Organo-Catalyzed Ring Opening Polymerization of a 1,4-Dioxane-2,5-dione Deriving from Glutamic Acid

The (3S)-[(benzyloxycarbonypethyl)-1,4-dioxan-2,5-dione (BED) was prepared in four steps starting from glutamic acid and bromoacetyl bromide. According to X-ray diffraction analysis, the pendant functional group is located in equatorial position and points away from the six-membered ring. The organo-catalyzed ring-opening polymerization of BED was promoted with 4-dimethylaminopyridine (DMAP) and the combination of thiourea TUCy and (-)-sparteine. PolyBED samples of number-average molar mass M-n up to 36000 and narrow polydispersity (M-w/M-n < 1.25) were thereby prepared in a controlled manner under mild conditions (dichloromethane solution, 30 degrees C), as substantiated by size-exclusion chromatography, matrix-assisted laser desorption ionization time-of-flight-mass spectrometry (MALDI-TOF-MS) and nuclear magnetic resonance (NMR) spectroscopy. The pendant functional group does not interfere with the polymerization and BED was even found to be slightly more reactive than lactide. Despite the strongly dissymmetric substitution pattern of the 1,4-dioxan-2,5-dione core, the ensuing polyBED polymers present a random distribution of glycolic-[(benzyloxycarbonyl)ethyl]glycolic (gly glu) units, as supported by a H-1-C-13 HMBC 2D NMR experiment. The preparation of 1:1 adducts with n-pentanol confirmed that ring-opening of BED occurs almost indifferently on either of the endocyclic ester groups. Poly(alpha-hydroxyacids) featuring pendant carboxylic acids were finally obtained by acetylation of the terminal OH groups followed by hydrogenolysis.