Switchable Reversible Addition–Fragmentation Chain Transfer (RAFT) Polymerization in Aqueous Solution, N,N-Dimethylacrylamide

Abstract

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...

Living Radical Polymerization by the RAFT Process – A Third Update

“…DMAm [532] St [530,531] BA [531] (NVC) [531] St-b-MA [530,531] St-b-VAc [530,531] St-b-MAgrad-VAc [530,531] 278 N S S CN N A* [531] BA [531] VAc [531] …”

“…[529] Further details of RAFT polymerization using 'switchable' RAFT agents that can be switched to offer good control over polymerization of both MAMs and LAMs and a route to poly-MAM-b-polyLAM has been reported. [530][531][532] [310] MA [182] BA [182] 348 [311] B/AN [312] 73 S S * St [313,314] BD [315] NIPAm [316] St/MAH [313] St [315] St-b-(St/BD) [314,315] St-b-(St/MAH) [313] ((St/MAH)-b-St) [313] 74 S S * MAA/EGDMA [291] 75 poly(3-hexylthiophene) macro-RAFT agent…”

“…Switchable RAFT agents have been proven effective in the aqueous solution polymerization of N,Ndimethylacrylamide (DMAm). [532] A quantitative study of the influence of acid type and concentration was performed for the polymerization of DMAm. [532] The conclusion was that best control over MAMs is achieved with stoichiometric amounts of a strong acid (e.g.…”

“…[532] The recent work of Destarac and co-workers suggests that this problem might be overcome with room temperature polymerization. [506,507] Block copolymers with LAMs were successfully prepared from the DMAm macro-RAFT agent in organic solution.…”

Universal Conditions for the Controlled Polymerization of Acrylates, Methacrylates, and Styrene via Cu(0)-RDRP

“…In contrast, research in the area of RAFT polymerization has made more progress towards the development of universal chain transfer agents (CTAs), potentially due to the simpler overall system. [17][18][19] Even after careful optimization of the reaction conditions of copper mediated ATRP, in order to maintain high end group fidelity one often has to stop the polymerization at moderate/low conversions (e.g. 60%) and extensively purify the macroinitiator product prior to performing a chain extension experiment which is a waste of materials and time consuming limiting commercial exploitation and attractiveness.…”