The controlled polymerization of vinyl acetate has been recently achieved by several techniques, but PVAc with targeted M-n and low dispersity up to very high monomer conversions and high degrees of polymerization was only obtained with Co(acac)(2) as controlling agent in the so-called CMRP, a type of organometallic mediated radical polymerization (OMRP). Other techniques (including ATRP, ITP, TERP, and RAFT/MADIX) have shown a more or less pronounced slowdown in the polymerization kinetics, which was attributed to the higher strength of the C-X bond between the radical PVAc chain and the trapping agent (X) in the dormant species and to a consequent slower reactivation after a less frequent head-to-head monomer addition. The reason for the CMRP exception is clarified by the present contribution. First, a detailed investigation by H-1, C-13 and multiplicity-edited HSQC and DEPT-135 NMR of the PVAc obtained by CMRP, in comparison with a regular polymer made by free radical polymerization under the same conditions, has revealed that Co(acac)(2) does not significantly alter the fraction of head-to-head sequences in the polymer backbone and that there is no accumulation of Co(acac)(2)-capped chains with a head-to-head omega end. Hence, both dormant chains (following the head-to-head and the head-to-tail monomer additions) must be reactivated at similar rates. A DFT study shows that this is possible because the dormant chains are stabilized not only by the C-Co sigma bond but also by formation of a chelate ring through coordination of the omega monomer carbonyl group. The head-to-head dormant chain contains an inherently stronger C-Co bond but forms a weaker 6-membered chelate ring, whereas the weaker C-Co bond in the head-to-tail dormant chain is compensated by a stronger 5-membered chelate ring. Combination of the two effects leads to similar activation enthalpies, as verified by DFT calculations using a variety of local, gradient-corrected, hybrid and "ad hoc" functionals (BPW91, B3PW91, BPW91*, M06 and M06L). While the BDE(C-X) of model H-VAc-X molecules [X = Cl, I, MeTe, EtOC(S)S and Co(acac)(2)] are functional dependent, the BDE difference between head-to-head and head-to-tail dormant chain models is almost functional insensitive, with values of 5-9 kcal/mol for the ATRP, ITP and TERP models, 3-6 for the RAFT/MADIX model, and around zero for CMRP.