A comprehensive mathematical model is developed for emulsion polymerisation of butyl acrylate to polybutyl acrylate in a semi batch reactor. The mathematical framework is based on the population balance approach. The model includes such mechanistic details as: (i) particle generation from radicals entering micelles, (ii) coupling of the radical concentration in the aqueous phase and particle phase, (iii) determination of the particle phase radical concentration by radical entry into, exit from and termination inside the particle, (iv) thermodynamic equilibrium between the monomer concentration in the aqueous phase and the particle phase. The population balance equations are solved efficiently using orthogonal collocation. Dynamic simulations are compared to the experimental data taken from the literature. The predictions for the various average diameters of the particle size distribution, polydispersity index, standard deviation and full particle size distribution are also given.