Spray polymerisation is a technique to carry out polymerisations in a highly efficient manner. As experimental optimisation of process parameters is tedious, simulations of the processes inside a single droplet are desirable. Today's models used in spray polymerisation assume ideal mixing and thus do not resolve the particle state along the radius. In this contribution we derive a single particle model for the case of free radical polymerisation, which spatially resolves the physical and chemical processes. The model allows for predicting inhomogeneity of the polymeric product as a function of drying conditions. The state of the polymer is calculated using the method of moments.