Stable, High-Molecular-Weight Poly(phthalaldehyde)

Amine-Catalyzed Chain Polymerization of Ethyl Glyoxylate from Alcohol and Thiol Initiators

Polyacetals have significant potential as degradable polymers, but aldehyde polymerizations are generally difficult to control. Here we show that polymerization of ethyl glyoxylate can be initiated from alcohols or thiols by activation with triethylamine to afford poly(ethyl glyoxylate) with controllable molecular weights and relatively low dispersities (Đ = 1.3-1.4), as evidenced by MALDI-TOF mass spectrometry. Stabilization against depolymerization by chain-capping with benzyl chloroformate was found to proceed without side reactions observed from chain-capping with tolyl isocyanate. The use of the stronger base DBU leads to competing side reactions that limit polymer molecular weight.

Flexible Cyclic-Poly(phthalaldehyde)/Poly(ε-caprolactone) Blend Fibers with Fast Daylight-Triggered Transience

Cyclic-poly(phthalaldehyde) (cPPHA) exhibits photo-triggerable depolymerization on-demand for applications like the photolithography of microfabricated electronics. However, cPPHA is inherently brittle and thermally sensitive; both of these properties limit its usefulness as an engineering plastic. Prior to this report, small molecule plasticizers are added to cPPHA-based films to make the polymer more flexible. But plasticizers can eventually leach out of cPPHA, then leaving it increasingly more brittle throughout product lifetime. In this research, a new approach to fabricating flexible cPPHA blends for use as spun fibers is achieved through the incorporation of poly (ε-caprolactone) (PCL) by a modified wet spinning method. Among blend compositions, the 50/50 cPPHA/PCL fiber shows fast transience (<50 s) in response to daylight while retaining the flexibility of PCL and mechanical properties of an elastomer (i.e., tensile strength of ≈8 MPa, Young's modulus of ≈118 MPa, and elongation at break of ≈190%). Embedding 2 wt% gold nanoparticles to cPPHA can further improve the transience rate of fibers comprising less than 50% cPPHA. These flexible, daylight-triggerable cPPHA/PCL fibers can be applied to an extensive range of applications, such as wearable electronics, intelligent textiles, and zero waste packaging for which modest mechanical performance and fast transience are desired.

Triggered Transience of Plastic Materials by a Single Electron Transfer Mechanism

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 (NMAPF6) 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.

Synthesis of Unzipping Polyester and a Study of its Photochemistry

Preparation of an unzipping polyester is reported. The monomer was prepared from benzoic acid in a four-step sequence. Step growth polymerization of the monomer provides the target polymer. Efficient depolymerization upon irradiation at 254 nm was confirmed with a quantum yield of >0.8. The photolysis mechanism was investigated, and the results of radical trapping experiments are consistent with an initial Norrish type I like homolysis followed by a radical mediated depropagation reaction driven by aromatization.

Enabling Applications of Covalent Adaptable Networks

The ability to behave in a fluidlike manner fundamentally separates thermoset and thermoplastic polymers. Bridging this divide, covalent adaptable networks (CANs) structurally resemble thermosets with permanent covalent crosslinks but are able to flow in a manner that resembles thermoplastic behavior only when a dynamic chemical reaction is active. As a consequence, the rheological behavior of CANs becomes intrinsically tied to the dynamic reaction kinetics and the stimuli that are used to trigger those, including temperature, light, and chemical stimuli, providing unprecedented control over viscoelastic properties. CANs represent a highly capable material that serves as a powerful tool to improve mechanical properties and processing in a wide variety of polymer applications, including composites, hydrogels, and shape-memory polymers. This review aims to highlight the enabling material properties of CANs and the applied fields where the CAN concept has been embraced.

Exceptional copolymerizability of o-phthalaldehyde in cationic copolymerization with vinyl monomers

o-Phthalaldehyde is copolymerized via the unique active species, which allows the controlled copolymerization with alkyl vinyl ethers, alternating copolymerization, and copolymerization with sterically hindered nonhomopolymerizable vinyl monomers.

Phototriggered Depolymerization of Flexible Poly(phthalaldehyde) Substrates by Integrated Organic Light-Emitting Diodes

We demonstrate phototriggered depolymerization of a low ceiling temperature ( T_c) polymer, poly(phthalaldehyde) (PPHA), via internal light emission from integrated organic light-emitting diodes (OLEDs) fabricated directly on flexible PPHA substrates with silver nanowire electrodes. The depolymerization of the PPHA substrates is triggered by absorption of the OLED emission by a sensitizer that activates a photoacid generator via energetically favorable electron transfer. We confirm with Fourier-transform infrared spectroscopy that the photon doses delivered by the integrated OLED are sufficient to depolymerize the PPHA substrates. We determine this critical dosage by measuring the operating lifetimes of the OLEDs whose failure is believed to be due to significant mechanical softening during the liquefaction of decomposed phthalaldehyde monomers.

Cyclic Poly(phthalaldehyde): Thermoforming a Bulk Transient Material

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.