Interpolymer hydrophobic association

In aqueous solutions of hydrophobically-modified water-soluble polymers, strong interpolymer hydrophobic association often leads to gelation or precipitation. It is well-known, however, that AB- and ABA-type block copolymers, where A and B represent hydrophilic and hydrophobic blocks, respectively, form micelles with a hydrophobic core and a hydrophilic corona (25). The formation of these polymer micelles is described by a closed association process (26-28), and at a thermodynamic... 

In aqueous solutions of amphiphilic polymers, which contain both hydrophilic and hydrophobic sequences, strong interpolymer hydrophobic association often leads to bulk-phase separation or gelation. However, there are classes of amphiphilic polymers that form well-organized associated structures in aqueous solution without accompanying macroscopic phase separation. This is generally characteristic of amphiphilic AB and ABA block copolymers, where A and B represent hydrophilic and hydrophobic sequences, respectively. These types of block copolymers and low molecular weight surfactant molecules have common features in their associating behavior. 

In contrast with ABA-type block copolymers, BAB-type block copolymers or multiblock copolymers, in which two or more hydrophobic block sequences are linked with hydrophilic block sequences in a polymer chain, tend to undergo multimolecular associations in which the number of polymers that can participate in the association process is not limited (Fig. 2). This type of interpolymer hydrophobic association, which may be termed... 

Fluorescent hydrophobes (naphthyl and pyrenyl groups) incorporated into the polysulfobetaines are a powerful tool for studying the formation of intra-and interpolymer aggregates in aqueous and aqueous salt solutions [85,229-231]. Intermacromolecular hydrophobic association is observed as an increase in the excimer emission relative to that of the monomer emission, where h/Iu is the ratio of intensities of excimer and monomer fluorescence which reflects the extent of inter/intra macromolecular interactions. Intramolecular micellization is easily monitored by the quenching efficiency of the thallium ions. The decrease of h/Iu reflects the breaking of the intra- and interchain associations in aqueous salt solutions, leading to chain expansion. 

Hydrophobic association is also enhanced in polymer systems. Although polymer surfactants are considered to form micelles via intrapolymer hydrophobic interaction, our recent study (2r,s) revealed that a polyionene bearing anthryl groups as the hydrophobic domain showed a clear cmc UHtical micelle concentration) at the segment concentration around 3 x 10 5m. Reference experiments with a polyionene without anthryl groups and the monomer and dimer model compounds have indicated that the cmc is particularly low for the polymer. Taking the excimer intensity of anthracene fluorescence as an index of interchromophore interaction, we confirmed the existence of interpolymer association by the concentration dependent excimer intensity. Under the same condition to the polymer, any model systems either monomeric or dimeric do not associate intermolecularly. 

Hydrophobic associations in random copolymers of sodium 2-(acrylamido)-2-methylpropanesulfonate and some methacrylamides and methacrylates substituted with bulky hydrophobes are described with a focus on preferential intrapolymer self-association which leads to the formation of single-macromolecular assemblies (i.e., unimolecular micelles). Structural parameters that critically determine the type of the macromolecular association (i.e., intra- vs. interpolymer associations) are discussed, which include the type of hydrophobes, their content in a polymer, sequence distribution of electrolyte and hydrophobic monomer units, and the type of spacer bonding. Functionalization of single-macromolecular assemblies with some photoactive chromophores is also presented. 

This chapter will discuss hydrophobic associations in random copolymers of AMPS and some hydrophobic methacrylamide and methacrylate comonomers with a focus on the intra- versus interpolymer self-association in connection witih the type of hydrophobes, their content in the polymers, and spacer bonding. A particular emphasis will be placed on intrapolymer association of hydrophobes which leads to single-molecular self-assemblies. Functionalization of the single-macromolecular assemblies with some photoactive chromophores will also be presented briefly. 

Hydrophobic associations in random copolymers of AMPS and iV-d ecylmethacrylamide (DodMAm) were investigated by viscometry, QELS, capillary electrophoresis (CE), NMR relaxation, and various fluorescence techniques (24,48), For fluorescence studies, the polymers that are singly-labeled with pyrene or naphthalene or doubly-labeled with pyrene and naphthalene (Chart 2) were employed (48). In this section, the self-association behavior of AMPS-DodMAm co lymers in water will be discussed with a focus on intra- and interpolymer self-associations as a function of the DodMAm content (/bod) in Ae copolymer. 

As the number of hydrophobic sequences in an amphiphilic polymer chain increases, intrapolymer hydrophobic association, as well as interpolymer association, becomes an important process to determine overall self-organized structures. This is particularly so with amphiphilic random or alternating copolymers in which hydrophobic and hydrophilic units are randomly or alternately distributed on a polymer chain. Intrapolymerassociating structures are of critical importance to determine interpolymerassociating structures. In general, intrapolymer hydrophobic association is dominant in dilute solutions, whereas interpolymer association also occurs in a semidilute or concentrated regimen. In random copolymers with a strong tendency for intrapolymer association, unimolecular micelles (unimer micelles) may be formed as a consequence of intrapolymer closed association. 

In general, unimer micelles are preferentially formed in highly dilute aqueous solutions. As the concentration is increased, however, the hydro-phobic association may not necessarily be an intrapolymer event. If interpolymer open associations occur between the hydrophobes or between the primary micelle units, multipolymer aggregates would be formed instead of the unimer micelles. Whether the intrapolymer closed association predominates over the interpolymer open association depends on primarily the chemical structure or the first-order structure of amphiphilic random copolymers. 

In the random copolymers shown in Scheme 1, hydrophobic associations occur completely in an intrapolymer mode [20-22] hence, the polymers form unimer micelles independent of the polymer concentration in water. The sequence distribution of electrolyte and hydrophobic monomer units in the polymer chain is a critical structural factor to determine whether the hydrophobic self-associations are an intra- or interpolymer mode. Block sequences of hydrophobic units have a strong tendency for interpolymer association, whereas random and alternating sequences tend to associate in an intrapolymer mode [20-22]. Even a subtle difference in the sequence distribution in random copolymers has a significant effect on the self-association mode [23]. If the sequence distribution is blocky in nature, there is a propensity for interpolymer association. Other important structural factors that induce polymers to form unimer micelles are that the hydrophobic groups should be bulky and have cyclic structures, such as cyclododecyl, adamantyl, and naphthyl groups, and their contents in the copolymers should be higher than about 30 mol%, as shown in Scheme 1 [20-22]. In addition. 

MYL Mylonas, Y., Bokias, G., lliopoulos, L, and Staikos, G., Interpolymer association between hydrophobically modified poly(sodium aciylate) and poly(Y-isopropyl-aciylamide) in water The role of hydrophobic interactions and polymer structure,... 

Any intermolecular interactions observable in small molecular systems such as hydrogen bonding, EDA interaction, Coulombic force, and hydrophobic interaction are operative in any interpolymer association phenomena. When any interacting groups are incorporated in polymers, the molecular interactions of the polymer with either small molecules or polymers are in general enhanced unless steric hindrance comes in effect. 

Interpolymer association has been most widely studied for polyelectolytes and hydrogen bonding polymers. Besides complex formation between polycation and polyanion represented by the complexes between polyionenes and poly(methacrylic acid) (7), the association of poly(carboxylic acid) with proton accepting polymers such as poly(ethylene oxide), poly(N-vinyl-2-pyrrolidone), and poly(vinyl alcohol) is the subject of active research (8). The main binding forces are attributed to Coulombic and hydrogen bonding interactions. The role of hydrophobic interaction cannot, however, be neglected. The different behaviors of poly(meth-acrylic acid) and poly(acrylic acid) in complex formation evidence the importance of hydrophobic interaction (9). 

The interpolymer association is a distinctively different property of exciplex forming polymers from that of excimer forming polymers. The reason must be attributed to the presence of the ground state interaction in the former polymers. Interpolymer excimer formation is, however, facilitated by the aid of hydrophobic interaction in water. Polyionenes bearing anthryl groups (J2) form both inter- and intrapolymer excimer in water (2s). The excimer intensity decreases with increasing hydrophobic interaction. All experimental results indicate that weak intermolecular interactions almost undetectable in small molecule systems are amplified enormously in polymer systems in dilute solutions. 

Olga E. Philippova s main research interests are polyelearolyte and ionomer behavior of polymer gels, linear and cross-linked polyelectrolytes with associating hydrophobic groups, polymer gel/surfactant interactions, interpolymer complexes, and polymer gels with entrapp l linear stiff-chain macromolecules. 

Complexation in its various forms plays a key role in the homo- and copolymerization of 1-alky 1-4-vinylpyridinium ions. Intermonomer associations are believed responsible for the enhanced poly-merizability of monomers with long alkyl chains (C , n > 6) on nitrogen, the ability of the title monomers to copolymerize with anionic and Ti-rich monomers, and the strong dependence on concentration for homopolymerization of all these cationic monomers. Hydrophobic interactions between lipophilic monomers, electrostatic attraction between cationic and anionic monomers, and charge-transfer complexation between Ti-rich and Ti-deficient monomers have all been observed to control polymer formation. Monomer organization/orientation on polyanion templates, at organic solvent-water interfaces and in ordered multiple-phase systems such as micelles, membranes, vesicles, and microemulsions have been used with limited success in attempts to control the microstructure (e.g. tacticity, monomer sequence) in the related polymers. Interpolymer complexes of poly(l-alky 1-4-vinylpyridinium ions) with natural and synthetic poly anions represent a rich resource for the development of selective electroanalytical methods, for efficient new separation procedures, for manipulation of biomembranes in drug dehvery, and numerous other applications. 

From what has been discussed above, it follows that there are at least three types of interpolymer cross-links that contribute to the formation of the three-dimensional network of an HM-CD solution (Figure 13.4). Apart from chain entanglements and associations between hydrophobic side groups, associations of hydrophobic segments of the polymer backbone also play an important role even if the contribution from hydrophobic backbone interactions is relatively small compared with the other two. 

Cl8 and azo benzene groups. Upon irradiation, the increase of the polarity of the chromophore and/or its change in shape would affect the stability of these cross-links. Both SDS and BSA can bind hydrophobic molecules. It was not suiprising to find that an increase of the polarity was resulting in a decrease of the association. The latter polymer was the only one to exhibit marked photoresponse in the presence of BSA. A complete study has now confirmed that the presence of interpolymer associations was required for the photoresponsiveness to emerge (45). 

---