## 1 Introduction  
Polymeric biomaterials (aliphatic polyesters, polyanhydrides, polyethers, polyamides, polyorthoesters or polyurethanes) signify one of the most interesting fields in current material chemistry. Among these materials, polylactide (PLA) is probably the most important biomedical polymer [1] and has previously been applied to the production of cell scaffolds, drug delivery systems (DDSs), sutures in tissue engineering and prostheses for tissue replacements [2; 3; 4; 5; 6; 7; 8]. Two methods for PLA preparation are commonly known: the polycondensation of lactic acid and the ring-opening polymerization (ROP) of lactide (LA) [1]. The polycondensation process is hampered by the typical limitations of step polymerization, whereas ROP of LA can be initiated by metal complexes and organic compounds or enzymes, both with and without alcohol [1; 9; 10; 11; 12]. Metal complexes are desirable because they can give rise to controlled polymerizations and can therefore yield materials with a well-defined number-average molecular weight (\(M_{\mathrm{n}}\)), as well as a narrow polydispersity index (_PD_) [1; 13]. These initiators are metal alkoxide or amide coordination compounds (sometimes formed _in situ_), which are particularly useful because of their selectivity, rate and lack of side reactions. However, metal residues are undesirable for medical or pharmaceutical applications and in these cases, a low toxicity organocatalytic or enzyme catalytic systems are favorable [1]. There are two primary mechanisms for the ROP of LA: the coordination insertion mechanism for metal complexes and the activated monomer mechanism for organo/cationic initiators [1; 13]. The key initiator or catalyst parameters are polymerization control, rate and stereocontl. Stereocontl is an important parameter, because the PLA's tacticity influences its properties (e.g., isotactic PLA is crystalline, whereas atactic PLA is amorphous). PLA tacticity is dependent on both the type of LA and the selected initiator or catalyst [14; 15; 16]. During initiator selection, the biocompatibility and toxicity of the initiator or catalytic system are important issues, especially in the case of medical or pharmaceutical applications. In general, the metal-based initiators or catalysts remain in the macromolecule and during degradation, are likely to be converted into an oxide or hydroxide. For example, some Sn-, Zn- or Zr-based initiator/catalyst systems are generally considered non-toxic [1; 13]. The development of reproducible and efficient DDS requires fine tailoring of the properties of the applied PLA. The microstructure of PLA (isotactic, syndiotactic, heterotactic and atactic) influences the kinetics of the biodegradation process [1; 13; 14; 15; 16]. Zinc compounds are attractive catalytic systems because they combine high activity with relatively low toxicity [1; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37]. The carboxylates, halides, amino acid salts, alkoxides, phenoxy-diamine, bis(phenoxy), phenoxy diamine, phenoxy-imine, phenoxy amine amine, guanidinate, bis(phenoxy), calixarene and amino bis(pyrazol) complexes of zinc as initiators have been investigated [19; 20; 21; 30; 31; 32; 33; 38; 39]. Furthermore, the oxides have been used in ROP of _rac_-LA as heterogeneous catalysts [1; 2]. In our recent study, we found catalytic systems composed of diethylzinc/gallic acid (ZnEt\({}_{2}\)/GAc) and diethylzinc/propyl gallate (ZnEt\({}_{2}\)/PGAc), synthesized for the first time, to be quite effective in the ROP of \(\varepsilon\)-caprolactone (CL). Polymerization in bulk at 40-80 \({}^{\circ}\)C produced poly(\(\epsilon\)-caprolactone) (PCL) with a high yield (_ca._ 100% in some cases). Most importantly, when the ROP of CL was carried out in the presence of ZnEt\({}_{2}\)/PGAc catalytic system at 40-60 \({}^{\circ}\)C within 48 h or at 80 \({}^{\circ}\)C within 6 h, no macrocyclic products were formed [40].