AbstractThe present work focuses on the evolution in the mechanical properties of an unsaturated polyester resin (UPR) on blending with itaconimide‐end terminated polyethers, namely, polypropylene glycol (I‐PPG), polyethylene glycol (I‐PEG), and polytetra methylene oxide (I‐PTMO). Blends of an unsaturated polyester (UPR, based on propylene glycol, terephthalic, and maleic acids) resin with different loading of itaconimide end‐capped telechelics were investigated for their mechanical and thermal properties. Blending with these additives enhanced the mechanical and thermal properties of the crosslinked UPR. The impact strength and fracture toughness values were improved by more than 100% by small quantities of the additives. The improvement in fracture properties was correlatable primarily to a decrease in overall crosslink density. The distribution of the polyether chains in the cured matrix as dictated by the reactivity ratios of styrene and polyether macromer was found to have a role in deciding the properties. The properties were found to be the best for the blend toughened with I‐PPG with a molecular weight 2000 g/mole at a loading of 2.5 parts per hundred parts. On comparison with the resin blended with a maleimide‐encapped polyether of same molecular weight, the itaconimide end‐capped polyether was found to provide a better toughening of the UPR matrix. This could a priori be explained based on a difference in distribution of the end‐capped polyether as a consequence of the difference in the copolymerization behavior of itaconimide and maleimide functionalized telechelics toward UPR. The itaconimide enters into a random copolymerization with styrene and the probability for formation of the continuous sequences of the itaconic group is about 60%. This will permit polyether segments to come close enough to form micro or even sub‐micron clusters of the polyether which eventually forms the micro crystallites of poly ether that act as a crack stopper. This possibility cannot be envisaged in maleimides which forms invariably an alternating sequence with styrene. The morphological features as reflected in scanning electron microscopic analyses tallied with these observations. This work could help identify the ways for obviating the inherent brittleness of the UPR systems.