From the invention of the first fully synthetic polymer by Leo Baekeland in 1907 until the development of engineering plastics for specialized applications, the synthesis of polymers underwent a tremendous development, mainly fuelled by the understanding and the development of chemistry over the past several decades. One of the key developments was the invention of controlled/living radical polymerization that started in the 70s, which allowed excellent control over the molecular weight, polymer composition and the polymer chain architecture. Besides the synthetic advantages, the capabilities of controlled/living radical polymerization are limited by the high sensitivity to oxygen. Atmospheric oxygen readily reacts with the propagating radicals and forms stabilized peroxy-radicals, inhibiting the polymerization reaction. Consequently, costly measures to remove atmospheric oxygen from the reaction are imperative for these reactions.

It is therefore not surprising that research on controlled/living radical polymerization focused lately on the development of oxygen tolerant systems, either by chemically trapping photosensitized oxygen or by employing sophisticated enzymatic systems which can convert oxygen into hydrogen peroxide.

In their recent communication in Angewandte Chemie, a team led by Cyrille A. Boyer reports a photocatalytic reversible addition-fragmentation chain-transfer (RAFT) polymerization  system based on a zinc(II) 2,3,7,8,12,13,17,18-(octaethyl)-5,10,15,20-(tetraphenyl) porphyrin (ZnOETPP) photoinitiator, which only exhibits polymerization when it is irradiated in presence of oxygen and a tertiary aliphatic amine. In this study, oxygen participates in the catalytic cycle and is neither consumed nor transformed into secondary species during the photocatalytic reaction, which is in stark contrast to previously reported oxygen tolerant photopolymerization systems which aim to convert oxygen or to remove it from the system. Consequently polymerization only takes place under aerobic conditions under illumination at 690 nm, making phot-RAFT more practical for a photopolymer community struggling with oxygen inhibition and on the permanent search for low energy wavelength photoinitiators. In the presence of a RAFT agent, amine, photocatalyst and oxygen, polymerization proceeds with a linear increase of M_n with monomer conversion and a decrease in dispersity. By performing chain extension experiments the authors confirmed the living character of the polymerization.

In the proposed mechanism ZnOETPP generates singlet oxygen (¹O₂) under 690 nm irradiation, which likely reacts with the aliphatic tertiary amine, forming superoxide (O₂^.-) and an amine radical cation. By employing DFT calculations the authors show that the reduction of the triplet state of ZnOETTP by the superoxide is energetically favourable, reforming the ground state oxygen and generating the photocatalyst radical anion which is capable of reducing the RAFT agent to produce radicals which participate in the RAFT process.

Interestingly, the necessity of catalytic oxygen and light to continuously generate the ZnOETTP triplet state paves the way for a dual-gated polymerization system. By switching the light on and off the polymerization can be initiated or stopped, a behaviour which is commonly observed in controlled/living photopolymerizations. Additionally, this catalytic system allows switching between an oxygenated and deoxygenated stated by purging the reaction with an inert gas. In the deoxygenated state polymerization stops under light irradiation but can be reinitiated when the inert gas is replaced by oxygen. This adds an additional temporal switch to the system.

The original article can be found here:

Zhang, L. Wu, C. Jung, K. Ng, Y. H. & Boyer, C. An Oxygen Paradox: Catalytic Use of Oxygen in Radical Photopolymerization. Angew. Chem. Int. Ed. 58 (2019) DOI: 10.1002/anie.201909014.

Image credit: Florian Glocklhofer