Low molecular weight hydroxy terminated polyethylene (HTPE) containing on average an ethyl group every 16-18 carbon atoms, and a hydroxy functionality of 2.6, has been used to prepare polyurethane copolymers and block copolymers which have good solvent resistance. The polymers show somewhat complicated thermal behavior, including T s r s at around -4OOC due to the HTPE and diffuse endothem between 40 and 6OOC. The simple copolymers, containing only the polyol and a diisocyanate, show infrared evidence for two phases in the case where CHDI (trans-l,4diisocyanatocyclohexane) was used, and poorer phase separation where other diisocyanates were used. Oynamic mechanical spectra show very broad tan delta transitions for the copolymers in the range of -9 to -23OC. All the polymers exhibit another transition in the G" curve above room temperature. SAXS reveals a microphase separated structure at 3OoC for the simple copolymers 'which increases in spacing, then disappears in the 60-70°C range. With cooling, the microphase separated structure reappears readily for the CHDI-based copolymer, while its reappearance shows a hysteresis resulting from rate effects for the other copolymers.