NacNac-Zn-pyridonate mediated ε-caprolactone ROP Jack W. J. Hughes , Michael J. Ingleson, Jennifer A. Garden School of Chemistry, The University of Edinburgh, UK
Zinc complexes, including those of β-diketiminate (NacNac) ligands, have been extensively studied as catalysts in the synthesis of bio-based and degradable polymer materials. 1–3 Carboxylate derivatives have shown poor activity for lactone ring-opening polymerisation (ROP) in the absence of an exogenous initiator, exhibiting poor control over polymerisation especially when compared to their alkoxide counter-parts. 4 Subsequent mechanistic studies have revealed the importance of the solution-state aggregation of these complexes in both ROP and ring-opening co-polymerisation to synthesise bio-based polymer materials. 5 Here, we report two new NacNac-zinc complexes bearing an amidate (2-pyridonate ( 1 , R = H) or 6-methyl-2-pyridonate ( 2 , R = Me)) co-ligand, which were tested for the ROP of ε-caprolactone (ε-CL) to form polycaprolactone (PCL) at room temperature. To the best of our knowledge this is the first reported instance of a pyridonate-initiated ROP, and the first example of amidate initiated ROP. DFT calculations and 1 H DOSY NMR studies suggest that while different solid-state structures are observed for 1 and 2 , ROP likely proceeds via a single-site monometallic mechanism in solution. High molecular weight PCL was synthesised in the absence of a co-initiator, with moderate to low dispersities. Major signals in the MALDI-ToF spectra were assigned to cyclic PCL species, which are proposed to form through intramolecular transesterification reactions facilitated by loss of the pyridonate unit. Pyridonate end-groups were also observed using both complexes, which contrasts with previous reports for analogous carboxylate complexes and suggests that the pyridonate units are more efficient initiators. A small increase in the pyridonate steric bulk ( Me vs H ) resulted in improved catalytic activity, facilitating complete conversion of 100 equiv. monomer within 1 h at 60 °C. References 1. R. H. Platel, L. M. Hodgson and C. K. Williams, Polymer Reviews , 2008, 48 , 11–63. 2. E. Fazekas, P. A. Lowy, M. A. Rahman, A. Lykkeberg, Y. Zhou, R. Chambenahalli and J. A. Garden, Chem. Soc. Rev. , 2022, 51 , 8793–8814. 3. E. Fazekas, P. A. Lowy, M. A. Rahman, A. Lykkeberg, Y. Zhou, R. Chambenahalli and J. A. Garden, Chem. Soc. Rev. , 2023, 52 , 1157–1157. 4. B. M. Chamberlain, M. Cheng, D. R. Moore, T. M. Ovitt, E. B. Lobkovsky and G. W. Coates, J. Am. Chem. Soc. , 2001, 123 , 3229–3238. 5. D. R. Moore, M. Cheng, E. B. Lobkovsky and G. W. Coates, J. Am. Chem. Soc. , 2003, 125 , 11911–11924.
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