Consistent with our previous report with Houk, density functional theory (DFT) calculations using the M06-2X functional revealed that cycloaddition (∆G ‡ CA = 18.2 kcal/mol) was the ratedetermining step during initiation rather than the more facile cycloreversion (∆G ‡ CR = 12.1 kcal/mol). More importantly the subsequent, propagating cycloadditions were predicted to be slower than that of the initiation (∆G ‡ propagate = 19.6 kcal/mol). These calculations support our observation of relatively narrow molar mass distributions (Đ ≤ 1.4), which are typically associated with well-controlled polymerizations.