Poly(o-xylylene) (PDX) has long been a challenging synthetic target despite its simple structure and potentially useful physical properties. In this report, we demonstrate a valence isomer strategy that leads to the formation of high molecular weight PDX via an intermediate polymer of a unique structure, namely poly(Dewar-o-xylylene) (PDOX). We show that the free radical polymerization of highly strained Dewar-o-xylylene (DOX) monomer afforded PDOX, a material with a high density of Dewar benzene units in the backbone through ring-retaining propagation. The thermal- and photoinduced isomerizations of PDOX to produce PDX were investigated. This chemistry yields PDXs that are difficult to obtain using traditional methods. Moreover, it also provides a potential entry into new reconfigurable materials featuring highly efficient postpolymerization main chain structural transformations.