Something that is not reactive to lesser wavelengths of light, specifically IR.
Methacrylates make nice starting materials for the lightinduced polymerization.
In methyl methacrylate (MMA), camphorquinone (a typical initiator) has an absorption maximum around $\lambda$ = 473 nm.
You mean "monomer" or "oligomer" not "polymer." You are making the polymer by polymerization or crosslinking. UV-curing optical adhesives. You choose nice (mix of) free radical-cure monomers, then dissolve a photoinitiator like camphorquinone, a diarylketone, a benzoin ether, or a benzil ketal. The molecule fragments to free radicals after absorbing a UV photon. That sets off the polymerization. Exclude oxygen (triplet diradical quench). Cage recombination of the intiator is your enemy.
A microwave-pumped electrodeless mercury bulb the size of your finger can output a kilowatt of UV. Beware of pinholes in your irradiation chamber, including entry and exit if continuous. And now, a second of silence for the Trommsdorff effect.
If you want a polymer as such to do be photoactive, then something like Prog. Polym. Sci. 28 729 (2003). Pull it up in a university library to get the graphics.