Fully-cycled depolymerization and repolymerization of a low ceiling temperature polymer, cyclic poly(phthalaldehyde) (cPPA), yielding high performance structural polymer is demonstrated. The facile conditions for cPPA depolymerization circumvent the extreme conditions required to break down and recycle traditional thermoplastics and thermosets. cPPA depolymerizes in as little as 14 min at 120 °C, with concurrent evaporation and quantitative recovery of the monomer. Polymerization of the recovered monomer produces cPPA with molecular and mechanical properties identical to the original material. Depolymerization of cPPA is also demonstrated in the presence of various carbon fiber reinforcements. Continuous carbon fibers retain 100% of their moduli and tensile strength through multiple generations of recycling, while fully recycled cPPA/carbon nanofiber composites exhibit mechanical properties equivalent to the original composite and show no degradation with cycling.

Cyclic poly(phthalaldehyde) (cPPA) is a metastable and stimuli responsive polymer that undergoes rapid solid state depolymerization and has been utilized as a packaging and encapsulating material for transient applications. However, the early onset thermal depolymerization of cPPA severely hinders the fabrication and processing of plastic parts. Herein, the thermally triggered depolymerization of cPPA was investigated and tailored to enable thermal processing and molding of cPPA at moderate temperatures below the thermal depolymerization temperature. Stabilization of cPPA at elevated temperature was accomplished by removal of the latent Lewis acid catalyst BF3 and by addition of radical inhibitors and a Lewis base. Addition of a plasticizer to the stabilized cPPA enabled the fabrication of a monolithic solid polymer via hot press molding. Importantly, it is shown that the thermally processed cPPA retains its stimuli responsive depolymerization capability and will enable future work in the fabrication of bulk plastic parts that depolymerize and disintegrate on demand.

The redox equilibrium method was used to determine accurate oxidation potentials in acetonitrile for 40 heteroatom-substituted compounds. These include methoxy-substituted benzenes and biphenyls, aromatic amines, and substituted acetanilides. The redox equilibrium method allowed oxidation potentials to be determined with high precision (≤ ±6 mV). Whereas most of the relative oxidation potentials follow well-established chemical trends, interestingly, the oxidation potentials of substituted N-methylacetanilides were found to be higher than those of the corresponding acetanilides. Density functional theory calculations provided insight into the origin of these surprising results in terms of the preferred conformations of the amides versus their cation radicals.

Transient polymers rapidly and controllably depolymerize in response to a specific trigger, typically by a chain-end unzipping mechanism. Triggers, such as heat, light, and chemical stimuli, are generally dependent on the chemistry of the polymer backbone or end groups. Single electron transfer (SET), in contrast to other triggering mechanisms, is achievable by various means including chemical, electrochemical, and photochemical oxidation or reduction. Here, we identify SET and subsequent mesolytic cleavage as the major thermal triggering mechanism of cyclic poly(phthalaldehyde) (cPPA) depolymerization. Multimodal SET triggering is demonstrated by both chemical and photoredox-triggered depolymerization of cPPA. Redox-active small molecules (p-chloranil and 1,3,5-trimethoxybenzene) were used to tune the depolymerization onset temperature of cPPA over the range 105−135 °C. Extending this mechanism to photoredox catalysis, N-methylacridinium hexafluorophosphate (NMAPF 6 ) was used to photochemically degrade cPPA in solution and thin films. Finally, we fabricated photodegradable cPPA monoliths with a storage modulus of 1.8 GPa and demonstrated complete depolymerization within 25 min of sunlight exposure. Sunlight-triggered depolymerization of cPPA is demonstrated and potentially useful for the manufacture of transient devices that vanish leaving little or no trace. Most importantly, this new mechanism is likely to inspire other SET-triggered transient polymers, whose development may address the ongoing crisis of plastic pollution.

Exciplexes of 9,10-dicyanoanthracene (DCA) with alkylbenzene donors in cyclohexane show structureless emission spectra, typical of exciplexes with predominantly charge-transfer (CT) character, when the donor has a relatively low oxidation potential (Eox ), e.g. hexamethylbenzene (HMB). With increasing Eox and stronger mixing with a locally excited (LE) state, vibrational structure begins to appear with 1,2,3,5-tetramethylbenzene and becomes prominent with p-xylene (p-Xy). A simple theoretical model reproduces the spectra and the radiative rate constants, and it reveals several surprises: Even in this nonpolar solvent, the fractional CT character of a highly mixed exciplex varies widely in response to fluctuations in the microscopic environment. Environments that favor the LE (or CT) state contribute more to the blue (or red) side of the overall spectrum. It is known that sparsely substituted benzene radical cations, e.g., p-Xy(•+) , are stabilized more in acetonitrile than the heavily substituted HMB(•+) . Remarkably, ion pairing with DCA(•-) in cyclohexane leads to even larger differences in the stabilization of these radical cations. The spectra of the low-Eox donors are almost identical except for displacements that approximately equal the differences in Eox , even though the exciplexes have varying degrees of CT character. These similarities result from compensation among several nonobvious, but quantified factors.