A mathematical model for free radical polymerizations initiated by tetrafunctional initiators is described in detail with comparisons to experimental results. Reactions involving the fate/efficiency of functional groups are properly accounted for, while in the past, kinetic models for difunctional initiators found in the literature have ignored this. Free volume theory is used to describe the diffusion-controlled regime. Based on model predictions, multi-radical concentrations were estimated to be several orders of magnitude smaller than mono-radical concentrations. Through various case studies, the model was able to demonstrate that the concentration and chain length of various polymer structures (i.e., linear, star or coupled stars) depend upon monomer type and reaction conditions. The model was found to be useful in explaining experimentally observed differences in the behaviour of a tetrafunctional initiator with styrene compared to methyl methacrylate (MMA). In both cases, higher reaction rates could be obtained when switching from a mono- to a tetrafunctional initiator: however, the influence on molecular weight was found to vary between the two systems. Work with styrene showed similar trends as with difunctional initiators, where the tetrafunctional initiator maintained similar molecular weights compared to a monofunctional initiator. Yet, for MMA, replacing the monofunctional initiator with its tetrafunctional counterpart decreased the molecular weight, (c) 2006 Elsevier Ltd. All rights reserved.