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]. 

The Inisurfs originally synthesized were susceptible to hydrolysis and required a multistep synthesis which both added to the manufacturing costs and affected the purity of the products. Products developed more recently have successfully addressed both issues. In... 

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. 

Maitre et al. [314] carried out anionic polymerization of phenyl glycidyl ether (PGE) in miniemulsion using didodecyldimethylamonium hydroxide as an inisurf comhindition initiator and surfactant). Long chain alcohols were used as the costabilizer and stable miniemulsions were created by sonication. Monomer conversion was low, as was the degree of polymerization, which only... 

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]. 

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... 

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

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%. 

Particle size, polymerization rate, ai solid content of the final latexes remain unchanged compared with polymerizations with the unreacted individual components of the inisurfe (water soluble non-surface-active azo-bis (di-isobutyric acid amidine) and a C15-alkylmonosulfonate as emulsifier in the case of Inisurf 4 and a potassium salt of co-aminoundecanoic acid as emulsifier in the case of Inisurf 5, respectively). 

The final latexes of the polymerizations with the inisurfs exhibit a lower electrolyte content as well as a lower foam stability both indicating a lower waste content in the latex serum than with polymerization with the individual components of the inisurf synthesis. 

Prepolymers of this type can also act as inisurfs for dispersion polymerizations in organic solvents by using prepolymers soluble in the particualr solvent. This can be determined by appropriate choice of the monomer in the polymerizations forming the prepolymer. So for instance the vinyl acetate prepolymer can be used to obtain stable polyacrylamide dispersions with a solid content of upto 50% by polymerization in methanol [47]. Another application of this type of polymeric azoinitiator mentioned very briefly is the preparation of graft and block copolymers [55,5Q. The chemical composition of the blocks as well as the polymerization technique employed can be matched over a wide range to obtain polymers with desired properties. 

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]

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. 

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