Stimuli-Responsive Depolymerization of Poly(Phthalaldehyde) Copolymers and Networks

This article describes a stimuli-responsive cross-linked network based on a poly(phthalaldehyde) (PPA) self-immolating polymer backbone which, upon removal of polymer end-caps, becomes degradable under ambient conditions. Self-immolating polymers (SIPs) are of particular interest due to their ability to undergo controlled depolymerization resulting in the elimination of the polymer structure and potential recovery and reuse of the original monomers. Linear copolymers of phthalaldehyde and unsymmetrical allylated-phthalaldehyde monomers are obtained via anionic polymerization with appreciable incorporation of the allyl-functional monomeric units. This approach enables crosslinking of the otherwise linear polymers via thiol-ene reactions with the allyl functional phthalaldehyde polymers. The polymer network thus formed unzips along the phthalaldehyde backbone to yield monomers and low molecular weight fragments in response to chemical (F-, H+) or physical (light, sonication) stimuli that remove the stabilizing functional endcaps on the phthalaldehyde polymers. Rheology is used to demonstrate gelation within 5 s of light exposure of the allylated-phthalaldehyde polymers reacted with pentaerythritol tetrakis(3-mercaptopropionate) and photoinitiator (3 wt%). Triggerable degelation makes this material well suited for photolithography and additive manufacturing as well as other applications that necessitate polymer network degradation or elimination. Further, a method is described for determining the degelation temperature of a self-immolative cross-linked network.