Polyethylene, polypropylene, polybutadiene, polyisobutylene, and poly(methyl methacrylate) tethered on a fresh surface of poly(tetrafluoroethylene) were produced by a block copolymerization of PTFE with their monomers in vacuum at 77 K. The tethered polymer chains can be regarded as isolated from other tethered chains because of an extremely low fraction of the tethered chains. We call them "isolated polymer chains". A molecular mobility of tethered polymer chains was studied by electron spin resonance (ESR) using a peroxy radical at the chain end as a label. Temperature-dependent ESR spectra of the peroxy radicals were successfully observed and the ESR spectra were composed of two spectra arising from two kinds of peroxy radicals, A-radical and B-radical, which were a rigid radical due to a train conformation and a mobile radical due to a tail conformation of tethered chains, respectively. The remarkable change with anisotropic g values of the peroxy radicals was analyzed by computer simulation. The train-tail transition temperature where the ratio of fractional amount of B-radical reached 50% was found to linearly increase with an increase in a conformer size related to a characteristic free volume of isolated polymer chains. Moreover, the flexibility of isolated polymer chains is dependent on the number of double bonds in the main chain and the size of side chain.