A circular approach to Si-based polymers: polymerising and depolymerising using an iron β-diketiminate catalyst Mirela A. Farcaş-Johnson 1,2 , Sara H. Kyne 2 , Ruth L. Webster 1 1 University of Bath, UK, 2 Monash University, Australia Silicon chemistry has had a renaissance in the 21 st century, with uses spanning from semiconductors to composite ceramics and high-performance polymers. Silicon-containing species are prevalent throughout the chemical, medicinal and materials industries and typically contain heteronuclear Si–E (E = C, N, O) bonds due to their attractive physical and chemical properties ( e.g. chemical inertness, thermal stability and good conductive properties). 1–4 However, the strength of the Si–E bond often results in persistent and difficult to recycle materials. Hence, the synthesis and activation of Si–E bonds has gained huge interest within the last decade. 5 In the context of Si-based polymers, these bond-breaking and bond-making transformations would translate into accessible and chemically-recyclable materials. With a strong interest in iron catalysis (due to its high abundance, low toxicity and favourability over precious metals), we have recently reported the straightforward dehydrocoupling reaction of silanes with amines, phosphines and alcohols using three-coordinate iron(II) β-diketiminate pre-catalyst ( 1 ). 6 Our procedure produced a variety of Si–E-type monomers and two Si–N and Si–O backbone small chain oligomers. Further investigations into silicon transformations found that 1 in the presence of a mild reductant, pinacol borane (HBpin), could also activate Si–O and Si–N bonds in siloxane and silazane monomers and polymers to perform Si–E bond-breaking desilylations. In the work presented here, we describe the optimised iron-catalysed synthetic procedure for a variety of Si–O and Si–N-based polymers through dehydrocoupling (including polymers consisting of bio-derived monomers) and the chemical recycling of these (and some commercially available examples) by desilylation using 1 and HBpin. Our study represents one of the few examples of catalytic Si–E bond activation, complete with mechanistic and kinetic data. We have adapted the iron(II)-mediated dehydrocoupling and desilylation processes to a circular approach for Si- based polymers, where successful polymerisation and depolymerisation of Si–E polymers was achieved to afford synthetically-useful boronic esters or amine-boranes and silane monomers. 7 References 1. J. Sołoducho, D. Zając, K. Spychalska, S. Baluta and J. Cabaj, Molecules , 2021, 26 , 2042. 2. Y. Nagasaki, F. Matsukura, M. Kato, H. Aoki and T. Tokuda, Macromolecules , 1996, 29 , 5859–5863. 3. J. H. Koo, M. J. Miller, J. Weispfenning and C. Blackmon, J. Compos. Mater. , 2010, 45 , 1363–1380. 4. A. Colas and J. Curtis, in Biomaterials Science: An Introduction to Materials in Medicine , eds. B. D. Ratner, A. S. Hoffman, F. J. Schoen and J. E. Lemons, Elsevier, Inc., 2nd edn., 2004, pp. 80–99. 5. E. M. Leitao, T. Jurca and I. Manners, Nat. Chem. 2013 510 , 2013, 5 , 817–829. 6. D. Gasperini, A. K. King, N. T. Coles, M. F. Mahon and R. L. Webster, ACS Catal. , 2020, 10 , 6102–6112. 7. D. E. Ryan, K. A. Andrea, J. J. Race, T. M. Boyd, G. C. Lloyd-Jones and A. S. Weller, ACS Catal. , 2020, 10 , 7443–7448.
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