Journal
MACROMOLECULES
Volume 52, Issue 21, Pages 8207-8216Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.macromol.9b01359
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Funding
- Northwestern University
- ISEN Fellowships
- Royal E. Campbell Terminal Year Fellowship
- 3M Fellowship
- NSF [CHE-1048773]
- Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource [NSF ECCS-1542205]
- State of Illinois
- International Institute for Nanotechnology (IIN)
- Walter P. Murphy Professorship
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Polythiourethane (PTU) can be synthesized by a type of click chemistry involving the reaction of thiols with isocyanates. To our knowledge, thiourethane dynamic chemistry has not been significantly explored from a fundamental standpoint and has only begun to be studied regarding its use in producing recyclable, reprocessable substitutes for traditional cross-linked polyurethane networks. Using model compounds, we demonstrated the dual nature of the mechanism associated with catalyzed thiourethane dynamic chemistry: at elevated temperature, thiourethane groups undergo exchange reactions with free thiol groups and thermal reversion to thiols and isocyanates. We used this chemistry to synthesize cross-linked PTU elastomers which, upon optimization, achieve full recovery of cross-link density and tensile properties after multiple, relatively rapid remolding cycles. We characterized stress relaxation as a function of temperature and stoichiometric imbalance to provide insight into the mechanism of the network structure change. A small level (10 mol %) of excess thiol groups reduces reversion, thereby suppressing undesired side reactions during reprocessing and promoting thiol-thiourethane exchange reactions, leading to excellent property recovery after multiple recycling steps. With a proof-of-concept demonstration, we also revealed the potential of recovering thiol monomer by solvolysis from PTU networks, which provides a second route for sustainable recycling. In addition to introducing thiourethane dynamic chemistry as a simple way to achieve high-value recyclability of polyurethane-type networks by reprocessing and/or monomer recovery, our study shows that tuning of the reaction stoichiometry may be a facile approach to optimize property recovery after reprocessing for some dynamic networks that exhibit property loss when at stoichiometric balance.
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