Surfactant macromonomers

Copolymers of acrylamide and surfactant macromonomers [e.g. alkyl polyetheroxyacrylates (R-PEO-AC)] do not require external surfactant for their preparation [4]. However, they too exhibit poor hydrolytic stability because of the hydrolysis of the amide and ester linkages upon heating in water or brine. 

Surfactant Macromonomers. Hydrophobic monomers that also have surfactant character, also termed surfomers. Example nonyl-... 

Better direct polymerization methods involve the use of micellar copolymerization or use of surfactant macromonomers. In the micellar copolymerization method [8,13], an oil-soluble acrylamide (e.g. Cg-alkyl acrylamide) is solubilized in the aqueous phase by appropriate surfactant(s) and copolymerized with water-soluble acrylamide (AM) monomer (Eq. 1.1). The resulting product is a copolymer of acrylamide and Cg-alkyl acrylamide (RAM). 

In the surfactant macromonomer method, [9-11, 17] a water-soluble (or dispersible) surfactant, nonylphenoxy poly(ethylene oxide), is built into an acrylate monomer. The surfactant macromer has both a hydrophilic head... 

Surfactant macromonomers such as nonylphenoxypoly(etheroxy) acrylates, R-EO-Ac (Fig. 3.1), are water soluble or water dispersible. The extent of water solubility depends upon the hydrophile-lipophile balance (HLB) of the surfactant and the temperature. Surfactant monomers, wherein R = nonylphenyl and n = 10, 20, and 40 have been synthesized. 

Above C, the solution viscosity of the surfomer copolymers is more sensitive to changes in the level of surfactant macromonomer than to changes in [M]/[I]. For example, doubling the surfomer level from 0.3 to 0.6 mol % greatly increases the polymer solution viscosity. However, doubling the [M]/[I] from 30 to 60 has almost no effect on the viscosity-concentration profile of the surfomer copolymer (Fig. 3.10) [13]. 

Transfer constants of the macromonomers arc typically low (-0.5, Section 6.2.3.4) and it is necessary to use starved feed conditions to achieve low dispersities and to make block copolymers. Best results have been achieved using emulsion polymerization380 395 where rates of termination are lowered by compartmentalization effects. A one-pot process where macromonomers were made by catalytic chain transfer was developed.380" 95 Molecular weights up to 28000 that increase linearly with conversion as predicted by eq. 16, dispersities that decrease with conversion down to MJM< 1.3 and block purities >90% can be achieved.311 1 395 Surfactant-frcc emulsion polymerizations were made possible by use of a MAA macromonomer as the initial RAFT agent to create self-stabilizing lattices . 

A novel approach to RAFT emulsion polymerization has recently been reported.461529 In a first step, a water-soluble monomer (AA) was polymerized in the aqueous phase to a low degree of polymerization to form a macro RAFT agent. A hydrophobic monomer (BA) was then added under controlled feed to give amphiphilic oligomers that form micelles. These constitute a RAFT-containing seed. Continued controlled feed of hydrophobic monomer may be used to continue the emulsion polymerization. The process appears directly analogous to the self-stabilizing lattices approach previously used in macromonomer RAFT polymerization (Section 9.5.2). Both processes allow emulsion polymerization without added surfactant. 

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]. 

The aggregation numbers Nagg is determined as 27 for C1-(EO)53-C4-VB and 38 for Cr(EO)53-C7-VB micelles by analysis of fluorescence curves. A micelle formation mechanism is proposed for nonionic polymeric surfactants with weakly hydrophobic groups. At low concentrations of PEO macromonomers, large loosely aggregated structures involving the PEO chains are formed. At higher concentrations normal micelles form. These are star-shaped, with a hydrophobic core surrounded by a corona of PEO chains. 

Short chain amphiphiles can be incorporated into the backbone of the polymer chains. The resulting graft macromolecules are able to form both intrachain and interchain aggregates. Polymeric surfactants assemble into a variety of intrachain micelles. These polymeric surfactants and/or amphiphilic polymacromonomers can also form mixed aggregates which incorporate free monomeric (macromonomer with a very small hydrophobic group) surfactants. 

In copolymerization of styrene, more monodisperse and smaller-size particles were obtained with the PEO-MA macromonomer with a longer alkylene spacer group, thus indicating their effectiveness as reactive surfactant increases in the series [71]... 

The colloidal stable polymer dispersions, the monodisperse polymer particles, and high conversions (85-100%) can be obtained with most of the other macromonomers (MAL,VB, and MA) of PEO (MW>PEO=2000)) [76]. Also, when macromonomers are used (3.1 wt% based on styrene), there is practically no coagulum produced. This is not the case in the presence of polymerizable PEO surfactants (surfmer I R1=CH3(CH2)11-, R2=H, n=34 and surfmer II R =CH3 (CH2)n-, R2=H, n=42) despite the higher amounts of stabilizer used (up to 60 wt% of coagulum). Furthermore, the particles are more monodisperse with PEO macromonomer (Dw/Dn=1.025 for PEO-MA and 1.13 for PVPo) compared to those with surfmer. Comparatively poorer results were obtained with conventional surfactants such as ethoxylated nonylphenol, even when used in large amounts. 

The emulsion copolymerization of BA with PEO-MA (Mw=2000) macromonomer was reported to be faster than the copolymerization of BA and MMA, proceeding under the same reaction conditions at 40 °C [100]. Polymerizations were initiated by a redox pair consisting of 1-ascorbic acid and hydrogen peroxide in the presence of a nonionic surfactant (p-nonyl phenol ethoxylate with 20 moles ethylene oxide). In the macromonomer system, the constant-rate interval 2 [9,10] was long (20-70% conversion). On the other hand, the interval 2 did not appear in the BA/MMA copolymerization and the maximum rate was lower by ca. 8% conversion min 1 and it was located at low conversions. 

The rate of polymerization was found to be independent of emulsifier concentration around CMC (1.8x10 4mol dm 3) and up to ca. 10 3 mol dm 3 and then strongly increased with increasing emulsifier concentration (Fig. 5). It can be seen that, for this system, the break in the dependence of the rate on surfactant concentration does not coincide with the CMC of either the surfactant or the surfactant/PEO-MA macromonomer. In fact, these two values are identical at room temperature at 50 °C the CMC of the surfactant is lower than at 20 °C. The kinetics of particle nucleation for the present nonionic polymerization of BA may not follow a micellar mechanism. 

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. 

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. 

Instead of conventional surfactant molecules, amphiphilic water soluble macromonomers, especially PEO macromonomers, have been used extensively as a reactive emulsifier and as steric stabilizer polymer, as summarized in Table 5. Generally speaking, however, the mechanism for the particle nucleation in the emulsion polymerization systems using macromonomers has been poorly established when compared to the dispersion copolymerizations with macromonomers as mentioned earlier. 

Capek described the use of a macromonomer in miniemulsion polymerization [54]. Lim and Chen used polyfmethyl methacrylate-fr-(diethylamino)ethyl methacrylate) diblock copolymer as surfactant and hexadecane as hydrophobe for the stabilization of miniemulsions [55]. Particles with sizes between about 150 and 400 nm were produced. It is possible to create stable vinyl acetate miniemulsions employing nonionic polyvinyl alcohol (PVA) as surfactant and hexadecane as hydrophobe [56]. 

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). 

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