PARTⅤ SOLVOLYSISEPOXIDATION AND DEGRADATION
OF NATURAL RUBBER
Xian-hong Jiang, Qian Zhou*, An-ke Du ,Tao Zhao, Yu-Zhong Wang
1Center for Degradable and Flame-Retardant Polymeric Materials (ERCEPM-MoE), Collegeof Chemistry, State Key Laboratory of Polymer Materials Engineering,
Sichuan University, Chengdu 610064, China;
Tel: +86-28-85410755;Fax: +86-28-85410755;
E-mail:qzhou@scu.edu.cn

1. Introduction
Effective recycling of waste tires is important for environmental protection and resourceutilization. Natural rubber (NR) is one of major components of tires. In order to achieveeffective recycling of waste tires, it is necessary to investigate the utilization of its main
rubber components. Epoxidation is a long-standing technique to improve the properties ofrubbers. Hydrogen peroxide is one of effective stable catalysts with a broad scope for olefin
epoxidations and alcohol oxidations because of its character of cheap, clean and give water as
the sole byproduct. Catalysts based on tungsten in situ activation of H2O2 for the epoxidationof olefins are particularly attractive since tungsten is a readily available and inexpensive
transition metal, with good catalytic stability and recycling efficiency without any speciallydesigned organic ligands.
In this paper, by combining sodium tungstate with the generally adopted acetic acid /hydrogen peroxide epoxidation system, natural rubber can be effectively oxidized to prepare
telechelic epoxidized liquid natural rubber (TELNR).
2. Experimental
The epoxidation and degradation reactions were carried out in air in a 250mL threenecked round-bottomed flask equipped with a magnetic stirrer and a thermometer. Typically,
4.08 g (60 mmol C=C) of natural rubbers were dissolved in toluene, then added 45 mol%acetic acid; 0.85 mol% Na2WO4·2H2O was dissolved in 10 ml H2O2 (30%) solution and added
dropwise to the polymer solution at 60℃ with stirring continuously. After reaction, theproduct were precipitated with ethanol, and then washed with distilled water; The products
were marinated in the 1% Na2CO3 solution for about 24 hour and then washed with distilledwater; The products were then dried in a vacuum oven at 50°C until constant weight has beenreached.The chemical structure of the products was characterized by FT-IR, 1H-NMR andGPC.
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3. Results and Discussion
By comparing the infrared spectrum of NR and DENR, it can be clearly seen that thecharacteristic signals of epoxide ring at 1241.4 (symmetric stretching of epoxide ring) and
874.6 cm-1 (asymmetric stretching of epoxide ring) appeared while the carbon-carbon doublebound at 3034.4 (stretching of carbon-carbon double bound) and 837.0 cm-1 (bending ofcarbon-carbon double bound) decreased after reaction. On the other hand, according to the1H-NMR spectrum for NR and TELNR, two signals at 1.2 and 2.7 ppm assigned to methyland methine protons of the epoxy group, respectively, appeared in the spectrum of TELNR.
These results strongly prove the NR has been successfully epoxidized.
As for the epoxidazation efficiency, it can be seen that the traditional peracetic acid only owns
low epoxidation efficiency for NR (Tab.1). The degree of epoxidation was no more than 1.0% at 60 ℃
after 6 h, and it only reached to 5.6% after reacting for as long as 24 h. Interestingly, by combing
acetic acid with Na2WO4, the epoxidation efficiency was sharply increased (6 h: 9.2%, 24 h: 37.2%),
proving the Na2WO4/CH3COOH/H2O2 system possesses high-efficiency for epoxidation of NR.

Tab. 1 Effects of sodium tungstate on the epoxidation and degradation of natural rubber.

a)

Na2WO4t

M w (×105)a)

M n (×105) a)

PDI(h)

Epoxidation(%)(% molar ratio)
---

5.77

4.15

1.39

00624

1.0
5.6

5.53
4.30

4.282.30

1.29
1.87

0.85
0.85

624

9.2
37.2

3.93
0.93

2.23
0.19

1.76
4.93

Caculated from GPC.

The weight and number-average molecular weight ( M w and M n ) and polydispersity
index (PDI) of the NR before and after reaction calculated according to the GPC curves werealso listed in Tab. 1.The decrease of the molecular weight has been observed for theNa2WO4/CH3COOH/H2O2 system, after reacting for 24 h, the M w of TELNR wassignificantly decreased from 5.77×105 Da to 0.93×105 Da.
4. Conclusions
By combining sodium tungstate with the geneally adopted acetic acid / hydrogen
peroxide epoxidation system, natural rubber can be effectively oxidized to prepare telechelicepoxidized liquid natural rubber (TELNR). In this reaction system, natural rubbers werereacted at 60 ℃ for 24 h, a TELNR with an epoxidation degree of 37.2% and weight averagemolecular weight of 0.93×105 has been obtained.

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Acknowledgements
The authors thank the financial support of the 863 program (Contract No:2007AA06Z325) and the National Science Fund for Distinguished Young Scholars(50525309).
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