Surface active initiators

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. 

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. 

Tauer, K. and Kosmella, S. (1993) Synthesis, characterisation and application of surface active initiators. Polym. Int., 30, 253. 

Surface active initiators with an azo group as radical generating functionality are another important class of inisurfs [16, 24, 37, 43-50]. 

Standing hydroxyl groups of the diester by conventional methods [57-59], leads to true inisurfs (Inisurf 10) [54], These inisurfs were diaracterized regarding their molecular weight [24] as well as regarding their surface activity [52]. The critical micelle concentration of these surface active initiators depends on the diol used. Inisurfe prepared with [K)ly(ethylene oxide) exhibit a higher critical micelle concentration than products prepared with a,[Pg.60]

The copolymerization in miniemulsion was not limited to systems for which the monomers were in the dispersed phase. Rather, copolymerization could also be carried out with monomers of opposite polarity - that is, with one comonomer in each phase - in both direct and inverse miniemulsion [26]. Water-soluble, surface active, and oil-soluble initiators were employed to start the polymerizations, as shown in Figure 15.2. Oil-soluble initiators were found to produce a higher yield of copolymers of acrylamide and methyl methacrylate with a low degree of blockiness than did water-soluble or surface-active initiators. In contrast, the surface-active polyethylene glycol (PEG) azo-initiator yielded polymers that were almost free from homopolymers, and with a low degree of blockiness, when acrylamide and styrene were copolymerized. At the interface, monomers that only copolymerize alternately [27] as water-soluble poly(hydroxy vinyl ether)s were also successfully polymerized with oil-soluble maleate esters, to yield polymer nanocapsules. 

Tauer, K. (2010) Polymer nanoparticles with surface active initiators and polymeric stabilizers, in Advanced Polymer Nanoparticles Synthesis and Surface Modifications (ed. V. Mittal), CRC Press, Taylor Francis, LLC, Boca Raton, FL, pp. 329-360. 

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. 

A third method to subdivide surface-active initiators is based on the chemical nature of the hydrophobic and hydrophilic groups. The hydrophilic group may be anionic or cationic, or an oxyethylene chain of an appropriate chain length. Hydrocarbon chains (alkyl, alkylphenol) or propylene oxide chains are used as hydropobic molecule components. Oxyethylene chains are also used as hydrophobic groups if they are in the neighborhood of ionic groups. 

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. 

Figure 1 demonstrates these effects by means of the particle size distribution of polystyrene latexes prepared with both types of inisurf. The polymerizations are carried out at 90 °C with 100 g of water, 10 g of styrene, and 0.25 g of 2,2 -azobis(N-2 -methylpropanoyl-2-amino-decyl-l-sulfonate) as gemini inisurf (Fig. la) and 1.292 g of bis(2-phenyl ethyl)-2,2 -azodiisobutyrate (Fig. lb) as conventional surface-active initiator, respectively. 

FIG. 1 Inisurfs. (a) Gemini inisurf (b) conventional surface active initiation. 

Stoffelbach, R, Griffete, N., Bui, C., Charleux, B. Use of a simple surface-active initiator in controlled/living free-radical miniemulsion polymerization under AGET tmd ARGET ATRP conditions. Chem. Commun. 39,4807- 809 (2008)... 

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