The effect of the monomer ratio on the microstructure of BA/MMA emulsion copolymers was investigated. Monomer ratios spanned from pure BA used for adhesives to 50150 wt/wt BA/MMA copolymers used for coatings. The gel content varied from 55% for pure BA to nil for the 50/50 copolymer. The branching level was reduced from 2.6% for pure BA to 0.3% for a 75/25 BA/MMA copolymer. The mechanisms responsible for the reduction of gel content can be found in the lower reactivity of the MMA terminated chains for hydrogen abstraction, the absence of abstractable hydrogens in the MMA units and the fact that MMA radicals terminate predominantly by disproportionation. The reduction of the level of branches is mainly due to the lower reactivity of MMA for intramolecular transfer and the lower instantaneous conversions that favored propagation over backbiting. (c) 2007 Elsevier Ltd. All rights reserved.