Photocatalyzed Cu-Based ATRP Involving an Oxidative Quenching Mechanism under Visible Light

Abstract

A new type of photocatalyzed Cu-based atom transfer radical polymerization (ATRP) is described, involving directly the light absorption of the activator form of the copper complex Cu(I). The selected catalyst was bis(1,10-phenanthroline)copper(I), Cu(phen) 2 + , due to its intense absorption in the visible domain, which permitted to use very soft irradiation conditions, consisting of a simple household blue LED at 0.9 W. An excellent control over the polymerization of methyl methacrylate (MMA) in dimethylformamide (DMF) was observed under irradiation in these conditions, using ethyl α-bromophenylacetate (EBPA) as the initiator, with polydispersity indexes (PDI) as low as 1.10 while using low catalyst content (80 ppm). The proposed mechanism implies first the formation under irradiation of the excited state of the activator form of the complex Cu(I)*. It can then rapidly undergo the oxidative quenching of the alkyl bromide, which results in its conversion into the deactivator form of the complex Cu(II)−Br along with the generation of a propagating radical. The setting up of the ATRP equilibrium ensues. Additionally, it was possible to complete the catalysis mechanism by adding triethylamine (TEA), which permitted a faster polymerization, thanks to a faster regeneration of the activator Cu(I). An excellent control over the polymerization was also observed in the presence of TEA, with PDI as low as 1.06. The addition of TEA allowed also to use a catalyst loading as low as 20 ppm, while maintaining a good controllability.

Photocatalysts in Polymerization Reactions

“…Different chemical mechanisms can be involved, which are probably affected by the ligand on the Cu complex. For example, the oxidation of Cu I by R–Br has been proposed in an oxidative catalytic cycle . But photo‐ATRP can also be directed by the reduction of Cu II Br 2 /L species (L=ligand) in the excited state under UV or visible irradiation and follows two mechanisms: (i) reductive elimination of a halide radical or (ii) oxidation of a sacrificial electron donor, in particular, aliphatic amine derivatives or methanol .…”

Copper and iron complexes as visible‐light‐sensitive photoinitiators of polymerization

“…Oxidation or reduction agents must be used in combination with the photocatalysts to ensure a catalytic cycle and the regeneration of the CuC (Table ). The mechanistic investigations that have been extensively detailed (see recent papers) will not be reviewed here.…”

Investigation into the photolytic stability of RAFT agents and the implications for photopolymerization reactions

“…RDRPs act by minimizing the time the chain end spends in its 'active' (i.e. Many examples of photochemically-mediated RDRP variants have emerged, including photo-ATRP, [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] photo-NMP, 30 and photo-RAFT (both photoinitiated [31][32][33][34][35][36][37][38] and photo-catalysed [39][40][41][42][43][44][45][46][47] ) with much success. 12 This can be achieved through two fundamental mechanisms: (1) the reversible activation/deactivation end-capping from a dormant to active state, and (2) a degenerative chain transfer mechanism.…”