SP–PLP–EPR Measurement of Iron-Mediated ATRP Deactivation Rate

Abstract

The rate of ATRP deactivation was measured for the amine−bis(phenolate)iron-mediated polymerization of 2-ethylhexyl methacrylate (EHMA). EHMA radical concentration is monitored by highly time-resolved EPR spectroscopy after pulsed-laser-induced radical production. In addition, the concentration of the iron(III) complex was measured via EPR. The SP−PLP−EPR method allows for the analysis of ATRP deactivation rate without interference by potential organometallic reactions. Toward higher temperature, the ratio of deactivation to propagation rate increases, which is beneficial for ATRP control. ■ INTRODUCTIONReversible-deactivation radical polymerization (RDRP) has been extensively used for synthesizing polymeric materials with precisely tailored topology, architecture, chain length, functionality, and narrow molar-mass distribution. 1−3 Fe catalysts are attractive for RDRP due to the low toxicity and broad availability of iron. 4,5 Moreover, Fe catalysts may be used for two RDRP techniques, i.e., for atom-transfer radical polymerization (ATRP) 6 and for organometallic-mediated radical polymerization (OMRP). 7−11 Both methods are based on a metal-mediated activation−deactivation equilibrium of propagating radicals involving the Fe catalyst in different oxidation states and may even operate simultaneously. 7,10,12−17 Because of the kinetic complexity, the precise knowledge of the mechanism and of the individual rate coefficients is necessary for improving Fe-mediated RDRP. EPR spectroscopy is particularly useful for investigations into the kinetics of radical polymerization, as the relevant radical species may be quantitatively monitored online. 18−20 Highly time-resolved EPR spectroscopy in conjunction with laser pulsing has emerged as the state-of-the-art method, since even very fast reaction steps such as termination or deactivation may be accurately monitored. 18,19 In single-pulse−pulsed laser polymerization (SP−PLP), a high concentration of primary radicals is almost instantaneously produced by the laser-induced decomposition of a photoinitiator. The chain length, i, of radicals increases according to the relation i = k p × c M × t + 1, where k p is the propagation rate coefficient, c M the monomer concentration, and t the time after applying the laser pulse. The term +1 represents the initiator fragment which starts chain growth. SP−PLP is similar to RDRP in that i progressively increases with time and molar mass distribution of the propagating radicals is narrow. Time-resolved monitoring may be carried out for different types of radical species which may evolve after laser-induced production of primary radical fragments. For example, in acrylate polymerizations the concentration of both secondary propagating radicals (SPRs) and midchain radicals, formed from SPRs via backbiting, 21−24 was determined by EPR analysis at separate magnetic field positions. 25,26 Metal complexes with unpaired electrons such as Cu II or high-spin Fe III are 27 With both the propagating radicals and the Fe III -catalyst species bein...

Bromoalkyl ATRP initiator activation by inorganic salts: experiments and computations

“…The moderating equilibrium is characterized by an equilibrium constant K, which is the ratio between the activation and deactivation rate constants ka and kda, respectively. In most ATRP implementations, a redox-active transition metal complex in its lower oxidation state has been used as activator species, with copper(I) 5,7,10,11 and iron(II) [12][13][14][15][16][17][18] being the most commonly used central metal ions, owing to their considerable versatility. Iron attracts considerable interest because of its lower cost, greater availability, lower toxicity and higher biocompatibility.…”

SP–PLP–EPR Measurement of CuII-Mediated ATRP Deactivation and CuI-Mediated Organometallic Reactions in Butyl Acrylate Polymerization

“…18 ■ RESULTS AND DISCUSSION Illustrated in Scheme 1 are the mechanistic scenarios for measuring (A) the Cu II -mediated ATRP deactivation rate and (B) the Cu I -mediated organometallic reaction with the starting components being marked in red. MMMP acts as the photoinitiator 27 for producing primary radicals which rapidly add to monomer molecules, M. Propagating radicals, R n • , of chain length, n, react with the associated Cu complex. The radicals may also terminate and produce dead polymer.…”

“…18,26−31 In the latter case, also complexes with unpaired electrons such as with Cu II or highspin Fe III may be monitored via EPR. 18,27,32 The earlier SP− PLP−EPR studies into Cu-mediated 18 and Fe-mediated 27 ATRP of dodecyl methacrylate (DMA) and ethylhexyl methacrylate (EHMA) were easier to be performed, as deactivation occurs at a much faster rate than termination with these methacrylates.In ATRP deactivation, the catalyst in the higher oxidation state, Cu II or Fe III , reacts with propagating radicals. It was also found 33,34 and quantified 26,35 that the catalyst in the lower oxidation state, Cu I or Fe II , may react in an organometallic fashion with propagating radicals, especially with acrylate-type radicals.…”

FeBr2-Catalyzed Bulk ATRP Promoted by Simple Inorganic Salts

“…Atom-transfer radical polymerization (ATRP) is one of the most successful reversible deactivation radical polymerization methods to control the chain growth of more activated monomers such as acrylates, methacrylates and styrenics, yielding well-defined polymers with predictable molecular weights as well as narrow molecular-weight distributions. − Copper complexes, in combination with ligands that can tune the moderating equilibrium constant (Scheme ) over more than 9 orders of magnitude, − offer the widest flexibility. ,,, However, there is growing interest in using iron-based catalysts, − in light of the greater availability and low cost of this metal, and especially because of its lower toxicity and higher biocompatibility. − …”