DOI: 10.1002/chem.201103677
Zinc-Catalyzed Depolymerization of Artificial Polyethers
[
a]
Stephan Enthaler* and Maik Weidauer
Polymers occupy an important and omnipresent role in
vinyl chloride, which can later on polymerize to yield fresh
high-quality materials. Based on this concept we present
[1]
[11]
human life. Current polymer feedstocks are based primari-
ly on monomers originated from fossil fuels, which evidently
herein the usefulness of an easy-to-adopt system composed
of catalytic amounts of zinc salts and stoichiometric amounts
of acid chlorides as nucleophiles for the depolymerization of
polyethers to yield valuable building blocks for polymer
[2]
cause several problems. On the one hand the confirmed
reserves of fossil fuels are steadily decreasing, and their de-
[3]
pletion is just a matter of time. On the other hand, huge
amounts of polymers are thermally decomposed after they
have accomplished the requested purposes, or alternatively
[12]
chemistry (Scheme 1).
Initially, as a model for the depolymerization of poly-
ethers the reaction of tetraethylene glycol dimethyl ether
(1) with benzoyl chloride (2) was studied (Table 1). In the
absence of any zinc source no formation of the depolymeri-
zation product 3 was detected (Table 1, entry 1). Addition of
catalytic amounts of zinc salts led to good yields of 3; a di-
[4]
they are converted to low-quality materials. As a conse-
quence, a negative impact on the environment is caused,
specifically by the emission of greenhouse gases. Owing to
these problems, an important issue for current research
must address to the development of methodologies for effi-
[4b,5]
cient recycling of polymers.
One interesting aspect could
minished yield was observed only in the case of Zn
ACHTUNGTRENNUNG( OAc)
2
be the depolymerization of polymeric materials to obtain
useful products, which can be reused in polymerization pro-
cesses to synthesize new polymeric materials (Scheme 1).
and Zn(acac) after 24 h at 1308C (Table 1, entries 2–10).
ACHTUNGTRENNUNG
2
Notably, no solvent is needed for this process. Methyl ben-
zoate (4) was also isolated as a side product; this originates
from the reaction of the me-
[6]
thoxy group of 1 with 2. More-
over the catalyst loading was in-
vestigated resulting in de-
creased yields at lower loadings
(
Table 1, entries 11–13). De-
creasing the amount of 2 to
three and one equivalent, re-
spectively, a diminished yield of
Scheme 1. Schematic view of the proposed depolymerization process.
3
was monitored (Table 1, en-
In this regard the application of transition-metal catalysts
offers an efficient possibility to realize this aim. A widely
applied class of polymers or co-polymers are polyethers
tries 14–16). Furthermore, it was found that the reaction
proceeds at elevated temperature, whereas at 1008C the re-
action is hampered (Table 1, entry 17). Additionally, the re-
action was performed on a higher scale of 45 mmol of 1
without any loss of activity (Table 1, entry 18). With suitable
[7]
(
e.g., polytetrahydrofuran, polyethylene oxide, polypro-
ACHTUNGTRENNUNGp ylene oxide). The repeating ether units could represent a
suitable function to coordinate to transition metals and by
this process an activation of the ether can occur. The addi-
tion of nucleophiles, such as acid chlorides, to the activated
species could allow the depolymerization of the polyether to
obtain as product defined monomeric chloroesters.
Those chloroesters can be appropriate sources for the syn-
thesis of monomers, such as vinyl esters, halohydrins or
reaction parameters in hand (5.0 mol% ZnCl , 2, 1308C,
2
24 h) we started to explore the scope and limitation of the
system in the depolymerization of different polyethers
(Table 2). In the first set of experiments, molecular defined
ethers were subjected to the depolymerization protocol
(Table 2, entries 1–5). Good results were obtained for ethers
with methoxy as well as hydroxyl functionality as end
groups. Furthermore, no difference was found between
cyclic and acyclic ethers. On the other hand, for the depoly-
merization of polyethylene glycol the chloroester 3 was iso-
lated in 70–78% yield (Table 2, entries 6 and 7). Moreover,
an excellent yield was monitored for the depolymerization
of polytetrahydrofuran, whereas for polypropylene oxide
two different products in a ratio of ꢀ1:1 were observed
(Table 2, entries 9–10). In addition, the properties of the
[8,9,10]
[
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)3031429732
E-mail: stephan.enthaler@tu-berlin.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103677.
1910
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 1910 – 1913