![]() Although this phenomenon is the main reason for voids, some other methods such as post-thermal treatment and removing phase-separated surfactants assist the structure in creating new voids and enlarging the size of existing voids. This phenomenon creates interconnections, known as pore throats (22) or voids, (23,24) making polymer foams highly permeable. (17−21) Notably, polyHIPE materials possess interconnected porous microstructures because the thin films between the droplets undergo shrinkage during the polymerization. Several studies have been reported on how emulsion stability affects the cellular morphology. ![]() (16) Therefore, the stability of the emulsion has a critical influence on the final structure of the polymer and is determined by some factors like surfactant type and its concentration, temperature, processing conditions, and miscibility of the phases. (14,15) In this technique, the morphology and pore structure of the monoliths are established by the state of the emulsion before the continuous phase is polymerized. (12,13) Polymerization of precursors in the continuous phase of HIPE results in constructing a macroporous polymer (polyHIPE) with a 3D polymer skeleton induced by the emulsion template. (9−11) Droplets in HIPEs are deformed because the fraction of the dispersed phase is greater than 74 vol %, which corresponds roughly to the most compact hexagonal arrangement of uniform spherical droplets. The high internal phase emulsions (HIPEs) technique is a versatile templating approach to produce porous polymers with interconnected channels and tunable pore size, which has been investigated extensively for the fabrication of porous materials with good moisture resistance, low cost, and a simple preparation process. (8) Highly concentrated emulsions can be used as templates for the preparation of highly ordered porous polymeric structures. (1−3) There are various methods to develop porous polymer: phase-separation micromolding, (4) high-internal phase emulsion polymerization, (5,6) block copolymer, (7) and particle or polymer leaching. Recently, porous polymers have been received much attention because they elegantly combine the features of both porous materials and organic polymers including high surface area, flexible backbone, well-defined porosity, and easy synthesis and processing in different forms such as thin films, tabular, or individual spheres. The excellent performance of the catalyst was attributed to the excellent chemical and physical properties of the developed support since it provides an elegant route for preparing site-isolated acid–base dual heterogenized functional groups and preventing their deactivation via chemical neutralization. It was shown that these catalysts were reusable for up to four consecutive runs with a very slight loss of activity. The optimized catalyst showed excellent catalytic performance with 100% substrate conversion and 100% yield of the final product in the one-pot production of β-nitrostyrene from benzaldehyde dimethyl acetal under cascade reactions comprising acid-catalyzed deacetalization and base-catalyzed Henry reactions. The catalyst demonstrated superior activity compared to the homogeneous catalysts, poly(St-DVB)-SO 3H+EDA, poly(St-VBC)-NH 2+chlorosulfonic acid, and poly(St-DVB)-SO 3H+poly(St-VBC)-NH 2 as the catalyst system. The poly(St-VBC)-NH 2-SO 3H(20) sample bearing 1.82 mmol/g of N (base site) and 1.16 mmol/g (acid site) showed the best catalytic activity. The catalytic activity of the poly(St-VBC)-NH 2-SO 3H series with different acid/base densities was assessed for one-pot cascade C–C bond-forming reactions involving deacetylation–Henry reactions. The prepared materials were characterized by Fourier transform infrared (FT-IR), CHNS elemental analysis, energy-dispersive X-ray (EDX), elemental mapping, field emission scanning electron microscopy (FE-SEM), and thermalgravimetric analysis (TGA). The role of various amounts of toluene as the porogenic solvent as well as the amount of 1,3-propane sultone (different ratio of acid/base sites) on the structure of the prepared materials have been carefully investigated. A macroporous dual-functional acid–base covalent organic polymer catalyst poly(St-VBC)-NH 2-SO 3H was prepared using high internal phase emulsion polymerization using vinylbenzyl chloride (VBC), styrene (St), and divinylbenzene (DVB) as substrates toluene as a porogenic solvent, and subsequent modification with ethylenediamine and 1,3-propane sultone.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |