PARTⅤ SOLVOLYSISEFFICIENT DEPOLYMERIZATION METHODSOF POLYAMIDES IN SUPERCRITICAL ALCOHOLSAND IONIC LIQUIDS
Akio Kamimura*,1 Kouji Kaiso,1,2 Shigehiro Yamamoto,1Tsunemi Shugimoto,2 Kohichi Kashiwagi2
1Department Applied Molecular Bioscience, Graduate School of Medicine, Yamaguchi
University, Ube 755-8611, Japan.
2
Ube Laboratory, Ube Industries Ltd., Ube 755-8633, Japan.
E-mail: ak10@yamaguchi-u.ac.jp

Abstract: Development of a new efficient method for the depolymerization of plastics is ofinterest for utilizing chemical recycling of waste plastics. In this presentation, we are going toshow our recent results for the depolymerization of polyamides, especially for nylon-6, by
using supercritical lower alcohols and ionic liquids. Use of supercritical secondary or tertiary
alcohols resulted in the clean depolymerization to give caprolactam in good yields. The
obtained caprolactam was usually highly pure. Use of ionic liquids provides anothersuccessful method for the depolymerization of nylon-6, in which caprolactam are usually
isolated in very pure form and the ionic liquids are repeatedly useful for the reaction. Thismethod is the first use of ionic liquids for this purpose and has opened a new perspective in
not only plastic recycling chemistry but also ionic liquids chemistry. Both of the new methods
provide useful methodologies for the chemical recycling of polyamides.

ON
H nnylon 6

supercritical alcohol
or ionic liquids

HN

O

caprolactam

1. Introduction
Development of a new efficient method for the depolymerization of plastics is of interestfor utilizing chemical recycling of waste plastics.[1] Polyamides such as nylons are one of theplastics that are used widely in our life. Depolymerization of nylon was explored from 30years ago and many efforts have been reported so far.[2] Use of supercritical fluids is one of
the fascinating methods.[3] Exposure of nylon to supercritical water, however, gave acomplex mixture due to the high reactivity of supercritical water. Supercritical alcohols areanother candidate for this purpose because its supercritical phase was achieved under much
- 233 -

The 5th ISFR (October 11-14, 2009, Chengdu, China)
milder conditions than that of water. Additionally, mild properties of supercritical alcohols areanother important point for the purpose to achieve selective depolymerization. In thispresentation, we will show our recent results for the depolymerization of polyamides,
especially for nylon-6, by using supercritical lower alcohols and ionic liquids[4]. Use of
supercritical secondary or tertiary alcohols resulted in the clean depolymerization to give
caprolactam in good yields.[5] Caprolactam was isolated by direct distillation from thereaction pot when ionic liquids were used as a depolymerization media. Obtained
caprolactams by both methods were usually highly pure.
2. Experimental
Treatment of nylon 6 with a variety of supercritical lower alcohols is examined. The
results are summarized in Scheme 1.

Nylon 6 underwent smooth
depolymerization on exposure tosupercritical alcohol (Scheme 1).
For
example,heterogeneous
mixture in supercritical MeOH
turned into homogeneous solution
after 1 h of the reaction. However,GC analyses revealed that thecontent of caprolactam 1 was only14%. Indeed this was a complex
mixture containing many sideproducts. To improve the yield of 1, Fig. 1 Depolymerization of nylon 6 in supercritical iPrOH
we examined various primaryalcohols for the reaction. Use of EtOH slightly improved the yield of 1 to 35%. As primaryalcohols with longer-alkyl chain was used, the yield of 1 increased. The best yield of 1reached 61% when C12 alcohol was used for the reaction.
The yield of 1 dramatically improved when secondary and tertiary alcohols wereemployed for the reaction. To our surprise, exposure of nylon 6 to iPrOH in 370 °C for 1.5 h
resulted in clean formation of 1 more than 90% yield. Other secondary alcohols and tertiary
alcohols also selectively gave 1 in very high yields. Simple concentration of the reaction
mixture afforded 1 in very pure form which showed the melting point at 68.7-71.3 °C (lit.
- 234 -

PARTⅤ SOLVOLYSIS
68-71 °C). It should be noted that tertiary alcohols in the reaction mixture sometimes spurted
out during opening the reaction vessel. This is probably due to partial decomposition oftertiary alcohols under the reaction conditions.
The present selective conversion to 1 depended on the reaction temperature (Fig. 1). The
depolymerization progressed smoothly when carried out at 350 °C or higher temperature.
However, the reaction became sluggish when the mixture was heated at lower temperature.
For example, treatment of nylon 6 at 330 °C only gave 1 in 60% yield. It should be noted thatthe yield of 1 was never improved when the reaction was prolonged to 5 hours.

We next examined ionic liquids for the depolymerization (Scheme 2).[6] At first we used[emim][BF4] as the reaction solvent because this ionic liquid was stable at 350 °C or lower. A
mixture of nylon 6 chip and [emim][BF4] was heated at 300 °C for 6 h. After 6 h,heterogeneous mixture became black homogeneous thick oil, so we thought most of nylon 6
had reacted under the present conditions. To our delight we succeeded to isolate 1 in 54%
yield after tedious 25 times of extraction with ether. This was somewhat problematic because
extraction is a kind of laborious work. Thus, we tried to improve an isolation method fromionic liquid. Fortunately, caprolactam was a small molecule that showed its bp at 136-138°C/10 mmHg. Ionic liquids were usually non-volatile and showed no boiling points even athigh temperature (>300 °C). As a result, ionic liquids would not vaporize even if exposed
under reduced pressure conditions. With this idea in hand, we examined direct distillation forthe present depolymerization reaction. Finally we found great improvement for the isolationand obtained 1 in 86% yield. In this case, addition of 5 wt% of DMAP promoted thedepolymerization effectively.

Fig. 2 Depolymerization of nylon 6 in ionic liquids- 235 -

The 5th ISFR (October 11-14, 2009, Chengdu, China)
The depolymerization took place smoothly in various ionic liquids that contains TFSA incounter anion. The reaction rate depended on the cationic part of the ionic liquids; ammonium
cations provided the good results. The depolymerization in imidazolium ionic liquid gave
only 35% of 1.
The reaction temperature was important for the efficient depolymerization. 300 °C was
the best temperature. When treated at 270 °C, most of nylon was still remaining in thereaction mixture and the yield of 1 decreased. Higher temperature caused another problem.
Treatment of nylon 6 at 330 °C provided caprolactam 1 along with N-alkylated side produces.
This clearly showed that cationic part of ionic liquids reacted with caprolactam as analkylating agent. Indeed, NMR spectra of used ionic liquids indicated some decomposition
happened when the reaction temperature was higher than 330 °C.
These ionic liquids were repeatedly useful at least five times for the reaction (Fig. 2). For
example, treatment with nylon 6 in [PP13][TFSA] gave 1 in 86% for the first time, and 78%
in the fifth time, although DMAP should be added at each times for the reaction. Thus, theionic liquids were possible for recycling useWith these results in hand, we supposed the reaction mechanism as the following way(Scheme 3). The depolymerization progresses through addition-elimination reaction at thecarbonyl carbon. When an external nucleophile such as methanolic oxygen atom attacks thecarbonyl carbon, a methyl ester at the C-terminal of nylon A and 6-aminocapronic ester B
should be formed (route 1). Caprolactam contained 7-membered ring which is not readily
formed from the open-chain precursor such as B. Additionally the amino group in B should bemethylated by methanol under the present reaction conditions.[8] On the other hand, theoxygen atom in secondary and tertiary alcohols is less nucleophilic than methanol. Thus, thereaction rate for route 1 should be much suppressed for the reaction employed with thesealcohols. As a result, the most nucleophilic species in the reaction mixture should be theterminal amino group of nylon-6, which internally attacks the carbonyl carbon to give
caprolactam D and next terminal amino group of the polymer C (route 2).[8]Thus, use of
secondary and tertiary alcohols affords efficient depolymerization to caprolactam.
The reaction in ionic liquids should be assumed in a similar manner. There are virtuallyno nucleophiles because the counter anion of ionic liquids is almost non-nucleophilic. Theroute 2 in Scheme 3 is the only way to progress the depolymerization in ionic liquids. Thus,
selective formation of caprolactam 1 was achieved by the back-bite mechanism
3. Conclusion
In conclusion, we have successfully depolymerized nylon 6 to its monomer caprolactam
in good yield. As iPrOH can be reused after distillation from the reaction mixture, almost nowaste comes out from the process. Ionic liquids can be used directly after the reaction and it
can be used at least 5 times repeatedly although the costs of ionic liquids are not very cheap
now. These new methodologies will open a new efficient strategy toward chemical recycling
for waste plastics.

- 236 -PARTⅤ SOLVOLYSIS
Route 1: Cleavage from O-terminal; intermolecular reaction
ROHHNO

ORROHORO+OO

H 2NORBA

Route 2: Cleavage from N-terminal; intramolecular reaction to give e-caprolactam
OONHScheme 3NH2NH 2C+NHDReferences[1] Research Association of Feedstock Recycling of Plastics, Technology for Feedstock
Recycling of Plastic Wastes, Japan, Ed. CMC books, Tokyo, Japan, 2005.
[2] S. R. Shukla, A. M. Harad, D. Mahato. J. Appl. Polym. Sci. 2006, 100, 186.
[3] R. E. N. De Castro, G. J. Vidotti, A. F. Rubira, E. C. Muniz, J. Appl. Polym. Sci. 2006,101, 2009.
[4] T. Welton, For recent review of ionic liquid, Chem. Rev. 1999, 99, 2071.
[5] A. Kamimura, Y. Oishi, K. Kaiso, T. Sugimoto, K. Kashiwagi. ChemSusChem, 2008, 1,82-84.
[6] A. Kamimura, S. Yamamoto.Org. Lett. 2007, 9, 2533-2535.
[7] T. Oku, T. Ikariya. Angew. Chem. Int. Ed. 2002, 41, 3476.
[8] D. M. Coltart, Similar intramolecular acyl transfer is reported in the field of peptidechemistry, Tetrahedron, 2000, 56, 3449.

- 237 -