End-capped poly(phthalaldehyde) [PPA] is a well-studied metastable polymer that has attracted interest due to its ease of synthesis and rapid depolymerization. PPA is limited, however, in the type of macromolecular architectures accessible, as functionalizable phthalaldehyde derivatives are not commercially available and their synthesis is cumbersome. To this end, a general route to phthalaldehyde-benzaldehyde copolymers was sought, as benzaldehyde comonomers with various pendant functionalities are readily available. It was found that copolymers are synthesized by an anionic initiated polymerization of phthalaldehyde and electron-deficient benzaldehydes. The comonomer reactivities are shown to be sensitive to the benzaldehyde electronics; the relative reactivity of phthalaldehyde benzaldehyde comonomer pairs strongly correlate with the Hammett values of the benzaldehyde monomer. These copolymers are then further modified to yield cross-linked, degradable polymer networks in just a two-step sequence. Phthalaldehyde benzaldehyde copolymers thus enable functionalization of metastable polymers that rapidly depolymerize upon exposure to acid, thereby facilitating the development of triggerable degradation of polymer networks.