South African Journal of Chemistry
On-line version ISSN 1996-840X
Print version ISSN 0379-4350
OJWACH, Stephen O. and ZACA, Thembisile P.. Ring-opening polymerization of lactides by (pyrazol-1-ylmethyl)pyridine Zn(II) and Cu(II) complexes: Kinetics, mechanism and tacticity studies. S.Afr.j.chem. (Online) [online]. 2015, vol.68, pp.07-14. ISSN 1996-840X. http://dx.doi.org/10.17159/0379-4350/2015/v68a2.
The kinetics, mechanism and polymer microstructure studies of ring-opening polymerization (ROP) of lactides (LA) by Zn(II) and Cu(II) complexes of (pyrazolylmethyl)pyridine ligands, 2-(3,5-dimethylpyrazol-1-ylmethyl)pyridine (L1) and 2-(3,5-diphenylpyrazol-1-ylmethyl)pyridine (L2) is described. The complexes [Zn(Ac)2(L1)] (1), [Cu(Ac)2(L1)] (2), [Zn(Ac)2(L2)] (3) and [Cu2(Ac)4(L2)2](4) form active initiators in the ROP of D,L-LA and L-LA monomers. Generally Zn(II) complexes 1 and 3 exhibit higher activities compared to the corresponding Cu(II) complexes 2 and 4. Polymerization kinetics of D,L-LA show higher rates compared to the L-LA reactions. All the polymerization reactions follow pseudo first-order kinetics with respect to monomer while 1 shows second-order dependency on the polymerization reactions. Molecular weights of the polymers range from 813 g mol-1 to 9207 g mol-1 and exhibit relatively narrow molecular weight distributions between 1.2 to 1.6. While poly(D,L-LA) are predominantly atactic, poly(L-LA) are largely isotactic. All polymerization reactions proceed through coordination insertion mechanism followed by hydrolysis of the end groups.
Keywords : Zinc and copper complexes; lactide; polymerization; kinetics; mechanism.