Highly elastic polythiourethanes were synthesized through sequential thiol reactions involving 1,6-hexanedithiol (HDT), 1.4-butanediol diacrylate (BDDA), and several diisocyanates (ISO). Thiol-terminated prepolymers prepared by the phosphine-catalyzed thiol Michael addition of HDT and BDDA form flexible thioether oligomers which were then incorporated as soft segments into polythiourethane main chains through a triethylamine-catalyzed thiol-isocyanate reaction with HDT and ISO to give polymers with both hard and soft segments. Real-time FTIR, used to investigate the kinetic conversion profiles of both reactions, and NMR showed that both the thiol Michael addition and the thiol-isocyanate reactions are very fast and efficient, having the chemical attributes generally associated with thiol-ene radical click reactions. The effects of the soft and hard segment length, soft/hard segment weight ratio, and chemical structure of the ISO on thermal and dynamic thermal mechanical properties was characterized in terms of microphase separation determined by DSC and DMA. Tensile properties of the polythiourethanes were measured and correlated with the degree of microphase mixing between the soft and hard segments.