Transurfs

Inisurfs, Transurfs and Surfmers may be used to reduce/avoid the use of conventional surfactants in emulsion polymerization. However, when Inisurfs and Transurfs are used, the stability of the system cannot be adjusted without affecting either the polymerization rate (Inisurfs) or the molecular weight distribution (Transurfs). Furthermore, the efficiency rate of Inisurfs is low due to the cage effect. It is therefore not obvious yet that these classes will become commercially significant. 

Polymerizable surfactants capable of working as transfer agents include thiosulfonates, thioalkoxylates and methyl methacrylate dimer/trimer surfactants. Thioalkoxylates with 17-90 ethylene oxide units were produced from ethoxylated 11 bromo-undecanol by replacing the bromine with a thiol group via the thiazonium salt route [8]. In the presence of water-soluble azo initiator the thio ended Transurfs (used at a concentration above the CMC) gave monodispersed latex particles in emulsion polymerization of styrene. However, the incorporation of the Transurf remained low, irrespective of the process used for the polymerization (batch, semibatch, seeded). The stability of the lattices when the surfactant and the transfer function were incorporated in the same molecule was better than when they were decoupled. 

When the same thioalkoxylates were combined with t-butyl hydroxyperoxide initiator the maximum incorporation yield of the Transurf was around 40%. Monomodal or multimodal molecular weight distributions were observed, depending upon the structure of the Transurf, the conversion of the monomer and the process used for feeding the reactor [9]. 

Monteiro et al. have used a RATF Transurf in the ab initio emulsion polymerization of methyl methacrylate at 70° C. The Transurf was synthesized by esterifying a methyl methacrylate dimer with 1,10 decandiol followed by sulfonation. The authors found that only a small amount of Transurf was incorporated and suggested that, in order to increase the Transurf incorporation, the ratio of monomer to Transurf should be kept as low as possible, as achieved, e.g. in starved-feed conditions [12]. 

In common with conventional surfactants, Inisurfs and Transurfs, Surfmers form micelles in aqueous solutions above the CMC. The organized monomer aggregates of colloidal dimension are microscopically heterogeneous and may affect polymerization kinetics and polymer structure and properties. 

Vidal, F., Guillot, J. and Guyot, A. (1995) Surfactants with transfer agent properties (transurfs) in styrene emulsion polymerisation. Colloid Polym. Sci., 273, 999-1007. 

Monteiro, M.J., Brussels, R. and Wilkinson, T.S. (2001) Emulsion polymerization of methyl methacrylate in the presence of novel addition-fragmentation chain-transfer reactive surfactant (transurf). /. Polym. Sci., A 39,2813-20. 

Microsphere Syntheses by Reactive Dispersants, Macromonomers, Inimers, and Transurfs... 

Dispersion polymerization in the presence of reactive surfactants including surfmers, inisurfs and transurfs is also a versatile method for producing functional microspheres [26]. For example, the macromonomeric azoinitiator 26 is an effective inisurf in the preparation of PS and PMMA particles [155]. 

Similarly, the thiol-ended transurf 27 shows higher stabilizing efficiency than PVP in the dispersion polymerization of styrene and MMA in water-ethanol [156]. 

A lot of mechanistic problems remain to be solved. It is not so clear, at the moment, why most of the inisurfs studied up to now have such a low efficiency, whatever their structure. On the other hand it is quite remarkable that, even with that low efficiency, they are able to allow the preparation of stable latexes under acceptable experimental conditions. Another problem is the control of the nucleation both with inisurfs and transurfs, there are indications that the particle number and size may not very sensitive to the amount of reactive surfactants, and more dependent on the amount of monomer used. Such behavior, up to now, has not been explainal Very few studies have been devoted to the reactivity of these surfactants, i.e., reactivity ratios for the surfmers, transfer constants for the transurfs and initiator efficiency for the inisurfs. Both the reactivities and the partition coefficient between the water and the organic phase have to be determined. In addition the reactivities may dependent on the other components of the recipe, for instance due to the effect of the ionic strength on the cmc. 

To be covalently linked to the particles, the stabilizer must carry a group able to participate in one of the key steps of the free radical polymerization process (initiation, propagation, termination, or transfer reactions). According to the reaction in which they are involved, the reactive surfactants are referred to as inisurf (initiation), surfmer (propagation) or transurf (transfer). In these particular cases, the surfactant not only plays a key role in the formation of the particles and the functionalization of their surface, but also becomes an actor in the synthesis of the polymer chains that will form the particles. 

Irreversible transfer reactions have been used in the past to generate block copolymers from hydrophilic or amphiphilic thiol molecules such as sodium 10-mercapto-1 -decanesulfonate [99], or functionalized PEO [100] or poly(VAc-ct)-vinyl alcohol) [101]. In fact, the very efficient transferring nature of the thiol function leads to a high reactivity of the transurf in the dispersing phase. Although the in situ formation... 

Figure 17.26. Structures of two inisurfs (a and b), i.e. polymerizable surfactants that also serve as polymerization initiators, and two transurfs (d and e), polymerizable surfactants with chain-tranfer capability. One conventional initiator (c) and one conventional chain-tranfer agent (f) are also given...

Some surfactants perform multiple functions. Reactive surfactants (surfmers) (214,289) chemically react with monomer to become bound at the surface of the particles, and caimot desorb or migrate to the surface of a latex film (44,168). Surfactants may also contain reactive groups that function as initiators (i.e., inisurfs) or chain transfer agents (i.e., transurfs). Anti-foaming agents (defoamers), which are typically silicones or hydrocarbons, may be incorporated in the emulsion recipe to counteract the foaming effects of the surfactant. 

Any of these surfactant classes can involve surfactants that react onto the latex particle. These surfactants are known as surfiners. If the surfactant also acts as a chain-transfer agent, it is dubbed a transurf. Reversible addition fragmentation agents can act as transurfs (120). 

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