Accessibility of Responsive End-Caps in Films Composed of Stimuli-Responsive, Depolymerizable Poly(phthalaldehydes)

Recent advances in self-immolative linkers and their applications in polymeric reporting systems

Interest in self-immolative chemistry has grown over the past decade with more research groups harnessing the versatility to control the release of a compound from a larger chemical entity, given...

Poly(carboxypyrrole)s That Depolymerize from Head to Tail in the Solid State in Response to Specific Applied Signals

This Article describes the design, synthesis, and analysis of a new class of polymer that is capable of depolymerizing continuously, completely, and cleanly from head to tail when a detection unit on the head of the polymer is exposed to a specific applied signal. The backbone of this polymer consists of 1,3-disubstituted pyrroles and carboxy linkages similar to polyurethanes. Diverse side chains or reactive end-groups can be introduced readily, which provides modular design of polymer structure. The designed depolymerization mechanism proceeds through spontaneous release of carbon dioxide and azafulvene in response to a single triggering reaction with the detection unit. These poly(carboxypyrrole)s depolymerize readily in nonpolar environments, and even in the bulk as solid-state plastics.

UV-Triggered Transient Electrospun Poly(propylene carbonate)/Poly(phthalaldehyde) Polymer Blend Fiber Mats

This work reports the first transient electrospun nanofiber mat triggered by UV-irradiation using poly(propylene carbonate) (PPC)/poly(phthalaldehyde) (cPPA) polymer blends. The ability to trigger room temperature transience of nanofiber mats without the need for additional heat or solvent expands its utility in nonbiological fields, especially for transient electronic devices. The addition of a photo-acid-generator to the system, working in combination with UV light, provides an acid source to enhance degradation because both polymer backbones are acid-sensitive. Electrospinning enables the production of PPC/cPPA composite nanofiber mats capable of significant degradation upon exposure to UV radiation while maintaining relatively high mechanical properties. An acid amplifier, an autocatalytically decomposing compound triggered by acid, was used to generate more acid and accelerate nanofiber degradation. The electrospun fiber mats can be post-annealed to achieve an improved mat with a mechanical strength of ∼170 MPa.

Modulating the Depolymerization of Self-Immolative Brush Polymers with Poly(benzyl ether) Backbones

We have synthesized a series of stimuli-responsive brush polymers by grafting azide-terminated side chains onto a self-immolative, alkyne-bearing poly(benzyl ether) backbone, which is prepared by anionic polymerization of quinone methide-based monomers. Upon exposure to a decapping reagent (Pd(0) or F^-), these brush polymers undergo an irreversible degradation cascade from head to tail to yield individual side chains. It is observed that several factors affect the depolymerization kinetics, including solvent polarity, type of counterion, the rate of the decapping chemistry, and interestingly, the rigidity of the side chains.

Synthesis and kinetics of disassembly for silyl-containing ethoxycarbonyls using fluoride ions

In this study, a series of silyl-containing ethoxycarbonates and ethoxycarbamates on electron poor anilines and phenols were synthesized and their kinetics of disassembly determined in real-time upon exposure to fluoride ion sources at room temperature. The results provide a greater understanding of stability and kinetics for silyl-containing protecting groups that eliminate volatile molecules upon removal, which will allow for simplification of orthogonal protection in complex organic molecules.

Rate constants of disassembly for synthesized silyl-containing ethoxycarbonyls were significantly faster than TEOC, a common protecting group.

Depolymerizable Poly(O-vinyl carbamate-alt-sulfones) as Customizable Macromolecular Scaffolds for Mucosal Drug Delivery

Interest in stimulus responsive materials and polymers has grown over the years, having shown great promise in a diverse set of applications. For drug delivery, stimulus-responsive polymers have been shown to encapsulate therapeutic cargo such as small molecule drugs or proteins, deliver them to specific locations in the body, and release them so that they can induce a therapeutic effect in the patient. Most hydrolytically degradable polymers are synthesized via nucleophilic, anionic, or cationic polymerization, which generally requires protection of nucleophilic or protic side chains prior to polymerization. Here, we report the synthesis of novel, alternating copolymers of sulfur dioxide and O-vinyl carbamate monomers that boast excellent functional group tolerance and pH-dependent instability. Alternating copolymers were synthesized containing pendant functionalities such as alcohol, carboxylic acid, ester, and azide without deprotection or post-polymerization modification. The copolymers were then formulated via nanoprecipitation into polymer nanoparticles capable of encapsulating small molecule dyes. The polymer nanoparticles were found to degrade rapidly at pH > 6 and were stable even in highly acidic conditions. Based on this observation, a proof-of-concept study for mucosal delivery was performed in polymer nanoparticles entrapped in a mucus model. At pH 8 the diffusion of encapsulated dye was found to be similar to free dye, while at pH 5 the diffusion coefficient was an order of magnitude lower. Cell viability was retained at 200 µg/mL particles after 24 h incubation. These polymers thus show promise as highly customizable scaffolds for mucosal drug delivery.

Synthetic Nano- and Micromachines in Analytical Chemistry: Sensing, Migration, Capture, Delivery, and Separation

Synthetic nano- and microscale machines move autonomously in solution or drive fluid flows by converting sources of energy into mechanical work. Their sizes are comparable to analytes (sub-nano- to microscale), and they respond to signals from each other and their surroundings, leading to emergent collective behavior. These machines can potentially enable hitherto difficult analytical applications. In this article, we review the development of different classes of synthetic nano- and micromotors and pumps and indicate their possible applications in real-time in situ chemical sensing, on-demand directional transport, cargo capture and delivery, as well as analyte isolation and separation.

Surface-accessible detection units in self-immolative polymers enable translation of selective molecular detection events into amplified responses in macroscopic, solid-state plastics

This Communication describes a strategy for incorporating detection units onto each repeating unit of self-immolative CDr polymers. This strategy enables macroscopic plastics to respond quickly to specific applied molecular signals that react with the plastic at the solid-liquid interface between the plastic and surrounding fluid. The response is a signal-induced depolymerization reaction that is continuous and complete from the site of the reacted detection unit to the end of the polymer. Thus, this strategy retains the ability of CDr polymers to provide amplified responses via depolymerization while simultaneously enhancing the rate of response of CDr-based macroscopic plastics to specific applied signals. Depolymerizable poly(benzyl ethers) were used to demonstrate the strategy and now are capable of depolymerizing in the context of rigid, solid-state polymeric materials.

End-capped poly(4,5-dichlorophthalaldehyde): a stable self-immolative poly(aldehyde) for translating specific inputs into amplified outputs, both in solution and the solid state

Poly(4,5-dichlorophthalaldehyde) is a new self-immolative CD_r polymer that is substantially more stable than poly(phthalaldehyde), yet retains its favorable attributes.

Multiresponsive Azobenzene End-Cap for Self-Immolative Polymers

Azobenzene was introduced as a new multiresponsive end-cap for self-immolative polymers. Using small-molecule model compounds, it was demonstrated that reducing agents including hydrazine and dithiothreitol could reduce the azobenzene to the corresponding hydrazobenzene, resulting in a 1,6-elimination reaction with the potential to initiate the depolymerization of self-immolative polycarbamates. An activated azobenzene derivative was then prepared, allowing for its incorporation as an end-cap for polycarbamates based on alternating N,N'-dimethylethylene diamine and 4-hydroxybenzyl alcohol. Upon treatment with hydrazine, depolymerization proceeded. To demonstrate the versatility of this end-cap, the chemistry was also applied to polycarbamates based on 4-aminobenzyl alcohol. In addition to allowing triggered depolymerization, the azobenzene end-cap also provides a visual signal upon triggering owing to the strong visible absorption of the azobenzene, which shifts to the UV range upon reduction. Furthermore, azobenzene is capable of undergoing trans-cis isomerization in response to UV light, providing multiple functions in a single end-cap.

Continuous Head-to-Tail Depolymerization: An Emerging Concept for Imparting Amplified Responses to Stimuli-Responsive Materials

Polymers that depolymerize continuously and completely from head-to-tail when a reaction-based detection unit is cleaved from the polymer provide both selective and amplified responses, a rare combination, to stimuli-responsive polymeric materials. This Viewpoint contextualizes this new class of depolymerizable polymers and outlines the key areas for growth and innovation.