cone-25,27-Dipropyloxy-26,28-dioxo-calix[4]arene titanium (IV) dichloride (1) has been assessed in the ring-opening polymerisation of rac-lactide (L,D-LA). The polymers formed (PLDA) turned out to display an isotactic stereoblock microstructure (determined by NMR) despite the fact that the catalyst has symmetry. Two techniques were applied for initiating the polymerisation reaction, microwave irradiation, and conventional thermal treatment. The polymers obtained were all characterised by NMR, IR, HPLC-SEC, DSC, and MALDI-TOF analysis. The use of microwave irradiation, applied for the first time to calixarene-based catalysts in the presence of the rac-lactide monomer, increased the polymerisation rate compared with that obtained by the other method. On the other hand, standard thermal treatment enabled a slightly better control than microwave irradiation over the molecular weight and molecular weight distribution of the polylactides formed. 1. Introduction Poly(lactic acids) (PLAs) are biocompatible and biodegradable materials with potential applications in medicine and agriculture, as well as packaging materials. The most efficient method for the synthesis of PLAs is the ring opening polymerisation (ROP) of lactide (LA), which possesses two chiral centers [1–4]. The stereochemistry of the polymer formed determines its physical and mechanical properties, as well as its rate of degradation, and it is therefore important to ensure stereocontrol of the polymerisation process [5–8]. Among the most used catalysts for the ring opening polymerisation of LA, notably in industry, are organotin compounds, for example, Sn(Oct)2 [9–12]. A major drawback to the use of these tin derivatives is their high toxicity. In this context, the development of new, performing catalysts based on non toxic metals (e.g., group IV transition metals) remains an important challenge [13–20]. Calix[ ]arenes are macrocyclic molecules made of four phenol units linked via methylene bridges connected to the ortho positions of the phenol rings. The presence of four oxygen atoms at the lower rim of these conical molecules provides a valuable platform for the synthesis of poly(phenoxy) metal complexes. Recently, we have reported on a calix[ ]arene titanium complex that was found to be active in the ROP of L-LA [21]. Here, the calixarene moiety behaves as a small oxo surface protecting one face of the complex, thereby confering high thermal stability to it [22–25]. In the present work, we describe the use of cone-25,27-dipropyloxy-26,28-dioxo-calix[ ]arene titanium (IV) dichloride ( ) in the
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