Synthesis and polymerization of tail-type cationic polymerizable surfactants and hydrophobic counter-anion induced association of polyelectrolytes

A series of tail-type cationic surface-active monomers with the cationic charge at the -end (1; ST-C_m-AB, m=5, 7, and 9, where ST is a styrenic group, C_m the alkylene chain at the 4-position of styrene, and AB is alkyltrimethylammonium bromide) have been synthesized as a novel cationic polymerizable surfactant. Their fundamental physicochemical properties such as critical micelle concentration (cmc) and weight-average aggregation number of the micelle (N_w(agg)) have been characterized in water at 25 °C by static light-scattering measurements. The cmc values determined for the tail-type surfactant monomers are two-orders of magnitude smaller than those of the corresponding head-type cationic surfactant monomers (2; ST-C₁-AC-C_m). The N_w(agg) of ST-C_m-AB is 68 for m=5, 156 for m=7, and 413 for m=9. Free-radical homopolymerization of ST-C₇-AB proceeds very rapidly in water as a result of organization in the micelle to afford the corresponding amphiphilic cationic polyelectrolyte with M_w=3.63×10⁶ and 23 nm hydrodynamic radius at 25 °C. Emulsion copolymerization of styrene with ST-C_m-AB also proceeds rapidly to afford very stable cationic polystyrene latex particles of 30–60 nm diameter. The amphiphilic cationic polyelectrolyte of poly(ST-C₇-AB) is likely to assume a compact conformation with high segment density in 0.1 mol L^–1 NaCl in water. Addition of hydrophobic aromatic counter-anions with an weak acid group, for example potassium hydrogen phthalate (PHK) and sodium salicylate, to a salt-free aqueous solution of poly(ST-C₇-AB) induces intermolecular aggregation and increases the solution viscosity substantially, often producing gels and precipitation at high polymer concentration.