Thermal and conductivity properties of poly(ethylene glycol)-based cyclopolymers

Article abstract of DOI:10.1039/b402677b

Cyclopolymers bearing a hexaethylene glycol-based malonate crown ether, have been synthesized and characterized from the point of view of their thermal properties and of their use, in thin films, either on their own or as blends with poly(ethylene glycol), in lithium ion battery applications.

Full text of DOI:10.1039/b402677b

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A R T I C L E
Thermal and conductivity properties of poly(ethylene glycol)-
based cyclopolymers{
Enrique Blazquez,a Piercarlo Mustarelli,*b Dario Pasini,*a Pier Paolo Righettia and
Corrado Tomasib
aDepartment of Organic Chemistry, University of Pavia, Viale Taramelli, 10 - 27100
Pavia, Italy. E-mail: dario.pasini@unipv.it; Fax: 139-0382-507323;
Tel: 139-0382-507312
bDepartment of Physical Chemistry ‘‘M. Rolla’’, University of Pavia, IENI-CNR and
INFM, Viale Taramelli, 16 - 27100 Pavia, Italy. E-mail: mustarelli@matsci.unipv.it;
Fax: 139-0382-507575; Tel: 139-0382-507205
Received 20th February 2004, Accepted 17th May 2004
First published as an Advance Article on the web 14th June 2004
Cyclopolymers bearing a hexaethylene glycol-based malonate crown ether, have been synthesized and
characterized from the point of view of their thermal properties and of their use, in thin films, either on
their own or as blends with poly(ethylene glycol), in lithium ion battery applications.
polymerization of a variety of malonate-derived difunctional
Introduction
acrylic-like monomers yields a polymer backbone comprised
exclusively of six-membered rings.7c In our design approach,
the spiro quaternary carbon center obtained after double
alkylation reaction on the malonate moiety should hold, after
polymerization, the crown ether portions in a stacked fashion,
so favoring packed channel-like structures (see Fig. 1).
We report here the synthesis of novel cyclo-copolymers
bearing a substantial content of their structure as poly(ethylene
glycol), their thermal and, when doped with Li1 ion salts,
their electrical properties either on their own or as blends
with PEO.9
Lithium electrolytes play an essential role in advanced battery
applications.1 Solid polymer electrolytes are composed of
poly(ethylene oxide) (PEO) as a blend with the lithium salt of a
large anion. If ion transport was originally thought to occur via
‘ion hopping’ along the PEO chains, it was later shown that
ionic mobility in the above system takes place primarily in the
amorphous region of the electrolyte.2 The currently accepted
model for ion transport involves a cooperative ion-polymer
motion activity. Recent variations in polymer electrolyte
composition and structure include both variations of the
counterions associated with the lithium salt, and the study of
macrocyclic compounds, both of the crown ether and cryptand
type,3 included as a blend with PEO. In a report, acrylic-like
polymers, with crown ethers covalently bound, via a flexible
ethylene glycol spacer of varying length, have been tested as
materials for lithium ion battery applications.4
We were interested in examining the properties of novel type
of materials in which rigid, pre-organized, channel-like struc-
tures could favor a high degree of intrachain mobility and
recognition affinity between the crown ether structure and the
lithium cation. Towards this objective, we have considered
the use of ‘malonate’ crown ethers, characterized by a poly-
(ethylene glycol) portion modified by the insertion of a malo-
nate ester functionality.5 { The modification of the malonate
unit is then easily achieved by mild organic functionalization of
the acidic CH2 carbon atom, for different purposes.6
Experimental
All commercially-available compounds were purchased from
Aldrich and used as received. THF (CaH2) and CH2Cl2 (CaH2)
were dried before use. Compound 1 and polymer 4 9 were
synthesized as previously reported. Flash chromatography was
carried out using silica gel (Merck 60, 0.040–0.063 mm). 1H and
13C NMR spectra were recorded from solutions in CDCl3 on
Bruker 200 or AMX300 spectrometers with the solvent residual
proton signal as a standard. Infrared spectra were recorded on
Cyclopolymerization procedures are very useful for achiev-
ing a high degree of structural control within the polymer
backbone of
a
certain macromolecular structure,7 and
have been used recently as a protocol for obtaining stereo-
regular polymers, or to impart peculiar materials properties.8
Mathias and co-workers have also demonstrated that the
{ Electronic supplementary information (ESI) available: 1H NMR
spectra and gel permeation chromatography traces of polymers 4, 5a
and 6 after purification by precipitation in the non-solvent. See http://
www.rsc.org/suppdata/jm/b4/b402677b/
{ There is no close resemblance between the size selectivity towards the
binding of metal ion shown by these series of compounds and that
shown by the 15-crown-5, 18-crown-6 and 21-crown-7 series; one
general trend is that their binding constants towards alkaline metal
cations were found to be generally lower than the latter series.
Fig. 1 One of the possible conformations of cyclopolymer 4.
2 5 2 4
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 5 2 4 – 2 5 2 9
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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an FT-IR PE Paragon 1000 spectrophotometer using potas-
sium bromide with a diffuse reflectance accessory. Elemental
analyses were carried out on a Carlo Erba 1106 elemental
analyzer. Size-exclusion chromatography was carried out on a
Perkin-Elmer chromatograph (Series 2000) equipped with a
DRI detector. Low polydispersity polystyrene standards
(Fluka) were used for the calibration curve and the mobile
phase was tetrahydrofuran (1 mL min21, 40 uC). A bank of
Differential scanning calorimetry measurements
The polymer samples used for the characterization and con-
ductivity experiments were the ones obtained in THF (entries 1
and 3 in Table 1). Modulated (MDSCTM) Differential Scan-
ning Calorimetry measurements were performed with an
MDSC 2910, equipped with
a 2000 Thermal Analyst
(TA Instruments Inc., USA). The MDSC was fitted out with
an auto-fill liquid nitrogen cooling accessory (LNCA). A
nitrogen gas flow of 30 mL min21 was used as the purge gas.
The measurements were performed at a heating rate of 5 uC
min21, in the temperature range 2100 uC to 100 uC,
by imposing a temperature modulation with a period of 40 s
and an amplitude of 0.5 uC. All samples were kept in a
desiccator and crimped in standard aluminium pans just before
measurements.
˚
four columns with porosities of 500, 1000, 10 000 A and mixed
was used.
Monomer and polymer preparation. Compound 2
Compound 2 was prepared similarly to a published procedure
for an ethylene glycol derivative.10 A solution of triethylene
glycol monomethyl ether (2 g, 1.95 mL, 12.2 mmol) and
triethylamine (3.37 g, 4.6 mL, 33.3 mmol) in CHCl3 (30 mL)
were cooled to 0 uC. Acryloyl chloride (1.01 g, 0.9 mL,
11.1 mmol) in CHCl3 (20 mL) was added to the solution
dropwise over a 30 min period. After 12 h of stirring at room
temperature, the reaction mixture was washed with brine, then
the aqueous phase washed with fresh chloroform and the
combined organic phases dried (Na2SO4). The product was
purified by flash column chromatography (cyclohexane/
AcOEt: 80/20) to yield 2 as a clear oil (1.2 g, 50%). IR
(cm21): 1735 (nCLO), 1640, 1250, 1225. 1H NMR (CDCl3,
200 MHz): d 6.38 (dd, 1H; H2CLCH–), 6.12 (dd, 1H; H2CLC–),
5.80 (d, 1H; H2CLC–), 4.30 (m, 2H; –COOCH2CH2O–),
3.8–3.5 (m, 10H; –OCH2CH2O–), 3.38 (s, 3H; –OCH3). 13C
NMR (CDCl3, 200 MHz): d 165.8, 130.7, 128.0, 71.6, 70.3,
68.8, 63.4, 58.7.
Thin film preparation
Films with thickness in the range 10–20 mm were prepared by
means of a Spincoater1 P6700 (Specialty Coating Systems,
USA). They were deposited on co-planar interdigited-comb
electrodes (IDE, surface area ca. 440 mm2, pitch 0.2 mm) in
order to perform the dielectric measurements (see below). The
IDEs were attached under vacuum at the spin-coater chuck
(2.54 mm diameter). Stoichiometric amounts of the synthesized
polymers, PEO and LiN(CF3SO2)2 (Sigma) were dissolved in
CH2Cl2 and quantities of solution of the order of 1026 dm3
were dropped onto the IDEs by means of a micro-syringe under
N2 atmosphere. The rotating speed ranged between 2000 and
4000 rpm.
Dielectric analysis
Free radical polymerization
Dielectric measurements were performed by means of a
DEA2970 analyzer (TA Instruments, USA), in the frequency
range from 1 to 105 Hz, and in the temperature range from
room temperature to 200 uC. A N2 flow of 500 mL min21 was
used to purge the analysis chamber (volume # 2 dm3). The film
thickness was continuously monitored during the measure-
ments by means of a Linear Variable Differential Transformer
(LVDT) with a 1 mm resolution.
The monomer and the initiator (AIBN), dissolved in the
solvent at the concentration and relative proportions outlined
in Table 1, were degassed with argon for 30 min and then
heated at 65 uC in a thermo-controlled bath for 48 h. Upon
cooling at room temperature, the reaction mixture was
examined by TLC to monitor the complete disappearance of
the starting monomer, the solvent removed in vacuo, the
remaining solid dissolved in the minimum amount of CH2Cl2,
and the solution added dropwise to a solution of the non-
solvent (20 times its co-solvent volume).
Results and discussion
Polymer and monomer synthesis
Polymer 5a. From monomer 1 (265 mg, 0.48 mmol) and
monomer 2 (26 mg, 0.12 mmol) in dry THF (2.4 mL) with
AIBN (1.97 mg, 0.012 mmol, 2% mol vs. total monomer con-
centration). Purified by precipitation in cyclohexane (175 mg,
83%). IR (cm21): 1730, 1450, 1350, 1250, 1130. Anal. calc. for
C22H34O11.4: C, 54.94; H, 7.01. Found: C, 54.96; H, 6.91%.
Monomer 1 (Scheme 1) could be prepared in large quantities
(up to 3 g) following a procedure we recently reported.9
Monomer 2 was obtained, after purification by column
chromatography, in fair yields by esterification reaction of
commercially-available triethylene glycol monomethyl ether,
according to a procedure already reported in the literature for
similar compounds.11
The cyclopolymerization and cyclo-copolymerization of
monomer 1 by AIBN-induced free radical polymerization
produced in all cases linear polymers soluble in a variety of
organic solvents. The polymers were purified by precipitation
in cyclohexane as the non-solvent, and characterized by NMR
and IR spectroscopies, and Gel Permeation Chromatography
(GPC). The 1H NMR of polymer 4, when compared with
monomer 1, showed complete disappearance of the signals
associated with the olefinic protons, indicating both complete
cyclization and efficient polymerization processes (see ESI{).
IR spectroscopy also indicated the complete disappearance
of the band at 1630 cm21, which was present in the monomer
1 and attributed to the unsaturated double bond olefinic
functionalities. We have also copolymerized monomer 1 with
increasing amounts of monomer 2: by possessing a highly
mobile ethylene glycol chain, our hope was that to introduce
a tunable glass transition temperature in the copolymer, and
Polymer 5b. From monomer 1 (265 mg, 0.48 mmol) and
monomer 2 (70 mg, 0.32 mmol) in dry THF (3.2 mL) with
AIBN (2.63 mg, 0.016 mmol, 2 mol% vs. total monomer
concentration). IR (cm21): 1730, 1450, 1350, 1250, 1130. 1H
NMR (CDCl3): d 4.5–4.0 (broad s; –COOCH2CH2O–), 3.9–3.5
(broad s; –COOCH3 and –OCH2CH2O–), 3.4 (broad s;
–OCH3), 1.8–2.2 (broad s; aliphatic –CH2–).
Polymer 6. From monomer 1 (200 mg, 0.37 mmol) and
N-phenylmaleimide 3 (64 mg, 0.37 mmol) in toluene (3 mL)
with AIBN (2.43 mg, 0.0148 mmol, 2 mol% vs. total monomer
concentration). Purified by precipitation in cyclohexane
1
(189 mg, 74%). IR (cm21): 2920, 1715, 1500, 1250. H NMR
(CDCl3): d 7.4–7.2 (broad m; Ph–H), 4.2–4.0 (broad m), 3.9–
3.2 (broad m), 3.0–1.2 (broad s). Anal. calc. for C35H45O15N:
C, 58.48; H, 6.30; N, 1.95. Found: C, 58.48; H, 6.13; N, 1.80%.
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 5 2 4 – 2 5 2 9
2 5 2 5

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Scheme 1 Synthesis of polymers 4–6.
therefore study the conductivity behavior as a function of the
glass transition temperature. Both copolymers 5a and 5b,
synthesized with, respectively, 20 and 40 mol% of comonomer 2
in the feed ratio, could be obtained as linear copolymers,
soluble in a variety of organic solvents. Whereas copolymer 5a
could be easily precipitated in cyclohexane, copolymer 5b
formed a gummy suspension difficult to separate and purify,
and therefore was not characterized further. The 1H NMR
spectrum of polymer 5a showed the resonances associated with
the terminal methyl ether as a broad signal, to confirm effective
incorporation of the comonomer 2.
to undergo free radical-induced alternating copolymerization
with acrylic and styrenic derivatives. Furthermore, other
malonate-derived acrylic-like difunctional monomers have
been shown to undergo a similar radically-induced alternating
process with maleic anhydride.8c Indeed, the polymer was
obtained in high yields after purification by precipitation in
cyclohexane. Incorporation of the maleimide comonomer is
clearly evident in the 1H NMR of the polymer: the relative
integration of the aromatics area with the area related to the
a-protons on the crown ether structure gave a fair corres-
pondence to the proposed 1:1 stoichiometry.
Copolymerization with N-phenylmaleimide 3 was also carried
out: maleic anhydride and maleimide derivatives are known
Elemental analyses revealed, both in the case of polymer 4,
5a and 6 a close correspondence between the feed and observed
Table 1 Cyclopolymerization of monomers 1 and 2 under free radical conditions.a
Solvent/[M]b
Mn
Mw
PDc
Yield (%)
c
c
Entry
Monomer
Polymer
1
2
3
4
1
1
1/2
1/3
4
4
5a
6
THF/0.25
10 700
20 900
6300
17 800
25 900
12 600
28 000
1.7
1.3
2.0
4.0
92
54
83
74
Toluene/0.25
THF/0.25
Toluene/0.25
6900
a Polymerizations were run for 30 h with a total monomer concentration indicated as [M]. Polymers were purified by precipitation in cyclohex-
ane. b All polymerizations were run with 2 mol% (vs. total monomer concentration) of AIBN as a radical initiator. c As determined by GPC
relative to polystyrene standards after purification. PD ~ polydispersity.
2 5 2 6
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 5 2 4 – 2 5 2 9

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Fig. 2 MDSC thermogram of polymer 4.
Fig. 4 Behavior vs. temperature of the permittivity, e’, at some
selected frequencies for the film 5a 1 PEO 1:1 wt. ($) 1 Hz, (#) 10 Hz,
(+) 100 Hz.
ratio of comonomers in the polymer samples (see Experimental
part). Characterization of the polymers by GPC is reported
in Table 1. Polymerization of monomer 1 in THF (entry 1,
Table 1) and in toluene (entry 2) revealed two very different
molecular weight distributions: in the former case, average Mw
and Mn values for 1 in THF are considerably lower than
toluene, as to be expected from a solvent with a relatively
higher chain transfer to solvent constant. As shown in entry 2,
an unusually low PDI was also recorded, but has to be attri-
buted to a fragmentation of the polymer composition obtained
by precipitation in cyclohexane.9 Incorporation of monomer
2 or the N-phenylmaleimide comonomer instead gave a
broader molecular weight distribution profile, together with
a somewhat lowered average degree of polymerization (entries
3 and 4, Table 1).
melting of the polymer backbone. It is interesting to note that
these polymers, and chiefly 5a, do not display the typical melt-
ing peaks of the PEO-based phases in the range 60–100 uC,12
although they contain a large amount of ethylene glycol
groupings.
Dielectric and conductivity properties of thin films
Our preliminary tests showed that both polymers 4 and 5a did
not form thick, stand-alone films by standard casting, but they
did on addition of quantities of PEO of the order of 30 wt%. In
the following we will report only the data obtained on thin films
obtained by spin-coating, both with and without the addition
of PEO to our synthesized polymers.
Thermal characterization of polymers 4 and 5a
Fig. 4 shows the behavior vs. temperature of the permittivity,
e’, at some selected frequencies for the film 5a 1 PEO 1:1 wt.
We observe a strong increase near 60 uC that calls for the
presence of a relevant quantity of crystalline PEO phase in the
thin film. The observed increase is likely due to interfacial
effects with the IDE contacts and it becomes negligible for
frequencies above 100 Hz. The subsequent decrease of per-
mittivity at temperatures of the order of 90 uC may be due to a
structural rearrangement of the polymer (accompanied by
small variations, of the order of 10–15%, of the film thickness)
following the PEO melting.
Figs. 2 and 3 show the MDSC thermograms of polymers 4 and
5a, respectively. In both cases, the continuous line represents
the total heat flow that is the signal given by a standard DSC,
whereas the dashed line is the reversing heat flow, which is due
to the thermal phenomena reversible in the time scale of the
temperature modulation.11 The total heat flow curves of both
the polymers show three thermal effects: the first endothermic
peak at ca. 290 uC is likely related to residual CH2Cl2 melting;
a second peak at ca. 5 uC is associated with the melting of
residual cyclohexane, which was used for allowing the sample
to precipitate; finally, the broad endotherm above ca. 50 uC is
due to solvent volatilization.
Fig. 5 shows the behavior of the ionic conductivity vs.
frequency at 100 uC for the sample 5a 1 PEO 1:1 wt and for the
The reversing signals, as expected, show only the peaks
related to the melting of solvents. In the examined temperature
range we did not observe any evidence of glass transitions or
Fig. 5 Behavior of the ionic conductivity vs. frequency at 100 uC for
the sample 5a 1 PEO 1:1 wt ($) and for the same sample doped with
LiTFSI (PEO:LiTFSI 8:1) (#).
Fig. 3 MDSC thermogram of polymer 5a.
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 5 2 4 – 2 5 2 9
2 5 2 7

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Table 2 Dielectric data at 50 uC and 1 kHz for formulations
containing polymers 4 and 5aa
Conclusions
We have reported the synthesis and characterization of a series
of new cyclo-copolymers containing unconventional crown
ethers embedded in their polymer chains. Even though they
incorporate a substantial portion of ethylene glycol units
(51 wt% for 4 and 56 wt% for 5a), their thermal properties are
substantially different from poly(ethylene glycols) since they do
not possess melting or glass transitions.
The new materials possess desirable film-forming character-
istics, and it was possible to prepare films in the micrometer
range by spin-coating. The conductivity of these films is not
yet enough to allow high-energy delivery (e.g. for lithium
batteries); however, the explored polymer architectures are of
interest because of the possibility to ‘tune’ their dimensions
and/or their order to allow new functional properties. Our
approach is indicative of how the molecular properties of the
recognition unit (i.e. crown ethers instead of open chain
glycols) cannot be discerned from their physical properties
(e.g. glass transition temperature). These new, robust polymers
could be useful in those applications of material science
requiring high thermal stability.
Samplea
e’
1.45
e@
s/V21 cm21
4
0.26
0.10
0.06
0.08
2.6 6 10211
2.6 6 10211
3.4 6 10211
4.1 6 10211
1.22 6 10210
1.31 6 10210
2.78 6 10210
6.91 6 10211
4.51 6 10210
4 1 Li (5:1)b
5a
1.60
2.78
3.35
2.68
9.45
15.32
2.83
7.45
5a 1 Li (5:1)b
5a 1 PEO
32.95
5a 1 PEO 1 Li (1:1)c
5a 1 PEO 1 Li (2:1)c
5a 1 PEO 1 Li (4:1)c
5a 1 PEO 1 Li (8:1)c
5020
109.7
32.4
74.65
a The ratio polymer/PEO is always 1:1 wt. b The amount of lithium
salt is expressed as the molar ratio obtained by considering the aver-
age molecular weight of the repeat monomer unit. c The amount of
lithium salt is expressed as the ratio between the moles of PEO and
those of the salt in the sample.
same sample doped with LiTFSI (PEO:LiTFSI 8:1). A plateau
giving the value of d.c. conductivity is observed at low
frequencies, followed by a dispersive behavior above 1 kHz,
which is probably related to the onset of slow re-orientational
motions of parts of the polymer backbone. Similar behaviors
are observed for all the other doped samples (see also Table 2).
The conductivity of the non-doped sample is likely due to
impurities introduced during the synthesis procedure (e.g.
Na1 ions) or to moisture. The addition of the lithium salt
causes only a minor change of the overall conductivity,
instead of the expected orders-of-magnitude increase which is
typical of salt-doped, PEO-based systems. This means that
the crown structures of the polymer 5a do not allow fast
diffusion of the lithium ions, and that the usual mechanisms
of conduction assisted by segmental motions, typical of
amorphous PEO–salt phases,13 are not effective. Therefore,
we can suppose that the highly regular structure of polymer
5a is able to hinder the growth of the amorphous phases
which are normally observed in PEO–salt systems near the
eutectics.
Table 2 reports the values of permittivity (e’), loss factor (e@)
and conductivity (s) at a specific frequency and temperature
for all the films we prepared. Generally speaking, we observe
that the addition of an ether group in order to increase
the amorphous fraction of the copolymer (sample 5a vs. 4)
does not practically affect the conductivity, whereas it makes
e’ change by a factor of two. On the other hand this result
may be somehow expected, since the added monomer contains
ethylene oxide groups, and it is well known from the
literature14 that the dielectric constant of amorphous PEO
is roughly twice that of the crystalline polymer. The addition
of PEO to both the polymers 4 and 5a determines an
orders-of-magnitude increase of the electric loss factor. As
far as concerns the transport properties, finally, the addition
of the lithium salt to the polymer blend does determine, at
most, a conductivity increase by a factor of two. Further
work is needed in order to prepare matrices allowing effective
ion transport. In particular, we stress that our chief aim in
this work was to see if highly ordered crown ether structures
could allow high lithium transport. As a consequence, the
addition of PEO was chiefly made to improve filmability and,
only as a ‘byproduct’, to add another mechanism for ion
motion.
Acknowledgements
We thank Dr Alessandro Galbiati (N.P.T. Polyurethane
Technologies, Gropello Cairoli, Italy) for performing the Gel
Permeation Chromatography analyses of the samples reported
here. Funding by the University of Pavia (FAR 2001-2003) is
acknowledged with thanks.
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In this frame, some developing strategies are: (i) changing the
crown ether dimension, (ii) changing the PEO MW (short chain
PEGs are recognized to work well in polymer electrolytes), and
(iii) adding plasticizers (e.g. ethylene carbonate). Another
important point in order to improve lithium transport is the
study of the relationships between crystallinity of the malonate
structures and ionic conductivity.
2 5 2 8
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 5 2 4 – 2 5 2 9

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9
A preliminary account on the synthesis and transport properties of
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reported: E. Cagnoni, D. Pasini, A. Galbiati, M. Ricci and
P. P. Righetti, Macromolecules, 2003, 36, 8894–8897.
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