Poly(phthalaldehyde), abbreviated as PPA, is a metastable stimuli-responsive polymer first synthesized in 1967. It has garnered significant attention during the past couple of years due to its ease of synthesis and outstanding transient and mechanical properties. for this reason, It has been exploited for a variety of applications including sensing, drug delivery, and EUV lithography. As of 2023, it is considered the only aromatic aldehyde polymerized through a living chain growth polymerization.