<p>The single-step synthesis of hydroxyl functionalized <i>cis</i>-1,4-polyisoprene telechelic was carried out by metathesis degradation of natural rubber in the presence of initiator <i>cis</i>-but-2-ene-1,4-diol and the Grubbs second generation catalyst (GII). The synthesis of hydroxytelechelic-based polyisoprene was assured by using FTIR and NMR spectroscopic techniques. The molecular weights of the resulting products with different concentrations of chain transfer agents were elucidated by gel permeation chromatography (GPC). It was investigated that the feed ratio of <i>cis</i>-but-2-ene-1,4-diol (NR)_o/(G-II)_o/(CTA)_o from 100/1/1 to 100/1/5 led to producing the lowest molecular weight telechelic. The hydroxytelechelic polyisoprene was used to synthesize poly(ε-caprolactone)-<i>block</i>-poly(isoprene)-<i>block</i>-poly(ε-caprolactone) through ring-opening polymerization. The synthesized ABA triblock copolymer was assured by measuring the molecular weight by GPC and the chemical evaluation was carried out by ¹H NMR. The synthesized ABA triblock copolymer was attuned to form phase-separated epoxy thermosets followed by reaction-induced phase-separation mechanism (RIMPS). The morphological studies of epoxy thermosets were carried out by transmission electron microscopy. The transition temperatures of epoxy thermosets containing different contents of triblock copolymer were investigated by dynamic mechanical analysis. The fracture toughness of phase-separated epoxy thermosets was measured in terms of the critical stress field intensity factor (<i>K</i>_<i>IC</i>). The comparison was discussed with phase-separated nanostructured epoxy thermosets containing polyisoprene chain in the form of ABA triblock copolymer and with the samples containing only polyisoprene chain.</p> Graphical abstract <p></p>