Thermal switching between solid- and liquid-like behavior of polyethylene additives such as semi-crystalline telechelics and their corresponding nanohybrids, both of which are uniformly dispersed as nanophases within the high density polyethylene (HDPE) matrix, enables temperature-induced crack healing of polyethylene by interfacial self-assembly. Key intermediate is oligo(cyclooctene) with different end groups having molar mass of 2000 g mol(-1) and melting at 60 degrees C prepared by ring-opening metathesis polymerization of cyclooctene in the presence of olefinic chain transfer agents (CTA). Whereas 1-octene as CTA leads to non-functionalized oligo (cyclooctene) (POC) the use of 1-undecylenic acid leads to monocarboxylic-acid-terminated oligo(cyclooctene) (cPOC). Neutralization of cPOC with diethyl zinc yields nanohybrids (iPOC) having a semi-crystalline POC shell and a zinc carboxylate core. Opposite to high molar mass POC, owing to their much lower melt viscosity and excellent compatibility with polyethylene, all oligo(cyclooctenes) are uniformly dispersed in polyethylene melts during extrusion forming crystalline nanophases which strongly adhere to the HDPE matrix. Upon annealing at temperatures above 60 degrees C but well below the HDPE melting temperature, they are rendered liquid and mobile within the solid HDPE matrix. As verified by microscopy, driven by pressure build-up due to their volume expansion resulting from POC melting they migrate to polyethylene cracks and heal them by means of interfacial self-assembly and cocrystallization upon cooling.