The triblock copolymer poly[6,8-dioxabicyclo[3.2.1]octane-b-(ethylene glycol)-b-6,8-dioxabicyclo [3.2.1]octane] was prepared by the controlled cationic ring-opening polymerization of 6,8-dioxabicyclo[3.2.1]octane (6,8-DBO) from a macroinitiator. The macroinitiator, poly(ethylene glycol) (PEG) di(1-chloroethyl ether), was prepared via the addition of HCl to PEG divinyl ether and was characterized with C-13 NMR, H-1 NMR, and gel permeation chromatography (GPC). Upon preparation, a small fraction of the chain ends underwent a cyclization reaction to form inactive chain ends. When the macroinitiator was used in polymerizations of 6,8-DBO with ZnI2 as an activator, linear kinetic plots were observed, a linear increase in the copolymer molecular weight with conversion was seen, and the molecular weight distributions of the copolymer samples remained constant at about 1.40. Confirmation of the triblock structure of the final product was obtained with H-1 NMR spectra, C-13 DEFT spectra, and GPC chromatograms.