Note pubs.acs.org/Macromolecules
Halogen Conservation in Atom Transfer Radical Polymerization Yu Wang, Mingjiang Zhong, Yaozhong Zhang, Andrew J. D. Magenau, and Krzysztof Matyjaszewski* Center for Macromolecular Engineering, Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States S Supporting Information *
Scheme 1. ATRP with Reduction Processa
I
n this Note, we introduce the principle of halogen conservation (PHC) which can be used as a tool to quantify the degree of termination in atom transfer radical polymerization (ATRP) systems and should lead to the development of more efficient ATRP procedures. A similar principle can be extended to other controlled radical polymerization (CRP) methods. ATRP has been conducted with a broad range of catalysts and numerous initiation systems and applied to polymerization of various monomers under reaction conditions to prepare (co)polymers with complex molecular architecture and site specific functionality.1 A prerequisite for materials prepared by a well-controlled ATRP, and other CRP methods, is the retention of chain end functionality.2 However, in all CRP processes, unavoidable biradical termination reactions always occur with a rate proportional to the square of the radical concentration (Rt = kt[P•]2). In addition, other side reactions may result in an additional loss of chain end functionality.1d,3 Although the loss of some fraction of chain end functionality cannot be completely avoided, proper reaction conditions can be selected to minimize their contribution. The principle of halogen (denoted as X) conservation dictates that the quantity of X must remain constant and the loss of halogen capped chain ends must result in transfer of X to other reagents within the reaction mixture. The loss of halogen chain end functionality in a normal ATRP conducted with a high concentration of catalyst must be correlated to the irreversible conversion of [CuIX/L] to [CuIIX2/L]. In ATRP processes involving the (re)generation of the activator (i.e., CuIX/L) by reduction of deactivator (CuIIX2/L), the loss of chain end functionality must be reflected by considering any change in [CuIX/L] and [CuIIX2/L] plus the amount of halogen atoms transferred in the reduction process (Scheme 1). This can be quantified by eq 1
a R-X: initiator; R-Pn-X: dormant chains with both α-R groups and ω-X groups; R-R, R=, and RH: termination products of primary radicals; R-Pn•: propagating radical; R-Pn-Pn-R, R-Pn=, and R-PnH: dead polymeric chains; RA: reducing agent; RA-X: the oxidized product of the reducing agent; L: ligand.
Tmol % (%) =
[R‐X]0 ‐[R‐X]t ‐[R‐Pn‐X]t (2)
Received: September 10, 2012 Revised: October 7, 2012
The molar percentage of terminated chains (Tmol %) can be defined by eq 3: © XXXX American Chemical Society
(3)
The overall efficiencies of several ATRP methodsnormal ATRP, supplemental activator and reducing agent (SARA) ATRP,4 and electrochemically mediated ATRP (eATRP)5 were analyzed using PHC, and the results are discussed below. The analysis of halogen retention/loss involved in initiators for continuous activator regeneration (ICAR) 6 ATRP and activators regenerated by electron transfer (ARGET) ATRP are provided in the Supporting Information. The retention (or loss) of chain end functionality is often determined by 1H NMR, MALDI-TOF, and GPC.2a,b,4c,7 These methods have some limitations when one seeks to precisely measure very low Tmol % (
