Abstract
Cure kinetics control of epoxy resins is critical for the realization of many structures and processes and is often manipulated by catalyst design. We here show an example of switchable Lewis base catalytic activity through ligand-controlled metal coordination. Divalent first-row transition-metal (Co, Ni, Cu, Zn) β-diketonates with methyl or trifluoromethyl end groups have found distinguished thermal latent curing behaviors in triphenylphosphine (TPP)-catalyzed epoxy resins, namely, a deceleration pattern for metal acetylacetonates (acac2) and an inhibition pattern for metal hexafluoroacetylacetonates (6Facac2). Comparative analysis exposed the major initiation mechanism as phosphine attack on epoxide rings, where the phosphine reactivity was regulated by metal coordination whose strength depends on the original diketone ligands. TPP further stabilized the metal chelates and suppressed their dissociation. Feed ratio studies of Co(II) chelates revealed an equilibrium built upon TPP, metal chelate, and the formed passivated complex through numerical analysis. Further, temperature dependence of the equilibrium constants suggested a reversed metal-base affinity evolution of the two chelates during heating, which determines the equivalent TPP concentration. Chemical and thermal characterizations on the formed complexation states identified structural changes during high-temperature treatment and, along with density functional theory (DFT) calculation, verified the Co-P binding energy that marks the TPP "effectiveness"in each stage to catalyze epoxy cure. It was found that the competition between incoming phosphine and original diketone ligands, depending on the basicity of the latter, dictates the initial relative affinity between metal and phosphine, while beyond phosphine ligand stabilization, the diketone ligand dynamics at elevated temperatures were accompanied by the respective Co-P affinity change. Across different metals, the deviation from the "natural order"in metal-phosphine affinity can also be qualitatively understood from the ligand competition concept, where the same ligand effects on the field stabilization schemes are expected as the distinctions caused by ligand fluorination were consistent throughout d7-d10 metal cations. The knowledge gained from this work could benefit future design of thermal latent catalysts and shed light on the capability of Lewis base reactivity control through adjusting transition-metal coordination spheres.
Original language | English |
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Pages (from-to) | 3280-3300 |
Number of pages | 21 |
Journal | Chemistry of Materials |
Volume | 34 |
Issue number | 7 |
DOIs | |
State | Published - Apr 12 2022 |
Externally published | Yes |
Funding
This work was supported by the Industry Consortium at the Georgia Tech Packaging Research Center (PRC). This work was performed in part at the Georgia Tech Institute for Electronics and Nanotechnology, a member of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation (ECCS-2025462).