Polyethers with phosphate pendant groups by monomer activated anionic ring opening polymerization: Syntheses, characterization and their lithium-ion conductivities

Article abstract of DOI:10.1016/j.polymer.2013.12.005

This paper describes the preparations and lithium-ion conductivities of various solid polymer electrolytes for potential use in high-energy density lithium-ion batteries. The ring opening polymerization of epoxides (M1-M6), catalyzed by Zn(II), Cu(II) and Cd(II) complexes in the presence of tetrabutylammonium bromide (TBAB), yielded polyethers (P1-P6) in which phosphates were attached as pendant groups. A reaction condition where Zn(II) catalyst used slightly excess to TBAB increased the polymerization rate remarkably and yielded the polyethers with higher molar masses in a short time. These polymerizations proceeded following a "monomer activated anionic ring opening polymerization" mechanism. These living like polymerizations also progressed according to "formation of polymer chain per initiator" model. The solid-state lithium-ion conductivities of these polymers were examined using lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). The conductivity of one of the solid polymer electrolytes with 40 wt% of LiTFSI was 5.2 × 10^-5 S cm^-1 at room temperature and 2.9 × 10^-4 S cm^-1 at 80 C.

Full text of DOI:10.1016/j.polymer.2013.12.005

Polymer 55 (2014) 83e94
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Polymer
journal homepage: www.elsevier.com/locate/polymer
Polyethers with phosphate pendant groups by monomer activated
anionic ring opening polymerization: Syntheses, characterization and
their lithium-ion conductivities
*
Heeralal Vignesh Babu, Krishnamurthi Muralidharan
School of Chemistry, University of Hyderabad, Hyderabad 500 046, India
a r t i c l e i n f o
a b s t r a c t
Article history:
This paper describes the preparations and lithium-ion conductivities of various solid polymer electro-
lytes for potential use in high-energy density lithium-ion batteries. The ring opening polymerization of
epoxides (M1eM6), catalyzed by Zn(II), Cu(II) and Cd(II) complexes in the presence of tetrabuty-
lammonium bromide (TBAB), yielded polyethers (P1eP6) in which phosphates were attached as pendant
groups. A reaction condition where Zn(II) catalyst used slightly excess to TBAB increased the polymeri-
zation rate remarkably and yielded the polyethers with higher molar masses in a short time. These
polymerizations proceeded following a “monomer activated anionic ring opening polymerization”
mechanism. These living like polymerizations also progressed according to “formation of polymer chain
per initiator” model. The solid-state lithium-ion conductivities of these polymers were examined using
lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). The conductivity of one of the solid polymer elec-
trolytes with 40 wt% of LiTFSI was 5.2 ꢀ 10ꢁ5 S cmꢁ1 at room temperature and 2.9 ꢀ 10ꢁ4 S cmꢁ1 at 80 ꢂC.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 12 September 2013
Received in revised form
28 November 2013
Accepted 2 December 2013
Available online 10 December 2013
Keywords:
Ring opening polymerization
Lithium-ion conductivities
Catalysis
1. Introduction
transported. Because of the conformational flexibility induced by
the presence of atoms of varying sizes in the polymer chains, the
Several variations have been tried to increase the ionic con-
ductivity of poly(ethylene oxide) (PEO) since its room temperature
ionic conductivity is low (10ꢁ7 S cmꢁ1) for practicable applications
[1]. Some effective variations are; the PEO based composites pro-
duced by mixing with polymer blends [2], inorganic fillers [3],
plasticizers [4] and ionic liquids [5]; PEO based network polymers
[6], cross-linked by siloxane polymers [7]; and block copolymers
[8]. One of the major reasons causing the low ionic conductivity at
ambient temperatures is the large crystalline area of PEO and less
conformational flexibility. Therefore, there are interests towards
decreasing crystallinity and increasing conformational flexibility.
Since the amorphous region is responsible for the ion-conductivity,
extending the amorphous phase can also improve the conductivity
of polymers [9]. The ionic conductivity in amorphous polymers is
caused by mobility of ions promoted by local segmental motion of
the polymer chains above Tg. Beyond Tg, the polymer’s super-
structures have disordered and flexible environment. Therefore,
when the chains are in constant motion, vacant spaces would
continually create and disappear through which ions can be
polyphosphazenes [10] and polysiloxanes [11] showed lithium ion
conductivities of 5.0 ꢀ 10ꢁ5 S cmꢁ1 and 4.5 ꢀ 10ꢁ4 S cmꢁ1
respectively. Our interest is impelling conformational flexibilities
and expanding the amorphous region; and examine the properties
like crystallinity, Tg and ionic conductivity of the polymer.
Lithium-ion batteries are the major power sources for portable
electronics. In the lithium-ion batteries, the electrodes are sepa-
rated by an inert, porous polymer separator which is usually
poly(propylene) soaked with low molecular weight organic liquids
containing a dissolved salt. At room temperature, the ionic con-
ductivity of liquid organic electrolytes is in the range of 10ꢁ3 S cmꢁ1
.
However, these organic liquids have potential fire hazard especially
with large batteries that are useful for electric vehicles [12]. Thus,
the research on finding solid polymer electrolyte (SPE) with flame-
retardant properties to replace organic solvents in high-energy
density lithium-ion batteries is always been in demand. The
flame-retardant property for polymer electrolytes so far achieved
by blending phosphorus containing molecules or covalently
attaching them directly to the polymer chains [13]. Varieties of
polymers in which phosphorus is a part of polymer chains are well-
known [14e16]. However, the present interest is producing poly-
ethers with bulky rigid pendant groups such as substituted phos-
phates. These rigid groups would be influencing the assembly of
* Corresponding author. Tel.: þ91 40 23012460.
E-mail addresses: kmsc@uohyd.ernet.in, murali@uohyd.ac.in (K. Muralidharan).
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.12.005

84
H.V. Babu, K. Muralidharan / Polymer 55 (2014) 83e94
chains in the polymer matrices and create perpetual free volume
for the mobility of lithium-ion.
reduced using SAINTPLUS and the structures were solved using
SHELXS-97 and refined using SHELXL-97 [23]. All hydrogen atoms
were refined anisotropically. The Netzsch STA 409 PC model was
used for thermogravimetric and differential thermal analysis (TG-
DTA) to examine the thermal stability. The decomposition behav-
iour of polymers was studied from 30 to 900 ꢂC under the nitrogen
flow with a heating rate of 10 ꢂC/min. The temperature of 5% weight
loss was chosen as an onset point of decomposition (Td). The glass
transition temperatures (Tg) of polyethers were measured in a
Differential Scanning Calorimeter (DSC) from PerkinElmer (Pyris
Diamond DSC 8000). The measurements were performed at a
heating rate of 10 ꢂC/min under the nitrogen. Tg was assigned as the
inflection point in the thermogram. The molecular weights of the
polymers were determined by the Gel Permeation Chromatography
(GPC) of Shimadzu 10AVP model equipped with refractive index
(RI) detector. The separation was achieved using a Phenogel mixed
bed column (300 ꢀ 7.80 mm) operated at 30 ꢂC with a flow rate of
0.5 mL/min using tetrahydrofuran (THF) as the eluent. The molec-
ular weight and molecular weight distributions were calculated
using polystyrene as a standard.
Poly(propylene oxide)s and PEO are important classes of poly-
mers [17], they gained importance in biological and industrial ap-
plications especially in lithium-ion batteries. These polymers are
produced from propylene oxides or EO respectively by cationic,
anionic or metal catalyzed ring opening polymerization (ROP). The
backbiting side reactions hinder the cationic polymerization lead-
ing to form low molecular weight oligomers. Alkali metal de-
rivatives are efficient initiators for the anionic polymerization of
cyclic ethers. However, the high basicity of propagating species
resulted in proton abstraction leading to form hydroxy ended
oligomer chains and allyl alkoxide initiator [18]. More recently, the
polyethers with high molecular weights were produced by mono-
mer activated anionic polymerization with restricted transfer to
monomer side reactions. This was realised in the presence of
combinations of alkali metal salts or ammonium salts and a tri-
alkylaluminium [19]. Similarly, a bimetallic cobalt catalyst/PPNX
was reported to be active in catalyzing enantioselective ROP of
propylene oxide [20].
Recently, we have reported the syntheses of Zn(II), Cd(II) and
Cu(II) complexes of 2,5-bis{N-(2,6-diisopropylphenyl)imino-
methyl}pyrrole and their effective catalytic activities for cyclic
carbonate synthesis from epoxide and CO2 at 1 atm [21]. Our in-
terests in the present research are threefold as follows. (i)
Designing and synthesis of polymers having conformational flexi-
bility, (ii) production of poly(ethylene oxide)s with phosphates as a
pendant group and by it creating suitable superstructure for
lithium-ion mobility, (iii) exploring the catalytic activities of above
complexes for ring opening polymerization of cyclic ethers.
The impedance measurements of polymer electrolytes were
performed in Zahner zennium electrochemical work station with
built in Thales software for data acquisition. The measurements
were done in the frequency range of 1 Hze4 MHz. Initially films
were made by dissolving polymer and LiN(SO2CF3)2 in THF at room
temperature, but the films of SPEs were brittle. Therefore, instead
of making films, pellets were made for measurements. The con-
sistency of result was checked by repeating the measurements
three times. The specimens in the form of pellets were sandwiched
between two gold plated electrodes housed in a homemade cell.
For variable temperature measurements, cell was equilibrated at
each temperature for 30 min before measuring. The conductivity
2. Experimental section
was calculated according to the equation
s
¼ d/(ARb), where d is the
2.1. Materials and instrumentation
thickness of the polymer electrolyte disc, A is the surface area of the
pellet and Rb is the bulk resistance value which can be obtained
from the Nyquist plot [24].
All manipulations involving air and moisture sensitive com-
pounds were carried out using standard Schlenk techniques under
the atmosphere of dry nitrogen. All solvents to be used under inert
atmosphere were thoroughly deoxygenated using freezeepumpe
thaw method before use. They were dried and purified by refluxing
over a suitable drying agent followed by distillation under nitrogen
2.2. Experimental procedure
2.2.1. Synthesis of chlorodi(4-methoxyphenoxy)phosphine oxide
(S3)
atmosphere.
The
compound,
3,30,5,50-tetra-tert-butyl-1,10-
A solution of 4-methoxyphenol (6.21 g, 50 mmol) in THF (10 mL)
was added to a solution of POCl3 (2.3 mL, 25 mmol) in THF (15 mL)
at 0 ꢂC. To this, NEt3 (7.0 mL, 50 mmol) was slowly added for 10 min
at 0 ꢂC. While adding NEt3, immediate formation of a white pre-
cipitate (NEt3HCl) was observed. This reaction mixture was stirred
for 2 h at 0 ꢂC. After this, the reaction mixture was filtered to
remove NEt3HCl and the filtrate was evaporated under vacuum. A
crude product was obtained as a colorless viscous oil. As monitored
by 31P{1H} NMR spectra, this crude product was not stable at room
temperature. This made isolation of pure S3 unsuccessful. There-
fore, it was used without purification for the synthesis of M3. 31P
biphenyl-2,20-diol was synthesized according to the literature
procedure [22]. The compounds, chlorodiphenylphosphine oxide
(S1), diphenyl phosphoryl chloride (S2), 4-methoxyphenol, 4-
phenylphenol, biphenyl-2,20-diol, glycidol and lithium bis(tri-
fluoromethanesulfonyl)imide LiN(SO2CF3)2 (LiTFSI) were pur-
chased from Aldrich and were used as received without
purification. The starting materials S3eS6 were prepared by
reacting POCl3 with suitable equivalents of respective compounds
4-methoxyphenol,
4-phenylphenol,
biphenyl-2,20-diol
and
3,30,5,50-tetra-tert-butyl-1,10-biphenyl-2,20-diol. Tetrabutylammo-
nium bromide (Merck) was dried at 50 ꢂC for 4 h under vacuum
prior to use.
NMR (162 MHz, CDCl3) of crude product (S3):
d
ꢁ3.83.
The structures of all the products were confirmed by 1H, 31P, 13
C
2.2.2. Synthesis of chlorodi(4-phenylphenoxy)phosphine oxide (S4)
A solution of 4-phenylphenol (8.51 g, 50 mmol) in THF (10 mL)
was added to a solution of POCl3 (2.3 mL, 25 mmol) in THF (15 mL)
at 0 ꢂC. To this, NEt3 (7.0 mL, 50 mmol) was slowly added for 10 min
at 0 ꢂC. While adding NEt3, immediate formation of a white pre-
cipitate (NEt3HCl) was observed. This reaction mixture was stirred
for 2 h at 0 ꢂC. After this, the reaction mixture was filtered to
remove NEt3HCl and the filtrate was evaporated under vacuum. A
crude product was obtained as a white powder which was not
stable in room temperature as seen in 31P{1H} NMR spectra. This
made isolation of pure S4 unsuccessful. Therefore, it was used
NMR spectra and HRMS data. All the NMR spectra were recorded on
a Bruker Avance 400 MHz FT NMR spectrometer at room temper-
ature using either CDCl3 or DMSO-d6 as a solvent. The chemical
shifts are reported in parts per million (d) relative to tetrame-
thylsilane as a reference for 1H and 13C{1H} NMR (100 MHz). The 31
P
{1H} NMR (162 MHz) spectra referenced to 85% H3PO4. HRMS
spectra were recorded on a Bruker maXis instrument using ESI
technique. Single crystal X-ray data collection was carried out at
298(2) K on a Bruker Smart Apex CCD area detector system (
l
(Mo
ꢀ
Ka
) ¼ 0.71073 A) with a graphite monochromator. The data were

H.V. Babu, K. Muralidharan / Polymer 55 (2014) 83e94
85
without purification for the synthesis of M4. 31P NMR (162 MHz,
CDCl3) of crude product (S4):
ꢁ5.21.
immediate formation of
a
white precipitate (NEt3HCl) was
d
observed. The reaction mixture was stirred for required time and
temperature. Then, the reaction mixture was filtered to remove
NEt3HCl and the filtrate was evaporated under vacuum to obtain
the products.
2.2.3. Synthesis of (1,10-biphenyl-2,20-dioxy)chlorophosphine oxide
(S5)
A solution of biphenyl-2,20-diol (4.66 g, 25 mmol) in THF (10 mL)
was added to a solution of POCl3 (2.3 mL, 25 mmol) in THF (15 mL)
at 0 ꢂC. To this, NEt3 (7.0 mL, 50 mmol) was slowly added for 10 min
at 0 ꢂC. While adding NEt3, immediate formation of a white pre-
cipitate (NEt3HCl) was observed. This reaction mixture was stirred
for 2 h at 0 ꢂC. After this, the reaction mixture was filtered to
remove NEt3HCl and the filtrate was evaporated under vacuum to
obtain pure product (S5). The product was obtained as a white
2.2.5.1. Synthesis of oxiran-2-ylmethyl diphenylphosphinate (M1).
Prepared according to the general procedure by using chlor-
odiphenylphosphine oxide (S1) (4.73 g, 20 mmol). After the addi-
tion of NEt3, the reaction mixture was stirred for 12 h at 25 ꢂC. The
product was obtained as colorless viscous oil (yield 96%). 1H NMR
(400 MHz, CDCl3): d 7.88e7.79 (m, 4H, eC6H5), 7.57e7.50 (m, 2H, e
C6H5), 7.50e7.42 (m, 4H, eC6H5), 4.35e4.27 (m, 1H, ePOCH2CH),
3.98e3.89 (m, 1H, ePOCH2CH), 3.31e3.25 (m, 1H, eCH2CHCH2),
2.85e2.80 (m, 1H, eOCH2CH), 2.68e2.63 (m, 1H, eOCH2CH). 13C
powder (yield 95%). 1H NMR (400 MHz, CDCl3):
eC6H4), 7.53e7.47 (m, 2H, eC6H4), 7.46e7.40 (m, 2H, eC6H4), 7.38e
d 7.60e7.56 (m, 2H,
7.33 (m, 2H, eC6H4). 13C NMR (100 MHz, CDCl3):
d 147.5 (d,
NMR (100 MHz, CDCl3):
d 132.4, 131.8e131.5 (m), 130.3 (d,
J ¼ 11.4 Hz), 130.5, 130.2, 128.1, 127.4, 121.5 (d, J ¼ 4.5 Hz). 31P NMR
J ¼ 16.3 Hz), 128.6 (d, J ¼ 12.8 Hz), 65.3 (d, J ¼ 5.3 Hz), 50.4 (d,
J ¼ 7.7 Hz), 44.7. 31P NMR (162 MHz, CDCl3):
d 32.95. HRMS (ESI) m/
(162 MHz, CDCl3):
d 9.92. HRMS (ESI) m/z: calcd. for C12H8ClO3P
z: calcd. for C15H15O3P [M þ Na]þ 297.0657, found 297.0655.
[M þ Na]þ 288.9797, found 288.9797. The molecular structure of S5
(Fig. 1) determined using X-ray crystallography clearly showed the
presence of a seven-membered ring in it.
2.2.5.2. Synthesis of oxiran-2-ylmethyl diphenylphosphate (M2).
Prepared according to the general procedure by using diphenyl
phosphoryl chloride (S2) (5.37 g, 20 mmol). After the addition of
NEt3, the reaction mixture was stirred for 12 h at 25 ꢂC. The product
was obtained as colorless viscous oil (yield 98%). 1H NMR (400 MHz,
2.2.4. Synthesis of (3,30,5,50-tetra-tert-butyl-1,10-biphenyl-2,20-
dioxy)chlorophosphine oxide (S6)
A
solution of 3,30,5,50-tetra-tert-butyl-1,10-biphenyl-2,20-diol
CDCl3):
d 7.38e7.31 (m, 4H, eC6H5), 7.26e7.17 (m, 6H, eC6H5),
(10.27 g, 25 mmol) in THF (10 mL) was added to a solution of POCl3
(2.3 mL, 25 mmol) in THF (15 mL) at 0 ꢂC. To this, NEt3 (7.0 mL,
50 mmol) was slowly added for 10 min at 0 ꢂC. While adding NEt3,
4.35e4.28 (m, 1H, ePOCH2CH), 4.19e4.10 (m, 1H, ePOCH2CH),
3.27e3.21 (m, 1H, eCH2CHCH2), 2.85e2.80 (m, 1H, eOCH2CH), 2.65
(dd, J ¼ 4.8 Hz and J ¼ 2.6 Hz, 1H, eOCH2CH). 13C NMR (100 MHz,
immediate formation of
a white precipitate (NEt3HCl) was
observed. This reaction mixture was stirred for 2 h at 0 ꢂC. After
this, the reaction mixture was filtered to remove NEt3HCl and the
filtrate was evaporated under vacuum to obtain pure product (S6).
The product was obtained as a white powder (yield 98%). 1H NMR
CDCl3):
d
150.4 (d, J ¼ 6.0 Hz), 129.9, 125.5, 120.1, 69.4 (d, J ¼ 5.7 Hz),
49.8 (d, J ¼ 7.7 Hz), 44.5. 31P NMR (162 MHz, CDCl3):
ꢁ11.81. HRMS
d
(ESI) m/z: calcd. for C15H15O5P [M þ Na]þ 329.0555, found
329.0555.
(400 MHz, CDCl3):
2H, eC6H2), 1.52 (s, 18H, eCH3), 1.36 (s, 18H, eCH3). 13C NMR
(100 MHz, CDCl3):
148.7, 144.3 (d, J ¼ 11.7 Hz),140.2 (d, J ¼ 4.9 Hz),
d
7.56e7.52 (m, 2H, eC6H2), 7.22 (d, J ¼ 2.4 Hz,
2.2.5.3. Synthesis of bis(4-methoxyphenyl)oxiran-2-ylmethyl phos-
phate (M3). Prepared according to the general procedure by using
chlorodi(4-methoxyphenoxy)phosphine oxide (S3) (6.57 g,
20 mmol). After the addition of NEt3, the reaction mixture was
stirred for 2 h at 0 ꢂC. The crude product was purified by column
chromatography on silica gel eluting with hexane/ethylacetate
(1:1) to give pure light brown color viscous oil (yield 88%). 1H NMR
d
129.8, 126.9, 125.8, 35.6, 34.8, 31.4. 31P NMR (162 MHz, CDCl3):
d
4.32. HRMS (ESI) m/z: calcd. for C28H40ClO3P [M þ H]þ 491.2482,
found 491.2486.
2.2.5. General procedure for the syntheses of monomers M1eM6
A solution of glycidol (1.6 mL, 24 mmol) in THF (10 mL) was
added to a solution of appropriate chloro compound (S1eS6)
(20 mmol) in THF (15 mL) at 0 ꢂC. NEt3 (5.6 mL, 40 mmol) was
slowly added to the reaction mixture at 0 ꢂC for 10 min. The
(400 MHz, CDCl3):
d 7.16e7.11 (m, 4H, eC6H5), 6.86e6.81 (m, 4H, e
C6H5), 4.47e4.40 (m, 1H, ePOCH2CH), 4.15e4.06 (m, 1H,
e
POCH2CH), 3.77 (s, 6H, eOCH3), 3.26e3.20 (m, 1H, eCH2CHCH2),
2.84e2.80 (m,1H, eOCH2CH), 2.64 (dd, J ¼ 4.8 Hz and J ¼ 2.6 Hz,1H,
eOCH2CH). 13C NMR (100 MHz, CDCl3):
d
157.0, 144.0 (d, J ¼ 6.6 Hz),
121.0, 114.7, 69.3 (d, J ¼ 5.8 Hz), 55.6, 49.8 (d, J ¼ 7.8 Hz), 44.5. 31
P
NMR (162 MHz, CDCl3):
d
ꢁ10.72. HRMS (ESI) m/z: calcd. for
C
17H19O7P [M þ H]þ 367.0947, found 367.0948.
2.2.5.4. Synthesis of dibiphenyl-4yl-oxiran-2-ylmethyl phosphate
(M4). Prepared according to the general procedure by using
chlorodi(4-phenylphenoxy)phosphine oxide (S4) (8.42 g,
20 mmol). After the addition of NEt3, the reaction mixture was
stirred for 2 h at 0 ꢂC. The crude product was purified by column
chromatography on silica gel eluting with hexane/ethylacetate
(3:1) to give pure white color powder (yield 81%). 1H NMR
(400 MHz, CDCl3):
d
7.60 (t, J ¼ 8.6 Hz, 8H, eC6H5), 7.47 (t, J ¼ 7.2 Hz,
4H, eC6H5), 7.42e7.34 (m, 6H, eC6H5), 4.62e4.52 (m, 1H, e
POCH2CH), 4.29e4.18 (m, 1H, ePOCH2CH), 3.37e3.30 (m, 1H, e
CH2CHCH2), 2.93e2.85 (m, 1H, eOCH2CH), 2.77e2.70 (m, 1H, e
OCH2CH). 13C NMR (100 MHz, CDCl3):
d
149.9 (d, J ¼ 7.2 Hz), 140.1,
138.8, 128.9, 128.6, 127.5, 127.1, 120.5e120.3 (m), 69.6 (d, J ¼ 5.8 Hz),
49.8 (d, J ¼ 7.6 Hz), 44.6. 31P NMR (162 MHz, CDCl3):
ꢁ11.53. HRMS
d
Fig. 1. ORTEP of the molecular structure S5. Thermal ellipsoids are shown at 30%
probability levels. Hydrogen atoms are omitted for clarity.
(ESI) m/z: calcd. for C27H23O5P [M þ H]þ 459.1361, found 459.1360.

86
H.V. Babu, K. Muralidharan / Polymer 55 (2014) 83e94
The molecular structure of M4 (Fig. 2) determined using X-ray
2.2.6.1. Synthesis of polymer P1. The ROP was carried out according
crystallography.
to the general procedure by using oxiran-2-ylmethyl diphenyl-
phosphinate (M1). Yield 92%. 1H NMR (400 MHz, CDCl3):
d 7.69e
7.57 (m, eC6H5), 7.26e7.13 (m, eC6H5), 4.29e4.07 (m, ePOCH2CH),
2.2.5.5. Synthesis of 2-(((1,10-biphenyl-2,20-dioxy)phosphine oxide)
methyl)oxirane (M5). Prepared according to the general procedure
by using (1,10-biphenyl-2,20-dioxy)chlorophosphine oxide (S5)
(5.33 g, 20 mmol). After the addition of NEt3, the reaction mixture
was stirred for 2 h at 0 ꢂC. The product was obtained as a pale
3.92e3.64 (m, eOCH2CH and eCH2CHCH2). 31P NMR (162 MHz,
CDCl3):
d 28.65.
2.2.6.2. Synthesis of polymer P2. The ROP was carried out according
to the general procedure by using oxiran-2-ylmethyl diphenyl-
yellow color powder (yield 90%). 1H NMR (400 MHz, CDCl3):
d 7.55
phosphate (M2). Yield 98%. 1H NMR (400 MHz, CDCl3):
d 7.43e7.09
(d, J ¼ 7.6 Hz, 2H, eC6H4), 7.49e7.43 (m, 2H, eC6H4), 7.39 (d,
J ¼ 7.5 Hz, 2H, eC6H4), 7.37e7.31 (m, 2H, eC6H4), 4.62e4.54 (m, 1H,
ePOCH2CH), 4.26e4.17 (m, 1H, ePOCH2CH), 3.36e3.30 (m, 1H, e
CH2CHCH2), 2.92e2.87 (m, 1H, eOCH2CH), 2.74e2.69 (m, 1H, e
(m, eC6H5), 7.03e6.75 (m, eC6H5), 4.33e3.99 (m, ePOCH2CH),
3.93-3.72 (m, eOCH2CH and eCH2CHCH2). 13C NMR (100 MHz,
CDCl3):
(162 MHz, CDCl3):
d
158.9, 129.8, 120.2, 115.1, 80.8, 70.1, 67.3. 31P NMR
d
ꢁ0.75.
OCH2CH). 13C NMR (100 MHz, CDCl3):
d
147.7 (d, J ¼ 14.9 Hz), 130.2,
129.9, 128.2, 126.6, 121.4, 69.8 (d, J ¼ 4.3 Hz), 49.8 (d, J ¼ 7.4 Hz),
2.2.6.3. Synthesis of polymer P3. The ROP was carried out according
to the general procedure by using bis(4-methoxyphenyl)oxiran-2-
ylmethyl phosphate (M3). Yield 95%. 1H NMR (400 MHz, CDCl3):
44.6. 31P NMR (162 MHz, CDCl3):
C
d 1.76. HRMS (ESI) m/z: calcd. for
15H13O5P [M þ H]þ 305.0579, found 305.0579.
d
6.99e6.78 (m, eC6H5), 6.75e6.62 (m, eC6H5), 4.43e4.15 (m, e
POCH2CH), 4.09e3.80 (m, eOCH2CH and eCH2CHCH2), 3.70e3.60
(m, eOCH3). 31P NMR (162 MHz, CDCl3):
ꢁ5.82.
2.2.5.6. Synthesis of 2-(((3,30,5,50-tetra-tert-butyl-1,10-biphenyl-2,20-
dioxy)phosphine oxide)methyl)oxirane (M6). Prepared according to
the general procedure by using (3,30,5,50-tetra-tert-butyl-1,10-
biphenyl-2,20-dioxy)chlorophosphine oxide (S6) (9.82 g, 20 mmol)
and NaH (40 mmol) as a base instead of NEt3. After the addition of
NEt3, the reaction mixture was stirred for 2 h at 0 ꢂC. The crude
product was purified by column chromatography on silica gel
eluting with hexane/ethylacetate (3:1) to give pure a white color
d
2.2.6.4. Synthesis of polymer P4. The ROP was carried out according
to the general procedure by using dibiphenyl-4yl-oxiran-2-
ylmethyl phosphate (M4). Yield 90%. 1H NMR (400 MHz, CDCl3):
d
7.66e7.50 (m, eC6H5), 7.44e7.35 (m, eC6H5), 7.32e7.19 (m, e
C6H5), 4.32e4.16 (m, ePOCH2CH), 3.95e3.73 (m, eOCH2CH and e
CH2CHCH2). 31P NMR (162 MHz, CDCl3):
ꢁ4.69.
powder (yield 84%). 1H NMR (400 MHz, CDCl3):
d 7.50 (s, 2H, e
d
C6H2), 7.22e7.19 (m, 2H, eC6H2), 4.48e4.40 (m, 1H, ePOCH2CH),
4.19e4.10 (m, 1H, ePOCH2CH), 3.28e3.22 (m, 1H, eCH2CHCH2),
2.85e2.80 (m,1H, eOCH2CH), 2.65 (dd, J ¼ 4.8 Hz and J ¼ 2.5 Hz,1H,
2.2.6.5. Synthesis of polymer P5. The ROP was carried out according
to the general procedure by using 2-(((1,10-biphenyl-2,20-dioxy)
phosphine oxide)methyl)oxirane (M5). Yield 78%. 1H NMR
eOCH2CH), 1.52 (d, J ¼ 3.4 Hz 18H, eCH3), 1.36 (s, 18H, eCH3). 13
C
(400 MHz, CDCl3):
3.65 (m), 3.59e3.44 (m), 3.41e3.19 (m). 13C NMR (100 MHz, CDCl3):
158.6, 157.6, 129.3, 128.2, 127.7, 127.5, 127.2, 126.7, 126.4, 120.8,
116.2, 115.4, 78.2, 73.2, 68.7, 63.6, 60.8. 31P NMR (162 MHz, CDCl3):
1.77, ꢁ0.72.
d 7.83e7.30 (m, eC6H5), 4.27e3.83 (m), 3.81e
NMR (100 MHz, CDCl3):
d
148.0, 144.4 (d, J ¼ 9.5 Hz), 140.1 (d,
J ¼ 4.5 Hz), 130.0, 126.7, 125.3, 69.8 (d, J ¼ 6.0 Hz), 49.8 (d,
J ¼ 7.0 Hz), 44.7, 35.5, 34.7, 31.4, 31.1. 31P NMR (162 MHz, CDCl3):
d
d
ꢁ3.01. HRMS (ESI) m/z: calcd. for C31H45O5P [M þ Na]þ 551.2902,
d
found 551.2903.
2.2.6.6. Synthesis of polymer P6. The ROP was carried out according
to the general procedure by using 2-(((3,30,5,50-tetra-tert-butyl-
1,10-biphenyl-2,20-dioxy)phosphine oxide)methyl)oxirane (M6).
2.2.6. General procedure for the ROP of epoxides
All the polymerization reactions were performed in the pressure
tube. To the solution of monomer (M1eM6) (10 mmol) in THF
(2.5 mL), catalyst (0.1 mmol) and TBAB (0.067 mmol) was added
under stirring. The pressure tube was heated to 70 ꢂC and stirred for
the required time. After the reaction time, methanol was added to
the reaction mixture to quench the polymerization and the solvent
was evaporated under vacuum. The product was purified by dis-
solving in THF and reprecipitated in hexane and this process was
repeated for three more times.
Yield 81%. 1H NMR (400 MHz, CDCl3):
d 7.53e7.49 (m, eC6H2), 7.33e
7.30 (m, eC6H2), 7.26e7.22 (m, eC6H2), 7.05e7.01 (m, eC6H2), 4.38e
4.21 (m), 4.20e4.07 (m), 4.04e3.84 (m), 1.48e1.42 (m, eCH3), 1.33e
1.27 (m, eCH3). 31P NMR (162 MHz, CDCl3):
d
ꢁ2.80, ꢁ5.61.
2.2.7. General preparation of solid polymer electrolytes SPEs
The polymers (P1, P2 and P4eP6) were dried at 60 ꢂC under
vacuum for 8 h. LiN(SO2CF3)2 was dried at 150 ꢂC under vacuum for
10 h before use. All manipulations were carried out in an MBraun
glove box filled with ultrapure nitrogen 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 ꢂC. After this, THF was
evaporated under vacuum and dried the residue at 60 ꢂC for 12 h.
The residue was loaded into a die and then pressed to make a pellet.
Specimens of 0.07e0.08 cm thickness and 0.9 cm diameter were
obtained for conductivity studies. These pellets were sandwiched
between two gold plated electrodes housed in a homemade cell for
conductivity studies. The consistency of results was checked by
repeating three times.
3. Results and discussion
Integrating phosphorus into polymer chains would increase the
Fig. 2. ORTEP of the molecular structure M4. Thermal ellipsoids are shown at 30%
probability levels. Hydrogen atoms are omitted for clarity.
conformational flexibility, thus leading to interesting chemical and

H.V. Babu, K. Muralidharan / Polymer 55 (2014) 83e94
87
O
of the bulky groups around tetrahedral phosphorus. The monomers
M1eM3 were liquids whereas M4eM6 were solids.
O
Br
n
O
P
HO
O
P
O
Catalyst / TBAB
R
R
O
Cl
O
O
NEt3, THF
THF, 70 o
C
R
R
R
P
3.2. Ring opening polymerization
R
R
Recently, we reported in detail about the Zn(II) 1, Cd(II) 2e3 and
Cu(II) 4 (Fig. 3) complexes of 2,5-bis{N-(2,6-diisopropylphenyl)
iminomethyl}pyrrole LH [21]. We also proved their efficiencies of
catalyzing cyclic carbonates formation from epoxides and CO2
(1 atm) in the presence of TBAB under mild temperature. They were
effective for converting monosubstituted terminal epoxides,
disubstituted terminal and internal epoxides. In our continuous
interest on these complexes (1e4), we have further explored their
catalytic activities for the ring opening polymerization of epoxides.
At first, the catalytic activity of Zn(II) complex 1 for the ROP of
oxiran-2-ylmethyl diphenylphosphate (M2) in the presence of
TBAB as initiator at 70 ꢂC in THF was tested. It was found to be a
good catalyst by yielding the polyether (P2) with narrow
polydispersity.
S1
S2
M1
P1
P2
O
M2
M3
O
O
S3
S4
P3
P4
O
M4
Scheme 1. Syntheses of phosphorus containing linear polyethers by ROP of epoxide.
physical properties including amorphous nature, high temperature
stability and flame-retardancy [25]. Although phosphorus con-
taining polymers are attractive mainly because of their flame-
retardant property [26], in recent years, their biodegradability is
A set of controlled experiments was performed in THF at 70 ꢂC to
understand the role of both catalyst and initiator (C and I). Either
catalyst or initiator alone did not catalyze the reactions. However,
when both of them are present in different molar ratios, we
observed the following; (i) when the C/I molar ratio was less than 1,
only subtle polymerization occurred. (ii) When the C/I molar ratio
equalled 1, the reaction yielded little more polyether (Table 1, en-
tries 3 and 4) but did not complete within 24 h as checked by 1H
NMR. (iii) When the C/I molar ratio exceeded slightly 1, say 1.1
(Table 1, entry 5), the ROP proceeded much faster and reached
completion within 7 h. So the reaction-condition was optimized by
varying the C/I (1/TBAB) molar ratio, temperature and time to
achieve good conversion and higher molecular weight with narrow
PDI (Table 1, entries 3e10). Among the different catalyst/initiator
loadings tried, the C/I molar ratio of 1.5 yielded the polymer within
4 h at 70 ꢂC with moderate properties (Table 1, entry 8). From here,
this molar ratio (C/I) was fixed for further investigations. Under this
reaction-condition, the catalysts 2e4 were examined for the ROP of
M2 in the presence of TBAB in THF at 70 ꢂC (Table 1, entries 11e13).
The ROP proceeded faster using 1 as a catalyst in comparison with
complexes 2e4. The presence of bulky groups was helpful to pro-
tect metal center as well as in controlling PDI by retarding the side
reactions. The molecular weight [Mn(exp)] measured by GPC
matched well with theoretical molecular weight [Mn(theo)]
calculated (based on a formation of one polymer chain per initiator)
from the initial monomer/initiator ratio, and their PDIs were also in
a narrow range.
also much sought. The biodegradable polymers
e
poly-
phosphoesters (PPE) [27] are promising for clinical and pharma-
ceutical applications, whereas the solid polymer electrolyte based
on PPE is unexplored [9c,28]. Our interest is producing polyethers
with phosphate pendant groups as solid polymer electrolyte for
lithium-ion transport.
3.1. Synthesis of monomers (M1eM6)
Chosen monomers M1eM6 having bulky groups were to get
preferably linear polymers in which the bulky groups would be side
groups. Because of the uneven shapes and sizes of these bulky
groups, they would influence the orientation of polymer chains and
thus expected to enlarge the amorphous region. Schemes 1 and 2
show the syntheses of monomers (M1eM6). The reactions of
starting materials S1eS5 with glycidol in THF in the presence of
NEt3 yielded the monomers M1eM5 respectively. Similarly, a re-
action of S6 with glycidol in THF but in the presence of NaH yielded
the monomer M6 in good yield. The monomers M1, M2 and M5
were isolated in pure by filtering the reaction mixtures followed by
evaporating the filtrates. The other monomers M3, M4 and M6
were purified by column chromatography. The monomers (M1e
M6) were characterized using 1H, 13C{1H} and 31P{1H} NMR spectral
data which showed the data were consistent with expected mo-
lecular structures. The crystal structure of M4 (Fig. 2) determined
using X-ray crystallography clearly showed the spatial orientations
The ROP of M2 was examined with a different initiator, bis(-
triphenylphosphine)iminium chloride (PPNCl) in the presence of 1
at 70 ꢂC in THF (Table 1, entry 16). Even after 24 h, the ROP did not
complete probably because of the poor solubility of PPNCl in THF.
Br
O
n
R
R
R
O
R
R
R
O
O
O
HO
THF, 0 o
O
O
P
Catalyst / TBAB
O
R
n
P
P
O
R
O
Cl
O
C
THF, 70 o
C
O
R
O
R
O
R
R
R
H
S5
S6
M5
M6
P5
P6
t-Bu
Scheme 2. Syntheses of phosphorus containing branched polyethers by ROP of epoxide.

88
H.V. Babu, K. Muralidharan / Polymer 55 (2014) 83e94
The scope of the catalyst was further explored for the ROP of
various substituted epoxides having phosphorus as monomers (M1,
M3 and M4). These reactions yielded the polymers P1, P3 and P4
respectively. Table 2 displays the properties of polymers obtained
from these reactions. The ROP of M1, M3 and M4 was also pro-
gressing according to the mechanism of polymer chain per initiator.
This was reflected by good agreement of measured molecular
weight [Mn(exp)] with calculated molecular weight [Mn(theo)].
Even though the active nucleophilic site (epoxide oxygen) was
away from substituents, electron releasing groups attached to
phosphorus indicated a trend in the rate of ROP. According to the
results displayed in Table 2 (entries 1e4), in the presence of catalyst
1, the rate of conversion of monomers to polymers was in the order
M3 > M4 > M2 > M1.
Ar =
N
H
N
N
Ar
Ar
LH
solv = CH3OH
N
N
N
N
N
N
Ar
Ar
Ar
Ar
Ar
M
M
solv
Ar
N
N
Ar
Ar
N
N
N
N
1, M = Zn
2, M = Cd
4, M = Cu
3, M = Cd
3.3. Branched polymers
Fig. 3. Structure of the ligand LH and its complexes 1e4.
The epoxide ring opening polymerization (in THF at 70 ꢂC) of
monomers M5 and M6 was interesting since they differed from the
other four monomers. These monomers yielded respective poly-
ethers (P5 and P6; Table 2, entries 5 and 6) whose measured mo-
lecular weights [Mn(exp)] were much lower than theoretical
molecular weights [Mn(theo)]. This suggested the ROP did not
follow the polymer chain per initiator mechanism while other
monomers following. The monomer M6 underwent faster poly-
merization than M5 showing the bulky groups in it did not disturb
the growth of PEO chain since the electronic effects dominated;
also the bulky group is much away from the growing chain. To
deduce the structure of polymers P5 and P6, we analysed the
following observations. (i) Both these polymers were soluble in
polar solvents. (ii) At start of the reaction, number of initiator
molecules were much less than monomer molecules. (iii) No
monomers remained unreacted in these reactions. (iv) The 31P{1H}
NMR spectra of P5 displayed two kinds of peaks at 1.77
and ꢁ0.72 ppm. Comparing with the chemical shift of the mono-
mer, the minor peak at 1.77 ppm might represent the phosphorus
at a terminal repeating unit of a chain, while the broad peak
at ꢁ0.72 ppm representing the phosphorus atoms in the chain [29].
In the 1H NMR of P5, the peaks of all aromatic protons merged
(Fig. 5) while the aliphatic protons merged among themselves. The
polymer P6 also presented a similar 31P{1H} and 1H NMR. The peaks
of aliphatic carbon were assigned in 13C NMR spectrum of P5
(Fig. 5). Due to the rigid structure of polymer the peaks were not
resolved clearly and so it was difficult to assign the peaks exactly to
other carbons.
So, the reaction was carried out in dichloromethane (DCM) at 40 ꢂC
(Table 1, entry 17). But since the high molecular mass polyether
(P2) was insoluble in DCM, once the reaction progressed, the
polymer precipitated out from the reaction mixture. Instead of 1/
TBAB, a well-known anionic initiator potassium tert-butoxide (t-
BuOK) was examined for the ROP of M2 (Table 1, entries 18 and 19).
In this case, the significant contribution of side reactions probably
resulted in low molecular weight polymer with broader PDI and
this also was consuming extensive time for polymerization. These
facts confirmed the usefulness of our catalyst in these ROP
reactions.
Table 1
Results of optimization studies for ROPa of M2 to form P2.
Entry Catalyst [C]0/[I]0 [M2]0 Reaction Properties of the P2
(C)
time (h)
Mn
Mn
PDId
(kg molꢁ1
(exp)b
)
(kg molꢁ1
(theo)c
)
1e
2f
3
4
5
e
1
1
1
1
1
1
1
1
1
2
3
4
1
1
1
1
e
e
e
100
100
100
100
100
100
100
100
100
100
100
100
100
200
300
100
100
120
120
24
24
7
6
24
4
e
e
e
e
e
e
e
0.5
1.0
1.1
1.3
1.5
1.5
1.8
2.0
1.5
1.5
1.5
1.5
1.5
1.5h
1.5h
e
30.6
30.6
33.6
39.8
45.9
45.9
55.1
61.2
45.9
45.9
45.9
91.8
137.8
45.9
45.9
e
e
e
e
31.4
40.2
43.1
46.9
57.8
64.2
42.3
43.2
46.0
90.6
136.7
e
1.21
1.18
1.08
1.15
1.23
1.28
1.27
1.34
1.19
1.18
1.20
e
6
7g
8
9
3
3
10
11
12
13
14
15
16
17i
18j
19j
14
16
10
15
48
24
10
168
72
Table 2
Results of syntheses of various phosphorus containing polyethers (P1eP6)a and its
properties.
Entry Epoxides Time Polymers Properties of polymers
16.3
16.5
20.8
1.25
1.64
1.85
(M)
(h)
(P)
t-BuOK 100
t-BuOK 100
Mn
Mn
PDId Td
(ꢂC)
Tg
(ꢂC)
e
(kg molꢁ1
(exp)b
)
(kg molꢁ1
(theo)c
)
a
Reactions were conducted in THF at 70 ꢂC in presence of catalyst (C) and TBAB (I)
as an initiator. The reactions were carried out up to 100% conversion based on 1H
NMR.
1
2
3
4
5
6
M1
M2
M3
M4
M5
M6
10
4
3
4
8
P1
P2
P3
P4
P5
P6
40.6
46.9
56.0
67.9
32.2
50.3
41.1
45.9
55.0
68.8
45.6
79.3
1.21 350.1 227.7
1.15 159.5 80.8
b
Molecular weight measured by GPC and calibrated against polystyrene
1.19 144.7 ꢁ10.6
standard.
1.16 159.3
1.20 167.9
1.18 203.4
82.5
98.4
74.7
c
Molecular weight calculated from ([M]0/[I]0)
ꢀ
molecular weight of
monomer ꢀ conversion.
6
d
Obtained from GPC analysis.
In the presence of TBAB alone.
In the presence of catalyst alone.
Reaction at 25 ꢂC.
In presence of PPNCl as an initiator instead of TBAB.
Conducted in DCM at 40 ꢂC.
Without catalyst.
e
a
Reaction condition: [M]0:[C]0:[I]0 ¼ 100:1:0.67 in THF at 70 ꢂC, [M]0 ¼ 0.94 M.
f
b
Molecular weight measured by GPC and calibrated against polystyrene
g
standard.
h
c
Molecular weight calculated from ([M]0/[I]0)
ꢀ
molecular weight of
i
monomer ꢀ conversion.
j
d
Obtained from GPC analysis.

H.V. Babu, K. Muralidharan / Polymer 55 (2014) 83e94
89
Fig. 4. Possible branching structures of P5 and P6.
From these observations we inferred as follows. At first, it
appeared the pendant rings opened by reaction with initiators were
taking part in side reactions with proximal pendant groups along
the same chain. This parallel chain could be formed because of
breaking of the PeO bonds in the seven membered rings in the
monomers. This side reaction would consume few initiator mole-
cules. Also, no monomers remained unconsumed. Breaking CeO
bond in the same ring was ruled out because such a case should
yield a positive-charge on the biphenyl group of tricyclic ring. In
these circumstances, according to the polymer chain per initiator
mechanism, the measured molecular weights should be higher
than the theoretical molecular weights. Thus the parallel chain
model would not explain the much lower measured molecular
weight than the theoretical value. Solubility of the polymers in
polar solvents ruled out the possibility of inter-chain side reaction
leading to traditional cross-linked polymers. Finally, we inferred
the polymers P5 and P6 as comb-like branched polymers (Fig. 4)
[30]. This was possible because of the breaking of PeO bonds in the
seven membered rings in the pendant ring and reacting with a new
monomer instead the proximal monomer leading the growth of
Fig. 5. 1H, 31P and 13C NMR spectra of P2 (left) and P5 (right).

90
H.V. Babu, K. Muralidharan / Polymer 55 (2014) 83e94
Fig. 9. Plot of ln([M2]0/[M2]t) versus time for the ROP of M2 catalyzed by 1 in the
Fig. 6. TGA plots of polyethers P1eP6.
presence of TBAB. Reaction condition: [M2]0:[C]0:[I]0 ¼ 100:1:0.67 in THF at 70 ꢂC,
[M2]0 ¼ 0.94 M.
peaks in the 31P NMR because there were two different kinds of
repeating units (in chain and terminal) in the pendant chain.
3.4. Properties of the polymers
The polymers P1, P2 and P4eP6 were solids, while P3 was a
liquid. All these polymers P1eP6 were soluble in polar solvents like
THF, dimethyl sulphoxide, and dimethyl acetamide. Table 2 lists the
glass-transition temperature (Tg), decomposition temperature (Td),
molecular weight (Mn) and PDI of all these polymers. The molecular
weights (Mn) of polymers P1eP6 were in the range 40e
80 kg molꢁ1. Thermogravimetric analyses (Fig. 6) disclosed the
polymers P2eP6 were thermally stable until heated up to 160 ꢂC
while P1 was stable up to 350 ꢂC. The Tg of the polymers P2 and P4
e P6 lies in the temperature range 75 ꢂCe100 ꢂC while the poly-
mers P1 and P3 have Tg at 227 ꢂC and ꢁ10 ꢂC respectively (Fig. 7).
All were amorphous polymers since the DSC graphs did not show
crystalline melting temperature peak for any polymers. Compared
with P2eP6, the P1 had higher thermal stability (Td) and high Tg.
This could be explained by the columnar stacking of phenyl rings
Fig. 7. DSC plots of polyethers P1eP6.
branched chain. This perception would explain the lower molecular
weight since the polymer chain branching would consume few
monomers. The comb-like polymer model would also explain two
Fig. 8. Plot of molecular weight (Mn) and polydispersity (PDI) versus [M2]0/[I]0 for the
Fig. 10. Plot of molecular weight (Mn) and polydispersity (PDI) versus monomer con-
version for the ROP of M2 catalyzed by 1 in the presence of TBAB. Reaction condition:
[M2]0:[C]0:[I]0 ¼ 100:1:0.67 in THF at 70 ꢂC, [M2]0 ¼ 0.94 M.
ROP of M2 catalyzed by
1 in the presence of TBAB. Reaction condition:
[M2]0:[C]0:[I]0 ¼ 100:1:0.67 in THF at 70 ꢂC, [M2]0 ¼ 0.94 M.

H.V. Babu, K. Muralidharan / Polymer 55 (2014) 83e94
91
R
-
(a)
ZnR2
Br
O
O
R2Zn
O
N+Bu4
+
ZnR2
ZnR2
+
+
Br
-
R
2
R
R2Zn
O
N+Bu4
(c)
O
NBu4Br
R
Propagation
(b)
R
-
O
R2Zn
O
N+Bu4
Br
n
H
ZnR2
+
Br
MeOH
n
R
R
Scheme 3. Proposed mechanism for the monomer activated anionic ROP of epoxide.
Fig. 11. a) TGA and b) DTG plots of SPE2 (10%e40%).
because of strong non-covalent interactions that would ensure a
space efficient packing. Thus the phosphate group attached to PEOs
provided significant changes in their properties especially the
crystallinity and Tg. Films of these polymers were observed to be
brittle probably because of chain rigidity. But their flexibility can be
improved by tuning the molecular structure of the polymers by
introducing various alkyl chain spacers. Some plasticizers can also
be added to improve the flexibility.
ring opening by the nucleophilic attack of bromide ions to form
initiating complex (b). Further, the excess zinc catalyst activated the
monomer (a) for the nucleophilic attack of initiating complex to
yield the growing species (c). Thus the propagation step proceeded
and polymerization happened rapidly. When the C/I molar ratio
was less than or equal to 1, the zinc catalyst involved in forming of
initiating complex only. Therefore, little excess of catalyst required
to trigger the polymerization. This was matching well with the
living like polymerization mechanism.
3.5. Mechanism of polymerization
3.6. Lithium-ion conductivity
The reaction optimization study showed that while increasing
the monomer/initiator molar ratio (M/I), the molecular weight of
the resultant polymers was increasing but with narrow PDI (Table 1,
entries 8, 14 and 15). In these reactions, existed linear-relationship
(Fig. 8) of molecular weight with the ratio of initial monomer to
initiator concentrations that is [M2]0/[I]0 also confirmed the
dependent of the molecular weight on monomer concentration. To
understand the order of ROP, reactions were monitored with time
in the presence of 1 and TBAB in THF at 70 ꢂC. Semi logarithmic plot
of ln[M2]0/[M2]t versus time (Fig. 9) revealed the linearity proving
the first order dependence of polymerization on monomer con-
centration. Further, the molecular weights of resultant polymers
were increasing linearly with increasing monomer conversion
which inferred that the polymerization proceeded with a living
polymerization manner (Fig. 10). All these observations revealed
the ROP underwent in a controlled living like polymerization and
clearly suggested the negligible contribution of “transfer to
monomer process” to the molecular weight as reflected by PDI.
Though we could not isolate any intermediate species, based on
our observations and earlier reports [16,18], we postulate a mech-
anism for ROP as shown in Scheme 3. The first step involved acti-
vating epoxide monomer by coordinating with a zinc catalyst. In
the presence of TBAB, the activated monomer underwent epoxide
Solid polymer electrolytes (SPE1, SPE2 and SPE4eSPE6) were
prepared by dissolving respective polymers (P1, P2 and P4eP6)
Fig. 12. DSC plots of SPE2 (10%e40%).

92
H.V. Babu, K. Muralidharan / Polymer 55 (2014) 83e94
Table 3
Properties of SPE2 (10%e40%) and SPE4 (10%e40%).
SPEs (wt% of LiTFSI)
SPE2
SPE4
O:Li
Conductivity
30 ꢂC
s
(S cmꢁ1
80 ꢂC
)
Td (ꢂC)
Tg (ꢂC)
O:Li
Conductivity
30 ꢂC
s
(S cmꢁ1
80 ꢂC
)
Td (ꢂC)
Tg (ꢂC)
SPE (10%)
SPE (20%)
SPE (30%)
SPE (40%)
42.2:1
18.8:1
10.9:1
7.0:1
2.7 ꢀ 10ꢁ6
3.3 ꢀ 10ꢁ6
5.3 ꢀ 10ꢁ6
5.2 ꢀ 10ꢁ5
1.1 ꢀ 10ꢁ5
2.6 ꢀ 10ꢁ5
3.2 ꢀ 10ꢁ5
2.9 ꢀ 10ꢁ4
167.5
168.9
177.4
188.8
89.7
108.3
124.1
133.5
28.2:1
12.5:1
7.3:1
3.9 ꢀ 10ꢁ9
2.5 ꢀ 10ꢁ8
7.7 ꢀ 10ꢁ7
4.8 ꢀ 10ꢁ5
3.8 ꢀ 10ꢁ8
2.0 ꢀ 10ꢁ7
6.5 ꢀ 10ꢁ6
1.8 ꢀ 10ꢁ4
166.7
168.4
180.0
182.1
99.2
107.0
110.1
130.3
4.7:1
Table 4
Properties of SPE5 (10%e40%) and SPE6 (10%e40%).
SPEs (wt% of LiTFSI)
SPE5
SPE6
O:Li
Conductivity
30 ꢂC
s
(S cmꢁ1
80 ꢂC
)
Td (ꢂC)
Tg (ꢂC)
O:Li
Conductivity
30 ꢂC
s
(S cmꢁ1
80 ꢂC
)
Td (ꢂC)
Tg (ꢂC)
SPE (10%)
SPE (20%)
SPE (30%)
SPE (40%)
42.5:1
18.9:1
11.0:1
7.1:1
1.2 ꢀ 10ꢁ8
4.0 ꢀ 10ꢁ8
2.7 ꢀ 10ꢁ7
9.8 ꢀ 10ꢁ7
1.8 ꢀ 10ꢁ7
3.9 ꢀ 10ꢁ7
2.7 ꢀ 10ꢁ6
1.0 ꢀ 10ꢁ5
183.7
190.0
193.9
198.7
110.0
119.4
122.1
130.3
24.5:1
10.9:1
6.3:1
2.2 ꢀ 10ꢁ9
3.1 ꢀ 10ꢁ8
4.5 ꢀ 10ꢁ8
5.0 ꢀ 10ꢁ8
1.1 ꢀ 10ꢁ8
3.2 ꢀ 10ꢁ7
3.5 ꢀ 10ꢁ7
4.7 ꢀ 10ꢁ7
210.4
215.8
220.6
228.5
102.0
111.2
123.5
130.7
4.1:1
Fig. 13. Temperature dependence Nyquist plots of SPE2 a) 10%, b) 20%, c) 30%, d) 40%. Arrhenius plot of temperature dependence conductivity of e) SPE1, f) SPE2, g) SPE4, h) SPE5,
i) SPE6 (10%e40%).

H.V. Babu, K. Muralidharan / Polymer 55 (2014) 83e94
93
and varying amounts of lithium salt (LiTFSI) in THF. Since the
polymer P3 was liquid we did not make electrolyte from it. After
evaporating and drying the solvent, the residues were made into
pellets for conductivity measurements. To understand the thermal
properties, TGA and DSC analyses on the solid electrolytes SPE1,
SPE2 and SPE4eSPE6 were performed. Figs. 11 and 12 depict the
TGA and DSC plots of SPE2 with varying amounts of lithium salt as
an example. Absence of peaks matching the melting temperature
(Tm) of lithium salt in DSC curves for any SPEs revealed the salt did
not exist as aggregates. The absence of melting point of salts might
because of solvation of lithium ions by polymers. The possibility of
polymer chain coordination with lithium ion would facilitate the
solvation. The SPEs with 40 wt% of LiTFSI [SPEs (40%)] also did not
show the peak for Tm which inferred the effective solvation by
polymers even for higher wt% of lithium salt.
weights. So far, other than poly(ethylene oxide), only few polymers
like polysiloxane, polyphosphazene have shown potential as
lithium ion conducting polymer electrolytes; but, both are liquids
at room temperature. However, the polyethers (P1eP6) produced
by us are stable, solids (except P3) at room temperature and
completely amorphous in nature. Therefore, the polymers P2 and
P4 demonstrated promising solid-state ionic conductivity. The solid
polymer electrolyte SPE2 (40%), prepared from P2, showed the
highest conductivity of 2.9 ꢀ 10ꢁ4 S cmꢁ1 at 80 ꢂC. Apart from
these, the presence of phosphorus in these polymers would impart
flame-retardant property to SPEs. These polyethers could find use
as solid polymer electrolytes in lithium-ion batteries.
Acknowledgement
The TGA and DTG analyses on SPEs revealed an increase in the
thermal stabilities of SPE1, SPE2 and SPE4eSPE6 compared with
their respective polymers P1, P2 and P4eP6 (Tables 3 and 4 and S2;
Fig. 11, S1eS4). The glass transition temperatures (Tg) of SPEs
having 10e40 wt% of LiTFSI respectively [SPEs (10%e40%)] deter-
mined through DSC analyses are shown in Tables 3and 4. As seen in
DSC plots, there were increase in Tg of SPEs compared with their
respective polymers. The increase was more with increasing salt
contents. The increased Td and Tg can be explained through coor-
dination of polymer chains with lithium ions by as follows. Coor-
dination of polymer with lithium ions would affect the stretching of
polymer chains thus restricting their segmental motion. Obviously,
more salt molecules would encourage more coordination by poly-
mer chains and increased control on the segmental motion of the
chains leading increased Tg. Similarly, because of the coordination
of polymer chains with lithium ions, it needs more energy to break
the coordination bonds at first followed by to break the polymer
chains.
HVB is thankful to CSIR India for a fellowship.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2013.12.005.
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