## 1 Introduction  
Over the last decade, biodegradable or bioresorbable polymers have been widely employed in biomedical applications such as implants, sutures, tissue scaffolds or various drug delivery systems [1; 2; 3; 4]. The most commonly used synthetic biodegradable polymers for medical and pharmaceutical applications are aliphatic polyesters (e.g., poly(\(\varepsilon\)-caprolactone), polyglycolide, polylactide or co- or terpolymers of \(\varepsilon\)-caprolactone, glycolide, l-lactide and _rac_-lactide) [5; 6; 7; 8]. Polycaprolactone (PCL) is an important biomedical polyester due to its physicochemical properties as well as good biocompatibility and biodegradability [9; 10]. PCL is a hydrophobic and semi-crystalline polymer. It was one of the earliest polymers synthesized, by the Carothers group in the early 1930s [9]. PCL can biodegradable in a time-span ranging from several months to several years, depending on its molecular weight, degree of crystallinity and the condition of biodegradation process [10]. Due to its multiple biomedical applications, the synthesis of PCL has received increased attention in the last years [9; 10]. There are two methods for the preparation of biomedical PCLs: the polycondensation of 6-hydroxyhexanoic acid and the ring-opening polymerization (ROP) of \(\varepsilon\)-caprolactone (CL) [9; 11]. The ROP process is more often used because of the higher molecular weight and lower polydispersity of the obtained polyesters [9]. The ROP of CL can be carried out in the presence of anionic or cationic initiators and coordinating or enzymatic catalysts [7; 8; 9; 12]. A wide range of metals have so far been studied for the ROP of CL through the coordination-insertion mechanism (e.g., aluminium, calcium, iron, magnesium, tin, titanium, zinc, lanthanides and rare earth metals) [13]. Zinc-based catalysts have been reported to be very effective initiators of the ROP of CL, resulting in a degree of polymerization of over 100 and a polydispersity index (_PD_) between 1.05 and 1.1. In this case, the ROP was carried out through a coordination-insertion mechanism relying on the cleavage of the acyl-oxygen bond of CL [9]. The polymerization mechanism involves the coordination of the CL molecule to the Lewis acidic-metal centre through the exocyclic oxygen. The carbonyl group of the lactose is therefore rendered more susceptible to nucleophilic attack [13]. It is known that a high temperature and long reaction time foster the inter- or intramolecular side-transesterification reaction [9; 13]. However, the polymerization of cyclic monomers by the ROP is free of these limitations and thus is preferred for synthesis of biomedical polyesters [14]. Many metal catalytic systems, such as Al, Ca, Fe, La, Mg, Nd, Sm, Sn, Ti, Zn, Zr, Y and Yb derivatives, are imparting control to the lactones or lactides polymerization process. The ROP of cyclic monomers in the presence of these catalytic systems is a controlled process that provides polyesters with narrow _PD_[14; 15]. Recently, the polymerization activity of various Zn/ligand catalytic systems has been studied [13]. Calixarenes, alkoxides, diamino phenolates, beta-diketiminates, phosphinophenolates, Schiff-base macrocycles, aryloxides, anilido imines, dialkoxo/diamines, phenoxy imines, heteroscorpionates, phosphinimino methanides, tripodal triamines, and pyridine imines have been used as ligands in the synthesis of coordination zinc catalysts [13]. However, to the best of our knowledge, there is no report on the application of Zn/gallic acid or Zn/propyl gallate as catalytic systems of the ROP of CL. For safety reasons, any residual metal content must be removed from the final materials before use in pharmaceutical or medical applications [14]. Another approach for producing biomedical polymers free of toxic metallic residues uses bioresorbable salts (e.g., Na\({}^{+}\), K\({}^{+}\), Mg\({}^{2+}\), Ca\({}^{2+}\), Zn\({}^{2+}\), and Fe\({}^{2+}\)) for the ROP of cyclic esters.