Homopolymers, polymeric surfactants

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. 

Perhaps the simplest type of a polymeric surfactant is a homopolymer, that is formed from the same repeating units, such as PEO or poly(vinyl pyrrolidone). These homopolymers have minimal surface activity at the O/W interface, as the homopolymer segments (e.g., ethylene oxide or vinylpyrroUdone) are highly water-soluble and have little affinity to the interface. However, such homopolymers may adsorb significantly at the solid/liquid (S/L) interface. Even if the adsorption energy per monomer segment to the surface is small (fraction of kT, where k is the Boltzmann constant and T is absolute temperature), the total adsorption energy per molecule may be sufficient to overcome the unfavourable entropy loss of the molecule at the S/L interface. 

Elemental analysis (EA) is a convenient method for determination of copolymer and blend composition if one homopolymer contains an element not present in the second one. For example, EA can be properly used to quantify nitrogen in copolymers containing acrylonitrile units and oxygen in polymeric surfactants such as poly(oxy-alkylene). Therefore, for a binary system, every element can be balanced according to the following equation ... 

Figure 16.1. Various conformations of polymeric surfactants adsorbed on a plane surface (a) random conformations of loops-trains-tails (homopolymer) (b) preferential adsorption of short blocks (c) chain lying flat on the surface (d) AB block copolymer with loop-train conformation of B and long tail of A (e) ABA block copolymer, as in (d) (f) BA graft with backbone B forming small loops and several tails of A ( teeth )...

This example shows that block copolymers can act as polymeric surfactants to stabilise polymeric mixtures of homopolymers, providing that the constituents of the blocks are similar to those constituting the homopolymers. 

To understand the solution behavior of polymeric surfactants of the block-and-graft type, it is essential to consider the solution properties of the more simple homopolymers. The solution behavior of homopolymers was considered in the thermodynamic treatment of Flory and Huggins. 

The second alternative for polymerizable surfactants is polymeric surfactants. The subject has been recently reviewed by Lachewsky [8]. Even more recently the same authors [135] compared polymerizable surfactant and their homopolymers (polysoaps) and showed that good results can be obtained from them. The same conclusion has been shown valid for the homopolymer of one of the first commercially available allylic surfmers [136]. Recently, core-shell particles have been prepared using an inisurfmer, containing both a polymerizable moiety and a peroxydic group. This compound has been used to cover a seed polymer particle and initiate, from the peroxide group, the polymerization of a shell of another polymer [137]. 

However, these low-molecular-weight surfactants are only efficient at high concentrations. For this reason, polymeric surfactants (homopolymers and copolymers) have been widely used for stabilization of dispersions and they usually have hydrophilic and hydrophobic regions so that they can act as low-molecular-weight surfactants. 

Polymeric surfactants can be homopolymers, random amphiphilic copolymers, and of the A-B (diblock), A-B-A (triblock), and BA (graft) types [23]. The A chain is referred to as the stabilizing chain (soluble in the medium), and the B chain is referred to as the anchor chain (insoluble in the medium with strong affinity to the surface). The simplest type of a polymeric surfactant is a homopolymer, such as PEO and poly(vinyl pyrrolidone) (PVP). Homopolymers are not the most suitable surfactants and it is better to use polymers with some groups that have affinity to the surface. The most employed copolymers are random amphiphilic copolymers, like poly(vinyl alcohol), diblocks of polystyrene-block-poly(vinyl alcohol) (PS-b-PVA), poly(ethylene oxide)-block-polystyrene (PEO-b-PS), and triblocks of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-b-PPO-b-PEO, Pluronic) (PPO resides at the hydrophobic surface, leaving the two PEO chains dangling inaqueous solution), andpoly(ethyleneoxide)-block-polystyrene-block-poly(ethylene oxide) (PEO-b-PS-b-PEO). The graft copolymer is referred to as a comb stabilizer Atlox 4913,... 

Perhaps the most common and well-studied regular polymeric surfactant family is that of the block copolymers. Denoting one monomer type as A and the other as B, typical basic compositions would be A-B, A-B-A, and B-A-B. The chemistry and technology for preparing such polymers are well developed. The most common addition process essentially involves a multistep reaction sequence in which one monomer (e.g.. A) is reacted with an acid or base catalyst to produce a homopolymer with a reactive terminal unit (A,i). When monomer A is used up or reacted to the desired extent, the reaction is treated with monomer B, which then begins to polymerize on the active terminal of After a suitable reaction time, the polymerization can be terminated to produce the A -B copolymer, or a second... 

A reactive surfactant shown next (RS) was used as a comonomer in a seeded polymerization. RS was easily adsorbed on seed particles due to their amphiphilicity. If dialkyl fumarate was preabsorbed in the particle, the polymerization proceeded quickly and resulted in the formation of skin layer of RS-fumarate copolymer. Because the vinyl group in RS is an allyl type, RS in aqueous phase hardly polymerizes and no water-soluble homopolymer was formed. The active ester group of RS on the skin layer was used for the preparation of functional microspheres (18). 

Poly vinylidene fluoride is polymerized under pressure at 25-150°C in an emulsion using a fluorinated surfactant to minimize chain transfer with the emulsifying agent. Ammonium persulfate is used as the initiator. The homopolymer is highly crystalline and melts at 170°C. It can be injection molded to produce articles with a tensile strength of 7000 psi (48 MPal. a modulus of elasticity in tension of 1.2 x 105 psi and a heat deflection of 3003F (149°C). 

It was previously reported that the homopolymer surfactant PFOA successfully stabilized poly(methyl methacrylate) (PMMA) dispersion polymerizations (DeSimone et al., 1994 Hsiao et ah, 1995), but was not successful for styrene dispersion polymerizations (Canelas et al., 1996). In these styrene polymerizations, the C02 pressure used was 204 bar. However, later studies showed that both PFOA and poly(l,l-dihydroper-fluorooctyl methacrylate) (PFOMA) could stabilize polystyrene (PS) particles (Shiho and DeSimone, 1999) when a higher pressure was used. These polymerizations were conducted at 370 bar, 65 °C, and the particle size could be varied from 3 to 10 pm by varying the concentration of stabilizer. These homopolymer surfactants are less expensive and easier to synthesize than block copolymer surfactants and provide access to a large range of particle sizes. 

The dispersions were obtained by emulsification via ultrasonication of a toluene solution of the unsaturated homopolymer in an aqueous surfactant solution. This was followed by exhaustive hydrogenation with Wilkinson s catalyst at 60°C and 80 bar H2 to produce a dispersion with an average particle size of 35 nm (dynamic light scattering and transmission electron microscopy analyses). The same a,co-diene was used as comonomer in the ADMET polymerization of a phosphorus-based monomer, also containing two 10-undecenoic acid moieties... 

For the stabilization of various insoluble hydrocarbon polymers in carbon dioxide, it has been found that no one surfactant works well for all systems. Therefore it has become necessary to tailor the surfactants to the specific polymerization reaction. Through variation of not only the composition of the surfactants, but also their architectures, surfactants have been molecularly-engineered to be surface active—partitioning at the interface between the growing polymer particle and the CO2 continuous phase. The surfactants utilized to date include poly(FOA) homopolymer, poly(dimethylsiloxane) homopolymer with a polymerizable endgroup, poly(styrene-b-FOA), and poly(styrene-b-dimethylsiloxane). Through the utilization of these surfactants, the successful dispersion polymerization of methyl methacrylate (MMA), styrene, and 2,6-dimethylphenol in CO2 has been demonstrated. 

To synthesize water-soluble or swellable copolymers, inverse heterophase polymerization processes are of special interest. The inverse macroemulsion polymerization is only reported for the copolymerization of two hydrophilic monomers. Hernandez-Barajas and Hunkeler [62] investigated the copolymerization of AAm with quaternary ammonium cationic monomers in the presence of block copoly-meric surfactants by batch and semi-batch inverse emulsion copolymerization. Glukhikh et al. [63] reported the copolymerization of AAm and methacrylic acid using an inverse emulsion system. Amphiphilic copolymers from inverse systems are also successfully obtained in microemulsion polymerization. For example, Vaskova et al. [64-66] copolymerized the hydrophilic AAm with more hydrophobic methyl methacrylate (MMA) or styrene in a water-in-oil microemulsion initiated by radical initiators with different solubilities in water. However, not only copolymer, but also homopolymer was formed. The total conversion of MMA was rather limited (<10%) and the composition of the copolymer was almost independent of the comonomer ratio. This was probably due to a constant molar ratio of the monomers in the water phase or at the interface as the possible locus of polymerization. Also, in the case of styrene copolymerizing with AAm, the molar fraction of AAm in homopolymer compared to copolymer is about 45-55 wt% [67], which is still too high for a meaningful technical application. 

Mixtures of two homopolymers (A and B) and their corresponding diblock copolymer (A-B) are polymeric counterparts of mixtures of water, oil and surfactant. The immiscible nature between water and oil is also observed in polymer blends due to the fact that most polymers are immiscible in each other. The addition of diblock copolymers into blends of homopolymers has effects similar to adding surfactants into water-oil mixtures. The resulting reduction in interfacial tension and formation of the preferred interfacial curvature yield a variety of self-assembled structures. 

These steric problems with their consequences should affect all vinylic surfactant polymers, independent of the inherent surface curvatures of the different models of polymeric micelles (see Sect. 4.2). Thus, vinylic surfactant homopolymers of other than tail end geometry should be of very limited use as polysoaps. In fact, very few exceptions to the geometry controlled model of solubility have been reported [82, 106, 128, 225, 289-291]. In these examples, the chemical integrity of the polymers prepared, the attributed structures, or the polymeric nature may be questioned considering the results obtained for very similar compounds [115,232,309], But even if these exceptions are real, this rule will help to design new monomers and polymers. 

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