Non-isocyanate polyurethane (NIPU) was synthesized via cyclic carbonate aminolysis using poly(ethylene oxide) (PEO)- and poly(tetramethylene oxide) (PTMO)-based soft segments, divinylbenzene dicyclocarbonate as hard segment, and diamine-diamide (DDA) chain extender. Characterization of the resulting segmented polyhydroxyurethanes (PHUs) reveals that the use of amide-based DDA chain extender leads to unprecedented improvements in nanophase separation and thermal and mechanical properties over segmented PHUs without DDA chain extender. With PEO-based soft segments, previously known to yield only phase-mixed PHUs, use of DDA chain extender yields nanophase-separated PHUs above a certain hard-segment content, as characterized by small-angle X-ray scattering. With PTMO-based soft segments, previously known to yield nanophase-separated PHUs with broad interphase, use of DDA chain extender produces nanophase-separated PHUs with sharp domain interphase, leading to wide, relatively temperature-independent rubbery plateau regions and much improved thermal properties with flow temperature as high as 200 degrees C. The PTMO-based PHUs with 19-34 wt % hard-segment content exhibit tunable mechanical properties with Young's modulus ranging from 6.6 to 43.2 MPa and tensile strength from 2.4 to 6.7 MPa, with similar to 300% elongation at break. Cyclic tensile testing shows that these PHUs exhibit elastomeric recovery with attributes very similar to conventional, isocyanate-based thermoplastic polyurethane elastomers.