Mild Approach for Non-Catalytic Hydrogenation of Liquid Natural Rubber Using 2,4,6-Trimethylbenzenesulfonyl Hydrazide as the Diimide Source Hamizah Md Rasid, †,‡ Nur Hanis Adila Azhar, † Naharullah Jamaluddin, † and Siti Fairus M. Yusoff †,§, * † School of Chemical Sciences and Food Technology, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia. *E-mail: sitifairus@ukm.edu.my ‡ Faculty of Applied Sciences, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia § Polymer Research Centre (PORCE), Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia Received August 6, 2015, Accepted February 20, 2016, Published online May 23, 2016 This article reports an efficient, mild-temperature method for the hydrogenation of liquid natural rubber (LNR). The hydrogenation of LNR was studied using diimide generated in situ from the thermolysis of 2,4,6-trimethylbenzenesulfonyl hydrazide (MSH) in o-xylene at 100  C. The effects of reaction tempera- ture, reaction time, solvent, and MSH/LNR weight ratio on the percentage of hydrogenation were evalu- ated. 1 H NMR analysis revealed that ~80% hydrogenation was achieved with a weight ratio of MSH: LNR = 1:1 at 100  C in o-xylene within 60 min. Keywords: Liquid natural rubber, Non-catalytic hydrogenation Introduction Natural rubber (NR) is a renewable material harvested from the sap of rubber trees, Hevea brasiliensis, and contains polyisoprene as its major component. 1 NR has been widely used in automobile and adhesive industries, due to its supe- rior tensile strength, excellent elasticity, high tear resist- ance, low permanent set, and waterproof nature. However, due to the large amount of double bonds in its chain struc- ture, NR is susceptible to oxidative and thermal degrada- tion, ozonolysis, and extreme weathering. In order to improve the thermal stability of NR, chemical modification via hydrogenation can be performed to produce highly satu- rated NR. The desired product of the hydrogenation reac- tion should withstand high temperature conditions and therefore could be used in various applications, such as rub- ber blending and vulcanization. 2 Typically, NR has an average weight molecular weight (M w ) of ~2.5 × 10 6 that must be reduced during manufac- turing processes. Low-molecular-weight NR can be easily modified into various useful products. One example is liq- uid natural rubber (LNR), which is a reduced form of NR with short polymeric chains and molecular weight lower than 10 5 . 3 The short polymeric chain of LNR is more advantageous than that of NR because it makes chemical modifications possible and thus expands the applicability of LNR in various fields. In general, hydrogenation involves the reaction of molec- ular hydrogen with alkenes/alkynes to reduce unsaturated organic compounds, normally in the presence of a catalyst. Considerable efforts have been made to hydrogenate NR using homogeneous catalysts that comprise transition metal complexes such as nickel, rhodium, iridium, or ruthenium complexes. 4–7 The catalytic hydrogenation of NR has the advantages of being highly selective toward the desired products and not being prone to macroscopic diffusion pro- blems. However, the classical catalytic hydrogenation reac- tion involving transition metal complexes is expensive. Moreover, the catalytic hydrogenation has to be conducted at high temperature and pressure, which requires special equipment. 8 On the other hand, non-catalytic hydrogenation methods use hydrogenation reagents, such as diimide (N 2 H 2 ), that convert unsaturated organic compounds into reduced alkane products. Diimide can be generated in a number of ways, but the widely used method is the thermolysis of p-toluenesulfonyl hydrazide (TSH). 8–12 Non-catalytic hydrogenation offers two major advantages: avoiding unnecessary handling of hydrogen gas and removal of the catalyst. However, this method takes place only at very high temperatures. Mahittikul et al. 9 studied the hydrogena- tion of NR using TSH in an in situ system at 135  C, and obtained hydrogenated NR in the form of an alternating ethylene–propylene copolymer. Azhar et al. 13 studied the hydrogenation of LNR using the same diimide source at 130  C and managed to achieve >90% hydrogenation yield. Even though NR and LNR can be successfully reduced, chain scission can also be promoted and often results in low-molecular-weight products due to the high reaction temperatures used. 14 In order to prevent these problems, it Article DOI: 10.1002/bkcs.10767 H. M. Rasid et al. BULLETIN OF THE KOREAN CHEMICAL SOCIETY Bull. Korean Chem. Soc. 2016, Vol. 37, 797–801 © 2016 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Wiley Online Library 797