Highly branched polymethacrylates have recently offered new perspectives in lithographic performance and drug delivery. The control of branching remains yet challenging and requires fundamental investigation to consider new applications. Therefore, an advanced study of the formation mechanism of branched polymers synthesized by self-condensing vinyl copolymerization (SCVCP) of a methacrylic AB* inimer, 2-(2-bromoisobutyryloxy)ethyl methacrylate (BIEM), with methyl methacrylate (MMA) via atom transfer radical polymerization (ATRP) has been performed. Evidence of branched structures was obtained with a conventional GPC apparatus equipped with a multiangle light scattering detector and detailed (1)H NMR analyses. A three-step reaction scheme is suggested according to the dependence of molecular weight with conversion. Controlled radical polymerization mainly occurs until moderate conversions, with the participation of inimer as chain initiator. Then the polymerization of small macromolecules, through consumption of polymerizable moiety, dramatically increases the molecular weight of polymer. Finally, a loss of control partially due to thermal decomposition of residual comonomers occurs at high conversion. This mechanistic methodology will allow, with adequate reaction process, the one-step preparation of controlled branched macromolecular architectures leading to functional materials.