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IntroductionCationic polymeric latexes have shown potential applications in many ®elds, including papermaking [1±6], paper coating [7,8], and adhesion [9]. Recent studies [4±6] indicated that cationically charged polystyrene latexes could be used as eective papermaking retention aids or sizing agents. However, it was found that the particle size and charge density of the polymeric latex are two critical factors aecting the performance of the latex. As a papermaking retention aid or sizing agent, it was found [4±6] that the most eective particle size of these latexes is in the range 30±50 nm. Although emulsion polymerization is the most convenient technology for making uniform latexes, this method can generally produce only a cationic polystyrene latex with a particle size larger than 150 nm, which is not eective as a papermaking additive agent. In order to reduce the particle size, a microemulsion polymerization technique has to be employed [4±6]. Although the particle size of cationic polystyrene-based latex can be well controlled by microemulsion polymerization, the high surfactant consumption and the diculty in latex clari®cation after polymerization are two major problems. Instead of directly synthesizing a latex using emulsion or microemulsion polymerization, it was shown in our previous study [6] that linear cationic polystyrene polymers obtained by solution polymerization of styrene and cationic vinylbenzyl trimethylammonium chloride (VBTMAC) monomers in ethanol could form stable colloidal aggregates in water with a particle size of 30± 50 nm and a similar structure as cationic polystyrene Abstract A series of polystyrenebased cationic copolymers was synthesized by two dierent methods:(1) solution copolymerization of styrene and vinylbenzyl trimethylammonium chloride (VBTMAC) in ethanol, and (2) surfactant-free emulsion copolymerization of styrene and vinylbenzyl chloride in water followed by reaction with trimethylamine. The results indicated that the dierent synthesis methods would result in dierent polymer structures and, therefore, aect the solubility and colloidal properties of the copolymers in water. For the copolymers prepared by method 2, partial crosslinking was observed. The copolymers made by this method are almost water-insoluble. In contrast, the copolymers made from direct copolymerization of styrene and VBT-MAC in ethanol are water-soluble or dispersible, but the solubility and the particle size of microaggregates formed by these copolymers in water strongly depend on charge density and temperature. One of the important results from this study is that uniform colloidal particles with a very small particle size (30±50 nm) can be obtained by dispersing polystyrene-based cationic copolymers in water without adding any surfactants.