The mechanism of the nitroxide mediated polymerization (NMP) is well understood, however less is known about the side-reactions that interfere and in certain cases severely compromise it. Experimental studies inevitably involve model fitting leading to at times contradictory conclusions as to which elementary side-reactions are behind the failure of a given NMP system. In the present work we use high-level quantum-chemical calculations to obtain the rate coefficients of the various side-reactions, both suggested previously and considered here for the first time, and first principles PREDICI kinetic simulations to identify the most deleterious side-reactions involved in the TEMPO, SG1 and DPAIO mediated polymerization of styrene, acrylate and methacrylate monomers. We show that the core mechanism for the thermal decomposition of alkoxyamines differs between the uni- and polymeric species, which often makes such experiments not suitable for modelling the NMP conditions. We also find that the main side-reaction responsible for the failure of TEMPO and SG1 in methacrylate homopolymerization is an intramolecular alkoxyamine decomposition (often referred to as 'disproportionation') via a Cope-type elimination, however in the case of SG1 the polymerization outcome is additionally affected by the equilibrium constant of alkoxyamine bond homolysis. On the basis of these findings, complemented by a thorough analysis of available experimental data, we define guidelines for minimising occurrence of the side-reactions and thus improving NMP. Finally, the accurate first principles rate parameters reported in this study should prove useful for subsequent kinetic modelling oriented at optimising different polymerization conditions.