Diacetylene-containing segmented copoly(ether urea) 1 was prepared via a polycondensation of 1,10-diisocyanato-deca-4,6-diyne with bis(3-aminopropyl)poly(tetrahydrofuran) (M-n approximate to 2300 g/mol). The reactive diacetylene spacer, which resides within the hard domains of the segmented elastomer, was cross-polymerized topochemically by means of UV radiation, without disrupting the flexible nature of the material, resulting in an intensely blue insoluble material. The extent of cross-polymerization was determined using solid-state C-13 NMR and was found to be similar to 10%. The optical properties of the cross-polymerized material could be modified mechanically 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.