Polymerization behaviour of butyl bis(hydroxymethyl)phosphine oxide: Phosphorus containing polyethers for Li-ion conductivity

Article abstract of DOI:10.1007/s12039-015-0819-9

Synthesis of phosphorus containing polyethers and their lithium-ion conductivities for the potential use as solid polymer electrolyte (SPE) in high-energy density lithium-ion batteries have been described. Co-polymerization of butyl bis(hydroxymethyl)phosphine oxide with three different dibromo monomers were carried out to produce three novel phosphorous containing polyethers (P1-P3). These polymers were obtained via nucleophilic substitution reactions and were characterized by ¹H, ³¹P NMR spectral data and gel permeation chromatography. SPEs were prepared using polyethers (P1 and P2) with various amounts of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). The lithium-ion conductivity of SPE2 containing 40 wt% of LiTFSI was 2.1 × 10^-5 S cm^-1 at room temperature and 3.7 × 10^-4 S cm^-1 at 80°C. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s12039-015-0819-9

c
J. Chem. Sci. Vol. 127, No. 4, April 2015, pp. 635–641. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-015-0819-9
Polymerization behaviour of butyl bis(hydroxymethyl)phosphine oxide:
Phosphorus containing polyethers for Li-ion conductivity
HEERALAL VIGNESH BABU, BILLAKANTI SRINIVAS,
KHEVATH PRAVEEN KUMAR NAIK and KRISHNAMURTHI MURALIDHARAN∗
School of Chemistry, University of Hyderabad, Gachibowli, Hyderabad 500 046, India
e-mail: kmsc@uohyd.ernet.in
MS received 30 October 2013; accepted 24 July 2014
Abstract. Synthesis of phosphorus containing polyethers and their lithium-ion conductivities for the poten-
tial use as solid polymer electrolyte (SPE) in high-energy density lithium-ion batteries have been described.
Co-polymerization of butyl bis(hydroxymethyl)phosphine oxide with three different dibromo monomers were
carried out to produce three novel phosphorous containing polyethers (P1–P3). These polymers were obtained
1
via nucleophilic substitution reactions and were characterized by H, 31P NMR spectral data and gel perme-
ation chromatography. SPEs were prepared using polyethers (P1 and P2) with various amounts of lithium
bis(trifluoromethanesulfonyl)imide (LiTFSI). The lithium-ion conductivity of SPE2 containing 40 wt% of
LiTFSI was 2.1 × 10−5 S cm−1 at room temperature and 3.7 × 10−4 S cm−1 at 80◦C.
Keywords. Phosphorus; polyether; solid polymer electrolyte; lithium-ion conductivity.
1. Introduction
retardant property for high-energy density lithium- ion
batteries is always in demand to replace the organic
liquid electrolyte.
Demand for efficient energy storage devices has
increased due to the requirements of higher energy stor-
age capacity and portability.1 Rechargeable lithium-ion
batteries are the major power sources for portable elec-
tronics. At present, the performance of lithium-ion bat-
tery for miniature portable electronic devices is sat-
isfactory. However, the current technology cannot be
promoted to automobile batteries due to the poten-
tial fire hazard of organic liquid electrolyte present in
batteries.2 Moreover, organic liquid electrolytes suf-
fer from volatility, pressure build-up or even explosion
hazard, limited operating temperature range and harm-
ful leakage. The fundamental solution to this problem
is substitution of the flammable organic liquid elec-
trolyte with non-flammable solid polymer electrolyte
(SPE). Besides the safety considerations, other inter-
esting properties such as flexibility, easy manipulation,
wide operating temperature range, high electrochemical
stability and light weight fabrication allows possibili-
ties for advanced lithium-ion polymer secondary bat-
teries. In addition, both ion conduction and mechanical
separation can be attained in a single solid electrolyte
membrane. Furthermore, a battery can be fabricated in
any desired shape and size due to the flexibility of poly-
mer membranes.3 Therefore, a better SPE with flame
Wright and co-workers revealed the ability of
poly(ethylene oxide) (PEO) to dissolve inorganic salts
and exhibiting ion conduction at room temperature.4
But, the room temperature ionic conductivity of PEO
based polymer electrolyte is too low (10−7 S cm−1)
for practicable applications. Our interest is to prepare
SPEs having good ionic conductivity with flame retar-
dant property. So, it is necessary to understand the con-
duction mechanism in order to improve the ionic con-
ductivity of SPEs. The ionic conductivity in amorphous
polymers depends on local segmental motions of poly-
mer chains and such a favourable situation was obtained
only at temperatures above Tg.5 Hence, an amorphous,
solid polymer with low glass transition temperature is
preferred for ionic conductivity.
Our aim is to induce conformational flexibilities by
incorporating phosphorus in the form of C-P-C bonds in
the chain. Integration of heteroatoms into the carbon–
carbon polymer chain can be an intriguing strategy to
tailor the properties to expand the applications of mate-
rials. However, incorporation of heavier main–group
element in the polymer backbone is synthetically chal-
lenging. We have used bis(hydroxymethyl)phosphine
oxides as monomer in a co-polymerization reaction to
lead to polyethers. Herein, we report the polymerization
behaviour of butyl bis(hydroxymethyl)phosphine oxide
∗For correspondence
635

636
Heeralal Vignesh Babu et al.
and the influence of phosphorus containing polyethers the eluent. The molecular weight and molecular weight
on ionic conductivity.
distributions were calculated using polystyrene as a
standard.
The impedance measurements of polymer elec-
trolytes were performed in Zahner zennium electro-
chemical work station with built-in Thales software for
data acquisition. The measurements were done in the
frequency range of 1 Hz to 4 MHz. The specimens in
the form of pellets were sandwiched between two gold
plated electrodes housed in a homemade cell. For vari-
able temperature measurements, cell was equilibrated
at each temperature for 30 min. before measuring. The
conductivity was calculated using the equation σ =
d/(ARb), where d is the thickness of the polymer elec-
trolyte disc, A is the surface area of the pellet and Rb
is the bulk resistance value which can be obtained from
the Nyquist plot.9
2. Experimental
2.1 Materials and instruments
All manipulations involving air and moisture sen-
sitive compounds were carried out using standard
Schlenk techniques under the atmosphere of dry nitro-
gen. All solvents to be used under inert atmosphere
were thoroughly deoxygenated using freeze-pump-
thaw method before use. They were dried and puri-
fied by refluxing over a suitable drying agent followed
by distillation under nitrogen atmosphere. The com-
pounds, tris(hydroxymethyl)phosphine (S1),6 1,4–bis
(bromomethyl)–2,3,5,6–tetramethylbenzene7 (M3), 1,4–
bis(bromomethyl)-2,3,5,6–tetramethoxybenzene8 (M4)
were synthesized according to the literature pro-
cedures. The compounds, tetrakis(hydroxymethyl)
2.2 Experimental procedure
phosphonium chloride (Acros); 1,5–dibromo-pentane 2.2a Synthesis of butyl bis(hydroxymethyl)phosphine
(M5), 1–iodobutane and lithium bis(trifluoromethane- oxide (M1): The synthesis of M1 was achieved
sulfonyl)imide LiN(SO2CF3)2 (LiTFSI) were pur- in three steps as follows: A solution of tris
chased from Aldrich and were used as received without (hydroxymethyl)phosphine (S1) (12.60 g, 0.1 mol) in
purification. K2CO3 (Merck) was dried at 100◦C for 6 h methanol (20 mL) was taken into a 500 mL two-necked
under vacuum prior to use.
flask under nitrogen. To this, deoxygenated mixture of
Structures of all the products were confirmed by 1–iodobutane (37.43 g, 0.2 mol) and methanol (140
1H, 31P and 13C NMR spectra. All the NMR spectra mL) was added dropwise at 0◦C for 30 min. The reac-
were recorded on a Bruker Avance 400 MHz FT NMR tion mixture was refluxed for 4 h and the solvent was
spectrometer at room temperature using either CDCl3 evaporated under high vacuum to get a viscous oily
or CD3OD as solvent. The chemical shifts are reported product. Purification of this product was unsuccessful
in parts per million (δ) relative to tetramethylsilane as because it was non-volatile oily liquid.
1
reference for H and 13C{1H} NMR (100 MHz). The
To the above mixture (25.38 g), dry triethylamine
31P{1H} NMR (162 MHz) spectra referenced to 85% (190 mL) was added in one portion under nitrogen with
H3PO4. The Netzsch STA 409 PC model was used stirring. The resulting mixture was then heated to 60◦C
for thermogravimetric and differential thermal analy- for 1 h and then allowed to cool to room temperature.
sis (TG-DTA) to examine the thermal stability. The The solid [NHEt3]Cl byproduct was filtered off and
decomposition behaviour of polymers was studied from triethylamine solvent was distilled out at atmospheric
30 to 900◦C under the nitrogen flow with a heat- pressure to give a crude product which was then heated
ing rate of 10◦C/min. The temperature of 5% weight to 90◦C for 5 h under reduced pressure. A viscous oily
loss was chosen as onset point of decomposition (Td). product was obtained.
The glass transition temperatures (Tg) of polyethers
To the above oily product (10.88 g) in methanol
were measured on a Differential Scanning Calorimeter (25 mL), 30% hydrogen peroxide solution (8.3 mL, 72
(DSC) from PerkinElmer (Pyris Diamond DSC 8000). mmol) was added dropwise at 15◦C and stirred for 2
The measurements were performed at a heating rate h. The solution was concentrated under vacuum which
of 10◦C/min under the nitrogen. Tg was assigned as was purified by column chromatography on silica gel
the inflection point in the thermogram. The molecular eluting with CH2Cl2/MeOH (6:1) to give a clear colour-
weights of the polymers were determined by Gel Per- less liquid butyl bis(hydroxy methyl)phosphine oxide
meation Chromotography (GPC) of Shimadzu 10AVP (M1). (TLC was analyzed by immersing the plate in
model equipped with refractive index (RI) detector. The 5% sulfuric acid in methanol followed by heating using
separation was achieved using a Phenogel mixed bed hot-air drier for 2 min. and spots were visualized by
1
column (300 × 7.80 mm) operated at 30◦C with a flow naked eye) Yield: 5.40 g, 45%. H NMR (CD3OD): δ
rate of 0.5 mL/min. using tetrahydrofuran (THF) as 4.86 (s, 2H, –OH), 3.97 (dd, J = 26.5, 14.1 Hz, 4H,

Synthesis of phosphorus containing polyethers
637
–OCH2P), 1.91–1.79 (m, 2H, –PCH2CH2), 1.71–1.56 2H, –PCH2), 1.67–1.50 (m, 6H, –CH2), 1.48–1.24 (m,
(m, 2H, –CH2CH2CH3), 1.53–1.41 (m, 2H, –CH2CH3), 4H, –CH2), 0.87 (t, 3H, –CH3). 31P NMR (CDCl3): δ
0.96 (t, J = 7.3 Hz, 3H, –CH3). 13C NMR (CD3OD): δ 44.92.
58.45 (d, J = 79 Hz, –PCH2O), 26.39 (d, J = 13 Hz,
–CH2), 24.98 (d, J = 4 Hz, –CH2), 24.13 (d, J =
2.2c General preparation of solid polymer elec-
62 Hz, –CH2), 15.07 (s, –CH3). 31P NMR (CD3OD):
trolytes SPE1 and SPE2: The polymers P1 and P2
δ 51.54. EI–MS: m/z 333 (2M++1, base peak). Anal.
were dried at 60◦C and 40◦C under vacuum for 8 h.
LiN(SO2CF3)2 was dried at 150◦C under vacuum for
10 h before use. All manipulations were carried out
in an MBraun glove box filled with ultrapure nitro-
gen gas. Electrolytes with different ratio were prepared
as follows: the polymer was dissolved in THF with
lithium salt and stir for 12 h at 25◦C. After this, THF
was evaporated under vacuum and dried the residue
at 60◦C for 12 h. The residue was loaded in to a die
and then pressed to make a pellet. Specimens of 0.07–
0.08 cm thickness and 0.9 cm diameter were obtained
for conductivity studies. These pellets were sandwiched
between two gold plated electrodes housed in a home-
made cell for conductivity studies. The consistency of
results was checked by repeating the experiment three
times.
calcd for C6H15O3P: C, 43.37; H, 9.10. Found: C,
43.28; H, 9.25. Further continuation of elution yielded
tris(hydroxymethyl)phosphine oxide (M2). Yield: 1.50
g, 12%. 1H NMR (CD3OD): δ 4.11 (s, 6H, –CH2), 2.55
(s, –OH). 13C NMR (CD3OD): δ 57.14 (d, J = 76 Hz,
–CH2).31P NMR (CD3OD): δ 46.00.
2.2b General procedure for the copolymerization
of M1 with M3−M5: A typical polymerization proce-
dure is as follows. The mixture of BuP(O)(CH2OH)2
(M1) (3 mmol), corresponding co-monomer (M3, M4
or M5) (3.3 mmol) and K2CO3 (7.2 mmol) in N,
N–dimethylacetamide (2.5 mL) was taken in a 100 mL
round-bottom flask equipped with a Dean-Stark trap. To
this, toluene (20 mL) was added as an azeotrope. The
reaction mixture was heated to 130◦C for 5 h in order to
remove water that formed during the reaction. Then, the
reaction mixture was heated to 160◦C and maintained
for 30 h. The viscous reaction mixture was poured into
ethanol and filtered off. The product was purified by
dissolving in THF and reprecipitating in hexane. This
process was repeated for three more times to obtain pure
polymers.
3. Results and Discussion
The following methods are generally used to achieve
flame retardant property for polymers: (a) blending
polymers with phosphorus containing molecules; (b)
covalently attaching phosphorus containing molecules
as pendant group to the polymer chains; (c) phospho-
rus containing polymers in which phosphorus present
in main chain of polymer.10 We are interested to syn-
thesize polyethers having phosphorus in main chain of
polymer. It is known that the hydroxymethyl groups
of alkyl bis(hydroxymethyl)phosphine RP(CH2OH)2
behaved like masked -PH2 group. But it behaves as a
normal diol when the phosphine was converted to phos-
phine oxide RP(O)(CH2OH)2.11 A variety of natural
and synthetic polymers where phosphorus is in main
chain with O-P-O linkages are known. However, poly-
mers with C-P-C linkages are challenging for synthesis.
Therefore, use of alkyl substituted bis(hydroxymethyl)
phosphine oxide as monomer to produce polyether is
attractive methodology to obtain polymers with C-P-C
links.
Polymer 1 (P1) (Copolymer of M1 and M3): After
purification, P1 was obtained as a white colour powder.
1
Yield: 83%. H NMR (CDCl3): δ 4.81–4.62 (m, 4H,
OCH2Ph), 3.98–3.82 (m, 4H, PCH2O), 2.45–2.18 (m,
12H, –CH3), 1.79–1.65 (m, 2H, –PCH2), 1.63–1.50 (m,
2H, –CH2), 1.48–1.31 (m, 2H, –CH2CH3), 0.88 (t, 3H,
–CH3). 31P NMR (CDCl3): δ 44.15.
Polymer 2 (P2) (Copolymer of M1 and M4): After
purification, P2 was obtained as a white color powder.
1
Yield: 80%. H NMR (CDCl3): δ 4.77–4.56 (m, 4H,
OCH2Ph), 4.32–3.91 (m, 4H, PCH2O), 3.90–3.68 (m,
12H, –OCH3), 1.93–1.78 (m, 2H, –PCH2), 1.66–1.51
(m, 2H, –CH2), 1.48–1.35 (m, 2H, –CH2CH3), 0.88
(t, 3H, –CH3).31P NMR (CDCl3): δ 45.75.
3.1 Synthesis of monomer (M1)
Polymer 3 (P3) (Copolymer of M1 and M5): After While tris(hydroxymethyl)phosphine (S1) was treated
purification, P3 was obtained as a colorless liquid. with 1–iodobutane in methanol, followed by the addi-
1
Yield: 82%. H NMR (CDCl3): δ 3.95–3.68 (m, 4H, tion of triethylamine and hydrogen peroxide yielded
OCH2P), 3.57–3.38 (m, 4H, –CH2O), 1.86–1.69 (m, M1 as a major product and M2 as a minor product

638
Heeralal Vignesh Babu et al.
(scheme 1). These compounds were separated by col- nucleophilic substitution reactions (scheme 2). The con-
umn chromatography. The compound P(CH2OH)3 S1 densation polymers were synthesized in the presence of
was known to release formaldehyde (HCHO) when K2CO3 as base in N, N–dimethylacetamide (DMAc).
heated. The HCHO thus formed in turn reacted with The polymerization procedure was carried out in two
unreacted P(CH2OH)3 similar to well-known Wittig stages. First, toluene was added to the reaction mixture
reaction forming M2. This is also a reason for low yield and azeotropic distillation was continued until water
of M1. The structures of M1 and M2 were confirmed that formed during the reaction was removed from the
by 1H, 13C and 31P NMR spectral data.
reaction mixture.12 In the later stage, temperature was
increased to 160◦C and polymerization was progressed
for required time. All the crude polymers were purified
by reprecipitation from THF in hexane.
3.2 Synthesis of polyethers (P1–P3)
1,4–bis(bromomethyl)–2,3,5,6–tetramethylbenzene (M3)
and 1,4–bis(bromomethyl)–2,3,5,6–tetramethoxybenzene
(M4) were synthesized from durene and 2,5-dihydroxy-
1,4-benzoquinone respectively according to the literature
procedures.7,8 The butyl bis(hydroxy methyl)phosphine
oxide M1 was copolymerized with different dibromo
monomers such as M3, M4 and 1,5–dibromopentane
M5 to obtain polyethers P1–P3 respectively via
The polyethers P1 and P2 are solids while P3 is liq-
uid and they are readily soluble in chloroform, THF and
dimethyl sulfoxide. The polyethers P1–P3 were charac-
terized by 1H and 31P NMR spectral data which showed
that the data were consistent with the expected molec-
ular structure. The 31P{1H} NMR spectra of polymers
P1–P3 showed a single peak and appeared at δ 44.15,
45.75 and 44.92 ppm respectively. The decomposition
Scheme 1. Synthesis of butyl bis(hydroxymethyl)phosphine oxide (M1).
Scheme 2. Synthesis of phosphorus containing polyethers P1–P3.
Table 1. Properties of polyethers P1–P3.
Entry Co-monomer Polymer (P) Td (◦C) Tg (◦ C) Mn (g mol−1) Mw (g mol−1) PDI
1
2
3
M3
M4
M5
P1
P2
P3
308.2
279.9
240.7 −40.5
65.1
43.4
9414
8420
10077
15156
14230
16425
1.61
1.69
1.63

Synthesis of phosphorus containing polyethers
639
temperature (Td), glass-transition temperature (Tg),
molecular weight (Mn and Mw) and PDI of polyethers
P1–P3 are listed in table 1. The copolymers P1–P3 have
molecular weight (Mw) in the range of 14.2–16.4 kg
mol−1. Thermogravimetric analyses (figure 1) revealed
the thermal stability of polymers P1–P3 lies in the range
of 240–308◦C. The polymers P1–P3 are completely
amorphous and they did not show any peak corresponds
to melting temperature (Tm) on DSC analyses (figure 2).
3.3 Solid polymer electrolyte (SPE)
P1 and P2 were utilized to prepare SPE1 and SPE2
using various amounts of lithium bis(trifluoro-
methanesulfonyl)imide (LiTFSI) (10, 20, 30 and 40
wt%). The liquid polymer P3 was not used to make
an electrolyte. The polymers and lithium salt was
dissolved in THF at 25◦C and maintained for 12 h.
THF was evaporated and dried under vacuum to obtain
SPEs. Compared to P1 and P2, thermal stability of
SPE1 and SPE2 was increased with increasing lithium
salt content from 10 wt% of LiTFSI (10%) to 40 wt%
of LiTFSI (40%) (figure 3 and S1; table 1 and 2). Like-
wise, Tg of SPE1 and SPE2 increased continuously
with the addition of lithium salt from 10% to 40%
(figure 4 and S2; table 1 and 2). The rise in Td and Tg
compared to their corresponding polymer might be due
to the effective coordination of polymer chains with
lithium-ion. Based on the following observations from
DSC analyses: (a) absence of peaks matching the melt-
ing temperature (Tm) of lithium salt; (b) a single Tg; (c)
increase in Tg with the addition of lithium salt; it was
concluded, LiTFSI is completely miscible in P1 and P2
that affords a homogeneous amorphous phase and did
not exist as aggregates. The absence of melting point
of salts is attributable to the solvation of lithium-ions
by coordination of polymer.
Figure 1. TGA plots of polyethers P1–P3.
Figure 2. DSC plots of polyethers P1–P3.
The conductivities of SPE1 with 10–40 wt% of
LiTFSI [SPE1(10–40%)] and SPE2 with 10–40 wt%
LiTFSI [SPE2(10–40%)] were calculated from the
bulk resistance determined from complex impedance
spectra.13 The ionic conductivities at 30◦C and 80◦C
of SPE1(10–40%) and SPE2(10–40%) are listed in
table 2. Figure 5 shows the temperature dependent
Figure 3. TGA plots of SPE1 (10–40%).
Table 2. Properties of SPE1 (10–40%) and SPE2 (10–40%).
SPE1
SPE2
Conductivity σ (S cm−1
)
Td (◦C) Tg (◦C)
Conductivity σ (S cm−1
)
Td (◦C) Tg (◦C)
SPEs (wt% of LiTFSI)
30◦C
80◦C
30◦C
80◦C
SPE (10%)
SPE (20%)
SPE (30%)
SPE (40%)
5.1 × 10−7
6.5 × 10−7
9.5 × 10−7
2.6 × 10−6
5.6 × 10−6
8.3 × 10−6
4.4 × 10−5
7.3 × 10−5
324.1
332.5
341.6
355.8
78.9
86.7
3.2 × 10−6
2.3 × 10−5
8.8 × 10−5
1.4 × 10−4
3.7 × 10−4
290.2
298.1
310.5
321.7
52.4
65.3
70.6
78.1
6.5 × 10−6
102.4 1.2 × 10−5
116.2 2.1 × 10−5

640
Heeralal Vignesh Babu et al.
coordinating group (-OMe) in P2 as well as low Tg of
P2 compared to P1 makes a better host polymer for
lithium-ion conductivity.
4. Conclusions
In conclusion, we have described the synthesis of
phosphorus containing polyethers wherein phosphorus
exists in the main chain of polymer with C-P-C
links. The utilization of alkyl substituted bis(hydroxyl-
methyl)phosphine oxide as monomer for polymer syn-
thesis was studied. The completely amorphous, highly
stable and flame retardant polymers (P1–P3) were
obtained by condensation polymerization. The solid
polymer electrolytes SPE1 and SPE2 were prepared
and their ionic conductivity behaviour was examined.
The SPE2 (40%), prepared from P2, showed the high-
est conductivity of 3.7 × 10−4 S cm−1 at 80◦C. SPE2
may be a promising SPE having good ionic conductivity
with possible flame retardant property that would find
potential application in larger batteries.
Figure 4. DSC plots of SPE1 (10–40%).
nyquist plots of SPE1(10–40%) and Arrhenius plots of
SPE1 (10–40%) and SPE2 (10–40%). While increas-
ing the lithium salt content progressively from 10%
to 40%, the ionic conductivity of SPE1 and SPE2
increased. Further, ionic conductivity of SPE1 (10–
40%) and SPE2 (10–40%) were increasing with tem-
perature. The room temperature ionic conductivity of
SPE1 (10–40%) and SPE2 (10–40%) lies in the range
of 5.1 × 10−7 −2.1× 10−5 S cm−1. The maximum con-
ductivity of 3.7 × 10−4 S cm−1 was attained for SPE2
Supplementary Information
(40%) at 80◦C. Among SPE1 and SPE2, SPE2 exhib- TGA, DSC and nyquist plots of SPE2, 1H, 13C and 31
P
ited better ionic conductivity with various amounts of NMR of M1 and M2,1H and 31P NMR of P1–P3 are
lithium salt and with temperature. The presence of more available at www.ias.ac.in/chemsci.
Figure 5. Temperature dependent nyquist plots of SPE1 (a) 10% (b) 20% (c) 30% (d) 40%. Arrhenius plots of temperature
dependent conductivities of e) SPE1 (10–40%) f) SPE2 (10–40%).

Synthesis of phosphorus containing polyethers
641
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Acknowledgements
HVB is thankful to CSIR India for a fellowship. The
authors acknowledge the DST, India for financial sup-
port of the DST fast track project No. SR/FTP/CS-
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