Poly(methyl methacrylate)-b-poly(styrene) (PMMA-b-PSt) block copolymer is synthesized successfully via seeded emulsion polymerization in the presence of 1,1-diphenylethylene (DPE). First, emulsion polymerization of MMA is carried out with KPS as initiator in the presence of DPE, giving a DPE-containing PMMA precursor with the ability of reactivation by simply heating. The emulsion polymerization behavior of MMA in the presence of DPE is investigated. The second monomer styrene (St) is then polymerized in PMMA seed emulsion and the block copolymer is successfully obtained. The formation of PMMA-b-PSt block copolymer is confirmed by 1H NMR and gel permeation chromatography. Dynamic light scattering is used to monitor the particle diameters, and suggests that the particles grow without secondary nucleation occurring.