Synthesis and tensioactive properties of PEO-b-polyphosphate copolymers

Article abstract of DOI:10.1039/c5ra02205c

Poly(ethylene oxide) (PEO)-b-polyphosphate copolymers made of hydrophilic PEO and hydrophobic polyphosphates are amphiphilic copolymers prone to self-assemble in water into nanoparticles. In this work, nanoparticles are obtained by the self-assembly of PEO-b-polyphosphate copolymers in water in the absence of any organic co-solvent whatever the length of the pendant alkyl chain (between 4 and 7 carbon atoms) of the polyphosphate block. Remarkably, this solvent-free process remains efficient even for the most hydrophobic polyphosphate blocks. The critical aggregation concentration (CAC) of the block copolymers was determined by pyrene probe fluorescence. Finally, the efficiency of these copolymer surfactants to decrease the air-water interface was measured by air-bubble tensiometry.

Full text of DOI:10.1039/c5ra02205c

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PAPER
Synthesis and tensioactive properties of PEO-b-
polyphosphate copolymers†
Cite this: RSC Adv., 2015, 5, 27330
S. Vanslambrouck,ac B. Clement,ac R. Riva,a L. H. Koole,b D. G. M. Molin,c G. Broze,a
´
ac
P. Lecomteac and C. Jerome
*
´ ˆ
Poly(ethylene oxide) (PEO)-b-polyphosphate copolymers made of hydrophilic PEO and hydrophobic
polyphosphates are amphiphilic copolymers prone to self-assemble in water into nanoparticles. In this
work, nanoparticles are obtained by the self-assembly of PEO-b-polyphosphate copolymers in water in
the absence of any organic co-solvent whatever the length of the pendant alkyl chain (between 4 and 7
carbon atoms) of the polyphosphate block. Remarkably, this solvent-free process remains efficient even
for the most hydrophobic polyphosphate blocks. The critical aggregation concentration (CAC) of the
block copolymers was determined by pyrene probe fluorescence. Finally, the efficiency of these
copolymer surfactants to decrease the air–water interface was measured by air-bubble tensiometry.
Received 20th February 2015
Accepted 10th March 2015
DOI: 10.1039/c5ra02205c
www.rsc.org/advances
At the end of the 1970s, Penczek and co-workers achieved a
pioneering work establishing that polyphosphates can be
Introduction
synthesized by ring-opening polymerization (ROP) of cyclic
phosphates.16,17 Organometallic compounds such as aluminum
alkoxides18 and tin octanoate (Sn(Oct)2)19 are commonly used to
promote the ROP of cyclic phosphates. Nowadays these metal-
based promoters are replaced by metal-free organo-
catalysts.20,21 Similarly to aliphatic polyesters (PCL, PLA,.),
polyphosphates are degraded by reaction with water. Indeed,
Baran and Penczek showed that the kinetics of the poly-
phosphates hydrolytic degradation is affected by a number of
parameters, such as temperature, pH and chemical structure of
the pendant chains.22 Narendran and Kishore reported on a
series of polyphosphates that exhibit a faster hydrolytic degra-
dation at basic pH, leading to a weight loss of 3.9% in six hours
(for the most stable polymer).23 Wang et al. described a slower
hydrolytic degradation for a PEO-b-polyphosphate amphiphilic
block copolymers at 37 ꢀC and pH 7.4 in a phosphate buffer
solution (a weight loss of 7% for the polyphosphate block in two
months).24 Consequently, polyphosphates turned out to be a
valuable alternative to aliphatic polyesters as a degradable
hydrophobic polymers for the elaboration of diblock amphi-
philic polymers. In terms of applications, these polyphosphates
appear particularly appealing for the fabrication of surfactants
or nanocarriers in the eld of Drug Delivery systems.25–28
Amphiphilic block copolymers hold a special place in macro-
molecular chemistry due to their dual nature with one block
exhibiting hydrophilicity and the other one lipophilicity. They
are widely used in diverse applications,1,2 including detergency,3
dispersion and foam stabilization4,5 and drug delivery systems6
in which they demonstrated some advantages such as a lower
critical micellar concentration as compared to small size
surfactants. These copolymers are able to self-assemble in water,
driven primarily by the hydrophobic effect but also by electro-
static interactions, inclusion complexes and hydrogen bonding.
The self-assembly of few non-ionic amphiphilic copolymers
was investigated compared to the cationic, anionic, or ampho-
teric ones. Among the non-ionic copolymers displaying micelli-
zation ability, the majority of the research was performed
on poly(ethylene oxide) (PEO) as the hydrophilic block with
poly(propylene oxide) (commercially called “Pluronics”)7,8 or
degradable aliphatic polyesters, i.e. poly(3-caprolactone)9,10 (PCL)
and poly(lactide)11 (PLA) as the hydrophobic block.12 Indeed,
such PEO-based non-ionic surfactants being biocompatible and
less toxic than ionic ones were extensively investigated for
diverse oral and parenteral pharmaceutical formulations.13,14
A
well-known example is Tween-80 which is less toxic, less hae-
molytic and maintains nearly physiological pH in solution.15
In contrast to aliphatic polyesters, the chemical structure of
polyphosphates is easily changed by simple modication of
lateral groups. This task is achieved by the implementation of
the ROP of cyclic phosphates prepared by the esterication
reaction of 2-chloro-1,3,2-dioxaphospholane 2-oxide (COP) with
the appropriate alcohol (Scheme 1).29,30 This strategy was
applied by Yang et al. in order to modify the hydrophobicity of
the polyphosphate block of amphiphilic copolymers.31 It turned
aCenter for Education and Research on Macromolecules, University of Liege, Chemistry
Department, B6a, Sart-Tilman, B-4000 Liege, Belgium. E-mail: c.jerome@ulg.ac.be
bFaculty of Health, Medicine and Life Science, Department of Biomedical Engineering/
Biomaterials Sciences, Maastricht University, Maastricht, The Netherlands
cBioMIMedics, Interred EMR IV-A Consortium: Lead Partner Maastricht University,
Universiteitssingel 50, 6229ER Maastricht, The Netherlands
† Electronic supplementary information (ESI) available: See DOI:
10.1039/c5ra02205c
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commercially available COP and the corresponding alcohol
(1-butanol, iso-butanol or 1-heptanol) in the presence of a stoi-
chiometric amount of TEA at 0 ꢀC for 7 hours (Scheme 1).
Aer ltration of the released triethylammonium hydro-
chloride salt, the monomers were puried by fractionated
distillation under vacuum with an average nal yield of 60% for
nBP and iBP monomers and 50% for nHP. The structure of the
different monomers was assessed by 1H NMR, by 13C NMR and
by 1H-decoupled 31P NMR analyses (see in the ESI†).
Scheme 1 Followed strategy for the synthesis of the PEO-b-poly-
phosphate copolymers.
out that the longer the lateral chains are, the more hydrophobic
the polyphosphate block is and the lower the Critical Aggrega-
tion Concentration (CAC) is. Nevertheless, in their work, the
nanoparticles were prepared by dropwise addition of water into
a solution of the amphiphilic diblock copolymer in an organic
solvent (DMSO). The higher hydrophilicity of polyphosphates
compared to aliphatic polyesters opens up the possibility to
exploit them to prepare nanoparticles in the absence of any
potentially toxic organic solvent,32 which is an important
advantage for several applications and especially for food,
biomedical and detergency applications.
Our work aims at investigating the self-assembly without
addition of organic co-solvent of PEO-b-polyphosphate copoly-
mers in water and at the air–water interface. For this purpose,
PEO-b-polyphosphate copolymers of increasing hydrophobicity
by increasing the number of carbon atoms of pendant group
were synthesized according to a similar strategy used by Yang
et al.31 Besides the length, the effect of the alkyl group archi-
tecture was investigated on the CAC of the block copolymers
determined by pyrene probe uorescence. Finally, the efficiency
of these copolymer surfactants to decrease the air–water inter-
face was measured by air-bubble tensiometry.
Synthesis of PEO-b-polyphosphate amphiphilic copolymers
Generally, hydrophilic PEO is used to synthesize non-ionic
amphiphilic copolymers. Among the degradable amphi-
philic copolymers, PEO-b-PCL copolymers are one of the
most described in the scientic literature. Previous
researches reported a molar mass of 5000 g molꢁ1 as typical
interesting length for the PEO block of such surfactant
copolymer,31,34,35 notable because of the possible renal
extraction when use in pharmaceutical elds.36 For sake of
comparison, this length was selected for the PEO macro-
initiator. Besides, a polymerization degree of 10 was tar-
geted for each polyphosphate block for the sake of compar-
ison with more commonly used PEO114-b-PCL8 copolymer
surfactants.
Accordingly, the synthesis of PEO114-b-PiBP10, PEO114-b-PnBP10
and PEO114-b-PnHP10 copolymers was investigated by
organocatalyzed ROP of cyclic phosphates in living conditions,
by using monomethoxypoly(ethylene oxide) (MeO-PEO-OH)
(5000 g molꢁ1) as macro-initiator. From literature data,21
a
mixture of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and
1-1-[3,5-bis(triuoromethyl)phenyl]-3-cyclohexyl-2-thiourea (TU)
appears as one of the most efficient catalytic system for poly-
merizing related cyclic phosphates. Based on these conditions
(CH2Cl2, 0 ꢀC and 60 min for iBP and nBP monomers or 70 min
for nHP monomer), the monomer to macro-initiator ratio was
set to 10 and polymerization let to full conversion. Accordingly,
the three targeted amphiphilic copolymers were successfully
obtained in a controlled fashion as depicted by the good
agreement between the theoretical and experimental
molar masses for each copolymer (Table 1) and the narrow
molecular weight distribution. The structures of the different
PEO-b-polyphosphate copolymers and the molar masses of the
corresponding polyphosphate block were determined by
1H NMR analysis. The typical 1H NMR spectrum of PEO-b-PnHP
Results and discussion
The rst step of our work is dedicated to the synthesis of
PEO-b-polyphosphate copolymers. It was decided to use poly-
phosphates substituted by pendant n-butyl, iso-butyl and
n-heptyl groups. The use of the ethyl group was discarded
because of the known hydrosolubility of the corresponding
polyphosphate,33 which is not suitable as the hydrophobic block
for our study. On the one side, polymers substituted by the
n-butyl and iso-butyl groups were selected in order to investigate
the inuence of the branching on the self-assembly of the cor-
responding amphiphilic copolymers in water. The tert-butyl
group was discarded because such tertiary carbon could affect
the stability of the resulting phosphate towards hydrolysis. On
the other side, the comparison of the self-assembly of copoly-
mers bearing pendant n-butyl and n-heptyl groups enabled the
investigation of the effect of the length of the substituent in the
absence of any organic co-solvent.
is shown Fig. 1 and MnHP ¼ 222 g molꢁ1 (MiBP ¼ MnBP
¼
180 g molꢁ1).
The molar mass of the PnHP block was measured by the
comparison of the intensity of the signal of the PEO block at
3.6 ppm (Fig. 1, peak C) and the typical signal of polyphosphate
block at 1.3 ppm (Fig. 1, peak F) according to eqn (1), where IC
and IF stand for the integral of the C and F peaks in Fig. 1.
Monomer synthesis
The synthesis of cyclic phosphates is described in many
works.21,31 Briey, the cyclic phosphates substituted by a n-butyl
group (nBP), an iso-butyl group (iBP) or a n-heptyl group (nHP)
were synthesized by the esterication reaction of the
IF
Mn;NMRðPnHP blockÞ ¼ DPPEO
ꢂ
ꢂ Mmonomer
(1)
2IC
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Table 1 Macromolecular characteristics of the different PEO-b-polyphosphate amphiphilic copolymers
a
b
c
Catalyst
DBU/TU ratio
(mmol)
Polym.
time
(min)
Mn,PEO
Mn,theo
PPhos
Mn,NMR
PPhos
Mn,SEC
total
b
macroinitiator
DPtheo
PPhos
DPexp
(g molꢁ1
)
(g molꢁ1
)
PPhos
(g molꢁ1
)
(g molꢁ1
)
Mw/Mn
HLBd
c
Monomer
iBP
nBP
nHP
2.7/1.2
2.7/1.2
2.7/1.2
60
60
70
5000
5000
5000
10
10
10
1800
1800
2200
8
9
8
1500
1600
1800
5200
5300
5700
1.10
1.10
1.10
15.4 (18.6)
15.2 (18.4)
14.7 (17.6)
a
Theoritical value for the molar mass of the polyphosphate block determined by the initial monomer/initiator ratio. b Polymerization degree and
molar mass of the polyphosphate block determined by 1H NMR according eqn (1). Molar mass (calculated for the main peak) and molar mass
distribution (calculated for the entire elugram) of the diblock copolymers determined by SEC in THF (PEO standards). HLB calculated by the
c
d
Griffin equation 20 ꢂ [1 ꢁ Mn(hydrophobic block)/Mn(total)] considering the phosphate block as hydrophobic, in bracket, HLB calculated by
considering only the alkyl group as hydrophobic.
Aqueous behaviour of PEO-b-polyphosphate diblock
copolymers
The main step of our work is to investigate the self-assembly of
the diblock copolymers in water without the use of organic
co-solvent. Indeed, the water contact angles of lms of PnBP
and PnHP homopolymers (ꢃ10.000 g molꢁ1) were measured to
be 7.6ꢀ and 14.8ꢀ, respectively (experimental details provided in
ESI†), showing the high hydrophilic character of these poly-
phosphates, even if both homopolymers are insoluble in water
at room temperature. This fact in combination with the low Tg
(<ꢁ50 ꢀC) and the absence of crystallinity observed for the
polyphosphate block of the copolymers, in contrast to the semi-
crystalline PCL, would favour such solvent-free process (DSC
traces of the amphiphilic copolymers are provided as ESI†).
The self-assembly of amphiphilic copolymers depends on
Fig. 1 1H NMR spectrum of PEO-b-PnHP copolymer in CDCl3.
the hydrophilic-lipophilic balance (HLB), usually calculated by
the Griffin equation. HLB calculated by considering as the
hydrophobic segments the only alkyl side-chain (10 butyl or
10 heptyl), and keeping the polyphosphate backbone
The 1H-decoupled 31P NMR spectrum (see ESI†) of the
(–CH2–CH2–O–PO3–) as part of the hydrophilic component are
PEO-b-PnHP copolymer conrmed the complete conversion of
reported in Table 1. Based on this consideration, the HLBalk
nHP monomer into the corresponding polymer by the shi of
obtained by the Griffin equation reaches 18.6, 18.4 and 17.6,
the signal from 18 ppm to ꢁ1 ppm.
while they were 15.4, 15.2 and 14.7 for the same equation but
SEC analysis conrmed also the success of the block copo-
considering the full polyphosphate block as hydrophobic
lymerization. Indeed, aer polymerization, a shi of the PEO
(Table 1). Due this strong hydrophilic character, it is question-
macro-initiator traces to higher apparent molar mass is
able to see whether these copolymers are able to self-assemble
observed in each case (Fig. 2). Moreover, no trace of residual
into nanoparticles in aqueous media. It turned out that all the
PEO is observed, which is in good agreement with a quantitative
three copolymers synthesized in our work can be directly dis-
initiation by the PEO macro-initiator.
solved in aqueous media (water or phosphate buffer) at room
However, although the SEC trace of MeO-PEO-OH is mono-
temperature without the help of any organic co-solvent.
modal, the SEC traces of the three different PEO-b-poly-
Hydrolytic stability. Before studying the surfactant proper-
phosphate copolymers are bimodal. Indeed,
a second
ties of the copolymers, the hydrolytic stability of these
polyphosphate-based copolymers was investigated in water. As
shown in Fig. 2, only the small peak of the triblock copolymers
is slightly shied to lower molecular weight. No shi of the
main peak of the SEC traces is observed aer 28 days evidencing
no signicant decrease of molar masses and thus no cleavage of
the polyphosphate backbone by hydrolysis under these condi-
tions. So, the diblock copolymers appear stable in milli-Q water
at least for 28 days at room temperature, i.e. in the conditions
used for the following studies.
population is observed at higher molar masses, which was
attributed to the presence of a triblock copolymer, poly-
phosphate-b-PEO-b-polyphosphate coming from the unavoid-
able initiation of the cyclic phosphate polymerization by
HO-PEO-OH, present in small amount in the commercial
MeO-PEO-OH. The separation of these copolymer mixtures
being tedious, they have been used without further purication
for the following step, knowing that it represents about 10% of
the sample.
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observed for the PEO-b-PnHP copolymer. The spherical
morphology was conrmed by Transmission Electron Micros-
copy (TEM) for PEO-b-polyphosphate self-assembled nano-
particles (TEM images in ESI†). As expected, the size of the
nanoparticles measured by TEM, i.e. under vacuum, appears
smaller than the diameter measured by DLS in aqueous
medium where the PEO shell is swollen.
Self-assembly in water by uorescence spectroscopy. In
order to conrm the formation of hydrophobic pockets above
the CAC, pyrene was added to the copolymer solutions of
increasing concentration and the pyrene uorescence spectrum
was recorded for each copolymer concentration. Indeed, the
uorescence spectrum of pyrene depends on its environment,
especially the intensities of the emission peak at 373 nm
(denoted I1), which corresponds to pyrene in aqueous environ-
ment, and at 383 nm (denoted I3), corresponding to pyrene in
hydrophobic environment, which is the case in the hydrophobic
core of the nanoparticles. The measurement of the I1/I3 ratio as
a function of the logarithm of copolymer concentration is
shown in the ESI.† From the slope break of these curves, the
CAC could be obtained for the three copolymers (Table 2). This
pyrene experiment conrms the presence of self-assembled
nanoparticles of the three PEO-b-polyphosphate copolymers
above a given concentration depending on the length of the
polyphosphate pendant groups.
In line with the recent report of Yang et al.,31 we can conclude
that the CAC decreases when the length of the pendant groups
increases and thus when the hydrophobicity of the poly-
phosphate block increases. In addition, varying the architecture
of the alkyl side-chain from iso-butyl to n-butyl appears to
decrease slightly the nanoparticles size and increase the CAC of
the copolymer of identical HLB. Remarkably, the used copoly-
mers are able to self-assemble in water without the help of
organic co-solvent in contrast to the report of Yang et al.31 on
PEO-b-polyphosphate and to PEO-b-PCL copolymers as reported
by Cajot et al.10
It is noteworthy that above the CAC, the size of the assem-
blies varies slightly with the concentration. The size generally
decreases when increasing the concentration (Table 2,
compares columns 2 and 4) except for the iBP based copolymer.
However, no dramatic changes of the nanoparticles size are
observed with the concentration by diluting or increasing it
above the CAC.
Fig. 2 SEC traces in THF of (a) PEO-b-PiBP and MeO-PEO-OH, (b)
PEO-b-PnBP and MeO-PEO-OH and (c) PEO-b-PnHP and MeO-
PEO-OH. The curved dotted line represents the SEC chromatograms
of the different copolymers after degradation in milli-Q water.
Self-assembly in water by dynamic light scattering. The
amphiphilic behaviour of the block copolymers was rst investi-
gated by measuring the intensity of scattered light of a concen-
trated solution for the different copolymers (3.8 ꢂ 10ꢁ4 mol Lꢁ1
)
in milli-Q water. The size and size distribution (PDI) of the
diffusers are reported in Table 2 as determined by Dynamic Light
Scattering (DLS).
Behaviour of the PEO-b-polyphosphate at the air–water
interface
These DLS results show that all the three copolymers directly
self-assemble in milli-Q water since diffusers have a size above
16 nm but remaining below 200 nm. Vangeyte et al.34 described
the self-assembly of a PEO114-b-PCL8 copolymer with a compa-
rable DP into micelles with a diameter of about 20 nm. By
comparison, the larger diameters observed for the PEO-b-PiBP
and PEO-b-PnBP copolymers indicate the formation of loose
aggregates rather than well-dened compact core–shell
micelles. In contrast, very small and well-dened micelles are
The self-association in water of the diblock copolymers being
demonstrated, the comparative ability to reduce the air–water
interfacial tension was measured in function of the copolymer
concentration. For all the three copolymers, a clear decrease of
the interfacial tension is observed upon increasing the copoly-
mer concentration (Fig. 3). A clear break in the slope is observed
in each case which occurs for quite similar concentration for
both butyl polyphosphate (ꢃ3 ꢂ 10ꢁ6 mol Lꢁ1) and corresponds
to a levelling of the interfacial tension down to 30 mN mꢁ1
.
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Table 2 Hydrodynamic diameter (Dh) and size distribution (PDI) of the PEO-b-polyphosphate assemblies in milli-Q water, determined by DLS
measured afor a copolymer concentration of 1.9 ꢂ 10ꢁ4 mol Lꢁ1 and bfor a copolymer concentration of 3.8 ꢂ 10ꢁ4 mol Lꢁ1
,
cCAC values
determined by pyrene probe fluorescence spectroscopy and dthe SCC values by tensiometry
Copolymers
Dha (nm)
PDIa
Dhb (nm)
PDIb
CACc (mol Lꢁ1
)
SCCd (mol Lꢁ1
)
PEO-b-PiBP
PEO-b-PnBP
PEO-b-PnHP
153 ꢄ 2
145 ꢄ 2
21 ꢄ 2
0.114
0.189
0.224
169 ꢄ 3
126 ꢄ 3
16 ꢄ 4
0.098
0.168
0.098
3.1 ꢂ 10ꢁ5
4.6 ꢂ 10ꢁ5
2.8 ꢂ 10ꢁ6
4.1 ꢂ 10ꢁ6
2.2 ꢂ 10ꢁ6
2.8 ꢂ 10ꢁ7
In the case of PEO-b-PnHP copolymer, this break appears at uorescence analysis. It is noteworthy that for PEO-b-PnHP
concentration of one order of magnitude lower copolymer, the interfacial tension does not stabilize aer the
a
(ꢃ3 ꢂ 10ꢁ7 mol Lꢁ1) as compared for PEO-b-PiBP and slope break. It rather keeps decreasing slowly when we further
PEO-b-PnBP in line with the lower CAC measured by pyrene increase the concentration, the interfacial tension remaining
above 35 mN mꢁ1
.
These slope change concentration (SCC) values were
compared with PEO-b-PCL amphiphilic block copolymer pre-
senting a similar DP for each block. Vangeyte et al.35 determined
by tensiometry the SCC value for PEO114-b-PCL8 copolymers
which is approximately 8 ꢂ 10ꢁ6 mol Lꢁ1. This value is similar
than the SCC values for PEO114-b-PiBP9 and PEO114-b-PnBP9
copolymers whereas the SCC value for the most hydrophobic
nHP system is much lower. Comparing the surface tension
values at the slope break with those for the PEO114-b-PCL8
copolymers (ꢃ51 mN mꢁ1),35 we observe that the surface
tension values for the PEO-b-polyphosphate at the slope break
are much lower, approximately 35 mN mꢁ1 for PEO-b-PiBP and
PEO-b-PnBP copolymers and 47 mN mꢁ1 for the most hydro-
phobic nHP system, showing the higher efficiency of PEO-b-
polyphosphate copolymers to stabilize the air–water interface as
compared to the PEO-b-PCL of the same DP.
Conclusion
This study demonstrated that the amphiphilic block copoly-
mers (PEO114-b-PiBP8, PEO114-b-PnBP9 and PEO114-b-PnHP8)
are able to self-assemble in water into nanoparticles without
the use of any organic co-solvent whatever the length of the
pendant group along the polyphosphate block and thus
whatever the hydrophobicity of the polyphosphate block. The
surfactant properties of these copolymers including aqueous
micellization and air–water interfacial tension investigations
were systematically studied. The CAC decreases when the
length of the pendant group increases and thus when the
polyphosphate block is more hydrophobic. Accordingly, a low
CAC can be achieved with a heptyl chain. In addition, we evi-
denced that the architecture of the alkyl chain while keeping
constant the HLB has an effect on the size and CAC of the self-
assemblies even if the particles size remains below 200 nm in
any case. The copolymers were synthesized by metal-free ring-
opening polymerization of the corresponding cyclic phos-
phates initiated by a PEO macro-initiator. Such copolymers
appear thus as a promising class of environmental-friendly
neutral surfactant copolymers of interest in various elds of
applications such as food, cosmetic, biomedical and deter-
gency applications.
Fig. 3 Plots of surface tension versus log[copolymer] for the PEO-b-
polyphosphate copolymers: (a) PEO-b-PiBP, (b) PEO-b-PnBP and (c)
PEO-b-PnHP.
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[TU]0 ¼ 1/10/1.3/0.5. In a glass reactor, TU (386 mg, 1.04 mmol)
and the cyclic phosphate monomer (20.8 mmol) were dried by
three successive azeotropic distillations with toluene and dis-
solved in CH2Cl2 ([monomer]0 ¼ 1 mol Lꢁ1) before being
transferred under inert atmosphere into a second glass reactor
containing 10 g (2.1 mmol) of MeO-PEO-OH, freshly dried by
three azeotropic distillations. Aer complete solubilisation of
PEO, 0.4 mL of DBU (2.7 mmol) was added under nitrogen
atmosphere with a freshly amed stainless steel capillary. Aer
60 minutes of polymerization (aer 70 minutes for nHP), the
solution was concentrated under vacuum and the polymer was
precipitated in cold diethyl ether and ltrated. Residual DBU
was eliminated overnight by dialysis against MeOH (Spectrum,
membrane porosity ¼ 1000 g molꢁ1). Aer elimination of
MeOH under vacuum, the residue was dissolved in THF and
the amphiphilic copolymer was recovered by precipitation in
cold diethyl ether before being dried under vacuum.
Experimental
Materials
2-Chloro-1,3,2-dioxaphospholane 2-oxide (COP, Acros), meth-
anol (MeOH, Acros), diethyl ether (Chem-lab) and calcium
hydride (CaH2, Aldrich) were used as received without further
purication.
2-Methylpropan-1-ol
(Aldrich),
butan-1-ol
(Aldrich), heptan-1-ol (Alfa Aesar), N,N-diethylethanamine (tri-
ethylamine, TEA, Aldrich) and 1,8-diazabicyclo [5.4.0]undec-7-
ene (DBU, Aldrich) were dried over CaH2 at room temperature
and puried by distillation under reduced pressure just before
use. Toluene (Chem-lab), tetrahydrofuran (THF, Chem-lab) and
dichloromethane (CH2Cl2, Chem-lab) were dried on molecular
sieves. Monomethoxy-PEO (MeO-PEO-OH, Mn ¼ 5000 g molꢁ1
,
Aldrich) was dried by three azeotropic distillations with anhy-
drous toluene before use. Milli-Q water was used for the micelle's
preparation. 1-1-[3,5-Bis(triuoromethyl)phenyl]-3-cyclohexyl-2-
thiourea (TU), 2-(2-methylpropoxy)-1,3,2-dioxaphospholane 2-
oxide (iBP) and 2-butoxy-1,3,2-dioxaphospholane 2-oxide (nBP)
monomers were synthesized as described in the literature
elsewhere.21,31,37
For PEO-b-PnBP. The 1H NMR spectrum is in good agreement
with the reported spectrum by Yang et al.31 1H-decoupled 31P
NMR (101 mHz, CDCl3, 25 ꢀC): ꢁ0.60 ppm. Mw/Mn (SEC) ¼ 1.10.
For PEO-b-PiBP. 1H NMR (250 MHz, CDCl3, 25 ꢀC, TMS): 0.95
ppm (d, 6H, –CH–(CH3)2), 1.95 ppm (m, 1H, –CH–(CH3)2), 3.65
ppm (s, 4H, –C–O–CH2–CH2–O–C–), 3.85 ppm (m, 2H, –O–CH2–
CH–(CH3)2), 4.3 ppm (m, 4H, –P–O–CH2–CH2–O–P–). 1H-
decoupled 31P NMR (101 mHz, CDCl3, 25 ꢀC): ꢁ0.60 ppm.
Mw/Mn (SEC) ¼ 1.10.
Structural characterization techniques
1H and 31P NMR analyses were performed on a Bruker Advance
ꢀ
250 MHz spectrometer in CDCl3 at 25 C in the FT mode. The
31P NMR measurement was carried out with the H powergate
1
For PEO-b-PnHP. 1H NMR (250 MHz, CDCl3, 25 ꢀC, TMS): 0.9
ppm (t, 3H, –CH2–CH3), 1.3 ppm (m, 8H, –O–CH2–CH2–(CH2)4–
CH3), 1.7 ppm (m, 2H, –O–CH2–CH2–(CH2)4–CH3), 3.65 ppm (s,
4H, –C–O–CH2–CH2–O–C–), 4.1 ppm (m, 2H, –O–CH2–CH2–
(CH2)4–CH3), 4.25 ppm (m, 4H, –P–O–CH2–CH2–O–P–). 1H-
decoupled 31P NMR (101 mHz, CDCl3, 25 ꢀC): ꢁ0.60 ppm.
Mw/Mn (SEC) ¼ 1.10.
decoupling method using a 30ꢀ ip angle. 1H NMR chemical
shis are reported in ppm relative to Me4Si. 31P NMR chemical
shis are reported in ppm relative to H3PO4. Size exclusion
chromatography (SEC) was carried out in THF at 45 ꢀC at a ow
rate of 0.7 mL minꢁ1 with Viscotek 305 TDA liquid chromato-
4
3
˚
˚
˚
graph. The PL gel 5 mm (10 A, 10 A and 100 A) columns were
calibrated with PEO standards.
Synthetic procedures
Characterization of the copolymer in water
Monomer synthesis. 2-Heptyloxy-1,3,2-dioxaphospholane
Hydrolytic stability. The hydrolytic stability of the copoly-
2-oxide (nHP) was synthesized by the esterication reaction of mers was monitored in Milli-Q water at room temperature over
COP with 1-heptanol in the presence of TEA. Typically, COP (100 a period of 1, 7, 14 and 28 days. 100 mg of copolymer were
g, 0.702 mol) in a THF solution (200 mL) was added dropwise, dissolved in 35 mL of Milli-Q water. The concentration of the
under stirring at 0 C, to a solution of the anhydrous alcohol polymer was set at 3 mg mLꢁ1. At the predetermined times, 3
ꢀ
and TEA in dry THF (200 mL) (COP/TEA/alcohol ¼ 1/1/1). Aer mL of the solution were collected, dried by lyophilisation before
the complete addition, the solution was stirred at 0 ꢀC for being analysed by SEC in THF.
4 hours and the resulting mixture was ltrated to remove the
Dynamic light scattering (DLS). Typically, 25 mg or 50 mg of
alkyl ammonium salt before concentrating the ltrate under the PEO-b-polyphosphate copolymer were directly dissolved in
vacuum. Finally, the monomer was puried by fractional 20 mL deionized water and stirred overnight. Particle size and
distillation under vacuum (10ꢁ4 Torr, Tb ¼ 105 ꢀC, yield ¼ 53%). size distribution were acquired from freshly prepared copoly-
1H NMR (250 MHz, CDCl3, 25 ꢀC, TMS): 0.9 ppm (t, 3H, mer solutions by dynamic light scattering (DLS). DLS
–CH2–CH3), 1.3 ppm (t, 8H, –O–CH2–CH2–(CH2)4–CH3), 1.7 ppm measurements were performed using a Beckman Coulter Delsa
2
(m, 2H, –O–CH2–CH2–(CH2)4–CH3), 4.1 ppm (dt, JHH ¼ 6.6 Hz Nano C Particle Analyzer and the data were treated by the Delsa
and 3JPH ¼ 8.6 Hz, 2H, –O–CH2–CH2–(CH2)4–CH3), 4.4 ppm (m, Nano UI 2.21 soware. The average size distribution of aqueous
4H, –O–CH2–CH2–O–). 1H-decoupled 31P NMR (CDCl3, micellar solutions was determined based on the CONTIN
101 MHz): 18.1 ppm.
method. All the measurements were carried out in a glass cell at
Polymerization from a PEO macro-initiator. The polymeri- 25 ꢀC at a measuring angle of 165ꢀ. Aqueous micellar solutions
zation of nBP, iBP or nHP was performed in CH2Cl2 at 0 ꢀC with were ltered with a microlter having an average pore size of
MeO-PEO-OH as a macro-initiator, DBU as a catalyst and TU 1.2 mm. All DLS measurements were repeated ve times in order
as a co-catalyst with a ratio [PEO-OH]0/[monomer]0/[DBU]0/ to check their reproducibility.
This journal is © The Royal Society of Chemistry 2015
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Acknowledgements
This study was performed within the Interreg Euregio Meuse-
Rhin IV-A consortium BioMIMedics (2011–2014). The Univer-
sity of Liege (Belgium), Maastricht University (The Nether-
lands), University of Hasselt (Belgium), RWTH Aachen
(Germany), and Fachhochschule Aachen (Germany) together
with several local small biotechnological enterprises cooperate
in BioMIMedics. This particular study was nanced through
generous contributions of the European Union (through Inter-
reg IV-A), the government of Wallonia (Belgium), and the
“Provincie Nederlands Limburg” (The Netherlands). CERM is
much indebted to IAP VII-05 “Functional Supramolecular
Systems” (FS). P.L. is Research Associate by the FRS-FNRS.
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RSC Adv., 2015, 5, 27330–27337 | 27337