Chemical recycling of plastic has attracted attention because the polymer can be more efficient in synthesis, retain better mechanical properties, be environmentally friendly and less expensive. Among the selected polymers, polydiketonenamine (PDK) is a novel polymer that can be highly chemically recycled. PDK is depolymerized by strong acid and forms triketone monomer and triamine with a recycling efficiency of above 90%. However, the properties of such polymers are less investigated.

In this study, we employed Raman spectroscopy to observe in-Situ PDK depolymerization in strong acid and sum-frequency generation (SFG) to monitor the interfacial structure of PDK. In our observation, hydrochloric acid (5M) rapidly reacted with PDK, resulting in a new peak, from C=O at 1590 cm^-1. Compared to depolymerized products, triketone monomer and triamine, we proposed this newly formed peak is due to the intermediate. Time-dependent study showed that the intermediate exists for at least ten hours. From our proposed reaction mechanism, we assign that the peak at 1590 cm^-1 is due to the single carbonyl group after the rapid hydrolysis of PDK.

Interfacial structure and interaction of anion of PDK is observed at PDK-aqueous interfaces by sum-frequency generation (SFG). Our SFG results indicate that the intensity of CH stretch of PDK changed depending on the anion identity. We observed the intensity of the asymmetric CH stretch of PDK at 2950 cm^-1 followed by the order of Cl^- > Br^- > HSO₄^-, representing the interfacial dipole of CH has decreased. In addition, a similar trend is also observed for the swelling rate of PDK in HCl, HBr and H₂SO₄. We proposed the intensity of CH stretch could be an indicator of reaction rate of PDK depolymerization.

Mutian Hua

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