Synthesis of Polyphosphazenes by a Fast Perfluoroaryl Azide-Mediated Staudinger Reaction

Article abstract of DOI:10.1021/acs.macromol.8b00618

We report the synthesis of polyphosphazenes by a fast Staudinger reaction between a bis-PFAA (perfluoroaryl azide) and a bis-phospine. Polymerization was completed within 30 min after mixing the two monomers (20 mM) in CH₃CN under ambient condi

Full text of DOI:10.1021/acs.macromol.8b00618

Article
Cite This: Macromolecules XXXX, XXX, XXX−XXX
Synthesis of Polyphosphazenes by a Fast Perfluoroaryl Azide-
Mediated Staudinger Reaction
Madanodaya Sundhoro,† Jaehyeung Park,†,‡ Bin Wu,† and Mingdi Yan*,†
†University of Massachusetts Lowell, 1 University Ave., Lowell, Massachusetts 01854, United States
‡Division of Advanced Materials Engineering, Dong-Eui University, Busan 47340, Korea
S
* Supporting Information
ABSTRACT: We report the synthesis of polyphosphazenes by a fast
Staudinger reaction between a bis-PFAA (perfluoroaryl azide) and a
bis-phospine. Polymerization was completed within 30 min after mixing
the two monomers (20 mM) in CH3CN under ambient conditions to
give polyphosphazene with molecular weight of up to over 59 000 and a
narrow dispersity (Đ) of 1.1−1.2. This is a significant improvement
over the polyphosphazenes obtained from the classic Staudinger
reaction which had lower molecular weight (M = 10 000−25 000) and
̅
n
higher Đ (1.7). This reaction applies to bis-PFAAs and bis-phosphines
with either an aromatic or an aliphatic linker. Polarity of the solvent
influenced the degree of polymerization, and more polar solvents such
as acetonitrile gave higher molecular weight polymer in comparison to
less polar solvents such as dichloromethane. The synthesized polyphosphazenes showed high thermal stability, with the aromatic
polyphosphazenes giving decomposition temperature and char yield of up to of 441 °C and 53%, respectively.
defined molecular weight and narrow size distribution.27
However, living cationic polymerization typically requires strict
polymerization conditions such as high vacuum and high purity
of the reagents.27
The Staudinger reaction, the reaction between an azide and a
phosphine, generates an iminophosphorane, i.e., phosphazene
product in almost quantitative yield under relatively mild
conditions.28 It has been used to synthesize polyphosphazenes.
For example, Herring reported a polymerization reaction
between 1,4-diazido-benzene with 1,4-bis(diphenylphosphino)-
benzene at room temperature for 4 h. However, the polymer
precipitated out of the solution, and the molecular weight was
impossible to analyze.29 The first high molecular weight
INTRODUCTION
■
Polyphosphazenes, inorganic−organic hybrid polymers consist-
ing of the PN repeating unit, possess several unique
properties such as unusual elasticity,1 high thermal stability,2
flame resistance,3 and high stability toward harsh chemicals
such as acids and bases.4 It has been used in many applications
including drug and vaccine delivery,5−8 microarray and
biosensors,9−11 superhydrophobic surface,12−14 and tissue
engineering.15−17
One of the early methods for the synthesis of polyphospha-
zenes was developed by Allcock and co-workers.18,19 It consists
of two steps, starting from the ring-opening of hexachlorocyclo-
triphosphazene to give polydichlorophosphazene having
10 000−15 000 repeating units, which is then treated with a
nucleophile such as amine or an alcohol to introduce the
desired substituent (R) to the polymer (Scheme 1A).20,21 This
method remains the most popular and is still in use today.3,19,22
However, this method suffers from several drawbacks such as
low hydrolytic stability of hexachlorocyclotriphosphazene and
polydichlorophosphazene.23 High temperature (250 °C) and
vacuum are required for the ring-opening polymerization which
may cross-link or degrade the polymer.24 In addition, the
nucleophilic substitution step may not give complete
replacement of all Cl atoms in the polymer, especially when
the substituent is large. Substituent exchange reaction can also
occur depending on the nucleophilicity and the steric
hindrance.25 Furthermore, the P−Cl bond is prone to
hydrolysis.24 Other methods have also been developed, such
as living cationic polymerization,26 where the polymerization
can be carried out at room temperature, yielding polymers with
polyphosphazene (M = 10 000−25 000) synthesized through
̅
n
Staudinger reaction was reported by Matyjaszweski and co-
workers.30 By mixing trimethylsilyl azide with bis-
(trifluoroethyl)(phenyl)phosphonite at 70 °C, the polyphos-
phazene product was obtained in 60−80% yield.
Perfluoroaryl azides (PFAAs), originally developed as highly
efficient photoaffinity labels owing to the increased lifetime of
the singlet perfluoroaryl nitrene generated by photoactivation
of the azide, have been applied for the functionalization of
materials and surfaces31,32 and for the conjugation of a wide
range of structures including drug molecules,33 polymers,34,35
carbohydrates,36−41 and nanoparticles.42−45 Additionally, the
azide in PFAA is a highly electrophilic dipole activated by the
Received: March 23, 2018
Revised: May 14, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.macromol.8b00618
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Scheme 1. Synthesis of Polyphosphazene: (A) Ring-Opening of Hexachlorocyclotriphosphazene Followed by Nucleophilic
Substitution with an Alcohol or an Amine; (B) PFAA-Mediated Staudinger Polymerization
temperature overnight. The reaction mixture was diluted with 20 mL
of water, and diethyl ether (20 mL) was added to the mixture. The
organic layer was washed with water (3 × 100 mL) and brine (100
mL) and dried over sodium sulfate. The collected organic phase was
concentrated under vacuum, and the crude product was purified by
column chromatography with hexanes as the eluent to obtain 1a as a
white solid powder (5.2 g, 91%). 19F NMR (188 MHz, CDCl3): δ
−134.69 to −134.88 (m, 4F), −147.58 to −147.71 (m, 4F). 13C NMR
multiple fluorine atoms. For instance, the LUMO of
pentaphenyl azide is 3.06 eV compared to 3.68 eV in the
case of phenyl azide.46,47 This electrophilic activation results in
orders of magnitude increase in the reaction rates and enables
new reactions of PFAAs that are unique and impossible with
phenyl azide, for example, azide−thioacid amidation,48 azide−
aldehyde amidation,49,50 azide−alkyne cycloaddition,46,51−53
and Staudinger reaction.54 In the PFAA-Staudinger reaction,
the reaction occurs under ambient conditions, and the rate
constant of the reaction between PFAA and triarylphosphine
reached 15 M−1 s−1.54 In addition, the iminophosphorane
product is stable toward hydrolysis and aza−ylide reactions. In
this work, we demonstrate the use of this reaction for the
synthesis of polyphosphazenes (Scheme 1B). The polymer-
ization occurs at room temperature in air to give
1
2
(126 MHz, CDCl3): δ 144.85 (dd, JCF = 253 Hz, JCF = 14 Hz),
1
2
2
140.73 (dd, JCF = 253 Hz, JCF = 16 Hz), 122.48 (t, JCF = 13 Hz),
2
102.31 (t, JCF = 15 Hz).
Synthesis of Ethane-1,2-diyl Bis(4-azido-2,3,5,6-tetrafluor-
obenzoate) (1b). Compound 1b was synthesized according to the
reported protocol.57 In brief, a solution of 4-azido-2,3,5,6-tetrafluoro-
benzoic acid58 (1.00 g, 4.30 mmol) in dry dichloromethane (20 mL)
was stirred with ethylene glycol (132 mg, 2.10 mmol) and DMAP (53
mg, 0.43 mmol) under Ar at room temperature for 30 min. Afterward,
EDAC (908 mg, 4.70 mmol) was added to the solution, and the
solution was further stirred at room temperature overnight. Then 20
mL of water was added, and the mixture was stirred for 30 min. The
mixture was extracted with 20 mL of CH2Cl2 for 3 times, and the
combined organic layers were washed with water (3 × 100 mL) and
brine (100 mL) and dried over sodium sulfate. The compound was
purified by column chromatography using hexanes:ethyl acetate (3:2,
polyphosphazenes with M of up to above 59 000 and narrow
̅
w
Đ (1.1−1.2). The scope of the polymerization was investigated
with regard to the structure of the monomers, and the impact
of the solvent was also studied. The resulting polyphosphazenes
showed high thermal stability, with the decomposition
temperature and char yield of up to 441 °C and 53%,
respectively.
1
v:v) as the eluent to give 1b as a white powder (729 mg, 70%). H
NMR (500 MHz, CDCl3): δ 4.71 (s, 4H). 19F NMR (188 MHz,
CDCl3): δ −136.10 (dd, 1JFF = 21 Hz, 2JFF = 10 Hz, 4F), −148.65 (dd,
1JFF = 19 Hz, 2JFF = 8 Hz, 4F). 13C NMR (126 MHz, CDCl3): δ 159.05
EXPERIMENTAL SECTION
■
Materials. Sodium azide (≥99.5%), cesium carbonate (99%),
copper(I) iodide (≥99.5%), ethylene glycol (≥99.5%), N′-ethyl-N-(3-
(dimethylamino)propyl)carbodiimide hydrochloride (EDAC)
(≥99%), 3-(diphenylphosphino)propionic acid (97%), 1,3-bis-
(diphenylphosphino)propane (97%), diphenylphosphine (98%),
decafluorobiphenyl (99%), DMAP (4-(dimethylamino)pyridine)
(≥99%), 1,4-diiodobenzene (99%), methyl pentafluorobenzoate
(99%), sodium chloride (≥99.5%), sodium sulfate (≥99%) and
poly(bis(phenoxy)phosphazene (PBPP) were purchased from Sigma-
Aldrich. N,N-Dimethylformamide (DMF), ethyl acetate, hexanes,
toluene, diethyl ether, acetone, methanol, methylene chloride, and
acetonitrile were purchased from Fisher Scientific. NMR spectra were
recorded on a Bruker Avance Spectrospin DRX 500 spectrometer (500
MHz) or a Bruker Avance Spectrospin DPX 200 spectrometer (200
MHz) referenced to either nondeuterated residual solvent peak or
tetramethylsilane peak (TMS, δ 0 ppm). Trifluoroacetic acid
(CF3COOH, δ −76.55 ppm) was used as the external reference for
19F NMR, and 85% phosphoric acid (H3PO4, δ 0 ppm) was used as the
1
2
3
(s), 145.57 (ddt, JC2F = 260 Hz, JCF = 13 Hz, JCF = 4 Hz), 140.56
1
2
3
(dd, JCF = 252 Hz, JCF = 16 Hz), 123.96 (tt, JCF = 12 Hz, JCF = 3
Hz), 107.04 (t, JCF = 15 Hz), 63.38 (s).
2
Synthesis of Ethane-1,2-diyl Bis(3-(diphenylphosphanyl)-
propanoate) (2b). Monomer 2b was synthesized in a similar manner
as monomer 1b from 3-(diphenylphosphino)propionic acid (595 mg,
2.30 mmol), ethylene glycol (71 mg, 1.15 mmol), EDAC (220 mg,
1.15 mmol), and DMAP (14 mg, 0.12 mmol) in dry CH2Cl2 (20 mL).
The crude product was purified by column chromatography using
hexanes:ethyl acetate (20:3, v:v) to give a thick yellow liquid (499 mg,
80%). 1H NMR (500 MHz, CDCl3): δ 7.45−7.38 (m, 8H), 7.36−7.28
(m, 12H), 4.20 (s, 4H), 2.48−2.30 (m, 8H). 31P NMR (81 MHz,
CDCl3): δ −14.12 (s). 13C NMR (126 MHz, CDCl3): δ 172.84 (d,
1
2
3JCP = 15 Hz), 137.73 (d, JCP = 13 Hz), 132.72 (d, JCP = 19 Hz),
128.84 (s), 128.57 (d, 3JCP = 7 Hz), 62.30 (s), 30.60 (d, 1JCP = 20 Hz),
2
22.99 (d, JCP = 13 Hz).
Synthesis of 1,4-Bis(diphenylphosphanyl)benzene (2c).
Monomer 2c was synthesized following the reported procedures.59
In brief, cesium carbonate (978 mg, 3.00 mmol) and copper iodide (46
mg, 0.24 mmol) were mixed in a flame-dried round-bottom flask.
Afterward, a solution containing 1,4-diiodobenzene (792 mg, 2.40
mmol) and diphenylphosphine (372 mg, 2.00 mmol) in toluene (5
mL) was added. After heating at 110 °C for 24 h, the mixture was
filtered through a packed Celite and rinsed with dichloromethane (3 ×
20 mL). The solution was then washed with water (3 × 100 mL) and
external reference for 31P NMR. Infrared spectra were recorded on a
Nicolet 6700 FT-IR spectrometer (Thermo Scientific, West Palm
Beach, FL).
Synthesis of 4,4′-Diazido-2,2′,3,3′,5,5′,6,6′-octafluoro-1,1′-
biphenyl (1a). Compound 1a was synthesized following a previously
developed protocol.55,56 To the solution of decafluorobiphenyl (5.0 g,
15 mmol) in 5 mL of DMF, a solution of sodium azide (1.95 g, 30.0
mmol) in 5 mL of DMF was added. The solution was stirred at room
B
DOI: 10.1021/acs.macromol.8b00618
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Scheme 2. (A) Bis-Azide (1a, 1b) and Bis-Phosphine (2a, 2b, 2c) Used in This Study; Syntheses of 1a (B), 1b (C), 2b (D), and
2c (E)
1
dried over sodium sulfate. After the solvent was removed on a
Rotovap, the crude was purified by column chromatography using
hexanes:dichloromethane (2:1, v:v) as the eluent to give 2c as a white
Polymer 3bb. Yield 28.9 mg, 37%. H NMR (500 MHz, CDCl3):
δ 7.74−7.70 (m, 8H), 7.55−7.51 (m, 4H), 7.48−7.44 (m, 8H), 4.55 (s,
4H), 4.12 (s, 4H), 2.85−2.80 (m, 4H), 2.55−2.50 (m, 4H). 31P NMR
(81 MHz, CDCl3): δ 9.12 (t, JPF = 5 Hz). 19F NMR (188 MHz,
3
1
powder (385 mg, 73%). H NMR (200 MHz, CDCl3): δ 7.47−7.35
(m). 31P NMR (81 MHz, CDCl3): δ −3.97 ppm.
CDCl3): δ −146.54 (m, 4F), −158.64 (m, 4F).
1
Polymer 3ac. Yield; 60.0 mg, 30%. H NMR (200 MHz, CDCl3):
General Polymerization Procedure. Polymers 3aa, 3ab, 3ba,
and 3bb were synthesized as follows. A solution of PFAA monomer 1a
or 1b (0.26 mmol) in CH3CN (6.5 mL) was added to a solution of
phosphine monomer 2a or 2b (0.26 mmol) in CH3CN (6.5 mL). The
solution was stirred at room temperature for 30 min. Typically,
precipitates were observed after stirring for 15 min. The precipitate
was collected by centrifugation, and the supernatant was removed. The
polymer was then redissolved in 1 mL of DMF and was precipitated by
the addition of diethyl ether (13 mL). The precipitation/centrifugation
process was repeated three times, and the obtained light yellow solid
was then dried under vacuum.
δ 7.78−7.62 (m, 10H), 7.49−7.36 (m, 14H). 31P NMR (81 MHz,
CDCl3): δ 9.67 (t, JPF = 5 Hz). 19F NMR (188 MHz, CDCl3): δ
3
−140.48 (m, 4F), −149.80 (m, 4F).
Thermal Stability. Thermal stability studies were conducted by
thermal gravimetric analysis (TGA 5500, TA Instruments, New Castle,
DE). Typically, about 2.5 mg of the polymer sample was added to the
TGA pan. The polymer was then heated under Ar at a heating rate of
10 °C/min until the temperature reached 800 °C.
RESULTS AND DISCUSSION
■
Polymer 3ac was synthesized as follows. A solution of PFAA
monomer 1a (0.26 mmol) in CH3CN/CH2Cl2 (1/1) (6.5 mL) was
added to a solution of phosphine monomer 2c (0.26 mmol) in
CH3CN/CH2Cl2 (1/1) (6.5 mL). The solution was stirred at room
temperature for 30 min. It was then purified following the purification
method above.
Polymer Synthesis. Bis-PFPAs and bis-phosphines with
aromatic or aliphatic linkers were used in this study (Scheme
2). The electron-deficient nature of the perfluorinated aryl
azide allows the straightforward synthesis of bis-PFPA 1a by
nucleophilic aromatic substitution of decafluorobiphenyl with
NaN3 at room temperature in high yield (91%) (Scheme 2).60
Monomer 1b was synthesized by esterification of EDAC-
activated 4-azido-2,3,5,6-tetrafluorobenzoic acid with 1,2-ethyl-
ene diol in 70% yield. Phosphine 2b was synthesized by
esterification of EDAC-activated 3-(diphenylphosphino)-
propanoic acid with 1,2-ethylene diol in 80% yield. Phosphine
2d was synthesized by a CuI-catalyzed cross-coupling of
diphenylphosphine and 1,4-diiodobenzene under toluene reflux
in 45% yield.59
Polymer 3aa. Yield: 71.6 mg, 34%. 1H NMR (500 MHz, CDCl3):
δ 7.67−7.64 (m, 8H), 7.46−7.45 (m, 4H), 7.41−7.38 (m, 8H), 2.74−
2.67 (m, 4H), 1.85−1.75 (m, 2H). 31P NMR (81 MHz, CDCl3): δ
3
7.56 (t, JPF = 5 Hz). 19F NMR (188 MHz, CDCl3): δ −147.76 (m,
4F), −159.02 (m, 4F).
Polymer 3ba. Yield: 88.4 mg, 37%. 1H NMR (500 MHz, CDCl3):
δ 7.67−7.64 (m, 8H), 7.46−7.45 (m, 4H), 7.41−7.38 (m, 8H), 2.74−
2.67 (m, 4H), 1.85−1.75 (m, 2H). 31P NMR (81 MHz, CDCl3): δ
3
9.75 (t, JPF = 5 Hz). 19F NMR (188 MHz, CDCl3): δ −146.37 (m,
4F), −158.81 (m, 4F).
Polymer 3ab. Yield: 136 mg, 56%. 1H NMR (500 MHz, CDCl3):
δ 7.78−7.74 (m, 8H), 7.52−7.49 (m, 4H), 7.46−7.43 (m, 8H), 4.12 (s,
4H), 2.86−2.81 (m, 4H), 2.58−2.53 (m, 4H). 31P NMR (81 MHz,
Monomers 1a and 2a were used as the model system to
determine the optimized conditions for the polymerization
(Figure 1A). The polymerization was carried out by mixing
equal mole amount of 1a and 2a in acetonitrile at room
temperature at the monomer concentration of 20, 30, or 40
3
CDCl3): δ 6.76 (t, JPF = 5 Hz). 19F NMR (188 MHz, CDCl3): δ
−147.92 (m, 4F), −158.86 (m, 4F).
C
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Figure 1. (A) Synthesis of polymer 3aa. (B) 31P NMR spectra of the polymerization mixture of 1a and 2a in acetonitrile (20 mM) over the course of
40 min. A spectrum was taken every 2 min. Spectrum 1 was that of 2a before 1a was added.
mM. Upon mixing the two monomers, precipitate started to
form immediately. After stirring for 30 min, the precipitate was
collected and was purified by three cycles of dissolving in DMF
and precipitating with diethyl ether.
formation of polyphosphazene, a broad peak at around 22.49
ppm appeared immediately after mixing the two monomers
(spectra 2−4). This peak can be assigned to the phosphazide
intermediate54,61 formed from the reaction between the azide
and the phosphine, as the Staudinger reaction proceeds through
a phosphazide intermediate before converting into phospha-
zene.62
The progress of polymerization was monitored using 31P
NMR at the monomer concentration of 20 mM over the course
of 40 min (Figure 1B). The 31P NMR spectrum of 2a
(spectrum 1) contains a single peak at −16.54 ppm which
corresponds to the P in 2a. Upon addition of 1a, the peak at
−16.54 ppm decreased, and a new peak at −17.11 ppm
appeared. This could be assigned the chain terminus of the
polymer which gradually decreased in relative intensity as the
polymer chain grew. The peak at −16.54 ppm continued to
decrease and was consumed by the end of the reaction. A new
peak at 14.89 ppm started to appear at 6 min into the
polymerization (spectrum 4). This peak is typical of
phosphazene formed from the PFPA-Staudinger reaction,54
which can be assigned to the P in the polyphosphazene product
3aa. As the high molecular weight polymer precipitated out
from the reaction, the phosphazene product detectable by
NMR in the supernatant solution should be low molecular
weight fractions. The intensity of the peak at 14.89 ppm
continued to grow with the progress of the polymerization and
plateaued at the end of 30 min (spectrum 16). Before the
The polymerization was also monitored by IR (Figure 2).
After polymerization, the characteristic azide peak at 2120/
2148 cm−1 in the IR spectrum of 1a (Figure 2A) disappeared,
indicating the full conversion of the monomer 1a (Figures 2C−
E). The NP band at 1175 cm−1, which was absent in both
monomers, appeared in all polymers (Figures 2C−E).63 The
aliphatic −CH and aromatic =CH stretching at around 2900−
3100 cm−1 in monomer 2a (Figure 2B) together with the
aromatic CC ring stretch around 1470 and 1640 cm−1 in
both monomers 1a (Figure 2A) and 2a (Figure 2B) also
appeared in the polymer (Figures 2C−E).
The product 3aa is furthermore characterized by 31P and 19
F
NMR (in CDCl3, Figure 3). The 31P NMR spectrum of the
polymer contained only one peak at 7.56 ppm, which belonged
to the P in the phosphazene structure (Figure 3A). The 31P
NMR spectra of the starting material 2a and the phosphazide
intermediate formed between 1a and 2a appeared at −22.3
D
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Table 1. Effect of the Monomer Concentration and Solvent
on the Molecular Weight and Dispersity of Polymer 3aa
monomer
concn
(mM)
solvent
polarity
a
w
a
n
a
entry
solvent
index64,65
M
̅
M
̅
Đ
1
2
3
4
5
20
30
40
20
20
CH3CN
CH3CN
CH3CN
EtOAc
6.2
6.2
6.2
4.3
3.1
59 516
54 141
57 041
41 254
21 706
54 051
44 303
47 730
29 706
15 384
1.1
1.2
1.2
1.4
1.4
CH2Cl2
a
Measured by GPC (THF, room temperature, polystyrene standard).
had higher molecular weight (M = 44 000−50 000) and
̅
n
narrower dispersity (Đ: 1.1−1.2) than those prepared by the
classic Staudinger reaction. The narrowest molecular weight
distribution of the polymer (Đ: 1.1, entry 1) was obtained from
the lowest monomer concentration used (20 mM), whereas Đ
increased to 1.2 at higher monomer concentrations of 30 and
40 mM (entries 2 and 3). The molecular weight of the polymer
was also slightly higher at the lowest monomer concentration
(entry 1 vs entries 2 and 3). The low dispersity can be
attributed to the fast reaction and the low solubility of the
polymer in the polymerization solvent CH3CN. The high
molecular weight polymer precipitated immediately upon
mixing the two monomers. The low molecular weight polymer
remained in the solution (Figure 1) and was removed during
the work-up. The collected polymer product contained only the
high molecular weight fractions with more uniform molecular
weight distribution. Increasing monomer concentration would
further accelerate premature termination of the growing
polymer chains, contributing to increased dispersity.
Figure 2. IR spectra of monomer 1a (A), 2a (B), and polymer 3aa
synthesized from 1a and 2a at 1:1 mole ratio at monomer
concentration of 20 (C), 30 (D), or 40 mM (E) in acetonitrile for
30 min.
The influence of solvent on the polymerization was next
tested while keeping the monomer concentration at 20 mM
(Table 1, entries 1, 4, and 5). The molecular weight of the
polymer increased with the polarity of the solvent. The
molecular weight was also more homogeneous for polymer
obtained in more polar solvent, giving Đ of 1.1 in CH3CN
(entry 1) and 1.4 in ethyl acetate and dichloromethane (entries
4 and 5). The Staudinger reaction goes through a polar
transition state that is more stabilized in the polar solvent and
hence faster reaction.62 In the PFAA-Staudinger reaction, the
reaction rate increased with the polarity of the solvent.54 We
expect that the rate of PFAA-Staudinger polymerization
increases with the polarity of the solvent in the order of
CH2Cl2, EtOAc, and CH3CN. The solubility of the polymer
differs considerably in different solvents. Acetonitrile is a poor
solvent for the polymer. When the polymerization was carried
out in acetonitrile, precipitate was formed immediately,
resulting from a combination of fast rate and poor solubility
of the polymer. CH2Cl2 is a less polar but better solvent for the
polymer. The slower polymerization as well as the higher
solubility of the polymer contributed to the lower molecular
weight and increased dispersity.
Figure 3. 31P NMR (A) and 19F NMR (C) spectra of polymer 3aa
synthesized from monomer 1a and monomer 2a (20 mM) at room
temperature for 30 min. (B) 31P NMR spectrum of 2a. (D) 19F NMR
spectrum of 1a. The insets are the enlarged peaks. All spectra were
taken in CDCl3.
ppm (Figure 3B) and ∼25 ppm,54 respectively. In addition, the
P peak in the polymer 3aa showed the triplet splitting pattern
(Figure 3A, inset), resulting from the coupling to the nearest
two F atoms, which is typical of the P from the phosphazene
structure that was not observed in the starting material 2a
(Figure 3B, inset).54 The formation of the polymer was
furthermore confirmed by the two distinct peaks at −147.76
and −159.02 ppm in the 19F NMR spectrum of polymer 3aa
(Figure 3C), which shifted upfield from those of the monomer
1a at −134.79 and −147.71 ppm, respectively (Figure 3D).
The molecular weight and molecular weight distribution of
polymer 3aa prepared at different monomer concentration in
CH3CN were analyzed by GPC (Table 1, entries 1−3). The
polyphosphazenes synthesized by PFAA-Staudinger reaction
To test the scope of the polymerization, monomers having
aliphatic, aromatic and ester linkers were used to investigate the
effect of the monomer structure on polymerization (Schemes 1
and 3). The polymerization was carried out by stirring 20 mM
of monomer 1 and monomer 2 in CH3CN at room
temperature for 30 min. Typically, precipitates were observed
after stirring for 15 min. The polymer was collected by
centrifugation, and was purified by repetitive dissolution/
E
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Scheme 3. Scope of PFAA-Staudinger Polymerization
precipitation from DMF/ether to give the polymer as light
yellow solids.
Table 2. Molecular Weight, Dispersity, Decomposition
Temperature, and Char Yield of Synthesized
Polyphosphazenes
Diphosphine 2c was chosen to test whether the aromatic
structure would affect the reactivity of the phosphine as
extended conjugation may reduce the reactivity of the
phosphine.66 Dichloromethane was initially used as the solvent
for the synthesis of polymer 3ac due to the lower solubility of
monomer 2c in CH3CN; however, the polymerization was
incomplete after 1 h as shown by the presence of the phosphine
monomer after purification. The slow reaction could be due to
the lower polarity of the solvent in addition to the conjugated
structure. The reaction was then carried out in 1:1
CH3CN:CH2Cl2. Under this condition, the reaction was
completed in 1 h as shown by the absence of the phosphine
peak in the polymer product (Figure S22).
decomposition
temperature (°C)
d
char
yield
a
a
a
e
entry polymer
M
M
Đ
Tonset Tmax
Tend
(%)
̅
̅
w
n
b
1
2
3
4
7
8
3aa
3ba
3ab
59 516 54 051 1.1
52 724 43 282 1.2
57 018 49 303 1.2
56 158 45 369 1.2
353
266
251
267
351
255
410
328
340
310
441
395
455
473
441
446
520
510
28
22
21
7
b
b
b
3bb
c
3ac
N.D.
N.D.
N.D.
53
21
PBPP
a
Measured by GPC (THF, room temperature, polystyrene standard),
N.D. = not determined because the polymer was insoluble in THF.
31P NMR analysis of the polymers 3aa, 3ba, 3ab, and 3bb
showed the phosphazene peak in the range of 6.8−9.8 ppm
(Figures S10, S13, S16, and S19), indicating that the
polyphosphazene products were successfully synthesized.
b
c
Synthesized in CH3CN. Synthesized in CH2Cl2:CH3CN (1:1, v:v).
d
Measured from differential thermogravimetric analysis (DTG, Figure
S24). Data are presented as the temperature at the onset (Tonset),
e
maximal (Tmax), and end (Tend) of decomposition. Measured by TGA
under argon from Figure 4.
Polymers 3aa, 3ba, 3ab, and 3bb have M in the range of
̅
w
52 724−59 516 and low Đ of 1.1−1.2 (Table 2, entries 1−4).
The molecular weight of polymers 3ac was not analyzed due to
its poor solubility in THF.
The thermal stability of the synthesized polyphosphazenes
was studied using TGA and DTG, from which the
decomposition temperature and char yield were obtained.
The polymer sample was gradually heated under argon and the
weight of the sample was monitored by TGA. The weight of
the sample was normalized against the initial sample weight,
and the percent weight was then plotted against the heating
temperature (Figure 4). The thermal decomposition temper-
atures were obtained from the differential thermogravimetric
curve shown in Figure S24,69 from which the decomposition
temperatures at the onset, the maximal rate, and the end of
decomposition were obtained (Table 2). Several factors
influence the thermal stability of the polymer, including the
Thermal Stability. The high nitrogen and phosphorus
contents in polyphosphazenes make them intrinsically fire
resistant.67 These polymers have high thermal stability, which
can be characterized by high thermal decomposition temper-
atures and high char yields. Thermal decomposition temper-
ature is the temperature that corresponds to a significant
decrease of the polymer weight.68 The char yield is defined as
the percentage of the polymer weight that remains unchanged
at the end of a heating process.67 For example, PBPP (Figure
4), a commercially available polyphosphazene, has a thermal
degradation temperature of 395 °C and a char yield of 37%.67
F
DOI: 10.1021/acs.macromol.8b00618
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Article
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.8b00618.
Experimental protocols; 1H, 19F, 31P, and 13C NMR
spectra of monomers and polymers (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail mingdi_yan@uml.edu (M.Y.).
ORCID
Figure 4. TGA thermograms of polymers tested. Samples were heated
at 10 °C/min under Ar.
Mingdi Yan: 0000-0003-1121-4007
Notes
The authors declare no competing financial interest.
structure of the monomer and the strength of the chemical
bond.70,71 Studies on the thermal degradation of polyphospha-
zenes conclude that the chain scission typically occurs
randomly followed by both depolymerization and some degree
of cross-linking.72−75 The decomposition temperatures of
polymers 3aa, 3ab, and 3ac synthesized from 1a having a
diphenyl linker are in general higher than polymers 3ba and
3bb synthesized from 1b that has an ester linker (entry 1 vs 2,
entry 3 vs 4, entry 5 vs 6). The ester bond typically decomposes
around 263−284 °C as compared to the Ph−Ph bond which
decomposes around ∼400 °C.76,77 The highest thermal
decomposition temperature was obtained from polymer 3ac
(441 °C, entry 7), which was synthesized from the aromatic
monomers 1a and 2c. Both polymers 3ac and 3aa have higher
thermal decomposition temperature than PBPP.
Similar to the decomposition temperature, the char yields of
polymers 3aa, 3ab, and 3ac synthesized from 1a were higher
than those of polymers 3ba and 3bb synthesized from 1b
(Table 2, entry 1 vs 2, entry 3 vs 4, entry 5 vs 6). This is also
consistent with other studies that associate highest char yields
with aromatic structures.67,78 With the exception of 3bb and
3ab, all other polyphosphazenes synthesized had the same or a
substantially higher char yield than PBPP (21%). Polymer 3ac
has the highest char yield (53%, entry 7), which is higher than
EYPEL-A rubber (20%), a material that had been used by US
Navy as a helicopter seating material.67
ACKNOWLEDGMENTS
■
This work was supported in part by the National Science
Foundation (CHE-1112436).
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