Surface-active cationic monomers

C Br, structure 6, Table 4.5) immobilized on the surface of silica gel [96b], Using a similar procedure, Yoshinaga [97] described the spontaneous formation of small polymer plots on the surface of silica particles by self-polymerization of a series of adsorbed cationic surface-active monomers in THF solutions. 

The surface active agents (surfactants) may be cationic, anionic or non-ionic. Surfactants commonly used are cetyltrimethyl ammonium bromide (CTABr), sodium lauryl sulphate (NaLS) and triton-X, etc. The surfactants help to lower the surface tension at the monomer-water interface and also facilitate emulsification of the monomer in water. Because of their low solubility surfactants get fully dissolved or molecularly dispersed only at low concentrations and at higher concentrations micelles are formed. The highest concentration where in all the molecules are in dispersed state is known as critical micelle concentration (CMC). The CMC values of some surfactants are listed in table below. 

The surface-active properties of the ii.onomers were also confirmed by surface-tension experiments. Figure 3 shows the variation of the surface tension 7 with the concentration of a cationic monomer (MADQUAT) (29) and a neutral one (AM) (Graillat, C.. Pichot, C. unpublished results). The surface-tension drops for example from 70 dyn./cm to 40 dyn/cm when the MADQUAT concentration varies from 10 to eibout 3M (a 3M is the monomer concentration based on the aqueous phase used in most polymerization reactions). However, the amphiphilic character of monomers is not sufficiently pronounced to give rise to micellization since no sharp transition corresponding to the C.M.C. is detectable. 

Fig. 21. Surface activity of surfactants and polysoaps in water at 25 °C, exemplified by a cationic methacrylate + = polysoap 53, X = corresponding surfactant monomer. (Data taken from [245])...

The polymerisation and the properties of the latexes depended sensitively on the emulsifier and on the charge of the initiator. There was no visible correlation between the properties of the final latexes and the properties of the emulsifiers such as surface activity, solubilisation capacity, or the ability to stabilise the initial monomer emulsion. When a cationic (2,2 -dimethyl-2,2 -azo-N-benzylpropionamidine hydrochloride, VA-552 from Wako Chemicals) initiator was employed, all emulsifiers lead to stable monodisperse latexes, except for polysoaps with low hydrophobe content. The polymerisable, as well as the polymeric emulsifiers, yielded latex solutions with very high surface tensions, different from the use of the standard surfactant. In contrast, the use of an anionic initiator (potassium persulphate) can pose difficulties. The... 

Nagai and co-workers [96] described, for example, the aqueous polymerization of a series of surface-active cationic monomers having different alkyl chain length... 

Another way for increasing filler-elastomer interactions could be the grafting of a polymer on the solid surface. A number of methods exist to secure the attachment of macromolecules to the surface of carbon black particles e.g., a polymeric chain may be grown on an initiation site on the surface, small molecules previously attached to the surface may be copolymerized with a monomer, a polymeric chain, either radical, cationic, or anionic in nature, may be terminated on an active site of the solid surface, etc. 55 63). 

In the copolymerization of trioxane with dioxolane, reactivity ratios of dissolved copolymer cations are quite different from those of active centers in the crystalline phase. The former strongly prefer addition of dioxolane. The difference in reactivity ratios between dissolved and precipitated active centers is attributed to the fact that in the solid phase, polymerization and crystallization of the copolymer are simultaneous. The cationic chain ends are assumed to be directly on the crystal surface. Determination of the equilibrium concentrations of formaldehyde confirms this conclusion dissolved copolymer has a higher tendency to cleave formaldehyde than crystalline polyoxymethylene. In the latter stages of copolymerization the soluble copolymer is degraded gradually to the dioxolane monomer which is incorporated into the crystalline copolymer in an almost random distribution. 

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