We recently discovered that the cationic polymerization of o-phthalaldehyde generates cyclic poly(phthalaldehyde) in high yield, high molecular weight, and a high degree of cyclic purity. Given this surprising result, we pursued the cationic polymerization of ethyl glyoxylate to determine if the macrocyclization outcome is, in fact, a general trend of low ceiling temperature polyacetals. Using NMR spectroscopy, MALDI-TOF mass spectrometry, and triple detection GPC, we have uncovered divergent macrocyclization mechanisms in the cationic polymerization of ethyl glyoxylate. Backbiting is observed either via the backbone acetal or via the pendant ester to give disparate polymer products and unique polymer architectures. The favored route for cyclization is found to depend on both the monomer concentration and the initiating species. Understanding the underlying mechanisms of polymerization and the ability to rigorously control polymer structure has important implications for the design of new transient materials.