ATRP with Alkyl Pseudohalides Acting as Initiators and Chain Transfer Agents: When ATRP and RAFT Polymerization Become One

Abstract

ATRP requires an alkyl halide or pseudohalide [17] (RÀX) as an initiator and a transition-metal complex (e.g., Cu, [10,18] Ru, [9,19] Fe, [20] Os, [21] Mo, [22] Ni, [23] Re, [24] etc. [2,25] ) as a catalyst. The most common catalysts for ATRP are complexes based on copper(I) halide and nitrogen-based ligands. [2,25] ATRP involves the homolytic cleavage of the RÀX bond by a transition-metal complex activator, Mt m -Y/ Ligand, that reversibly generates an alkyl radical, P n * , and the corresponding higher oxidation state metal halide deactivator, X-Mt m + 1 -Y/Ligand (Scheme 2). P n * can then propagate with a vinyl monomer (M), be deactivated by X-Mt m + 1 -Y/Ligand, or terminate with another P n * , at which point two equivalents of deactivator accumulate as persistent radicals. Radical termination is diminished in controlled polymerizations obeying the persistent radical effect (PRE) [26] as the equilibrium becomes strongly shifted toward the dormant species.The PRE is a particular kinetic feature that provides a self-regulating mechanism in radical reactions. According to the PRE, when two radicals are generated at the same rate (for example from the same source, RÀY) and one of them is persistent (e.g., Y) while the other is transient (R * ), the persistent radical eventually becomes the dominating species in solution due to the self-termination of R * . This has the effect of steering the system toward the cross-termination of the two radicals (i.e., formation of RÀY), strongly inhibiting the self-termination of transient radicals. Therefore, the PRE is a key feature of all CRP techniques based on an activation/deactivation process.Polymerization kinetics and degree of control in ATRP largely depend on the appropriate equilibrium between the activation process (generation of radicals, k act ) and the deactivation process (formation of alkyl halides, k deact ). [25] The rate constants and their ratio (K ATRP = k act /

Living Radical Polymerization by the RAFT Process – A Third Update

“…[44] The first reports involved the use of N,N-dialkyl dithiocarbamate derivatives as ATRP initiators. Dithiocarbamates do not provide effective control over the polymerization [241] MMA, St 456 Propylamine Network cleavage [603] BA, St 441 Butylamine/TCEP/BA In situ thiol-ene multiblock cleavage [604] PEGA 47 Butylamine/TCEP/divinyl sulfone In situ thiol-ene [281] MA, DMAm, NIPAm, St, C 2 H 5 S (121) Hydrazine - [375] St, PEGMA Ph (54, 18) Hydrazine - [375] St Ph (18) Hydrazine Used in thiol-epoxide [193] St Ph (18) DEAm (18) Hydrazine In situ thiol-epoxide [193] St/AcS Ph (11) Hydrazine Au nanoparticles [176] EGDMA, 421, 420 Ph (18) Hexylamine Used in thiol-ene or other reaction [588] St PhCH 2 S 1) KOH, 2) Zn/AcOH Au nanoparticles [338] St PhCH 2 S (89) Ethylene diamine Initiator for ROP C [339] HPMAm Ph (11) NaBH 4 /PBu 3 In situ thiol-ene (AA) [161] HPMAm Ph (68) Butylamine MeOH/degassed used in thiol-ene [307] NIPAm Ph (18) NaBH 4 Used in thiol-ene [247] MMA Ph (18) Hexylamine In situ thiol-ene [220] HPMAm Ph (18) Hexylamine In situ thiol-ene [220] NIPAm CH 2 (CO 2 H)CH 2 S (179) Hexylamine In situ thiol-ene [220] NIPAm C 4 H 9 S (159) Hexylamine/DMPP Used in thiol-ene [446] NIPAm C 12 H 25 S (123) Hexylamine/Bu 3 Ph Used in thiol-ene [398] Am Ph (18) NaBH 4 Used in thiol ene [244] DEAm Ph ( StMe OEt Ozone --(C¼O)SOEt [606] BA OEt Ozone --(C¼O)SOEt [607] iBoA 2-pyridyl Air AIBN, THF/Ph 3 P -OH [605] MMA, acrylates Ph Air AIBN, THF/Ph 3 P -OH [182,605] St, acrylates P n S Air AIBN, THF/Ph 3 P -OH [182] MA, BA CH 2 Ph Air AIBN, THF/Ph 3 P -OH [182] St CH 2 Ph Air AIBN, THF/Ph 3 P -OH …”

“…include those on the kinetics and mechanism of RAFT polymerization, [26,27] RAFT agent design and synthesis, [28] the use of RAFT to probe the kinetics of radical polymerization, [29] microwaveassisted RAFT polymerization, [30,31] RAFT polymerization in microemulsion, [32] end-group removal/transformation, [33][34][35][36] the use of RAFT in organic synthesis, [37] the combined use of RAFT polymerization and click chemistry, [38] the synthesis of star polymers and other complex architectures, [39][40][41][42] the synergistic use of RAFT polymerization and ATRP, [43,44] the synthesis of self assembling and/or stimuli-responsive polymers, [45][46][47] and the use of RAFT-synthesized polymers in green chemistry, [48] polymer nanocomposites, [49][50][51] drug delivery and bioapplications, [41,46,47,[52][53][54][55][56][57][58][59][60] and applications in cosmetics [61] and optoelectronics. [62] The process is also given substantial coverage in most recent reviews that, in part, relate to polymer synthesis, living or controlled polymerization or novel architectures.…”

“…Recently, more active RAFT agents such as N,Ndiaryldithiocarbamates, 1-pyrrolecarbodithioates, trithiocarbonates, [387] and dithioesters (1-dithionaphthalates [206] and dithiobenzoates [168] ) have been used as initiators in these polymerizations. [44] Since these are effective RAFT agents with the monomers used, it is pertinent to question the mechanism of the process. While one should not exclude ATRP participation, it seems best to consider these processes as RAFT polymerizations with ATRP initiation.…”

Fenton‐Chemistry‐Mediated Radical Polymerization

“…With the emergence of RDRP methods such as atom transfer radical polymerization (ATRP), nitroxide‐mediated polymerization (NMP), and reversible addition–fragmentation transfer (RAFT) there has been significant effort to control these techniques via different physical and chemical stimuli . Of these, RAFT holds particular promise due to its compatibility with a diverse range of solvents, monomers, and operating conditions . Among RDRP methods, one might argue that RAFT is the most similar to traditional FRP, with the simple addition of a thiocarbonylthio‐containing compound chain transfer agent (CTA, termed as RAFT agent) that can mediate the polymerization, allowing the synthesis of well‐defined polymers ( Scheme ).…”

One‐pot deprotection and functionalization of polythiol copolymers via six different thiol–X reactions

“…Dithioesters are common reversible addition–fragmentation chain transfer agents. They can also be used as alkyl pseudohalides, that is, initiators or dormant species in concurrent ATRP/RAFT . The latter process relies on both activation/deactivation and degenerative chain transfer.…”