Polymerization hydrophobically associating polymers

Polymerization of Hydrophobically Associating Polymers. The following procedure for the preparation of 0.75 mol % N-4-butylphenylacrylamide-acrylamide polymer is typical of all the polymers discussed in this report. 

Hydrophobically associating polymers consist primarily of water-soluble monomer units with a small number of water-insoluble monomer units. Synthesis of high-molecular-weight random copolymers of acrylamide and alkylacrylamides required a novel aqueous surfactant micellar solution polymerization (2-4) because of the mutual immiscibility of the water-soluble and hydrophobic monomers. The use of surfactant micelles enabled solubilization of the hydrophobic monomer (alkylacrylamide [R]) into the aqueous phase containing the water-soluble monomer (acrylamide [AM]). The resulting RAM polymer after isolation provided homogeneous aqueous solutions. 

The third approach to hydrophobically associating polymers avoided the need for a surfactant during polymerization by employing a water dispersible surfactant like monomer such as an alkylaryl poly(etheroxy)acrylate in conjunction with acrylamide to form the PAM-SURF copolymers.With this approach the relative amounts of ethylene oxide to hydrophobic alkylaryl group were used to provide a balance between water dispersibility of the hydrophobic monomer and association in solution. The incorporation of this monomer and sequencing depended on the relative amounts of ethylene oxide and hydrophobic portion of the monomer. The use of an arene group in the hydrophobic portion enabled UV spectroscopy to be used to determine monomer incorporation. 

Rheological studies also indicated differences in behavior as a function of concentration for these two polymers. The plots of relative and reduced viscosities for the copolymer prepared in surfactant were similar to the profile in Fig. 2.1, indicating that it behaved as a hydrophobically associating polymer in deionized water and 2% NaCl. The solution polymer, in contrast, was shown to behave as a polymeric microsuspension in these solvents. 

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... 

We now know that emulsion polymerization is not just another polymer synthesis method and that the complexity of the interactions, whether chemical or physical, must he considered before any control is possiUe over the outcome of the reaction. The creation and nucleation of particles, for example, is not necessarily and simply explained by the presence or or absence of micelles, but needs the understanding of interactions of all the ingredients present. Variables such as hydrophilic and hydrophobic associations or repulsions, polarity of the monomers, chemical structure of the surfactants, have to lx taken into account. 

Similarly, Wasserman and coworkers have studied a wide selection of polymeric materials in aqueous solution that are associative of some kind, i.e., that form some sort of self-assembly through non-covalent interactions [96]. Their study mainly deals with hydrogels of hydrophobically modified polymers, aqueous solutions of polymeric micelles created by block copolymers, and hydrogels based on poly (acrylic acid) and macrodiisocyanates. The spin probes of choice were hydrophobic, such as 5- and 16-DSA (see Eig. 2) or even spin labeled polymers. It was, e.g., possible to screen for the effect of chemical stmcture on the gel formation by recording and understanding the local mobility of the hydrophobic, long chain spin probes as a function of temperature. 

Hydrophobically associating acrylamide based polymers were explored as a means of alleviating the salt sensitivity observed in the block systems. A micellar polymerization technique was developed to enable preparation of random copolymers of acrylamide and N-n-alkylacrylamide. When these copolymers were dissolved in an aqueous solvent, the hydrophobic groups associated to minimize their exposure to water. The hydrophobic associations provided an additional dimension to polymer molecular weight and chain expansion by ionic groups for the control of aqueous fluid rheology. 

Surfactant concentration (varied after polymerization) greatly affects the viscosity of associating polymer systems. Iliopoulos et al. studied the interactions between sodium dodecyl sulfate (SDS) and hydrophobically modified polyfsodium acrylate) with 1 or 3 mole percent of octadecyl side groups [85]. A viscosity maximum occurred at a surfactant concentration close to or lower than the critical micelle concentration (CMC). Viscosity increases of up to 5 orders of magnitude were observed. Glass et al. observed similar behavior with hydrophobically modified HEC polymers. [100] The low-shear viscosity of hydrophobically modified HEC showed a maximum at the CMC of sodium oleate. HEUR thickeners showed the same type of behavior with both anionic (SDS) and nonionic surfactants. At the critical micelle concentration, the micelles can effectively cross-link the associating polymer if more than one hydrophobe from different polymer chains is incorporated into a micelle. Above the CMC, the number of micelles per polymer-bound hydrophobe increases, and the micelles can no longer effectively cross-link the polymer. As a result, viscosity diminishes. 

Non-ionic (hydrophobic) association of the polymeric catalyst-substrate has also been shown to increase the catalytic activity. Thus, the rate of hydrolysis of 3-nitro-4-acyloxybenzoic acid (6) by polyvinylimidazole, increases with increasing bulk of acyloxy group (Over-berger and Sannes, 1974). It was observed that the deacylation step of the intermediate polymeric acylimidazole is rate-determining. Since the acylated polymer becomes more hydrophobic, because of the extra group attached to the polymer, apolar associations increase with the chain length of the acyl group, and, hence increase the catalytic activity. 

Compared with the massive increase in tjo observed with increasing polymer concentration, the ratio /e//m was fotmd to increase to a much lesser extent. While t]o increased by seven orders of magnitude when the Py(35)-HASE concentration was increased from 1 to 45 g the /e//m ratio increased by less than threefold. Nevertheless, the increase in 7e//m was noticeable and indicated that intermolecular interactions occurred. Furthermore, it took place in the same concentration range where ijo increased suggesting that the increase in intermolecular hydrophobic association between the pyrene labels coincided with the increase in r/o, an expected result if it was due to the formation of an extended polymeric network between Py-HASE molecules. 

The solution properties of polymeric amphiphilic molecules are strongly influenced by molecular aggregation and have been a subject of widespread research [4-19]. These polymers contain a major amount of polar, water-soluble groups and a minor amount of nonpolar hydrocarbon chains. The amphiphilic nature of these structures causes polymer aggregation in aqueous media which results in unique and useful solution properties. This chapter describes the solution properties of hydrophobically associating water-soluble polymers. In addition, the relationship between these properties and performance attributes in a variety of technological applications will be described. 

Hydrophobic associations in aqueous solution have been recognized in polymeric systems for some time. Their ability to enhance viscosity has led to research directed toward modifying existing water soluble polymers with alkyl substituents capable of associating in solution. These have included studies by Strauss et al. on polysoaps from polyvinylpyridine [7], Schulz [8] and McCormick... 

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. 

The self-assembling character of bilayer membranes is demonstrated by the formation of free-standing cast films from aqueous dispersions of synthetic bilayer membranes. The tendencies for association are sufficiently strong to allow the addition of guest molecules (nanoparticles, proteins, and various small molecules) to these films where the connective forces are secondary in nature and not primary. Synthetic polymer chemists have made use of these self-assembling tendencies to synthesize monolayer films. In particular, a monomer that contains both reactive groups and hydrophobic and hydrophilic areas is cast onto an appropriate template that self-assembles the monomer, holding it for subsequent polymerization. Thus, a bilayer structure is formed by... 

---