This study examined linear viscoelasticity (LVE) of stereo-regular poly(1-butene) (PB) and its random copolymer with polyethylene, with number fraction of ethylene comonomer <= 10%, using a conventional shear rheometer combined with a quartz resonator. This combination resulted in the detection of LVE in a broad frequency range from Rouse region to terminal relaxation. LVE, thus, determined was fit to the double reptation model through utilizing inputting molecular weights determined by gel permeation chromatography equipped with refractive index and two-angle laser light scattering monitors, and the segmental time and entanglement molecular weight as the two fitting parameters. A quantitative prediction was finally achieved when the two fitting parameters changed systematically by the increase of ethylene content, a decrease of segmental time quantified a plasticizing effect, and a decrease of entanglement molecular weight reflected enhanced the chain flexibility. The entanglement molecular weight of the PB samples was found to be lower than those reported for stereo-irregular PB, suggesting the important effect of stereo-regularity on the chain flexibility. (C) 2019 The Society of Rheology.