DFT-supported Mechanistic Understanding of the Ring-opening Polymerization of Cyclic Trithiocarbonates Mediated by Organic Base

Sulfur-rich polymers are endowed with several enhanced features, such as high refractive index and excellent thermal and mechanical properties, owing to the incorporation of sulfur atoms into the polymer main chain. As an important category of sulfur-rich polymers, polytrithiocarbonates can be efficiently synthesized through ring-opening polymerization(ROP) of cyclic trithiocarbonates(CTCs) mediated by 1,5,7-triazabicyclo[4.4.0] dec-5-ene(TBD) and benzyl mercaptan(BnSH). However, the mechanistic investigations of this reaction are limited. In this study, we attempted to understand this reaction by simulating chain initiation, propagation, and termination in the ROP of CTCs using density functional theory(DFT) calculations. As demonstrated, the acceleration of the ROP of δ-CTC by BnSH can be attributed to the smaller size of BnS− compared to TBD when nucleophilic attacking, and [TBDH]+ enhancing the interaction with the monomer and stabilizing the generated active species in the initiating process. In addition, the ability of BnSH to tune the topological structures of the final polymers is ascribed to the suppressed transesterification between the thiolate species and the first trithiocarbonate unit initiated by BnS−. Furthermore, the limitations that hinder the ROP of five- and six-membered CTCs under general conditions are the unfavored thermodynamic factors, wherein the ROP of six-membered CTC is possible below −75 °C.