We report the bulk cationic ring-opening polymerization of renewably sourced 2-methyl-1,3-dioxan-4-one (MDO) to yield a polyester with hydrolytically and thermally sensitive linkages that facilitate degradation. Neat monomer was successfully polymerized using a variety of protic acids as catalysts. We discovered that, with these catalysts, the cationic polymerization of MDO proceeds via two distinct mechanistic routes, namely, the activated monomer (AM) and active chain-end (ACE) mechanisms. The kinetics of these competing mechanistic avenues were investigated by employing diphenylphosphoric acid (DPP) with or without an alcohol initiator. Without an exogenous initiator, the polymerization propagates via a dioxacarbenium ion that rapidly adds more MDO to produce high-molar-mass poly(2-methyl-1,3-dioxan-4-one) (PMDO). However, we found no clear relationship between [MDO](0)/[protic acid](0) and resultant molar mass, suggesting that the ACE mechanism is not well controlled. This conclusion was further supported by the production of cyclic PMDO arising from unimolecular backbiting reactions as the system approached equilibrium. With an exogenous alcohol initiator, the polymerization proceeds primarily via an AM mechanism and affords a mixture of linear PMDO and a small amount of macrocyclics derived from the competing ACE mechanism. Consistent with this interpretation of competing mechanisms, a linear relationship between theoretical and observed molar mass was observed when the initial ratio of monomer to added initiator was <80.