Abstract
The activity of the heterometallic alkoxide [FeCl{Ti2(OiPr)9}] (1) towards polymerization of rac-lactide (rac-LA) and e-caprolactone (e-CL) was investigated in toluene solution and in bulk at various temperatures, monomer/heterometallic alkoxide molar ratio and reaction times. The alkoxide 1 was active in solution for e-CL and in bulk for both monomers. Polymers were obtained in good yields with molecular weights ranging from 3890 to 15000 g moL-1 and polydispersity indexes (PDI) values varying from 1.3 to 2.5. Based on the 1H nuclear magnetic resonance (NMR) end-group analysis of polymers, a coordination-insertion mechanism was suggested for both monomers. The average number of growing chains per molecule of initiator (4 to 5 for rac-LA and 7 to 8 for e-CL) indicates that both bridging and terminal isopropoxides are active initiating groups. Kinetic studies with e-CL indicated that the polymerization rate is first order with respect to monomer concentration. The catalytic properties of 1 were compared to those found for other titanium(IV) and iron(II) complexes using e-CL as a model monomer.
Highlights
Biodegradable and biocompatible polyesters such as poly(ε-caprolactone) (PCL), poly(rac-lactide) (PLA)and their copolymers have been extensively employed in the biomedical field.[1,2] The synthesis of these polymers requires the use of an initiator, usually a metal alkoxide or carboxylate, based on a choice of different metals.[3]
The resulting solution received the addition of ε-caprolactone (1.00 g, 8.76 mmol) and the system was heated to the appropriate temperature for the desired time, after which the solvent was removed under vacuum
In the majority of the essays, narrow ranges of molecular weight (Mn) and polydispersity indexes (PDI) values were observed; Mn values are in good agreement with those calculated by 1H nuclear magnetic resonance (NMR)
Summary
Biodegradable and biocompatible polyesters such as poly(ε-caprolactone) (PCL), poly(rac-lactide) (PLA). Iron alkoxides are still barely reported and only few of them have been assessed for their activity towards ROP.[24,25,26] Promising examples come from bimetallic iron(II) and sodium complexes with alkoxide and aryloxide ligands that were able to promote ring-opening polymerization of rac-lactide efficiently at room temperature and afforded good control of the molecular weight.[27]. Titanium(IV) alkoxide [FeCl{Ti2(OiPr)9}]28 (1, Figure 1) as initiator of ring-opening polymerization was investigated both in bulk (for rac-lactide and ε-CL) and in solution (for ε-CL). This complex is one of the few examples of heterometallic alkoxides that contain biocompatible metals (iron and titanium) in the same molecular framework. Experiments were carried out according to previous literature reports.[34]
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