This Perspective reviews the design and synthesis of RAFT agents. First, we briefly detail the basic design features that should be considered when selecting a RAFT agent or macro-RAFT agent for a given polymerization and set of reaction conditions. The RAFT agent should be chosen to have an optimal C tr (in most circumstances higher is better) while at the same time it should exhibit minimal likelihood for retarding polymerization or undergoing side reactions. The RAFT agent should also have appropriate solubility in the reaction medium and possess the requisite end-group functionality for the intended application. In this light we critically evaluate the various methods that have been used for RAFT agent synthesis. These methods include reaction of a carbodithioate salt with an alkylating agent, various thioacylation procedures, thiation of a carboxylic acid or ester, the ketoform reaction, thiol exchange, radical substitution of a bis­(thioacyl) disulfide, and radical-induced R-group exchange. We also consider methods for synthesis of functional RAFT agents and the preparation of macro-RAFT agents by modification of, or conjugation to, existing RAFT agents. The most used methods involve esterification of a carboxy functional RAFT agent, azide–alkyne 1,3-dipolar cycloaddition, the active ester–amine reaction, and RAFT single unit monomer insertion. While some of these processes are described as “click reactions”, most stray from that ideal. The synthetic method of choice is strongly dependent on the structure of the desired RAFT agent. Finally, we outline some of the current challenges in RAFT agent design and synthesis.

The discovery of reversible-deactivation radical polymerization (RDRP) has provided an avenue for the synthesis of a vast array of polymers with a rich variety of functionality and architecture. The preparation of block copolymers has received significant focus in this burgeoning research field, due to their diverse properties and potential in a wide range of research environments. This tutorial review will address the important concepts behind the design and synthesis of block copolymers using reversible addition-fragmentation chain transfer (RAFT) polymerization. RAFT polymerization is arguably the most versatile of the RDRP methods due to its compatibility with a wide range of functional monomers and reaction media along with its relative ease of use. With an ever increasing array of researchers that possess a variety of backgrounds now turning to RDRP, and RAFT in particular, to prepare their required polymeric materials, it is pertinent to discuss the important points which enable the preparation of high purity functional block copolymers with targeted molar mass and narrow molar mass distribution using RAFT polymerization. The key principles of appropriate RAFT agent selection, the order of monomer addition in block synthesis and potential issues with maintaining high end-group fidelity are addressed. Additionally, techniques which allow block copolymers to be accessed using a combination of RAFT polymerization and complementary techniques are touched upon.

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Reversible additionÀfragmentation chain transfer (RAFT) polymerization, 1À4 mediated by thiocarbonylthio chain transfer agents (or RAFT agents, 1) (Scheme 1), possesses several advantages over other forms of reversible deactivation radical polymerization (RDRP), 5 such as atom transfer radical polymerization (ATRP) 6 and nitroxide mediated polymerization (NMP). 7,8 These include the ability to control the polymerization of a broad range of functional monomers (including vinyl esters and amides) and the absence of potentially toxic transition metal catalysts. RAFT polymerizations are simple to implement, because experimental conditions can mirror those of conventional radical polymerization; differing only by the addition of a RAFT agent. 2À4 To achieve optimal control over a RAFT polymerization, addition of the monomer derived propagating radical (P n • ) to the thiocarbonyl of a RAFT agent 1 and subsequent fragmentation of the RAFT intermediate 2 must occur efficiently. To facilitate this, the selection of a RAFT agent suitable for the monomer system is critical. Because propagating radicals of "more-activated" monomers (MAMs) (e.g., methacrylic, acrylic and styrenic monomers) are somewhat stabilized by conjugation, they are less reactive than propagating radicals derived from the "less-activated" monomers (LAMs) (e.g., vinyl esters and vinylamides). As such, these two classes of monomer require RAFT agents that are tailored to their differing reactivity. For effective control over polymerization of MAMs, dithioesters (Z = alkyl or aryl) or trithiocarbonates (Z = SR) are generally used. When these RAFT agents are used in the polymerization of LAMs, inhibition/retardation is observed, as fragmentation of the more reactive LAMs derived propagating radical is slow with respect to propagation. 9 The presence of O or N as the "Z" group adjacent to the thiocarbonyl, as is the case with xanthates (Z = OR) or dithiocarbamates (Z = NR 2 ), both slows addition of radicals to the RAFT agent and promotes the subsequent fragmentation such that it is not the rate determining step in chain transfer. Hence, these RAFT agents are regularly used for control over LAMs polymerization. Generally, xanthates and dithiocarbamates are relatively unreactive toward MAM derived propagating radicals, 10 making them ineffective control agents for these monomers. However, they may be effective for MAMs when the substituent is part of an aromatic heterocycle 10 or when highly electron withdrawing groups are present on the heteroatom. 11 As the reactivity of commonly used RAFT agents are tailored to either MAMs or LAMs, preparation of low dispersity polyMAM-block-polyLAM is not possible using the conventional RAFT process. While some RAFT agents, such as the N,N-diaryldithiocarbamates, have been reported by Destarac et al. 12 and Malepu et al. 13 for the homopolymerization of both MAMs and LAMs, these RAFT agents give only moderate control with both monomer classes. 12,13 When they were used for the preparation of poly(methyl acrylate)-bloc...