Low-temperature iron-catalyzed depolymerization of polyethers

Article abstract of DOI:10.1002/cssc.201200125

Iron will: The iron-catalyzed depolymerization of a range of polyethers is studied. The products of the depolymerization reactions are chloroesters, which can be used as starting materials for new polymers. In the presence of simple iron salts extraordinary catalyst activities and selectivities are feasible at low temperature. Copyright

Full text of DOI:10.1002/cssc.201200125

DOI: 10.1002/cssc.201200125
Low-Temperature Iron-Catalyzed Depolymerization of Polyethers
Stephan Enthaler* and Maik Weidauer[a]
Dedicated to Professor Matthias Beller on the occasion of his 50th birthday.
Nowadays synthetic polymers (or: plastics) play a fundamental,
ubiquitous role in everyday life.[1] In view of the steadily in-
creasing demand for polymeric materials (growing rate ca. 9%
p.a.), rising amounts of plastic waste represents the downside
of this success.[2] Many polymers are based on monomers pro-
duced from fossil fuels, which are gradually decreasing, and
their degradation is simply a matter of time.[3] Moreover, waste
polymers are commonly thermally decomposed for energy
production (thermal recycling) or converted to low-quality ma-
terials (downcycling).[4] Only a small fraction of the polymer
waste is submitted to degradation and conversion into new
and high-quality materials.[5] Accordingly, the recycling of poly-
mers continues to be a significant and topical subject in
chemistry.[6]
obtain suitable products (chloroesters) appropriate for follow-
up chemistry (Figure 1).[15]
Initial studies on the influence of the reaction conditions
were carried out with tetraethylene glycol dimethyl ether (1)
as the model substrate for polyethers and benzoyl chloride as
nucleophile (Table 1). Firstly, different iron salts were examined
Table 1. Iron-catalyzed depolymerization of tetraethylene glycol dimethyl
ether (1).
In this context low-temperature depolymerization methods
are an interesting approach towards using plastic waste as
a source of raw materials. Specifically, the polymer is converted
to functional synthons or monomers, which can reprocessed
as starting materials in polymerization chemistry.[7] Interesting
polymeric materials for this method are polyethers (e.g., poly-
tetrahydrofuran, polyethylene glycol, polyethylene oxide, poly-
propylene oxide), which are omnipresent in modern life. To re-
alize a low-temperature approach the application of catalysts
can be useful.[8,9] In the presence of suitable transition metals
the ether functionality can be activated by coordination to the
metal centre and following this event an attack with nucleo-
philes (e.g., acid chlorides) is feasible (Figure 1).[10,11] By contin-
uous diminution of the polymer by this procedure an increas-
ing amount of the corresponding chloroester will be obtained
as well-defined molecule. Chloroesters can be easily trans-
formed into other compounds (e.g., vinyl esters, halohydrins,
vinyl chloride) that can be applied as monomers in polymeri-
zation processes.[12] Moreover, the selection of the catalyst is of
great importance because modern research focuses on the
substitution of expensive and toxic transition metals. Indeed,
during the last years the chemistry of cheap, abundant, and
low-toxicity “bio”metals as catalyst core (e.g., iron) has been re-
discovered and numerous exciting reactions have been report-
ed.[13,14] In this regard, we describe the first iron-catalyzed de-
polymerization method, applying widely available iron salts to
Entry[a]
Iron
source
Amount
[mol%]
T
[8C]
Conv. (1)
[%]
Yield (3)[b]
[%]
1
2
3
4
5
6
7
8
9[c]
10
11
12
13
–
–
5
5
5
5
5
2.5
1.0
5
5
5
5
5
130
130
130
130
130
130
130
130
130
100
80
<1
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
<1
<1
87
81
69
71
73
82
68
84
86
69
25
<1
FeCl2·4H2O
FeCl3
FeBr3
Fe(ClO4)2·H2O
Fe(ClO4)3·4H2O
FeCl2·4H2O
FeCl2·4H2O
FeCl2·4H2O
FeCl2·4H2O
FeCl2·4H2O
FeCl2·4H2O
FeCl2·4H2O
60
40
[a] Reaction conditions: 1 (0.72 mmol), iron source (1.0–5.0 mol%), 2
(7.2 mmol), 40–1308C, 24 h. [b] Determined by GC methods using anisole
as an internal standard. Products were compared with authentic samples.
[c] 12 h.
as catalyst precursor under non-inert conditions (entries 1–6).
In all cases full conversions of 1 were observed and no signifi-
cant difference was noticed for FeII or FeIII salts. An excellent
yield for the desired chloroester was obtained with FeCl2·4H2O
as precatalyst at 1308C, while in the absence of an iron source
no product was detected (entries 1 and 2). Moreover, when
the catalyst loading was reduced to 1.0 mol% there was only
a slight reduction of the activity in comparison to 5.0 mol%
(entries 7 and 8). In addition, the influence of the reaction tem-
perature was studied (entries 10–13). Decreasing the tempera-
ture to 1008C resulted in an excellent yield, while at 608C
a lower yield was noticed accompanied by the detection of
various intermediates. Notably, at 408C the reaction was ham-
pered (entry 13). Remarkably, the iron-based catalyst showed
an excellent performance at low temperature (<1008C), while
a recently reported zinc-based protocol required a reaction
temperature of 1308C.[10b]
[a] Dr. S. Enthaler, M. Weidauer
Technische Universitꢀt Berlin
Department of Chemistry
Cluster of Excellence “Unifying Concepts in Catalysis”
Straße des 17. Juni 115, 10623 Berlin (Germany)
Fax: (+49)30-31429732
E-mail: stephan.enthaler@tu-berlin.de
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201200125.
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Figure 1. Depolymerization of polyethers in the presence of catalysts and acid chlorides and potential follow-up chemistry.
With the appropriate reaction conditions in hand (5.0 mol%
FeCl2·4H2O, 2, 1008C, 24 h) we investigated the scope and limi-
tation of the protocol in the depolymerization of diverse poly-
ethers (Table 2). First, various molecularly defined ethers were
fragmented by the depolymerization procedure (entries 1–5).
Good results were achieved for ethers with methoxy or hy-
droxyl end-groups. Moreover, no
significant difference in the yield
Table 2. Iron-catalyzed depolymerization of polyethers—scope and limitations.
of
3 was detected between
Entry[a]
Substrate
Product
Yield [%][b]
79[c]
cyclic and acyclic ethers (en-
tries 3 and 5). After having dem-
onstrated the potential of the
iron catalyst in the depolymeri-
zation of well-defined ethers, we
directed our attention to poly-
1
5
6
1
7
3
3
3
3
2
3
4
81[c]
78[c]
73
meric
ethers
(entries 6–15).
Indeed, the system was capable
to produce the corresponding
chloroethers in yields of up to
95%. For polyethylene glycol
with a number-average molecu-
lar weight of up to 1000000, the
chloroester 3 was obtained in
89–95% yield (entries 6 and 7).
A good performance was no-
ticed for the depolymerization of
polytetrahydrofuran, whereas for
polypropylene oxide two differ-
ent products in a ratio of ca. 1:1
were observed (entries 9 and
10). Additionally, the influence of
the nucleophile was studied (en-
tries 12–15). Notably, later on the
nature of the alkyl or aryl func-
tion of the acid chloride can
control the properties of the
new polymer. Applying 4-chloro-
benzoyl chloride as well as 4-
methylbenzoyl chloride resulted
in similar yields as for unmodi-
fied benzoyl chloride. Moreover,
myristic acid chloride was ap-
plied as nucleophile, which is
easily obtained from renewable
resources, to depolymerize poly-
ethylene glycol (entry 15).
5
8
3
82
6
7
9 (Mn ꢀ100000)
3
3
89
95
10 (Mn ꢀ1000000)
8
9
11 (TRITON X-100)
12 (Mn ꢀ1000)
3
91
13
15
16
3
83
68
68
92
92
89
10[d]
14 (Mn ꢀ2500)
11
17 (Mn ꢀ2500)
12 (Mn ꢀ1000)
12 (Mn ꢀ1000)
12[e]
13[f]
18
19
14[f]
9 (Mn ꢀ100000)
21 (Mn ꢀ300)
20
22
91
78
The usefulness of the protocol
was demonstrated in a scale-up
experiment (45.0 mmol). Polyeth-
ylene glycol (21) was reacted
with benzoyl chloride (2) in the
presence of catalytic amounts of
15[g]
[a] Reaction conditions: substrate (0.72 mmol, based on the subunits), FeCl2·4H2O (5.0 mol% per subunit), 2
(1.5 equiv per subunit), 1008C, 24 h. [b] Isolated product yield. [c] As side product 4 was detected. [d] A mixture
of two isomers in a ratio of ca. 1:1 was observed. [e] 4-Chlorobenzoyl chloride was used as nucleophile.
[f] 4-Methylbenzoyl chloride was used as nucleophile. [g] Myristic acid chloride was used as nucleophile.
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sumed.[16] With regard to the underlying processes in
Scheme 2 a reaction mechanism is proposed. First, the oxygen
functionality of the ether is coordinated to the metal center
(A), effecting an activation of the oxygen and the adjacent
carbon.[17] In the next step a chloride is transferred from the
metal center (B) to the activated carbon under cleavage of the
CÀO ether bond, to produce an alkyl chloride fragment and an
iron alkoxide (C). Afterwards the iron alkoxide (C) reacts with
the acid chloride via intermediate D. Consequently, a new CÀO
bond is formed to obtain an ester fragment. The chloride of
the acid chloride is abstracted by the iron, which allows regen-
eration of A. Overall, the former ether is cleaved, creating two
fragments that will feed again into the catalytic cycle, finally re-
sulting in the formation of monomeric chloroesters.
In summary, we report an efficient methodology for the de-
polymerization of artificial polyethers using acid chlorides as
nucleophiles in the presence of catalytic amounts of abundant
and cheap iron salts under mild reaction conditions. Notewor-
thy, well-defined chloroesters are obtained as major product,
which are valuable building blocks in polymerization chemistry
for the synthesis of new polymers. Future investigations will
focus on further evaluation of the reaction mechanism and the
application of the protocol to the conversion of polyethers ori-
ginated from biomass (e.g., lignin).
Scheme 1. Depolymerization of polyethylene glycol 21.
FeCl2·4H2O at 1008C (Scheme 1). After 1 h full conversion was
reached, accompanied by 87% isolated yield. A sample taken
after 30 min and analyzed by GC–MS showed the presence of
intermediates 23, 24, and 25 with decreased chain length
compared to the starting material.
To investigate the effect of the counterion of the iron on the
reaction outcome, substoichiometric amounts of FeBr3
(0.5 equiv) were added to a mixture of 1 and 2 (see conditions
Table 1, entry 3). After performing the reaction for 24 h the re-
action mixture was analyzed by GC–MS, revealing the forma-
tion of chloroester 2 and to some extent (5%) the correspond-
ing bromoester. Based on that result an abstraction of the
chloride from the acid chloride by the metal can be as-
Experimental Section
General procedure for the depolymerization of tetraethylene
glycol dimethyl ether (1): A pressure tube was charged with an
appropriate amount of iron precursor (0.036 mmol, 5.0 mol%), tet-
raethylene glycol dimethyl ether (0.72 mmol), and benzoyl chloride
(5.0 equiv, 3.6 mmol). The reaction
mixture was stirred in a preheated
oil bath at 1308C for 24 h. The
mixture was cooled on an ice bath
and anisol (internal standard) was
added. The solution was diluted
with dichloromethane and an ali-
quot was taken for GC analysis
(30 m Rxi-5 ms column, 40–3008C).
Acknowledgements
Financial support from the Cluster
of Excellence “Unifying Concepts
in Catalysis” (funded by the Deut-
sche Forschungsgemeinschaft and
administered by the Technische
Universitꢀt Berlin) is gratefully ac-
knowledged. The authors thank
Prof. M. Drieß (Technische Univer-
sitꢀt Berlin) for general discus-
sions. Furthermore, D. Bombaj
and F. Schrçder (Technische Uni-
versitꢀt Berlin) are thanked for
their support.
Scheme 2. Mechanistic proposal for the iron-catalyzed depolymerization of polyethers.
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Keywords: depolymerization · homogeneous catalysis · iron ·
polyethers · polymers
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Received: February 20, 2012
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COMMUNICATIONS
Iron will: The iron-catalyzed depoly-
merization of a range of polyethers is
studied. The products of the depoly-
merization reactions are chloroesters,
which can be used as starting materials
for new polymers. In the presence of
simple iron salts extraordinary catalyst
activities and selectivities are feasible at
low temperature.
S. Enthaler,* M. Weidauer
&& – &&
Low-Temperature Iron-Catalyzed
Depolymerization of Polyethers
ChemSusChem 0000, 00, 1 – 4
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
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ÞÞ
These are not the final page numbers!