Facile Synthesis of Polyorganosiloxanes via Photo-Induced Anionic Ring-Opening Polymerization Using a Latent Catalyst

Precise, controlled, and living polymerization of cyclosiloxanes is garnering considerable attention due to the distinct properties and promising applications of polyorganosiloxanes in various fields. In this contribution, photo-induced living anionic ring-opening polymerization of cyclosiloxanes is achieved by developing an efficient photobase generator (PBG). Structure-photoactivity analysis of the photo-latent catalyst is undertaken by synergistic density functional theory (DFT) calculations and experimental investigations. DFT calculations predict the superior photoactivity of the synthesized PBG2 (i.e., 2-[(9-oxo-9H-thioxanthen-2-yl)oxy]acetic-1,5,7-riazabicyclo[4.4.0]dec-5-ene) and provide insights into the structure-catalytic activities, which is confirmed through the experiments. Polyorganosiloxanes with predetermined molar masses and low dispersities (Đ < 1.30) are produced through on-demand cleavage of PBG2 under various light intensities, different PBG2 loadings, and sequential addition of monomers. On-off light switching enables the polymerization in a rate-controlled manner. Later on, mechanistic insights by DFT calculations identify the anionic species for nucleophilic attack and reveal that the reversible equilibrium involving anionic species, catalyst cations, and ion pairs contributes to precise control over chain growth during photopolymerization. Furthermore, deterministic kinetic simulation disentangles the effects of the cleavage rate and loading of PBG2 on polymerization kinetic behaviors. This developed photopolymerization strategy shows promising potential to expand the application of polyorganosiloxanes in advanced manufacturing fields.

Photomediated Cationic Ring-Opening Polymerization of Cyclosiloxanes with Temporal Control

Precision synthesis of polyorganosiloxanes and temporal control over the polymerization process during ring-opening polymerization (ROP) of cyclosiloxanes remain challenging due to the occurrence of side reactions, e.g., intramolecular transfer (backbiting) and intermolecular chain transfer, and irreversible catalyst transformation. In this study, a merocyanine-based photoacid catalyst is developed for cationic ROP of different cyclosiloxanes. A series of well-defined cyclotrisiloxane polymers with predetermined molar masses and low dispersities (Đ < 1.30) are successfully synthesized under various conditions (i.e., different catalyst loadings, initiator concentrations, solvents, and monomer types). Mechanistic insights by experiments and theoretical calculations suggest that the cationic active species, siloxonium ions, are combined with the catalyst anions to form tight ion pairs, thereby attenuating the reactivity of active species and subsequently minimizing side reactions. An efficient photocatalytic cycle is established among the catalyst, monomer, and polymer chain due to the rapid and reversible isomeric phototransformation of the catalyst, which endows the polymerization process with excellent temporal control. Successful in situ chain extension further confirms the controlled characteristics of photomediated CROP. This as-developed polymerization strategy effectively addresses long-standing challenges in the field of polyorganosiloxane synthesis.