The molecular motion of polyethylene (PE) chain ends tethered to a fresh surface of poly(tetrafluoroethylene) (PTFE) in vacuo was studied. The tethered PE chain ends were produced by a block copolymerization of PTFE with ethylene in vacuo at 77 K. Each of the tethered PE chains has an unpaired electron at the growing chain end. The mobility of the tethered PE chain ends was observed in the range of 2.8-95 K by an electron spin resonance (ESR) spectrometer with the unpaired electron as a probe. The inter exchange motion between two a protons at the chain ends probably occurs at 2.8 K. The site exchange motion between two conformations at the chain ends occurs above 7 K and is more clearly observed at 15 K. Moreover, the torsional oscillation of the beta proton occurs above 30 K. Furthermore, the rotation of the chain end about the chain axis of PE probably occurs at 95 K. The assignment of molecular motion modes was based on spectral simulations. The high mobility of the tethered PE chain ends is attributed to (1) a large space around the chains; (2) the presence of the vacuum; (3) the lack of chain aggregation, because the concentration of the chain ends is low and they are tethered; and (4) the immiscibility of PE and PTFE. The ends of PE chains tethered on the PTFE in vacuo probably behave as isolated PE chain ends in vacua, and may reveal the mobility intrinsic to PE chain ends.