Diacetylene-contg. segmented copoly(ether urea) 1 was prepd. via a polycondensation of 1,10-diisocyanato-deca-4,6-diyne with bis(3-aminopropyl)poly(tetrahydrofuran) (Mn ~ 2300 g/mol). The reactive diacetylene spacer, which resides within the hard domains of the segmented elastomer, was cross-polymd. topochem. by means of UV radiation, without disrupting the flexible nature of the material, resulting in an intensely blue insol. material. The extent of cross-polymn. was detd. using solid-state 13C NMR and was found to be .apprx.10%. The optical properties of the cross-polymd. material could be modified mech. and thermally as well as by changing the solvent. A study into the mechanochromic behavior-a color change from blue to yellow upon elongation-showed that the color changes are irreversible at strain levels greater than 80%. Polarized UV spectroscopy indicated that upon deformation the polydiacetylene chains orient parallel to the deformation axis. Thermochromic and solvatochromic behavior of 1 was completely irreversible.