AbstractTwo hydroxy‐functionalized liquid rubbers, one a commercially available polybutadiene (PB) and the other a specially synthesized polymyrcene (PM), have been converted into homopolyurethane elastomers by reaction with 4,4prime;‐diphenylmethane diisocyanate (MDI). Additionally, PB and PM, each in admixture with various amounts of 1,4‐butane diol, were reacted with MDI to yield two series of segmented copolyurethanes having different hard‐block contents (0–30 w/w). The physical properties of these elastomers have been compared by stress–strain, thermal, and dynamic mechanical analyses, and by swelling experiments. The two series of segmented copolyurethanes have similar morphologies being almost completely phaseseparated and variations in physical properties have been empirically related to hard block contents. The PM‐based elastomers exhibited higherTgvalues, ultimate elongations, and larger swelling ratios, but were softer and possessed lower tensile strenghts in comparison with elastomers based on PB. These differences have been related to solfraction contents, the nature and distribution of molecular species present in the parent liquid rubbers and hence to polyol functionalities (f̄n). Analysis of the stress–strain data from the homopolyurethanes using the Mooney–Rivlin expression enabled the relationship betweenf̄nand elastomer structure to be quantified in terms of the molar mass (M̄c) of the polyurethane network chains for
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