The Bronsted acid-catalyzed polytransacetalization of hydroxymethylbenzaldehyde dimethylacetal (1), a commercially available AB(2)-type monomer, led to hyperbranched polyacetals (HBPA's) with a degree of branching (DB) around 0.5 by forming methanol as byproduct. In sharp contrast, the polyacetalization of the nonprotected homologue, namely, hydroxymethylbenzaldehyde (2), yielded HBPA's with DB = 1, by forming water as byproduct, under the same acidic conditions. This major difference arises from the instability of the initially formed hemiacetal intermediates, which react faster than aldehyde moieties, driving the polyacetalization toward the quantitative formation of dendritic acetal units. This represents a rare example of defect-free hyperbranched polymer synthesis utilizing a very simple AB(2)-type monomer. Bronsted acid catalysts included p-toluenesulfonic, camphorsulfonic, and pyridinium camphorsulfonic acids. Trapping of the water generated during polyacetalization of 2 was accomplished using molecular sieves regularly renewed, which allowed achieving polymers of relatively high molar masses. These HBPA's with DB = 1 featuring multiple aldehyde functions at their periphery were further derivatized into PEGylated HBPA's, using linear amino-terminated poly(ethylene oxide)s of different molar masses. This led to submicrometric sized HBPA's with a core shell architecture. Finally, HBPA derivatives could be readily hydrolyzed under acidic conditions (e.g., pH = 4), owing to the acid sensitivity of their constitutive acetal linkages.Read less <