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. Discovery and history Poly(phthalaldehyde) was first reported in 1967 by Chuji Aso and Sanae Tagami from the department of Organic Synthesis at Kyushu University by an addition homopolymerization reaction of aromatic o-phthalaldehyde (Figure 1). This polymer, consisting of a polyacetal main chain, is still to date, the only aromatic aldehyde that can be homopolymerized through a chain-growth polymerization method. It is a white brittle solid with a low ceiling temperature and significant self-immolative properties. It has gathered significant attention in recent years especially in the development of novel responsive materials and applications. A summary of the main milestones representing the first report of the applications of PPA is illustrated in Figure 2. Synthesis techniques Since its first inception in 1967, many synthesis techniques have been developed and employed for the polymerization of o-phthalaldehyde. Most notably, living polymerization methods are among the most common and promising techniques used, as can be seen in the high number of publications in the literature depicting their usage in poly(phthalaldehyde) preparation. Nonetheless, three different synthesis methods will be described in this section. Living cationic polymerization (LCP) History and main idea Aso and Tagami were the first to report the polymerization of o-phthalaldehyde in 1967 using the cationic living polymerization technique. This technique, which was initially thought to require the usage of a strong Brönsted acid to initiate polymerization in addition to a strong nucleophile to depress polymerization and endcap the polymer chain was proven successful in a number of polymerization processes reported earlier. Interestingly, the authors were able to produce this polymer without using an initiator nor a terminator and determined the polymer’s structure to be cyclic. In fact, they worked at liquid nitrogen temperature and relied on Boron trifluoride etherate catalyst which was sufficient to produce a polymer stable enough at room temperature for a few days. Current trends In the following years, polymer chemists started studying the characteristics of this polymer and worked on enhancing its thermal stability and mechanical properties.