Development of Cyclic Tetrasiloxane Polymer as a High-Performance Dielectric and Hydrophobic Layer for Electrowetting Displays

Cyclic tetrasiloxane polymer (CTP) has recently garnered interest as a hydrophobic material with unique properties. This study aims to enhance the dielectric constant of CTP films by introducing excess Si-H groups and to explore the impact of synthesis and processing conditions on the resulting properties. The film demonstrates high hydrophobicity, with contact angles of 107° in air and 165° in n-decane, along with a notable dielectric constant of 5.1°. Furthermore, the CTP film displays reversible electrowetting behavior with low contact angle hysteresis (2°) and possesses good transparency (∼99%) and thermal stability. As such, the CTP film has significant potential as a material for the electric wetting of hydrophobic dielectric layers and may serve as a promising alternative in electrowetting applications.

Tris(pentafluorophenyl)borane-catalyzed Hydride Transfer Reactions in Polysiloxane Chemistry-Piers-Rubinsztajn Reaction and Related Processes

Tris(pentafluorophenyl)borane (TPFPB) is a unique Lewis acid that catalyzes the condensation between hydrosilanes (Si-H) and alkoxysilanes (Si-OR), leading to the formation of siloxane bonds (Si-OSi) with the release of hydrocarbon (R-H) as a byproduct-the so-called Piers-Rubinsztajn reaction. The analogous reactions of hydrosilanes with silanols (Si-OH), alcohols (R-OH), ethers (R-OR') or water in the presence of TPFPB leads to the formation of a siloxane bond, alkoxysilane (Si-OR or Si-OR') or silanol (Si-OH), respectively. The above processes, often referred to as Piers-Rubinsztajn reactions, provide new synthetic tools for the controlled synthesis of siloxane materials under mild conditions with high yields. The common feature of these reactions is the TPFPB-mediated hydride transfer from silicon to carbon or hydrogen. This review presents a summary of 20 years of research efforts related to this field, with a focus on new synthetic methodologies leading to numerous previously difficult to synthesize well-defined siloxane oligomers, polymers and copolymers of a complex structure and potential applications of these new materials. In addition, the mechanistic aspects of the recently discovered reactions involving hydride transfer from silicon to silicon are discussed in more detail.

Spatially Controlled Highly Branched Vinylsilicones

Branched silicones possess interesting properties as oils, including their viscoelastic behavior, or as precursors to controlled networks. However, highly branched silicone polymers are difficult to form reliably using a “grafting to” strategy because functional groups may be bunched together preventing complete conversion for steric reasons. We report the synthesis of vinyl-functional highly branched silicone polymers based, at their core, on the ability to spatially locate functional vinyl groups along a silicone backbone at the desired frequency. Macromonomers were created and then polymerized using the Piers–Rubinsztajn reaction with dialkoxyvinylsilanes and telechelic HSi-silicones; molecular weights of the polymerized macromonomers were controlled by the ratio of the two reagents. The vinyl groups were subjected to iterative (two steps, one pot) hydrosilylation with alkoxysilane and Piers–Rubinsztajn reactions, leading to high molecular weight, highly branched silicones after one or two iterations. The vinyl-functional products can optionally be converted to phenyl/methyl-modified branched oils or elastomers.

Well-defined hydrogen and organofunctional polysiloxanes with spiro-fused siloxane backbones

Structurally well-defined macrocyclic polysiloxanes with unique spirosiloxane units and regularly arranged Si–H groups were synthesized by B(C₆F₅)₃-catalyzed dehydrocarbonative cross-couplings.

Precise synthesis of linear polysiloxanes with a polar side-chain structure by organocatalytic controlled/living ring-opening polymerization of (3-cyanopropyl)pentamethylcyclotrisiloxane

Organocatalytic controlled/living ring-opening polymerization of (3-cyanopropyl)pentamethylcyclotrisiloxane using 1,3-trimethylene-2-methylguanidine as the catalyst produced various linear polysiloxanes with nitrile groups on the side chains.

Facile Synthesis of Sequence-Defined Oligo(Dimethylsiloxane-co-Diphenylsiloxane)s

1,1,3,3,5,5,7,7-Octamethyltetrasiloxane (^H MD₂ M^H ), which is reported to release Me₂ SiH₂ via a B(C₆ F₅ )₃ -catalyzed redistribution, acts as a good Me₂ SiH₂ precursor in the B(C₆ F₅ )₃ -catalyzed dehydrocarbonative condensation of alkoxysilanes. A series of oligo(dimethylsiloxane-co-diphenylsiloxane)s that are uniformly sized and sequence-defined at the atomic level are synthesized by a one-pot controlled iteration of a B(C₆ F₅ )₃ -catalyzed dehydrocarbonative condensation of alkoxysilanes with ^H MD₂ M^H or Ph₂ SiH₂ and a B(C₆ F₅ )₃ -catalyzed hydrosilylation of carbonyl compounds, followed by the subsequent B(C₆ F₅ )₃ -catalyzed dehydrogenative condensation of silanols.

Catalytic Synthesis of Oligosiloxanes Mediated by an Air Stable Catalyst, (C6F5)3B(OH2)

The utility of (C₆F₅)₃B(OH₂) as catalyst for the simple and environmentally benign synthesis of oligosiloxanes directly from hydrosilanes, is reported. This protocol offers several advantages compared to other methods of synthesizing siloxanes, such as mild reaction conditions, low catalyst loading, and a short reaction time with high yields and purity. The considerable H₂O-tolerance of (C₆F₅)₃B(OH₂) promoted a catalytic route to disiloxanes which showed >99% conversion of three tertiary silanes, Et₃SiH, PhMe₂SiH, and Ph₃SiH. Preliminary data on the synthesis of unsymmetrical disiloxanes (Si-O-Si') suggests that by modifying the reaction conditions and/or using a 1:1 combination of silane to silanol the cross-product can be favored. Intramolecular reactions of disilyl compounds with catalytic (C₆F₅)₃B(OH₂) led to the formation of novel bridged siloxanes, containing a Si-O-Si linkage within a cyclic structure, as the major product. Moreover, the reaction conditions enabled recovery and recycling of the catalyst. The catalyst was re-used 5 times and demonstrated excellent conversion for each substrate at 1.0 mol% catalyst loading. This seemingly simple reaction has a rather complicated mechanism. With the hydrosilane (R₃SiH) as the sole starting material, the fate of the reaction largely depends on the creation of silanol (R₃SiOH) from R₃SiH as these two undergo dehydrocoupling to yield a disiloxane product. Generation of the silanol is based on a modified Piers-Rubinsztajn reaction. Once the silanol has been produced, the mechanism involves a series of competitive reactions with multiple catalytically relevant species involving water, silane, and silanol interacting with the Lewis acid and the favored reaction cycle depends on the concentration of various species in solution.

Novel clickable and fluorescent poly(siloxane amine)s for reusable adhesives and reprocessable elastomers

A series of clickable and fluorescent poly(siloxane amine)s were firstly reported and used as dynamic adhesives and elastomers.

Synthesis of Structurally Precise Polysiloxanes via the Piers⁻Rubinsztajn Reaction

Silicone materials are widely used, from daily life to the military industry. With the advancement of science and technology and the increasing demands of industry, the requirement for high-performance precise structural silicone materials has increased. Therefore, the most important aspect in this field is finding a breakthrough in the synthetic methods. In this review, the latest research developments in controllable morphological structure and composite structure optimized synthesis of silicone materials using the Piers⁻Rubinsztajn (PR) reaction are summarized. The advantages of the PR reaction compared with traditional synthetic routes to silicone materials are presented. The highly controllable spatial structure of silicone materials and the structural combination of biomass or inorganic materials with silicone materials results in an improvement in performance or function. The morphological control of more complex silicone materials and the synthesis of non-traditional silicone materials with composite structures through the PR reaction will be the main research directions for the development of silicone materials in the future.

Polymerization of Polar Monomers Mediated by Main-Group Lewis Acid-Base Pairs

The development of new or more sustainable, active, efficient, controlled, and selective polymerization reactions or processes continues to be crucial for the synthesis of important polymers or materials with specific structures or functions. In this context, the newly emerged polymerization technique enabled by main-group Lewis pairs (LPs), termed as Lewis pair polymerization (LPP), exploits the synergy and cooperativity between the Lewis acid (LA) and Lewis base (LB) sites of LPs, which can be employed as frustrated Lewis pairs (FLPs), interacting LPs (ILPs), or classical Lewis adducts (CLAs), to effect cooperative monomer activation as well as chain initiation, propagation, termination, and transfer events. Through balancing the Lewis acidity, Lewis basicity, and steric effects of LPs, LPP has shown several unique advantages or intriguing opportunities compared to other polymerization techniques and demonstrated its broad polar monomer scope, high activity, control or livingness, and complete chemo- or regioselectivity, as well as its unique application in materials chemistry. These advances made in LPP are comprehensively reviewed, with the scope of monomers focusing on heteroatom-containing polar monomers, while the polymerizations mediated by main-group LAs and LBs separately that are most relevant to the LPP are also highlighted or updated. Examples of applying the principles of the LPP and LP chemistry as a new platform for advancing materials chemistry are highlighted, and currently unmet challenges in the field of the LPP, and thus the suggested corresponding future research directions, are also presented.

Fabrication of Reactive Poly(Phenyl-Substituted Siloxanes/Silsesquioxanes) with Si‒H and Alkoxy Functional Groups via the Piers⁻Rubinsztajn Reaction

Poly(phenyl-substituted siloxanes/silsesquioxanes) are obtained by the Piers⁻Rubinsztajn (PR) reaction of hydrogen-containing siloxanes (HCS) with diphenyldialkoxysilanes such as diphenyldimethoxysilane and diphenyldiethoxysilane catalyzed by tris(pentafluorophenyl)borane. ²⁹Si nuclear magnetic resonance (NMR) spectroscopy, gel permeation chromatography, and refractive index analysis revealed that apart from phenyl substituents and complex structures such as molecular bridges composed of D₂^Ph2[(C₆H₅)₂Si(OSi)₂], structures also existed in these polymers, having high refractive indexes (above 1.50) and high molecular weights (75.60 KDa·mol^-1). As revealed by thermogravimetric analysis, these polymers have high thermal stability as well, with temperature at 5% mass loss (T_5%) increasing by 182.5 °C and R_w (residual weight ratio) increasing by 5.17 times from 14.63% to 75.60%, as compared to HCS, exhibiting its potential application as resins for resisting strong heat. Such high-refractive-index and temperature-resistant poly(phenyl-substituted siloxanes/silsesquioxanes) with Si⁻H and alkoxy functional groups can be used as a good addition-type crosslinking agent with adhesion-promoting properties or a special curing agent that can solidify silicone materials through simultaneous addition and condensation reactions, which has potential application in the light-emitting diode (LED) packaging industry.

Molecular Strategies for Improved Dielectric Elastomer Electrical Breakdown Strengths

Dielectric elastomer transducers (actuators and generators) possess great commercial potential because they allow for novel transducer designs and applications due to-amongst others-their flexibility and low weight. On the other hand, the flexibility and inherent softness of dielectric elastomers also pose restrictions on their use, since the thin elastomers may undergo destructive deformations under large loads or in large electrical fields. In order to design better dielectric elastomers, it is crucial to understand the underlying phenomena of how thin and elastic dielectric elastomer films undergo electrical breakdown. This understanding will allow for the design of dielectric elastomers with high electrical breakdown strength and thus open up the use of films in transducers at higher electrical fields and forces. Here, the study couples intrinsic electrical breakdown strengths with well-described polymer and network characteristics, namely Kuhn parameters and cross-linking density. The universality of the developed model is illustrated by comparison over a wide range of silicone-based elastomers, such as prestretched elastomers and synthesized cross-linked bottlebrush polymers, representing both filled and unfilled elastomers. This study paves a robust way for the molecular design of elastomers into high-intrinsic electrical breakdown strength dielectric elastomers.

New Control Over Silicone Synthesis using SiH Chemistry: The Piers-Rubinsztajn Reaction

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(C₆ F₅ )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.

Polar Elastomers as Novel Materials for Electromechanical Actuator Applications

Dielectric elastomer actuators are stretchable capacitors capable of a musclelike actuation when charged. They will one day be used to replace malfunctioning muscles supposing the driving voltage can be reduced below 24 V. This focus here is on polar dielectric elastomers and their behavior under an electric field. Emphasis is placed on all the features that are correlated with the molecular structure, its synthetic realization, and its impact on properties. Regarding the polymer class, the focus, to some degree, is on polysiloxanes because of their attractively low glass transition temperatures. This enables introduction of highly polar groups to the backbone while maintaining soft elastic properties. The goal is to provide a few guidelines for future research in this emerging field that may be useful for those considering entering this fascinating endeavor. Because of the large number of materials available, a few restrictions in the selection have to be applied.

Enhancing the electro-mechanical properties of polydimethylsiloxane elastomers through blending with poly(dimethylsiloxane-co-methylphenylsiloxane) copolymers

In this work, improved electro-mechanical properties of silicone-based dielectric elastomers are achieved by means of adding so-called “voltage-stabilisers” prepared from phenyl-functional copolymers prepared using oxyanionic ring-opening polymerisation of octamethylcyclotetrasiloxane (D4) and either tetramethyltetraphenylcyclotetrasiloxane (T4) or octaphenylcyclotetrasiloxane (O4). The concentration of the voltage stabiliser was varied both by changing the molar ratio between methyl and phenyl groups in the copolymer and also by varying the amount of copolymer mixed into a PDMS-based elastomer. The phenyl-functional copolymers were generally found to disperse homogeneously in the PDMS matrix and this resulted in networks with improved mechanical and electrical properties. The developed elastomers were inherently extensible with enhanced tensile and tear strengths, due to phenyl-rich microphases acting as reinforcing domains. Furthermore, addition of phenyl-functional copolymers resulted in elastomers with increased relative permittivity and electrical breakdown strength compared to control elastomers while retaining a low dielectric loss. This demonstrates their efficiency as voltage stabilisers.

Improved electro-mechanical properties of silicone-based dielectric elastomers are achieved by means of adding so-called “voltage-stabilisers” prepared from phenyl-functional copolymers prepared using oxyanionic ring-opening polymerisation.

Synthesis of novel cyclosiloxane monomers containing push–pull moieties and their anionic ring opening polymerization

The synthesis of three novel tetracyclosiloxane monomers modified either with a nitroaniline (NA) or with a Disperse Red 1 (DR1) push–pull group and their ring opening polymerization reaction in the presence of tetramethylammonium hydroxide are presented. The prepared monomers and polymers were characterized by different spectral methods and gel permeation chromatography. For the crystalline monomers, the structures were further proven by single crystal X-ray diffraction. Dynamic scanning calorimetry shows that the polymers that carry NA groups have a glass transition temperature (T_g) well below room temperature (RT), while the one that carries DR1 groups melts at 55 °C. The transition temperatures have a strong effect on permittivity as indicated by broadband impedance spectroscopy measurements conducted at different temperatures and frequencies. The polymers modified with NA groups have a high permittivity (maximum value of 17.3) at RT, suggesting the polar groups to be mobile and orientation polarization to be effective. However, the polar groups of the polymer modified with DR1 are frozen and thus cannot contribute to the permittivity via orientation polarization. Consequently, the permittivity is only 8.8 at RT, but increases to 22 above the melting temperature, where the dipoles are mobile. Because of the high dielectric permittivity and rather low T_g, the polymers modified with NA are attractive as active dielectric materials in actuators, capacitors, and stretchable electronics, whereas the polymer modified with DR1 may be of interest in nonlinear optical devices.

The synthesis of polysiloxanes modified with a nitroaniline or with a Disperse Red 1 push–pull group and their dielectric properties are reported.

Silicone elastomers with covalently incorporated aromatic voltage stabilisers

When optimising dielectric elastomers (DEs) a conflict exists, namely that for large achievable actuation strains softness is required, but with increased softness electrical breakdown strength decreases.

A facile synthetic strategy to polysiloxanes containing sulfonyl side groups with high dielectric permittivity

Novel polysiloxanes containing sulfonyl side groups with high dielectric permittivity by thiol–ene post-polymerization reactions are presented.

Voltage-stabilised elastomers with increased relative permittivity and high electrical breakdown strength by means of phase separating binary copolymer blends of silicone elastomers

Increased electro-mechanical properties of silicone-based dielectric elastomers are achieved by means of the addition of so-called voltage-stabilisers prepared from PDMS–PPMS copolymers.

A new soft dielectric silicone elastomer matrix with high mechanical integrity and low losses

Alkyl chloride-functional siloxane copolymers are synthesised as the basis for a new soft matrix for dielectric elastomers.

Fine-tuning of the dielectric properties of polysiloxanes by chemical modification

Novel polysiloxanes with dielectric properties which are systematically fine-tuned by utilizing thiol-ene post-polymerization reactions of a polymethylvinylsiloxane with various ratios of 1-butanethiol and 3-mercaptopropionitrile is presented.