The ring-opening polymerizations of trimethylene (TMC) and tetramethylene (7CC) carbonates using [(BDI^iPr)Zn(N(SiMe₃)₂)] (BDI^iPr = 2-((2,6-diisopropylphenyl)amido)-4-((2,6-diisopropylphenyl)-imino)-2-pentene) as catalyst precursor, in the presence of benzyl alcohol (BnOH) as co-initiator, have been investigated by means of density functional theory (DFT) calculations. This computational study highlights that the zinc-amido precursor is by itself a poor catalyst whereas the alkoxide analogue, obtained upon alcoholysis of the Zn–N bond by BnOH, is very active. For both TMC and 7CC, the initiation and the first propagation steps have been computed. In each case, the reaction proceeds via two transition states, involving nucleophilic attack of the alkoxide group followed by the ring-opening of the cyclic carbonates. Thus TMC and 7CC undergo a similar ROP process with accessible energy barriers. The analysis of the reaction mechanisms and energy profiles indicates that the relaxation of the growing polymer chain is an important feature, making the overall reaction exergonic (7CC) or athermic (TMC). This exothermicity difference predicts that 7CC is slightly more easily polymerizable than TMC, which is consistent with what is already known in the case of six- and seven-membered related lactones and previous experimental investigations on TMC and 7CC.