Synthesis and Organocatalytic Ring-Opening Polymerization of Cyclic Esters Derived from L-Malic Acid

The synthesis of 3-(S)-[(benzyloxycarbonyl)methyl]-1,4-dioxane-2,5-dione (BMD) and 3,6-(S)-[di(benzyloxycarbonyl)methyl]-1,4-dioxane-2,5-dione (malide) from commercially available L-malic acid is reported. Ring-opening polymerization (ROP) studies of BMD are reported showing that the controlled ROP of this monomer is possible in the absence of transesterification side reactions, despite the presence of side-chain esters, using 1-(3,5-bis(trifluoromethyl)phenyl)-3-cyclohexylthiourea and (-)-sparteine to catalyze the polymerization. The ROP of malide with this system was ineffective. Investigation of the effect of initiating species revealed that the electronic nature of the alcohol had a greater effect on the ultimate molecular weight and hence initiator efficiency than steric considerations. Deprotection of the resultant poly(BMD) using H-2 and Pd/C resulted in hydrophilic poly(glycolic-co-malic acid)s (PGMAs) that were able to undergo autocatalytic degradation in dilute H2O solution such that complete degradation was observed within 6 days.