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In a 200 L pressure-resistant vessel filled with dry nitrogen, 75 kg of cyclohexane as a solvent and 21 g of sec-butyllithium as a polymerization initiator were added and, subsequently, 1.7 kg of styrene was added. After polymerization at 50° C., 0.59 kg of tetrahydrofuran as a Lewis base was added and, subsequently, 17.3 kg of isoprene and 1.7 kg of styrene were added in this order. When polymerization was carried out, a triblock copolymer of styrene-isoprene-styrene type was obtained. The block copolymer had a branching degree (α) of 0.335. The block copolymer obtained was subjected to hydrogenation at a hydrogen pressure of 2 MPa and a reaction temperature of 100° C. in cyclohexane using Pd/C as a catalyst, yielding a styrene-hydrogenated isoprene-styrene block copolymer (SEBS). The resulting block copolymer had a weight average molecular weight of 83000, a styrene content of 16% by weight, and a ratio of hydrogenation of 92 mol %.

1. customAfter polymerization at 50° C.
2. customWhen polymerization