Polymers that depolymerize back to monomers can be repeatedly chemically recycled, thereby reducing their environmental impact. Polyphthalaldehyde is a metastable polymer that can rapidly and quantitatively depolymerize due to its low ceiling temperature. However, the effect of substitution on the physical and chemical properties of polyphthalaldehyde derivatives has not been systematically studied. Herein, we investigate the cationic polymerization of seven o-phthalaldehyde derivatives and demonstrate that judicious choice of substituent results in materials with a wide range of ceiling temperatures (<-60 to 106 °C) and decomposition temperatures (109-196 °C). We anticipate that these new polymers and their derivatives will enable researchers to access degradable materials with tunable thermal, physical, and chemical properties.