Ring-opening polymerization of lactides by (pyrazol-1-ylmethyl)pyridine Zn(II) and Cu(II) complexes: Kinetics, mechanism and tacticity studies

Stephen O Ojwach,Thembisile P Zaca

doi:10.17159/0379-4350/2015/v68a2

Abstract

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 [Cu 2 (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.

Highlights

Biopolymers derived from renewable resources have attracted growing attention over the last decades as biocompatible and biodegradable alternatives to petrochemical-based plastics.[1,2,3,4,5] An excellent example of such a biodegradable polymer is polylactide (PLA), which can be obtained by ring-opening polymerization (ROP) of lactide monomers
We explore the potential use of these breeds of complexes as initiators in the ROP of Mesolactide (D,L-LA) and L-LA monomers
Initial screening of complexes 1–4 (Scheme 1) as initiators in the ring-opening polymerization (ROP) reactions of lactides were performed at 110 °C in toluene using [M]/[I] ratio of 50

Summary

Introduction

Biopolymers derived from renewable resources have attracted growing attention over the last decades as biocompatible and biodegradable alternatives to petrochemical-based plastics.[1,2,3,4,5] An excellent example of such a biodegradable polymer is polylactide (PLA), which can be obtained by ring-opening polymerization (ROP) of lactide monomers. PLAs display desirable mechanical, optical and chemical properties such as optical transparency, ease of processing and ease of microbial decomposition or degradation. Due to these unique features, PLAs have wide range of applications in food packaging6a as well as in pharmaceutical industries, tissue engineering and drug delivery devices.6b,c. One major drawback of most of these metal-based catalysts is their relative toxicity, which limits the application of the resultant PLAs in medical fields.6c,d In addition, the high cost of transition metal-based catalysts has dissuaded their industrial appeal

Experimental

Results and Discussion

Kinetics of LA Polymerization Reactions

Conclusions