A joint experimental and theoretical study has been carried out on reversible addition-fragmentation chain transfer polymerisation (RAFT). We have performed density functional theory calculations at the (Perdew-Burke-Ernzerhof) PBE/triple zeta plus polarisation level to analyse the RAFT mechanisms corresponding to these compounds. Global and local reactivity indices have been calculated to investigate the effect of the addition of methyl, cyanomethyl and styryl radicals on the double bond C=S of thionoester compounds producing an adduct radical. This mechanism is shown to be difficult when the cyanomethyl is used contrarily to the methyl and styryl radicals, in agreement with experimental results. The activation barrier of fragmentation of adduct radicals does not correlate well with the length of fragmented bond (O-C-alpha). The bond topological analysis of radical adduct predicts that the distance between the oxygen and a critical point (O-CP) in the fragment bond is a good parameter to estimate the activation energy of the fragmentation mechanism. It is shown that the nature of the free radicals is more selective than that of the thionoester compounds. With an overall large agreement with experiments, these theoretical results afford an explanation of the efficiency for the RAFT mechanism.