Synthesis of lactide from lactic acid and its esters in the presence of rare-earth compounds

Article abstract of DOI:10.1134/S1070428017030058

A procedure is described for the synthesis of lactide by dehydration of L-lactic acid and subsequent depolymerization of its oligomer mixture in the presence of yttrium(III) and praseodymium(III) oxides, as well as of cerium(III) chloride heptahydrate. The catalytic activity of yttrium and praseodymium sesquioxides was determined at different temperatures at the oligomerization and deoligomerization stages. Ethyl lactate was prepared in the presence of Purolite C100 MB cation exchange resin and subjected to oligomerization followed by thermal decomposition of oligoester and oligolactic acid mixture in the presence of yttrium(III) and praseodymium(III) oxides and aqueous cerium(III) chloride.

Full text of DOI:10.1134/S1070428017030058

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2017, Vol. 53, No. 3, pp. 344–350. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © E.A. Poryvaeva, T.A. Egiazaryan, V.M. Makarov, M.V. Moskalev, D.A. Razborov, I.L. Fedyushkin, 2017, published in Zhurnal
Organicheskoi Khimii, 2017, Vol. 53, No. 3, pp. 346–352.
Synthesis of Lactide from Lactic Acid and Its Esters
in the Presence of Rare-Earth Compounds
E. A. Poryvaevaa, T. A. Egiazaryana, V. M. Makarova, M. V. Moskaleva, b,
D. A. Razborova, b, and I. L. Fedyushkina, b*
a Koz’ma Minin Nizhny Novgorod State Pedagogical University,
ul. Ul’yanova 1, Nizhny Novgorod, 603002 Russia
b Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences,
ul. Tropinina 49, Nizhny Novgorod, 603137 Russia
*e-mail: igorfed@iomc.ras.ru
Received December 6, 2016
Abstract—A procedure is described for the synthesis of lactide by dehydration of L-lactic acid and subsequent
depolymerization of its oligomer mixture in the presence of yttrium(III) and praseodymium(III) oxides, as well
as of cerium(III) chloride heptahydrate. The catalytic activity of yttrium and praseodymium sesquioxides was
determined at different temperatures at the oligomerization and deoligomerization stages. Ethyl lactate was
prepared in the presence of Purolite C100 MB cation exchange resin and subjected to oligomerization followed
by thermal decomposition of oligoester and oligolactic acid mixture in the presence of yttrium(III) and
praseodymium(III) oxides and aqueous cerium(III) chloride.
DOI: 10.1134/S1070428017030058
Polymeric materials obtained from renewable plant
sources are valuable due to not only their capability for
biodegradation in the environment but also bio-
compatibility, which opens prospects of using such
polymers in medicine. An example of such materials is
polylactic acid often called polylactide. As early as
1932, Carothers et al. [1] showed the possibility of
synthesizing polylactide with a molecular weight of
~3 kDa from lactic acid (2-hydroxypropanoic acid).
However, it is necessary to achieve high conversion of
lactic acid to obtain polylactide with a molecular
weight of several tens kilodaltons [2]. The resulting
polymer is characterized by a high polydispersity
index, and materials based thereon have unsatisfactory
physicochemical characteristics. At present, preference
is given to the catalytic synthesis of polylactic acid by
ring-opening polymerization of its dimer, lactide
(3,6-dimethyl-1,4-dioxane-2,5-dione) [3–8]. Magne-
sium derivatives of 1,2-bis(2,6-diisopropylphenyl)-
iminoacenaphthene (dpp-bian) [9, 10] were shown to
efficiently catalyze polymerization of lactide with
formation of polylactic acid with a high molecular
weight (57–78 kDa) and low polydispersity index
(1.38–1.46).
Removal of water from a 80% solution of lactic
acid leads to its oligomerization which is accompanied
by formation of a small amount of lactide. The known
methods for the synthesis of lactide generally include
preparation of a mixture of lactic acid oligomers
[11–13] or a mixture of oligoesters (from lactic acid
esters) [14, 15] and subsequent catalytic or thermal de-
polymerization with formation of lactide (Scheme 1).
A high yield of lactide was achieved with the use of
tin [11, 13–15], antimony [12], and lead compounds
[15]. However, the presence of even small amounts of
these heavy metals in polymeric materials excludes
their further use in medicine or food industry. There-
fore, search for low-toxic catalysts ensuring synthesis
of lactide in high yield from lactic acid or its esters
is an important problem. Herein we report the effect of
yttrium(III) and praseodymium(III) oxides and
cerium(III) chloride on the yield of lactide from lactic
acid, ethyl lactate, and oligomers of the latter. We
believe that our results may be useful for the design of
catalytic systems for the synthesis of lactide of food
and medicinal purity.
Initially, we examined the possibility of using
yttrium(III) and praseodymium(III) oxides and ceri-
344

SYNTHESIS OF LACTIDE FROM LACTIC ACID AND ITS ESTERS
345
Scheme 1.
Me
Me
O
130–180°C, catalyst
ROH
OR
O
HO
HO
OR
O
O
Me
n
Me
Me
Lactic ester
Oligoester mixture
180–250°C
catalyst
O
O
ROH
–H2O
O
O
Me
O
Me
Me
Lactide
130–180°C, catalyst
H2O
O
OH
OH
HO
O
HO
O
Me
OH
O
n
Lactic acid
Lactic acid oligomer mixture
um(III) chloride to catalyze oligomerization of lactic
acid with subsequent depolymerization of the resulting
oligomers to lactide. In the first stage, a solution of
lactic acid was subjected to dehydration in the pres-
ence of the above catalysts at different temperatures
(Tolig; Table 1). The efficiency of the process was
estimated by the degree of oligomerization (DPn) of
nitrogen. Two fractions were collected: fraction 1 was
a mixture of lactic acid and lactide which crystallized
in the reflux condenser, and fraction 2 is the residue
remained in the reactor (oligomeric and polymeric
compounds). The weight fractions ω1 and ω2 were
calculated relative to the initial weight of the oligomer.
The yield of lactide (fraction 1) was determined by
1H NMR.
1
lactic acid on the basis of the H NMR data. The
degree of oligomerization was calculated as the ratio of
the overall intensity of CH signals of lactic acid and
oligomers to the overall intensity of CH signals of
lactic acid and CHOH signals of oligomers [16]. The
results were compared with those obtained in blank
experiments carried out without a catalyst. As follows
from the data in Table 1, the DPn value increases as the
oligomerization temperature rises. However, catalytic
effect was observed only in the presence of praseo-
dymium(III) oxide and cerium(III) chloride at 180°C;
the degree of oligomerization reached a value of 4.7 or
4.2, respectively (run nos. 9, 10). Both these catalysts
are soluble in aqueous lactic acid solution. Praseo-
dymium(III) oxide is likely to be converted to the cor-
responding lactate which catalyzes oligomerization of
lactic acid. Cerium(III) chloride gives rise to solvates
with variable compositions, which are less active than
praseodymium lactate. Yttrium(III) oxide is insoluble
in aqueous lactic acid (run nos. 4–6), and it does not
catalyze oligomerization of lactic acid.
The data in Table 1 show that in each series of
experiments performed at Tdep 180°C the yield of
lactide increases in parallel with the DPn value for the
initial oligomer. An exception was the blank run where
thermal degradation of oligomer (DPn 1.4) afforded
20% of lactide (run no. 1). Presumably, in this case the
rate of dehydration of lactic acid to lactide and
removal of the latter from the reaction zone together
with lactic acid and water vapor exceeds the rate of
polymerization. This follows from the low yield of
fraction 2 (10%) and the absence of polymer fraction.
Depolymerization of oligomers with close DPn
values (1.4 and 1.5) in the presence of yttrium(III) and
praseodymium(III) oxides, respectively (run nos. 4, 7),
gave a lower yield of lactide (7% in the presence of
Y2O3 and 13% in the presence of Pr2O3). Here, the
contribution of polymerization of lactic acid oligomers
increases; this is reflected in increased yield of fraction
2 (25 and 33% for Y2O3 and Pr2O3, respectively) which
contains a small amount of polylactide (≤4 wt % rela-
tive to the initial oligomers) with an average molecular
weight Mw of ~2 kDa (run nos. 2, 5, 8). Yttrium(III)
oxide showed no catalytic activity at Tdep = 180°C, as
followed from comparison of the yields of lactide in
run nos. 2 and 5, as well as from the weights of
fractions 1 and 2. The yield of lactide in the presence
of Pr2O3 was higher by 8%, and of fraction 2, lower by
Oligomer samples obtained as described above
were subjected to thermal depolymerization at 180 and
200°C (Tdep) without purification from rare earth
compounds (Table 1). Thermal degradation of oligo-
mers was carried out under reduced pressure
(0.05 mm) in a setup consisting of a reactor, air-cooled
reflux condenser, and receiver cooled with liquid
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 53 No. 3 2017

PORYVAEVA et al.
346
Table 1. Oligomerization of lactic acid and thermal decomposition of lactic acid oligomers in the presence of rare earth
catalysts
Run no.a
Catalystb
Tolig, °C
DPn
Tdep, °C
ω1, %
ω2, %
Yield of lactide, %
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
–
–
–
130
150
180
130
150
180
130
150
180
180
180
130
150
180
180
1.4
2.0
3.2
1.4
1.9
3.2
1.5
2.1
4.7
4.2
3.2
1.5
2.1
4.7
4.2
34
30
29
13
30
30
44
31
28
31
60
70
73
80
47
10
59
64
25
61
64
33
46
68
58
33
21
17
16
15
20
14
19
7
180
Y2O3
Y2O3
180
180
15
19
13
22
27
25
24
08
19
52
46
Y2O3
Pr2O3
Pr2O3
Pr2O3
CeCl3 ·7H2O
–
180
200
Pr2O3
Pr2O3
200
Pr2O3
CeCl3 ·7H2O
200
a
In run nos. 11–15 oligomers obtained in the first stage in run nos. 3 and 7–10, respectively, were subjected to depolymerization.
Amount of the catalyst: 0.8 mol % of Y2O3 or Pr2O3 or 1.6 mol % of CeCl3·7H2O (relative to lactic acid).
b
13% than in blank experiment. Presumably, Pr2O3
exhibits higher depolymerizing activity. Fractions 2 of
all three samples contained polylactide (Mw ~3 kDa).
DPn = 1.5 and 2.1 led to reduced yields compared to
experiments at Tdep = 180°C (run nos. 7, 8 and 12, 13,
respectively), despite lower amount of fraction 2.
Presumably, removal of lactic acid from the reactor
predominates over its dehydration due to high rate of
removal of low molecular weight components from the
reactor at 200°C. However, the yield of lactide in the
depolymerization of oligomers with DPn = 4.7 and 4.2
at Tdep = 200°C was higher than at Tdep = 180°C by 25
and 22% for Pr2O3 and CeCl3, respectively (run nos. 9,
14, 10, 15). Thermal decomposition of the oligomer
with DPn = 3.2 containing no rare earth compounds
showed an insignificant increase of the yield (by 5%;
run nos. 3, 11). In this case, the residue in the reactor
contained polylactide with Mw ≈ 13 kDa, whereas the
average molecular weights of the polymeric residues in
run nos. 14 and 15 were only 2.4 and 2.8 kDa, respec-
tively. These data indicate catalytic character of the
depolymerization of oligomers synthesized in the
presence of praseodymium(III) oxide and cerium(III)
chloride.
The results of depolymerization of oligomer
samples with DPn = 3.2 at Tdep = 180°C (run nos. 3, 6)
showed that yttrium(III) oxide does not affect the
thermal degradation; in both cases, the yield of lactide
increases on the average by 5% in comparison to
depolymerization of oligomers with DPn ~2 (run
nos. 2, 5). The thermal decomposition of the oligomer
with DPn = 4.7 (obtained in the presence of Pr2O3; run
no. 9) was also characterized by increase of the yield
of lactide by 5% compared to the oligomer obtained at
Tolig = 150°C using the same catalyst (run no. 8). The
yield of lactide in the presence of cerium(III) chloride
(25%, run no. 10) was comparable with that obtained
with praseodymium(III) oxide.
The amount of undepolymerized product (still
residue in the reactor) after thermal degradation of
oligomers with DPn ≥2 was fairly large. In order to
improve the yield of lactide, the depolymerization
temperature was raised to 200°C. Experiments were
carried out with oligomers obtained in the presence of
Pr2O3 and CeCl3 and with a sample with DPn = 3.2,
which was synthesized without a catalyst (run nos. 11–
15). It was found that increase of the temperature of
depolymerization of Pr2O3-containing samples with
We failed to obtain lactide in a high yield by con-
densation of lactic acid, followed by thermal decom-
position of oligomers. Therefore, we tried to syn-
thesize it by oligomerization of alkyl lactates and sub-
sequent depolymerization of oligoesters. This method
was successful with the use of tin and lead catalysts
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SYNTHESIS OF LACTIDE FROM LACTIC ACID AND ITS ESTERS
347
[14, 15], and the best yields of both oligoesters and
lactide were achieved with ethyl lactate as initial
compound [14].
a small amount of ethyl lactate (14% of the theoretical
yield).
The still residue boiling above 150°C contained
ethyl lactate in a mixture with oligoesters and unre-
acted lactic acid and its oligomers. This residue was
subjected to oligomerization in the presence of yttrium
and praseodymium oxides and cerium chloride; the
results are given in Table 2. The amount of the catalyst
for oligomerization of the still residue was calculated
for the initial lactic acid (less lactic acid removed as
ethyl lactate in fractions boiling below 150°C). The
oligomerization was carried out at 160°C under nitro-
gen (780 mm) in the setup used previously for dehy-
dration of lactic acid. After removal of alcohol–water
mixture, the pressure was reduced to 10 mm, and the
mixture was kept for 2 h at 160°C. The catalytic effect
was estimated by comparing the degree of oligomer-
ization of components of the still residue obtained in
the esterification of lactic acid with that in the blank
experiment (Table 2). The degree of oligomerization
It is known that the equilibrium in esterification
reactions is shifted toward ester formation in acid
medium; the use of sulfuric acid to catalyze the
synthesis of ethyl lactate was reported [17]. However,
isolation of the ester from the reaction mixture by
distillation without preliminary neutralization of the
acid was accompanied by tarring. The known method
for the synthesis of esters using molecular iodine as
catalyst [18] involves laborious procedures for isola-
tion of the product and separation of the catalyst.
Efficient methods have been reported for the syn-
thesis of ethyl lactate in the presence of cation ex-
changers [19–22], including those containing phospho-
tungstic and phosphomolybdic heteropolyacids [23].
Preyssler acid [24], zeolite membranes [25], and
combinations of some acid catalysts with pervapora-
tion zeolite membranes [26] were also used to catalyze
the synthesis of ethyl lactate. Among the above
methods, the most convenient is esterification
catalyzed by Amberlyst-15 cation exchanger, which
afforded satisfactory yield of ethyl lactate. Taking into
account similar proton capacities of Amberlyst-15 and
Purolite C100 MBH (52–57 and 51–55%, respec-
tively), we used more accessible Purolite C100 MBH
as catalyst in an amount of 5 wt % relative to the
overall amount of ~100% lactic acid and ~80% aque-
ous ethanol at a molar ratio of 1:4. The conversion of
lactic acid to esterification products (ethyl lactate and
di- and trilactic acid esters) was 70% in 6 h. Apart
from unreacted lactic acid, the reaction mixture con-
tained its oligomers. The esterification products were
1
was determined by H NMR. While interpreting the
1H NMR spectra, it was taken into account that the
region δ 4.22–3.98 ppm comprises both CH proton
signals and signals of methylene protons of the ethoxy
groups. Therefore, the intensity of the CH signals was
calculated by subtracting the intensity of OCH2 signals
corresponding to methyl proton signals in the region
δ 1.19–1.13 ppm from the overall signal intensity in
the region δ 4.22–3.98 ppm (see figure).
As follows from the data in Table 2, yttrium(III)
oxide does not affect the degree of oligomerization to
an appreciable extent (run nos. 1, 2). However, the
yield of oligomer in the presence of Y2O3 was higher
by 12% than in blank experiment, indicating higher
conversion of ethyl lactate and di- and trilactic acid
esters, DPn = 6.3.
1
1
analyzed by H NMR spectroscopy, including H–1H
COSY experiments. The results insignificantly differed
from those obtained with the use of semipermeable
zeolite membrane [25], which confirmed the efficiency
of our method.
The highest degree of oligomerization was
observed for the oligomer obtained in the presence of
praseodymium(III) oxide (run no. 3). The yield was
close to that in the absence of a catalyst. Presumably,
lactide formed at the oligomerization stage is removed
to the receiver under reduced pressure. The DPn value
obtained with cerium(III) chloride as catalyst occupies
an intermediate position (run no. 4). In this case, the
maximum yield of oligomer was reached, indicating
more complete and selective conversion of ethyl
lactate to nonvolatile compounds without formation of
an appreciable amount of lactide.
The products can readily be separated from the
catalyst, but we failed to isolate pure ethyl lactate in
high yield from the product mixture. The largest
amount of ethyl lactate (92%) was present in the
fraction collected at 150–160°C on distillation of the
reaction mixture, while its overall yield did not exceed
31%. Distillation at a temperature higher than 160°C
led to oligomerization of components of the still
residue, and the overall yield of ethyl lactate de-
1
creased. As shown by H NMR, the fraction collected
The resulting oligomers were subjected to depoly-
merization at 200°C (5 mm) without purification from
in the temperature range from 78 to 150°C contained
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PORYVAEVA et al.
348
5.2
5.1
5.0
4.9
4.2
4.1
4.0
1.5
1.4
1.3
1.2
δ, ppm
1H NMR spectrum (400 MHz, 296 K, DMSO-d6) of oligomers obtained in the presence of yttrium(III) oxide from the still residue in
the esterification of lactic acid catalyzed by Purolite C100 MBH.
rare earth compounds in the same setup as that used for
thermal decomposition of lactic acid oligomers.
Likewise, two fractions were obtained: fraction 1 con-
tained volatile ethyl lactate, oligoeesters, and lactide,
and fraction 2 was the still residue. Their weight
fractions ω1 and ω2 were calculated on the initial
oligomer. The yield of lactide in fraction 1 was deter-
side formation of polylactide which cannot undergo
depolymerization under the given conditions; this
follows from fairly large amounts of the still residue.
In the depolymerization of oligomer with lower DPn
value in the presence of Y2O3, fast removal of volatile
oligoesters from the reaction zone also leads to reduc-
tion of the yield of lactide.
1
mined by H NMR. All rare earth compounds showed
The maximum yield of lactide was achieved in the
presence of CeCl3 (run no. 4), and this catalyst
displayed high selectivity in depolymerization of both
oligoesters (97%) and lactic acid oligomers (98%) at
200°C (run no. 15). The catalytic activity of ceri-
um(III) chloride in the synthesis of lactide from lactic
acid oligomers is comparable with the activity of some
tin and lead compounds [14, 15].
catalytic activity at the oligomer depolymerization
stage, as seen from the high yields of fraction 1 and
lactide in comparison to blank experiment. However,
despite comparable yields of lactide-containing frac-
tion 1, the selectivity for lactide increased in the series
Y2O3 < Pr2O3 < CeCl3. Presumably, the lower yield of
lactide in the presence of Y2O3 and Pr2O3 is related to
Table 2. Oligomerization of the still residue obtained after esterification of lactic acid and thermal decomposition of
oligoester mixture in the presence of rare earth catalysts
Run no.
Catalysta
–
DPn
Yield of oligomer, %
ω1, %
ω2, %
Yield of lactide, %
1
2
3
4
6.5
6.3
8.0
7.3
68
80
70
83
36
83
83
87
62
14
12
2
27
55
64
84
Y2O3
Pr2O3
CeCl3 ·7H2O
a
Amount of the catalyst: 0.8 mol % of Y2O3 or Pr2O3 or 1.6 mol % of CeCl3·7H2O.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 53 No. 3 2017

SYNTHESIS OF LACTIDE FROM LACTIC ACID AND ITS ESTERS
349
Thus, praseodymium(III) oxide and cerium(III)
chloride heptahydrate can be used to catalyze the syn-
thesis of lactide from lactic acid and mixtures of ethyl
lactate with lactic acid oligoesters. To improve the
yield of lactide it is necessary to optimize the condi-
tions for the transformation of lactic acid to lactide, in
particular optimal catalyst concentration, pressure, tem-
perature, reactant contact time, etc., should be found.
Esterification of lactic acid. Concentration of
250 g of a 79% aqueous solution of lactic acid for 1 h
at 90°C under reduced pressure on an IKA RV 10 basic
rotary evaporator afforded 100% lactic acid after
removal of 52.5 g of water. A mixture of 50 g
(0.56 mol) of 100% lactic acid, 129 g of 80% aqueous
ethanol, and 7.5 g of Purolite C100 MBH (5 wt % of
the overall amount of lactic acid and ethanol) was
refluxed for 6 h with continuous stirring. The mixture
was cooled to room temperature, the catalyst was
filtered off, and the mixture was distilled under
atmospheric pressure to collect a fraction boiling in the
range from 78 to 150°C.
EXPERIMENTAL
1
The H NMR spectra were recorded on Bruker
DPX 200 and Bruker Avance III 400 spectrometers.
The degree of oligomerization of lactic acid and/or its
esters (DPn) was determined from signal intensity
ratios in the 1H NMR spectra. The molecular weight of
polylactide was determined by gel permeation chroma-
tography at 20°C with a Knauer Smartline chromato-
graph [Phenogel Phenomenex column, 300×7.5 mm,
grain size 5 μm; refractometric and UV detectors
(λ 254 nm)]; standard polystyrene samples with
medium and high molecular weights (2 to 2570 kDa)
were used for calibration. The experimental molecular
weights were corrected with account taken of Mark–
Houwink coefficient (0.58) [27, 28]. Commercially
available aqueous solution of L-lactic acid (79%,
Galacide Food) was used.
Oligomerization of components of the still
residue obtained after esterification of lactic acid.
The reactions were carried out in a setup analogous to
that used for oligomerization of lactic acid. A flask was
charged with 0.578 g (2.56 mmol) of Yt2O3, 0.844 g
(2.56 mmol) of Pr2O3, or 1.907 g (5.12 mmol) of
CeCl3·7H2O, 33.1 g of the still residue was added, and
the mixture was heated at 160°C under nitrogen
(780 mm) until condensate no longer accumulated in
the receiver. The system was then evacuated to
a residual pressure of 10 mm and kept for 2 h at 160°C
to remove volatile compounds. The composition of the
condensate collected at 780 and 10 mm was deter-
1
mined by H NMR. Oligomer mixture in the still
Oligomerization of lactic acid. Oligomerization of
lactic acid in the presence of yttrium(III) and praseo-
dymium(III) oxides and cerium(III) chloride, as well as
in the absence of a catalyst, was carried out under
continuous stirring under nitrogen (780 mm) in a flask
maintained at a required temperature. The flask was
connected with an air-cooled reflux condenser
equipped with a bottom plate for removal of con-
densate. The reactions were carried out at 130, 150,
and 180°C for each catalyst. In each run, 0.569 g
(2.52 mmol) of yttrium(III) oxide, 0.831 g (2.52 mmol)
of praseodymium(III) oxide, or 1.885 g (5.06 mmol) of
cerium(III) chloride heptahydrate was added to 36 g of
a 79% aqueous solution of lactic acid. The mixture was
heated for 4 h at a constant temperature. The con-
densate was removed from the plate receiver, and the
still residue (lactic acid oligomers) was dissolved in
residue was dissolved in DMSO-d6 and analyzed by
1H NMR to determine the degree of oligomerization
(DPn). The yield of oligomers was determined as the
ratio of the oligomer weight to the theoretical weight
with account taken of its average molecular weight
calculated from the degree of oligomerization.
Depolymerization and isolation of lactide. The
mixture of lactic acid oligomers or oligoeesters and
catalyst obtained after oligomerization was subjected
to thermal decomposition. For this purpose, oligomer
mixture, 3 g, was placed in a flask connected to an air-
cooled reflux condenser (receiver) and a trap cooled
with liquid nitrogen. The flask was heated for 4 h at
180 or 200°C at a residual pressure of 5 or 0.05 mm.
The sublimate collected in the receiver was analyzed
1
by H NMR. The yield was calculated as the weight
ratio of lactide and initial oligomer. The concentration
of lactide in the sublimate was determined by 1H NMR
from the signal intensity ratio of lactide and naphtha-
lene used as standard. The unsublimed residue (poly-
lactide) was dissolved in tetrahydrofuran (3 mL),
precipitated with methanol (100 mL) or diethyl ether
(100 mL), and dried under reduced pressure. The
1
DMSO-d6 and analyzed by H NMR. The degree of
oligomerization (DPn) was determined from the ratio
of the overall intensity of the CH proton signals of
lactic acid and oligomers to the overall intensity of the
CH proton signals of lactic acid and CHOH signals of
oligomers according to [16].
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 53 No. 3 2017

PORYVAEVA et al.
350
13. Jiang, B., Tantai, X., Zhang, L., Hao, L., Sun, Y.,
molecular weight distribution of polylactide was deter-
mined by gel-permeation chromatography.
Deng, L., and Shi, Z., RSC Adv., 2015, no. 5, p. 50747.
14. Upare, P.P., Hwang, Y.K., Chang, J.-S., and
Hwang, D.W., Ind. Eng. Chem. Res., 2012, vol. 51,
p. 4837.
This study was performed under financial support
by the Ministry of Education and Science of the
Russian Federation (project no. 4.70.2014).
15. Upare, P.P., Lee, M., Hwang, D.W., Hwang, Y.K., and
Chang, J.-S., Catal. Commun., 2014, vol. 56, p. 179.
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