| dc.description.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. | en_US |
