Synthesis of amphiphilic poly((R,S)-β-malic acid)-graft-poly(ε-caprolactone):: "Grafting from" and "grafting through" approaches

The synthesis of novel amphiphilic graft copolyesters based on hydrophobic poly(epsilon-caprolactone) (PCL) segments grafted all along a hydrophilic poly((R,S)-beta-malic acid) (PMLA) backbone is proposed. These PMLA-g-PCL graft copolymers have been prepared in a controlled way using either the "grafting through" or the "grafting from" technique. Both approaches involve two different well-controlled ring-opening polymerization (ROP) mechanisms: anionic ROP of beta-lactones, i.e., benzyl or allyl beta-malolactonate (MLABz or MLAAllyl), and living ROP of c-caprolactone (CL) through a so-called coordination-insertion mechanism. The grafting through approach relies upon the anionic copolymerization of MLABz and omega-malolactonate-PCL macromonomer as initiated by potassium carboxylic acid salt in the presence of 1 equiv of 18-crown-6 ether. The second route follows a four-step synthesis involving the anionic ROP of MLABz and MLAAllyl, the conversion of allylic groups into pendant hydroxyl functions owing to a free-radical reaction with thioethanol in the presence of 2,2'-azobis(2-methylpropionitrile), and then the CL polymerization as initiated from these pendant -OH groups after adequate activation into aluminum alkoxide active species. Whatever the synthetic approach, the final step consists of the deprotection of the MLABz repeating units along the backbone via a soft catalytic hydrogenation reaction. The amphiphilic character of the so-prepared graft copolyesters has been evidenced by some preliminary interfacial tension experiments using the pendant drop method.