Poly(phthalaldehyde)
[3] 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.
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.
[9] In particular, Moore and coworkers conducted rigorous mechanistic studies on poly(phthalaldehyde) by modifying the type of catalyst used, as well as the starting monomer concentration in an effort to control the molar mass, decrease the polydispersity index, and increase the polymer's purity.
While LCP was the first and sole method used to produce poly(phthalaldehyde), its usage nowadays has dramatically decreased in favor of other polymerization techniques which allow a better control over the polymer properties including molar mass and thermal stability.
[11] In general, LAP involves the usage of a strong nucleophile to initiate polymerization in addition to the employment of an electrophile as a terminator to endcap the polymer chain.
Moreover, most of the published research articles describing PPA synthesis between 2008 and 2023 revolve around the usage of LAP, rendering it the most common and effective polymerization technique.
The major advantage this polymerization technique presents over LCP lies in the fact that the polymer can be end capped on both sides of the chain with stimuli-responsive groups.
Professor Hisaya Tani from the Department of Polymer Science at Osaka University was the first to report a stereospecific polymerization of o-phthalaldehyde by employing dimeric dimethylaluminumoxybenzylideneaniline [Me2AlOCMeNPh]2 as catalyst and water as a co-catalyst.
Nonetheless, due to the inability to endcap the polymer with functional groups, this technique is rarely utilized at present and the mechanism of formation of PPA remains ambiguous and not well studied.
Although processing linear PPA requires highly sensitive reaction conditions and is more time demanding, this type of polymer has many advantages over its cyclic counterpart.
Various linear PPA with distinct end groups have been reported and studied for a variety of applications including sensing, drug delivery, and lithography.
[15] For instance, once these end groups are cleaved as a response to the exposure of PPA to a specific stimulus, the polymer will sequentially disassemble from head to tail through an unzipping reaction to form the monomer in short times that can be as low as a few minutes.
It was not until 2013 that polymer chemists proved that the structure is cyclic using a combination of characterization techniques including Nuclear Magnetic Resonance (NMR), Fourier Transform Infrared Spectroscopy (FT-IR), Gel Permeation Chromatography (GPC), and Mass Spectrometry (MS).
[20] Furthermore, the polymer can be isolated without the addition of pyridine nor methanol nor a strong base terminator, which in general makes this polymerization technique easy, fast, and cheap.
[9] With the insurgence in the usage of PPA during the past few years for various applications, the need to ameliorate the transient properties and enhance the mechanical features of this polymer has come to surface.
PPA is known to be brittle; it possesses a large storage modulus, and a glass transition temperature that is above its thermal degradation point, which renders the polymer unsuitable for a broad range of applications.
[9] One way to ameliorate its intrinsic properties is via the addition of a plasticizing agent that can disrupt the polymer's intermolecular packing, and thus making it more flexible, decreasing its storage modulus, depressing its glass transition temperature, and increasing its shear strength.
[9] In another study published by the same research group, the effect of diethyl adipate (DEA) plasticizer on the glass transition temperature of cyclic PPA was investigated.
By varying DEA concentration, the authors were able depress Tg to 12.5 °C demonstrating the importance of plasticizers in enhancing the mechanical flexibility and thermal properties of PPA.
[25][26] Similarly, Phillips et al. proved that the substituted and end-capped poly(4,5-dichlorophthalaldehyde) possesses higher thermal degradation temperatures than its unsubstituted counterparts.
The high solubility and stability of PPA in organic solvents have allowed its investigation as a base material in first generation amplified photoresist for lithography in the early 80s by three scientists, Grant Willson, Jean Fréchet, and Hiroshi Ito who were working at IBM at the time.
[35] Another similar application in transient electronics was reported where an organic light-emitting diode (OLED) was integrated on the PPA substrate and can cause depolymerization in the presence of a PAG.
This has been reported by Phillips and his research group, where they used an allyl formate endcap that stoichiometrically depolymerized within minutes upon its exposure to a catalytic amount of tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4).
