Mechanical and viscoelastic properties of precision polyolefins, poly(4-phenylcyclopentene) (P4PCP) and its hydrogenated analog (H-2-P4PCP) containing atactic phenyl branches at exactly every five carbons along the backbone are explored. Both materials are amorphous with a glass transition temperature of similar to 17 +/- 3 degrees C. Rheological investigations determined that P4PCP has an entanglement molar mass (M-e = 10.0 kg mol(-1) ) much higher and doser to polystyrene than H-2 -P4PCP (M-e = 3.6 kg mol(-1). Both materials have elastomeric and shape memory properties at ambient temperatures, which were further explored through strain hysteresis measurements. H-2 -P4PCP has an elastic recovery of similar to 95 at max strain values up to 500% as determined by uniaxial tensile testing. Time-temperature superposition analysis, Williams-Landel-Ferry constants, and further mechanical analysis are discussed and compared to previously reported ethylene-styrene copolymers of similar phenyl-branch content within the microstructure.