There is a strong imperative to synthesize polymers with highly controlled structures and narrow property ranges. Silicone polymers do not lend themselves to this paradigm because acids or bases lead ...to siloxane equilibration and loss of structure. By contrast, elegant levels of control are possible when using the Piers–Rubinsztajn reaction and analogues, in which the hydrophobic, strong Lewis acid B(C6F5)3 activates SiH groups, permitting the synthesis of precise siloxanes under mild conditions in high yield; siloxane decomposition processes are slow under these conditions. A broad range of oxygen nucleophiles including alkoxysilanes, silanols, phenols, and aryl alkyl ethers participate in the reaction to create elastomers, foams and green composites, for example, derived from lignin. In addition, the process permits the synthesis of monofunctional dendrons that can be assembled into larger entities including highly branched silicones and dendrimers either using the Piers–Rubinsztajn process alone, or in combination with hydrosilylation or other orthogonal reactions.
Activation of SiH compounds by B(C6F5)3 permits the controlled synthesis of a broad range of polymers, materials and composites. Of particular import is the ability to create complex, precise structures at scale without competition from the usual acid/base decomposition pathways.
There is a strong imperative to synthesize polymers with highly controlled structures and narrow property ranges. Silicone polymers do not lend themselves to this paradigm because acids or bases lead ...to siloxane equilibration and loss of structure. By contrast, elegant levels of control are possible when using the Piers–Rubinsztajn reaction and analogues, in which the hydrophobic, strong Lewis acid B(C6F5)3 activates SiH groups, permitting the synthesis of precise siloxanes under mild conditions in high yield; siloxane decomposition processes are slow under these conditions. A broad range of oxygen nucleophiles including alkoxysilanes, silanols, phenols, and aryl alkyl ethers participate in the reaction to create elastomers, foams and green composites, for example, derived from lignin. In addition, the process permits the synthesis of monofunctional dendrons that can be assembled into larger entities including highly branched silicones and dendrimers either using the Piers–Rubinsztajn process alone, or in combination with hydrosilylation or other orthogonal reactions.
Polymers with dynamically exchanging crosslinks possess the ability to be repurposed, in whole or part, following a stimulus. The ability of imines to undergo dynamic exchange reactions under mild ...conditions was utilized to chain extend or crosslink silicone polymers. Schiff-base crosslinked polysiloxanes were prepared by reacting aminopropyl-functionalized polydimethylsiloxane with aromatic aldehydes. The reactions occur efficiently and the water byproduct spontaneously separates from the silicone if the weight fraction of silicone is sufficiently high. The physical properties of the products were readily tunable by crosslink density; the most practical reaction partners were commercial pendant aminopropylsilicones with terephthaldehyde, although telechelic silicones and triformylbenzene were also utilized. Complete breakdown of the polymers occurred with addition of excess amine. Silicone imine cleavage with excess aldehyde also occurred, but the two processes followed different pathways. The rate of the dynamic processes was increased with good solvents, or acid or amine catalysts. However, even in the absence of catalysts the dynamic transimination was efficient. Simply placing two strips of the elastomer in contact led to an adhesive bond that, depending on contact surface area, was stronger than the cohesive strength of the rubber. The process of self-healing was followed using rheology and was shown to be complete within an hour. Surface modification of elastomers with amines occurred in water, or at aminated solid surfaces.
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•Aromatic imines rapidly form under mild conditions from aminopropylsilicones.•Polyimines are thermally stable and stable to moisture.•The polyimines exhibit a chemoplastic behavior via dynamic bond exchange in the presence of amines or acid catalysts.•Strong adhesion and self-healing are exhibited by these polymers.
Gold nanoparticle (AuNP)-based colorimetric biosensing assays have recently attracted considerable attention in diagnostic applications due to their simplicity and versatility. This Minireview ...summarizes recent advances in this field and attempts to provide general guidance on how to design such assays. The key to the AuNP-based colorimetric sensing platform is the control of colloidal AuNP dispersion and aggregation stages by using biological processes (or analytes) of interest. The ability to balance interparticle attractive and repulsive forces, which determine whether AuNPs are stabilized or aggregated and, consequently, the color of the solution, is central in the design of such systems. AuNP aggregation in these assays can be induced by an "interparticle-crosslinking" mechanism in which the enthalpic benefits of interparticle bonding formation overcome interparticle repulsive forces. Alternatively, AuNP aggregation can be guided by the controlled loss of colloidal stability in a "noncrosslinking-aggregation" mechanism. In this case, as a consequence of changes in surface properties, the van der Waals attractive forces overcome interparticle repulsive forces. Using representative examples we illustrate the general strategies that are commonly used to control AuNP aggregation and dispersion in AuNP-based colorimetric assays. Understanding the factors that play important roles in such systems will not only provide guidance in designing AuNP-based colorimetric assays, but also facilitate research that exploits these principles in such areas as nanoassembly, biosciences and colloid and polymer sciences.
Silicone sealants and adhesives are widely used to prevent the ingress of water. However, silicones must normally be cured in air, as excess water inhibits or prevents cure from occurring. It is ...reported that aqueous solutions of the aliphatic aldehydes glutaraldehyde, glyoxal or, particularly, formaldehyde rapidly react without catalysts with a variety of aminopropyl‐modified silicone polymers to give silicone elastomers, even underwater. These products, whose properties are readily tailored by controlling the density of amino groups in the starting materials, may be 3D printed, or used both as adhesives and sealants in air or water.
Aqueous solutions of the small aldehydes glutaraldehyde, glyoxal, and especially formaldehyde, react rapidly with aminopropylsilicones to create silicone elastomers in air or under water, that can act as sealants or adhesives, including in 3D printing applications.
Many strategies have been adopted to prepare silica materials with highly controlled structures, typically using sol-gel chemistry. Frequently, the alkoxysilanes used in sol-gel chemistry are based ...on monoalcohols, e.g., Si(OEt)
. The structural control over silica synthesis achieved by these precursors is highly sensitive to pH and solvency. Alkoxysilanes derived from the sugar alcohol glycerol (diglycerylsilane) react more slowly and with much less sensitivity to pH. We report that, in the presence of cooled aqueous starch solutions, glyceroxysilanes undergo transesterification with the sugars on starch, leading to (hollow) microtubules resembling worms of about 400 nm in diameter. The tubes arise from the pre-assembly of starch bundles, which occurs only well below room temperature. It is straightforward to treat the first-formed starch/silica composite with the enzyme amylase to, in a programmed fashion, increasingly expose porosity, including the worm morphology, while washing away untethered silica and digested starch to leave an open, highly porous materials. Sintering at 600 °C completely removes the starch silane moieties.
Rolling circle amplification (RCA) is an isothermal, enzymatic process mediated by certain DNA polymerases in which long single-stranded (ss) DNA molecules are synthesized on a short circular ssDNA ...template by using a single DNA primer. A method traditionally used for ultrasensitive DNA detection in areas of genomics and diagnostics, RCA has been used more recently to generate large-scale DNA templates for the creation of periodic nanoassemblies. Various RCA strategies have also been developed for the production of repetitive sequences of DNA aptamers and DNAzymes as detection platforms for small molecules and proteins. In this way, RCA is rapidly becoming a highly versatile DNA amplification tool with wide-ranging applications in genomics, proteomics, diagnosis, biosensing, drug discovery, and nanotechnology.
Silicone surfactants are widely used in many industries and mostly rely on poly(ethylene glycol) (PEG) as the hydrophile. This can be disadvantageous because commercial PEG examples vary ...significantly in polydispersity—constraining control over surface activity of the surfactant—and there are environmental concerns associated with PEG. Herein, we report a three-step synthetic method for the preparation of saccharide-silicone surfactants using the natural linker, cysteamine, and saccharide lactones. The Piers–Rubinsztajn plus thiol-ene plus amidation process is attractive for several reasons: if employed in the correct synthetic order, it allows for precise tailoring of both hydrophobe and hydrophile; it permits the ready utilization of natural hydrophiles cysteamine and saccharides in combination with silicones, which have significantly better environmental profiles than PEG; and the products exhibit interesting surface activities.
Most silicone elastomers are thermosets. As a response to the new paradigm of polymer recyclability, the development of silicone elastomers that can be reversibly and repeatedly cured and ...uncrosslinked using redox conditions is reported. Thiopropyl‐modified silicones are oxidized to elastomers with disulfide crosslinks using the organosoluble oxidant PhI(OAc)2. As with any elastomer, mechanical properties can be tuned by varying crosslink density. Thermal stabilities in air show that the products are comparable to traditional silicone thermosets, with degradation only starting over 300 °C. Uncrosslinking back to the same thiopropyl‐modified silicones involves reductive S–S bridge cleavage using a Piers–Rubinsztajn reaction with hydrosilanes catalyzed by B(C6F5)3; HSiMe2OSiMe3 is identified as a convenient reducing agent. The initially formed silicone‐(CH2)3S‐SiMe2OSiMe3 products need deprotection with water in isopropanol/water to completely regenerate the thiopropylsilicones. This oxidation/reduction crosslinking/uncrosslinking cycle is practiced thrice, with a yield of 89% per cycle, with essentially no change in the Young's moduli of the elastomers, or 1H NMR spectra of the uncrosslinked fluids after reduction. Further oxidation of disulfide groups on the elastomer surface permanently and significantly improved water wettability.
Silicone elastomers may be repeatedly, iteratively, formed by oxidation of thiopropylsilicone oils and then reduced using hydrosilanes under Piers Rubinsztajn conditions. Overoxidation of the elastomer surfaces using bleach permits more wettable surfaces to be formed that are less susceptible to hydrophobic recovery.
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