Emulsion polymerization inisurfs

Utilization of another function of the macroinitiator was tried in emulsion polymerization [30]. An MAI composed of PEG (molecular weight of a segment is 1000) linked with AGP units was confirmed to be usable as a surface active initiator (Inisurf) for preparing PSt-b-PEG [30]. A higher molecular weight block copolymer was obtained in comparison with the case of solution copolymerization. 

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. 

Surface active initiators or Inisurfs have the advantage of reducing the number of ingredients in an emulsion polymerization recipe to water, monomer and initiator, at least in the initial stages of the process. However, the surface active properties of the Inisurfs may be reduced on formation of the radicals and additional surfactant must be added to stabilize the latex if high solid levels are wanted. 

The key feature of Inisurfs is their surfactant behavior. They form micelles and are adsorbed at interfaces, and as such they are characterized by a critical micelle concentration (CMC) and an area/molecule in the adsorbed state. This influences both the decomposition behavior and the radical efficiency, which are much lower than those for conventional, low molecular weight initiators. Tauer and Kosmella [4] have observed that in the emulsion polymerization of styrene, using an Inisurf concentration above the CMC resulted in an increase in the rate constant of the production of free radicals. This was attributed to micellar catalysis effects as described, for example, by Rieger [5]. Conversely, if the Inisurf concentration was below the CMC the rate constant of the production of free radicals decreased with an increase in the Inisurf concentration, which was attributed to enhanced radical recombination. Also note that a similar effect of the dependence of initiator efficiency on concentration was reported by Van Hook and Tobolsky for azobisisobutyronitrile (AIBN) [6]. 

One advantage of inisurfs is the possibility of reducing the ingredients of an emulsion polymerization to the components monomer, water, and initiator. The accessory content of the final latex can be decreased by this way considerably. 

Problems exist with the chemical and structural purity of the inisurfs especially from the colloidal point of view. One must always bear in mind that impurities are present in most systems investigated. Nevertheless, the results known so far clearly show the pecularities of inisurfs compared to conventional initiators for emulsion polymerizations like water-soluble peroxides or AIBN. 

Furthermore, the authors pointed out that they obtained in the emulsion polymerization of styrene (monomer to water ratio 1 2) with an inisurf concentration of 5.4 X 10 mol/1 water in the presence of an alkylated poly (oxyethyl-ene) emulsifier (alkyl chain length C16 -18 and 20 oxyethylene units 4% by weight related to water) the same overall rate of polymerization as with water-soluble initiators in the concentration range 10 to 10 mol/1 water. The polymer produced in the presence of inisurf has a molecular weight of some of 10 g/mol mainly due to the lowered termination rate constant. 

Ivancev and Pavljuchenko published a series of papers [36-40] regarding the emulsion polymerization with surface-bonded radical generation. Their pioneering work, however, is related only to peroxy inisurfs. 

Other examples of peroxy inisurfs can also be found in Russian scientific papers. As for instance in Ref. [41] Voronov et al. describe a polymeric surfactant with peroxy side chains for application as inisurfs in emulsion polymerization. They obtained the polymeric inisurf (Inisurf 2) by copolymerization of a peroxide containing monomer (dimethyl-vinylethinyl-methyl-tm-butyl-peroxide) with acrylic or methacrylic acid or 2-methyl-5-vinyl pyridine with benzoyl peroxide as initiator in the presence of dodecylmercaptan as chain transfer agent. The resulting copolymers are water soluble at appropriate pH-values, surface active, and exhibit a critical micelle concentratioiL... 

Some examples of monomeric surface-active azo initiators as well as their synthesis, application in emulsion polymerization, and resulting latex properties are described in Refs. [43, 44]. These inisurfs are symmetrical and exhibit a methylene chain as hydrophobic part of the molecule and an ionic hydrophilic group (Inisurf 4, Inisurf 5). The main residts of these investi tions may be summarized as follows ... 

The inisurfs can be used in emulsion polymerization without addition of surfactants to obtain, for example, butyl acrylate-styrene-copolymer dispersions with a solid content of about 35%. 

In conclusion we can say that the inisurfs known today have different chemical structures and consequently different properties. Experimental data are available showing that emulsion polymerization is posssible using inisurfs without any additional emulsifiers, thus reducing the electrolyte content in the latex serum as well as foam formation. From a more technical point of view problems existing today concern the low initiator efficiencies as well as the fact that the solid content of the latexes is restricted to approximately 40% without coagulum formation. 

There is also surface active initiators which are called as "inisurfs", for example bis[2-(4 -sulfophenyl)alkyl]-2,2 -azodiisobutyrate ammonium salts and 2,2 -azobis(N-2 -methylpropanoyl-2-amino-alkyl-l-sulfonate)s. The initiators of this type carry stabilizing groups in their structures, and emulsion polymerization can be successfully carried out in the presence of them, without additional stabilizers up to more than 50% in solid content [37]. Moreover, the free radicals needed to initiate the emulsion polymerization can be produced by ultrasonically, or radiation-induced. Co y radiation is the most widely used as radiation-induced initiation system in the emulsion polymerizations. 

ABS) and polyvinyl chloride (PVC). In the emulsion polymerization, reactive surfactants can be used as inisurfs when they can replace both the initiator and the surfactants, as transurfs when they may be used to control the molecular weight, and finally as surfmers when they are used as comonomers which are polymerizable. 

Recently, new surface-active initiators have been synthesised and tested in emulsion polymerizations in the framework of a European network. The inisurfs known before the start of the network have one main drawback their susceptibility to hydrolysis due to the presence of ester linkages (R1-CO-OR2 or R3-0S03" or both). Another drawback is that their preparation requires a multistep synthesis. Considerable progress has been made in overcoming these drawbacks. 

The general structures of new surface active initiators are given in Table 1 (A1 and A2). With both Inisurfs it is possible to carry out emulsion polymerizations without additional stabilizers up to more than 50% solid contents. In particular, structure A2 fulfills all of the demands with respect to chemical stability against hydrolysis and ease of preparation (one-step synthesis via a modified Ritter reaetion). Strueture A1 has still ester bonds, but a sulfonate instead of a sulfate hydrophilie group. The synthesis occurs via a two-step procedure where by the first step is the preparation of the corresponding bis(phenyl alkyl)-2,2 -azobisisobutyrates (Pinner reaction) and the second step is the sulfonation of the phenyl ring. 

There is another important difference between the two types of surfactant. The phenylalkylsulfonates (Al) represent simple dimeric surfactants, whereas the A2 compounds are examples of a new class of surfactants, the gemini surfactants. Both types of surfactants and consequently both types of inisurfs differ considerably in their properties. The gemini inisurfs are able to stabilize a much larger surface area per molecule than conventional and simple dimeric surfactants. Consequently, the gemini inisurfs should lead in an emulsion polymerization to smaller particle sizes than the other inisurfs. 

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