Hydrophobe modification water-soluble polymers

Chain-Growth Associative Thickeners. Preparation of hydrophobically modified, water-soluble polymer in aqueous media by a chain-growth mechanism presents a unique challenge in that the hydrophobically modified monomers are surface active and form micelles (50). Although the initiation and propagation occurs primarily in the aqueous phase, when the propagating radical enters the micelle the hydrophobically modified monomers then polymerize in blocks. In addition, the hydrophobically modified monomer possesses a different reactivity ratio (42) than the unmodified monomer, and the composition of the polymer chain therefore varies considerably with conversion (57). The most extensively studied monomer of this class has been acrylamide, but there have been others such as the modification of PVAlc. Pyridine (58) was one of the first chain-growth polymers to be hydrophobically modified. This modification is a post-polymerization alkylation reaction and produces a random distribution of hydrophobic units. 

Rheology modification Clays, amide modified alkyds, hydrophobically associating water soluble polymers, cellulosics, titanium chelates... 

Hydrophobically associating water soluble polymers can be prepared either by copolymerization of a hydrophilic and a hydrophobic monomer or by modification of a precursor water-soluble polymer. A typical example of materials prepared by the first method is the hydrophobic derivatives of polyacrylamide (PAM) [10-15]. The copolymerization occurs in the presence of a surfactant in order to increase the solubility of the hydrophobic component in the water and the resulting polymers exhibit a rather block structure the hydrophobic groups have a rather block distribution on the PAM chain [10, 13-15]. The size of the hydrophobic blocks can be controlled by the quantity of the surfactant added (see also the contribution of Prof. McCormick group in this book). Unfortunately this copolymerization technique does not allow the simultaneous control of all the molecular parameters (i.e. polymer molecular weight, degree of modification and alkyl group distribution). 

The concept of hydrophobe modification of water-soluble polymers may have arisen from the hydrophobic half-esters of maleic anhydride-methyl vinyl ether copolymers, which were commercially available products in the 1960s (i). Replacement of the methyl vinyl ether with styrene produced more stable copolymers in general (2), but hydrophobes attached through the ester... 

An early discovery, which has had abundant subsequent confirmation, was that anionic surfactants are much more reactive than cationic surfactants toward simple uncharged water soluble polymers (3). Several explanations of this interesting difference in reactivity have been attempted, but none is completely satisfactory, as is evident in various review articles (3,5,9). In a similar vein it is also now accepted that nonionic surfactants are generally unreactive toward simple uncharged (and also charged) polymers. These effects are summarized in Table 2, which also includes other combinations of polymer and surfactant. For example. Table 2 shows that an anionic surfactant will react strongly with a polycation but will not react, or will react only weakly, with a polyanion, illustrating the potent effect of electrostatic forces. It must be stressed that Table 2 applies to polymers that are not hydrophobically modified as pointed out already, such modification can com-... 

Liquid-liquid extraction using water-soluble polymer solutions provides a mild, nondenaturing extraction system for proteins [1,7]. The most familiar example is partition between a relatively hydrophilic dextran phase and a relatively hydrophobic PEG phase. The selectivity of the two phases can be modulated by addition of salts very high selectivity can be achieved by binding an affinity ligand to a fraction of the PEG polymers. In the case of affinity liquid-liquid extraction, recovery of protein requires modification of the phase to dissociate the bound protein from the polymer, e.g., by a pH change. 

Starch, a hydrophilic polymeric material produced by plants as a food reserve, is produced in amounts in excess of that required for nutritionally balanced diets. In the United States about 4 billion pounds are used annually in industrial markets where a water-soluble polymer is needed, as in paper sizes and pastes. Interest has been shown in chemical modification of starch to make it hydrophobic to increase its potential as a replacement or extender for certain polymeric materials derived from nonrenewable resources. 

As seen from the above examples, most of these hydrophobe modification processes involve introducing a hydrophobe onto a water soluble polymer (WSP) backbone. However, Bock and Valint [30] have reversed the process they have added solubilizing ionic moieties to a hydrophobic block copolymer, poly(t-butylstyrene-h-styrene), backbone by selective sulfonation of the polystyrene block (Scheme 1.5). The resulting material is a novel hydrophobically associating block copolymer. 

The conversion of dextran with 1,2-epoxy-3-phenoxypropane, epoxyoctane or epoxydodecane may be exploited for the preparation of amphiphilic dextran derivatives. Polymeric surfactants prepared by hydrophobic modification of polysaccharides have been widely studied, starting with the pioneering work of Landoll [261]. Neutral water-soluble polymeric surfactants can be obtained by reaction of dextran with 1,2-epoxy-3-phenoxypropane in 1 M aqueous NaOH at ambient temperature (Fig. 35, [229,233]). The number n of hydrophobic groups per 100 Glcp units varies between 7 and 22 depending on the reaction conditions. 2-Hydroxy-3-phenoxy propyl dextran ethers (DexP) behave like classical associative polymers in aqueous solution. In dilute solution, the intrinsic viscosity decreases significantly whereas... 

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