The present invention relates to a composition comprising an aqueous dispersion of first polymer particles functionalized with structural units of t-butyl methacrylate or t-butyl acrylate, and imbibed with a catalyst of Structure (I), where R, R1, R2 and R3 are as defined herein, and a process for making the composition. The present invention also relates to an aqueous dispersion of core-shell polymer particles, wherein the first polymer particles are encapsulated in a shell having a high Tg. The imbibed first polymer particles and the core-shell polymer particles can be expanded in the dry state at significantly lower temperatures than reported in the prior art.