Biosourced Amphiphilic Degradable Elastomers of Poly(glycerol sebacate): Synthesis and Network and Oligomer Characterization

Glycerol (G, a triol) and sebacic acid (S, an alpha,omega-dicarboxylic acid) were condensed in the bulk to obtain poly(glycerol sebacate) (PGS) cross-linked elastomers which were characterized in terms of their swelling, thermal, and mechanical properties. The soluble precursors to the elastomers were characterized in terms of their size, size distribution, and composition. In particular, G-S mixtures of five different compositions (molar G:S ratio = 2:1, 2:2, 2:3, 2:4, and 2:5) were copolymerized in the bulk at 120 degrees C in a three-step strategy (first step under inert gas atmosphere, followed by two steps in vacuo). When the G:S molar ratio was equal to (2:3) or close to (2:4), the stoichiometrically matched, network formation took place from the second condensation step, whereas three reaction steps were necessary for network formation far from stoichiometry, at G:S molar ratios equal to 2:2 and 2:5; at a G:S molar ratio of 2:1, no network formation was observed at all. Network composition also proved to be an important structural property, directly influencing the swelling and thermomechanical behavior of the elastomers. In particular, at the stoichiometrically matched G:S ratio of 2:3, corresponding to the cross-linking density maximum, the sol fraction extracted from the elastomers and the elastomer degree of swelling in aqueous media and in organic solvents presented a minimum, whereas the storage moduli of PGS elastomeric membranes in the dry state, measured within the temperature range between 35 and 140 degrees C, exhibited a maximum. The molecular weights of all soluble network precursors were found to be below 5000 g mol(-1) (gel permeation chromatography), containing but traces of ring oligomers (electron-spray ionization mass spectrometry). H-1 NMR spectroscopy indicated that the precursor composition was close to that expected on the basis of the G:S feed ratio and that monomer-to-polymer conversion increased from the first to the second condensation step.