Methyl methacrylate/styrene (MMA/S), ethyl methacrylate/styrene (EMA/S) and butyl methacrylate/styrene (BMA/S) feeds (> 90 mol% methacrylate) were copolymerized in 50 wt% p‐xylene at 90° C with 10 mol% of additional SG1‐free nitroxide mediator relative to unimolecular initiator (BlocBuilder®) to yield methacrylate rich copolymers with polydispersities M w/M n= 1.23–1.46. kpK values (kp= propagation rate constant, K= equilibrium constant) for MMA/S copolymerizations were comparable with previous literature, whereas EMA/S and BMA/S copolymerizations were characterized by slightly higher kpK's. Chain extensions with styrene at 110° C initiated by the methacrylate‐rich macroinitiators (number average molecular weight M n= 12.9–33.5 kg mol− 1) resulted in slightly broader molecular weight distributions with M w/M n= 1.24–1.86 and were often bimodal. Chain extensions with glycidyl methacrylate/styrene/methacrylate (GMA/S/XMA where XMA= MMA, EMA or BMA) mixtures at 90° C using the same macroinitiators resulted frequently in bimodal molecular weight distributions with many inactive macroinitiators and higher M w/M n= 2.01–2.48. P (XMA/S) macroinitiators (M n= 4.9–8.9 kg mol− 1), polymerized to low conversion and purified to remove “dead” chains, initiated chain extensions with GMA/MMA/S and GMA/EMA/S giving products with M w/M n∼ 1.5 and much fewer unreacted macroinitiators (< 5%), whereas the GMA/BMA/S chain extension was characterized by slightly more unreacted macroinitiators (∼ 20%).© 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2574–2588, 2009