title: Structural and kinetic studies of the polymerization reactions of ε-caprolactone catalyzed by (pyrazol-1-ylmethyl)pyridine Cu(II) and Zn(II) complexes.
abstract: The structural and kinetic studies of polymerization reactions of ε-caprolactone (ε-CL) using (pyrazolylmethyl)pyridine Cu(II) and Zn(II) complexes as initiators is described. Reactions of 2-(3,5-dimethylpyrazol-1-ylmethyl)pyridine (L1) and 2-(3,5-diphenylpyrazol-1-ylmethyl)pyridine (L2) with Zn(Ac)2·2H2O or Cu(Ac)2·2H2O produced the corresponding complexes [Zn(Ac)2(L1)] (1), [Cu(Ac)2(L1)] (2), [Zn(Ac)2(L2)] (3) and [Cu2(Ac)4(L2)2] (4) respectively. Solid state structures of 1 and 4 confirmed that complexes 1 and 4 are monomeric and dimeric respectively and that L1 is bidentate in 1 while L2 is monodentate in 4. X-band EPR spectra of 2 and 4 revealed that complex 2 is monomeric both in solid and solution state, while the paddle-wheel structure of 4 is retained in solution. Complexes 1-4 formed active initiators in the ring opening polymerization of ε-CL. Zn(II) complexes 1 and 3 exhibited higher rate constants of 0.044 h(-1) and 0.096 h(-1) respectively compared to rate constants of 0.017 h(-1) and 0.031 h(-1) observed for the corresponding Cu(II) complexes 2 and 4 respectively at 110 °C. All the polymerization reactions follow pseudo first-order kinetic with respect to ε-CL monomer. Initiator 1 showed first-order dependency on the polymerization reactions and utilizes only one active site as the initiating group. The molecular weights of the polymers produced range from 1982 g mol(-1) to 14,568 g mol(-1) and exhibited relatively broad molecular weight distributions associated with transesterification reactions.
authors.affiliation: School of Chemistry and Physics, University of KwaZulu-Natal, Pietermaritzburg Campus, Private Bag X01 Scottsville, 3209, South Africa. ojwach@ukzn.ac.za
authors.forename: Stephen O
authors.initials: SO
authors.lastname: Ojwach
authors.forename: Teddy T
authors.initials: TT
authors.lastname: Okemwa
authors.forename: Nelson W
authors.initials: NW
authors.lastname: Attandoh
authors.forename: Bernard
authors.initials: B
authors.lastname: Omondi
mesh_terms: D002208:Caproates / Q000737:chemistry*; D002384:Catalysis; D003300:Copper / Q000737:chemistry*; D007700:Kinetics; D007783:Lactones / Q000737:chemistry*; D009942:Organometallic Compounds / Q000737:chemistry*; D058105:Polymerization; D011108:Polymers / Q000737:chemistry; D011720:Pyrazoles / Q000737:chemistry*; D011725:Pyridines / Q000737:chemistry*; D015032:Zinc / Q000737:chemistry*
chemical_list: D002208:Caproates; D007783:Lactones; D009942:Organometallic Compounds; D011108:Polymers; D011720:Pyrazoles; D011725:Pyridines; C031280:pyrazole; C121056:caprolactone; D003300:Copper; D015032:Zinc