Solvothermal Alcoholysis Routes for Recycling Polylactide Waste as Lactic Acid Esters

Article abstract of DOI:10.1021/acscatal.6b01009

In this work, we investigated the possible use of polylactide (PLA), a biodegradable polymer obtained from renewable biofeedstock, to produce a range of industrially useful lactic acid esters. We describe a simple and convenient solvothermal alcoholysis method for large-scale recycling of PLA resins or residues from disposable packaging in the presence of the appropriate alcohol under catalyst-free or catalytic conditions. This process proceeds easily both without and with a catalyst. The results show that the best catalytic activities involve magnesium and calcium alkoxides synthesized in situ from organometallic or metallic precursors and an alcohol. We determined the crystal structure of the chiral mononuclear postcatalyst [Ca(LAc)₂(EL)₂] (1; LAc = lactic acid anion, EL = ethyl lactate), obtained directly from the reactor. Particular emphasis is placed on the operating conditions and high activity of the catalyst used. Key factors that affect the catalytic activity and reaction mechanism are also highlighted.

Full text of DOI:10.1021/acscatal.6b01009

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Article
Solvothermal Alcoholysis Routes for Recycling
Polylactide Waste as Lactic Acid Esters
Rafa# Petrus, Dominik Bykowski, and Piotr Sobota
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01009 • Publication Date (Web): 01 Jul 2016
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Solvothermal Alcoholysis Routes for Recycling
Polylactide Waste as Lactic Acid Esters
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Rafał Petrus, Dominik Bykowski, and Piotr Sobota*
Faculty of Chemistry, University of Wrocław, 14 F. JoliotꢀCurie, 50ꢀ383 Wrocław, Poland
ABSTRACT
In this work, we investigated the possible use of polylactide (PLA), a biodegradable polymer
obtained from renewable bioꢀfeedstock, to produce a range of industrially useful lactic acid
esters. We describe a simple and convenient solvothermal alcoholysis method for largeꢀscale
recycling of PLA resins or residues from disposable packaging in the presence of the appropriate
alcohol under catalystꢀfree or catalytic conditions. This process proceeds easily both without and
with a catalyst. The results show that the best catalytic activity have magnesium and calcium
alkoxides synthesized in situ from organometallic or metallic precursors and an alcohol. We
determined the crystal structure of the chiral mononuclear post catalyst, [Ca(Lac) (EL) ] (1) (Lac
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= lactic acid anion, EL = ethyl lactate), obtained directly from the reactor. Particular emphasis is
placed on the operating conditions and high activity of the catalyst used. Key factors that affect
the catalytic activity and reaction mechanism are also highlighted.
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KEYWORDS
polylactide, polymer recycling, alcoholysis reaction, solvothermal synthesis, green solvents,
alkyl lactates, ethyl lactate
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1.
INTRODUCTION
In recent years there has been growing interest in the use of biomass as the feedstock for the
production of chemical intermediates, because this is an attractive alternative to the use of
traditional fossilꢀfuelꢀderived materials. A number of highꢀpotential platform molecules have
been created as precursors for valueꢀadded chemicals and building blocks. Much attention has
focused on lactic acid and its alkyl esters, which are produced commercially from a range of
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agricultural residues and food byꢀproducts. Alkyl lactates (ALs) are important ecofriendly
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chemicals, referred to as green solvents, they are attractive alternatives to petrochemical
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solvents. They are therefore a popular choice in many industrial scenarios, because they can be
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used safely without affecting the environment. Because of their particular physicochemical
properties, ALs have a number of industrial and commercial applications, e.g., as solvents or
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diluents for polymeric resins, dyes, oils, paints, inks, and pigments, components of cleaning or
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agriculturally useful compositions, degreasers for paints, graffiti removal, and adhesives, lowꢀ
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temperature lubricants, emulsifiers or dispersants in pharmaceutical compositions, and
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components of transdermal drugꢀdelivery systems or skinꢀcare formulations . Lactate esters
have a number of advantages that make them excellent starting material for the synthesis of a
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range of chemicals. Ethyl lactate (EL) is particularly useful and is one of the most promising
green chemicals. EL is used in a multitude of mechanical and microelectronic applications and
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meets the highꢀpurity requirements of the semiconductor industry.
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Currently, bulk production of lactate esters is based on esterification of lactic acid with
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the appropriate alcohol in the presence of acid catalysts. The process is technologically
complicated because the presence of water affects the reaction equilibrium and limits conversion,
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or selfꢀesterification of lactic acid under prolonged heating. The use of excess alcohol,
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catalysts, and separation processes are therefore essential for efficient AL synthesis. The recent
significant increase in industrial applications of bioplastics has also increased interest in the
development of alternative processes based on chemical recycling of polylactides (PLAs). PLA
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recycling via thermal cracking to lactides, conversion to functionalized macromolecules, and
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hydrolytic degradation to lactic acid has been extensively studied. The conversion of PLA
waste by alcoholysis therefore has potential as an economically viable and environmentally
sustainable method for producing ALs. The concept of lactic acid ester synthesis from PLA is
well established and has been widely explored. Filiachione (1945) established that AL can be
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obtained by the acidꢀcatalyzed reaction of PLA oligomers with aliphatic alcohols. In an
alternative method, developed by Du Pont, the reaction is performed at 150–190 °C, using 2–3
equiv of alcohol (ROH for R = Me, Et, nBu) per polymer unit, under the autogenic pressure of
the reactants, with H SO as the catalyst; 69–87% conversion of highꢀmolecularꢀweight PLA to
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AL is achieved within 2 h. Several studies have been devoted to the synthesis of AL using ionic
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liquids, strong organic bases, metal carboxylates, halides, and alkoxides as catalysts, but
these methods, which are performed in solution, and have low conversion rates or long reaction
times, are unattractive as largeꢀscale recycling techniques. Various routes for solvolytic recycling
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of PLA, such as conventional heating, microwave irradiation and ultrasound treatment, have
also been proposed. However, our understanding of PLA alcoholysis needs to be improved; for
example, there have been few direct investigations of PLA alcoholysis using metalꢀbased
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catalysts, and the information available in the literature mainly covers detailed analysis of the
postꢀreaction composition, rather than the catalyst itself.
Recently, our group reported a method that enables the exclusive preparation of lactic
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acid esters by chemoselective alcoholysis of
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ꢀlactide mediated by magnesium alkoxides. In
this study, based on this concept, we developed a green and efficient technique for the
transformation of PLA into a wide variety of industrially useful lactate esters. This method was
designed specifically for chemical recycling of used, postꢀindustrial, and postꢀconsumer PLA.
2.
RESULTS AND DISCUSSION
.1 Catalyst-Free PLA Alcoholysis. We investigated the utility of our method by performing
ꢀlactide) (PLLA), poly( ꢀlactide) (PDLA), and poly( ꢀlactide)
PDLLA). These materials had been used as commercial granules or were residues from
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PLA alcoholysis of poly(
L
D
D,L
(
disposable packaging. The detailed specifications of the PLA resins used in the alcoholysis
reactions are presented in Table S1 in the Supporting Information. Typical reactions were
performed using a steel pressure reactor of capacity 150 mL and then with a reactor of capacity 5
L equipped with a mechanical stirrer. We first conducted control experiments in the absence of a
catalyst source. The PLLA and alcohol were placed in a reactor and heated at 200 °C for 1 h,
using 4 equiv of EtOH per ester unit, la = −[OCH(CH )CO]−, in the polymer. The results show
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that PLLA alcoholysis occurs easily under solvothermal conditions, giving polymer degradation
to a liquid consisting of lactic acidꢀbased esters (Figure S1 in SI). Because little work has been
done on PLA alcoholysis under solvothermal conditions in the absence of a catalyst, we
examined this process first, because it opens up a new simple method for the synthesis of lactic
acid esters. We therefore performed a series of catalystꢀfree PLA alcoholysis reactions. In the
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standard procedure, the polymer and alcohol were placed in a reactor and heated at 180–260 °C
for 1 h, using EtOH/la ratios ranging from 1 to 10 (see Table S2 in SI).
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2
.1.1. Detailed Analysis of PLA Alcoholysis Products and Reaction Conditions. H NMR
spectroscopic analysis of the obtained products showed three different methine peaks, at 5.05,
4.22, and 4.09 ppm, characteristic of PLLA oligomers endꢀcapped with ethoxy groups (ELn),
ethyl lactyllactate (EL ), and EL, as shown in Figure 1.
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Figure 1. H NMR spectrum in C D of the PLLA degradation products by the alcoholysis
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reaction.
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Matrixꢀassisted laser desorption/ionization timeꢀofꢀflight mass spectrometry studies of
depolymerized samples confirmed the presence of lactyl oligomers separated by increments of
72 Da, with ethoxy end groups (Figure 2), suggesting that extensive transesterification of the
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initial PLLA sample had occurred. Gelꢀpermeation chromatography analysis of the products of
the reaction of PLLA with EtOH at 200 °C, using the stoichiometry la/EtOH = 1/1–10, showed
that the weightꢀaverage molecular weight of the starting material decreased from an initial value
of 137.9 kDa to 500–800 Da, corresponding to EL , with polydispersity indexes of 1.39–1.51.
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Figure 2. MALDI spectrum of the potassium cationized lactyl oligomers with a
incorporated ꢀOEt end group formed in the PLLA alcoholysis reaction.
These results show that as the reaction progressed, the amount of PLA decreased, with
simultaneous increases in the amounts of EL oligomers and the final products, i.e., EL and EL .
n
2
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The experimental data, summarized in Table S2, suggest that excess alcohol, and a high
temperature are necessary for the efficient synthesis of EL. Graphical analysis of the obtained
results (Figure 3) shows that for a given reaction stoichiometry, the amount of EL gradually
increased with increasing temperature; for example, when the reaction was performed with the
stoichiometry la/EtOH = 1/1 at 260 °C for 1 h, the maximum PLLA conversion to EL was 50%;
increasing the amount of EtOH in the reaction system 2ꢀ, 4ꢀ, and 10ꢀfold increased the
conversions to 77%, 96%, and 99%, respectively.
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la/EtOH =
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Figure 3. Effect of temperature and alcohol amount on ethanolysis of PLLA.
The amount of alcohol influences the reaction kinetics, and therefore it is important to reach a
compromise that avoids the need to remove a large excess of alcohol during the following
purification steps. For economic reasons and to achieve a high yield of EL, we recommend
performing the reaction with a stoichiometry of la/EtOH = 1/4 at 220–260 °C. It is worth noting
that the upper limit of the reaction temperature can be higher than 260 °C, but should not exceed
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the decomposition temperature of the polymer used. Thermogravimetric analysis/differential
thermal analysis showed that weight loss and degradation of the PLLA resin began at around 330
ºC (for details see Table S1, and Figure S2 in SI). Timeꢀdependent alcoholysis reactions with a
reactant stoichiometry la/EtOH = 1/4 at 220 °C showed that the slowest reaction step is the
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transformation of EL to EL; for example, when the reaction was performed for 15 min, the
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PLLA conversions to EL/EL were 69%/24%, whereas for a reaction time of 40 min, the
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conversion were 90%/10%, but the conversions did not change on further increasing the reaction
time to 60 min. Leibfarth reported slow transesterification of EL , taking over 12 min to produce
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90% EL, using triazabicyclodecene (TBD) as a catalyst. We used various PLA resins as
starting materials and observed that the reaction rate strongly depended on the molecular weight,
microstructure and material properties of the used polymer; for example, for reactions performed
using PDLA with the stoichiometry la/EtOH = 1/4 at 220 °C conversion to EL was 58%,
whereas for PLLA and PDLLA, the conversions were 90% and 96%, respectively. (Tables S2 –
S3 in SI).
We optimized EL formation, and obtained significant quantities, by varying the reactant
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stoichiometry and the reaction temperature. These experiments show that to achieve significant
amounts of EL it is necessary to perform the reaction using less than 2 equiv of alcohol per la
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unit at 200 °C (Table S2 in SI). It is worth noting that this method enables the synthesis of EL2
with ca. 40% yield of product that is in industrial demand and that is currently difficult to
produce in commercial quantities.
2.1.3. Summary. We have developed a method for the catalystꢀfree production of lactic acid
esters by breaking down the ester linkages between the constituent units of PLA, followed by
protonation of the carbonyl group and nucleophilic attack on the ion pair by the alcohol, as
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shown in Scheme 1. The carbonyl group is protonated by the carboxylic acid end group of the
polymer chain. However, it is not possible to exclude the formation of lactic acid resulting from
moisture from the polymer or alcohol.
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Scheme 1. Mechanism of carboxylic acidꢀcatalyzed PLA alcoholysis reaction.
O
OH
OH
O
H+
O
O
+
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O
Also, commercial PLAs are synthesized by ringꢀopening polymerization of lactides initiated by
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Sn(Oct)2, and therefore the final products are contaminated with Sn residues from the
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catalyst. Inductively coupled plasma optical emission spectroscopy showed that the amount of
Sn in the used PLAs ranged from 3.5 to 16.5 mg of Sn per 1 kg of polymer (Table S1). We
therefore cannot exclude the participation of catalyst residues in the investigated reactions.
2.2. PLA Alcoholysis in Presence of Catalyst. An important economic aspect of the present
process is to achieve rapid depolymerization under mild reaction conditions, including a low
pressure, temperature, and alcohol content. Once we had found a suitable system for the
alcoholysis of PLA to the corresponding AL, we investigated the use of catalysts to achieve high
product yields under gentler reaction conditions, which has financial benefits. We decided to
explore the use of metal alkoxides synthesized in situ from organometallic or metallic precursors
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in direct reactions with alcohols under solvothermal conditions as catalysts. The use of
magnesium and calcium alkoxides is preferred because they are highly reactive, do not damage
human health or the environment, and can be synthesized in oneꢀpot reactions from safe and
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easily stored compounds. First, we performed PLLA alcoholysis, using Mg(OEt) prepared in
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situ from MgBu and EtOH as the catalyst, with stoichiometries of la/EtOH/Mg = 1/1–10/0.01 in
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the temperature range 80–200 °C for 1 h (Tables S4ꢀ5 in SI). The results (Figure 4) show that
78–96% conversion of the polyester to EL can be achieved at 140 °C using 2–10 equiv of
alcohol per la unit.
la/EtOH/Mg =
1/1/0.01
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Figure 4. Effect of temperature and alcohol amount on ethanolysis of PLLA using MgBu as
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precatalyst.
However, to prevent excessive use of alcohol, we suggest conducting the PLLA alcoholysis at
temperatures above 160 °C with stoichiometries of la/EtOH/Mg = 1/2/0.01, or above 140 °C
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with la/EtOH/Mg = 1/4/0.01. A comparison of these results with those for catalystꢀfree reactions
shows that the use of a magnesium alkoxide enables 90% conversion to be achieved in reactions
performed at temperatures lower than 80–100 °C and under pressures several times lower. The
results in Table S6 show that the amount of catalyst also significantly affects the yield of the
final product. Magnesium ethoxide works efficiently when the amount used is 0.75–1 mol% per
la unit for a stoichiometry of la/EtOH = 1/2, and from 0.5–1 mol% per la unit for a
stoichiometry of la/EtOH = 1/4.
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2.2.1. Alcoholysis of EL . Results of our study show that the main difficulty in this process is
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not only achieving polymer degradation but also getting the high conversion of EL to EL.
2
Therefore, to better understand the test process, we additionally conducted transesterification
reactions of EL with EtOH at stoichiometry EL /EtOH/Mg = 1/1ꢀ10/0.01 at 25 and 60 °C and
2
2
1
monitored these reactions in the function of time by H NMR spectroscopy. The results
presented in Figures S5ꢀ6 (SI) show significant increase in conversion of EL to EL with
2
increasing reaction temperature and amount of the EtOH added. This effect is particularly visible
at stoichiometry of the reactants EL /EtOH/Mg: 1/1/0.01 and 1/10/0.01. Used of 10ꢀfold excess
2
of alcohol or increasing the reaction temperature to 60 °C caused fourꢀfold increase in
conversion. For example, at 25 °C with EtOH/EL ratio of 1 after 2h the conversion was 3% and
2
for ratio 10 equals 11%. While the reaction was carried out at 60 °C, also after 2 h for the same
molar ratios the conversion reached the value of 13% and 51%, respectively. The performed
kinetic analysis indicating that alcoholysis proceeds with a secondꢀorder dependence on EL2
concentration (Figures S7ꢀ8 in SI). These observations are more typical for the ROP of LA
3
4
initiated by magnesium alkoxide, than for existing data for chemical recycling of polymers.
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2
.2.2. Kinetic Studies of PLA Alcoholysis. The kinetic studies for safety reason have been
carried out in toluene at 90 °C for reactants concentration: [la] = 0.5 M; [EtOH] = 0.5, 1, 2, 5 M;
Mg] = 2.5, 5, 7.5, 10, 12.5 mM. The influence of catalyst and alcohol amount on the PLLA
[
0
1
2
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4
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9
0
1
2
3
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9
0
1
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0
1
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9
0
ethanolysis process have been presented on the plots of CEL vs. time (Figures S9ꢀ10 in SI). The
plots of 1/[la] ꢀ 1/[la] vs. time for a wide range of [Mg] or [EtOH] are linear, indicating the
t
0
second order dependence on la concentration (Figures S11ꢀ12 in SI). Thus, the rate expression
2
n
m
n
m
can be written as ꢀd[la]/dt = k [la] , where k = k [Mg] [EtOH] = k [Mg] = k [EtOH] .
obs
obs
p
1
2
From the plots of ln(k ) vs. ln[Mg] the order of [Mg] is 4.15 (Figure S13 in SI). While from the
obs
plot of ln(k ) vs. ln[EtOH] the order of [EtOH] is 2.19 (Figure S14 in SI). The PLLA
obs
2
4.15
2.19
alcoholysis process demonstrates the following rate law: d[la]/dt = k [la] [Mg] [EtOH]
.
p
These studies show that the use of MgBu in combination with EtOH leads to formation of a
2
tetranuclear catalyst adduct, whereupon two PLLA unit can attach to it to be converted to EL.
2
.2.3. PLA Alcoholysis Using C1–C8 Alcohols. We broadened our research by investigating
a wide range of aliphatic C1–C8 alcohols in PLLA transesterification, using la/ROH/Mg =
/2/0.01 at 200 °C as the standard conditions. The results, which are listed in Table 1, show that
1
methanol, nꢀpropanol, nꢀbutanol, secꢀbutanol, isobutanol, nꢀpentanol, isopentanol, nꢀhexanol,
and allyl alcohol provided fast and efficient depolymerization. The alcohol reactivity in PLLA
degradation decreases with increasing number of carbon atoms in the main chain; for example, to
achieve a PLLA conversion of about 70% using nꢀbutanol it was necessary to conduct the
reaction for 1.5 h, whereas for nꢀpentanol and nꢀoctanol, the time increased to 2 and 6 h,
respectively. The slowest reaction was observed when isopropanol was used.
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Table 1. Synthesis of a wide range of AL by alcoholysis of PLLA using metallic Mg or MgBu2
a
as precatalyst.
b
b
c
No.
Ester
t (h)
C (%)
AL
CAL2 (%)
Y (%)
AL
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
O
O
O
O
O
O
O
O
O
O
O
O
d
1
1.5
88
87
57
73
66
89
85
66
65
91
68
11
81
80
50
64
59
82
79
60
60
85
59
HO
HO
HO
HO
HO
HO
HO
HO
HO
HO
HO
HO
O
d
2
1.5
10
1.5
1.5
2
12
33
19
24
7
O
d
3
O
O
O
O
O
O
O
O
O
O
4
d
5
6
7
8
9
2
12
19
26
7
2
2
1
0
3
e
1
1
6
26
e
1
2
6
88
11
82
a
Alcoholysis conditions: la/ROH/Mg = 1/2/0.01, 30g of PLLA (IngeoTM 2003D). Reaction
performed using pressure reactor with a capacity of 150 mL, under N atmosphere and
autogenous pressure of reactants, at temperature 200 °C. Obtained from H NMR analysis
according equation CAL = [AL]/[PLLA] and C
2
b
1
c
= 2[AL ]/[PLLA] .
Reaction yields
0
AL2
2
0
d
calculated based on the amount of isolated AL. Reaction performed using metallic Mg as
e
precatalyst. Reaction performed using 15g of PLLA.
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2
2
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5
6
In order to determine the influence of alcohols structure and steric effect for PLLA alcoholysis
process, we have conducted detailed studies using different butanol isomers, including nꢀbutanol,
isobutanol, secꢀbutanol, and tertꢀbutanol. PLLA butanolysis reactions were carried out with the
stoichiometry la/ROH/Mg = 1/2/0.01 at 160 and 200 °C for 1 h. The results showed that the
reactivity of butanols decreases in the order isobutanol > nꢀbutanol > secꢀbutanol > tertꢀbutanol
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(Table S7 in SI). The lower reactivity secꢀ and tertꢀbutanol may be explained by the influence of
steric effect, while the higher reactivity of isobutanol than nꢀbutanol, can be elucidated by
alcohols’ main chain length, formally C3 vs. C4. A similar effect was also observed in reactions
of 2ꢀethylhexanol and nꢀoctanol, wherein the first one was much more active. These observations
may be also explained by the fact that primary alcohols with shorter main chains can better
penetrate the surface of polymer and simultaneously due to their lower boiling points exert a
higher pressure in the reactor, what result in a higher conversion of PLLA. Moreover, PLLA
alcoholysis tests carried out using cyclopentanol and cyclohexanol confirmed that the presence
of a sterically hindered aliphatic groups in the alcohols caused lower PLLA conversion, which in
both cases at 200 °C were approximately 30% (Table S7 in SI). In order to better understand the
effect of ROH substituents on the reactivity in PLLA alcoholysis, we choose electron rich and
poor alcohols as reagents. According to the results shown in Table S8 in SI reactivity of used
alcohols has decreased in the following order: (CH ) NCH CH OH > CH OCH CH OH >
3
2
2
2
3
2
2
PhCH CH OH > CF CH OH > PhCH OH > Cl CH OH. These studies confirmed that the
2
2
3
2
2
3
2
reaction of PLLA with primary ethanol derivatives is accelerated by electronꢀdonating
substituents that promote nucleophilic attack of the ROH on the carbon atom of the ester group.
Summarizing the above results, we have concluded that PLLA conversion depends not only on
structural and electronic effects of the alcohol but can be also influenced by various factors such
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as the ramification of ROH, solubility of the resulting polymer in alcohol, or catalyst
aggregation. These explain, among other things weak reactivity of PhCH OH.
2
2.2.4. Synthesizes EL of various optical activity. In asymmetric synthesis, EL can be used as
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35
the starting material for the preparation of a wide range of optically active compounds. We
therefore used our method to synthesize lactic acid esters with various optical activities. These
were determined by the PLA microstructure; for example, we used isotactic PLLA or PDLA to
prepare the corresponding LꢀEL or DꢀEL esters. D,LꢀEL (racemic mixture) were prepared from
atactic PDLLA as the substrate. The Mgꢀbased catalysts did not cause significant epimerization
of the resulting lactate ester during the recycling process. This was verified using circular
dichroism (Figure 5) and optical purity determination by polarimetry. The optical rotation
measurements were used to determinate the enantiomeric excess, which were equal to 92% for
LꢀEL and 93% for DꢀEL.
Figure 5. CD spectra of LꢀEL, DꢀEL and D,LꢀEL obtained using PLA resins with different
chain microstructure.
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2
.2.5. Upscaling of PLA Alcoholysis. After optimization of the PLLA alcoholysis parameters,
we increased the reaction scale 100ꢀ and 150ꢀfold, using a 5 L reactor and Mg chips as the
precatalyst (Table 2). The developed technique was successfully used to achieve 89% conversion
of PLA and isolated EL yields of 86% (2.05 L) starting from 1.5 kg of polyester. The scale of the
reaction did not affect the composition of the reaction products and the performance of the
process.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
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4
5
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9
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1
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9
0
1
2
3
4
5
6
7
8
9
0
a
Table 2. Large scale synthesis of EL by alcoholysis of PLLA using metallic Mg as precatalyst.
d
d
e
f
No.
T (°C)
160
t (h)
C (%)
CEL2 (%)
Y (%)
Pmax (bar)
EL
EL
b
1
1
1
2
1
82
88
88
89
16
11
9
79
85
ꢀ
10
14
14
18
b
2
200
b
3
200
c
4
200
10
86
a
b ꢀ c
Alcoholysis conditions: la/EtOH/Mg = 1/2/0.01.
Reaction performed using 1 (b) or 1.5 kg
(
c) of PLLA (IngeoTM 2003D) in pressure reactor with a capacity of 5 L under N atmosphere
and autogenous pressure of reactants. Obtained from H NMR analysis according equation C
[EL]/[PLLA] and C = 2[EL ]/[PLLA] . Reaction yields calculated based on the amount of
isolated EL. The maximum pressure in the reactor observed throughout the course of the
reaction.
2
d
1
EL
e
=
0 EL2 2 0
f
2.2.6. Screening Activities of Various Catalysts in PLLA Alcoholysis. We examined the
catalytic activities of various catalysts in PLLA alcoholysis, by performing reactions using
selective metal halides, hydroxides, carboxylates, alkoxides, metallic chips, and organometallic
compounds. Table 3 shows results of the compounds having the highest catalytic activity in the
examined process. We found that metal alkoxides generated in situ from organometallic or
metallic precursors have one of the highest catalytic activities in the ethanolysis of PLLA. When
metallic Mg and Ca were used as precatalysts, the PLLA conversions and product yields were
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36
comparable to those achieved industrially using Sn(Oct) . Good catalytic activities were also
2
observed for SnCl , ZnCl , and TBD.
2
2
2
.2.7. Catalyst characterization. In these studies, for most of the alcoholysis reactions, the
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catalyst residues were isolated as insoluble precipitates, which made it impossible to perform
spectroscopic studies in solution. It was difficult to obtain crystals of the used catalyst suitable
for Xꢀray diffraction (XRD) from the postꢀreaction mixture, because amorphous solid materials
rapidly formed. However, after multiple repeated syntheses, and using a slow crystallization
technique, we managed to obtain, for the first time, crystals appropriate for XRD studies directly
from the reactor, using metallic Ca as the catalyst precursor. The crystal structure of the
mononuclear complex [Ca(LAc) (EL) ] (1, 42%) was determined. In the molecular structure of
2
2
2
+
−
chiral compound 1, the Ca ion is coordinated with two lactic acid anions LAc and two neutral
EL ligands, forming an eightꢀcoordinated polyhedron with surrounding O donors, as shown in
8
−
Figure 6 (see also 1.cif file). The LAc ligands around the central atom display enantiomeric
disorder, and two equivalent positions for the methine group are observed. Racemization of the
37
lactate unit was previously observed during thermal degradation of Caꢀendꢀcapped PLLA. We
dissolved CaCl in an EL/EtOH mixture (1:4) to check whether EtOH molecules can also
2
2
+
coordinate with the Ca center in the presence of EL. A previously unknown crystalline ionic
compound [Ca(EL) ]Cl (2, 80%) was formed. The solidꢀstate structure of 2 is similar to that of
4
2
1, and contains a Ca1 ion eightꢀcoordinated with O donor atoms from four EL ligands. The
8
absence of an alcohol molecule in the metal coordination sphere suggests that the used
3
8
compounds function as typical transesterification catalysts.
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
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4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
a
Table 3. Screening the catalytic activity of different catalysts in PLLA ethanolysis reaction.
b
b
c
No.
1
Catalyst/Precatalyst
C (%)
CEL2 (%)
15
Pmax (bar)
EL
Li
Na
82
79
83
89
91
85
81
80
83
78
81
87
83
85
87
64
77
77
86
85
81
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
16
7
3
K
12
7
4
Mg
11
13
14
10
9
5
Ca
7
6
Sr
10
7
Ba
13
8
Na(OEt)
K(OEt)
Ca(OMe)2
Sn(OEt)4
Sn(Oct)2
LiMe
ZnEt2
SnEt Cl
14
13
13
20
23
10
12
13
15
21
15
13
12
16
11
9
12
10
11
12
13
14
15
19
18
12
15
14
11
2
2
16
AlEt3
29
1
7
NaOH
KOH
SnCl2
ZnCl2
TBD
17
18
16
1
9
12
20
15
2
1
13
a
Alcoholysis conditions: la/EtOH/M = 1/2/0.01, 10 g of PLLA (IngeoTM 2003D). Reaction
performed using pressure reactor with a capacity of 150 mL, under N atmosphere and
autogenous pressure of reactants, at temperature 200 °C through 1h. Obtained from H NMR
2
b
1
c
analysis according equation CEL = [EL]/[PLLA] and CEL2 = 2[EL ]/[PLLA] . The maximum
0
2
0
pressure in the reactor observed throughout the course of the reaction.
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Figure 6. The molecular structures of [Ca(LAc) (EL) ] (1, left) and [Ca(EL) ]Cl (2, right);
2
2
4
2
hydrogen atoms and disorders of ligands are omitted for clarity [symmetry code: (i) −x+1, y,
−z+1].
The solidꢀstate structures of 1–2 give insights into the possible metalꢀbinding environments of
PLA, and provide molecular models that help us to understand the formation of reaction
intermediates. The structural data confirm that during PLA alcoholysis calcium lactate species
are formed, which act as discrete catalysts in the investigated reactions. It was established that
the first step involves the formation of a calcium–lactic acid complex via the reaction of Ca(OR)2
with the −COOH end group in the polymer, as shown in Scheme 2. On the basis of these results,
we propose that PLA alcoholysis proceeds as shown in Scheme 3 During alcoholysis, the O=C
group of the PLA molecule coordinates with the metal center, and this increases the positive
charge on the carbonyl carbon. This facilitates attack by the oxygen atom in ROH on the carbon
atom of the ester group, resulting in breakage of the C−O bond and lactyl oligomer formation.
The oligomers react further with ROH to generate the main products, AL and AL . However, we
2
do not rule out the participation of moisture from the polymer or alcohol in the formation of
–
(COO) Ca species.
2
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2
2
2
2
2
2
2
2
2
3
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3
3
4
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4
4
4
4
4
4
4
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5
5
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5
5
5
5
5
5
6
Scheme 2. Proposed scenarios formation of metal lactate species as the results of reaction of
Ca(OR) with the −COOH end group in polymer.
2
O
O
O
O
O
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
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9
0
1
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2
3
4
5
6
7
8
9
0
O
2
O
OH
+
Ca(OR)2
Ca
+
2 ROH
O
O
O
Scheme 3. The proposed course of PLA alcoholysis reaction catalyzed by calcium lactate species
synthesized in situ from metallic Ca.
O
O
O
O
O
O
O
O
O
O
Ca
O
Ca
O
O
O
O
Ca
O
O
+
ROH
O
O
O
O
O
O
O
H
O
O
ROH
O
O
R
O
O
O
O
O
O
O
O
O
Ca
Ca
O
O
HO
O
O
O
OR
ROH
O
+
O
+
O
O
O
OR
HO
O
H
O
-1
O
R
O
O
HO
OR
O
O
When we conducted the process of PLLA alcoholysis in the presence of compounds 1 and 2, at
stoichiometry of the reactants la/EtOH/Ca = 1/2/0.01 at 200 °C for 1 h, the results were similar
to catalyst free process. It follows from this that both compounds are not active in PLLA
alcoholysis. Lack of reactivity of both compounds is due to their high coordination number
around the calcium atoms which is 8. It is especially visible on the example of compound 1
where the coordination of two additionally chelating EL ligands to {Ca(lac) } unit, maked it
2
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catalytically inactive. We confirmed this experimentally by using commercial [Ca(lac) ]·5H O
2
2
as a catalyst. In this case the conversion of PLLA to EL was 58%.
CONCLUSION
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In this work, we developed a simple and environmentally friendly method for largeꢀscale
chemical recycling of PLA by alcoholysis under solvothermal conditions to lactic acid esters.
This method can be used in either the absence or presence of a catalyst. The use of a metal
catalyst enables the reaction to be conducted at lower temperatures, i.e., 80–100 °C, and
pressures, with conversions comparable to those achieved under catalystꢀfree conditions. The
crystal structure of 1 strongly suggests that metal–lactic acid complexes act as discreet catalysts
in PLA alcoholysis when metal alkoxides are used as precatalysts. We believe that the current
results represent an important advance in understanding the course of polyester alcoholysis
mediated by metal complexes and explain the flexibility in the formation of active forms of the
catalyst. Our study will be helpful in the design of new efficient catalysts for the transformation
of green polymers and other polyesters to many useful chemicals. This could have direct impacts
on their use and result in financial benefits.
EXPERIMENTAL SECTION
All reactions were performed under N atmosphere using a steel pressure reactors with a capacity
2
of 150 mL (Paar) or of 5 L (Amar) equipped with a mechanical stirrer. All reagents were
purchased from commercial sources. PLA resins: PLLA (IngeoTM 2003D, NatureWorks LLC;
or Naturesse® PLA cup, Naturesse), PDLA (PURASORB® PD24, Purac), PDLLA (BIOCOP®,
BioMatPol); metallic Mg (Chempur), MgBu solution 1.0 M in heptanes, toluene (99.7%,
2
VWR), CD Cl and C D (SigmaꢀAldrich). Mentioned below alcohols were purchased in the
2
2
6
6
anhydrous form (water impurities < 0.005%) from SigmaꢀAldrich and were used without any
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purification: nꢀpropanol (99.7 %), isopropanol (99.5 %), nꢀbutanol (99.8 %), isobutanol (99.5
%), secꢀbutanol (99.5%), nꢀpentanol (≥99%), isopentanol (≥99%), nꢀhexanol (≥99 %), nꢀoctanol
(≥99%) and benzyl alcohol (99.8%). While the 2ꢀphenylethanol (>99%, Merck), 2,2,2ꢀ
0
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trifluoroethanol (99.8%, ROTH) and 2,2,2ꢀtrichloroethanol (99%, SigmaꢀAldrich) were dried by
storage in an inert atmosphere through 3ꢀ4 days with molecular sieves (3Å, SigmaꢀAldrich),
which were before thermal activated under vacuum, in 280 °C for 24 hours. In turn, methanol
(99.8%, Chempur), ethanol (99.8%, Chempur), tertꢀbutanol (≥99%, ROTH), 2ꢀethylhexanol
(≥99%, SigmaꢀAldrich,), N,Nꢀdimethylꢀ2ꢀaminoethanol (99.5%, SigmaꢀAldrich,), 2ꢀ
methoxyethanol (≥99.5%, SigmaꢀAldrich), cyclopentanol (99%, POCH) and cyclohexanol (99%,
POCH) were dried by reflux and distillation over metallic magnesium, usually through from 5 to
12h. 2,2,2ꢀtribromoethanol (98%, POCH) was three times purified by crystallization from
hexane. Allyl alcohol and phenol (≥99%, SigmaꢀAldrich) because of their high toxicity and very
high irritation effect were used without any purification. Reagents were purified by standard
methods: toluene was distilled over Na, CD Cl was distilled over P O . Nuclear magnetic
2
2
2
5
resonance (NMR) spectra were recorded at room temperature using a Bruker Avance 500 MHz
spectrometer. Chemical shifts are reported in parts per million and referenced to the residual
protons in deuterated solvents. Fourierꢀtransform infrared (FTꢀIR) spectra were recorded as
Nujol mulls using a Bruker 66/s FTꢀIR spectrometer. Elemental analysis was determined on a
PerkingꢀElmer 2400 CHN Elemental Analyzer. Thermogravimetric analysisꢀdifferential thermal
analysis (TGAꢀDTA) was performed under N atmosphere using a SETSYS 16/18 system
2
−1
(
SETARAM); a heating rate of 10 °C min was used. Thermal decompositions of PLA were
performed at 500 °C using a NT 1313 Furnace equipped with a KXP3+ thermostat
(NEOTHERM). The concentrations of the metal ions in polymer resins after thermal
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decompositions were determined by inductively coupled plasma atomic emission spectrometry
(
ICPꢀOES) using an ARL 3410 sequential spectrometer (Fisons Instruments). Size exclusion
chromatography (SEC) traces were recorded using an Agilent 1100 Series isocratic pump, a
degasser, an autosampler thermostatic box for columns, and a set of TSK Gel columns (2 ×
PLGel 5 microns MIXEDꢀC) at 30 °C. A Wyatt Optilab rEX interferometric refractometer and a
MALLS DAWN EOS Laser Photometer (Wyatt Technology Corp., USA) were applied as
detectors. Methylene chloride was used as the eluent, at a flow rate of 0.8 mL min−1. Mn and
Mw/Mn were calculated from the experimental traces using a Wyatt ASTRA v 4.90.07 program.
The value of molecular weight and PDIs index for high molecular weight polymers were
determinated by SECꢀMALLS technique. While the molecular weight of PLA oligomers were
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determined relative to polystyrene standards and the obtained M values were multiplied by
w
39
correction factor of 0.58. Matrixꢀassisted laser desorption ionization time of flight (MALDIꢀ
TOF) mass spectra were registered using a Voyager Elite mass spectrometer (PerSeptive
Biosystems, USA) equipped with a N laser (337 nm, 4ꢀns pulse width) and a timeꢀdelayed
2
extraction ion source. Mass spectra were obtained in the linear mode. The sample solution in
CH Cl (5 mg/mL) were mixed with a CH Cl solution of the matrix (dithranol 10 mg/mL). For
2
2
2
2
the enhancement of ion formation KTFA was added to the matrix. The circular dichroism spectra
were recorded on a JASCO Jꢀ715 spectrometer at 25°C in a 0.1cm path length quartz cuvette in
EtOH at sample concentration of 0.01 M. Optical rotation were measured on a Jasco DIPꢀ1000
polarimeter at 25°C using a Na lamp (589 nm) in a 50 mm cell length quartz cuvette in EtOH at
sample concentration of 3%. Because of the large discrepancies in the literature values of
2
5
specific rotation of LꢀEL, the reference value [α]D ꢀ10.5 (3.00, EtOH) was calculated based on
optical rotation measurements of commercial products with ee of 98 and 99% (SigmaꢀAldrich,
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VWR). Xꢀray diffraction (XRD) data were collected at 80 K for 1, and 100 K for 2, using
4
0
Xcalibur Ruby or Xcalibur PX diffractometer. The experimental details and crystal data are
given in Table S9 (SI). The structures were solved by direct methods and refined by the fullꢀ
0
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matrix leastꢀsquares method on F using the SHELXTL package. Nonꢀhydrogen atoms were
refined with anisotropic thermal parameters. All hydrogen atoms were positioned geometrically
and added to the structure factor calculations, but were not refined. The molecular graphics were
4
2
created using Diamond, version 3.1e. Crystallographic data for the structural analyses reported
in this paper have been deposited with the Cambridge Crystallographic Data Centre (CCDC),
numbers CCDC 1465242ꢀ1465243. Copies of the information may be obtained free of charge
from the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: +44ꢀ1223ꢀ336033;
Eꢀmail: deposit@ccdc.cam.ac.uk; Homepage: http://www.ccdc.cam.ac.uk).
General alcoholysis procedure. A typical reaction procedure was exemplified by the
synthesis of EL. Typically 10 g of PLA resin were placed in steel pressure reactor with a
capacity of 150 mL and dried under vacuum through 15 ꢀ 30 min. Next under N atmosphere
2
from 8.1 to 81.1 mL anhydrous ethanol were introduced in the amount from 1 to 10 equivalents
of EtOH per ester unit in polymer (la). Optionally, in a next step we can introduce MgBu as
2
precatalyst in an amount of 1 mol% per la unit. Then reaction were performed under autogenous
pressure of reactants, through 1h, at varying temperature range depends on the type of studied
reaction. For catalyst free reaction at temperature range from 200 to 260 °C, and from 80 to 200
1
°
C when MgBu was used. Conversion of PLA into lactate esters were determined from H
2
NMR spectra in C D by integrating methine peaks at 5.05, 4.22 and 4.09 ppm characteristic for
6
6
PLA and/or their oligomers endꢀcapped with ethoxy group (EL ), ethyl lactyllactate (EL ) and
n
2
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ethyl lactate (EL). The samples for NMR studies were taken from the reactor at 60 – 80 °C when
the postreaction mixtures were homogeneous.
Kinetic studies of EL alcoholysis. MgBu (0.1 mL, 0.1 mmol) was added to the 1.9 g of EL
2
2
2
10
11
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(
10 mmol) and from 0.58 to 5.8 mL of EtOH at stoichiometry of reactants EL /EtOH/Mg = 1/1,
2
1
2, 4, 10/0.01 at 25 and 60 °C. The reactions were monitored in time by H NMR spectroscopy.
Aliquots (0.1 mL) were periodically withdrawn from the reaction mixture, cooled in liquid N2,
and stopped by removing EtOH in vacuo. The residues were dissolved in C D and analyzed by
6
6
1
H NMR spectroscopy at room temperature.
Kinetic studies of PLLA alcoholysis. A typical PLLA alcoholysis procedure was exemplified
using MgBu as precatalyst. To a solution of PLLA (1.44g, 20 mmol): a) in 37.6 mL of toluene,
2
2.34 mL of EtOH, and from 0.1 to 0.5 mL of MgBu ([la] = 0.5 M; [Mg] = 2.5, 5, 7.5, 10, 12.5
2
0
0
mM); or b) in from 38.84 to 28.4 mL of toluene, from 1.16 to 11.6 mL of EtOH, and 0.2 ml of
MgBu were added ([la] = 0.5 M, [EtOH] = 0.5, 1, 2, 5 M). For all performed reactions, starting
2
0
total volume of toluene and EtOH were equal to 40 ml. The reactions were carry out in round
bottom flask with a volume of 150 mL under reflux at 90 °C. Aliquots (0.2 mL) were
periodically withdrawn from the reaction mixture, cooled in liquid N , and stopped by removing
2
1
EtOH in vacuo. The residues were dissolved in C D and analyzed by H NMR spectroscopy at
6
6
room temperature.
General AL purification procedure. After PLA alcoholysis, the individual components of
the reaction mixture were separated by distillation under reduced pressure using a rotary
evaporator. First, the alcohol was distilled off, then the corresponding AL and finally the AL2.
1
13
Then, isolated AL was purified by double distilling and characterized by H and C NMR
spectroscopy (Figures S15ꢀ42 in SI).
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1
methyl
L-lactate. H NMR (500 MHz; C D ): δ 4.02 (1H, qd, J = 6.9, 5.3 Hz, HOCH), 3.20
6 6
13
(
3H, s, OCH ), 2.77 (1H, d, J = 5.3 Hz, OH), 1.21 (3H, d, J = 6.9 Hz, CH ). C NMR (125
3
3
−
1
MHz; C D ): δ 176.1 (1C, C=O), 66.8 (1C, HOCH), 51.8 (1C, OCH ), 20.4 (1C, CH ). IR (cm ,
6
6
3
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
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1
2
3
4
5
6
7
8
9
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2
3
4
5
6
7
8
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2
3
4
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Nujol mull): 3448 (vs), 2987 (s), 2957 (s), 2910 (m), 2853 (m), 2588 (w), 2340 (w), 2065 (w),
1958 (w), 1889 (w), 1742 (vs), 1455 (s), 1438 (s), 1408 (w), 1373 (m), 1271 (s), 1223 (vs), 1133
(
vs), 1083 (w), 1048 (m), 980 (m), 918 (w), 843 (w), 758 (w), 649 (w), 535 (w), 494 (w).
1
ethyl
L
-lactate. H NMR (500 MHz; C D ): δ 4.05 (1H, qd, J = 6.9, 5.3 Hz, HOCH), 3.83 (2H,
6
6
m, OCH CH ), 2.78 (1H, d, J = 5.3 Hz, OH), 1.24 (3H, d, J = 6.9 Hz, CH ), 0.82 (3H, t, J = 7.1
2
3
3
13
Hz, OCH CH ). C NMR (125 MHz; C D ): δ 175.7 (1C, C=O), 66.8 (1C, HOCH), 61.2 (1C,
2
3
6
6
−
1
OCH CH ), 20.5 (1C, CH ), 14.0 (1C, OCH CH ). IR (cm , Nujol mull): 3458 (vs), 2985 (s),
2
3
3
2
3
2940 (m), 2909 (m), 2879 (w), 2816 (w), 2782 (w), 2676 (w), 2360 (w), 2339 (w), 2247 (w),
074 (w), 1979 (w), 1877 (w), 1737 (vs), 1451 (s), 1393 (s), 1372 (s), 1268 (vs), 1215 (vs), 1133
vs), 1097 (s), 1047 (s), 1020 (s), 932 (s), 860 (s), 796 (w), 757 (m), 649 (m), 582 (w), 534 (w),
2
(
5
03 (w), 451 (w), 437 (w), 421 (w), 402 (w), 388 (w), 374 (w).
1
ethyl
L
-lactyllactate. H NMR (500 MHz; C D ): δ 5.01 (1H, q, J = 7.1 Hz, OCH), 4.18 (1H,
6
6
q, J = 6.9 Hz, HOCH), 3.84 (2H, qd, J = 7.1, 2.2 Hz, OCH2CH3), 2.63 (1H, s, OH), 1.41 (3H, d,
J = 6.9 Hz, HOCHCH ), 1.16 (3H, d, J = 7.1 Hz, OCHCH ), 0.83 (3H, t, J = 7.1 Hz, OCH CH ).
3
3
2
3
1
3
C NMR (125 MHz; C D ): δ 175.3 (1C, (C=O)OC H ), 170.1 (1C, HOCH(C=O)), 69.5 (1C,
6
6
2
5
OCH), 66.9 (1C, HOCH), 61.3 (1C, OCH CH ), 20.7 (1C, HOCHCH ), 16.6 (1C, OCHCH ),
2
3
3
3
−1
1
3.9 (1C, OCH CH ). IR (cm , Nujol mull): 3484 (s), 2987 (s), 2942 (m), 2910 (m), 2881 (m),
2 3
2
612 (w), 2406 (w), 2339 (w), 2248 (w), 2081 (w), 1992 w), 1746 (vs), 1452 (s), 1394 (m), 1379
(
s), 1372 (s), 1349 (m), 1305 (m), 1273 (s), 1201 (vs), 1133 (vs), 1097 (vs), 1050 (s), 1020 (m),
9
43 (w), 918 (w), 863 (m), 761 (w), 749 (w), 685 (w), 576 (w), 545 (w).
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n-propyl
L-lactate. H NMR (500 MHz; C D ): δ 4.08 (1H, m, HOCH), 3.81 (2H, m,
6 6
OCH CH CH ), 2.85 (1H, d, J = 4.9 Hz, OH), 1.28 (2H, m, OCH CH CH ), 1.26 (3H, d, J = 6.9
2
2
3
2
2
3
13
Hz, CH ), 0.62 (3H, t, J = 7.4 Hz, OCH CH CH ). C NMR (125 MHz; C D ): δ 175.9 (1C,
3
2
2
3
6
6
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
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58
59
60
C=O), 66.8 (1C, HOCH), 66.7 (1C, OCH CH CH ), 22.0 (1C, OCH CH CH ), 20.5 (1C, CH ),
2
2
3
2
2
3
3
−
1
10.1 (1C, OCH CH CH ). IR (cm , Nujol mull): 3463 (vs), 2972 (vs), 2940 (s), 2898 (m), 2882
2
2
3
(m), 2684 (w), 2444 (w), 2349 (w), 2339 (w), 1968 (w), 1736 (vs), 1462 (s), 1391 (m), 1381 (m),
1372 (m), 1350 (w), 1311 (m), 1268 (s), 1213 (s), 1132 (vs), 1083 (m), 1045 (m), 977 (w), 951
(m), 930 (w), 911 (w), 892 (w), 855 (w), 837 (w), 756 (m), 648 (w), 540 (w), 457 (w).
1
isopropyl
L
-lactate. H NMR (500 MHz; C D ): δ 4.90 (1H, hept, J = 6.3 Hz, OCH(CH ) ),
6
6
3 2
4
.09 (1H, q, J = 6.9 Hz, HOCH), 3.05 (1H, d, s, OH), 1.27 (3H, d, J = 6.9 Hz, CH ), 0.93 (6H,
3
13
dd, J = 6.3, 1.0 Hz, OCH(CH ) ). C NMR (125 MHz; C D ): δ 175.4 (1C, C=O), 68.9 (1C,
3
2
6
6
−1
OCH(CH ) ), 66.9 (1C, HOCH), 21.5, 21.4 (2C, OCH(CH ) ), 20.5 (1C, CH ). IR (cm , Nujol
3
2
3 2
3
mull): 3472 (vs), 2983 (vs), 2940 (vs), 2880 (m), 2738 (w), 2081 (w), 1733 (vs), 1647 (w), 1469
s), 1457 (s), 1387 (s), 1376 (vs), 1271 (vs), 1219 (vs), 1183 (s), 1134 (vs), 1108 (vs), 1043 (s),
(
983 (w), 942 (s), 911 (m), 862 (m), 822 (m), 759 (m), 636 (w), 508 (w), 464 (w), 426 (w).
1
allyl
L
-lactate. H NMR (500 MHz, C D ): δ 5.59 (1H, ddt, J = 17.2, 10.5, 5.7 Hz,
6
6
OCH CH=CH ), 5.01 (1H, ddd, J = 10.4, 2.6, 1.3 Hz, OCH CH=CH ), 4.92 (1H, ddd, J = 10.4,
2
2
2
2
2
.6, 1.3 Hz, OCH CH=CH ), 4.33 (2H, qdt, J = 13.2, 5.7, 1.4 Hz, OCH CH=CH ), 4.10 (1H, m,
2 2 2 2
1
3
HOCH), 3.03 (1H, d, J = 5.2 Hz, OH), 1.26 (3H, d, J = 6.9 Hz, CH ). C NMR (125 MHz;
3
C D ): δ 175.4 (1C, C=O), 132.1 (1C, OCH CH=CH ), 118.2 (1C, OCH CH=CH ), 66.9 (1C,
6
6
2
2
2
2
−
1
HOCH), 65.7 (1C, OCH ), 20.4 (1C, CH ). IR (cm , Nujol mull): 3457 (vs), 3088 (m), 2986 (s),
2
3
2941 (m), 2886 (m), 2586 (w), 2453 (w), 2339 (w), 2112 (w), 1968 (w), 1740 (vs), 1649 (m),
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2
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2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
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4
4
4
4
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456 (s), 1425 (m), 1414 (m), 1374 (m), 1273 (s), 1209 (vs), 1131 (vs), 1088 (m), 1045 (s), 980
(s), 931 (s), 858 (w), 837 (w), 757 (m), 656 (w), 556 (w).
1
n-butyl
L
-lactate. H NMR (500 MHz; C D ): δ 4.08 (1H, q, J = 6.9 Hz, HOCH), 3.88 (2H,
6
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
qt, J = 10.8, 6.6 Hz, OCH (CH ) CH ), 2.83 (1H, s, OH), 1.30 – 1.23 (m, 2H,
2
2 2
3
OCH CH CH CH ), 1.26 (3H, d, J = 6.9 Hz, CH ), 1.07 (2H, dq, J = 14.8, 7.4 Hz,
2
2
2
3
3
1
3
O(CH ) CH CH ), 0.70 (3H, t, J = 7.4 Hz, O(CH ) CH ). C NMR (125 MHz; C D ): δ 175.9
2
2
2
3
2 3
3
6
6
(1C, C=O), 66.8 (1C, HOCH), 65.1 (1C, OCH (CH ) CH ), 30.7 (1C, OCH CH CH CH ), 20.5
2 2 2 3 2 2 2 3
−
1
(1C, CH ), 19.1 (1C, O(CH ) CH CH ), 13.6 (1C, O(CH ) CH ). IR (cm , Nujol mull): 3465
3
2 2
2
3
2 3
3
(
vs), 2962 (vs), 2937 (vs), 2876 (s), 2740 (w), 2669 (w), 2447 (w), 2396 (w), 2349 (w), 2306
w), 2172 (w), 1737 (vs), 1650 (w), 1643 (w), 1632 (w), 1573 (w), 1563 (w), 1461 (m), 1435
(
(w), 1391 (m), 1383 (m), 1371 (m), 1270 (vs), 1212 (vs), 1132 (vs), 1084 (m), 1060 (m), 1044
s), 1019 (m), 997 (m), 965 (m), 944 (m), 843 (m), 809 (w), 757 (m), 739 (m), 651 (w), 532 (w),
(
5
11 (w), 484 (w), 469 (w), 453 (w), 437 (w), 421 (w), 388 (w).
1
isobutyl
L
-lactate. H NMR (500 MHz; C D ): δ 4.09 (1H, qd, J = 6.9, 4.8 Hz HOCH), 3.76
6
6
(1H, dd, J = 10.6, 6.7 Hz, OCH CH(CH ) ), 3.66 (1H, dd, J = 10.6, 6.7 Hz, OCH CH(CH ) ),
2 3 2 2 3 2
2.88 (1H, d, J = 4.8 Hz, OH), 1.62 (1H, dp, J = 13.4, 6.7 Hz, OCH CH(CH ) ), 1.27 (3H, d, J =
2
3 2
1
3
6
.9 Hz, CH ), 0.65 (6H, dd, J = 6.7, 1.6 Hz, OCH CH(CH ) ). C NMR (125 MHz; C D ): δ
3 2 3 2 6 6
175.8 (1C, C=O), 71.2 (1C, OCH CH(CH ) ), 66.8 (1C, HOCH), 27.8 (1C, OCH CH(CH ) ),
2
3 2
2
3 2
−
1
20.6 (1C, CH ), 18.8 (1C, OCH CH(CH ) ), 18.7 (1C, OCH CH(CH ) ). IR (cm , Nujol mull):
3
2
3 2
2
3 2
3
464 (vs), 2965 (vs), 2897 (s), 2877 (s), 2728 (w), 1736 (vs), 1471 (s), 1395 (m), 1379 (n), 1371
(
n), 1343 (w), 1268 (s), 1213 (vs), 1131 (vs), 1083 (m), 1046 (s), 988 (s), 968 (m), 947 (m), 935
m), 874 (w), 835 (w), 820 (w), 799 (w), 757 (w), 650 (w), 540 (w), 432 (w).
(
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ACS Catalysis
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1
sec-butyl
1H, m, HOCH), {2.90, 2.88 (1H, d, J = 5.3 Hz, OH)}, {1.32, 1.19 (m, 2H,
OCH(CH )CH CH )}, {1.27, 1.26 (3H, d, J = 6.9 Hz, CH )}, {0.94, 0.93 (3H, d, J = 6.3 Hz,
L-lactate. H NMR (500 MHz; C D ): δ 4.80 (1H, m, OCH(CH )CH CH ), 4.09
6 6 3 2 3
(
3
2
3
3
10
11
12
13
14
15
16
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19
20
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52
53
54
55
56
57
58
59
60
13
OCH(CH )CH CH ), {0.65, 0.63 (3H, m, OCH(CH )CH CH )}. C NMR (125 MHz; C D ): δ
3
2
3
3
2
3
6
6
{175.7, 175.6 (1C, C=O)}, 73.5 (1C, OCH(CH )CH CH ), {66.9, 66.8 (1C, HOCH)}, {28.9,
3
2
3
2
8.7 (1C, OCH(CH )CH CH )}, {20.7, 20.6 (1C, CH )}, {19.3, 19.2 (1C, OCH(CH )CH CH )},
3 2 3 3 3 2 3
−
1
{
9.5 (1C, OCH(CH )CH CH )}. IR (cm , Nujol mull): 3471 (vs), 2977 (vs), 2939 (s), 2882 (s),
3
2
3
2741 (w), 2338 (w), 2079 (w), 1980 (w), 1732 (vs), 1458 (s), 1380 (s), 1268 (vs), 1219 (vs),
179 (m), 1128 (vs), 1114 (vs), 1094 (s), 1043 (s), 1027 (m), 995 (m), 968 (m), 937 (m), 886
1
(
m), 865 (s), 829 (w), 782 (w), 758 (w), 652 (w), 575 (w), 542 (w), 493 (w), 435 (w).
1
n-pentyl
L
-lactate. H NMR (500 MHz; C D ): δ 4.11 (1H, qd, J = 6.9, 5.4 Hz, HOCH), 3.90
6
6
(
2H, m, OCH (CH ) CH ), 2.97 (1H, d, J = 5.4 Hz, OH), 1.32 (2H, m, OCH CH (CH ) CH ),
2 2 3 3 2 2 2 2 3
1
.28 (3H, d, J = 6.9 Hz, CH ), 1.07 (4H, m, O(CH ) (CH ) CH ), 0.77 (3H, t, J = 7.1 Hz,
3 2 2 2 2 3
13
O(CH ) CH ). C NMR (125 MHz; C D ): δ 175.9 (1C, C=O), 66.9 (1C, HOCH), 65.4 (1C,
2
4
3
6
6
OCH (CH ) CH ), 28.4 (1C, OCH CH (CH ) CH ), 28.1 (1C, O(CH ) CH CH CH ), 22.4 (1C,
2
2 3
3
2
2
2 2
3
2 2
2
2
3
−
1
O(CH ) CH CH ), 20.6 (1C, CH ), 14.0 (1C, O(CH ) CH ). IR (cm , Nujol mull): 3465 (s),
2
3
2
3
3
2 4
3
2
959 (vs), 2935 (vs), 2874 (s), 2863 (s), 2735 (w), 1736 (vs), 1467 (s), 1459 (s), 1380 (m), 1372
(
m), 1265 (s), 1210 (vs), 1132 (vs), 1080 (m), 1046 (s), 969 (m), 930 (w), 868 (w), 757 (w), 730
(w), 648 (w), 517 (w), 467 (w).
1
isopentyl
L
-lactate. H NMR (500 MHz; C D ): δ 4.11 (1H, m, HOCH), 3.95 (2H, qt, J =
6
6
1
0.9, 6.8, OCH CH CH(CH ) ), 2.95 (1H, d, J = 3.5 Hz, OH), 1.40 (1H, m, O(CH ) CH(CH ) ),
2 2 3 2 2 2 3 2
1
.28 (3H, d, J = 6.9 Hz, CH ), 1.23 (2H, ddd, J = 14.0, 6.9, 1.3 Hz, OCH CH CH(CH ) ), 0.70
3
2
2
3 2
13
(
6H, dd, J = 6.7, 1.7 Hz, O(CH ) CH(CH ) ). C NMR (125 MHz; C D ): δ 175.9 (1C, C=O),
2 2 3 2 6 6
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