Selective Synthesis of Polyoxyethylene–Polyoxypropylene Block Copolymer (Poloxamer) Fatty Acid Monoesters Over Homogeneous Organotin Catalyst
Selective Synthesis of Polyoxyethylene-Polyoxypropylene Block Copolymer (Poloxamer) Fatty Acid Monoesters Over Homogeneous Organotin Catalyst
Janusz Nowicki 0 1 2
Julia Woch 0 1 2
Małgorzata Mos´cipan 0 1 2
Andrzej Robaszkiewicz 0 1 2
Rafał Grabowski 0 1 2
Izabela Semeniuk 0 1 2
Karol Erfurt 0 1 2
Monoesters Fatty 0 1 2
0 Faculty of Chemistry, Silesian University of Technology , Gliwice , Poland
1 Institute of Heavy Organic Synthesis ''Blachownia'' , Ke ̨dzierzyn-Koz ́le , Poland
2 & Janusz Nowicki
The synthesis of selected polyoxyethylenepolyoxypropylene block copolymer (poloxamer) fatty acid monoesters is presented. Organotin homogeneous catalyst Sn bis(2-ethylhexanoate) effectively catalyzed the esterification reaction of (EO)-(PO)-(EO) block copolymer (poloxamer) with fatty acids. The reaction proceeded in high yield and high selectivity to monoesters. Content of diesters in final products was below 1 wt%. The new protocol opened up a high yield and high selective method for the synthesis of poloxamer fatty acid monoesters. These products are potentially interesting for industrial applications, e.g. in lubricants, cosmetics and, in particular, as potential emulsifying agents compatible with hydrocarbon bases, such as paraffin.
Poloxamer; Esterification acid; Organotin catalyst
Polyoxyalkylenene glycol esters are a group of nonionic
surfactants with a vast field of application.
Polyoxyethylene and polyoxypropylene glycol fatty acid esters,
prepared by esterification of fatty acids with corresponding
polyoxyalkylene glycols, are classified as non-toxic and
non-irritating nonionic emulsifiers. Depending on their
molecular weight and water solubility, esters can be
applied in both aqueous and non-aqueous systems.
Polyoxyalkylene esters of fatty acids are widely used as
emulsifiers in cosmetics and textiles [
]. Fatty acid
polyol esters have also been recently used for removal of
soil (or water) contamination caused by spilled oil. Herein,
their relatively low toxicity for marine fauna plays a
significant role [
]. Important representatives of discussed
group—laurates, stearates and oleates, primarily used as
emulsifiers in cosmetics and pharmaceutics, are also
applied as components for drilling and cutting oils [
Application as lubricants and filament cohesion agents for
fiber finishing is less relevant, because of their limited
solubility in water.
Polyol esters are produced by chemically catalyzed
]. Fatty acid esters of polyoxyethylene
glycols, in general, are prepared either via oxyethylation or
esterification route . For increased selectivity of
monoesters, a large excess of polyoxyethylene glycol to
fatty acid, even 6–12 molar ratio, is recommended [
After the reaction, excess of polyoxyethylene glycol is
washed out with concentrated salt solution [
original method of the selective synthesis of monoesters
using boric acid was reported by Hartman, but this method
is of little preparative significance [
]. Many of the
esterification reactions are carried out at elevated
temperatures in presence of homogeneous acidic, amphoteric or
basic catalysts [
]. Sodium or potassium hydroxide or
ptoluenesulphonic acid are the most preferred catalysts. In
order to favorably shift the equilibrium, esterification water
is removed by applying vacuum or using a flow (stream) of
Fatty acid esters of polyoxyethylene glycols can be also
produced from fats or plant oils in transesterification
processes in presence of strong basic catalysts. The reaction
product in general is a mixture of mono-, di- and
triglycerides, glycerol, unreacted polyoxyethylene glycol and also
a mixture of mono- and diesters [
]. Because of the
very complex character of the product this method is not
Diesters of polyalkylene glycols can be selectively
synthesized with high yields by using a high molar excess of
fatty acid. Synthesis of monoesters, which are characterized
by more interesting surface active properties (e.g. as
emulsifier for textile industry) [
], is possible, but requires the
use of high molar excess of glycol [
]. Equal activity of both
hydroxyl groups in polyoxyethylene glycols leads to
esterification of both of hydroxyl groups. As reported by Corma
et al. better miscibility of fatty acids with the
polyoxyalkylene glycol monoesters compared to glycol
promotes the reaction of fatty acids with hydroxyl group of
monoesters, rather than with polyoxyethylene glycol, which
is immiscible with fatty acids. In addition to that,
disproportionation of monoester formed in the first step by
transesterification also leads to increased amounts of diester [
Fatty acid monoesters of poly(oxyalkylene) glycols can
be also obtained in oxyalkylation route. Direct
oxyalkylation of fatty acids with conventional basic catalysts yields a
complex mixture of mono- and diesters, as well as various
polyethylene glycols as by-products, with a wide range of
polyethylene glycol units. The final product can be used as
emulsifiers in food, cosmetics and other technical
In recent years, enzyme-catalyzed high yields synthesis
of fatty acid esters and fatty acids and polyoxyalkylene
glycols through esterification and interesterification
processes have attracted particular attention. The catalytic
activity of lipases toward hydroxy fatty acid esters was
well studied by Hayes [
]. In the esterification of
several hydroxyl fatty acids Rhizomucor miehei lipase was
also adopted [
]. Steffen et al. used R. miehei and
Candida antarctica lipases as biocatalyst for the synthesis
of monoglycerides of 17-hydroxy- and 12-hydroxystearic
acids with high yield [
]. A promising alternative for this
processes could be the use of homogeneous organometallic
Lewis compounds as catalysts, which are used in many
other commercial processes, e.g. synthesis of polyesters.
Our previous experiments on the esterification reaction of
higher fatty acids and polyols conducted over various Ti,
Zr and Sn catalysts have shown that homogeneous Sn
bis(2-ethylhexanoate) is characterized by the highest
activity and selectivity to the desired products [
In this paper, the novel high selective method of the
synthesis of monoesters of selected hydrophobic
block copolymers (poloxamer) is presented. In the
literature, there is practically no detailed information on the
synthesis of poloxamer fatty acid esters. Only in the FDA
report can find information, that poloxamer fatty acid esters
are safe and can be used to manufacturing materials, that
come in contact with food [
]. A homogeneous Sn
bis(2ethylhexanoate) catalyst was used. The described method
under optimal reaction conditions results in the desired
monoesters with high selectivity, above 99%.
Oleic acid 90?% was purchased from Croda Int. as
Priolene 6936. Stearic acid 99?%, lauric acid 99?% and
octanoic acid 99?% were purchased from Sigma Aldrich.
block copolymer (poloxamer) was purchased from BASF
as Pluronic PE3100 (mol mass = 1000; molar mass of
propylene glycol block = 850; polyethylene glycol
content = 10 wt%; hydroxyl value = 105 mg KOH/g).
Commercially available homogeneous Sn
bis(2-ethylhexanoate) purchased from PMC Organometallic as Fascat
2003 was used as catalyst. Cyclohexane and potassium
phosphate analytical grade (Pure) were purchased from
Gel permeation chromatography (GPC) was carried out
on a L-7100 series pump (Merck-Hitachi) equipped with
degasser (Knauer) and three column series from Polymer
Laboratories, Inc. (Amherst, MA, USA) consisting of PLgel
3 lm Guard (7.5 9 50 mm), PLgel 5 lm MiniMIX-E
(7.5 9 250 mm, molecular weight range 500–30,000 g/mol)
and PLgel 5 lm MiniMIX-E (7.5 9 250 mm, molecular
weight range 500–30,000 g/mol) columns. The system was
fitted with a VISCOTEK VE 3580 differential refractometer
detector and anhydrous tetrahydrofuran was used as the
mobile phase (0.3 mL min flow rate). Data were collected
and processed using GRAMS/386 for Chromatography
software and calibrated against polystyrene standards.
Analyses were performed at 30 C.
1H-NMR spectra were recorded on Varian Unity Inova
Plus spectrometer (CDCl4, 400 MHz) FT-IR spectra were
recorded on MATTSON 3000 spectrometer (Unicam) at
500–4000 cm-1. Kinematic viscosity was determined on
Brookfield DV-II? viscometer. Density of esters at 25 C
was determined using 1 cm3 micro-pycnometer.
Hydrophile-lipophile balance values (HLB) of polyol esters were
calculated according to Davies method [
Synthesis of Polyol Monoesters (General Procedure)
In a 200 mL glass reactor, equipped with mechanical
stirrer, an electronic temperature controller, glass capillary
and a receiver to collect esterification water were placed
180 g of Pluronic PE3100 and the required amount of fatty
acid (according to desired COOH:OH molar ratio
calculated on the basis of hydroxyl value). The reactor was
heated up to 150 C and then a proper amount of catalyst
was added into the reactor. Temperature was raised to
220 C and the reaction mixture was then stirred for 9 h.
Nitrogen introduced into the reactor (glass capillary) helps
to remove the water and also provides a protective
atmosphere. Esterification was considered complete when the
acid number of the reaction mixture was below 1 mg KOH/
g. Crude esters were then diluted with cyclohexane in order
to decrease the viscosity and density. This solution was
washed at first by 5 wt% of K3PO4 water solution and
twice with deionized water to reach a neutral pH. Solvent
was removed under reduced pressure on a rotary
Results and Discussion
It is well known that the esterification reaction is strongly
influenced by the efficiency of water removal. In the case
of low molecular weight alcohols (methanol, ethanol),
which do not form heteroazeotropic mixtures with water,
the process is difficult. In such type of alcohols the
satisfactory results are achieved by using a large excess of
alcohol (e.g. direct esterification of fatty acid with an
excess of methanol). A more convenient situation is
observed in the case of higher aliphatic alcohols ([ C4).
They form heteroazeotropic mixtures with water, which
facilitates the removal of the esterification water.
Unfortunately, for higher aliphatic alcohols ([ C10) and low
volatility polyols the above method is not practical. In
these cases water can be removed with the use of neutral
solvents (e.g. toluene) or by running the esterification
reaction at elevated temperatures. This method is applied in
the esterification of polyols, also polyoxyalkylene glycols,
with fatty acids. However, because of relatively high
melting points of polyols and their limited miscibility with
fatty acids, an esterification temperature above 150 C is
To obtain an efficient emulsifying agent, a higher
content of monoester in the reaction product is preferred
(higher hydrophilicity and higher HLB value). The choice
of catalyst and reaction conditions are crucial for high
selectivity of monoesters. Monoesters of polyxyalkylene
glycols are characterized by better emulsification potential,
especially in terms of w/o emulsions. EO/PO/EO block
copolymers are a mixture of oligomers with various
molecular masses characterized by different values of
The aim of this study was too selectively obtain fatty
acid monoesters of EO–PO–EO block copolymer (Pluronic
PE3100). To find the optimal COOH:OH molar ratio, a
series of esterification reactions with different molar ratios
were performed. As a control parameter, acid value of the
crude ester was adopted. The catalyst (tin octanoate) did
not affect the overall acid number in post-synthesis
mixtures, so the acid value of the esterification product
catalyzed by metalorganic Lewis catalysts depended mainly
on the free fatty acid content.
Experiments were conducted using fatty acid:OH ratio
of 1:0.45–1:0.55 in the presence of Sn
bis(2-ethylhexanoate) as a catalyst at 220 C. The esterification reaction
in the above adopted reaction conditions resulted mainly in
monoesters according to the scheme presented in Fig. 1.
Similarly to esterification of polyols, the molar ratio of
fatty acid and OH group is the key factor affecting the
composition of post-synthesis mixtures. In EO–PO–EO
block copolymers, the terminal groups are primary
hydroxyl groups, so this type of polyols in esterification
reaction react similarly to most aliphatic a,x-glycols.
However, polyoxyalkylene glycols are a mixture of glycols
with various oxyalkylene units and, in consequence, of
various molar mass (Gauss rule).
The amount of added catalyst is considered as one of the
important parameter in the esterification reaction. The
results of preliminary studies of the esterification reaction
of oleic acid and Pluronic PE3100 in relation to the amount
of added Sn catalyst are presented in Table 1. The amount
of added catalyst had noticeable less importance on final
acid values of crude reaction product. Acid value of crude
esters obtained for the amount of catalyst at the level
0.7–0.9 wt% was very similar, so in the following
experiments the catalyst was added at 0.7 wt% only.
In Table 2 are presented the results of esterification
reaction of oleic acid and Pluronic PE3100 in relation to
COOH:OH molar ratio. Results presented in Table 2 shows
that product containing the higher amount of monoester
was obtained for a molar ratio below the theoretical value
(0:0.5). For the molar ratio COOH:OH = 0:0.45 the
product contained almost entirely monoester. Assuming the
previously established optimum reaction parameters the
esterification of Pluronic PE3100 with the series of fatty
acids was then conducted. The results of these experiments
are presented in Table 3.
Results presented in Table 3 shows that both the acid
values and the mono/diester composition for fatty acids
used are similar to oleic acid monoester. The acid value of
crude products was 0.5–0.6 mg KOH/g and the content of
diesters was below 1 wt%. Results presented in Table 3
clearly shows that the use of organotin catalyst in the
synthesis of EO–PO–EO block copolymers allows for
obtaining monoesters with high selectivity, above 99%.
These results, especially the selectivity towards monoesters
in the presented case, were significantly higher than results
described in the literature [
]. In the cited study
polyoxyethylene glycol was used as polyol. EO–PO–EO
glycols contain terminal hydroxyethyl groups, so they can be
considered as polyoxythylene glycols analogs.
The main analytical method widely adopted for
determination of oxyalkylene fatty acid ester is gel permeation
chromatography (GPC), also used in this study [
results of the esterification of Pluronic PE3100 with oleic
acid is presented in Fig. 2 (for GPC analysis of all esters
see Supporting Information). As demonstrated in Fig. 2,
the crude post-reaction mixture contains only one type of
compounds (mixture of glycol monoesters). In order to
confirm the contents of reaction products, the reaction
mixture was compared with the raw materials used in the
synthesis (polyol, fatty acids). Figure 2 clearly shows that
the product of esterification consists mainly of monoesters.
Small amount of diesters were detected in the case of oleic
acid esterification. The lack of free oleic acid confirms high
conversion rate of the fatty acid.
Crude esters were purified in order to remove the
catalyst and to decrease the amount of free fatty acids in the
product. Among various washing methods, the one
incorporating a dilute solution of K3PO4 was adopted. The
amount of basic phosphate was a three-fold molar excess
relative to the acid values of the crude esters. This method
was considered as the best for purification of crude fatty
acid esters of various polyols [
Chemical structures of obtained esters were confirmed
by instrumental analysis. The 1H-NMR spectra recorded
for EO–PO–EO glycol were typical for analogous fatty
acid esters. In 1H-NMR spectra recorded for Pluronic
PE3100 fatty acid monoesters several characteristic signals
(ppm): 0.88 (terminal –CH3); 2.30 (ester –CH2CO–); 4.22
(ester –CO–OCH2–) and additional 5.34 (–CH–CH–) for
oleic acid ester can be found (see Supporting Information).
1H-NMR spectrum of polyoxyalkylene glycol oleic acid
ester is very similar to another oleic acid esters [
In the FT-IR spectra recorded for Pluronic PE3100 fatty
acid monoesters, several characteristic bonds can be found.
The strong intensity OH stretching vibrations in the region
Reaction conditions: temp.—220 C; reaction time—9 h; COOH:OH molar ratio—0:0.45; catalyst
Acid value, mg KOH/g
of 3485–3475 cm-1 corresponding to terminal OH groups
confirm the presence of glycol monoesters. The strong
intensity bond at 1736 cm-1 corresponds to ester carbonyl
group. The strong intensity bond at the region
1112–1109 cm-1 corresponds to polyol ether groups. In all
FTIR spectra two characteristic bonds at 1015 and
929 cm-1 are observed that corresponds to oxypropylene
segment (see Supporting Information).
Density of purified esters was at the level of
0.878–0.882 g/cm3. Kinematic viscosity of Pluronic
PE3100 monoesters depended on the fatty acid used.
Viscosity values, measured at 25 C, ranged from 100 to
152 mPa s. Physicochemical data of Pluronic PE3100 fatty
acid monoesters are presented in Table 4.
HLB values of synthesized monoesters were determined
according to the modified additive Davies method. For
surfactant containing CH2, EO, PO can be written as:
HLB ¼ 7 þ NðCH2Þ nCH2 þ NðEOÞ nðEOÞ þ NðPOÞ
nðPOÞ þ R other hydrophilic group
þ R other hydrophobic groups,
where N(CH2), N(EO), and N(PO) are the number of
groups CH2, EO and PO. n(CH2), n(EO) and n(PO)
represent the chain lengths for CH2, EO and PO groups.
Beside of CH2, EO and PO groups, Pluronic monoesters
contains also ester C(O)O and OH group as other
hydrophilic group and terminal CH3 as other hydrophobic
groups. Above equation for synthesized esters can be
HLB ¼ 7 þ NðCH2Þ
nðCH2Þ þ NðEOÞ nðEOÞ
nðPOÞ þ NðCOOÞ þ NðOHÞ
N values have been adopted from widely published data
as follow: N(EO) = 0.33; N(ester) = 2.4; N(OH) = 1.9;
N(PO) = -0.15; N(CHx) = -0.475 [
]. The results
of HLB calculations were collected in Table 4. According
to widely adopted applications for nonionic surfactants
within the HLB ranges, octanoic and lauric acid monoester
of Pluronic PE3100 can be classified as W/O emulsifier and
oleic and stearic acid monoester as defoamers .
The results described in this study show that homogeneous
organotin catalyst is characterized by high activity and
selectivity in the synthesis of fatty acid monoesters of
Pluronic PE3100 EO–PO–EO block copolymer. Sn
bis(2ethylhexanoate) proved to be useful in the synthesis of
oxyalkylene glycols with longer fatty acid chains that are
potentially suitable as W/O emulsifier and defoamers. In
optimal reaction conditions polyoxyalkylene glycol fatty
acid esters with high selectivity to monoesters (99%) were
obtained. Simple synthesis procedure, relatively mild
reaction conditions, commercial availability of catalyst and
also no need for difficult and expensive purification stages
enable, in our opinion, application of this method in
Acknowledgements The authors wish to thank the National Centre
for Research and Development (Poland) for providing the financial
support for the realization of these studies. This work has been done
within the BIOSTRATEG II (ID 298537/16) project ‘‘New types of
food packages comprising renewable raw materials and innovative
Open Access This article is distributed under the terms of the
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Janusz Nowicki assistant professor in the Institute of Heavy Organic
Synthesis ‘‘Blachownia’’, Ke˛dzierzyn-Koz´le, graduated from
Wrocław University of Technology, Wrocław, Poland, in 1978. He
completed his Ph.D. in chemistry from Ło´dz´ University of
Technology, Ło´dz´, and D.Sc. degree in chemical technology from Wrocław
University of Technology, Wrocław, Poland. Currently, he works as a
senior researcher in the Renewable Raw Materials Department. He
has co-authored 70 original papers. The main areas of his scientific
interest are chemistry of renewable raw materials, modern
heterogeneous catalysis, and synthesis and application of ionic liquid. He is a
member of the Polish Chemical Society.
Julia Woch graduated from Silesian University of Technology,
Faculty of Chemistry, in 2010. She is a researcher in the Institute of
Heavy Organic Synthesis ‘‘Blachownia’’ in Ke˛dzierzyn-Koz´le,
Poland, and a Ph.D. student in the Opole University. The field of
her studies covers aggregation of surfactants in aqueous solutions and
chemistry of industrial colloids.
Małgorzata Mos´cipan graduated from the Opole University in 2009
(Faculty of Chemistry). She received her Ph.D. degree in analytical
chemistry in 2013. She works in the Analytical Department of the
Institute of Heavy Organic Synthesis ‘‘Blachownia’’. Her research
work is concentrated on developing analytical methods for the
determination of the analyte in various types of samples.
Andrzej Robaszkiewicz graduated from the University of Science
and Technology in Bydgoszcz, Poland, in 1998. He completed his
Ph.D. in chemistry from the Gdan´sk University of Technology,
Gdan´sk, Poland, in 2004. He works in the Institute of Heavy Organic
Synthesis ‘‘Blachownia’’ in Ke˛dzierzyn-Koz´le, Poland, as a research
assistant. His work focuses on organic synthesis and chemical
Rafał Grabowski graduated from the Silesian University of
Technology, Faculty of Chemistry, in 2013. He works in the Institute of
Heavy Organic Synthesis ‘‘Blachownia’’ in Ke˛dzierzyn-Koz´le,
Poland, as a research technical specialist. His work focuses on
organic synthesis and chemical technology.
Izabela Semeniuk graduated from the Silesian University,
Departament of Biochemistry, Katowice, in 1987. She specializes in
Karol Erfurt graduated from the Silesian University of Technology,
Faculty of Chemistry, in 2011. After graduation, he remained at the
SUT as a technical worker. In his work, he deals with the synthesis of
ionic liquids based on natural compounds.
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