“Greener” Coupling of Poly(methyl methacrylate) and Poly(methyl acrylate) Chains using Activators Generated by Electron Transfer and Radical Traps

Abstract

Monobrominated versions of poly(methyl methacrylate) (PMMABr) and poly(methyl acrylate) (PMABr) are prepared by atom transfer radical polymerization and subjected to a variety of chain end‐coupling reactions, with the goal of achieving high extents of coupling with minimal metal content. By using atom transfer radical coupling (ATRC) with radical trap assistance in conjunction with activators generated by electron transfer, high extents of coupling (Xc) could be achieved for both polymer classes without requiring copper(I) as an initial reactant, and replacing elemental copper with environmentally benign reducing agents (Xc > 0.85). Specifically, the highest extents of coupling are observed with ascorbic acid as the reducing agent along with copper(II) bromide as the pre‐catalyst. Overall, similar extents of coupling are observed while using ≈1% of the total metal of a traditional ATRC reaction for PMABr and ≈3% for PMMABr.

Universal Chain-End Coupling Conditions for Brominated Polystyrenes, Polyacrylates, and Polymethacrylates

“…One difficulty involved is that two of the three polymers above resist coupling by radical-radical termination in traditional ATRC sequences; PMA and PMMA require intervention to induce chain-end coupling of their radicals. In the case of PMA, both styrene-assisted ATRC [18] or radical trap-assistance [19] have proven capable of leading to high extents of coupling. Similarly, the bulky PMMA radicals, which favor disproportionation, require radical trap-assistance to undergo substantial dimerization [12,19].…”

“…In the case of PMA, both styrene-assisted ATRC [18] or radical trap-assistance [19] have proven capable of leading to high extents of coupling. Similarly, the bulky PMMA radicals, which favor disproportionation, require radical trap-assistance to undergo substantial dimerization [12,19]. On the other hand, PS radicals can be successfully coupled into dimers [6,8,[20][21][22][23][24] or macrocycles [10,20,25] under a wider range of ATRC-type conditions and are less sensitive to the presence [21,22] or structure of a radical trap [26].…”