The branching copolymerization of 2-hydroxypropyl methacrylate ( HPMA) with either ethylene glycol dimethacrylate (EGDMA) or bisphenol A dimethacrylate (BPDMA) as the branching agent has been carried out using atom transfer radical polymerization (ATRP) in methanol at 20 C. With EGDMA, soluble branched copolymers were obtained provided that less than one branching agent was incorporated per primary chain: higher levels of EGDMA led to gelation, as expected. Analysis of the changes in the molecular weight and polydispersity of the polymers shows that the formation of highly branched chains occurs only at high (> 90%) conversions. Chain coupling is close to the ideal behavior predicted by the Flory-Stockmayer theory, suggesting that all double bonds are equally reactive and that there is no significant cyclization, in contrast to conventional free radical polymerization. This analysis is confirmed by comparison of the consumption of the EGDMA branching agent with predictions from both theory and simulation. With BPDMA as the branching agent, similar results are obtained although branching is slightly less efficient.