Clip Chemistry: Diverse (Bio)(macro)molecular and Material Function through Breaking Covalent Bonds.

In the two decades since the introduction of the "click chemistry" concept, the toolbox of "click reactions" has continually expanded, enabling chemists, materials scientists, and biologists to rapidly and selectively build complexity for their applications of interest. Similarly, selective and efficient covalent bond breaking reactions have provided and will continue to provide transformative advances. Here, we review key examples and applications of efficient, selective covalent bond cleavage reactions, which we refer to herein as "clip reactions." The strategic application of clip reactions offers opportunities to tailor the compositions and structures of complex (bio)(macro)molecular systems with exquisite control. Working in concert, click chemistry and clip chemistry offer scientists and engineers powerful methods to address next-generation challenges across the chemical sciences.

Converting Commodity Polyolefins to Electronic Materials through Borane-Catalyzed Alkene Isomerization.

Novel approaches to the functionalization of commodity polymers could provide avenues for the synthesis of materials for next-generation electronic devices. Herein, we present a catalytic method for the conversion of common unsaturated polymers such as polybutadiene, polyisoprene, and styrene-butadiene copolymers [e.g., polystyrene-block-polybutadiene-block-polystyrene and poly(styrene-stat-butadiene)] to poly(acetylene) (PA)-based multiblock copolymers with conjugation lengths of up to ∼20, making them potentially suitable for electronics applications. Additionally, we demonstrate the application of this method to the formal conversion of polyethylene─the most widely produced thermoplastic─into PA-containing multiblock materials.

Biodegradable and pH-responsive nanoparticles designed for site-specific delivery in agriculture.

We report the synthesis and characterization of pH-responsive polysuccinimide-based nanoparticles. Polysuccinimide (PSI), a precursor to biodegradable poly(aspartic acid), was synthesized from the condensation of l-aspartic acid and subsequently functionalized with primary amines to form random amphiphilic copolymers. The copolymers formed stable nanoparticles in aqueous medium via nanoprecipitation and were subsequently loaded with a model hydrophobic molecule to demonstrate their potential as controlled-release delivery vehicles. It was found that above pH 7, the hydrophobic succinimidyl units of the PSI nanoparticles hydrolyzed to release encapsulated materials. The release rate significantly increased at elevated pH and decreased with an increasing degree of functionalization. Finally, plant toxicity studies showed that the polymer materials exhibit little to no toxic effects at biologically relevant concentrations.

Self-healing hydrogels containing reversible oxime crosslinks.

Self-healing oxime-functional hydrogels have been developed that undergo a reversible gel-to-sol transition via oxime exchange under acidic conditions. Keto-functional copolymers were prepared by conventional radical polymerization of N,N-dimethylacrylamide (DMA) and diacetone acrylamide (DAA). The resulting water soluble copolymers (P(DMA-stat-DAA)) were chemically crosslinked with difunctional alkoxyamines to obtain hydrogels via oxime formation. Gel-to-sol transitions were induced by the addition of excess monofunctional alkoxyamines to promote competitive oxime exchange under acidic conditions at 25 °C. The hydrogel could autonomously heal after it was damaged due to the dynamic nature of the oxime crosslinks. In addition to their chemo-responsive behavior, the P(DMA-stat-DAA) copolymers exhibit cloud points which vary with the DAA content in the copolymers. This thermo-responsive behavior of the P(DMA-stat-DAA) was utilized to form physical hydrogels above their cloud point. Therefore, these materials can either form dynamic-covalent or physically-crosslinked gels, both of which demonstrate reversible gelation behavior.

Efficiency of Biodegradable and pH-Responsive Polysuccinimide Nanoparticles (PSI-NPs) as Smart Nanodelivery Systems in Grapefruit: In Vitro Cellular Investigation.

Biodegradable pH-responsive polysuccinimide nanoparticles (PSI-NPs) are synthesized for directly delivering agrochemicals to plant phloem to improve their efficacy. The PSI-NPs have an average size of 20.6 nm with negative charge on the surface. The desired responsiveness to changes in pH is demonstrated by release efficiency of the model molecule (Coumarin 6), which increases with increasing pH over 24 h. The internalization of PSI-NPs into grapefruit cells occurs in 10 min, and into nucleus in 2 h, with most of the PSI-NPs being distributed in cytoplasm and nucleus. The proportion of PSI-NPs in plant cells significantly increases with time, from 19.1% at 10 min to 55.5% at 2 h of administering. The PSI-NPs do not show significant inhibitory effects on soil microbial growth and activity. These results indicate that this smart nanodelivery system has potential of application in agriculture for mitigating phloem-limited diseases, such as citrus huanglongbing.

Radical Ring-Opening Copolymerization of Cyclic Ketene Acetals and Maleimides Affords Homogeneous Incorporation of Degradable Units.

Radical copolymerization of donor-acceptor (D-A) monomer pairs has served as a versatile platform for the development of alternating copolymers. However, due to the use of conventional radical polymerization, the resulting copolymers have generally been limited to nondegradable vinyl polymers. By combining radical D-A copolymerization with radical ring-opening polymerization (rROP), we have synthesized an alternating copolymer with a high incorporation of degradable backbone units. Copolymerization of N-ethyl maleimide (NEtMI) with the cyclic ketene acetal (CKA) 2-methylene-4-phenyl-1,3-dioxolane (MPDL) was demonstrated to proceed in an alternating fashion, and controlled polymerization was achieved using reversible addition-fragmentation chain transfer (RAFT) polymerization. Spontaneous copolymerization, in the absence of an exogenous initiating source, occurred when the mixture of monomers was heated, presumably due to the large electron disparity between the comonomers. Chain-extension with styrene afforded well-defined P(MPDL-alt-NEtMI)-b-polystyrene copolymers, and degradation of the homopolymers and block copolymers showed complete breakdown of the alternating copolymer.

Macromolecular metamorphosis via stimulus-induced transformations of polymer architecture.

Macromolecular architecture plays a pivotal role in determining the properties of polymers. When designing polymers for specific applications, it is not only the size of a macromolecule that must be considered, but also its shape. In most cases, the topology of a polymer is a static feature that is inalterable once synthesized. Using reversible-covalent chemistry to prompt the disconnection of chemical bonds and the formation of new linkages in situ, we report polymers that undergo dramatic topological transformations via a process we term macromolecular metamorphosis. Utilizing this technique, a linear amphiphilic block copolymer or hyperbranched polymer undergoes 'metamorphosis' into comb, star and hydrophobic block copolymer architectures. This approach was extended to include a macroscopic gel which transitioned from a densely and covalently crosslinked network to one with larger distances between the covalent crosslinks when heated. These architectural transformations present an entirely new approach to 'smart' materials.

Boronic Acid Linear Homopolymers as Effective Emulsifiers and Gelators.

We report emulsion studies using poly(vinylphenyl boronic acid) (PVPBA) linear homopolymer as an effective emulsifier and gelator. Two stabilizing regimes were identified depending on the pH of PVPBA aqueous solutions, i.e., emulsions stabilized by the hompolymer nanoparticles (Pickering emulsions) at pH < pKa and emulsions stabilized by the homopolymer unimers at pH > pKa. In both cases, gelled emulsions were obtained from medium to high internal phase volume fractions with the unimers exhibiting more effective emulsification and gelling properties. Hydrogen bonding between the boronic acid units is proposed to account for the high strength of the emulsions. The emulsions were shown to be pH- and sugar-responsive. Finally, the stable emulsions were used as templates to directly prepare PVPBA macroporous materials and to fabricate multilayered capsules. This remarkable observation that a simple homopolymer can serve as an effective emulsifier and gelator may dramatically extend the scope of potential emulsifiers and inspire further research in the design of new types of efficient emulsifying agents.

Synthesis of amphiphilic polysuccinimide star copolymers for responsive delivery in plants.

While polymeric nanocarriers are widely used in medicine for controlled release and site-specific delivery, few reports have applied such delivery methods within agriculture, despite the urgent need for specific delivery of pesticides and nutrients. We report the synthesis of stimuli-responsive and biodegradable polymeric nanocarriers designed for delivery to the phloem of plants and describe methods employed to evaluate their toxicity in plant cells.