Design of biodegradable amphiphilic polymers: Well-defined amphiphilic polyphosphates with hydrophilic graft chains via ATRP

Article abstract of DOI:10.1021/ma049043g

Novel biodegradable amphiphilic polymers composed of Hydrophobic polyphosphate grafted with well-defined hydrophilic poly[2-methacryloyloxyethyl phosphorylcholine (MPC)] were newly synthesized. 2-Isopropyl-2-oxo-1,3,2- dioxaphospholane and 2-(2-oxo-1,3,2-dioxaphosphoroyloxyethyl-2-bromoisobutylate) (OPBB) were copolymerized by ring-opening polymerization using triisobutylaluminum as an initiator. Polyphosphates (PIBr) whose molecular weights were 3.4× 10⁴ g/mol with 3.0 units of OPBB and 3.7× 10⁴ g/mol with 10.5 units of OPBB were obtained. MPC was grafted from the OPBB sites in PIBr via atom transfer radical polymerization (ATRP). The molecular weights of the graft copolymers [PIBr-g-poly(MPC) (PMPC)] and the amphiphilicity of the graft copolymers were well controllable with polymerization time. The solution properties of various amphiphilic PIBr-g-PMPCs were investigated by the methods of surface tension measurement, light scattering, and fluorescence probe. PIBr-g-PMPC showed high surface activity. The critical micelle concentration (cmc) of the surface tension of the PIBr-g-PMPCs increased with an increase in the molecular weight and density of the PMPC chain. Above the cmc of PIBr-g-PMPC with 3.0 units of PMPC per PIBr, the polymer associated and formed nanosize-hydrogels (nanogels) (R_g = 26.2 nm.) observed by a light scattering method and a fluorescence probe method. Degradation of the PIBr-g-PMPC was also investigated under various pH conditions. In a basic condition, the rate of hydrolysis pf the graft copolymer was quite large. The polyphosphate of PIBr-g-PMPC completely degraded after only 3 days. PIBr-g-PMPCs were novel biodegradable amphiphilic polymers.

Full text of DOI:10.1021/ma049043g

Macromolecules 2004, 37, 7637-7642
7637
Design of Biodegradable Amphiphilic Polymers: Well-Defined
Amphiphilic Polyphosphates with Hydrophilic Graft Chains via ATRP
Ya su h ik o Iw a sa k i* a n d Ka zu n a r i Ak iyosh i
Institute of Biomaterials and Bioengineering, Tokyo Medical and Dental University,
2-3-10 Kanda-surugadai; Chiyoda-ku, Tokyo 101-0062, J apan
Received May 14, 2004; Revised Manuscript Received J uly 20, 2004
ABSTRACT: Novel biodegradable amphiphilic polymers composed of hydrophobic polyphosphate grafted
with well-defined hydrophilic poly[2-methacryloyloxyethyl phosphorylcholine (MPC)] were newly syn-
thesized. 2-Isopropyl-2-oxo-1,3,2-dioxaphospholane and 2-(2-oxo-1,3,2-dioxaphosphoroyloxyethyl-2-bro-
moisobutylate) (OPBB) were copolymerized by ring-opening polymerization using triisobutylaluminum
as an initiator. Polyphosphates (PIBr) whose molecular weights were 3.4 × 104 g/mol with 3.0 units of
OPBB and 3.7 × 104 g/mol with 10.5 units of OPBB were obtained. MPC was grafted from the OPBB
sites in PIBr via atom transfer radical polymerization (ATRP). The molecular weights of the graft
copolymers [PIBr-g-poly(MPC) (PMPC)] and the amphiphilicity of the graft copolymers were well
controllable with polymerization time. The solution properties of various amphiphilic PIBr-g-PMPCs were
investigated by the methods of surface tension measurement, light scattering, and fluorescence probe.
PIBr-g-PMPC showed high surface activity. The critical micelle concentration (cmc) of the surface tension
of the PIBr-g-PMPCs increased with an increase in the molecular weight and density of the PMPC chain.
Above the cmc of PIBr-g-PMPC with 3.0 units of PMPC per PIBr, the polymer associated and formed
nanosize-hydrogels (nanogels) (Rg ) 26.2 nm) observed by a light scattering method and a fluorescence
probe method. Degradation of the PIBr-g-PMPC was also investigated under various pH conditions. In
a basic condition, the rate of hydrolysis of the graft copolymer was quite large. The polyphosphate of
PIBr-g-PMPC completely degraded after only 3 days. PIBr-g-PMPCs were novel biodegradable amphiphilic
polymers.
In tr od u ction
reported, including ring-opening polymerization,25,26
polycondensation,27 transesterfication,28,29 and enzy-
matic polymerization.30 Novel biodegradable amphiphilic
polymers with polyphosphates can be then obtained
because the solubility of polyphosphates can be con-
trolled with the structures of their side chains.
Recently, we have synthesized polyphosphates bear-
ing isopropyl and methacryloyl groups.31 These poly-
phosphates are soluble in ethanol, tetrahydrofuran
(THF), and chloroform, but not in water. In the synthe-
sis of amphiphilic polymers in this study, polyphos-
phates were used as hydrophobic polymers and water-
soluble poly[2-methacryloyloxyethyl phosphorylcholine
(MPC)] was selected as the hydrophilic graft chain.
We have been studying MPC polymers synthesized
as biomimetics in biomembrane structures.32-35 The
MPC polymers exhibit a property that resists nonspe-
cific interaction with plasma proteins and cells.36,37
Further, it has been shown that the activation and
inflammatory response of cells in contact with MPC
polymers are not induced.38,39
Controlled/”living” radical polymerization has been
explored as a means of producing well-defined poly-
mers.1 Atom transfer radical polymerization (ATRP) is
one of the best methods to accomplish this because it
can be applied to the polymerization of a wide variety
of monomers.2-5 ATRP has great synthetic power to
control the molecular architecture of polymers and is
an exceptionally robust method of producing block or
graft copolymers.1,2,6-12
Amphiphilic block and graft copolymers consisting of
hydrophilic and hydrophobic segments are self-assem-
bling materials, which are capable of forming polymeric
associates in aqueous solutions, and have been used
extensively in both research and technology. Vari-
ous amphiphilic polymers have been synthesized via
ATRP,3,13-20 and their amphiphilic properties can be
well controlled. However, extensive design of biodegrad-
able amphiphilic polymers has not yet been performed
via ATRP.
There has been a great deal of interest in polyphos-
phates, which are biodegradable through hydrolysis and
possibly through enzymatic digestion of phosphate
linkages under physiological conditions.21 Biodegradable
polyphosphates appear interesting for biological and
pharmaceutical applications because of their biocom-
patibility and structural similarities to naturally occur-
ring nucleic and teichoic acids. Recently, polyphosphates
have been proposed for use in the field of tissue
Here, we report the synthesis of a novel biodegradable
amphiphilic polymer consisting of polyphosphates pre-
pared by ring-opening polymerization and the well-
defined PMPC graft chain. In addition, solution prop-
erties of the amphiphilic graft copolymers are investi-
gated.
Ma ter ia ls a n d Meth od
engineering as scaffolds and as gene carriers.22-24
A
Ma ter ia ls. 2-Propanol, THF, acetnitrile, and diisopropyl-
amine (DIPA) were purified by conventional distillation.
2-Chloro-2-oxo-1,3,2-dioxaphospholane (COP) was synthesized
according to the method of Edmunson, purified by distillation
under reduced pressure, and the fraction of bp 98 °C (1 mmHg)
[lit.: bp 79 °C (0.4 mmHg)] was used.40 2-Isopropyl-2-oxo-1,3,2-
variety of synthetic routes for polyphosphates has been
* Corresponding author. Telephone: +81-3-5280-8026. Fax:
+81-3-5280-8027. E-mail: yasu.org@tmd.ac.jp.
10.1021/ma049043g CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/28/2004

7638 Iwasaki and Akiyoshi
Macromolecules, Vol. 37, No. 20, 2004
extrapolation to zero angle and concentration for a series of
measurements for five solutions at angles ranging from 50 to
130°.
The absolute Mw, the root-mean-square of gyration (Rg), and
the second virial coefficient (A2) for graft copolymer associates
were determined from MALLS analysis using the following
equation:
F igu r e 1. Chemical structure of MPC.
Sch em e 1. Syn th etic Rou te of P olyp h osp h a te Bea r in g
Br om oisobu tyla te (P IBr )
16π2
3λ2
θ
2
Kc
1
2
)
1 +
Rg sin2 + A2c
(1)
(
)
Rλ Mw
where c denotes the weight concentration of polymer, Rλ is the
Rayleigh ratio, θ is the scattering angle, λ is the wavelength
of light in the medium, and K is given as
4
K ) (4λ2/λ0 NA)n02(dn/dc)2
(2)
where λ0 is the wavelength of light in a vacuum, NA is
Avogadro’s number, n0 is the refractive index of the medium,
and dn/dc is the change in refractive index with the concentra-
tion of polymers. The mole fraction of the OPBB unit in the
PIBr was calculated from 1H NMR data.
dioxaphospholane (IPP) was synthesized as previously re-
ported,40 purified by vacuum distillation, and stored under
argon at -30 °C until use. MPC (Figure 1) was synthesized
by the method previously described and purified by recrystal-
lization from acetonitrile.41 Ethylene glycol was purchased
from Kanto Chemical Co., Ltd., J apan. Copper(I) bromide, 2,2′-
bipyridine (bpy), and 2-bromoisobutyryl bromide were all
purchased from Aldrich and used without further purification.
8-Anilino-1-naphthalene sulfonic acid sodium salt (ANS) was
purchased from Tokyo Kasei Co., Ltd., J apan, and used
without further purification. Distilled water was obtained by
purification using a Millipore Milli-Q system that involves
reverse osmosis, ion exchange, and filtration (18.2 MΩ).
Syn th esis of Cyclic P h osp h a te for ATRP In itia tor .
2-Hydroxyethyl-2′-bromoisobutylate (HEBB) was synthesized
from the reaction of ethylene glycol and 2-bromoisobutyryl
bromide and purified by vacuum distillation.42 COP (0.047 mol)
was added dropwise to a THF solution (300 mL) of DIPA (0.047
mol) and HEBB (0.047 mol) at -30 °C for a period of 1 h. The
reaction was allowed to continue at -30 °C for another 2 h.
The reaction mixture was filtered to correct the precipitate
including the diisopropylammonium chloride. The solvent was
then reduced by half, and the remaining THF solution was
cooled to 0 °C. 2-(2-Oxo,1,3,2-dioxaphospholoyloxy) ethyl-2′-
bromoisobutylate (OPBB) was obtained by recrystallization in
67.1% yield. The structure of OPBB was confirmed by 1H NMR
(R-500, J EOL, Tokyo, J apan) and FT-IR spectroscopy (FT-500,
J asco, Tokyo, J apan).
ATRP of MP C fr om P olyp h osp h a te. Because of the
sensitivity of the Cu(I) complex to air, all reactions were
performed under an argon gas atmosphere. The ethanol used
as solvent was first heated to its boiling point and then stored
with an argon bubble at room temperature. A given amount
of the PIBr (OPBB unit: 0.067 mmol) was desorbed in the
ethanol and argon gas was passed through the solution for 30
min to eliminate oxygen. CuIBr (9.5 mg, 0.067 mmol) and bpy
ligand (21.0 mg, 0.135 mmol) were added to the stirred solution
under argon. MPC (2.00 g, 6.73 mmol) was then added to the
reaction mixture under argon. The solution was stirred at room
temperature for 12 h. After polymerization, the graft copolymer
[PIBr-g-poly(MPC) (PMPC)] was precipitated into THF, then
dissolved in water, and passed through a silica gel column to
remove any residual ATRP catalyst. In addition, the elution
product was dialyzed for 1 day to remove unpolymerized MPC.
The absolute Mw of the graft copolymers was calculated from
the 1H NMR data and the Mw of PIBr determined by the
MALLS analysis.
Am p h ip h ilic P r op er ties of P IBr -g-P MP C. The surface
tension of the PIBr-g-PMPC aqueous solutions at various
concentrations was measured by the Wilhelmy method with
a dynamic contact angle meter (DCA-100, ORIENTEC, Co.,
Ltd., Tokyo, J apan). The critical micelle concentration (cmc)
of the surface tension of the PIBr-g-PMPC aqueous solution
was obtained from the middle point of the transition state, as
summarized in Table 2.
The absolute molecular weight (Mw) of the PIBr-g-PMPCs
was measured by the MALLS method at the higher concentra-
tion of the cmc of the surface tension. To determine the
absolute Mw for polymeric associates, the cmc subtracted from
the total concentration, c of eq 1. Distilled water was used as
the solvent.
Interaction of ANS with a polymer associate was studied to
estimate the polarity of the hydrophobic domain inside the
polymeric associate. A fixed amount of PIBr-g-PMPC was
dissolved in 1.0 × 10-5 M ANS aqueous solution. The PIBr-
g-PMPC solution was further diluted with ANS aqueous
solution (1.0 × 10-5) to a desired concentration of polymer.
The internal polarity of the polymer aggregates was evaluated
by maximum wavelength from the fluorescence spectra of ANS
(λex ) 350 nm, measurement range 420-650 nm).
1H NMR (500 MHz, CDCl3), δ ) OPBB: 1.97 (s; -CH3, 6H),
4.43 (m; -OCH2CH2O-, 4H), 4.35-4.48 (m; -OCH2CH2O-
in cyclic phosphate, 4H).
IR: 2910 (-CH2-), 1235 (-OPO-), 1089 (-OPO-CH2-)
cm-1
.
Syn th esis of P olyp h osp h a tes. Given amounts of IPP and
OPBB were placed into a thoroughly dried 50-mL round-
bottomed flask equipped with a three-way stopcock. After the
mixture was dried under reduced pressure for 2 h, triisobutyl
aluminum was added under an argon gas atmosphere. The
reaction was continued until the magnetic stirrer was stopped.
Dry THF was then added to dilute the reaction mixture. The
polyphosphate (PIBr) was purified by reprecipitation from
diethyl ether. Scheme 1 and Table 1 show the chemical
structure and the synthetic results of the PIBr reaction,
respectively.
The weight-averaged molecular weights of the PIBrs were
measured using gel-permeation chromatography (GPC) through
a Shodex KF-803 column using a calibration curve based on
linear polystyrene standards. THF was the GPC solvent. The
absolute molecular weight (Mw) of the PIBr was determined
by multiangle laser light scattering (MALLS) analysis in batch
mode using a Wyatt Dawn DSP-F detector at a flow rate of 1
mL/min in ethanol. The refractive index increment (dn/dc) for
the PIBr was measured using a Wyatt OPTILAB DSP detector
with a 690 nm light. The absolute Mw was determined by Zimm
The size of the polymeric associate of PIBr3-g-PMPC12,
which was synthesized 12 h after ATRP of the MPC from the
PIBr3, was determined by dynamic light scattering (DLS;
Otsuka Electronics, Co., Ltd., Tokyo, J apan). The scattering
was performed with a vertically polarized incident beam at a
wavelength of 488 nm supplied by an argon ion laser. The
measurements were carried out with a scattering angle of 90°,
the measurement temperature was 25 °C, and the concentra-
tion of the PIBr3-g-PMPC12 was 0.5 g/dL.

Macromolecules, Vol. 37, No. 20, 2004
Amphiphilic Biodegradable Graft Copolymers 7639
Ta ble 1. Syn th etic Resu lt of P olyp h osp h a te
OPBB/IPP 9 mol %)
solubility
no. of OPBB
b
polyphospate
PIBr3
in feed
in copolymera
yield (%) Mw (×10-4
)
Mw/Mn
per PlBr moleculed
H2O
EtOH
THF
CH3Cl
2.0/98.0
1.5/98/5
76.1
3.9b
3.4b
3.1b
3.7c
1.4
3.0
-
+
+
+
+
PIBr11
6.0/94.0
5.0/95.0
49.2
1.4
10.5
-
+
+
a
b
Determined by 1H NMR. Apparent molecular weight: determined by GPC. c Absolute molecular weight: determined by MALLS
with batch mode Calculated from MALLS and 1H-NMR analyses.
d
Ta ble 2. Tr a n sition P oin t of Su r fa ce Ten sion for
P IBr -g-P MP C Aqu eou s Solu tion
graft copolymer
mol wta (×10-4
)
cmc (g/dL)
PIBr3-g-PMPC4
PIBr3-g-PMPC6
PIBr3-g-PMPC12
PIBr11-g-PMPC4
PIBr11-g-PMPC6
PIBr11-g-PMPC12
8.4
9.8
13.6
14.8
18.5
25.8
3.5 × 10-3
3.9 × 10-3
8.6 × 10-3
1.1 × 10-2
2.2 × 10-2
2.3 × 10-1
a
Absolute molecular weight: determined by MALLS and 1H
NMR analyses.
Hyd r olysis of P IBr -g-P MP C. Hydrolysis of the PIBr3-g-
PMPC12 was evaluated by soaking the polymers (0.02 g) in
an aqueous media (10 mL) at 37 °C. The pH of the media was
adjusted to 4.0 (citric acid/NaH2PO4), 7.4 (phosphate buffer
solution), or 11.0 (NaOH/NaH2PO4). After the polymer was
soaked in water for the given time period, the molecular
weights were measured and recorded with GPC under aqueous
conditions. The chemical structures of the degradation prod-
ucts were confirmed by 1H NMR analyses (R-500, J EOL,
Tokyo, J apan).
The weight-averaged molecular weights and molecular
weight distribution of PIBr3-g-PMPC and degradation prod-
ucts were measured with a Tosoh GPC system with a refrac-
tive index detector and size-exclusion columns, Shodex, SB-
804 HQ with a poly(ethylene glycol) (PEG, Tosoh standard
sample) standard in distilled water containing 10 mM LiBr.
F igu r e 2. Change in number of units of MPC in a graft chain
during ATRP: (circle) PIBr3-g-PMPC; (square) PIBr11-g-
PMPC.
Ta ble 3. Molecu la r Weigh t of P IBr -g-P MP C befor e a n d
a fter Hyd r olysis
mol wt (×10-4
)
graft
copolymer
synthesized
polymera
polymer after soaking
in basic buffer for 3 days
PIBr3-g-PMPC12
PIBr11-g-PMPC12
13.6
25.8
3.4a
3.1 (1.2)b
2.1a
2.4 (1.2)b
Absolute molecular weight: determined by MALLS and 1H
NMR analyses. b Apparent molecular weight: determined by GPC.
Molecular weight distribution is given in parentheses.
a
Resu lts a n d Discu ssion
Syn th esis of P olyp h osp h a tes. The chemical struc-
ture and synthetic results of polyphosphates are shown
in Scheme 1 and Table 1, respectively. Polymerization
was homogeneously performed by a solvent-free reac-
tion. As indicated in Table 1, the composition of each
monomer unit could be controlled by the feed. The
polyphosphates synthesized in this study were soluble
in ethanol, THF, and chloroform, but not in water and
diethyl ether. The polymer was stored in dry ethanol
solution at -80 °C until use. The molecular weights of
the polyphosphates were 3.9 × 104 and 3.1 × 104 g/mol
by GPC and 3.4 × 104 and 3.7 × 104 by multiangle laser
light scattering (MALLS). PIBr3 and PIBr11 contain 3.0
and 10.5 initiator sites per macromolecule, respectively.
The PIBrs were novel biodegradable multimacroinitia-
tors for ATRP.
molecular weights of PMPC were evaluated by hydroly-
sis experiments on PIBr-g-PMPC. PIBr of PIBr-g-PMPC
degraded in a basic buffer after 3 days (Table 3). From
1
the results of MALLS and H NMR analyses, the Mw of
the PMPCs grafted from PIBr3 and PIBr11 were 3.4 ×
104 and 2.1 × 104, respectively. The molecular weights
of the graft chains cleaved from PIBr3 and PIBr11
determined by GPC were calculated to be 3.1 × 104 and
2.4 × 104, respectively, and coincident to the calculated
values of PMPC, as shown in Table 3. Moreover, the
molecular distribution of PMPC grafted from PIBr was
quite narrow (Mw/Mn ) 1.2). The polymerization pro-
ceeded homogeneously in every initiator site in PIBr.
Figure 3 is a schematic representation of PIBr and PIBr-
g-PMPC synthesized in this study. A wide variety of
polymer architectures can be well controlled.
Syn th esis of Gr a ft Cop olym er s via ATRP . ATRP
of the MPC from polyphosphate was carried out in an
ethanol solution. Figure 2 shows the average number
of MPC units in a graft chain of PIBr3-g-PMPC and
Su r fa ce Activity of Gr a ft Cop olym er s. Figure 4
shows the surface tension of graft copolymers in water
as a function of polymer concentration. As the polymer
concentration increased, the surface tension decreased
from 72 to about 40 dyn/cm. Changes in the surface
tension are typically observed in the micelle formation
of polymer amphiphiles. The cmc of the graft copolymers
was evaluated from the change in the surface tension.
The cmc increased with an increase in the Mw of the
graft copolymer and the density of PMPC, as shown in
Table 2. Typical examples of the concentrations of
PIBr3-g-PMPC12 and PIBr11-g-PMPC12 are 8.6 × 10-3
and 2.3 × 10-1 g/dL, respectively. Although the density
1
PIBr11-g-PMPC as determined by H NMR. The num-
bers increased linearly with an increase in the duration
of polymerization. The increment in the number (y) of
MPC units in a graft chain with polymerization time
(x) (dy/dx) for PIBr3-g-PMPC and PIBr11-g-PMPC were
7.4 and 4.8, respectively. The rate of polymerization
decreased with graft density. This might be due to the
steric effect of the bulky MPC. To obtain further
information about graft polymerization of PIBr, the

7640 Iwasaki and Akiyoshi
Macromolecules, Vol. 37, No. 20, 2004
F igu r e 3. Schematic representation of PIBr and PIBr-g-PMPC.
fluorescence was observed in the ANS aqueous solution
without the PIBr3-g-PMPC12. However, the fluores-
cence intensity of the ANS solution increased as the
polymer concentration increased, and the peak shifted
to a shorter wavelength. Figure 6 shows the dependence
of the maximum fluorescence peak wavelength of ANS
in the PIBr3-g-PMPC12 and PIBr11-g-PMPC12 aqueous
solutions on its concentration. The behavior change in
the wavelength of ANS in the polymer solutions was
similar to that of their surface tensions.
The maximum fluorescence wavelength of ANS is
influenced by the polarity of the microenvironment43
and is lower with the lower polarity. The results in
Figure 6 suggest the formation of hydrophobic domains
upon the association of hydrophobic polyphosphates of
both graft copolymers. The relationship between polarity
and the maximum fluorescence peak wavelength of ANS
is known.44 From these data, the polarity of the hydro-
phobic domain is similar to the polarity of methanol.
The hydrodynamic radii (Rh) of the solution of PIBr3-
g-PMPC12 and PIBr11-g-PMPC12 above cmc were
measured by DLS. Rh of PIBr11-g-PMPC12 was not
determined due to the low intensity of light scattering.
PIBr11-g-PMPC12 probably exists as a monomer as
does the unimer micelle because of the brush-type
structures with long hydrophilic PMPC chains. In
contrast, Rh in PIBr3-g-PMPC12 was 16.4 nm although
the molecular weight of PIBr3-g-PMPC12 (Mw ) 13.6
× 104) is less than that of PIBr11-g-PMPC12 (Mw ) 25.8
× 104). PIBr3-g-PMPC12 may form nanoparticles by the
intermolecular association. To obtain further informa-
tion on the nanoparticles, static light scattering (MALLS)
was performed.
F igu r e 4. (a) Surface tension of PIBr3-g-PMPC in aqueous
solution: (gray) PIBr3-g-PMPC4; (white) PIBr3-g-PMPC6;
(black) PIBr3-g-PMPC12. (b) Surface tension of PIBr11-g-
PMPC in aqueous solution: (gray) PIBr11-g-PMPC4; (white)
PIBr11-g-PMPC6; (black) PIBr11-g-PMPC12.
and molecular weight of the PMPC affected the cmc of
the PIBr-g-PMPC, the equilibrium surface tension
reached 40 dyn/cm in every graft polymer. The poly-
phosphates of the graft copolymers act as hydrophobic
polymers of the polymer surfactants. The cmc can be
controlled with retaining high surface activity by their
well-controlled molecular architectures.
Solu tion P r op er ty of Gr a ft Cop olym er s in Wa -
ter . Micelle formations were also investigated by the
fluorescence probe method. Figure 5 shows typical
fluorescence spectra of ANS in an aqueous solution of
the PIBr3-g-PMPC12 at different concentrations. Weak
Figure 7 shows the Zimm plot of PIBr3-g-PMPC12
at a concentration of 0.1-0.5 g/dL. The dn/dc of PIBr3-
g-PMPC12 in aqueous solution was 0.119 and the effect
of molecular weight PMPC on dn/dc was not observed.
On the basis of this plot, Mw and Rg of the polymeric
associate were 94.1 × 104 and 26.2 nm, respectively. The
molecular weight of PIBr3-g-PMPC12 can be estimated
at 13.6 × 104 from the absolute molecular weight of
PIBr3 and 1H NMR analysis. Thus, the association

Macromolecules, Vol. 37, No. 20, 2004
Amphiphilic Biodegradable Graft Copolymers 7641
F igu r e 5. Fluorescence spectra of ANS incorporated in PIBr3-
g-PMPC12 in aqueous solution.
F igu r e 8. Degradation of profiles of PIBr3-g-PMPC12 in
various pH aqueous media: (gray) pH 4.0; (white) pH 7.4;
(black) pH 11.0.
containing PMPC moieties have been synthesized.53,54
However, a few graft copolymers have been reported to
form micelles.43,55-57 Moreover, most graft copolymers
reported in micelle formation have hydrophilic back-
bones. This is probably due to difficulties in synthesizing
amphiphilic graft copolymers with controlled structures.
Graft copolymers with hydrophobic backbones are rarely
reported.58-60 PIBr-g-PMPCs form polymer micelles by
hydrophobic interaction of hydrophobic polyphosphates.
The hydrophobicity of the polyphosphate may be con-
trollable by changing the chemical structure of the side
chain instead of the 2-isopropyl groups. The present
synthetic procedure for PIBr-g-PMPCs is very suitable
for synthesizing well-defined amphiphilic graft copoly-
mers. PIBr3-g-PMPC12 formed novel nanogels of graft
copolymers with biodegradable hydrophobic backbones.
In biotechnology and medicine, nanogels with the
characteristics of both nanoparticles and hydrogels have
attracted growing interest as nanobiomaterials.61,62
Hyd r olysis of Na n ogels of Gr a ft Cop olym er s. A
degradation profile of nanogels of PIBr3-g-PMPC12 was
investigated in aqueous media with various pH values,
as shown in Figure 8. In an acidic medium, the loss of
molecular weight of the graft copolymer was observed
as being less and degradation occurred markedly after
50 days of soaking. Under physiological pH conditions,
the weight-averaged molecular weight of the PIBr-g-
PMPC decreased from 15.6 × 104 to 11.0 × 104 after 90
days. Under basic conditions, the polymers degraded
nearly completely within 3 days. Although the basic
condition used (pH 11.0) is not a physiological condition,
we chose the optimal pH to characterize the degradation
behavior of polyphosphates in a relatively short period.
Under acidic conditions (pH 4.0), hydrolysis of the PIBr
was slow. In contrast, under basic conditions (pH 11.0),
the PIBr was completely degraded in only 3 days.
Penczek et al. reported the hydrolysis of poly(methyl
ethylene phosphate) in the range from pH 1 to pH 12.63
Under basic conditions, the rate of hydrolysis of the
main chain (km) was dramatically faster (1.32 × 10-5
s-1 at pH 11.16) than that under acidic conditions (1.14
× 10-8 s-1 at pH 3.78). The nanogel structure may be
significant in the hydrolyzation of hydrophobic poly-
phosphates. Nanogels of PIBr3-g-PMPC12 are novel
biodegradable nanoparticles.
F igu r e 6. Concentration dependence of maximum fluores-
cence wavelength of incorporated ANS: (circle) PIBr3-g-
PMPC12; (square) PIBr11-g-PMPC12.
F igu r e 7. Zimm plot of associated PIBr3-g-PMPC12.
number of the PIBr3-g-PMPC12 was 6.9. It is well-
known that the Rg/Rh ratio changes from infinity to
0.775 when the polymer structure changes from rod to
sphere, with values from 1.3 to 1.5 for random coils.45
The Rg/Rh ratio of the PIBr3-g-PMPC12 associate was
1.59, suggesting a slightly extended conformation of the
polymer chain.
The average polymer density (øg) was calculated by
Rh and Mw of the polymer by the following equation:
Mw 4
-1
3
φg )
πRh
(3)
(
)
NA
3
The density was estimated to be 8.4 wt %. The nano-
particles can be considered nanosized hydrogels (nano-
gels) containing 8.4 wt % polymers and 91.6 wt % water.
The associate domains of the hydrophobic polyphos-
phates are cross-linking points of hydrogels although
the detailed structure is not yet clear.
Considerable study has been given to the synthesis
and structures of micelles from amphiphilic block
copolymers.46-50 In particular, there are interests in
biomedical applications,51,52 and novel block copolymers
Con clu sion s
PIBrs were synthesized as novel biodegradable mul-
timacroinitiators for ATRP. The novel biodegradable
amphiphilic graft copolymers, which consist of a hydro-
phobic polyphosphate backbone and hydrophilic PMPC
graft chains, were synthesized via ATRP. The graft

7642 Iwasaki and Akiyoshi
Macromolecules, Vol. 37, No. 20, 2004
(28) Pretura, J .; Kaluzynski, K.; Szymanski, R.; Penczek, S. J .
Polym. Sci., Part A: Polym. Chem. 1999, 37, 1365-1381.
(29) Myrex, R. D.; Farmer, B.; Gray, G. M.; Wright, Y.-J .; Dees,
J .; Bharara, P. C.; Byrd, H.; Branham, K. E. Eur. Polym. J .
2003, 39, 1105-1115.
(30) Wen, J .; Zhuo, R. X. Macromol. Rapid Commun.i. 1998, 19,
641-642.
copolymers with varied densities of PMPC could be
obtained and the molecular weight of PMPC was mono-
disperse and well controllable. The polymers associate
in aqueous solutions and form polymer micelle or
nanogels. The associative properties were controlled by
the architecture of the graft copolymer. PIBr-g-PMPCs
are novel biodegradable building blocks for constructing
polymer self-assembly. The polymer is hydrolyzable in
aqueous solutions and PMPC could be obtained as a
degradation product. We have reported that the PMPC
has excellent biocompatibility.64 The graft copolymers
synthesized might be novel functional polymeric bio-
materials, which could be used for diagnostic and drug
delivery applications.
(31) Iwasaki, Y.; Komatsu, S.; Narita, T.; Akiyoshi, K.; Ishihara,
K. Macromol. Biosci. 2003, 3, 238-242.
(32) Iwasaki, Y.; Mikami, A.; Kurita, K.; Yui, N.; Ishihara, K.;
Nakabayashi, N. J . Biomed. Mater. Res. 1997, 36, 508-515.
(33) Iwasaki, Y.; Sawada, S.; Nakabayashi, N.; Khang, G.; Lee,
H. B.; Ishihara, K. Biomaterials 1999, 20, 2185-2191.
(34) Ishihara, K.; Ishikawa. E.; Iwasaki, Y.; Nakabayashi, N. J .
Biomater. Sci. Polym. Ed. 1999, 10, 1047-1061.
(35) Iwasaki, Y.; Nakabayashi, N.; Ishihara, K. J . Biomed. Mater.
Res. 2001, 57, 72-78.
(36) Ishihara, K.; Oshida, H.; Endo, Y.; Ueda. T.; Watanabe, A.;
Nakabayashi, N. J . Biomed. Mater. Res. 1992, 26, 1543-
1552.
(37) Ishihara, K.; Nomura, H.; Mihara, T.; Kurita, K.; Iwasaki.
Y.; Nakabayashi, N. J . Biomed. Mater. Res. 1998, 39, 323-
330.
(38) Iwasaki, Y.; Sawada, S.; Ishihara, K.; Khang, G.; Lee, H. B.
Biomaterials 2002, 23, 3897-3903.
(39) Sawada, S.; Sakaki, S.; Iwasaki, Y.; Nakabayashi, N.; Ishi-
hara, K. J . Biomed. Mater. Res. 2003, 64A, 411-416.
(40) Edmundson, R. S. Chem. Ind. (London) 1962, 1828.
(41) Ishihara, K.; Ueda, T.; Nakabayashi, N. Polym. J . 1990, 22,
355-360.
(42) J ayachandran, K. N.; Takacs-Cox, A.; Brooks, D. E. Macro-
molecules 2002, 35, 4247-4257.
(43) Ghosh, S.; Basu, M. K.; Schweppe, J . S. Biochim. Biophys.
Acta 1974, 337, 395-403.
(44) Ishihara, K.; Iwasaki, Y.; Nakabayashi, N. Polym. J . 1999,
31, 1231-1236.
(45) Buchard, W.; Schmidt, M.; Stockmayer W. H. Macromolecules
1980, 13, 1265-1272.
(46) Alexander, V. K.; Irina, R. N.; Irina, V. A.; Elena, V. B.;
Valery, Y. A.; Alexander, A. Y.; Victor, A. K. Macromolecules
1995, 28, 2303.
(47) Balsara, N. P.; Tirrel, M. Lodge, T. P. Macromolecules 1991,
24, 1975.
(48) Nguyen-Misra, M.; Mattice, W. L. Macromolecules 1995, 28,
1444.
(49) Kwon, G.; Naito, M.; Yokoyama, M.; Okano, T.; Sakurai, Y.;
Kataoka, K. Langmuir 1993, 9, 949.
(50) Wilhelm, M.; Zhao, C.-L.; Wang, Y.; Xu, R.; Winnik, M. A.;
Mura, J .-L.; Riess, G.; Croucher, M. D. Macromolecules 1991,
24, 1033.
(51) Kumar, N.; Ravikumar, M. N. V.; Domb, A. J . Adv. Drug
Deliv. Rev. 2001, 53, 23-44.
(52) Rosler, A.; Vandermeulen, G. W. M.; Klok, H. A. Adv. Drug
Deliv. Rev. 2001, 53, 95-108.
(53) Lobb, E. J .; Ma, I.; Billingham, N. C.; Armes, S. P.; Lewis, A.
L. J . Am. Chem. Soc. 2001, 123, 7913-7914.
(54) Ma, Y.; Tang, Y.; Billingham, N. C.; Armes, S. P.; Lewis, A.
L.; Lloyd, A. W.; Salvage, J . P. Macromolecules 2003, 36,
3475-3484.
Ack n ow led gm en t. We gratefully acknowledge the
valuable assistance provided by Dr. Kazuhiko Ishihara,
The University of Tokyo.
Refer en ces a n d Notes
(1) Matyjaszewski, K.; Davis, T. P. Handbook of Radical Polym-
erization; Wiley-Interscience: Hoboken, NJ , 2002.
(2) Wang, J .-S.; Matyjaszewski, K. J . Am. Chem. Soc. 1995, 117,
5614-5615.
(3) Pattern, T. E.; Xia, J .; Abernathy, T.; Matyjaszewski, K.
Science 1996, 272, 866-868.
(4) Matyjaszewski, K.; Xia, J . Chem. Rev. 2001, 101, 2921-2990.
(5) Kamigaito, M.; Ando, T.; Sawamoto, M. Chem. Rev. 2001, 101,
3689-3745.
(6) Paik, H-J .; Gaynor, S. G.; Matyjaszewski, K. Macromol.
Rapid Commun. 1998, 19, 47-52.
(7) Buchmeiser, M. R. Chem. Rev. 2000, 100, 1565-1604.
(8) Hong, S. C.; Pakula, T.; Matyjaszewski, K. Macromol. Chem.
Phys. 2001, 202, 3392-3402.
(9) Borner, H. G.; Beers, K.; Matyjaszewski, K.; Sheiko, S. S.;
Moller, M. Macromolecules 2001, 34, 4375-4383.
(10) Hong, S. C.; J ia, S.; Teodorescu, M.; Kowalewski, T.; Maty-
jaszewski, K.; Gottfried, A. C.; Brookhart, M. J . Polym. Sci.,
Part A: Polym. Chem. 2002, 40, 2736-3749.
(11) Bo¨rner, H. G.; Matyjaszewski, K. Macromol. Symp. 2002, 177,
1-16.
(12) Boyes, S. G.; Brittain, W. J .; Weng, X.; Cheng, S. Z. D.
Macromolecules 2002, 35, 4960-4967.
(13) Mu¨hlebach, A.; Gaynor, S. G.; Matyjaszewski, K. Macromol-
ecules 1998, 31, 6046-6052.
(14) Matyjaszewski, K.; Beers, K. L.; Kern, A.; Gaynor, S. G. J .
Polym. Sci., Part A: Polym. Chem. 1998, 36, 823-830.
(15) Lee, S. B.; Russell, A. J .; Matyjaszewski, K. Biomacromol-
ecules 2003, 4, 1386-1393.
(16) Ohno, K.; Tsujii, Y.; Fukuda, T. J . Polym. Sci., Part A: Polym.
Chem. 1998, 36, 2473-2481.
(17) Robinson, K. L.; de Paz-Banez, M. V.; Wang, X. S.; Armes, S.
P. Macromolecules 2001, 34, 5799-5805.
(18) Cheng, G.; Boker, A.; Zhang, M.; Krausch, G.; Muller, A. H.
E. Macromolecules 2001, 34, 6883-6888.
(19) Lee, S. B.; Russell, A. J .; Matyjaszewski, K. Biomacromol-
ecules 2003, 4, 1386-1393.
(55) Ma, Y.; Cao, T.; Webber, S. E. Macromolecules 1998, 31, 1773.
(56) Kikuchi, A.; Nose, T. Macromolecules 1996, 29, 6770.
(57) Carrot, G.; Hilborn, J .; Knauss, D. M. Polymer 1997, 38, 6401.
(58) Chang, J . Y.; Park, P. J .; Han, M. J . Macromolecules 2000,
33, 321-325.
(59) Breitenkamp, K.; Emrick, T. J . Am. Chem. Sci. 2003, 125,
12070-12071.
(60) J eong, B.; Lee, K. M.; Gutowska, A.; An, Y. H. Biomacromol-
ecules 2002, 3, 865-868.
(61) Nomura, Y.; Ikeda, M.; Yamaguchi, N.; Aoyama, Y.; Akiyoshi,
K. FEBS Lett. 2003, 533, 271-276.
(62) Kuroda, K.; Fujimoto, K.; Sunamoto, J .; Akiyoshi, K. Lang-
muir 2002, 18, 3780-3786.
(63) Baran, J .; Penczek, S. Macromolecules 1995, 28, 5167-5176.
(64) Iwasaki, Y.; Ijuin, M.; Mikami, A.; Nakabayashi, N.; Ishihara,
K. J . Biomed. Mater. Res. 1999, 46, 360-367.
(20) Ma, Y.; Tang, Y.; Billingham, N. C.; Armes, S. P.; Lewis, A.
L. Biomacromolecules 2003, 4, 864-868.
(21) Renier, M. L.; Kohn, D. H. J . Biomed. Mater. Res. 1997, 34,
95-104.
(22) Wan, A. C.; Mao, H. Q.; Wang, S.; Leong, K. W.; Ong, L. K.;
Yu, H. Biomaterials 2001, 22, 1147-1156.
(23) Wang, J .; Zhang, P. C.; Lu, H. F.; Ma, N.; Wang, S.; Mao, H.
Q.; Leong, K. W. J . Controlled Release 2002, 83, 157-168.
(24) Huang, S. W.; Wang, J .; Zhang, P. C.; Mao, H. Q.; Zhuo, R.
X.; Leong, K. W. Biomacromolecules 2004, 5, 306-311.
(25) Libiszowski, J .; Kaluzynski, K.; Penczek, S. J . Polym. Sci.,
Part A: Polym. Chem. 1978, 16, 1275-1283.
(26) Pretula, J .; Kaluzynski, K.; Penczek, S. Macromolecules 1986,
19, 1797-1799.
(27) Richards, M.; Dahiyat, B. I.; Arm, D. M.; Lin, S.; Leong, K.
W. J . Polym. Sci., Part A: Polym. Chem. 1991, 29, 1157-
1165.
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