Hydrophilic Aliphatic Polyesters:  Design, Synthesis, and Ring-Opening Polymerization of Functional Cyclic Esters

The synthesis and homo- and copolymerization of new cyclic esters containing protected functional groups (hydroxyl, bishydroxyl, amino, and carboxyl) are described. Each of the ε-caprolactone derivatives was generated by the Baeyer−Villiger oxidation of the corresponding cyclohexanone derivative. Monoprotection of 1,4-cyclohexanediol by benzylation or esterification was accomplished in moderate yields by reaction of benzyl bromide or 2,2‘-bis(phenyldioxymethyl)propionyl chloride. Each could be oxidized with pyridinium chlorochromate to yield the respective protected hydroxyl and bis(hydroxyl) functional cyclohexanones. The benzyl ether and benzyl ester protecting groups are readily removed by catalytic hydrogenolysis using Pd/C. Alternatively, ethyl-4-ketocyclohexylcarboxylate was hydrolyzed, and the free carboxylic acid was reesterified to the benzyl or tert-butyl 4-ketocyclohexylcarboxylate by esterification with either benzyl bromide or tert-butyl alcohol. These protecting groups were chosen because they are readily cleaved to the respective carboxylic acids under mild conditions. The aza-cyclohexanone derivative was produced from the commercially available ethylene ketal of 4-piperidone by acetylation with trifluoroacetic anhydride followed by transketalization using p-toluenesulfonic acid in excess acetone. The trifluoroacetyl protecting group is easily removed by NaBH4 reduction. Polymerization of the new monomers was accomplished either in bulk (110 °C) initiated from benzyl 2,2‘-bis(hydroxymethyl)propionate in the presence of stanneous 2-ethyl hexanoate (Sn(Oct)2) or from Al(O i Pr)3 in toluene (0 °C), yielding polymers close to their targeted molecular weights (5000−15 000) with modestly narrow polydispersities (1.20−1.35). Removal of the protecting groups on the aliphatic polyesters yielded the functionalized polymers.