Availability of monomer

Availability of Monomer. Under most solution conditions employed for gel formation, monomer is rapidly consumed to form a distribution of oligomeric species (12). Thereafter the only possible source of monomer is that which is produced by hydrolytic (or alcoholic) depolymerization of siloxane bonds (dissolution). According to Her (241 the dissolution of a-Si02 above pH 2 is catalyzed by OH ions which are able to increase the coordination of silicon above 4... 

Vinyl fluoride and 1,1,1-trifluoropropene have shown a great efficiency in telomerisation with various telogens,but concerning the other ones, less interest has been focused on. This may be due to the lack of availability of monomers, and consequently studies on monoadduct were mostly investigated. 

The rate of plasma polymerization depends on the nature of the monomer gas. In addition, such parameters as flow rate, pressure, power, frequency, electrode gap and reactor configuration also strongly influence the polymerization rate for a given monomer. Generally at low flow rates there is an abundance of reactive species so the polymerization rate is limited only by the availability of monomer supply. At high flow rates, however, there is an overabundance of monomer concentration and the polymerization rate now depends on the residence time. At intermediate flow rates these two competing processes result in a maximum. This behavior is illustrated in Figure 1 for ethane, ethylene, and acetylene (11). These data also demonstrate the effect of increased unsaturation in... 

The high thermodynamic polymerizability of oxiranes (due to the relatively large ring strain) and the availability of monomers such as ethylene oxide (EO), propylene oxide (PO) or epichlorohydrin (ECH) led to considerable efforts directed toward the preparation of high molecular weight polymers. Both cationic and anionic polymerizations were explored and it soon became clear that for several monomers only anionic polymerization gives high polymers. 

The understanding of the mechanism of this reaction is important because there is no definite borderline between the solid-state and solution or bulk polymerization of TXN. In all these processes the polymer forms a solid phase from the beginning of polymerization and growth proceeds inside and/or on the surface of the solid particles. Thus, the difference between solid-state and solution processes stems from the different mobility and availability of monomer molecules being incorporated into the crystal lattice in the former and being in a liquid phase in the latter system. 

It is informative to consider the rate of a semi-batch emulsi< i polymerization in terms of the maximum rate attainable under the given ccmditions, i.e. for the specific particle number concentration, rate of generation of free radicals, etc. The maximum rate of polymerization, occurs when the saturation concentradon, [ jmax, Qf monomer in the particles is achieved. For all semi-batch processes, the rate, R, at which monomer is introduced to the reaction vessel is of crucial importance because it controls the availability of monomer for polymerization and hence the concentration, [M]p, of monomer in the particles. Two scenarios can be considered ... 

There is no doubt that the supply of relatively low cost dimethyldichlorosilane has been key to the rapid growth of the silicone industry, but it is also apparent that the availability of monomers has focused attention on systems primarily based on polydimethylsiloxane. Many would argue that this is of no concern because this is obviously the material of choice as shown by the very rapid growth in silicone science and technology. Others, who see a materials science based on silicon that potentially parallels that based on carbon, observe the profusion of silicon monomers that have been made and wonder if there are other silicon based polymer systems of equal importance to polydimethylsiloxane which now need to be developed and commercialized to maintain rapid growth in this Industry. 

This sequence of condensation requires both depolymerization (ring opening) and the availability of monomers, which are in solution equilibrium with the oligomeric species and/or are generated by depolymerization (reverse of Eqs. 9 and 10). However in alcohol-water solutions normally employed in sol-gel processing, the depolymerization rate is lower than in aqueous media, especially at low pH [1]. Under these conditions. Her [1] proposed ... 

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