Insertion of degradable thioester linkages into styrene and methacrylate polymers Matthew Rix 2 , Samuel Higgs 1 , Eleanor Dodd 2 , Simon Coles, 2 Nathaniel Bingham 1 and Peter Roth 1 1 The University of Surrey, UK, 2 University of Southampton, UK In recent years, there has been considerable interest in the synthesis of novel thionolactone (C(=S)O) monomers and inserting them into polymers to make degradable thioester (C(=O)S) units. This process is called Thiocarbonyl Addition Ring-Opening (TARO) polymerisation and has been headlined by a first-generation thionolactone, dibenzo[ c , e ]oxepine-5(7 H )-thione (DOT). 1,2 Now, second-generation thionolactones are being synthesised at a rapid pace to find one which copolymerises with more challenging vinyl monomers such as methacrylates. 3,4 Our work has shown that the thionolactone monomer, 3,3-dimethyl-2,3-dihydro-5H-benzo[e][1,4]dioxepine-5- thione (DBT), is able to homopolymerise, and for the first time, copolymerise with methacrylates ( e.g. PEGMA). 5,6 Polymerisation kinetics has been at the forefront of these studies and has helped understand how DBT copolymerises, especially in terms of composition and speed. The consumption of DBT in radical polymerisations with styrene has proved so rapid that a starve-fed semi-batch setup had to be employed, though further optimisation is required. Conversely, the consumption of DBT in polymerisations with methacrylate was sluggish and led to few thioester linkages being introduced; this has led to the feed of DBT being increased to force thionolactone incorporation. Whilst DBT monomer can be handled at room temperature, elevated temperatures (+110 °C) or the presence of acid can compromise its stability. This is because DBT has a propensity to rearrange in-situ , transforming from a 7- to a 6-membered thionolactone. The rearrangement appears to be novel. With regards to the DBT homopolymer, aminolysis of the thioester linkage proceeded in an unexpected fashion; it seems that the thiol-intermediate underwent a 3-exo-tet reaction to form an N -functional hydroxybenzamide and thiirane. This meant that the DBT linkage was cleaved at two locations, the thioester and ortho -ether. It is expected that this discovery will aid in the design of next generation thionolactones to maximise degradability. References 1. (a) Both groups discovered DOT independently of each other: N. M. Bingham and P. J. Roth, Chem. Commun. (Cambridge, UK) , 2019, 55 , 55-58. DOI: 10.1039/c8cc08287a; (b) R. A. Smith et al. , J. Am. Chem. Soc. , 2019, 141 , 1446-1451. DOI: 10.1021/jacs.8b12154. 2. M. P. Spick et al. , Macromolecules , 2020, 53 , 539-547. DOI: 10.1021/acs.macromol.9b02497. 3. (a) O. Ivanchenko et al ., Polym. Chem. , 2021, 12 , 1931-1938. DOI: 10.1039/D1PY00080B; (b) C. M. Plummer et al ., ACS Applied Polymer Materials, 2021, 3 , 3264-3271. DOI: 10.1021/acsapm.1c00569. 4. G. R. Kiel et al ., J. Am. Chem. Soc. , 2022, 144 , 12979–12988. DOI: 10.1021/jacs.2c05374. 5. M. Rix et al ., ACS Macro Lett. , submitted. A preprint version is available at ChemRxiv TM , DOI: 10.26434/chemrxiv-2022- cdt52. 6. Independent of each other, Guillaneuf et al. and ourselves reported the novel copolymerisation of styrene using thionolactone species: N. Gil et al. , Macromolecules , in press. DOI: 10.1021/acs.macromol.2c00651
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