Organocatalysts in Solving Challenges in Polymer Synthesis

INTRODUCTION Polymeric materials are ubiquitous in our daily life and play a major role in scientific discoveries for constructing next generation smart materials, bioelectronics, novel drug carriers and so on. Numerous polymerizations methods have been developed to access functional polymeric materials, with metalcatalyzed polymerizations proven one of the most successful. However, some inherent limitations include metal contamination hinders the application of polymers synthesized in these methods. This report summarizes the development of metal-free atom transfer radical polymerization (ATRP) and the recent advance of H-bonding catalyzed ring-opening polymerization (ROP) to discern the potential of using organocatalysts to overcome challenges associated with metal catalyzed polymerization reactions. PHOTOINDUCED METAL FREE ATRP ATRP is one of the most studied methods for control radical polymerization (CRP). The broad scope and versatility makes ATRP the most utilized methods to convert vinyl monomers to a plethora of polymers for a board range of applications. One major drawback of traditional copper-catalyzed ATRP is the inevitable contamination with toxic and colorful metal residue, which has hindered the application of polymers synthesized by this method in biomedical and electronic fields. Early attempts such as introducing sacrificial reducing agents and using photoredox catalysts have been utilized to enhance catalyst reactivity and allow low catalyst loading. However, a complete metal-free ATRP system remains absent. Inspired by the success of photomediated iridium catalyzed ATRP, perylene and phenothiazine were first identified as organic photocatalysts for metal-free ATRP to polymerize methyl methacrylate (MMA) due to their strong reduction potential at photoexcited state (-1.68 and -2.1 V vs SCE respectively). Kinetic studies for both cases suggested a controlled polymerization process and polymerization only processes in the presence of light. The structureactivity relationship and reaction mechanism of phenothiazine and its derivatives were then extensively studied by different groups to direct the development of more efficient catalyst system. Currently, the most efficient system consists of highly conjugated phenoxazine, which polymerizes Scheme 1. Proposed mechanism for metal-free photomediated ATRP (Cat = catalyst, Pn = polymer chain). Cat Pn Br Cat*