Selective formation of diblock copolymers using radical trap‐assisted atom transfer radical coupling

Abstract

Polystyrene (PSt) radicals and poly(methyl acrylate) (PMA) radicals, derived from their monobrominated precursors prepared by atom transfer radical polymerization (ATRP), were formed in the presence of the radical trap 2‐methyl‐2‐nitrosopropane (MNP), selectively forming PSt‐PMA diblock copolymers with an alkoxyamine at the junction between the block segments. This radical trap‐assisted, atom transfer radical coupling (RTA‐ATRC) was performed in a single pot at low temperature (35 °C), while analogous traditional ATRC reactions at this temperature, which lacked the radical trap, resulted in no observed coupling and the PStBr and PMABr precursors were simply recovered. Selective formation of the diblock under RTA‐ATRC conditions is consistent with the PStBr and PMABr having substantially different KATRP values, with PSt radicals initially being formed and trapped by the MNP and the PMA radicals being trapped by the in situ‐formed nitroxide end‐capped PSt. The midchain alkoxyamine functionality was confirmed by thermolysis of the diblock copolymer, resulting in recovery of the PSt segment and degradation of the PMA block at the relatively high temperatures (125 °C) required for thermal cleavage. A PSt‐PMA diblock formed by chain extenstion ATRP using PStBr as the macroinitiator (thus lacking the alkoxyamine between the PSt‐PMA segements) was inert to thermolysis. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3619–3626

Synthesis of Nitroxide End‐Labeled Polymers by Capturing Polystyrene Radicals with Spin Traps

“…Two radical traps, MNP and nitrosobenzne (NBz) were added in varying equivalents to redox active solutions of PSBr. Previous research by our group has shown that both MNP and/or NBz act as radical traps in ATRC‐like reactions leads to high amounts of coupling for end‐halogenated polystyrene (PS), poly(methyl methacrylate), and poly(methyl acrylate), producing either coupled polymer dimers or cyclic polymers. This so‐called RTA‐ATRC is summarized in Scheme , along with the desired nitroxide‐capped intermediate (boxed).…”

Effect of Trapping Agent and Polystyrene Chain End Functionality on Radical Trap-Assisted Atom Transfer Radical Coupling

“…For instance, PMMABr can be dimerized by RTA-ATRC using nitrosobenzene (NBz), due to disproportionation being the favored bimolecular termination pathway [18][19][20][21]. Selective coupling is also possible using RTA-ATRC by activating halogenated polymers with different KATRP values, preferentially trapping Step 1: Rate = k 1 …”

“…Step 2: Rate = k 2 PSBr the more stable polymer radical as a nitroxide-capped polymer before the other chain is activated and participates in the coupling reaction, ultimately creating diblock copolymers [1].…”

“…Percent monomer conversion values for ATRP reactions were determined by 1 H NMR on the raw mixture in CDCl3. 1 H NMR measurements were taken on a Varian (Palo Alto, CA, USA) 400 MHz FT (Fourier Transform)-NMR at room temperature.…”

“…Radical trap-assisted atom transfer radical coupling (RTA-ATRC) is simply ATRC performed with the inclusion of a radical trapping agent, altering both the mechanistic pathway and the kinetics of the coupling reaction [1][2][3]. In traditional ATRC (Scheme 1, top), polymer radicals are formed in the absence of monomer, forcing a radical-radical termination reaction that is second order with respect to chain end radical concentration [4][5][6].…”