeXAcOH may be due to the stronger elec-
tron withdrawing effect of the methoxy group among the
four alkoxy groups including methoxy, ethoxy, isopropoxy,
and octyloxy groups. The strongest electron withdrawing
effect of the methoxy group was confirmed by the 1H and
13C NMR spectra, as the methylene protons and carbon in
the SACH2ACOOH group of MeXAcOH showed the highest
chemical shift at 4.01 and 37.8 ppm, respectively, compared
to those at 3.93-3.97 and 37.3-37.5 ppm, respectively, for the
other three other investigated CTAs. However, the difference
in the RAFT polymerizations by using the four different CTAs
Synthesis of TiO2-PVAc Nanocomposites via a One-Pot
Process
Traditionally polymer composites are prepared by mixing
the fillers with polymer resins. Functionalization or surface
activation of the fillers before mixing is often needed to
achieve uniform dispersion of fillers in the polymer matrix.
Preparation of polymer nanocomposites without the func-
tionalization of nanofillers results in aggregation and leach-
ing of the nanofillers. Both “grafting-to” and “grafting-from”
polymerization methods usually require multiple steps such
as dissolving, purifying, functionalizing, and drying some
intermediates. However, a one-pot method is advantageous
by enabling energy-saving, waste-reduction, and cost-
reduction. Our bifunctional synthesized xanthate CTAs con-
tain a carboxyl group which could be utilized for coordina-
tion bonding of the polymer chains to the metal oxide
nanoparticles. To test this hypothesis, an experiment was
designed to synthesize TiO2-PVAc nanocomposites via a one-
pot process where the xanthate CTAs, VAc, initiator AIBN,
and TiO2 nanoparticles were introduced into a reactor simul-
taneously. The unattached PVAc was isolated and character-
ized with GPC (Table 2), showing similar molecular weights
(Mn 5 2620–3130 Da) to but slightly higher dispersities
-
-
(D 5 1.87–2.05) than those (Mn 5 2610–3500 Da, D=1.69–
1.87) without introducing nano-TiO2 under the similar reac-
-
tion conditions. The comparable Mns and Ds suggest living
polymerization of VAc even in the presence of nano-TiO2.
Thermo-analysis
The amount of polymer grown from the surface of TiO2
nanoparticles was measured by TGA (Fig. 6), which revealed
that 7.9–18.1 wt % of PVAc was attached under the experi-
mental conditions using the four xanthate CTAs. As shown in
Figure 6 for the investigated CTAs, the TGA results show two
obvious regions when the temperature was increased: the
FIGURE 6 TGA curves of the synthesized TiO2-PVAc nanocom-
posites. (Reaction conditions: nano-TiO2 0.500g; VAc 50 mmol;
xanthate CTA 0.5 mmol; AIBN 0.25 mmol; THF 7.5 mL; T 5 60
ꢁC; t 5 24 h).
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FIGURE 8 TEM images of nano-TiO2 and the synthesized TiO2-PVAc nanocomposite dispersed in THF. (Reaction conditions: nano-
TiO2 0.500g; VAc 50 mmol; EtXAcOH 0.5 mmol; AIBN 0.25 mmol; THF 7.5 mL; T 5 60 ꢁC; t 5 24 h).
first region started from 30 ꢁC and ended at 200 ꢁC with a
relatively small slope (due to evaporation of the entrapped
water or solvent); while the second region started at 250 C
2), showing similar molecular weights to but slightly higher
PDIs than those without introducing nano-TiO2. The low
amount of PVAc attached to the surface of nano-TiO2 may
also be partially due to the hindered polymerization on the
surface of nanoparticles.46
ꢁ
and ended at 450 ꢁC with a large slope indicating more
weight loss, and decomposition of some organic groups of
the PVAc and RAFT agent components. It ꢁbecomes flat again
after the temperature increases over 450 C. The weight loss
of TiO2/PVAc at the region of 250–450 ꢁC gives strong evi-
dence regarding the polymer layers grafted onto nano-TiO2
as the polymer, and TiO2 components have distinct thermal
stabilities.19,45 Compared to the introduced weight ratio of
VAc to nano-TiO2 at 8.6:1, the measured amount of PVAc on
the surface of TiO2 nanoparticles is rather low, which can in
part be also due to some removal of unattached/free PVAc
during the centrifugation purification process. The unat-
tached PVAc was isolated and characterized with GPC (Table
In addition, DSC was employed in the characterization of the
synthesized nanocomposites. Figure 7 compares the DSC
plots of nano-TiO2, PVAc, and the synthesized TiO2-PVAc
nanocomposites. There was not any transition peak between
ꢁ
270 and 95 C for the nano-TiO2 (Figure 7a). In contrast to
the transition of the PVAc sample at 24.0 ꢁC (Figure 7b),
TiO2-PVAc nanocomposites showed much higher transition
temperature at 14.4-16.5 ꢁC (Figure 7c-d), suggesting a
chemical bonding between the nano-TiO2 particles and the
PVAc chains. In comparison with the commonly reported
FIGURE 9 Dispersion of nano-TiO2 (a) without/(b-d) with PVAc coating in the mixture of water and chloroform.
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47
ꢁ
glass transition temperature of PVAc at 28-31 C, the lower
(ethoxycarbonothioylthio) acetic acid on the polymerization
was examined, indicating that higher concentration of CTA led
to lower molecular weights and narrower dispersities. Higher
concentrations of initiator AIBN in the RAFT polymerization
of VAc resulted in quicker conversion of the monomer. The
bifunctionality of the synthesized xanthate CTAs was utilized
in the synthesis of TiO2-PVAc nanocomposites using a one-pot
process, demonstrating their potential application in the syn-
thesis of polymer nanocomposites via RAFT polymerization.
ꢁ
transition temperature of the synthesized PVAc at 24.0 C is
due to the low molecular weight (Mn 5 3160 Da).
Coordination of nano-TiO2 to PVAc chains
To verify the coordination of nano-TiO2 and PVAc chains, the
distribution of TiO2 in the polymer nanocomposites was
examined using TEM. Figure 8 compares the distribution of
nano-TiO2 before and after being incorporated into the poly-
mer matrix. As expected, the nano-TiO2 with a particle size
of ca 25 nm aggregated before being introduced into the
RAFT polymerization (Figure 8a). After the polymerization,
the distribution of the nano-TiO2 was slightly improved (Fig-
ure 8b), indicating the successful coordination of PVAc
chains to the nanoparticles.
ACKNOWLEDGMENT
The authors thank Dr. Wei Wu and Mr. Md. Abdul Mumin for
their assistance in the DSC and TEM measurements, respec-
tively. This work was supported by the Canadian Natural Scien-
ces and Engineering Research Council (NSERC) Idea to
Innovation (I2I) Program.
There are three possible structures of carboxylate absorbed
on to TiO2 surface, i.e., monodentate, chelating bidentate,
and bridging bidentate.26 These modes could be distin-
guished in infrared spectra by the difference between the
carboxylate stretching bands (DV).48 FTIR spectra of the syn-
thesized TiO2-PVAc nanocomposites were measured and
compared with that of nano-TiO2 (see supporting informa-
tion). In contrast to the almost flat spectrum of nano-TiO2,
there are a few major peaks appearing in the spectrum of
TiO2-PVAc nanocomposites at 1738 (VC@O), 1433 (dCH3(as)),
1373 (dCH3(s)), 1244 (VCOC(as)), 1122 (VCOC(s)), 1043, 1024
(qCH3), and 947 cm21. These peaks confirm the presence of
poly(vinyl acetate) in the TiO2-PVAc nanocomposites. How-
ever, these carboxylate stretching bands were hardly
observed, maybe due to the low concentration of carboxyl
group in the polymer chains. The characteristic C 5 S stretch-
ing mode at 1047–1065 cm21 could not be confirmed due to
its overlapping with the rocking mode of methyl group.
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