RAFT-approach to well-defined telechelic vinyl polymers with hydroxyl terminals as polymeric diol-type building blocks for polyurethanes

Article abstract of DOI:10.1002/pola.26380

A new trithiocarbonate 1 bearing two hydroxyl moieties was synthesized and employed as a RAFT agent for radical polymerization of vinyl monomers. 1 mediated RAFT polymerizations of styrene and ethyl acrylate to give the corresponding polymers with predictable molecular weights and narrow molecular weight distributions. Structural analyses of the polymers with NMR and MALDI-TOF mass techniques revealed that they were telechelic ones, of which both chain ends were endowed with hydroxyl groups inherited from trithiocarbonate 1. Usefulness of these telechelic polymers as polymeric diol-type building blocks was demonstrated in their polyaddition with diisocyanates, which gave the corresponding polyurethanes.

Full text of DOI:10.1002/pola.26380

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RAFT-Approach to Well-Defined Telechelic Vinyl Polymers with Hydroxyl
Terminals as Polymeric Diol-Type Building Blocks for Polyurethanes
Atsushi Sudo, Takaaki Hamaguchi, Naoto Aoyagi, Takeshi Endo
Molecular Engineering Institute, Kinki University, Iizuka, Fukuoka 820-8555, Japan
Correspondence to: T. Endo (E-mail: tendo@moleng.fuk.kindai.ac.jp)
Received 11 July 2012; accepted 6 September 2012; published online 5 October 2012
DOI: 10.1002/pola.26380
ABSTRACT: A new trithiocarbonate 1 bearing two hydroxyl
moieties was synthesized and employed as a RAFT agent
for radical polymerization of vinyl monomers. 1 mediated
RAFT polymerizations of styrene and ethyl acrylate to give
the corresponding polymers with predictable molecular
weights and narrow molecular weight distributions. Struc-
tural analyses of the polymers with NMR and MALDI-TOF
mass techniques revealed that they were telechelic ones, of
which both chain ends were endowed with hydroxyl groups
inherited from trithiocarbonate 1. Usefulness of these tele-
chelic polymers as polymeric diol-type building blocks was
demonstrated in their polyaddition with diisocyanates,
C
V
which gave the corresponding polyurethanes.
2012 Wiley
Periodicals, Inc. J Polym Sci Part A: Polym Chem 51: 318–
326, 2013
KEYWORDS: reversible addition fragmentation chain transfer
(RAFT); living polymerization; block copolymers; polyurethanes
INTRODUCTION Telechelic polymers, which are polymers
endowed with reactive groups at both the a and x terminals,
have been recognized as important building blocks for mac-
romolecular architectures.1 Based on the efficient use of
their terminal reactive groups, polymers with unique shapes
and topologies have been synthesized and investigated as
new functional materials.2–4
NMP with employing nitroxides with hydroxyl groups,19,20 (5)
NMP followed by reductive scission of OAN bond of terminal
alkoxyamine,21 (6) synthesis of polymers with azide terminals
followed by azide-alkyne ‘‘click’’ coupling,22 and (7) RAFT
polymerization with employing hydroxyl-functionalized thio-
carbonyl compounds.8,23–25
Herein, we report an approach to well-defined telechelic
polymers with hydroxyl terminals based on RAFT polymer-
ization with employing a new trithiocarbonate 1 as a chain
transfer reagent. Despite the potential applicability to con-
struction of a wide range of macromolecular architectures,
there have been only a few RAFT agents bearing hydroxyl
groups that are referred above. The synthesis of 1 reported
herein involves a method for highly efficient synthesis of tri-
thiocarbonates, that is, potassium carbonate-mediated reac-
tion of alkyl halides with carbon disulfide, which was
reported by us previously.26 Application of the obtained a,
x-dihydroxyl-functionalized polymer as a polymeric diol to
polyurethane synthesis is also demonstrated.
Among several approaches to telechelic polymers reported
so far, those based on controlled/living radical polymeriza-
tion are highly attractive from the viewpoints of reliability,
facile operation, and tolerance to a wide range of conditions
and functional groups of monomers.1 Reversible addition-
fragmentation transfer (RAFT) polymerization, which has
been appreciated as a convenient, versatile, and highly reli-
able tool for building complex polymer architectures,5–10 is
one of the controlled radical polymerization techniques that
have been applied to the synthesis of well-defined telechelic
polymers.8
Hydroxyl-functionalized telechelic polymers have gained
their importance because of the versatility of hydroxyl group
as a reactive function that offers selective and precise modi-
fications of polymer terminals. They have been synthesized
via several approaches involving (1) iniferter polymerization
EXPERIMENTAL
Materials
All the reagents and solvents were purchased from Wako
Pure Chemical Industries (Osaka, Japan). Styrene, ethyl acry-
late (EA), and chlorobenzene were distilled over calcium
hydride prior to use. Other reagents and solvents were used
as received.
with employing
a bifunctional dithiocarbamate bearing
hydroxyl groups,11 (2) metal-mediated controlled radical poly-
merizations such as ATRP followed by terminal coupling,12–17
(3) ATRP followed by postfunctionalization of chain end,18 (4)
C
V
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Measurements
added dropwise for 20 min at ambient temperature. After
stirring the mixture at ambient temperature for 2 h, the mix-
ture was filtered with washing the residue with toluene (30
mL). The filtrate was washed successively with 2 M hydro-
chloric acid (50 mL), saturated aqueous solution of sodium
carbonate (50 mL), water (100 mL) and saturated aqueous
solution of sodium chloride (50 mL). The toluene solution
was concentrated under reduced pressure, and the resulting
residue was fractionated by silica gel column chromatogra-
phy with elution by chloroform/methanol (50/1) to obtain
N-ethyl-N-(2-acetoxyethyl)-4-(chloromethyl)benzamide (EACMB;
5.22 g, 18.4 mmol, 92%) as a colorless oil.
1H (400 MHz) and 13C NMR (100.6 MHz) spectra were
recorded on a Varian NMR spectrometer model Unity INOVA,
using tetramethylsilane (TMS) as the internal standard. Fou-
rier transform infrared (FTIR) spectra were recorded on a
JASCO FTIR 460 plus spectrometer. Number-average molecu-
lar weight (Mn) and weight-average molecular weight (Mw)
were estimated by size-exclusion chromatography (SEC) on a
TOSOH HLC-8220 system equipped with three consecutive
polystyrene gel columns [TSK-gels (bead size, exclusion lim-
ited molecular weight); super-AW4000 (6 lm, >4 ꢀ 105),
super-AW3000 (4 lm, >6 ꢀ 104), and super-AW2500 (4
lm, >2 ꢀ 103)] and refractive index and ultraviolet detec-
1H NMR (in CDCl3) d 1.12–1.26 (3H, m, NCH2CH3), 2.08 (3H,
s, C(O)CH3), 3.33–3.53 (2H, m, NCH2CH3), 3.60–3.74 (2H, m,
NCH2CH2OAc), 4.11–4.36 (2H, m, NCH2CH2OAc), 4.60 (2H, s,
ArCH2Cl), 7.37 (2H, d, J ¼ 8.0, ArH), 7.43 (2H, d, J ¼ 8.0,
ArH).
ꢁ
tors at 40 C. The system was operated at a flow rate of 0.5
mL/min, using THF or a N, N-dimethylformamide (DMF) so-
lution of lithium bromide (10 mM) as an eluent. Polystyrene
standards were employed for calibration. Matrix-assisted
laser desorption/ ionization time-of-flight mass spectrometry
(MALDI-TOF MS) was carried out on a PerSeptive Biosystems
Voyager DE Pro Bio Spectrometry workstation. The samples
for MALDI-TOF MS were prepared as follows: A THF solution
of polymer (10 g/L), that of dithranol (20 g/L) and that of
sodium trifluoroacetate (10 g/L) were mixed in a volume ra-
tio of 2:1:10. From the resulting solution, 2 lL of it was
taken and dropped on a sample plate and was dried under
air. The sample was irradiated with 337-nm nitrogen laser.
The resulting ionized polymers were detected in a positive
mode at 20 kV.
Step 3
Potassium carbonate (6.29 g, 19.3 mmol), N, N-dimethylfor-
mamide (DMF; 14 mL), and carbon sulfide (1.47 g, 19.3
ꢁ
mmol) were mixed with heating at 50 C for 15 min. To the
resulting mixture, a solution of EACMB (4.97 g, 17.5 mmol)
in DMF (3.5 mL) was added at 50 ꢁC. The mixture was
stirred at the same temperature for 24 h, and then poured
into cold water (100 mL). The mixture was extracted with
ethyl acetate (100 mL) twice, and the combined organic
layers were washed successively with water (100 mL) and
saturated aqueous solution of sodium chloride. The ethyl ac-
etate solution was dried over sodium sulfate, filtered, and
then concentrated under reduced pressure. The resulting
residue was fractionated by silica gel column chromatogra-
phy with elution by chloroform/methanol (50/1) to obtain
bis[4-(ethyl-2 -(acetoxyethyl)aminocarbonyl)-benzyl]trithio-
carbonate (AcOTTC; 4.92 g, 8.13 mmol, 93%) as a yellow oil.
Synthesis of Trithiocarbonate 1 Bearing Hydroxyl
Groups (Scheme 1)
Step 1
To a solution of 2-(ethylamino)ethanol (8.91 g, 100 mmol) in
THF (50 mL), 4-chloromethylbenzoyl chloride (4.73 g, 25.0
mmol) was added dropwise with maintainiꢁng the tempera-
ture of the solution in a range from 0 to 10 C. The resulting
solution was allowed to warm to ambient temperature with
stirring. After 4 h, the volatiles were removed under reduced
pressure. To the resulting residue, cold water (50 mL) and
35% hydrochloric acid (5 mL) were successively added to
acidify the aqueous layer. This mixture was extracted with
ethyl acetate (100 mL) twice, and the combined organic
layers were washed with saturated aqueous solution of
sodium chloride. The resulting ethyl acetate solution was
dried over sodium sulfate, filtered, and then concentrated
under reduced pressure to obtain N-ethyl-N-(2-hydrox-
yethyl)-4-(chloromethyl)benzamide (EHCMB; 5.95 g, 24.6
mmol, 98%) as a colorless oil.
1H NMR (in CDCl3) d 1.12–1.25 (6H, m, NCH2CH3), 2.08 (6H,
s, C(O)CH3), 3.32–3.52 (4H, m, NCH2CH3), 3.59–3.74 (4H, m,
NCH2CH2OAc), 4.11–4.35 (4H, m, NCH2CH2OAc), 4.64 (4H, s,
ArCH2S), 7.33 (4H, d, J ¼ 8.5, ArH), 7.38 (4H, d, J ¼ 8.5,
ArH).
Step 4
To a solution of AcOTTC (3.02 g, 4.99 mmol) in THF (60
mL), 1 M aqueous solution of potassium hydroxide (30 mL)
was added dropwise for 30 min at 0 ꢁC. The mixture was
allowed to warm to ambient temperature and was stirred
for 4 h. Then, the mixture was diluted with water (50 mL)
and extracted with dichloromethane (100 mL) twice. The
combined organic layers were dried over sodium sulfate, fil-
tered, and concentrated under reduced pressure. The result-
ing residue was fractionated by silica gel column chromatog-
raphy with elution by chloroform/methanol (30/1) to obtain
bis[4-(ethyl-2-(hydroxyethyl)aminocarbonyl)-benzyl]trithiocar-
bonate (1; 2.25 g, 4.32 mmol, 87%) as a yellow solid: Mp ¼
1H NMR (in CDCl3) d 1.15 (3H, m, NCH2CH3), 3.33 (2H, m,
NCH2CH3), 3.59 (1H, s, OH), 3.70 (2H, m, NCH2CH2OH), 3.89
(2H, m, NCH2CH2OH), 4.60 (2H, s, ArCH2Cl), 7.40–7.45 (4H, m,
ArH4).
Step 2
To a solution of crude EHCMB (4.83 g, 20.0 mmol) in toluene
(64 mL), triethylamine (3.04 g, 30.0 mmol) and 4 -(dimethy-
lamino)pyridine (171 mg, 1.40 mmol) were added. To the
resulting solution, acetyl chloride (2.35 g, 37.6 mmol) was
ꢁ
121.8–122.5 C.
1H NMR (in CDCl3) d 1.16 (6H, br m, NCH2CH3), 3.34 (4H, br
m, NCH2CH2OH), 3.60 (2H, br s, OH), 3.69 (4H, br m,
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SCHEME 1 Synthesis of trithiocarbonate 1 bearing hydroxyl groups.
NCH2CH2OH), 4.70 (4H, s, ArCH2S), 7.36 (8H, br s, ArH); 1H
NMR (in DMSO-d6, at 120 ꢁC) d 1.11 (6H, t, J ¼ 7.1,
NCH2CH3), 3.40 (8H, br m, NCH2CH2OH), 3.57 (4H, br m,
NCH2CH3), 4.74 (4H, s, ArCH2S), 7.33 (4H, d, J ¼ 7.9, ArH),
Polyaddition of 1a with Diisocyanate (Scheme 4)
A typical procedure: Polystyrene 3a, of which Mn and Mw/
Mn were 4600 and 1.17, respectively, was used. To a solution
of polystyrene 3a (651 mg, the amount of hydroxyl group
calculated by using Mn value ¼ 0.65 mmol) in chlorobenzene
(2 mL), dibutyltin dilaurate (10 mg) and 4,40-methylenebi-
s(phenyl isocyanate) (82.0 mg, 0.328 mmol) in chloroben-
zene (1.6 mL) were successively added, and the resulting
solution was stirred at 50 ꢁC. After 48 h, the solution was
poured into methanol (30 mL). The resulting precipitate was
collected by filtration with suction, dried under vacuum with
heating at 40 ꢁC to obtain the corresponding polyurethane
5b (601 mg).
7.42 (4H, d, J ¼ 7.9, ArH);13C NMR (in DMSO-d6, at 120 C)
ꢁ
d 12.49 (NCH2CH3), 39.87 (NCH2CH3), 41.34 (SCH2Ar), 48.22
(NCH2CH2OH), 58.55 (NCH2CH2OH), 126.17 (Ar), 128.23
(Ar), 135.47 (Ar), 136.33 (Ar), 169.66 (ArC(O)N), 221.85
(SC(S)S).
RAFT Polymerization (Scheme 2)
A typical procedure: A mixture of 1 (104 mg, 0.200 mmol),
2,20-azobis[2-methyl-N-(2-hydroxyethyl)propionamide] 2 (290
mg, 1.00 mmol), chlorobenzene (40 mL), and styrene (10.4 g,
100 mmol) was placed in a flask, degassed under vacuum for
1 min, and flashed with nitrogen. The resulting homogeneous
RESULTS AND DISCUSSION
Synthesis of Trithiocarbonate 1 Bearing Hydroxyl
Groups
ꢁ
mixture was heated at 110 C for 24 h. After cooling, the mix-
ture was poured into methanol (2.0 L). The resulting precipi-
tate was collected by filtration with suction and dried under
vacuum with heating at 40 ꢁC to obtain the corresponding
polystyrene 3a (10.1 g).
Scheme 1 shows the route to trithiocarbonate 1, which
involves a reaction step for the formation of a trithiocarbon-
ate moiety by a condensation reaction of a benzyl chloride
derivative with carbon disulfide that can be mediated by car-
bonates of alkali metals.26 As the starting material, 4-(chloro-
methyl)benzoyl chloride was chosen. To its acid chloride
moiety, the amino group of 2-(N-ethylamino)-1-ethanol
reacted selectively to afford the corresponding amide bearing
hydroxyl group. The hydroxyl group thus introduced was
acetylated to give the corresponding benzyl chloride deriva-
tive bearing amide function. Treatment of this benzyl
chloride derivative with carbon disulfide in the presence of
potassium carbonate resulted in the selective formation of
the corresponding trithiocarbonate, of which acetate moieties
were hydrolyzed selectively under basic conditions to obtain
Block Copolymerization (Scheme 3)
A typical procedure: Polystyrene 3a (840 mg), EA (1.00 g,
10.0 mmol), and diazo compound 2 (29 mg, 0.10 mmol)
were dissolved in chlorobenzene (4 mL). The solution was
placed in a glass ampoule, which was sealed off under vac-
uum. The solution was heated at 90 ꢁC for 24 h. After cool-
ing the mixture to ambient temperature, the solution was
poured into hexane (200 mL). The resulting precipitate was
collected by filtration with suction, dried under vacuum with
heating at 40 ꢁC to obtain the corresponding block copoly-
mer 4a (1.75 g).
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TABLE 2 RAFT Polymerization of St with Varying
Polymerization Timea
Mn (Mw/Mnc) of
c
Time (h)
Conversion (%)b
Crude Polymer
3
32
47
65
72
82d
2,000 (1.24)
2,400 (1.23)
3,200 (1.21)
4,100 (1.18)
3,500d (1.26d)
6
9
12
24
a
Conditions: [St]0:[1]0:[2]0 ¼ 100:2:1; 2.5 M in PhCl, 110 ꢁC for 24 h; in a
sealed tube.
b
Determined by 1H NMR.
c
Estimated by SEC.
d
The data are same as those shown in Table 1, Run 2.
weight (Mn) was 4400, was obtained in a reasonable yield. Its
polydispersity index (Mw/Mn) was 1.14, which was enough
low to demonstrate the ability of trithiocarobonate 1 to control
the polymerization. It is noteworthy that the polymerization
was up-scaled easily: In Run 3, with using 100 mmol of St, the
polymerization was carried out in a flask under nitrogen
atmosphere, while 10 mmol of St was polymerized in a sealed
tube in the other runs. As a result, more than 8 g of the corre-
sponding polymer with a reasonably low polydispersity index
was obtained. On the other hand, lower polymerization tem-
perature lead to the insufficient monomer conversion (Entry
1) and the higher initial concentrations lead to less precise
control in molecular weight and thus broader distribution
(Entries 4 and 5). Then, the polymerization under the condi-
tions in Run 2 was investigated further in detail with varying
polymerization time. The corresponding monomer conversions
and Mn as well as Mw/Mn of the resulting polymers are shown
in Table 2 with using which the dependences of Mn and Mw/
Mn on monomer conversion and first-order kinetic plots were
visualized in Figure 1(a,b), respectively. Figure 1(a,b) tells us
that the polymerization was well-controlled until 6 h, where
monomer conversion reached to ꢂ70%.
SCHEME 2 RAFT polymerization of vinyl monomers with
using 1.
the target trithiocarbonate 1 bearing hydroxyl groups. All
the reactions that composed this route did not require spe-
cial caution to oxygen and moisture, and proceeded almost
quantitatively. The overall yield starting from 4 -(chlorome-
thyl)benzoyl chloride was 73%.
RAFT Polymerization of Vinyl Monomers
With a new trithiocarbonate bearing hydroxyl groups at hand,
RAFT polymerization of two vinyl monomers, styrene (St) and
EA, was investigated (Scheme 2). As a radical source, azo com-
pound 2 was used, because the introduction of its fragment
into the initiating end can also endow the formed polymer
with hydroxyl terminal. The conditions for the RAFT polymer-
ization of St and the corresponding results are summarized in
Table 1. In all the experiments, the feed ratio [St]0:[1]0:[2]0
was 100:2:1. The optimum conditions were those for Run 2,
where the initial concentration of St, solvent, and temperature
were 2.5 M, chlorobenzene and 110 ꢁC, respectively. The
resulting polystyrene, of which number average molecular
Next, RAFT polymerization of EA with using trithiocarbonate
1 was investigated. The results are summarized in Table 3.
TABLE 1 RAFT Polymerization of Styrene (St) Under Various Conditionsa
Isolated Polymer
(Methanol-Insoluble Parts)
c
c
Conversion
of St (%)b
Mn, Mw/Mn of
c
c
Run
Conditions
Crude Polymer
Yield (%)
Mn (Mw/Mn
)
1
2.5 M in PhCl, at 90 ꢁC
2.5 M in PhCl, at 110 ꢁC
2.5 M in PhCl, at 110 ꢁC
34
82
85
ND
18
2,200 (1.17)
4,400 (1.14)
4,500 (1.21)
2
3d
3,500, 1.26
3,600, 1.39
75
81 (70)e
[4,600 (1.17)]e
4,200 (1.26)
3,400 (1.47)
4
5
a
5.0 M in PhCl, at 110 ꢁC
In bulk, at 110 ꢁC
92
90
3,500, 1.36
NDf
82
87
e
f
Conditions: [St]0:[1]0:[2]0 ¼ 100:2:1; for 24 h; in a sealed tube.
Determined by 1H NMR.
Reprecipitation was done twice.
Not determined.
b
c
Estimated by SEC (eluent ¼ THF, calibrated with polystyrene standards).
100 mmol of St was used.
d
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TABLE 4 RAFT Polymerization of EA with Varying
Polymerization Timea
c
Conversion
(%)b
Mn (Mw/Mnc) of
Time (h)
crude polymer
1.5
3
11
44
67
80
97d
1,300 (1.12)
2,900 (1.11)
3,600 (1.16)
4,500 (1.21)
5,300 (1.17d)
4.5
6
24
a
FIGURE 1 (a) Dependences of Mn and Mw/Mn on conversion of
monomer and (b) first-order kinetic plots for the RAFT poly-
merization of styrene with using trithiocarbonate 1.
Conditions: [EA]0:[1]0:[2]0 ¼ 100:2:1; 2.5 M in PhCl, 90 ꢁC for 24 h; in a
sealed tube.
b
Determined by 1H NMR.
c
Estimated by SEC.
d
The data are same as those shown in Table 3, Run 1.
Because of the intrinsically higher polymerization ability of EA
than St, the polymerization was performed at a lower tempera-
ture, 90 ꢁC. The other parameters such as feed ratio, initial
concentration of monomer, solvent, and polymerization time,
were same as the optimum ones for the polymerization of St.
In Run 1, EA was consumed almost quantitatively, leading to
the successful formation of the corresponding polymer with a
low polydispersity index. In Run 2, the polymerization was up-
scaled with using 100 mmol of EA and performing the poly-
merization not in a sealed tube but in a flask filled with nitro-
gen gas. The corresponding polymer with a reasonably low
polydispersity index was obtained in an excellent yield. On the
other hand, the polymerization in bulk was poorly controlled
as is indicated by the higher polydispersity index. The time-
dependences of monomer conversion, Mn, and Mw/Mn in the
polymerization of EA under the conditions in Run 1 was inves-
tigated and summarized in Table 4. With using those values,
the dependence of Mn on monomer conversion and first-order
kinetic plots were visualized in Figure 2(a,b), respectively. Fig-
ure 2(a,b) shows that the polymerization was controlled
throughout by the RAFT mechanism.
able to the chain ends were clearly observed. Their chemical
shifts were almost same as those observed in Figure 3(a), to
imply that the fragments of trithiocarbonate 1 were intro-
duced into the chain ends of the polymers. There was no
clear signal for the CH adjacent to the trithiocarbonate cen-
ter. It may be due to its serious broadening. The signals
appeared below 1 ppm can be attributable to the methyl
groups originated from the initiator.
Figure 4 shows the MALDI-TOF mass spectrum of poly
(styrene) 3a. In the spectrum, a series of signals (Series A)
with a regular interval was observed: The m/z values for the
signals were in good agreement with those calculated based on
the equation, (formula weight of terminal structure) ꢀ 2 þ
(formula weight of styrene) ꢀ n þ (formula weight of cen-
tral trithiocarbonate) þ (atomic weight of H), implying that
the polymers inherited the central trithiocarbonate and the
terminal hydroxyl groups from the RAFT agent 1 and were
ionized in a protonated form. Besides, there was another
series of signals (Series B), of which mass numbers were
agreed with the polymer structure that can be expected from
the RAFT polymerization of styrene mediated by 1. The sig-
nals in Series B shifted from those in Series A in about 22
Da toward higher molecular weight, implying that the poly-
mers were ionized with bearing sodium cation. In addition
to Series A and Series B, there were other series of small sig-
nals. Although the corresponding terminal structures were
not characterized fully, the occurrence of these signals would
Chemical Structures of the Obtained Telechelic Polymers
1
The H NMR spectra of the obtained telechelic poly(styrene)
3a and poly(EA) 3b are shown in Figure 3(b,c), respectively.
These spectra were measured in CDCl3 at ambient tempera-
ture. In addition, the spectrum of trithiocarbonate 1, which
was measured under the same conditions, is shown in Figure
1(a). In Figure 3(b, c), besides the major signals attributable
to the polymer main chains and side chains, those attribut-
TABLE 3 RAFT Polymerization of EA Under Various Conditionsa
Isolated Polymer
(Methanol-Insoluble Parts)
c,
c
Conversion
of St (%)b
Mn Mw/Mn
c
c
Run
Conditions
of Crude Polymer
Yield (%)
Mn (Mw/Mn
)
1
2d
2.5 M in PhCl, 90 ꢁC
2.5 M in PhCl, 90 ꢁC
In bulk, 90 ꢁC
97
5,300, 1.17
4,200, 1.21
NDe
82
95
92
5,500 (1.15)
4,600 (1.19)
5,100 (1.37)
>99
96
3
a
d
e
Conditions: [EA]0/[1]0/[2]0 ¼ 100/2/1; for 24 h; in a sealed tube.
Determined by 1H NMR.
100 mmol of EA was used.
Not determined.
b
c
Estimated by SEC (eluent ¼ THF, calibrated with poly(methyl methac-
rylate) standards).
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FIGURE 2 (a) Dependences of Mn and Mw/Mn on conversion of
monomer and (b) first-order kinetic plots for the RAFT poly-
merization of EA with using trithiocarbonate 1.
be due to some fragmentation reactions during ionization in
the MALDI-TOF mass analysis.27–29
Two Stage Polymerization and Block Copolymerization
To prove the presence of the trithiocarbonate moiety at the
center of the telechelic polymers 3a and 3b, their potential
FIGURE 4 MALDI-TOF mass spectrum of 3 a obtained by the
RAFT polymerization of styrene with using 1.
as macro chain transfer agents (macro CTA) was investigated
(Scheme 3). To poly(styrene) 3a (Mn ¼ 3600, Mw/Mn
¼
1.39), styrene (50 equiv) and diazo compound 2 (0.5 equiv)
were added and heated at 110 ꢁC. As a result, a chain
extended poly(styrene) 3a0 was successfully obtained. As
shown in Figure 5(a), its SEC profile was unimodal, to prove
the trithiocarbonate moiety of 3a mediated the RAFT poly-
merization of styrene. A similar experiment with using pol-
y(EA) 3b as a macro CTA gave the corresponding chain
extended poly(EA) 3b0. However, as shown in Figure 5(b), its
Mw/Mn was much larger than that of 3b, to imply the pres-
ence of some mechanisms that avoided the perfectly RAFT-
controlled polymerization of EA.
FIGURE 3 1H NMR spectra (in CDCl3) of (a) trithiocarbonate 1,
(b) poly(styrene) 3a, and (c) poly(EA) 3b.
SCHEME 3 Two-stage
polymerizations
and
block
copolymerizations.
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ties of the polymers drastically. There are two approaches to
such segmented polymers: The first one relies on (1) synthe-
sis of prepolymers bearing homolytically dissociable bonds
in the main chain by step-growth polymerizations followed
by (2) radically induced insertion polymerizations of vinyl
monomers into the main chain.24,30–36 An interesting
approach to prepolymers based on a radical system involving
a ring-opening reaction of a cyclic trithiocarbonate has been
also reported.37 The second strategy relies on (1) synthesis
of well-defined telechelic polymers followed by (2) chain
extension of the telechelic polymers by their step-growth
polymerizations through reactions of the polymer terminals
with appropriate reagents.38–49 The telechelic polymers
obtained in the present work are potentially adoptable
for the second strategy. To prove the feasibility of this idea,
polyaddition of telechelic polystyrene 3a (Mn ¼ 4600, Mw/
FIGURE 5 SEC profiles for the polymers in the chain extension
experiments: (a) Polymerization of St with using a poly(St)
bearing a trithiocarbonate center; (b) polymerization of EA with
using a poly(EA) bearing a trithiocarbonate center.
Mn
¼
1.17) and several diisocyanates was performed
(Scheme 4). The total amount of the hydroxyl groups was
calculated based on the SEC-estimated Mn, and the total
amount of isocyanate groups was adjusted to it. Figure 7
shows the SEC profiles of the formed polyurethanes 5, which
proved the successful chain extension through the polyaddi-
tion with using the terminal hydroxyl groups of 3a as the re-
active sites. The IR spectrum clarified the successful forma-
tion of urethane linkage (Figure 8). Table 5 summarizes the
yields of the polyurethanes and their Mn and Mw/Mn values.
Even though these polyaddition experiments were carried
out under nonoptimized conditions, Mw exceeded 14.9 ꢀ
105, which was about 30 times larger than that of 3a. Opti-
mization of feed ratio between 3 and diisocyanate, concen-
tration, solvent, and temperature would allow more efficient
chain extension to give polyurethanes with larger molecular
weights.
By combining 3a and EA, an ABA-type block copolymer was
synthesized. Figure 6(a) shows the SEC profiles of 3a and
the resulting copolymer 4a. The latter indicated a shoulder,
to imply a part of 3a did not participate in the block copoly-
merization. RAFT polymerization of St with using poly(EA)
3b as a macro CTA gave the corresponding ABA-type block
copolymer 4b. As shown in Figure 6(b), its SEC profile was
unimodal and the corresponding Mw/Mn was reasonably low.
Although there is no further analytical data for the polymer
structure, the relatively successful copolymerization experi-
ments implied that the obtained block copolymers 4a and
4b would have inherited the central trithiocarbonate and the
terminal hydroxyl groups from the macro CTAs 3a and 3b.
Polyaddition of Polymeric Diol 3 and Diisocyanates
Segmented polymers, which are consisted of well-defined
polymer segments and functional moieties connecting the
polymer segments, have been targets of polymer synthesis.
The functional moieties are regularly located in the polymer
main chain with defined intervals, to modulate the proper-
SUMMARY
Telechelic vinyl polymers bearing hydroxyl groups at both
the chain ends were precisely synthesized by RAFT polymer-
ization of vinyl monomers with using a trithiocarbonate
FIGURE 6 SEC profiles for the polymers in the block copolymerization experiments: (a) Polymerization of EA with using a poly(St)
bearing a trithiocarbonate center; (b) polymerization of St with using a poly(EA) bearing a trithiocarbonate center.
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SCHEME 4 Polyaddition of 3a and diisocyanates.
FIGURE 7 SEC profiles for the polyurethanes obtained by poly-
addition of 3a with diisocyanates.
FIGURE 8 IR spectrum of the polyurethane obtained by the polyaddition of 3a with TDI.
bearing hydroxyl group. The RAFT system allowed chain
extension of polymers and synthesis of block copolymers,
and thus is expected to be potentially applicable to tailored
synthesis of telchelic vinyl polymers with properties adjusted
specifically to various applications. As was demonstrated by
their successful application to polyaddition with diisocya-
nates, the versatility of hydroxyl group as a reactive site
would promise their usefulness as building blocks for con-
structing a wide range of macromolecular architectures.
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