Add time:07/17/2019         Source:sciencedirect.com

The novel complexes [LnCoCl2] (Ln = LA − LH), containing the N,N-chelating ligands N,N-bis((1H-pyrazol-1-yl)methyl)aniline (LA), N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)-3,5-dimethylaniline (LB), N,N-bis((1H-pyrazol-1-yl)methyl)-3,5-dimethylaniline (LC), N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-3,5-dimethylaniline (LD), N,N-bis((1H-pyrazol-1-yl)methyl)-4-methoxyaniline (LE), N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-4-methoxyaniline (LF), N,N-bis((1H-pyrazol-1-yl)methyl)-4-fluoroaniline (LG) and N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-4-fluoroaniline (LH), have been synthesized and structurally characterized. The molecular structures of these Co(II) complexes showed a distorted tetrahedral geometry. No interaction exists between the Namine atom and the Co(II) centre in the [LnCoCl2] complexes, thus resulting in the formation of an eight membered chelate ring. [LDCoCl2] exhibited the highest catalytic activity (4.02 × 104 g PMMA/molCo h) for the polymerization of methyl methacrylate (MMA) in the presence of modified methylaluminoxane (MMAO) at 60 °C and yielded poly(methylmethacrylate) (PMMA) with high molecular weight (Mw) (10.5 × 105 g/mol). Syndiotactic enriched PMMA was obtained with Tg in range 123–130 °C. The MMA polymerization activity of the complexes in the current study should not be solely considered as a function of the total steric hindrance from the ligands around the metal center. The dimethyl derivatives [LnCoMe2] (Ln = LA − LH), generated in situ, effectively polymerized rac-lactide (rac-LA) and yielded polylactide (PLA) with good number-average molecular weights and narrower polydispersity indices (PDIs). The electronic density around the metal center in these dimethyl cobalt initiators seemed to enhance the activity, while the stereoselectivity is negatively affected. Thus, the presence or absence of a methyl group in the pyrazole ring moiety influences not only the activity of the Co(II) complexes in the MMA polymerization, but also the stereoselectivity of the Co(II) initiators in the ROP of rac-LA.

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