Durable, broadband-transparent polyoxalamide- based optics and methods of making and using the same

Some variations provide an optic (such as an optical lens or an optical window) comprising at least 1 wt % of a polyoxalamide, wherein the optic is characterized by at least 40% average transmission of infrared radiation, and wherein the polyoxalamide comprises: first segments containing at least one repeat unit that includes (i) a branched, aliphatic hydrocarbon species and (ii) first amide groups at internal ends of the repeat unit, wherein the first amide groups are part of oxalamide groups; polymer end groups containing second amide groups that are each covalently bonded directly to one of the first amide groups, wherein the second amide groups are also part of the oxalamide groups, and wherein the oxalamide groups contain -N-C(═O)-C(═O)-N- sequences; and a reacted form of multifunctional amine chain extenders or crosslinkers with an amine functionality of 3 or greater. Methods of making and using the optic are described.