Block copolymers of poly(styrene) and poly(acrylic acid) of various molar masses, topologies, and compositions prepared via controlled/living radicalpolymerization. Application as stabilizers in emulsion polymerization

A series of well-defined diblock, triblock, and star-block copolymers compo sed of polystyrene and poly(acrylic acid) were synthesized by controlled/li ving radical polymerization and used as stabilizers in emulsion polymerizat ion under alkaline conditions. The structure of the copolymers, the size of the blocks, and the composition were varied and their efficiency as stabil izers was correlated with their structural characteristics. The block lengt h was varied from 10 to 30 units for the polystyrene block and from 13 to 2 66 units for the poly(acrylic acid) block. The copolymers appeared to be ef ficient stabilizers down to a block copolymer-to-monomer ratio of less than 0.5 wt %. From the comparison of the effect of the different structures an d compositions, it was shown that the diblock copolymers were particularly efficient and that the optimal composition was about 10 styrene units and a maximum of 50 acrylic acid units. The triblock and star-block copolymers w ith external hydrophilic blocks did not behave much differently than dibloc k copolymers. In contrast, for the triblock copolymers with an internal hyd rophilic segment, the efficiency strongly depended on the respective length of both blocks. The evolution of the number of latex particles, N-p, with the concentration of surfactant was also studied and N-p was shown to be pr oportional to [surfactant](alpha) over a wide range of surfactant concentra tions. The value of a was a function of the block copolymer composition irr espective of the individual block lengths: it was 1 for block copolymers wi th a poly(acrylic acid) content lower than 75 mol % and decreased to 0.4 wh en the hydrophilic content was increased. This trend was correlated with th e exchange dynamics of the stabilizer. The results obtained with Various in itiator concentrations, temperatures, and ionic strengths corroborated the previous observation that the important point to explain the evolution of c t with the copolymer composition was the competition between nucleation of the micelles and exchange of the block copolymers between the micelles and the continuously created polymer /water interfaces in the system. The time scale of this exchange (which is very fast for small-molecule surfactants) was on the same order of magnitude as the nucleation step for emulsion poly merizations carried out in the presence of block copolymers.