Narrow disperse polymers using amine functionalized dithiobenzoate RAFT agent and easy removal of thiocarbonyl end group from the resultant polymers

Article abstract of DOI:10.1002/pola.24572

A novel amine functionalized RAFT agent, 2-cyanoprop-2-yl(4-N,N- dimethylaminophenyl) dithiobenzoate has been synthesized and used to control the polymerization of vinyl monomers. This dithiobenzoate RAFT agent, although air sensitive, controlled the poly

Full text of DOI:10.1002/pola.24572

Narrow Disperse Polymers Using Amine Functionalized
Dithiobenzoate RAFT Agent and Easy Removal of Thiocarbonyl
End Group from the Resultant Polymers
SATYASANKAR JANA, ANBANANDAM PARTHIBAN, CHRISTINA L. L. CHAI
Institute of Chemical and Engineering Sciences (ICES), Agency for Science, Technology and Research (A*STAR), 1 Pesek Road,
Jurong Island, Singapore 627833
Received 22 September 2010; accepted 4 January 2011
DOI: 10.1002/pola.24572
Published online 24 January 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: A novel amine functionalized RAFT agent, 2-cyano-
prop-2-yl(4-N,N-dimethylaminophenyl) dithiobenzoate has
simplified the removal procedure of the color causing end
thiocarbonyl group from the RAFT derived polymers and
thereby leading to polymers with improved appearance. The
removal of end group from the polymer was confirmed by
been synthesized and used to control the polymerization of
vinyl monomers. This dithiobenzoate RAFT agent, although air
sensitive, controlled the polymerization of MMA and St very
well in an inert atmosphere and the polymerization results
obtained were marginally better than using the most popular
2-cyanoprop-2-yl dithiobenzoate RAFT agent. The living nature
of these polymerizations was confirmed by kinetics study and
chain extension reactions to yield narrow disperse di-block
copolymers. Most importantly, use of this novel RAFT agent
1H NMR and UV-vis spectroscopic techniques.
2011 Wiley
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Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 1494–
1502, 2011
KEYWORDS: dithiobenzoate; end group removal; kinetics
(polym.); living radical polymerization; reversible addition frag-
mentation chain transfer (RAFT)
may lead to toxicity (although depend on the type of dithio-
carbonyl end group and cells used for the study4) and odor
associated with the final polymers produced.1a,c,d,5 There are
indeed a number of different approaches used to remove the
thiocarbonylthio end groups include hydrolysis,6 aminolysis,7
thermolysis,8 reduction,9 and radical induced process5,10 etc.
However, most of these processes require an additional rea-
gent to react with the thiocarbonylthio end group. In addi-
tion, such reagents are used in large excess in order to
ensure complete removal of the end groups. Hence, the dis-
covery of a RAFT agent, which controls the polymerizations
well and at the same time simplifies the removal procedure
of end thiocarbonyl group from the resultant polymer in the
post polymerization process, preferably without any addi-
tional use of reagents or in a simple and mild condition will
be of great importance for the RAFT process.
INTRODUCTION In the past few decades significant develop-
ments in controlled or living radical polymerization (CRP/
LRP) techniques have allowed increased control over the
polymerization processes and polymerized products. Reversi-
ble addition fragmentation chain transfer (RAFT) mediated
polymerization is believed to be one of the most versatile
CRP methods in the view of the variety of monomers used,
tolerance towards different functional groups and simplicity
of the polymerization procedure.1 RAFT mediated polymer-
ization works by simple addition of RAFT agents, usually thi-
ocarbonylthio compounds (S¼¼C(Z)-SR), to a free radical po-
lymerization system. Dithioesters are one of the most
popular RAFT agents among all and are explicitly useful for
controlling polymerization of more activated vinyl monomers
like methyl methacrylate (MMA), styrene (St) etc.1,2 However
dithioester RAFT agents are colored and the polymers syn-
thesized using these RAFT agents are also colored due to the
presence of RAFT agent derived thiocarbonylthio (S¼¼CASA)
group attached to the end of the polymer chains. This chain
end thiocarbonylthio group could be interesting to synthe-
size different end functional polymers as reported in the lit-
erature.3 However, there is still concern over this color caus-
ing thiocarbonylthio (S¼¼CASA) end group of RAFT derived
polymers especially when the polymer is to be applied for
certain color related applications directly after synthesis and
Herein, we report the synthesis of novel 2-cyanoprop-2-yl(4-
N,N-dimethylaminophenyl) dithiobenzoate (CPDADB) RAFT
agent and its use for the homopolymerization and copoly-
merization of MMA and styrene (St). This amine functional-
ized dithiobenzoate RAFT agent controls the polymerization
of MMA and St in an excellent manner and simplifies the re-
moval procedure of color causing thiocarbonyl end group
from the resulting polymers to produce polymers with
improved appearance.
Additional Supporting Information may be found in the online version of this article. Correspondence to: S. Jana (E-mail: satyasankar_jana@ices.
a-star.edu.sg) or C. L. L. Chai (E-mail: christina_chai@ices.a-star.edu.sg)
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stored in glove box before use. Yield 65%. 1H NMR (400
MHz, CDCl3, d, ppm): 1.94 (s, 6H, 2 ꢀ CACH3), 3.07 (s, 6H, 2
ꢀ NACH3), 6.57 (d, 2H, C6H4), 8.03 (d, 2H, C6H4). 13C NMR
(400 MHz, CDCl3, d, ppm): 27.0, 40.1, 41.1, 110.5, 121.0,
129.1, 133.1, 154.1, 217.2. HRMS (m/z): 287.06470 [M þ
Na]þ; Calcd. C13H16N2S2: 287.06471. Calcd. C13H16N2S2: C,
59.35; H, 6.10; N, 10.59; S, 24.25%; Found C, 59.68; H, 6.09;
N, 10.83; S, 23.21%.
EXPERIMENTAL
Materials and Characterizations
Methyl methacrylate (MMA, 99%), styrene (St, 99þ%), car-
bon disulfide (99.9%), and 4-bromo-N,N-dimethylaniline
(97%) were obtained from Aldrich, magnesium chips
(99þ%), phenylacetyl chloride (98%), and 4-methylstyrene
(98%) were obtained from Alfa Aesar, potassium hexacyano-
ferrate (III) (K3Fe(CN)6, 99%) was obtained from Merck and
2,20-Azobisisobutyronitrile (AIBN) was obtained from Hallo-
Chem Pharm. The monomers MMA and St were purified by
passing through neutral alumina column and stored in
freezer before polymerizations. AIBN was purified by recrys-
tallization from absolute alcohol.
Typical RAFT Polymerization of MMA Using CPDADB
(P3)
AIBN (4.4 mg, 2.67 ꢀ 10ꢂ5 mol) and CPDADB (14.1 mg,
5.342 ꢀ 10ꢂ5 mol) were weighed into a 25 mL Schlenk tube
inside a glove box. Nitrogen purged MMA (2 mL, 18.69
mmol) and dry toluene (0.7 mL) were then added to the
Schlenk tube and immediately frozen on a liquid N2 bath.
The solution was degassed with three freeze-pump-thaw
cycles using N2 and warmed to room temperature and then
1H and 13C NMR spectra were recorded on a 400 MHz
Bruker UltraShield AVANCE 400SB spectrometer. Residual
solvent peaks were used as internal standard. UV-vis spectra
were recorded on SHIMAZDU UV-2550 spectrophotometer.
Elemental microanalysis was performed using Eurovector
E300 elemental analyzer. High resolution mass spectra were
recorded on Agilent 6210 TOF/LCMS. The THF run SEC sys-
tem was equipped with Waters 515 HPLC pump, 717plus
autosampler, and 2414 refractive-index detector. The follow-
ing Styragel GPC columns were arranged in series: guard,
HR5E (ꢀ2, 4.6 mm ID ꢀ 300 mm), HR1 and HR0.5. The
eluant flow rate was 0.3 mL/min and the columns were
ꢁ
heated at 60 C for the required time. The stoichiometries of
reagents are MMA:CPDADB:AIBN ¼ 350:1:0.5. Samples from
the reaction mixture were collected for NMR and GPC analy-
sis, when required. Polymer samples were analyzed by GPC
as collected from the reaction mixture. Finally, polymeriza-
tion was stopped by cooling the content to room tempera-
ture and exposing in air. Bright yellow powdery polymer
sample was collected on precipitation from excess hexane,
which was dried in a vacuum oven.
ꢁ
maintained at 40 C.
Typical RAFT Polymerization of St Using CPDADB (P7)
A mixture of styrene (3 mL, 26.2 ꢀ 10ꢂ3 mol), AIBN (2.46
mg, 1.495 ꢀ 10ꢂ5 mol), CPDADB (19.78 mg, 7.48 ꢀ 10ꢂ5
mol) and anisole (5 drops) were taken into a 25 mL Schlenk
tube. The solution was degassed with three freeze-pump-
thaw cycles using N2 and then heated to 80 ꢁC for the
required time. The stoichiometries of the reagents are
St:CPDADB:AIBN ¼ 350:1:0.2. Samples from the reaction
mixture were collected for NMR and GPC analysis, when
required. Polymer samples were analyzed by GPC as col-
lected from the reaction mixture. Polymerization was
stopped by cooling the content to room temperature and
exposing in air and finally polymer was precipitated out
from excess methanol to produce bright yellow powder.
Synthesis of 2-Cyanoprop-2-yl(4-N,N-
dimethylaminophenyl)dithiobenzoate (CPDADB)
4-Bromo-N,N-dimethylaniline (2.0 g, 9.995 mmol) was slowly
added to a dispersion of Mg chips (0.267 g, 11 mmol) in dry
THF (40 mL) under N2 atmosphere. The mixture was then
heated to 40 ꢁC for 1.5 h. The light blue color of 4-bromo-
N,N-dimethylaniline solution dissipated slowly and a color-
less solution was obtained. Carbon disulfide (3.131 g, 41.1
mmol) was added dropwise to the solution above at room
temperature. The reaction mixture was raised to 40 ꢁC and
allowed to react for 1 h. The resultant red solution was
poured into an acidified ice/water mixture (300 mL) and the
yellow precipitate of corresponding dithiobenzoic acid
formed was extracted with DCM (250 mL). The DCM phase
was concentrated to 150 mL and the product from DCM
phase was extracted with 2N NaOH solution (2 ꢀ 150 mL)
as its sodium salt. This sodium salt was converted to corre-
sponding disulfide by the addition of aqueous solution (50
mL) of K3Fe(CN)6 (3.29 g, 9.995 mmol). The precipitate
formed was filtered, washed with water, and then dried in a
vacuum oven at 40 ꢁC for 18 h to yield bis(4-N,N0-dime-
thylthiobenzoyl) disulfide as a brick red solid (1.65 g). A
part of this compound (1.0 g, 2.547 mmol) was mixed with
excess AIBN (0.627 g, 3.82 mmol) in ethyl acetate (50 mL)
and the rꢁesultant heterogeneous mixture was heated under
N2 at 90 C for 18 h to produce homogeneous solution. The
solution was filtered and the filtrate was dried in vacuo to
yield an orange solid. The product was purified by column
chromatography using 30% ethyl acetate in hexane under
nitrogen to yield a bright orange solid. The product was
Synthesis of Poly(methyl methacrylate)-block-
poly(styrene) (PMMA-b-PS) Copolymer by Chain
Extension Reaction
At first MMA was polymerized similar to the process men-
tioned above for P3. After 7 h, the polymerization reaction
was stopped and the excess MMA was removed by high vac-
uum to yield a yellow-orange colored solid polymer (conver-
sion 46.8%; obtained Mn,GPC 21,900, PDI 1.1). Next, the re-
sultant polymer was dissolved thoroughly in St (3 mL, 0.026
mol) with the addition of AIBN (1.5 mg) and anisole (5
drops), degassed with three freeze-pump-thaw cycles using
N2 and then heated at 80 ꢁC for 20 h to yield poly(methyl
methacrylate)-block-poly(styrene) (PMMA-b-PS) copolymer
(St conversion 50.9%; obtained Mn,GPC 58800; PDI 1.22).
Polymer samples were analyzed by GPC as collected directly
from the reaction mixture.
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Synthesis of Poly(styrene)-block-poly(4-methylstyrene)
(PS-b-P4MS) Copolymer by Chain Extension Reaction
At first St (2 mL, 17.45 ꢀ 10ꢂ3 mol) was polymerized for 46
h at 80 ꢁC in presence of AIBN (1.63 mg, 9.932 ꢀ 10ꢂ6
mol), CPDADB (13.18 mg, 4.98 ꢀ 10ꢂ5 mol) and anisole (5
drops) (St: CPDADB:AIBN ¼ 350:1:0.2). Excess St was
removed by high vacuum to yield a yellow-orange colored
solid polymer (conversion 72.7%; calculated Mn,Theor 26760;
obtained Mn,GPC 25,900 and PDI 1.07). Next, the resultant
polymer was dissolved thoroughly in 4-methylstyrene (4MSt,
3 mL) with the addition of AIBN (1.5 mg) and anisole (5
drops), degassed and then heated at 80 ꢁC for 30 h under
N2 to yield a poly(styrene)-block-poly(4-methylstyrene) (PS-
b-P4MS) copolymer. 4MSt conversion 49%, obtained Mn,GPC
55,700; PDI 1.12.
yellow polymer solution). A portion of the solution
(ꢃ10 mL) was used to precipitate out polymer from excess
methanol. The bright yellow solid was dried in vacuum oven
at 60 ꢁC overnight and characterized by 1H NMR spectros-
copy and GPC (colored PS, Mn,GPC 24,900 and PDI 1.08). The
remaining 10 mL solution was purged with compressed air
for 5–10 min and then heated at 60 ꢁC for 2 days. A color-
less solution was obtained which when precipitated from
excess methanol produced completely white polymer proꢁd-
uct. The white polymer was dried in a vacuum oven at 60 C
overnight and characterized by 1H NMR spectroscopy and
GPC (white PS, Mn,GPC 24,100 and PDI 1.07). Similarly a col-
orless polymer solution (and white powder) was also
obtained when the THF solution (5 mL) of bright yellow PS
ꢁ
(200 mg) was heated at 60 C for 2 days.
Typical Thiocarbonyl End Group Removal
Typical Thiocarbonyl End Group Removal from
from CPDADB Derived PS in THF
CPDADB Derived Low Molecular Weight PMMA in
THF Using Con. HCl Followed by Functionalization
A small portion (200 mg) of CPDADB derived bright yellow
colored PMMA (P4 in Table 1, Mn,GPC 11,200 and PDI 1.18)
was dissolved in 5 mL THF. Concentrated HCl (5 drops) was
added and stirred for 5 min at room temperature. The mix-
First, PS was synthesized as the process for P7 (St:CPDAD-
B:AIBN ¼ 350:1:0.2, at 80 ꢁC, 46 h). The final product was
then mixed with about 20 mL THF. (a) A portion (5 mL) of
the polymer solution was used to precipitate out PS from
methanol. The bright yellow powder solid was characterized
1
ꢁ
by H NMR spectroscopy and GPC (colored PS, Mn,GPC 24,700
ture was then heated to 60 C for 1 h to yield a colorless so-
and PDI 1.04). (b) Rest of the polymer solution was purged
compressed air for 5–10 min and then kept at room temper-
ature in a closed bottle until it became colorless after 2
months. White fine powder PS was obtained on precipitation
from methaꢁnol. The white PS sample was dried in vacuum
oven at 60 C overnight and characterized by 1H NMR spec-
troscopy and GPC (white PS, Mn,GPC 23,900 and PDI 1.08).
lution. White polymer was precipitated out from cold metha-
nol and dried in a vacuum oven at 60 ꢁC for 18 h and
characterized by 1H NMR spectroscopy and GPC (white
PMMA, Mn,GPC 12,200 and PDI 1.17).
The resultant white PMMA (100 mg) was dissolved in dry
CHCl3 (2 mL) and purged with N2. Triethylamine (2 drops)
and phenylacetyl chloride (2 drops, ꢃ15 mg, excess) were
added by syringe and the stirring was continued for 24 h.
Polymer was precipitated out from excess methanol and col-
lected after centrifugation. The polymer was washed
Another batch of PS was synthesized by the above procedure
(St:CPDADB:AIBN ¼ 350:1:0.2, 80 ꢁC, 47 h) and the final
product was similarly mixed with about 20 mL THF (produce
TABLE 1 RAFT Polymerization Results of MMA and St
RAFT
Monomer:
RAa:AIBN
Time
Conversionb
(%)
Entry
Monomer
Agent (RA)
(h)
Mn,Theor
Mn,GPCc; PDIc
226,000 2.32
P1
P2
MMA
MMA
None
CPDB
350:0:0.5
350:1:0.5
4
2
24
NA
9.8
3,750
32,740
4,650
33,990
10,230
NA
5,000
25,900
5,400
1.33
1.20
1.31
1.15
1.18
1.97
1.10
1.07
1.09
1.08
1.15
1.06
20
2
92.8
12.5
96.2
99.5
39.5
46
P3
MMA
CPDADB
350:1:0.5
20
20
6
26,700
11,200
97,900
17,500
5,900
P4
P5
P6
P7
MMA
St
CPDDDB
None
100:1:0.5
350:0:0.2
350:1:0.2
350:1:0.2
St
CPDB
23
2
16,990
5,850
20,230
26,740
70,580
8,220
St
CPDADB
15.3
54.8
72.6
70.2
76.4
23
47
48
30
19,300
24,900
67,800
8,200
P8
P9
St
St
CPDADB
CPDADB
1000:1:0.2
100:1:0.2
b
MMA was polymerized in toluene solution (with ꢃ74 volume % MMA)
Determined by 1H NMR spectroscopy.
In THF using PS calibration.
c
at 60 ꢁC and St was polymerized in bulk at 80 ꢁC.
a
RA is RAFT agent used.
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CPDADB degrades 10 and 55% in 24 h when stored in solid
state and solution state (15 mg/mL in CDCl3) respectively
1
proved by H NMR spectroscopy. However, we are unable to
identify the degradation products and the mechanism by LC-
MS or GC-MS studies.
CPDB has only one absorption maximum in its UV-vis spec-
trum in between 250–600 nm, that is, at ꢃ303 nm but
CPDADB has two absorption maxima at ꢃ333 nm and ꢃ423
nm possibly due to the extended conjugation by para-N,N-
dimethylamino group through the phenylene unit of
CPDADB. The conjugation of para-N,N-dimethylamino group
in CPDADB was also further supported by the fact that the
C¼¼S resonance of CPDADB appears at highfield (217 ppm)
in 13C NMR spectrum in compare to the same one in CPDB
(223 ppm). The absorption maximum of CPDADB at ꢃ423
nm was the most intense one and therefore was more appro-
priate to monitor the presence or absence of end thiocar-
bonyl group in the CPDADB derived polymers.
FIGURE 1 Dithiobenzoate RAFT agents used in this study.
thoroughly for 5 times with excess MeOH before drying in a
vacuum oven at 60 ꢁC overnight to produce white powder
(functionalized white PMMA, Mn,GPC 12,600 and PDI 1.16).
RESULTS AND DISCUSSION
Synthesis of RAFT Agents
2-Cyanoprop-2-yl dithiobenzoate (CPDB) was synthesized
using a similar procedure reported in the literature11 while
the novel 2-cyanoprop-2-yl(4-N,N-dimethylaminophenyl)
dithiobenzoate (CPDADB) (Fig. 1) was synthesized, following
a modified method because of the different solubility behav-
ior of the intermediates and air sensitivity of the final prod-
uct. These RAFT agents were purified by column chromatog-
raphy and characterized by 1H (Fig. 2) and 13C NMR
spectroscopic techniques, high resolution mass spectroscopy,
elemental microanalysis, and UV-vis spectroscopy. CPDB was
used as a reference RAFT agent for the polymerization stud-
ies. Noteworthy is the observation that CPDADB is sensitive
to air and was therefore synthesized and purified under
nitrogen atmosphere and finally stored in a glove box before
using in polymerization reactions. On exposure to air
RAFT Mediated Synthesis of Homopolymers
and Copolymers
The novel amine functionalized dithioester RAFT agent,
CPDADB, was independently used for the polymerization of
MMA and St. The polymerization of MMA in toluene using
this RAFT agent in the presence of AIBN as initiator was
well controlled. The polymerization using CPDADB followed
first order kinetics [Fig. 3(A)] and showed a linear increase
in molecular weight with monomer conversion [Figs. 3(B,C)].
The polydispersity index (PDI, Mw/Mn) of the poly(methyl
methacrylate) (PMMA; P3 in Table 1) obtained was very low
in compare to the polymerization without using the RAFT
agent (P1 in Table 1) and slightly lower in compare to using
CPDB (P2 in Table 1). The experimental molecular weight
from GPC (Mn,GPC) for these polymerizations matched quite
FIGURE 2 1H NMR spectrum of
2-cyanoprop-2-yl(4-N,N-dimethyl-
aminophenyl)
(CPDADB).
dithiobenzoate
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FIGURE 3 (A) Kinetic plots, (B) evolution of Mn and PDI with monomer conversion and (C) GPC traces of MMA polymerization
using CPDADB; (D) evolution of Mn and PDI in St polymerization using CPDADB.
well with the theoretical values (Mn,Theor) at different mono-
mer to initiator ratios [P3 and P4 in Table 1 and Fig. 3(B)].
block copolymers by chain extension reactions (Fig. 4). At
first, narrow disperse PMMA (Mn,GPC 21,900 and PDI 1.1)
and PS (Mn,GPC 25,900 and PDI 1.04) were synthesized sepa-
rately using CPDADB. The PMMA chain was then extended
with the polymerization of St to yield a narrow disperse di-
block copolymer poly(methyl methacrylate)-block-poly(sty-
rene) (PMMA-b-PS; Mn,GPC 58,800 and PDI 1.22). The chain
extension reaction of PS (Mn 25,900 and PDI 1.04) in 4-
methylstyrene (4MS) yielded poly(styrene)-block-poly(4-
methylstyrene) (PS-b-P4MS; Mn,GPC 55,700 and PDI 1.12)
(Scheme 1). In all chain extension reactions pure di-block
copolymers were obtained as indicated by the monomodal
GPC chromatograms (Fig. 4) of polymers. Chain extension
reaction of PMMA (Mn,GPC 20,700 and PDI 1.13) by its own
monomer was also successful (resultant PMMA: Mn,GPC
62,900 and PDI 1.24) as indicated by the monomodal GPC
chromatograms of respective polymers.
Interestingly,
a
better controlled polymerization was
observed for the bulk polymerization of St at 80 ꢁC using
CPDADB to yield poly(styrene) (PS) with very low PDI [P7
in Table 1 and Fig. 3(D)]. The Mn,GPC of the polymers was
very close to the theoretical values (Mn,Theor) and the PDI
values of these polymers were marginally lower relative to
the polymers obtained using CPDB under similar conditions.
Indeed, we have also synthesized relatively high and low mo-
lecular weight PS of very low PDI separately using different
monomer to CPDADB ratios (P8 and P9 in Table 1). In gen-
eral, less retardation was observed for the CPDADB mediated
polymerizations of both MMA and St in comparison to CPDB
mediated ones. For example, the monomer conversions at 2
h in St polymerizations using CPDB and CPDADB (P6 and P7
in Table 1) were 10.3% and 15.3%, respectively where as
monomer conversion at the same time without using any
RAFT agent was 15.8% (P5 in Table 1). This was most prob-
ably due to the better re-initiation for the CPDADB mediated
polymerizations because of the p–conjugations of para-ami-
nophenyl group, which slightly destabilizes the intermediate
polymer-RAFT agent adduct radicals in the equilibrium steps
of the polymerization reaction.
End Group Analysis RAFT Derived Polymers and End
Group Removal Procedures
Like other dithioester derived polymers, polymers synthe-
sized using CPDADB RAFT agent (both before and after pre-
cipitation from nonsolvent) were bright yellow in color [Fig.
5(A)] due to the presence of (4-N,N-dimethylaminophenyl)
dithiobenzoate end group. The presence of this end group in
polymer P7 (Table 1) (after reprecipitation from methanol)
was further confirmed by the appearance of proton
Furthermore, the living nature of the polymerization of MMA
and St using CPDADB was confirmed by the synthesis of di-
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FIGURE 4 GPC traces of (A) PMMA and PMMA-b-PS copolymer and (B) PS and PS-b-P4MS copolymer synthesized by respective
chain extension reactions.
resonances of N,N-dimethylamine (AN(CH3)2, 6H) functional
group and phenylene group (AC6H4A, 2H, ortho to thiocar-
bonylthio group) at 3.03 and 8.02 ppm, respectively with
ꢃ3:1 ratio in 1H NMR spectrum [Fig. 5(A)]. The resonances
of other two protons of phenylene group (meta to thiocarbo-
nylthio group) overlapped with the region of the phenyl
resonances of styrene repeat units. The UV-vis spectrum of
the same sample in THF also revealed the characteristic
absorption maxima at ꢃ339 nm and ꢃ417 nm correspond-
ing to the (4-N,N-dimethylaminophenyl) dithiobenzoate end
group [Fig. 6(B)] similar to the maxima for CPDADB RAFT
agent. The absorption maximum at ꢃ417 nm was the most
intense one and therefore used to monitor the presence or
absence of end thiocarbonyl group in the CPDADB derived
polymers. The absorption maximum ꢃ260 nm is apparently
due to the presence of styrene repeat units of the polymer.5a
Similar characteristic NMR spectra and UV-vis absorption
maxima were observed for all CPDADB derived PMMA and
PS samples.
air for 5–10 min, a colorless polymer solution (and white
powder when precipitated from methanol) was obtained af-
ter 2 months of storage at room temperature with very little
change of its Mn,GPC and PDI (yellow PS, Mn,GPC 24,700 and
PDI 1.06; white PS obtained after treatment, Mn,GPC 23,900
and PDI 1.08). Similar colorless polymer solution (and as
white polymer when precipitated from methanol) was
obtained [Fig. 5(B)] only after 2 days when the THF solution
of a yellow PS sample, P7 was heated at 60 ꢁC and again
without any major change in molecular weight characteris-
tics (yellow PS, Mn,GPC 24,900, PDI 1.08 and white PS, Mn
24,100, PDI 1.07; see Figure S2 in Supporting Information).
The UV-vis spectrum of the precipitated white PS sample
produced by the heating process mentioned above also con-
firmed the disappearance of absorption maxima at ꢃ339 nm
and ꢃ417 nm corresponding to the (4-N,N-dimethylamino-
phenyl) dithiobenzoate end group (Fig. 6). However, similar
attempts of heating P7 in other solvents, for example,
dichloroethane, toluene, and DMF failed to produce colorless
solutions. These above observations made us assume that
the discoloration may be due to the THF-generated peroxide-
induced oxidation of dithioester (S¼¼CASA) end group to
yield sulfine (O¼¼CASA) end group similar to the observa-
tions reported by Vana et al.10a However, surprisingly, 1H
NMR study of the resultant white PS, P7, obtained from the
It was previously mentioned that CPDADB was sensitive to
air. Therefore, it was expected that this air sensitivity of (4-
N,N-dimethylaminophenyl) dithiobenzoate end group can be
exploited to produce polymers with easily removable end
functional groups. Gratifyingly, when the THF solution of a
yellow colored PS sample, P7 was purged with compressed
SCHEME 1 Synthesis of di-block copolymer by chain extension reaction.
A NOVEL AMINE FUNCTIONALIZED RAFT AGENT, JANA, PARTHIBAN, AND CHAI
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 5 Photograph and 1H NMR spectra of PS samples (after precipitation from methanol) (A) P7 (yellow) and (B) P7 after the
removal of thiocarbonyl end group (white) obtained by heating the THF solution at 60ꢁC for 2 days (sol ¼ solvent). [Color figure
can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
above heat treatment confirmed the absence of proton
resonances of N,N-dimethylamine (AN(CH3)2, 6H) functional
group and phenylene group (AC6H4A, 2H, ortho to thiocar-
bonylthio group) at 3.03 ppm and 8.02 ppm, respectively
(Fig. 5(B)], which would not happen in the case of sulfine
formation through the oxidation of dithioester group. There-
fore the discoloration of PS, P7 in THF solution was due to
the removal of (4-N,N-dimethylaminophenyl) attached thio-
carbonyl end group and the most plausible explanation for
the observations is the hydrolysis of dithioester end group.
high molecular weight shoulder corresponding to the double
(Mn,GPC 17,000) of the original molecular weight of the poly-
mer (see Supporting Information Figure S3). 1H NMR
For relatively low molecular weight PS and PMMA (P9 and
P4 respectively in Table 1) only 50–60% end thiocarbonyl
group was removed when the THF solutions of individual
ꢁ
polymer were separately heated at 60 C for 3 days, as con-
firmed by 1H NMR spectroscopy. However, the addition of a
few drops of concentrated hydrochloric acid to the same PS
solution (P9 in Table 1) speeded up the thiocarbonyl end
group removal, which was almost completed in 1 day at 60
ꢁC to produce colorless polymer solution (and white polymer
when precipitated from methanol) with very slight increase
of its original Mn,GPC and PDI (P9: yellow PS before end
group removal, Mn,GPC 8200 and PDI 1.06; white PS after
end group removal, Mn,GPC 8300 and PDI 1.1). The closer
look on the GPC chromatogram reveled the appearance of
FIGURE 6 UV-vis spectra of THF solution of (A) CPDADB
(0.0125 mg/mL); precipitated PS samples (0.5 mg/mL) (B) P7
(yellow) and (C) P7 after the removal of thiocarbonyl end group
(white) obtained by heating the THF solution at 60ꢁC for 2
days.
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ARTICLE
spectra of white PMMA showed complete disappearance of
absorption maximum at ꢃ417 nm corresponding to thiocar-
bonylthio end group. Previously Xu et al.12 have proposed
the formation of thiolactone end group for the aminolysis
product of RAFT derived PMMA. However in our case, the
end group remains as thiol as indicated by the subsequent
reaction with phenylacetyl chloride (Scheme S2 and Figure
S5 in Supporting Information).
Our efforts to establish the exact nature of end group by
MALDI-TOF analysis of these polymers were inconclusive
most likely due to the transient nature of the end group of
the polymers.
CONCLUSIONS
In conclusion, we have synthesized a novel amine functional-
ized dithiobenzoate RAFT agent to be used for the RAFT
polymerization of vinyl monomers. This dithiobenzoate RAFT
agent controlled the polymerization of MMA and St in excel-
lent manner and the polymerization results obtained were
marginally better than using the most popular 2-cyanoprop-
2-yl dithiobenzoate RAFT agent. The living nature of the
polymerizations was confirmed by kinetic study and the syn-
thesis of di-block copolymers by chain extension reactions.
Most importantly, we have demonstrated that the air sensi-
tivity of this RAFT agent can be exploited to produce poly-
mers with easily removable color causing RAFT agent
derived thiocarbonyl end functionality. The thiocarbonyl end
functional group of the poly(styrene) and poly(methyl meth-
acrylate) synthesized using the amine functionalized RAFT
agent was easily removed either simply by heating the THF
solution of the polymer or by the addition of few drops con-
centrated hydrochloric acid. The removal of end group from
the polymer was confirmed by 1H NMR and UV-vis spectro-
scopic techniques.
FIGURE 7 Photograph of relatively low molecular weight
PMMA: (A) THF solution of yellow P4, (B) the same solution af-
ter heating at 60ꢁC for 1 h in presence of five drops of conc.
HCl and (C) overlay GPC chromatograms of yellow P4 before
end group removal (red line) and white PMMA after end group
removal (blue line) obtained by the HCl treatment.
spectrum of the resultant polymer indicated that more than
90% of (4-N,N-dimethylaminophenyl) attached thiocarbonyl
end group was removed in this process (see Figure S4 in
Supporting Information). The UV-vis spectra of white PS
obtained after acid treatment showed complete disappear-
ance of absorption maximum at ꢃ417 nm corresponding to
thiocarbonylthio end group. All these observations made us
believe that the this end group removal procedure of PS
(P9) would have happened by the acid promoted hydrolysis
of dithioesters group to yield thiol end functional group
(Scheme S1 in Supporting Information) partly which may be
subsequently oxidized or hydrolyzed.
This work was supported by the Science and Engineering
Research council of Agency for Science, Technology and
Research (A*STAR), Singapore.
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