Multiply functional hydroxyl telechelic poly(cyclooctene-s-5-norbornene-2- methylene methacrylate) was synthesized by ring opening metathesis (co)polymerization of cis-cyclooctene and 5-norbornene-2-methylene methacrylate using the second generation Grubbs catalyst in combination with a symmetric chain transfer agent bearing hydroxyl functionality. The resulting hydroxyl-telechelic polymer was used as a macroinitiator for the ring opening transesterification polymerization of d,l-lactide to form reactive poly(lactide)-b-poly(cyclooctene-s-5-norbornene-2-methylene methacrylate)-b- poly(lactide) triblock polymers. Subsequently, the triblocks were crosslinked by free radical copolymerization with several vinyl monomers including styrene, divinylbenzene, methyl methacrylate, and ethyleneglycol dimethacrylate. Certain conditions led to optically transparent thermosets with mesoscale phase separation as evidenced by small angle X-ray scattering, differential scanning calorimetry and transmission electron microscopy. Disordered, bicontinuous structures with nanoscopic domains were generated in several cases, rendering the samples attractive for size-selective membrane applications.