Hydrophilic aliphatic polyesters: Design, synthesis, and ring-opening polymerization of functional cyclic esters

The synthesis and homo- and copolymerization of new cyclic esters containin g protected functional groups (hydroxyl, bishydroxyl, amino, and carboxyl) are described. Each of the epsilon-caprolactone derivatives was generated b y the Baeyer-Villiger oxidation of the corresponding cyclohexanone derivati ve. Monoprotection of 1,4-cyclohexanediol by benzylation or esterification was accomplished in moderate yields by reaction of benzyl bromide or 2,2'-b is(phenyldioxymethyl)propionyl chloride. Each could be oxidized with pyridi nium chlorochromate to yield the respective protected hydroxyl and bis(hydr oxyl) functional cyclohexanones. The benzyl ether and benzyl ester protecti ng groups are readily removed by catalytic hydrogenolysis using Pd/C. Alter natively, ethyl-4-ketocyclohexylcarboxylate was hydrolyzed, and the free ca rboxylic acid was reesterified to the benzyl or tert-butyl 4-ketocyclohexyl carboxylate by esterification with either benzyl bromide or tert-butyl alco hol. These protecting groups were chosen because they are readily cleaved t o the respective carboxylic acids under mild conditions. The aza-cyclohexan one derivative was produced from the commercially available ethylene ketal of 4-piperidone by acetylation with trifluoroacetic anhydride followed by t ransketalization using p-toluenesulfonic acid in excess acetone. The triflu oroacetyl protecting group is easily removed by NaBH4 reduction. Polymeriza tion of the new monomers was accomplished either in bulk (110 degrees C) in itiated from benzyl 2,2'-bis(hydroxymethyl)propionate in the presence of st anneous a-ethyl hexanoate (Sn(Oct)(2)) or from Al((OPr)-Pr-i)(3) in toluene (0 degrees 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 functional ized polymers.