Enhancing Temporal Control and Enabling Chain‐End Modification in Photoregulated Cationic Polymerizations by Using Iridium‐Based Catalysts

Abstract

Gaining temporal control over chain growth is a key challenge in the enhancement of controlled living polymerizations. Though research on photocontrolled polymerizations is still in its infancy, it has already proven useful in the development of previously inaccessible materials. Photocontrol has now been extended to cationic polymerizations using 2,4,6-triarylpyrylium salts as photocatalysts. Despite the ability to stop polymerization for a short time, monomer conversion was observed over long dark periods. Improved catalyst systems based on Ir complexes give optimal temporal control over chain growth. The excellent stability of these complexes and the ability to tune the excited and ground state redox potentials to regulate the number of monomer additions per cation formed allows polymerization to be halted for more than 20 hours. The excellent stability of these iridium catalysts in the presence of more nucleophilic species enables chain-end functionalization of these polymers.

PET-RAFT Polymerization Catalyzed by Small Organic Molecule under Green Light Irradiation

“…Hawker et al reported that Ir(ppy) 3 could be utilized in visible light-induced atom transfer radical polymerization of methacrylates [9], acrylates [10], and to fabricate a 3D polymer brush [11]. Besides, many other metallic materials [12,13,14,15,16,17,18,19] have been developed to achieve living/controlled polymerization under irradiation by visible light as well.…”

Progress and Perspectives Beyond Traditional RAFT Polymerization

“…[62,63] As with radical photoRAFT, temporal control is readily achieved by switching ON/OFF light irradiation, which can be further enhanced by selection of a photocatalyst with greater stability (i.e., iridium complexes). [64] An interesting application to emerge from this technology has been the ability to combine orthogonal cationic and radical RAFT polymerizations. Combining these mechanistically distinct transformations enables the synthesis of unique block and star polymers consisting of cationically and radically polymerizable monomers, which cannot be accessed by a single polymerization in isolation.…”

“…[ 62,63 ] As with radical photoRAFT, temporal control is readily achieved by switching ON/OFF light irradiation, which can be further enhanced by selection of a photocatalyst with greater stability (i.e., iridium complexes). [ 64 ]…”

Degenerative chain‐transfer process: Controlling all chain‐growth polymerizations and enabling novel monomer sequences

“…More recently, Fors reported a visible light‐regulated cationic RAFT polymerization in which the cationic propagating species is generated upon oxidation of the dormant thioester bond by a photoredox catalyst such as a pyrylium salt or an iridium complex (Fig. ) . Furthermore, by simultaneously using both oxidizing and reducing photocatalysts (PCs) for the common dormant species (trithiocarbonate) in the copolymerization of vinyl ethers and acrylates, both cationic and radical RAFT polymerizations were active and generated copolymers; the monomer compositions and sequences of these copolymers can be tuned by the wavelength of the light .…”