Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Versatility of Organocatalyzed Atom Transfer Radical Polymerization and CO2‑Switching for Preparing Both Hydrophobic and Hydrophilic Polymers with the Recycling of a Photocatalyst Xin Su,†,‡ Philip G. Jessop,*,§ and Michael F. Cunningham*,‡
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Polymer Research Institute, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, Sichuan 610065, China ‡ Department of Chemical Engineering, Queen’s University, 19 Division Street, Kingston, Ontario, K7L 3N6 Canada § Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario, K7L 3N6 Canada S Supporting Information *
ABSTRACT: A new approach was designed to prepare both hydrophobic and hydrophilic polymers by organocatalyzed atom transfer radical polymerization (O-ATRP). The method is based on using a recoverable photocatalyst whose properties can be switched using only CO2 addition and removal as triggers. The effectiveness of the CO2switching approach in O-ATRP is demonstrated using a new CO2switchable photoinitiated catalyst, which can be extracted from the polymer and reused. The residual catalyst in the polymer is reduced to less than 15 ppb. The feasibility of recovering and reusing the photoinitiated catalyst for subsequent polymerizations is also established.
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INTRODUCTION Atom transfer radical polymerization (ATRP) is an important synthetic method for preparing advanced polymer materials with novel microstructures and morphologies.1−4 However, as metal complexes are used in the reaction, its application in industry has thus far been restricted.5 The material cost of catalyst complexes (primarily ligands), the toxicity of the compounds, and post-polymerization purification costs related to the removal of metal complexes have posed challenges to the commercial application of ATRP. Removing or reducing copper ions and catalysts in the final product remains a challenge for ATRP.6 Copper ions will harm the human body and will not decompose in the environment.6 Therefore, they will accumulate in vivo when present in water or soil. Considerable efforts have been made to address this challenge. Many proposed solutions rely on the purification stage after the reaction, which brings about additional processing steps and can significantly increase the overall reaction cost. These methods include the use of ion-exchange resins, twophase systems, fixed/immobilized catalysts, and fixed/soluble mixed catalyst systems.7 A more preferable strategy is to reduce the catalyst concentration used in the polymerization process. The ATRP rate does not depend on the absolute catalyst concentration but on the ratio of the activator concentration to deactivator concentration. Activator regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP) uses a smaller catalytic quantity of the activating catalyst than ATRP.8 The deactivator formed due to the termination of free © XXXX American Chemical Society
radicals is continuously regenerated into the activator through a redox process. ARGET ATRP can produce a well-defined polymer using very little catalyst and common reducing agents. For example, 25 ppm of copper can catalyze reasonably wellcontrolled polymerizations.8 In e-ATRP, the traditional catalyst with the structure X-CuII/L can be reduced to CuI/L through electrochemical reduction to initiate controllable free-radical polymerizations.9−12 A reduction process can also be used to obtain the required catalyst and ligand. Other recent studies have addressed the issue of reducing the amount of copper used by strategies including the use of high-activity initiators, ultrasonication, and ion-pair catalysis in miniemulsions.13−15 CO2-switching refers to inducing a reversible property change in a material, such as a transition between hydrophobic and hydrophilic states, by adding or removing CO2.16−18 We recently proposed a CO2-switching method for removing the copper salt and ligand from a polymer synthesized by ordinary ATRP or ARGET ATRP.19 Me6TREN, a common ATRP ligand, exhibits CO2 responsiveness. Following the ATRP reaction, a CO2-switching method was used not only to remove the copper (
