TY - JOUR
T1 - Reprocessable Polymer Networks via Thiourethane Dynamic Chemistry
T2 - Recovery of Cross-link Density after Recycling and Proof-of-Principle Solvolysis Leading to Monomer Recovery
AU - Li, Lingqiao
AU - Chen, Xi
AU - Torkelson, John M.
N1 - Funding Information:
We acknowledge support of Northwestern University via discretionary funds associated with a Walter P. Murphy Professorship (J.M.T.), ISEN Fellowships (L.L., X.C.), a Royal E. Campbell Terminal Year Fellowship (L.L.), and a 3M Fellowship (X.C.). We thank Dr Zhe Qiang (Northwestern University) for assistance in the synthesis of thiourethane small molecules and Dr Kailong Jin (University of Minnesota Twin Cities) for helpful discussion. This work made the use of the IMSERC at Northwestern University, which has received support from the NSF (CHE-1048773), Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205), the State of Illinois, and International Institute for Nanotechnology (IIN). After the submission of our manuscript, Fortman et al. (64) published a study, which indicated that several Lewis acid catalysts mediate urethane exchange, likely via a dissociative mechanism. Using low molecular weight diisocyanate and multifunctional alcohols, they observed moderate (39–67%) recovery of (room-temperature) tensile strengths. At 90 °C, tensile testing revealed quantitative recovery of tensile strength for one of the catalysts.
Publisher Copyright:
Copyright © 2019 American Chemical Society.
PY - 2019/11/12
Y1 - 2019/11/12
N2 - 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.
AB - 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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U2 - 10.1021/acs.macromol.9b01359
DO - 10.1021/acs.macromol.9b01359
M3 - Article
AN - SCOPUS:85074493585
SN - 0024-9297
VL - 52
SP - 8207
EP - 8216
JO - Macromolecules
JF - Macromolecules
IS - 21
ER -