Anionic polymerization of octamethylcyclotetrasiloxane in miniemulsion II. Molar mass analyses and mechanism scheme

Preparation and characterization of high molecular weight vinyl-containing poly[(3,3,3-trifluoropropyl)methylsiloxane

Prior to crosslinking and vulcanization, fluorosilicone rubber is a linear polymer. This linear polymer contains 3,3,3,-trifluoropropyl methyl siloxane links, a few methyl vinyl siloxane links, and is formed by co-polymerization of 1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl) cyclotrisiloxane (D3F) with 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (V4) under alkaline conditions. To improve the performance of fluorosilicone rubber, three key points should be considered during the synthesis of vinyl-containing high-molecular-weight linear fluorosilicone polymers (fluorosilicone raw rubber): first, avoid the generation of low molecular weight equilibrium by-products; second, eliminate the influence of impurities; and third, increase the copolymerization participation rate of monomer V4. From the three aspects above, this study optimized the reaction conditions for the synthesis of high-molecular-weight linear fluorosilicone polymers containing vinyl. Various factors influencing polymerization were thoroughly investigated. These factors include the initiation system, accelerator, equilibrium reaction, feeding ratio, feeding sequence, neutralization mode, impurity content, etc.

Photoinitiated Cationic Ring-Opening Polymerization of Octamethylcyclotetrasiloxane

Photochemical techniques have recently been revitalized as they can readily be adapted to different polymerization modes to yield a wide range of complex macromolecular structures. However, the implementation of the photoinduced cationic methods in the polymerization of cyclic siloxane monomers has scarcely been investigated. Octamethylcyclotetrasiloxane (D4) is an important monomer for the synthesis of polydimethylsiloxane (PDMS) and its copolymers. In this study, the cationic ring-opening polymerization (ROP) of D4, initiated by diphenyl iodonium hexafluorophosphate (DPI), has been studied. Both direct and indirect initiating systems acting at broad wavelength using benzophenone and pyrene were investigated. In both systems, photochemically generated protonic acids and silylium cations are responsible for the polymerization. The kinetics of the polymerization are followed by viscosimetry and GPC analyses. The reported approach may overcome the problems associated with conventional methods and therefore represents industrial importance for the fabrication of polysiloxanes.

Cationic Polymerization of Hexamethylcyclotrisiloxane in Excess Water

Ring-opening ionic polymerization of cyclosiloxanes in dispersed media has long been discovered, and is nowadays both fundamentally studied and practically used. In this short communication, we show some preliminary results on the cationic ring-opening polymerization of hexamethylcyclotrisiloxane (D₃), a crystalline strained cycle, in water. Depending on the catalyst or/and surfactants used, polymers of various molar masses are prepared in a straightforward way. Emphasis is given here on experiments conducted with tris(pentafluorophenyl)borane (BCF), where high-molar polymers were generated at room temperature. In surfactant-free conditions, µm-sized droplets are stabilized by silanol end-groups of thus generated amphiphilic polymers, the latter of which precipitate in the course of reaction through chain extension. Introducing various surfactants in the recipe allows generating smaller emulsions in size with close polymerization ability, but better final colloidal stability, at the expense of low small cycles’ content. A tentative mechanism is finally proposed.

Flow lithography in ultraviolet-curable polydimethylsiloxane microfluidic chips

Flow Lithography (FL) is the technique used for the synthesis of hydrogel microparticles with various complex shapes and distinct chemical compositions by combining microfluidics with photolithography. Although polydimethylsiloxane (PDMS) has been used most widely as almost the sole material for FL, PDMS microfluidic chips have limitations: (1) undesired shrinkage due to the thermal expansion of masters used for replica molding and (2) interfacial delamination between two thermally cured PDMS layers. Here, we propose the utilization of ultraviolet (UV)-curable PDMS (X-34-4184) for FL as an excellent alternative material of the conventional PDMS. Our proposed utilization of the UV-curable PDMS offers three key advantages, observed in our study: (1) UV-curable PDMS exhibited almost the same oxygen permeability as the conventional PDMS. (2) The almost complete absence of shrinkage facilitated the fabrication of more precise reverse duplication of microstructures. (3) UV-cured PDMS microfluidic chips were capable of much stronger interfacial bonding so that the burst pressure increased to ∼0.9 MPa. Owing to these benefits, we demonstrated a substantial improvement of productivity in synthesizing polyethylene glycol diacrylate microparticles via stop flow lithography, by applying a flow time (40 ms) an order of magnitude shorter. Our results suggest that UV-cured PDMS chips can be used as a general platform for various types of flow lithography and also be employed readily in other applications where very precise replication of structures on micro- or sub-micrometer scales and/or strong interfacial bonding are desirable.

Looking over liquid silicone rubbers: (1) network topology vs chemical formulations

This study proposes a comprehensive study on liquid silicone rubber (LSR) formulations to unravel which components (among functional polydimethylsiloxane polymers and modified silica fillers) improve the mechanical properties of the final materials. In this first part, various industrial products have been deformulated using conventional chemical analyses. The silica content and their surface chemistry were assessed by TGA. Architecture and molar mass of polymers were deduced from (29)Si NMR and SEC in toluene, respectively. Relative concentrations of hydride and vinyl reactive groups and stoichiometric imbalance (r = nSiH/nSiVi) were quantified by proton NMR. Stoichiometric imbalance is slightly higher than 1.5 for cross-linker with hydride functions well redistributed along the chain, whereas for some formulations, r's as high as 3.7 were implemented. These variations has strong implications on the cross-linking density of the final material, since the remaining hydride groups react together and decrease the molar mass between cross-links. From the comparison between formulations, it was shown that hardness adjustment is mainly performed by playing on two parameters: filler content and molar mass between cross-linking points for hardness ranging from 20 to 30 Shore A. Above this limit, it is necessary to modify the silica surface with reactive groups, such as vinyl functions. Surprisingly, two formulations were shown to use a dual cross-linking catalysis systems, peroxide and platinum, leading to efficient and full cure even at lower temperature (typically 140 °C). Network topologies were estimated from the predicted chemistry of the materials in a final discussion part.

Soft Core–Hard Shell Silicone Hybrid Nanoparticles Synthesized by Miniemulsion Polymerization: Effect of Silicone Content and Crosslinking on Latex Film Properties

Composite nanoparticles consisting of ‘soft’ silicone oil and ‘hard’ polyacrylate with core–shell morphology were obtained by a one-step synthesis via the miniemulsion process. Various parameters, i.e. the viscosity and amount of the silicone, the surfactant content, the (co)monomers affecting the particle size and morphology were studied. With an optimum hydrophilicity of the polymer shell, composite particles possessing a well-defined core–shell morphology were obtained as determined by transmission electron microscopy. The fully encapsulated silicone oil (40 wt-%) in a slightly crosslinked polyacrylate shell showed good film formation as revealed by atomic force microscopy. The ability to highly confine silicone oil in the composite film could clearly be evaluated from contact angle measurements. By finely changing the crosslink concentration in the polymeric shell, tunable hydrophobic properties of films cast from silicone core–polyacrylate shell latexes could be achieved. In addition, the high thermal resistance and excellent water-resistant properties of the film were shown by thermal gravimetric analysis and water swelling determination. These composite latexes are presented as new alternatives for practical utility in waterborne coatings.

Miniemulsion polymerization as a versatile tool for the synthesis of functionalized polymers

The miniemulsion technique is a particular case in the family of heterophase polymerizations, which allows the formation of functionalized polymers by polymerization or modification of polymers in stable nanodroplets. We present here an overview of the different polymer syntheses within the miniemulsion droplets as reported in the literature, and of the current trends in the field.

Influence of PEG-12 Dimethicone addition on stability and formation of emulsions containing liquid crystal

Oil/water emulsions, containing liquid crystals, were developed employing Andiroba oil, PEG-12 Dimethicone and Crodafos CES. It was evaluated the influence of silicone surfactants on the emulsions stability and on the formation of liquid crystalline phases and therefore, physicochemical characteristics, such as rheology and zeta potential, were evaluated. Emulsions were prepared by the emulsions phase inversion method. All the formulations presented lamellar liquid crystalline phases. The PEG-12 Dimethicone addition did not change microscopically the liquid crystalline phases. The emulsions containing silicone demonstrated lower viscosity than those without the additive. This is an important feature, as the silicone did not change the rheological profile; however, the addition of silicone still can be used as a viscosity controller. The formulations had their viscosity increased 15 and 150 days after their preparation. This characteristic shows that the emulsions have their organization increased along the storing time. In the analysis of zeta potential, we could verify that all formulations presented negative values between -39.7 and -70.0 mV. Within this range of values, the emulsion physical stability is high (Fig. 10). It was concluded that the addition of PEG-12 Dimethicone kept the liquid crystalline phase of the emulsion obtained with Crodafos CES, influencing in a positive way in the system stability.

Cationic polymerization of vinyl monomers in aqueous media: from monofunctional oligomers to long-lived polymer chains

Polymer latexes are easily prepared on a multimillion ton scale in industry using free radical initiated emulsion and suspension polymerizations in water, a cheap, nonviscous, heat-controlling, and environmentally benign solvent. Until recently, researchers had done little investigation into ionic polymerization because even a small amount of water would easily deactivate the conventional catalysts used in these processes. In the last decade, however, cationic polymerization in aqueous media has emerged as a new and attractive method for controlling the polymerization reactions using mild experimental conditions. This Account reviews the current science of and future outlook for cationic polymerization of vinyl monomers in aqueous media. We particularly emphasize the design and evolution of catalytic systems and the precision synthesis of functional polymers. Early work to tailor the suspension and emulsion cationic polymerizations of reactive monomers such as p-methoxystyrene and vinyl ethers used long-chain strong acids, called INISURF for their dual roles as initiators and surfactants, and lanthanide triflates. These polymerization processes shared two main features: (i) all reactions (initiation, propagation, and termination) occurred at the particle interface; (ii) synthesized polymers have limits on their molecular weight, attributed to the "critical DP" effect, related to the entry of oligomers inside the particles as they become increasingly hydrophobic. The next generation of catalysts, named "Lewis acid-surfactant combined catalysts" (LASC), shifted the polymerization locus from the interface to the inside of the monomer droplets, allowing for the production of long polymer chains. Recently, catalytic systems based on boranes, (BF(3)OEt(2), B(C(6)F(5))(3), (C(6)F(4)-1,2-[B(C(6)F(5))(2)]), and (C(6)F(4)-1,2-[B(C(12)F(8))](2))), have shown great potential in controlling the cationic polymerization in "wet" solution, containing an excess of water relative to Lewis acid, or aqueous media of such industrially important monomers as styrene, cyclopentadiene, and even isobutylene.

High molar mass polymers by cationic polymerisation in emulsion and miniemulsion

The generation of poly(p-methoxystyrene) with a molar mass of several thousand g mol(-1) by cationic polymerisation in emulsion, is described here for the first time. Such a striking result was achieved by carrying out the polymerisation inside monomer droplets, thus preventing fast transfer reactions with water.

Synthesis of chitosan-stabilized polymer dispersions, capsules, and chitosan grafting products via miniemulsion

The potential of chitosan as an emulsion stabilizer is combined with the miniemulsion technique to generate oil droplets, hollow capsules, and latex particles in the diameter range of 100-300 nm carrying a functional biopolymer surface. It turned out that chitosan alone, independent of its molecular weight, is just moderately efficient, which is speculatively attributed to its stiff polysaccharide structure. The addition of biocompatible costabilizers with higher flexibility either to the oil phase or to the water phase, such as Jeffamine or Gluadin (a peptide), eliminates these deficiencies, and very small nanocapsules made of biopolymer hybrids can be obtained. NMR analysis shows that the costabilizer is effectively grafted/cross-linked to the chitosan which makes the miniemulsion route also effective for the modification of hard-to-handle, amphiphilic biopolymers either from the water or from the oil phase.