Chemical Recycling of End-of-Life Poly(lactide) via Zinc-Catalyzed Depolymerization and Polymerization

Article abstract of DOI:10.1002/open.202000243

The chemical recycling of poly(lactide) was investigated based on depolymerization and polymerization processes. Using methanol as depolymerization reagent and zinc salts as catalyst, poly(lactide) was depolymerized to methyl lactate applying microwave heating. An excellent performance was observed for zinc(II) acetate with turnover frequencies of up to 45000 h^?1. In a second step the monomer methyl lactate was converted to (pre)poly(lactide) in the presence of catalytic amounts of zinc salts. Here zinc(II) triflate revealed excellent performance for the polymerization process (yield: 91 %, M_n ～8970 g/mol). Moreover, the (pre)poly(lactide) was depolymerized to lactide, the industrial relevant molecule for accessing high molecular weight poly(lactide), using zinc(II) acetate as catalyst.

Full text of DOI:10.1002/open.202000243

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doi.org/10.1002/open.202000243
ChemistryOpen
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Chemical Recycling of End-of-Life Poly(lactide) via Zinc-
Catalyzed Depolymerization and Polymerization
Even Cheung, Christoph Alberti, and Stephan Enthaler*[a]
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The chemical recycling of poly(lactide) was investigated based
on depolymerization and polymerization processes. Using
methanol as depolymerization reagent and zinc salts as catalyst,
poly(lactide) was depolymerized to methyl lactate applying
microwave heating. An excellent performance was observed for
zinc(II) acetate with turnover frequencies of up to 45000 hÀ 1. In
a second step the monomer methyl lactate was converted to
(pre)poly(lactide) in the presence of catalytic amounts of zinc
salts. Here zinc(II) triflate revealed excellent performance for the
polymerization process (yield: 91%, Mn ~8970 g/mol). More-
over, the (pre)poly(lactide) was depolymerized to lactide, the
industrial relevant molecule for accessing high molecular
weight poly(lactide), using zinc(II) acetate as catalyst.
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Plastics derived from renewable resources can be valuable for a
future circular and resource-efficient chemistry/economy. More-
Scheme 1. Chemical recycling concept for PLA.
over, this type of plastics have some advantages compared to
fossil resource-based plastics.[1,2] In recent times, poly(lactide)
plastics (PLA, 1, Scheme 1) have been established as most
important representative produced on a ~0.2 m t/y scale.[3] PLA
is accessible by a multi-step process. Initially, in biological
transformation carbon dioxide, water and solar energy are
converted to biomass, which is subsequently converted to lactic
acid.[4] Afterwards, lactic acid is subjected to polycondensation
to generate polymer/oligomer 1. Another option is the
oligomerization of lactic acid and subsequent degradation to
lactide, which can be subjected to ring-opening polymerization
to form 1. PLA can be used in a wide range of applications (e.g.,
food packaging, pharmaceuticals). Nevertheless, after complet-
ing the purpose PLA is designated as End-of-Life PLA (EoL-PLA)
and it has to be treated as waste. As a value of PLA the bio-
degradability has been discussed.[5] However, the dwell time in
composting plants is too short for complete degradation. As an
alternative EoL-PLA can be degraded by incineration to release
the stored energy and produce CO2 and water, which can go
through the processes again (vide supra). In contrast to fossil
resource-based plastics, a circular process and carbon-neutrality
is attainable on a short time scale.[6,7] Nevertheless, a drawback
of this approach is the necessity for substitution of the EoL-PLA
by fresh PLA, which requires land use and cultivation time,
therefore a competition with other agricultural goods can
occur. To overcome these issues/limitations a recycling of PLA
plastics can be a useful tool. In this regard, the chemical
recycling based on depolymerizations and polymerizations can
offer benefits.[8] In more detail, the depolymerization process
transforms the polymer to the monomer, while the polymer-
ization regenerates the polymer from the monomer (Scheme 1).
Numerous chemical recycling methods for EoL-PLA have been
accounted.[9,10,11] Especially, the alcoholysis of EoL-PLA has been
studied. In more detail, the EoL-PLA is reacted with methanol to
generate methyl lactate (2) containing the monomeric unit of
PLA. The methyl lactate can easily be converted to lactic acid
the starting material for 1 (industrially established route). On
the other hand, 2 can be polymerized to PLA (less investigated
route).[12,13] In both processes methanol is formed, which can be
resent to the depolymerization process. Notably, for performing
the depolymerization as well as the polymerization catalysts are
essential. In case of methanolytic depolymerization catalytic
amounts of zinc complexes among others have been success-
fully applied.[11] However, the application of complexes requires
the upstream synthesis of the complex and the corresponding
ligand. Therefore, the use of simple zinc salts can be beneficial
with respect to catalyst costs and resource-efficiency. Indeed,
[a] E. Cheung, C. Alberti, Dr. S. Enthaler
Universität Hamburg
Institut für Anorganische und Angewandte Chemie
Martin-Luther-King-Platz 6, D-20146 Hamburg (Germany)
E-mail: stephan.enthaler@chemie.uni-hamburg.de
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/open.202000243
© 2020 The Authors. Published by Wiley-VCH GmbH. This is an open access
article under the terms of the Creative Commons Attribution Non-Com-
mercial NoDerivs License, which permits use and distribution in any med-
ium, provided the original work is properly cited, the use is non-commercial
and no modifications or adaptations are made.
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Collinson and coworker reported the use of simple zinc(II)
acetate as catalyst (1.4 mol%) resulting in a yield of 70% of 2
after 15 hours at reflux.[11c] Moreover, for the synthesis of 1
starting from 2 metal catalysis has been reported.[13]
However, based on the general reaction path we wonder if
catalysts based on simple zinc salts can enable the depolymeri-
zation as well as the polymerization, which can add some value
to a circular and resource-efficient chemistry/economy.
Initially, as model for optimization of the depolymerization
reaction conditions transparent end-of-life plastic cups contain-
ing poly(lactide) (1a, Mn =57,500 g/mol, Mw =240,800 g/mol,
D=4.2, PLLA) as major component was investigated (Table 1).
Small pieces of the PLA 1a (2.78 mmol with respect to the
monomer unit, implying that 1a contains 100% of PLA), an
excess of methanol (67.5 equivalents with respect to the
monomer unit of 1a) and catalytic amounts of Zn(OTf)2
(1.0 mol% with respect to the monomer unit of 1a) were added
to a microwave glass vial. After sealing the vial the mixture was
entry 2). The yield of 2 was calculated by relating the integral of
the CH3CHOH-function of 2 to the integral of the CH3CHOR-
function of the polymer/oligomer leftover (~5.17 ppm). More-
over, other zinc salts were tested revealing also excellent yield
for Zn(OAc)2, while zinc halides showed an inferior performance
(Table 1, entries 3–6).[14] Importantly, in the absence of any zinc
salt no product formation was observed (Table 1, entry 1).
In case of Zn(OAc)2 chemical 2 was isolated in 92% yield
from the reaction mixture. In detail, the methanol and
compound 2 were separated from the catalyst by distillation
under vacuum to circumvent side/back processes (polymer-
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°
ization). Afterwards, the mixture of 2 (b.p. ~144 C) and
°
methanol (b.p. ~65 C) was separated by distillation.Next the
°
reaction temperature was decreased stepwise to 100 C show-
°
ing still a good yield of 91% at 100 C (Table 1, entries 9–11).
Interestingly, with a Zn(OAc)2 loading of 0.25 mol% an
excellent performance was still noticed, which corresponds to a
turnover frequency (TOF) of 2400 hÀ 1 (Table 1, entry 13). By
°
°
stirred and reacted at 160 C applying microwave heating (MW)
increasing the temperature to 180 C a further increase of the
for 10 min, meanwhile the PLA was completely dissolved
(Table 1, entry 2). After cooling to ambient temperature an
aliquot was added to CDCl3 for determination of the yield/
TOF to 45000 hÀ 1 was observed (Table 1, entry 16). Noteworthy,
the Zn(OAc)2 catalyst outperform our earlier reported systems,
e.g. DMAP (~178 hÀ 1), KF (~816 hÀ 1), bismuth subsalicylate (~
13800 hÀ 1) or Sn(Oct)2 (~39600 hÀ 1).[10j–m] Finally, the reaction
was performed under conventional heating under refluxing
conditions (Table 1, entry 18). Here 2 was obtained in >99%
yield after 24 hours.
1
conversion by H NMR analysis. The existence of the depolyme-
rization product 2 was proven by the presence of a singlet at δ
= 3.78 ppm, which corresponds to the methyl ester function-
ality (-C(=O)OCH3). Additionally, a doublet at δ = 1.41 ppm
(3JHH =6.91 Hz, CH3CH-function) and a quartet of doublets at δ
= 4.28 ppm (3JHH =6.91 Hz, 3JHH =5.20 Hz, CH3CHOH-function)
were observed, which matches to compound 2. For Zn(OTf)2 as
catalyst an excellent yield of >99% was detected (Table 1,
Afterwards, the protocol was tested in the depolymerization
of different PLA-products (Table 2). In all cases excellent
performance with yields >99% was observed for the Zn(OAc)2
catalyst. Moreover, the depolymerization of PLA 1a was studied
Table 1. Depolymerization of 1a using zinc catalysis – optimization of reaction conditions.
[c]
Entry
Catalyst
Catalyst loading [mol%]
MeOH [equiv.]
T [ C]
t [min]
Yield 2 [%][b]
TOF [hÀ 1
]
°
1
2
3
4
5
6
–
–
67.5
67.5
67.5
67.5
67.5
67.5
67.5
67.5
67.5
67.5
67.5
67.5
67.5
67.5
67.5
67.5
56.3
67.5
160
160
160
160
160
160
160
160
140
120
100
160
160
160
160
180
160
64.7
10
10
10
10
10
10
10
10
10
10
10
10
10
5
<1
>99
60
<1
600
360
168
600
600
<1
Zn(OTf)2
ZnCl2
ZnBr2
Zn(OAc)2
Zinc(II) methacrylate
Poly(zinc(II) methacrylate)
Poly(zinc(II) methacrylate)
Zn(OAc)2
Zn(OAc)2
Zn(OAc)2
Zn(OAc)2
Zn(OAc)2
Zn(OAc)2
Zn(OAc)2
Zn(OAc)2
Zn(OAc)2
Zn(OAc)2
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
0.25
0.5
0.25
0.1
0.5
1.0
28
>99 (92)[d]
>99
<1
7
8[e]
9
<1
<1
>99
>99
91
>99
>99
>99
72
75
>99
>99
600
600
546
1200
2400
2400
3456
45000
600
4
10
11
12
13
14
15
16
17
18[f]
5
1
10
1440
[a] Conditions: 1a (transparent PLA cup), 2.78 mmol with respect to the monomer unit, it is presumed that 1a is composed of 100% of PLA), catalyst (0-
1.0 mol%, 0–0.0278 mmol with respect to the monomer unit of 1a), MeOH (56.3-67.5 equiv. with respect to the monomer unit of 1a), temperature: 100–
1
À 1
°
180 C (microwave heating), time: 1–10 min. [b] The yield of 2 bases on H NMR spectroscopy. [c] The TOF was calculated: (mole product/mole catalyst)*h
The TOF was calculated using the yield of 2 after the designated time. [d] Isolated yield. [e] Anisole (1 g) or 1,2,4-trichlorbenzene (1 g) or THF (3 g) as
cosolvent. [f] Reaction was performed under reflux using oil bath heating.
.
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with an equimolar amount of another polymer (based on its
repeating unit) and the mixture was subjected to depolymeriza-
tion. In all cases excellent catalyst performance was observed
for the conversion of PLA to methyl lactate. Importantly, most
additional polymers were recovered. Only in case of poly
(bisphenol A carbonate) the depolymerization products bi-
sphenol A and dimethyl carbonate were detected by 1H NMR.
Next the depolymerization product 2 was applied as
starting material for the resynthesis of polymer 1 (Table 4). In
this regard, a mixture of methyl lactate and catalytic amounts
Table 2. Zn(OAc)2 catalyzed depolymerization of PLA goods.
1
2
3
4
5
6
7
8
9
Entry[a] Product
Yield of 2
[%][b]
1
2
3
4
5
6
7
8
transparent cup (1 oz) (1a)
>99
>99
>99
>99
>99
>99
>99
>99
transparent cup (250 mL) (1b)
transparent disposable food box (1c)
transparent sushi box cover (1d)
transparent bottle (1e)
drinking straw with green strips (1f)
disposable fork with talcum powder (1g)
lid for espresso mugs (contains ~20-30% talcum
powder) (1h)
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°
of Zn(OTf)2 was heated at 130 C for 24 hours (Table 1, entry 2).
After cooling to ambient temperature an aliquot was taken for
1
determination of the yield/conversion by H NMR analysis. The
existence of the polymerization product 1b was proven by the
presence of a multiplet at δ = ~5.10 ppm, which corresponds
to the CH3CHO-functionality of the polymer, while the CH3CHO-
functionality of the monomer can be detected at δ = 4.28 ppm
(vide supra). Additionally, a quartet was observed at δ =
5.02 ppm (J=6.66 Hz, 1H, CH3CH-), which corresponds to L-
lactide (3).[15] In consequence a conversion of 90% and a NMR
yield of 85% of 1b and 5% of 3 was calculated. Moreover, a
number average molecular weight (Mn) of ~4614 g/mol was
calculated based on the end-groups of the PLA. In addition,
different zinc(II) salts were tested as catalysts (Table 4, entries
3–5). Good yields 69–70% for 1b with Mn 576-1243 g/mol and
2–15% for 3 were observed for all zinc salts, while in the
absence of any zinc source no product formation was noticed
(Table 4, entry 1). Reducing the loading of the catalyst revealed
a decrease of product yield accompanied by an increase of
9
lid for coffee mugs (1i)
black lid for coffee mugs (1j)
sushi box (black base) (1k)
>99
>99
>99
>99
10
11
12
blue ice cream spoon (1l)
[a] Conditions: 1a–1l (200.0 mg, 2.78 mmol with respect to the monomer
unit), Zn(OAc)2 (5.1 mg, 1.0 mol%, 0.0278 mmol with respect to the
monomer unit of 1), MeOH (6.0 g, 187.3 mmol, 67.5 equiv. with respect to
°
monomer unit of 1), temperature: 160 C (microwave heating), time:
10 min. [b] The yield of 2 bases on H NMR spectroscopy. The amount of
substance of 2 was linked to the amount of substance of PLA.
1
in the presence of different kinds of polymers, which can be
beneficial for plastic separation (Table 3). Therefore, best
conditions for 1a were applied (1.0 mol% Zn(OAc)2, 160 C,
°
67.5 equiv. methanol, 5 min). In more detail, PLA 1a was mixed
Table 3. Zn(OAc)2 catalyzed depolymerization of PLA in the presence of additional polymers.
Entry[a]
Additional polymer B
Yield of 2 [%][b]
Yield [%][c]
1
2
3
–
>99
>99
>99
<1
<1
<1
Poly(ethylene terephthalate) (PET)
Poly(ɛ-caprolactone) (PCL)
(Mn ~80,000 g/mol)
Poly((R)-hydroxybutyric acid)
Poly(bisphenol A carbonate)
Nylon 6
4
5
6
7
8
>99
>99
>99
>99
>99
<1
87[d]
<1
<1
<1
Poly(phenylene sulfide) (PPS)
Poly(ethylene) (PE)
(Mn ~1,700 g/mol)
Poly(styrene) (PS)
9
>99
>99
>99
>99
>99
>99
<1
<1
<1
<1
<1
<1
(Mw ~35,000 g/mol)
Poly(vinyl chloride) (PVC)
(Mw ~48,000 g/mol)
Poly(vinyl alcohol)
(Mw ~67,000 g/mol)
Poly(ethylene glycol) methyl ether
(Mn ~5,000 g/mol)
Epoxy resin
(Mn ~1,750 g/mol)
Poly(dimethylsiloxane)
10
11
12
13
14
[a] Conditions: 1a (200.0 mg, 2.78 mmol with respect to the monomer unit), polymer B (2.78 mmol with respect to the monomer unit), Zn(OAc)2 (5.1 mg,
1.0 mol%, 0.0278 mmol with respect to the monomer unit of 1), MeOH (6.0 g, 187.3 mmol, 67.5 equiv. with respect to monomer unit of 1), temperature:
1
°
160 C (microwave heating), time: 5 min. [b] The yield of 2 bases on H NMR spectroscopy. The amount of substance of 2 was linked to the amount of
substance of PLA. [c] The yield was determined by 1H NMR for depolymerization product(s) of the additional polymer. [d] Bisphenol A and dimethylcarbonate.
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Table 4. Polymerization of 2 using zinc catalysis – optimization of reaction conditions.[a]
1
2
3
4
5
6
7
8
9
°
Entry
Catalyst
Catalyst loading [mmol]
T [ C]
t [h]
Conv. 2 [%][b]
Yield 3 [%][b]
Yield 1b [%][b]
Mn (1b) [g/mol][c]
1
2
3
4
5
7
8
9
10
11
12[d]
13
–
–
130
130
130
130
130
130
130
130
130
130
111
80
24
24
24
24
24
24
24
24
24
48
24
24
<1
90
85
79
72
93
83
67
35
91
69
10
<1
5
15
9
2
7
15
5
<1
5
10
0
<1
85
69
70
70
86
68
62
35
86
59
10
<1
Zn(OTf)2
ZnCl2
ZnBr2
1.0
1.0
1.0
1.0
0.5
0.1
0.5
0.25
1.0
1.0
1.0
n ~53.4; Mn ~4614
n ~6.5; Mn ~576
n ~14.2; Mn ~1243
n ~8.4; Mn ~739
n ~14.9; Mn ~1295
n ~8.7; Mn ~762
n ~3.5; Mn ~319
n ~1.8; Mn ~168
n ~104.0; Mn ~8970
n ~10.3; Mn ~904
-
10
11
12
13
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23
24
25
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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
Zn(OAc)2
Zn(OTf)2
Zn(OTf)2
Zn(OAc)2
Zn(OAc)2
Zn(OTf)2
Zn(OAc)2
Zn(OAc)2
1
°
[a] Reaction conditions: 2 (1.92 mmol), catalyst (0–0.0192 mmol), 80–130 C, 24–48 h. [b] The conversion was determined by H NMR. [c] Calculation based on
1H NMR data. [d] Toluene as solvent (2.0 mL).
°
lactide formation (Table 4, entries 7–10). Increasing the reaction
time revealed the formation of a higher molecular weight PLA
(Mn ~8970 g/mol) (Table 4, entry 11). Moreover, at lower
temperature a reduced yield was noticed (Table 4, entry 13).
Moreover, for plastic cups (1a) a scale-up reaction was
carried out (Scheme 2). In more detail, the reaction was
performed in accordance to the reaction conditions stated in
Table 1 entry 15 by using 20.0 g of 1a. After purification/
isolation by distillation 25.7 g of 2 were isolated, which
corresponds to a yield of 98%. Next, the isolated compound 2
was subjected to the polymerization protocol. Two grams of 2
were reacted with catalytic amounts of Zn(OTf)2 at 130 C for
24 hours. Polymer 1b was obtained in 92% NMR yield and
lactide in 2% NMR yield. In addition, the mixture of polymer 1b
and lactide (3) was reacted with catalytic amounts of Zn(OAc)2
°
at 210 C and 6 mbar. As product lactide was isolated in 49%
yield (L-lactide: 44%).[16]
In summary, we have established an easy-to-adopt chemical
recycling method for EoL poly(lactide) (PLA). In more detail, the
method bases on the zinc-catalyzed depolymerization of PLA
applying methanol as depolymerization reagent to obtain
methyl lactate as product. Noteworthy, as catalyst simple zinc
salts are applied to realize excellent yields of 2 (>99%) within
short reaction times (10 min) under microwave heating. Apply-
ing Zn(OAc)2 as catalyst turnover frequencies up to ~45000 hÀ 1
were obtained. The principle of operation of the concept was
successfully proven in the conversion of a variety of PLA goods.
In a second part methyl lactate was reacted with zinc salts to
generate new low molecular weight PLA. Here good perform-
ance was observed for Zn(OTf)2 with yields of 91% and Mn
~8970 g/mol. The synthesized PLA was converted to lactide by
zinc catalyzed depolymerization. The lactide can be used to
access high molecular weight PLA.
Acknowledgements
Financial support from the Universität Hamburg is gratefully
acknowledged. We thank Prof. Dr. Axel Jacobi von Wangelin, Dr.
Felix Scheliga and Dr. Dieter Schaarschmidt (all UHH) for general
discussions and support.
Conflict of Interest
Scheme 2. Scale-up of end-of-life PLA depolymerization and polymerization
of methyl lactate.
The authors declare no conflict of interest.
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Keywords: green chemistry · catalysis · polymers · recycling ·
depolymerization
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Manuscript received: August 25, 2020
Revised manuscript received: October 12, 2020
ChemistryOpen 2020, 9, 1224–1228
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