Ring opening polymerization of d,l-lactide and ε-caprolactone catalysed by (pyrazol-1-yl)copper(ii) carboxylate complexes

1,2-Bis{(3,5-dimethylpyrazol-1-yl)methyl}benzene (L) reacts with [Cu(OAc)(2)] and C(6)H(5)COOH, 4-OH-C(6)H(4)COOH, 2-Cl-C(6)H(4)COOH and (3,5-NO(2))(2)-C(6)H(3)COOH to afford the copper complexes [Cu(2)(C(6)H(5)COO)(4)(L)(2)] (1), [Cu(2)(4-OH-C(6)H(4)COO)(4)(L)(2)] (2), [Cu(2)(2-Cl-C(6)H(4)COO)(4)(L)(2)](n) (3) and [Cu{(3,5-NO(2))(2)-C(6)H(3)COO}(2)L](n) (4) which are characterised by IR, mass spectrometry, elemental analyses, and X-ray crystallography. The structural data revealed two geometries that are adopted by the complexes: (i) paddle wheel in 1, 2·7H(2)O, 3 and (ii) regular chains in 3 and 4. Magnetic studies show strong antiferromagnetic couplings in the paddle wheel complexes and a weak antiferromagnetic coupling in the monometallic chain one. Catalysis studies performed with these complexes (1–4) showed that they initiate ring opening polymerization (ROP) of ε-caprolactone (ε-CL) under solvent-free conditions and d,l-lactide in toluene at elevated temperatures. Polycaprolactone (PCL) and poly(d,l-lactide) (PLA) obtained from the polymerization reactions are of low molecular weights (858 for PCL and 602 Da for PLA for initiator 1) and polydispersity indices (typically 2.16 for PCL and 1.64 for PLA with 1 as the initiator). End group analysis of the polymers, determined by MALDI-ToF MS, indicates that the polymers have benzoate, hydroxyl, methoxy and cyclic end groups.