There has been a resurgence of research related to poly(phthalaldehyde) (PPHA) in the past several years because of the demonstration of long room-temperature lifetime and its low ceiling temperature facilitating the ability to rapidly depolymerize the polymer at ambient conditions. This rapid depolymerization upon triggering of PPHA mixtures makes them well suited for fabricating transient devices or stimuli-responsive materials. The copolymerization of phthalaldehyde (PHA) with other aldehydes provides a route to incorporate chemical or physical functionality into PHA-based polymers while still maintaining the favorable degradation properties. Although the anionic copolymerization of PHA with benzaldehydes has been described, only a limited number of examples have been reported on the cationic copolymerization of PHA with other aldehydes. In this study, the synthesis and reactivity behavior of PHA copolymers is reported. Aliphatic aldehydes were chosen for their prevalence and favorable degradation properties. Dichloromethane solvent and the boron trifluoride diethyl etherate Lewis acid catalyst showed the best monomer conversions and highest polymer molecular weights. It was found that the aliphatic aldehyde reactivity for copolymerization increased with the electron-withdrawing nature of the aldehyde, which correlates with the aldehyde hydration equilibrium constant. The molecular weight and copolymer yield decrease with an increase in the aliphatic aldehyde feed concentration. Results indicate that the polymerization conditions used in this study (ca. -78 degrees C) are above the ceiling temperature of the aliphatic aldehyde comonomers, but this does not prevent copolymerization of the comonomer with PHA. Postpolymerization modifications were performed to introduce functional groups into the PHA-based copolymer that are incompatible with the polymerization chemistry. PPHA copolymers were cross-linked using a radiation-induced thiol-ene click chemistry to show that the mechanical properties can be improved even though the copolymer has a lower molecular weight.