Macromolecules, Vol. 39, No. 24, 2006
PN-PPG-PN Triblock Copolymers 8335
Scheme 1
at room temperature. Following addition of the salt, the reaction
mixture was stirred at room temperature for 12 h. After completion
of the reaction, the mixture was centrifuged for 30 min to remove
sodium chloride. The solution was transferred to a round-bottom
flask, and the solvent was removed at reduced pressure by rotary
evaporation. The remaining solution was vacuum distilled to
produce a colorless liquid. The product was dissolved in benzene
(200 mL) and cooled to 10 °C. To this solution was added bromine
(29.61 g) in benzene (50 mL), and the mixture was stirred at 10
°C for 2 h and allowed to warm to room temperature. The solvent
was removed at reduced pressure, and the crude product was
vacuum distilled at least twice to remove unreacted bromine and
produce a colorless liquid (34.86 g, yield 40%). 1H NMR (CDCl3):
δ 0 (s, 9H), 4.23 (q, 4H). 31P NMR (CDCl3): δ -33.93.
Scheme 2
Synthesis of Trifluoroethoxyphosphoranimine 3. To a solution
of bromophosphoranimine 1 (11.63 g, 29.36 mmol) in THF (50
mL) was added a solution of sodium trifluoroethoxide (32.30 mmol)
at -78 °C. The reaction mixture was stirred at -78 °C for 1 h and
allowed to warm to room temperature. After completion of the
reaction, the reaction mixture was passed through a pad of Celite
to remove precipitated sodium chloride. The filtrate was collected,
and the solvent and excess trifluoroethanol were removed at reduced
pressure. The crude product was purified by vacuum distillation
(at 40 °C) to yield a colorless liquid (6.10 g, yield 50%). 1H NMR
(CDCl3): δ 0.00 (s, 9H), 4.23 (q, 6H). 31P NMR (CDCl3): δ -11.90.
Synthesis of Chlorophosphoranimine 4.10 A solution of LiN-
(SiMe3)2 (53.53 g, 0.32 mol) in 700 mL of diethyl ether was cooled
to 0 °C, and PCl3 (43.95 g, 0.32 mol) was added dropwise over 30
min. The reaction mixture was allowed to warm to room temper-
ature and was stirred for 2 h. Sulfuryl chloride (43.19 g, 0.32 mol)
was added slowly, and the reaction mixture was stirred at 0 °C for
3 h. After completion of the reaction, the salt was removed by
filtration. The crude product was purified by vacuum distillation
(30 °C, 0.3 mmHg) to yield a colorless liquid (25.15 g, yield 35%).
1H NMR (CDCl3): δ 0.00 (s, 9H). 31P NMR (CDCl3): δ -54.10.
Synthesis of Triblock Copolymers 6. A mixture of NH2-PPG-
NH2 (2.10 g, 0.53 mmol) and triethylamine (0.10 g) in THF (100
mL) was cooled to 0 °C. To this solution was added dropwise (CF3-
CH2O)2BrPdNSiMe3 (0.42 g, 1.05 mmol) over a 30 min period.
The reaction mixture was stirred for 12 h at room temperature. All
volatiles and solvent were removed under reduced pressure to
produce a yellow viscous liquid. This end-functionalized product
was dissolved in CH2Cl2 for further reaction. In a separate reaction
vessel, PCl5 (0.44 g, 2.10 mmol) was dissolved in 50 mL of distilled
CH2Cl2 at room temperature. The end-capper reagent (CF3-
CH2O)3PdNSiMe3 (0.44 g, 14.70 mmol) was added to the solution,
which was stirred for 1 h at room temperature. The monomer,
Cl3PdNSiMe3 (3.30 g, 9.62 mmol), was then added to the reaction
mixture which was stirred for 2 h to generate “living” poly-
(dichlorophosphazene) chains. The solution of PPG-phosphoran-
imine in CH2Cl2 was then added to the polyphosphazene solution,
and the mixture was stirred for 10 h at room temperature to
terminate the polymerization. The CH2Cl2 was removed from the
reaction mixture under reduced pressure, and the polymer was
redissolved in 25 mL of freshly distilled THF. An excess of
NaOCH2CF3 (34.65 mmol) in THF was added to the polymer
solution to replace the labile chlorine atoms in the phosphazene
blocks. The reaction mixture was stirred at room temperature until
31P NMR spectroscopy indicated complete replacement of the
chlorine atoms. The reaction solution was then concentrated and
precipitated repeatedly from THF into deionized water and into
hexanes. The polymer was purified again by dialysis against EtOH/
H2O (4/1 vol/vol), followed by drying under reduced pressure to
give a viscous yellowish liquid.
(1.2 × 10-3 M) was added to Nanopure water to give a pyrene
concentration of 12 × 10-7 M, and THF was removed using a rotary
evaporator at 30 °C for 2 h. The pyrene solution was mixed with
the triblock copolymer solutions to obtain copolymer concentrations
from 2.5 × 10-4 to 1.25 × 10-4 g/L. The pyrene concentration of
the samples was 6.0 × 10-7 M. All the samples were sonicated for
15 min and were allowed to stand for 2 days before fluorescence
measurements.
Fluorescence Measurements. The fluorescence spectra were
obtained using a Perkin-Elmer LS 55 spectrofluorometer. For the
measurement of pyrene excitation spectra, emission and excitation
bandwidths were set at 5 nm each, and the emission wavelength
was set at 393 nm.
Sizes and Size Distributions. The sizes and size distributions
of the triblock copolymer micelles were evaluated by dynamic light
scattering (DLS) using a particle size analyzer (BI-90Plus,
Brookhaven Instruments Corp., Holtsville, NY) with a scattering
angle of 90°. Samples were filtered through a 0.45 µm syringe filter
before measurement of particle size for each sample. Measurements
were conducted at room temperature (25 °C).
Results and Discussion
A series of new ABA triblock copolymers of a polyphos-
phazene and poly(propylene glycol) were prepared by the
synthetic procedure illustrated in Schemes 1 and 2. Phospho-
ranimines such as (CF3CH2O)2BrPdNSiMe3 (1) readily undergo
bromine replacement reactions in the presence of amines to
produce phosphoranimines such as RNH(CF3CH2O)2PdN-
SiMe3.6,7 These species can then be utilized as initiators and/or
terminators in the cationic living polymerization of Cl3Pd
NSiMe3. Stoichiometric amounts of amine-terminated poly-
(propylene glycol), such as H2N-PPG-NH2 were allowed to
react with 1 in THF in the presence of triethylamine (Scheme
1). This process produced the phosphoranimine-terminated poly-
(propylene glycol), Me3SiNdP(OCH2CF3)2NH-PPG-NH(CF3-
CH2O)2PdNSiMe3 (2).
Living polyphosphazene 5 can be terminated with triorga-
nophosphoranimines, which allows the controlled introduction
of two functional units at the termini of the polyphosphazene
chain.11,12 Thus, compound 2 was employed as a PPG macro-
molecular terminator for the living polymerization of polyphos-
phazenes. As illustrated in Scheme 2, triblock copolymers were
prepared by the addition of 2 to 5. Following termination, the
Sample Preparation. To prepare micellar solutions, Nanopure
water with a conductivity of 18.2 MΩ/cm (10 mL) was added
dropwise to a stirred THF solution of the triblock copolymer (10
mL). The THF was removed on a rotary evaporator at 30 °C for 2
h. The micellar solution was diluted with Nanopure water to obtain
a concentration range from 5 × 10-4 to 1 × 10-4 g/L. For the
measurement of fluorescence spectra, a pyrene solution in THF