Silica radical polymerization

The silica microspheres provide some diversity but not enough for many complex discrimination tasks. To introduce more sensor variety, hollow polymeric microspheres have been fabricated8. The preparation of these hollow microspheres involves coating silica microspheres by living radical polymerization, using the surface as the initiation site. Once the polymer layer forms on the silica microbead surface, the silica core is removed by chemical etching. These hollow spheres can be derivatized with the dye of interest. The main advantage of these polymer microspheres is the variety of monomers that can be employed in their fabrication to produce sensors with many different surface functionalities and polymer compositions. 

Mandal T. K., Fleming M. S., Walt D. R., Production of hollow polymeric microspheres by surface-confined living radical polymerization on silica templates, Chem. Mater. 2000 12 3481-7. 

Radical polymerization of styrene was carried out in the presence of bare silica particles, and of the HPC-coated silica particles in water by using potassium persulfate as an initiator. Table 2 gives the typical ingredients used for these polymerizations. The HPC-coated silica particles were prepared under the same conditions as in the adsorption experiments. The polymerization temperature was kept at 1+5 °C to protect the adsorption layer of HPC, and polymerized for 2l+ hrs in the same manner as that... 

The precipitated silica (J. Crosfield Sons) was heated in vacuo at 120° for 24h. before use. Two grades of surface areas 186 and 227 m g l (BET,N2), were used during this project. Random copolymers, poly(methyl methacrylates) and polystyrene PS I were prepared by radical polymerization block polymers and the other polystyrenes were made by anionic polymerization with either sodium naphthalene or sodium a methylstyrene tetramer as initiator. The polymer compositions and molecular weights are given in Table I. 

An optically active polymethacrylate (2) having a binaphthol moiety in the side chain was synthesized by radical polymerization. This polymer coated on silica gel resolved several racemates.50 However, no data on the influence of the stereoregularity of the main chain on resolution have been reported. The chiral recognition by this polymer may simply arise from the binaphthyl group. 

While only a few reports concern the in situ preparation of monolithic CEC columns from silica, much more has been done with porous polymer monoliths and a wide variety of approaches differing in both the chemistry of the monomers and the preparation technique is currently available. Obviously, free radical polymerization is easier to handle than the sol-gel transition accompanied by a large decrease in volume. 

We have repeated similar degrafting experiments for brush formation via ATRP. While there have been reports on degrafting using conventional radical polymerization [10,58], this discussion will be limited to brush formation by ATRP. In unpublished work [59], we immobilized an ATRP initiator, (1 l-(2-bromo-2-methyl)propionyloxy)undecyltrichlorosilane) on StOber silica and conducted a styrene polymerization. Degrafting of the PS brushes was conducted by etching of the silica cores with HE From TGA analysis of the immobilized initiator and the corresponding PS brush system, we determined that there are 4.8 initiator molecules/nm and / = 0.06. The initiator density corresponds well to the values of 2.4-5.0 reported by Patten and co-workers [56,57] for the immobilization of (2-(4-chloromethylphenyl)ethyl)dimethylethoxysilane on a similar support. 

FIGURE 1.3 Schematic representation of the silanization procedure of borosilicate or fused silica capillary column inner walls, (a) Surface etching under alkaline conditions, (b) attachment of reactive groups by condensation with silanol, (c) chemical linkage of polymer (PS/DVB considered as example) by free radical polymerization. 

The preferentially employed approach for the fabrication of inorganic (silica) monolithic materials is acid-catalyzed sol-gel process, which comprises hydrolysis of alkoxysilanes as well as silanol condensation under release of alcohol or water [84-86], whereas the most commonly used alkoxy-silane precursors are TMOS and tetraethoxysilane (TEOS). Beside these classical silanes, mixtures of polyethoxysiloxane, methyltriethoxysilane, aminopropyltriehtoxysilane, A-octyltriethoxysilane with TMOS and TEOS have been employed for monolith fabrication in various ratios [87]. Comparable to free radical polymerization of vinyl compounds (see Section 1.2.1.5), polycondensation reactions of silanes are exothermic, and the growing polymer species becomes insoluble and precipitates... 

Tsubokawa et al. (12-14) have introduced radical sources of azo or peroxy groups by another methods, and successively conducted the radical polymerization of vinyl compounds, such as styrene or methyl methacrylate, to give polymer-grafted particles see Reaction (3). The grafting by the radical polymerization of methyl methacrylate, initiated from a peroxy group introduced on silica, takes place at relatively high efficiency, compared with those from azo group-introduced particles. 

According to Tsubokawa et al. (18), the molecular weight of grafted polystyrene from radical polymerization at the surface of silica is considerably higher than that from cationic polymerization. 

Oligomers prepared by radical polymerization of 2-vinylpyridine could be separated into tactic isomers on a silica column (250 x 4.6 dQ = 10 nm dP = 10 pm) with a gradient pentane/methanol (concave 10 to 40% in 30 min). The experiments were... 

The concept of using the functional groups of electrode surfaces themselves to attach reagents by means of covalent bonding offers synthetic diversity and has been developed for mono- and multi-layer modifications. The electrode surface can be activated by reagents such as organosilanes [5] which can be used to covalently bond electroactive species to the activated electrode surface. Recently, thermally induced free-radical polymerization reactions at the surfaces of silica gel have been demonstrated [21]. This procedure has been applied to Pt and carbon electrode surfaces. These thermally initiated polymer macromolecules have the surface Of the electrode as one of their terminal groups. Preliminary studies indicate that the... 

Furthermore, Fu et al.140 developed a transport system that responds to thermal stimuli. This system is based on chains of poly-iV-isopropylacrylamide (a known thermosensitive polymer), which exists in a collapsed, hydrophobic state when exposed to heat, but in an expanded, hydrophilic state in the cold. In this way, samples of mesoporous, spherical silica particles (particle diameter 10 p,m) that were lined and coated with the thermosensitive polymer by atom transfer radical polymerization... 

The use of a polymer species as a way to control diffusion to the inside of mesoporous silica was also employed by Lopez and coworkers.67 In this work the researchers polymerized iV-isopropyl acrylamide on mesoporous silica by atom transfer radical polymerization, and took advantage of the changes the polymer experiences upon thermal treatment. The authors discovered that the hybrid material could take up more fluorescein than nonfunctionalized material at temperatures above 45°C. At that temperature the polymer is in a collapsed hydrophobic state and partially covers the negatively charged surface of silica that otherwise repels the negatively charged fluorescein dye. At temperatures below 30°C the polymer exists in a hydrated state in which the chains are expanded. Interestingly, the fluorescein loaded hybrid particles were... 

An example of the first approach is the integration of hydrogels into nanostructured silica films by addition of a suitable monomer (e.g., methyl methacrylate, /V-isopropyl acrylamide, etc.) and an initiator for radical polymerization to a solution containing a structure-directing surfactant and a prehydrolyzed silica precursor. During self-assembly, the monomers partition within the hydrophobic core of the surfactant mesophase postsynthesis polymerization (for instance, by UV treatment) followed by solvent washing to remove the surfactant template yields a polymer-silica nanohybrid. 

Recently an aromatic capillary monolithic acrylate material was prepared via a thermally initiated free radical polymerization in the confines of 200-pm inner diameter fused silica capillaries. The material proved to be very robust and was applied to IPC of oligodeoxynucleotides with good reproducibility [21]. Similarly a derivatized poly[(trimethylsilyl-4-methylstyrene)-co-bis(4-vinylbenzyl)dimethylsi-lane] capillary monolith was used for the same purpose [22],... 

Summary Silylester fimctionalized copolymers have been synthesized by free radical polymerization of alkenyl ethoxysilanes and vinyl acetate. The alkenyl ethoxysilane monomers and the preformed polymers have been applied to silica surfaces with hydroxy flmctions. Both treatments led to covalently modified samples of different properties as shown by DRIFT spectroscopy. 

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