Synthesis of cyclic polymers and block copolymers by monomer insertion into cyclic initiator by a radical mechanism

Article abstract of DOI:10.1021/ma021371y

Cyclic poly(methyl acrylate) with controlled ring size and narrow ring size distribution was successfully prepared by the ⁶⁰Co γ-ray-induced polymerization of methyl acrylate at -30 °C in the presence of cyclic initiator 3. GPC, ¹H N

Full text of DOI:10.1021/ma021371y

5960
Macromolecules 2003, 36, 5960-5966
Synthesis of Cyclic Polymers and Block Copolymers by Monomer
Insertion into Cyclic Initiator by a Radical Mechanism
Ta o He, Gen -Hu a Zh en g, a n d Ca i-yu a n P a n *
Department of Polymer Science and Engineering, University of Science and Technology of China,
Hefei, Anhui, P.R. China 230026
Received August 22, 2002; Revised Manuscript Received May 28, 2003
ABSTRACT: Cyclic poly(methyl acrylate) with controlled ring size and narrow ring size distribution was
successfully prepared by the 60Co γ-ray-induced polymerization of methyl acrylate at -30 °C in the presence
of cyclic initiator 3. GPC, 1H NMR, and MALDI-TOF confirmed the cyclic structure of the obtained
polymers. With the same method and using cyclic PMA as macroinitiator instead of cyclic initiator 3,
amphiphilic cyclic block copolymers poly(N-isopropylacrylamide-b-methyl acrylate-b-N-isopropylacryla-
mide)s were obtained. The structures and compositions of the block copolymers obtained were investigated
by GPC and NMR measurements. The living nature of the polymerization was supported by the linear
evolution of molecular weight with conversion, constant concentration of chain radicals, and narrow
molecular weight distribution. Thus, ring sizes and narrow distribution of cyclic polymers and cyclic block
copolymers can be controlled.
In tr od u ction
fore, elaborate purification procedures are required to
remove the acyclic contaminant. In addition, it is
difficult to prepare the amphiphilic cyclic block copoly-
mers using the coupling method. When they are pre-
pared by end-to-end ring closure reaction of amphiphilic
diblock copolymers, it may be difficult for two end
groups to encounter each other due to the incompat-
ibility of the two blocks. Therefore, the synthesis of cyclic
polymers and block copolymers with narrow ring size
distribution and controlled ring size and structure is
exceedingly challenging.
Cyclic polymers have novel properties different from
their corresponding linear polymers.1-6 Compared to
other polymers with special architectures, such as star
polymers, hyperbranched polymers, and dendrimes, the
synthetic routes to cyclic polymers are less investigated.
Cyclic oligomers have been known as minor byproducts
in step-growth polymerizations due to ring-chain equi-
librium reactions7 and also in ring-opening polymeriza-
tions through backbiting reactions.8,9 A series of cyclic
oligo(alkylidene isophthalate)s have been prepared by
the cyclo-depolymerization of the corresponding linear
polymers in the presence of appropriate metal
catalysts.10-13 Colquhoun and co-workers14 developed
this method to prepare cyclic poly(ether sulfone)s con-
taining from eight to at least 60 aromatic rings. Another
method for preparation of cyclic polymers is through
end-to-end ring closure reactions of linear polymer
precursors in very low concentration.15-18 Recently, this
method is improved greatly.19-21 Ishizu et al. synthe-
sized cyclic poly(oxyethylene)s by Williamson etherifi-
cation.19 Tezuka et al.20 suggested a remarkably efficient
polymer cyclization method in which bifunctional poly-
(tetrahydrofuran)s (poly(THF)s) with terminal pyrroli-
dinium salt groups carrying dicarboxylate counteranions
were prepared by an electrostatic self-assembly and
covalent fixation strategy. Subsequent heat treatment
of this ionic polymer precursors afforded cyclic poly-
(THF)s. Lepoittain et al.21 prepared ring-shaped poly-
styrenes (PSt) by coupling two-end living PSt dianions
Recently, a new synthetic route to cyclic polyethylenes
has been developed, in which methylene groups were
repetitively inserted into the carbon-boron bond24 or
the ends of growing polymer chains remain attached to
a metal complex throughout the entire polymerization
process.25 However, these methods cannot be applied in
the polymerization of conventional monomers, such as
styrene (St) and methyl (meth)acrylates. In this paper,
we report a new synthetic strategy for the synthesis of
cyclic polymers and block copolymers by monomer
insertion into a cyclic initiator, and the cyclic poly-
(methyl acrylate) (PMA) and cyclic triblock copolymer
have been synthesized.
Exp er im en ta l Section
Ma ter ia ls. o-Bromophenol (The First Shanghai Chemical
Reagent Plant, >99%), 1,4-dibromobutane (Shanghai Chemical
Reagent Co., 99.5%), and R,R′-dibromo-p-xylene (ACROS
Organics) were used as received. Tetrahydrofuran (THF) was
refluxed for 24 h over sodium and distilled prior to use. MA
(Shanghai Chemical Reagent Co.) was dried over CaH2 and
distilled under reduced pressure prior to use. N-Isopropylacry-
lamide (NIPAAM, Kohjin Chemical Co. Ltd.) was recrystallized
from the mixture of cyclohexane and benzene (65/35, v/v) and
dried. All other reagents were of analytical grade and used as
received.
precursor with
1,3-bis(1-phenylethylenyl)benzene
(DDPE). To enhance the intramolecular over intermo-
lecular reactions, Deffieux et al.22,23 synthesized linear
R-A and ω-B heterodifunctional polymers, and then
intramolecular end-to-end cyclization reactions were
performed under high dilution. All of these preparations
are based on coupling reactions of telechelomers in
dilute solution. Incomplete cyclizations or undesirable
side reactions are common for the preparations; there-
Syn th esis of 1,4-Bis(o-br om op h en oxy)bu ta n e (1). Into
a solution of o-bromophenol (20.76 g, 0.12 mol) in acetone (100
mL), K2CO3 (20.7 g, 0.15 mol) was added, and the mixture was
stirred under a nitrogen atmosphere for 1 h. Then 1,4-
dibromobutane (21.6 g, 0.1 mol) was dropped. The mixture was
stirred at refluxing temperature for another 16 h and then
* To whom all correspondence should be addressed. E-mail:
pcy@ustc.edu.cn.
10.1021/ma021371y CCC: $25.00 © 2003 American Chemical Society
Published on Web 07/08/2003

Macromolecules, Vol. 36, No. 16, 2003
Cyclic Polymers and Block Copolymers 5961
cooled to room temperature. After most of acetone added was
removed, CH2Cl2 (30 mL) and water (50 mL) were added. The
organic layer was separated, and the aqueous phase was
extracted with CH2Cl2 three times (3 × 20 mL). The combined
extracts were washed with distilled water until neutral and
then dried over anhydrous magnesium sulfate overnight. After
the solvent was removed under reduced pressure, the crude
product was recrystallized from benzene, and then the pure
graph (GPC) equipped with Ultrastyragel columns (500, 103,
104 Å) at 30 °C, using monodisperse polystyrene as calibration
standard. THF was used as eluent at a flow rate of 1.0 mL/
min. MALDI-TOF mass spectra of PMA were performed on
a Bruker BIFLEX III equipped with a nitrogen laser (337 nm).
The accelerating potential is 20 kV. Matrix (20 mg, 2,5-
dihydroxybenzoic acid) and 1 mg of PMA were dissolved in
0.5 mL of THF. A 1 mL portion of the solution was deposited
onto the sample target and allowed to dry in air at room
temperature. Internal standards (peptide derivatives) were
used to calibrate molecular weight.
1
product was obtained as white crystal (34.5 g, 90% yield). H
NMR (500 MHz, CDCl3) δ (TMS, ppm): 1.8 (4H, 2 OCH2CH2);
3.9 (4H, 2 OCH2CH2); 7.0-7.4 (aromatic protons).
Syn th esis of Cyclic In itia tor 3. The Grignard reagent of
dibromide 1 was prepared according to the method described
in ref 26. Magnesium (0.33 g, 13.6 mmol) in a 250 mL three-
necked flask was “activated” by purging purified nitrogen while
stirring, until the magnesium became gray-black in color. Then
THF (200 mL) was added, and compound 1 (2.727 g, 6.82
mmol) in THF (8 mL) was dropwise added in 1 h. The mixture
was warmed to 40 °C. Into the reaction mixture, carbon
disulfide (1.216 g, 16 mmol) was added in 30 min. After being
maintained at 40 °C for 4 h, R,R′-dibromo-p-xylene (1.8 g, 6.82
mmol) in THF (8 mL) was added slowly in 1 h. The temper-
ature was raised to 50 °C and maintained at this temperature
for 2 days. Ice water was added, the organic layer was
separated, and the water phase was extracted with diethyl
ether (total 500 mL). The extracts and organic phase were
combined, washed with water until neutral, and dried over
anhydrous magnesium sulfate. After removal of solvent, the
residue was purified on a silica column with dichloromethane/
petroleum ether (30-60 °C) (v/v) as eluent. The cyclic initiator
3 was obtained as red solid (1.94 g, 57.6% yield); mp 91.7 °C
(DSC measurement). 1H NMR (500 MHz, CDCl3), δ (TMS,
ppm): 6.87-6.97, 7.24-7.36 (8H, aromatic Hs), 4.68 (4H, 2S-
CH2), 3.92 (4H, 2O-CH2), 1.74 (4H, C-CH2CH2-C). Mass
spectrum for C26H24O2S4: 496.0660 (26.37, 496.0659), 360.0300
(25.47), 328.0609 (22.32), 264.1145 (41.91), 221.0586 (14.33),
189.0406 (26.15), 137.0058 (100), 108.0042 (35.61), 104.0638
(25.30).
Resu lts a n d Discu ssion
F or m a tion of Cyclic P olym er s a n d P olym er iza -
tion Mech a n ism . As we mentioned, the cyclic poly-
ethylenes were prepared by repetitive methylene inser-
tions into the carbon-boron bond of B-thexylborocane.24
In this paper, we also prepare cyclic polymers by
monomer insertion into a cyclic initiator. As shown in
Scheme 1, using heat, UV, or γ-ray irradiations, the
cyclic initiator is homolytically split into active and
stable radicals. The former radical initiates the polym-
erization of monomer, forming propagating radical. In
a local medium, the active chain radical propagates or
terminates with the original stable radical, and they are
competition reactions. When the propagation reaction
is too fast, it is difficult for the propagating radical to
react with the original stable radical because they
diffuse apart. Fortunately, it will not occur because the
rate constant (kt) for termination is 102-106 times that
(kp) for propagation (generally, kp ) 102-104 and kt )
106-108 L/(mol s); for methyl acrylate, kp ) 2.09 × 103
and kt ) 0.95 × 107 L/(mol s)).27 Therefore, after the
propagating radical reacts with several monomers, the
propagating chain will reversibly terminate with the
original stable radical. When the cyclic chain conforma-
tion is adjusted, this process will repeat again until a
cyclic polymer with predetermined ring size is formed.
Each initiating site has the same possibility to ho-
molytically split, propagate, and terminate; thus, the
ring size and its distribution can be controlled.
Although we have no ideas for designing a cyclic
initiator for atom transfer radical polymerization (ATRP),
we can design the cyclic initiator for nitroxide-mediated
radical polymerization. Unfortunately, the polymers
with complicated structure were obtained. Probably, the
active and stable radicals, which are produced from
homolytical decomposition of the cyclic initiator, diffuse
thermally too fast. As a result, the reversible termina-
tion reaction with the original stable radical is impos-
sible to occur. Obviously, decreasing the reaction tem-
perature is necessary for reducing the diffusion rate and
for suppressing the side reactions, such as chain-
transfer reactions. Therefore, we synthesize the cyclic
initiator 3 according to Scheme 2 and used it in the
polymerization of MA at -30 °C under 60Co irradiation,
since γ-ray-induced polymerization is easier to operate
at low temperature than with UV irradiation. For
comparison, a polymerization was also carried out at
30 °C. The conditions and results are listed in Table 1.
Considering the low active energy (29.7 kJ /mol) of
propagation reaction of MA,27 it is reasonable that the
yields of polymers prepared at -30 °C are not much
lower than those at 30 °C (see PMA1, PMA6, PMA3,
and PMA7 in Table 1).
P olym er iza tion of MA. The polymerization was carried
out in sealed tubes. The general synthetic procedure is as
follows. MA (0.8 g, 9.3 mmol), 3 (0.04 g, 0.08 mmol), and THF
(8 mL) were added into a 10 mL glass tube. After the mixture
was degassed by three freeze-evacuate-thaw cycles, the tube
was sealed under vacuum and then subjected to 60Co γ-ir-
radiation at 80 Gy/min for 4 h. The tube was opened, and the
polymer was precipitated by pouring a polymer solution in
THF into excess petroleum ether (30-60°C) while stirring,
obtained by filtration, and then dried in a vacuum at room
temperature overnight. The cyclic PMA was obtained in 52%
with Mn,NMR ) 5730 and Mw/Mn ) 1.32. 1H NMR (500 MHz,
CDCl3), δ (TMS, ppm): 7.35-7.18, 6.84 (aromatic Hs), 4.85
(S-CH-COOMe), 3.90 (Ph-O-CH2-), 3.62 (COOCH3),
2.71 (Ph-CH2-), 2.35 (-CH-COOMe), 2.12-1.35 (PhO-C-
CH2CH2-C-OPh, -CH2- in main chain).
Block Cop olym er iza tion of NIP AAM. Into a 10 mL glass
tube, the cyclic macroinitiator PMA (Mn,NMR ) 5730, 0.278 g,
0.0485 mmol), NIPAAM (0.5 g, 4.42 mmol), and THF (4 mL)
were added. The tube was sealed under vacuum after three
freeze-evacuate-thaw cycles and subjected to 60Co γ-irradia-
tion at 80 Gy/min for 4 h. The final polymer was obtained by
pouring the reaction mixture into excess petroleum ether (30-
60 °C) while stirring and dried in a vacuum overnight. The
cyclic PMA-b-PNIPAAM was obtained in 38% yield with
Mn,NMR ) 9600 and Mw/Mn ) 1.34. (MA)67-b-(NIPAAM)34. 1H
NMR (500 MHz, CDCl3), δ (TMS, ppm): 7.37-6.9 (aromatic
Hs), 6.38-5.6 (NH), 4.79 (S-CH-CO), 4.04 (N-CH(Me2)), 3.80
(Ph-O-CH2), 3.65 (COOCH3), 2.70 (Ph-CH2-), 2.31(-CH-
COOMe, -CH-CON-), 2.11-1.41 (PhO-C-CH2CH2-C-
OPh, -CH2- in main chain), 1.14 (CH3 in NIPAM).
Ch a r a cter iza tion . 1H NMR spectra were measured on a
Bruker DMX-500 nuclear magnetic resonance instrument with
CDCl3 as solvent and tetramethylsilane (TMS) as internal
reference. The molecular weight and polydispersity indexes
were determined on a Waters 150C gel permeation chromato-
Ch a r a cter iza tion of th e Cyclic P olym er s. For
determining cyclic structure of the polymers prepared

5962 He et al.
Macromolecules, Vol. 36, No. 16, 2003
Ta ble 1. Con d ition s a n d Resu lts of γ-r a y-In d u ced P olym er iza tion s of MA Usin g Cyclic In itia tor 3a
c
d
e
e
no.
temp (°C)
time (h)
yield b (%)
Mn,th × 10-3
Mn,NMR × 10-3
Mn,GPC × 10-3
Mw/Mn
PMA 1
PMA 2
PMA 3
PMA 4
PMA 5
PMA 6
PMA 7
-30
-30
-30
-30
-30
30
1
2
4
6
8
1
4
14
30
52
66
77
16
59
1.89
3.53
5.64
7.11
8.13
1.84
3.55
5.73
7.19
8.22
1.3
2.7
4.3
5.4
6.1
1.32
1.30
1.32
1.31
1.33
30
a
b
c
Polymerization conditions: [MA]/[3] ) 116/1 (molar ratio), MA: 0.8 g; dose rate ) 80 Gy/min. Calculated by weight method. Mn,th
) conv × ([M]0/[I]0) × 86 + 496, where [M]0 and [I]0 are the initial molar concentrations of MA and 3; 86 and 496 are the molecular
weights of MA and 3, respectively. Calculated by eq 1. e Determined by GPC relative to polystyrene.
d
Sch em e 1
Sch em e 2
F igu r e 1. GPC curves of (a) cyclic PMA prepared at -30 °C
(no. 2 in Table 1) and cyclic triblock copolymer, PNIPAAM-b-
PMA-b-PNIPAAM, prepared at -30 °C (no. 2 in Table 2); (b)
at -30 °C, their GPC traces were measured, and one
typical GPC curve of PMA prepared at -30 °C is shown
in Figure 1a. In principle, a single and symmetrical peak
indicates that the polymerization proceeds via one
the PMA prepared at 30 °C (no. 7 in Table 1).
molytical cleavage to form benzyl radical and a stable
sulfur radical. The former radical initiates the polym-
erization, forming the propagation chain, which termi-
nates with the original stable sulfur radical. The
polymerization continues to form cyclic polymers. If the
polymerization mechanism. On the basis of our previous
28-31
investigations,
under 60Co irradiation, the weak
bond, C-S bond, in cyclic initiator 3 undergoes ho-

Macromolecules, Vol. 36, No. 16, 2003
Cyclic Polymers and Block Copolymers 5963
Sch em e 3
Sch em e 4
propagating radical is far from the original stable sulfur
radical, coupling reactions and/or termination reactions
with impurities in the polymerization system may
compete to form a linear polymer, as shown in Scheme
3.
large broad peak and a small peak. This is consistent
with the polymers formed at -30 °C being cyclic because
increasing the temperature will enhance the linear
polymerization rate, forming more linear polymers.
For determining further the cyclic structure of the
polymer, 1H NMR spectra of the cyclic PMA were
measured, and a typical spectrum is shown in Figure
2b. Except for characteristic peaks of PMA: δ ) 1.4-
1.9 (a), 2.3 (b), and 3.6 (c), the signal at δ ) 4.8 (d)
corresponds the methine proton of MA unit next to
sulfur, the peak of the methlyene protons between
benzene and the sulfur groups at δ ) 4.7 in Figure 2a
is completely disappeared, and moves to δ ) 2.74 (f) in
Figure 2b. This suggests that all the cyclic initiator 3
participated in the initiation. In addition, the signal
appeared at δ ) 3.9 (e) indicates the existence of
methylene groups next to ether oxygen in the 3 unit.
The integration ratio of the peaks at δ ) 4.8, 3.9, and
2.74 is 1/2/2, which is a strong evidence to support that
the cyclic initiator is completely transferred to the
polymer. Comparison with the 1H NMR spectrum of the
PMA prepared at 30 °C in Figure 2C, the small signals
at δ ) 4.06 and 4.22 may represent two terminal
methine protons respectively next to alkoxy and hy-
droxyl groups; other small signals at δ ) 4.66, 5.29, and
5.90 are not clear now. All these signals must be related
to the ring opening of cyclic PMA to form linear
polymers. Assuming that each cyclic polymer contains
one cyclic initiator unit, the number-average molecular
When considering the chain-transfer reactions with
thiocarbonate group, as shown in Scheme 4, the poly-
dispersity index of the polymers obtained will become
broader as the conversion increases because the transfer
reaction of one propagating radical with one cyclic will
form one chain containing more than one initiator
residue. Further chain transfer reactions will produce
polymers with quite different molecular weights. The
higher the conversion, the higher the molecular weight
of the polymer obtained. The polydispersity index of the
polymers obtained from the chain-transfer reactions
becomes broader. The data listed in Table 1 show that
the molecular weight distributions remain low and
almost constant with increasing conversion. Thus, the
chain-transfer reactions shown in Scheme 4 are negli-
gible for the polymerization at -30 °C.
Although the linear polymerization may not produce
the polymer with narrow molecular weight distribution
(Scheme 3), its GPC curve may also show one single,
symmetric peak. To identify which mechanism occurred
for the polymerization at -30 °C, we carried out the
polymerization at 30 °C since increasing the tempera-
ture will increase the diffusion rate of the radicals and
favor the formation of linear polymers. The GPC curve
of the polymers obtained at 30 °C in Figure 1b shows a

5964 He et al.
Macromolecules, Vol. 36, No. 16, 2003
F igu r e 3. MALDI-TOF mass spectrum of cyclic PMA pre-
pared by γ-ray-induced polymerization of MA in the presence
of cyclic initiator 3 at -30 °C (no. 1 in Table 1). The molecular
weight of each peak, Mn ) 496 + (n × 86) + 1, where 496 and
86 are molecular weights of cyclic initiator and MA, 1 is the
atomic weight of H+, and n is the degree of polymerization.
the MALDI-TOF mass spectrum of PMA1 (PMA1 in
Table 1) and shows only one distribution consisting of
a number of peaks with a peak-to-peak mass increase
of 86 g mol-1, which corresponds to the molecular weight
of MA unit. This distribution could be assigned to the
cyclic structure of PMA: [3-(MA)n-H]+, in which 3 and
MA are cyclic initiator 3 and MA unit, respectively, and
n is the degree of polymerization. The molecular weight
of each peak can be calculated according to Mn ) 496 +
(n × 86) + 1. The calculated values agree well with the
corresponding experimental values, indicating that the
polymer contains only one cyclic initiator 3 unit. There-
fore, the pure cyclic PMA is ready formed. As we
mentioned, raising the temperature will increase the
diffusion rate of chain radicals, leading to the formation
of linear polymers. As a comparison, the γ-ray-induced
polymerization of MA was performed at 30 °C for 1 h,
and the typical MALDI-TOF MS of the polymer ob-
tained is shown in Figure 4. There are at least three
distributions: one is assigned as cyclic PMA, [3-(MA)n-
H]+; another two correspond to two linear PMAs,
[3-(MA)n-THF-H]+ and [3-(MA)n-H2O-H]+, which must
result from the termination reactions of linear propaga-
tion radicals with THF and H2O, respectively. When
comparing the linear PMA, 3-(MA)n-THF, the relative
amount of 3-(MA)n-H2O is very small because THF is a
solvent of the polymerization, but H2O is a small
amount of impurity in the polymerization system.
To confirm that the ring size of the polymers obtained
can be controlled and its distribution is narrow, it is
important to study whether the polymerization is of
“living” nature. The relationship between ln([M]0/[M])
and polymerization time at -30 °C is a straight line,
as shown in Figure 5a, indicating constant concentration
of propagating radicals during the polymerization. A
linear evolution of Mn with conversion as shown in
Figure 5b suggests that the molecular weight of cyclic
PMA obtained can be controlled by the molar ratio of
monomer/initiator in the feed and polymerization time.
Further evidence for the “living” nature is the narrow
molecular weight distribution (MWD) (Figure 5b) and
the single and symmetric peak (Figure 1a).
F igu r e 2. 1H NMR spectra of (a) cyclic initiator 3, (b) cyclic
PMA (no. 1 in Table 1), and (c) cyclic PNIPAAM-b-PMA-b-
PNIPAAM.
weight, Mn,NMR, can be calculated according to eq 1
Mn,NMR ) [(I3.6/3)/(I4.8/2)] × 86 + 496
(1)
where I3.6 and I4.8 are the integral values of the peaks
at δ ) 3.6 and 4.8; 86 and 496 are the molecular weights
of MA and 3, respectively. The Mn,NMRs of the polymers
obtained were calculated and are listed in Table 1. The
data listed in Table 1 demonstrate good agreement of
M
n,NMRs with theoretical number-average molecular
weight Mn,th, which cannot be resulted from linear
polymerization (see Scheme 3). In addition, it is reason-
able that M n,GPC are lower than M n,NMR or Mn,th (Table
1) because the cyclic polymers have more compact
structure in comparison with the corresponding linear
polymers having similar molecular weight. This is
another evidence of cyclic polymers formed.
A powerful evidence confirms that the polymers
formed at -30 °C is their MALDI-TOF mass spectra.
If the polymerization proceeds via monomer insertion
into cyclic initiator, the cyclic polymers obtained contain
only one cyclic initiator 3 unit. When the propagation
radical is far from the original sulfur radical, the linear
polymerization will occur; the resulting product should
have end groups except cyclic initiator 3. Figure 3 is
Syn th esis of Cyclic Tr iblock Cop olym er , P oly-
(NIP AAM-b-MA-b-NIP AAM). With the same proce-
dures of synthesizing cyclic PMA, the cyclic poly-

Macromolecules, Vol. 36, No. 16, 2003
Cyclic Polymers and Block Copolymers 5965
Ta ble 2. Con d ition s a n d Resu lts of Block Cop olym er iza tion of NIP AAM u n d er 60Co γ-r a y Ir r a d ia tion a t -30 °C Usin g
P MA3 a s Ma cr ocyclic In itia tor a
c
d
e
e
no.
time (h)
yieldsb (%)
Mn,th(copolymer) × 10-3
Mn,NMR(copolymer) × 10-3
Mn,GPC(copolymer) × 10-3
Mw/Mn
1
2
3
4
5
2
4
6
8
12
21
38
51
61
76
7.86
9.57
10.9
12.0
13.5
7.9
9.6
11.0
12.1
13.6
5.8
7.2
7.9
8.6
9.5
1.33
1.34
1.33
1.35
1.34
a
b
[NIPAAM]/[PMA3] ) 100 (molar ratio); NIPAAM ) 1 g, polymerization temperature: -30 °C; dose rate ) 80 Gy/min. Calculated
with the gravimetrical method. c Calculated based on Mn,th(copolymer) ) conversion × ([M]0/[I]0) × 103 + 5730, where [M]0 and [I]0 are the
initial concentrations of NIPAAM and macrocyclic initiator PMA3; 103 and 5730 are the molecular weights of NIPAAM and PMA3,
d
respectively. The Mn,NMR(copolymer)s were calculated by equation Mn,NMR(copolymer) ) [I4.1/(I4.8/2)] × 103 + 5730, where I4.1 and I4.8 are integral
values of peaks at δ ) 4.1 and 4.8, respectively. e Determined by GPC instrument.
F igu r e 5. Relationship of ln([M]0/[M]) vs polymerization time
(a) and Mn vs conversion (b) for polymerization of MA under
60Co irradiation at -30 °C in the presence of cyclic initiator 3.
F igu r e 4. MALDI-TOF mass spectrum of PMA prepared by
γ-ray-induced polymerization of MA in the presence of cyclic
initiator 3 at 30 °C (no. 6 in Table 1) Full scan (a). Part of full
scan (b). Cyclic PMA, horizontal oval: molecular weight of each
peak Mn ) 496 + (n × 86) + 1; linear polymer, square: Mn
)
496 + (n × 86) + 72 + 1, where 72 is molecular weight of THF;
vertical oval: Mn ) 496 + (n × 86) + 18 + 1, where 18 is the
molecular weight of H2O.
(NIPAAM-b-MA-b-NIPAAM) was prepared at -30 °C
using macrocyclic PMA as initiator instead of cyclic
initiator 3. The results and conditions are listed in Table
2.
For confirming the formation of triblock copolymers,
their 1H NMR spectra were measured, and a typical
spectrum is shown in Figure 6. The characteristic peaks
of PNIPAAM can be observed. The signal at δ ) 1.1 (a)
is ascribed to the methyl protons, the signal at δ ) 4.1
(b) belongs to the methine proton next to nitrogen, and
the broad peak at δ ) 5.8-6.5 (c) corresponds to N-H.
The signal of ester methyl in the PMA unit appears at
F igu r e 6. 1H NMR spectrum of poly(NIPAAM-b-MA-b-
NIPAAM) (no. 2 in Table 2).
δ ) 3.6 (g). These facts evidence the formation of triblock
copolymers. The signals at δ ) 4.8 (d) and 2.74 (h)
correspond to the methine proton next to sulfur and
methylene protons adjacent to the phenyl group from
initiator 3, respectively, and their integral ratio is 1/2.
This indicates that all the cyclic initiator 3 residues were
transferred into the block copolymers. The number-
average molecular weight Mn,NMR of PNIPAAM block
could be calculated on the basis of the integration ratio

5966 He et al.
Macromolecules, Vol. 36, No. 16, 2003
PMA as initiator. The key point for preparing cyclic
polymers is the reduced diffusion rate, which we ob-
tained at lower polymerization temperature, such as
-30 °C. The polymerization is proposed via repetitive
monomer insertion into the cyclic initiator. The γ-ray-
induced homopolymerization of MA in the presence of
cyclic initiator 3 and the block copolymerization of
NIPAAM with macrocyclic initiator are of “living”
nature. The ring size of cyclic polymers and block
copolymers can be controlled by the initial molar ratio
of monomer to initiator and polymerization time. The
ring size distribution is narrow.
Ack n ow led gm en t. The author is thankful for the
support of the National Nature Science Foundation of
China (Grant 50173025).
Refer en ces a n d Notes
(1) Gan, Y.; Dong, D.; Carlotti, S.; Hogen-Esch, T. E. J . Am.
Chem. Soc. 2000, 122, 2130.
(2) Cooke, J .; Viras, K.; Yu, G. E.; Yonemitsu, T.; Ryan, A. J .;
Price, C.; Booth, C. Macromolecules 1998, 31, 3030.
(3) Kusmartssev, F. V. Phys. Rev. 1992, 46, 7674.
(4) Iwata, K. Macromolecules 1985, 18, 115.
(5) Cherayil, B. J .; Bwendi, M. G.; Miyake, A.; Freed, K. F.
Macromolecules 1986, 19, 2770.
(6) Di Marzio, E. D. Macromolecules 1993, 26, 4613.
(7) Fawcett, A. H.; Mee, R. A. W.; McBride, F. W. Macromolecules
1995, 28, 1481.
F igu r e 7. Relationship of ln([M]0/[M]) vs polymerization time
(a) and Mn vs conversion (b) for block copolymerization of
NIPAAM under 60Co irradiation at -30 °C using PMA3 as
macrocyclic initiator.
(8) Clarkson, S. J .; Semlyen, J . A.; Horska, J .; Stepto, R. F. T.
of the peak at δ ) 4.1 to that at δ ) 4.8. The results are
listed in Table 2. The lower value of Mn,GPC (copolymer)
than that of Mn,NMR for the block copolymers is an
evidence that the triblock copolymers obtained are
cyclic, since cyclic polymers have compact structure in
comparison with the corresponding linear polymers.
The formation of cyclic polymers can be further
confirmed by the method similar to that of identifying
the cyclic PMA with GPC measurements. A typical GPC
curve of the block copolymer obtained at -30 °C is
shown in Figure 2a (no. 2 in Table 1) and demonstrates
a single and symmetrical peak. This might be resulted
from the cyclic polymerization.
Polymer 1986, 27, 31.
(9) Sigwalt, P.; Masure, M.; Moreau, M.; Bischoff, R. Macromol.
Chem. Rapid Commun. 1993, 73, 147.
(10) Hall, A. J .; Hodge, P.; McGrail, C. S.; Rickerby, J . Polymer
2000, 41, 1239.
(11) Bryant, J . J . L.; Semlyen, J . A. Polymer 1996, 38 2475.
(12) Hamilton, S. C.; Semlyen, J . A.; Haddleton, D. M. Polymer
1998, 39, 3241.
(13) Hubbard, P. A.; Brittain, W. J .; Mattice, W. L.; Brunelle, D.
J . Macromolecules 1998, 31, 1518.
(14) Colquhoun, H. M.; Lewis, D. F.; Hodge, P.; Ben-Haida, A.;
Williams, D. J .; Baxter, I. Macromolecules 2002, 35, 6875.
(15) Roovers, J .; Toporowski, P. M. Macromolecules 1983, 16, 843.
(16) Ohtani, H.; Kotsuji, H.; Momose, H.; Matsushita, Y.; Noda,
I.; Tsuge, S. Macromolecules 1999, 32, 6541.
The block copolymerization also shows the living
nature, which is supported by the following evidence:
the linear relationship between the polymerization time
and ln([M]0/[M]) (Figure 7a); the linear increase of
Mn,NIPAAM with the increasing NIPAAM conversion
(Figure 7b); the relatively low polydispersity indexes
(Mw/Mn ∼ 1.34). Thus, the ring size of the polymer
obtained can be controlled by monomer conversion. The
narrow distribution of polymer ring size might be
obtained.
The mechanism of the controlled cyclic block copo-
lymerization at -30 °C using the macrocyclic PMA
initiator under 60Co γ-irradiation is similar to that of
the homopolymerization of MA under the same condi-
tions; the key point for the synthesis of cyclic block
copolymers is reducing the diffusion rate, which can be
achieved by lowering temperature.
(17) Iatrou, H.; Hadjichristidis, N. Polym. Prepr. 1999, 40, 1059.
(18) Gan, Y. D.; Zo¨ller, J .; Yin, R.; Hogen-Esch, T. E. Macromol.
Symp. 1994, 77, 93.
(19) Ishizu, K.; Akiyama, Y. Polymer 1997, 38, 491.
(20) Oike, H.; Mouri, T.; Tezuka, Y. Macromolecules 2001, 34,
6592.
(21) Lepoittevin, B.; Douges, M. A.; Masure, M.; Hemery, P.;
Baran, K.; Cranail, H. Macromolecules 2000, 33, 8218.
(22) Rique-Lurbet, L.; Schappacher, M.; Deffieux, A. Macromol-
ecules 1994, 27, 6318.
(23) Deffieux, A.; Schappacher, M.; Rique-Lurbet, L. Polymer
1994, 35, 4562.
(24) Shea, K. J .; Lee, S. Y.; Busch, B. B. J . Org. Chem. 1998, 63,
5746.
(25) Bielawski, C. W.; Benitez, D.; Grubbs, R. H. Science 2002,
297, 2041.
(26) Moad, G.; Rizzardo, E.; Thang, S. H. PCT/US97/12540, 30.
(27) Odian, G. Principles of Polymerization, 3rd ed.; J ohn Wiley
& Sons: New York, 1991; pp 274-275.
(28) Bai, R. K.; You, Y. Z.; Pan, C. Y. Macromol. Rapid Commun.
2001, 22, 315.
Con clu sion
(29) You, Y. Z.; Bai, R. K.; Pan, C. Y. Macromol. Chem. Phys. 2001,
202, 1980.
(30) Bai, R. K.; You, Y. Z.; Zhong, P.; Pan, C. Y. Macromol. Chem.
Phys. 2001, 202, 1970.
(31) Hong, C. Y.; You, Y. Z.; Bai, R. K.; Pan, C. Y. J . Polym. Sci.,
Part A: Polym. Chem. 2001, 39, 3934.
The γ-ray-induced radical polymerization of MA at
-30 °C using cyclic initiator 3 yielded pure cyclic PMA
without specific purification procedure. The triblock
copolymers, PNIPAAM-PMA-PNIPAAM, were suc-
cessfully synthesized by the controlled γ-ray-induced
radical polymerization of NIPAAM with macrocyclic
MA021371Y