Syntheses and Application of Nitrogen - Donor Zinc(Ii) and Copper(Ii) Complexes as Catalysts In Ring Opening Polymerization of Lactides and Caprolactones

Abstract

Petrochemicalderived plastics have numerous advantages, however, there are several drawbacks arisingfromnon-renewable sources and their accumulation in the environment due to their nondegradablenature. This has led to the focus on biodegradable polymers obtained from renewable resources.Examples of biodegradable polymers are polylactides and polycaprolactones obtained by catalyticring-opening polymerization of cyclic lactides and caprolactones obtained from byproductsof sugar and com. Use of these materials would result in increased benefits of the materials,and this could offer a boost to the local producers of the plant materials. Although industrialprocesses currently employ a tin(II)bis-2-ethylhexanoate catalytic system, tin is toxic thuslimitsthe application of tin-derived polymers in biological areas. Design of less toxic and biocompatible metal catalysts is currently attracting much attention. Diaceto {2-(3,5- dimethylpyrazol-l-ylmethyl)pyridine} zinc(II) (Dl), diaceto {2-(3,5-dimethylpyrazol-lI ylmethyl)pyridine}copper(II) (D2), diaceto {2-(3,5-diphenylpyrazol-I-ylmethyl)pyridine} zinc(II) (03) and diaceto{2-(3,5-diphenylpyrazol-l-ylmethyl)pyridine} copper(II) (D4) were synthesised and used in catalytic ring-opening polymerization of D,L-Iactides and s-caprolactones. CompoundsDl and D3 formed monometallic complexes while D2 and D4 were bimetallic. The ligandsadopt a bidentate coordination mode in the monometallic complexes Dl and D3, but they are monodentate in the bimetallic complexes D3 and D4. Complexes Dl-D4 were active catalysts in the ring opening polymerization of D,L-Iactides and s-caprolactones to give polylactidesand polycaprolactones with low to moderate molecular weights between 1,402- 14.568 g/mol. Relatively narrow molecular weight distributions (1.23-1.64) were observed for polylactides whereas broad molecular weight distributions (2.0-3.9) were reported for poiycaprolactone.At 110°C the Zn(II) complexes Dl and D3 exhibited higher rate constants of 0.29h· i (0.044 h'l) and 0.17 h'l(0.096 h'l) respectively compared to 0.06 h-1 (0.017 h'l) and 0.04 h 1(0.031 h'l) rates reported for the corresponding Cu(II) complexes D2 and D4 respectively for ROPof D,L-Iactide (s-caprolactone). All the polymerization reactions follow pseudo first-order kineticwith respect to monomer. Higher activities were observed using methanol as solvent, elevatedtemperature and lower monomer/catalyst ratio.