Graft copolymers, polymeric surfactants

Most reports on emulsion polymerisation have been limited to commercially available surfactants which, in many cases, are relatively simple molecules such as sodium dodecyl sulphate and simple nonionic surfactants. However, studies on the effects of surfactant structure on latex formation have revealed the importance of the structure of the molecule. Block and graft copolymers (polymeric surfactants) are expected to be better stabilisers when compared to simple surfactants. The use of these polymeric surfactants in emulsion polymerisation and the stabilisation of the resulting polymer particles is discussed below. 

Another effective graft copolymer is hydrophobicaUy modified inuUn this is a linear polyfructose chain A (with degree of polymerisation >23) onto which several alkyl chains have been grafted. The polymeric surfactant adsorbs onto several alkyl chains via multipoint attachment. 

Emulsion polymerizations of vinyl acetate in the presence of ethylene oxide- or propylene oxide-based surfactants and protective coUoids also are characterized by the formation of graft copolymers of vinyl acetate on these materials. This was also observed in mixed systems of hydroxyethyl cellulose and nonylphenol ethoxylates. The oxyethylene chain groups supply the specific site of transfer (111). The concentration of insoluble (grafted) polymer decreases with increase in surfactant ratio, and (max) is observed at an ethoxylation degree of 8 (112). 

Macromonomers afford a powerful means of designing a vast variety of well-defined graft copolymers. These species are particularly useful in the field of polymer blends as compatibilizers and/or stabilizers (surfactants). When macromonomer itself is an amphiphilic polymer, then its polymerization in water usually occurs rapidly as a result of organization into micelles. In copolymerizations, important factors for macromonomer reactivity are the thermodynamic repulsion or incompatibility between the macromonomer and the trunk polymer and its partitioning between the continuous phase and the polymer particles [4,5]. 

Most dispersion polymerizations in C02, including the monomers methyl methacrylate, styrene, and vinyl acetate, have been summarized elsewhere (Canelas and DeSimone, 1997b Kendall et al., 1999) and will not be covered in this chapter. In a dispersion polymerization, the insoluble polymer is sterically stabilized as colloidal polymer particles by the surfactant that is adsorbed or chemically grafted to the particles. Effective surfactants in the dispersion polymerizations include C02-soluble homopolymers, block and random copolymers, and reactive macromonomers. Polymeric surfactants for C02 have been designed by combining C02-soluble (C02-philic) polymers, such as polydimethylsiloxane (PDMS) or PFOA, with C02-insoluble (C02-phobic) polymers, such as hydrophilic or lipophilic polymers (Betts et al., 1996, 1998 Guan and DeSimone, 1994). Several advances in C02-based dispersion polymerizations will be reviewed in the following section. 

The emulsion polymerization process involves the polymerization of liquid monomers that are dispersed in an aqueous surfactant micelle-containing solution. The monomers are solubilized in the surfactant micelles. A water-soluble initiator catalyst, such as sodium persulfate, is added to the aqueous phase. The free radicals generated cause the dispersed monomers to react to produce polymer molecules within the micellar environment. The surfactant plays an additional role in stabilizing dispersion of the produced polymer particles. Thus, the surfactants used both provide micelles to house the monomers and macroradicals, and also stabilize the produced polymer particles [193,790], Anionic surfactants, such as dodecylbenzene sulfonates, are commonly used to provide electrostatic stabilization [193], These tend to cause production of polymer particles having diameters of about 0.1-0.3 pm, whereas when steric stabilization is provided by, for example, graft copolymers, then diameters of about 0.1-10 pm tend to be produced [790,791]. 

Use of macromonomers as reactive (copolymerizable) surfactants in heterogeneous systems such as emulsion and dispersion constitutes an increasingly important application in the design of polymeric microspheres, as will be discussed later in Sect. 6. Here the macromonomers copolymerize in situ with some of the substrate comonomers to afford the graft copolymers, the grafts (branches) of which serve as effective steric stabilizers by anchoring their backbone onto the surfaces of the particles. In general, however, the copolymerization reactivities of macromonomers in such systems are not well understood yet. 

In the next section, therefore, we review recent studies of simpler cases, i.e., homopoly(macromonomers), star- and comb-shaped polymers, followed by some interesting properties of the graft copolymers to be used as polymeric surfactants, surface modifiers, and compatibilizers for blends. 

Diblock copolymers consisting of soluble and insoluble parts (Fig. 2b) act much as grafted chains once they are adsorbed on the surface. However, the thermodynamics of the initial solution, consisting primarily of micelles, and the conformation of the insoluble blocks on the surface affect the coverage in ways not well understood (e.g., Munch and Gast, 1988 Marques et al., 1988 Gast, 1989). Many dispersants or polymeric surfactants are synthesized in this way (Reiss et al, 1987). 

Emulsion polymerization has become an important process for the production of a large number of industrial polymers in the form of polymer colloids or latexes. They are the base of adhesives, paints and especially of waterborne coatings. An interest has been developed in recent years in emulsion polymerization systems in which the classical low molecular weight surfactaints are replaced by polymeric surfactants, either hydrophilic-hydrophobic block and graft copolymers (1-4) or functionalized oligomers (5). 

Polymerization of the alkoxyallene with macromonomers having a poly (ethyleneglycol) group by [(7r-allyl)Ni(OCOCF3)]2/PPh3 produces a graft copolymer with narrow molecular weight distribution [129]. The products serve as polymeric surfactants in the polymer blend system of polystyrene and poly(methyl methacrylate). 

The most convenient polymeric surfactants are those of the block and graft copolymer type. A block copolymer is a linear arrangement of blocks of variable... 

Three main mechanisms of stabilisation can be considered (i) electrostatic, as produced by ionic surfactants (ii) steric, as produced by nonionic polymeric surfactants of the A-B, B-A-B, A-B-A or AB graft copolymers (where A is the anchor chain and B is the stabilising chain and (iii) electrosteric, as produced by polyelectrolytes. 

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