Hundreds of billions of pounds of polyolefins are manufactured per year. Despite this scale and widespread use of these materials, the applications of polyolefins have been limited by their lack of functional groups. The value of polyolefins arises, in part, from their resistance to chemical reactions. Thus, a grand challenge has been to modify polyolefins by selective functionalization of their C–H bonds. We report the selective oxidation of polyethylenes to incorporate functional groups that imbue physical properties unseen in the unmodified materials. Although further work is needed to create catalysts that reach commercially suitable lifetimes with sustainable oxidants to fully realize this potential, our demonstration that these properties can be created by the direct modification of existing polyolefins enables one to reconsider the range of applications of these materials and modes for recycling polyolefin-based materials that could exploit the installed functionality.