Shielding effects of the surrounding arms and the chain on the encounter probability of reactive sites taking part in the Z-RAFT star polymerization are investigated (a) by use of lattice based Monte Carlo simulations in combination with exact enumeration techniques and (b) by direct simulation applying an off-lattice coarse-grained molecular dynamics method (dissipative particle dynamics, DPD). Making use of the former method the chain-length dependence of the shielding factors is discussed for a broad range of thermodynamic conditions and compared to DPD results for the athermal (good solvent) and theta (bad solvent) case, i.e., for the limiting solvent qualities evaluated. In addition, changes of the size and shape of the reaction partners on approach and penetration are discussed in some detail. The results of both simulation methods fairly well coincide and reveal that shielding is smaller and chain-length dependence less pronounced under theta conditions as compared to good solvent conditions. Experimentally determined polydispersities of polystyrene generated by Z-RAFT star polymerizations in the poor solvent cyclohexane were found to be smaller than with the good solvent toluene, which is in full accordance to the theoretical predictions. (C) 2010 Elsevier Ltd. All rights reserved.