RESEARCH ARTICLE Preparation of phenyl‐modified natural rubber in latex stage Nuorn Choothong 1 | Shintaro Shioyama 1 | Yoshimasa Yamamoto 2 | Seiichi Kawahara 1 1 Department of Materials Science and Technology, Faculty of Engineering, Nagaoka University of Technology, 1603‐1 Kamitomioka Nagaoka, Niigata 940‐2188, Japan 2 Department of Chemical Science and Engineering, National Institute of Technology, Tokyo Collage, 1220‐2 Kunigida‐cho, Hachioji, Tokyo 193‐0997, Japan Correspondence Seiichi Kawahara, Department of Materials Science and Technology, Faculty of Engineering, Nagaoka University of Technology, 1603‐1 Kamitomioka Nagaoka, Niigata 940‐2188, Japan. Email: kawahara@mst.nagaokaut.ac.jp Funding information Scientific Research (A) from the Japan Society for the Promotion of Science and JST‐JICA SATREPS, Grant/Award Number: 16H02291 Phenyl‐modified natural rubber was prepared in latex stage by bromination of deproteinized natural rubber followed by Suzuki‐Miyaura cross‐coupling reaction. First, the bromination of natural rubber was carried out using N‐bromosuccinimide in latex stage. The bromine atom content increased as amount of N‐bromosuccinimide increased. Second, the allylic bromine atom was replaced with a phenyl group using phenyl boronic acid in the presence of a palladium catalyst, according to the Suzuki‐ Miyaura cross‐coupling reaction in latex stage. The resulting products were character- ized by nuclear magnetic resonance (NMR) spectroscopy. Signal at 7.13 ppm was assigned to the phenyl group of the product, while signals at 3.98, 4.14, and 4.44 ppm were assigned to the remaining allylic brominated cis‐1,4‐isoprene units. The estimated phenyl group content and the conversion of the Suzuki‐Miyaura cross‐coupling reaction were 1.32 and 23.7 mol%, respectively. Glass transition tem- perature (T g ) of deproteinized natural rubber increased from -62°C to -46.7°C, when the phenyl group was introduced into the rubber. KEYWORDS brominated natural rubber latex, bromination with NBS, phenyl‐modified natural rubber latex, Suzuki‐Miyaura cross‐coupling reaction 1 | INTRODUCTION Phenyl‐modified natural rubber is expected to be a prominent visco- elastic material, which may achieve outstanding mechanical properties necessary for raw rubber as a source to prepare various kinds of rubber products. 1 This may be due to a well controllable dynamic mechanical property on frequency since a glass transition temperature (T g ) of natu- ral rubber may be controlled by inserting phenyl group into the rubber; that is, T g is altered by changing the content of phenyl group as in the case of styrene‐butadiene rubber (SBR). For instance, intentional adjustment of T g to -30°C may meet the requirement as a criterion of raw rubber to prepare tire that value of loss tangent must be smaller at 50°C while it is larger at 0°C. In addition, the phenyl group, which is a chemically stable, may attain significant mechanical energy dissipation as a bulky pendant group 2-5 because of its rapid rotation. It is, thus, nec- essary to prepare the phenyl‐modified natural rubber by direct insertion of phenyl group into the rubber, which may be a promising key technol- ogy for preparation of materials from natural rubber as a source. Phenyl‐modified natural rubber may be prepared by forming chemical linkages between natural rubber and the phenyl groups. It is considered to be one of the homologation reactions, the Suzuki‐ Miyaura cross‐coupling reaction, which make possible to introduce the phenyl groups into natural rubber. Suzuki‐Miyaura cross‐coupling reaction is known to be a palladium‐catalyzed cross‐coupling reaction between organoboronic acid and organohalides 6-8 so that the phenyl groups may be introduced into natural rubber under mild condition by reacting phenyl boronic acid with natural rubber having allylic bro- mide groups in the presence of palladium catalyst. Thus, hydrogen atoms at allylic position of natural rubber must be replaced with bro- mine atoms before the cross‐coupling reaction. In addition, the bromi- nation and the cross‐coupling reaction must be performed in latex stage since natural rubber is isolated as a latex from Para rubber tree (Hevea brasiliensis). In the previous work, 1 we performed Suzuki‐Miyaura cross‐ coupling reaction on natural rubber after bromination. Its reaction scheme is shown in Figure 1. First, the bromination of natural rubber was made with N‐bromosuccinimide (NBS) under nitrogen atmosphere in dichloromethane. Hydrogen atoms at the allylic position of natural rubber were replaced to bromine atoms, according to the radical mech- anism. 9-11 Second, allylic bromine atoms of the resulting brominated Received: 30 August 2018 Revised: 14 December 2018 Accepted: 18 December 2018 DOI: 10.1002/pat.4537 Polym Adv Technol. 2019;1–7. © 2019 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/pat 1