DEPOLYMRIZATION OF POLYMERS IN IONIC LIQUIDS
Akio Kamimura1,*, Shigehiro Yamamoto1, Tomoki Okagawa1, Tamami Otsuka1 and Makoto Yoshimoto1
1Department of Applied Molecular Bioscience, Graduate School of Medicine, Yamaguchi University Ube 755-8611, Japane-mail: ak10@yamaguchi-u,ac,jp; TEL/FAX +81-836-85-9231

Abstract
Use of ionic liquids for the depolymerization of synthetic polymer was examined. Polyamides were readily converted intomonomeric lactam in good yields and recovered ionic liquids were useful for iteration use. Polyamides were isolated by adirect distillation or an extraction from aqueous mixture. To accelerate the reaction, microwave irradiation was effective.
For example, unsaturated polyester in fiber-reinforced plastics was readily depolymerized to give glass fiber andmonomeric material under the conditions of microwave heating at 340 ºC. This improved method was successfully
applied to not only synthetic plastics but also natural polymers for their conversion of monomeric materials.

Keywords: ionic liquids, depolymerization, microwave irradiation, polyamides, FRP

1. Introduction
Conversion of plastics or polymers into monomericmaterial is recognized a key technology for achieving aneffective chemical recycling. We have recently developed
a new conversion for polymers into monomers using
supercritical alcohols.[1] Although these methodologies
provided useful and effective reactions, use of
supercritical fluids always requires a high-pressure
apparatus and special cares to treat them. Ionic liquids
are a new type of solvent. They have interesting
properties that have never been seen in any otherorganic solvents.[2] For example, they are lessflammable by contacting with fire and non-volatile undervery high temperature conditions. We were interested inthese properties that would be very suitable for thedepolymerization of polymers into monomers. Ionic
liquids were usually ready for recycling use, andexpected for using them several times in the same
reaction. Thus, we started our investigation of using ionicliquids for the depolymerization of plastics and bioderived polymers. Here we will show our recent
developments on conversion of polymers into monomers
using ionic liquids for achieving effective chemical
recycling chemistry.

2. Materials and Methods
Ionic liquids were purchased from Kanto Kagaku Co.
Ltd., Tokyo, Japan. The reactions were carried out in ausual glass apparatus. A glass tube oven or a sand bath
was used for conventional heating of the reaction
mixture. Microwave irradiation was achieved by using a
microwave reactor made by Shikoku Keisoku, Japan.
The reaction temperature was monitored and controlled
by a thermo couple. All of monomeric products by thedepolymerization reaction were isolated and checked by
NMR and GC analyses. Ionic liquids were recovered andused for several times for the depolymerization reaction.

The ionic liquids used for several times were purified bytreatment with and alumina column and charcoal.

3. Results and Discussion
Depolymerization of polyamides was carried out in[PP13][NTf2]. A mixture of nylon 6 and [PP13][NTf2] washeated in a sand bath for 1 h at 300 ºC. Nylon 6 wasdissolved in the ionic liquids and the reaction mixture
became homogeneous black oil. Then the mixture washeated by a glass tube oven at 300 ºC for additional 6 hunder reduced pressure. Caprolactam was isolated inpure form and [PP13][NTf2] was recovered as a residue.
The yield of caprolactam was 55%. The presence ofcatalytic amounts of DMAP (N,N-dimethylaminopyridine)promoted the reaction smoothly and the yield of themonomer was improved to 86% (Scheme 1).[3]Scheme 1.
We examined a variety of ionic liquids and found that
[PP13][NTf2] gave the best results. The reaction
progressed effectively at 300 ºC that was the mostsuitable reaction temperature; the depolymerization
progressed in sufficiently when nylon 6 was treatedunder 300 ºC and higher reaction temperature provided
side products such as N-methylcaprolactam. NMR chartsof ionic liquids indicated that partial decomposition of[PP13][NTf2] happened when the reaction was carriedout under more than 300 ºC conditions. Recovered[PP13][NTf2] was useful at least five times for thedepolymerization reaction. The depolymerization reaction

7th International Symposium on Feedstock Recycling of Polymeric Materials (7th ISFR 2013)
New Delhi, India, 23-26 October 2013

of nylon 6 in modified ionic liquids were also
examined.[4]We next examined use of the microwave irradiation forthe heating method for the depolymerization.
Unsaturated polyester was widely used as a material for
fiber-reinforced plastics (FRP), which is recognized as amost formidable plastic for the recycling. Treatment of
FRP chips in ionic liquids at 340 ºC for 1 h in a sand bathfailed the depolymerization and most of the starting FRP
chip remained in its original shape. On the other hands,
heating by microwave irradiation changed the reaction
dramatically. Most of FRP chip dissolved in the ionicliquids and depolymerization progressed within 2 min ofheating at 340 ºC. Glass fibers and phthalic anhydride,
one of the monomeric materials of the unsaturated
polyester, were recovered in good yields (Scheme 2).[5]Scheme 2[PP13][NTf2] were recovered and used for several times
for the depolymerization reaction. However, in this casenon-volatile residue remained in the ionic liquids after thereaction. As a result the weight and the viscosity of[PP13][NTf2] increased and their handling became noteasy. We attempted their purification. The ionic liquids
after several time uses in the depolymerization reaction
were diluted with acetone and the resulting solution waspassed through alumina column chromatography. Thenthe treatment of the solution with charcoal resulted in thedecolorization of the ionic liquids. [PP13][NTf2] wererecovered in more than 90% and used for thedepolymerization reaction again.
The microwave irradiation was also effective for thedepolymerization of nylon 6. The reaction time wasdramatically reduced and extraction work-up enabled toreduce energy consumption for the depolymerization
reaction.
The present methodology was extended to thedepolymerization of bio-derived polymers. For example,
cellulose, massive amounts of which abandons in theworld, is expected as a new chemical and energy
sources today. We examined its depolymerization in ionicliquids. Although many efforts for the conversion ofcellulose to monomeric materials in ionic liquids havebeen developed so far, most of the reports employ ionicliquids containing chloride anion in the anion part.
However it is not easy to isolate glucose, one of the
expected monomeric materials, from the reaction mixture
because such ionic liquids are water-soluble. Use ofhydrophobic ionic liquids would provide a good solution,
but such ionic liquids do not solve cellulose.

Scheme 3
We used [TMPA][NTf2] as the hydrophobic ionic liquids
and examined LiCl and HCl as an additive for thereaction mixture. Use of HCl converted cellulose toglucose, but the yield was quite low. Combination use ofLiCl and HCl in the reaction mixture improved theconversion. The optimal reaction temperature was 100ºC and the microwave irradiation shortened the reaction
time to 15 min. We finally succeeded to efficientconversion of cellulose to glucose in 40 – 50% yields
when a large excess amount of LiCl was used.[6] As
expected, glucose was readily isolated from the reaction
mixture by simple extraction work-up. [TMPA][NTf2] wasalso recovered and employed for iteration use. LiCl wasalso separated from glucose by treatment with acetoneiPrOH that solves only LiCl. The present method opens a
new use of ionic liquids for feedstock recycling chemistryfrom biomass.

4. Conclusions
Ionic liquids were useful for the depolymerization ofplastics. Their less flammable and non-volatile properties
provide an easy depolymerization method for plastics.
The depolymerization in ionic liquids was performed
under normal pressure so that no special apparatus suchas an autoclave is necessary. The less flammableproperty of ionic liquids reduces a risk of fire during themanipulation of the reaction. Ionic liquids are readily
recovered and reused for several times. Although theprices of ionic liquids are relatively high at the moment,these properties will progress the more use of ionicliquids for the plastic recycling chemistry as well as theconversion of bio-resources for useful materials.

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[2] T. Welton. Chem. Rev. 99 (1999) 2071 – 2083.
[3] A. Kamimura and S. Yamamoto. Org. Lett. 9 (2007)
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[4] S. Yamamoto and A. Kamimura. Chem Lett. 39(2009) 1016 – 1017.
[5] A. Kamimura, S. Yamamoto and K. Yamada.
ChemSusChem 4 (2011) 644 – 649.
[6] A. Kamimura, T. Okagawa, N. Oyama, T. Otsuka andM. Yoshimoto. Green Chem. 14 (2012) 2816 – 2820.

7th International Symposium on Feedstock Recycling of Polymeric Materials (7th ISFR 2013)
New Delhi, India, 23-26 October 2013