Diminishing catalyst concentration in atom transfer radical polymerization with reducing agents

Abstract

The concept of initiators for continuous activator regeneration (ICAR) in atom transfer radical polymerization (ATRP) is introduced, whereby a constant source of organic free radicals works to regenerate the Cu(I) activator, which is otherwise consumed in termination reactions when used at very low concentrations. With this technique, controlled synthesis of polystyrene and poly(methyl methacrylate) (Mw/Mn < 1.2) can be implemented with catalyst concentrations between 10 and 50 ppm, where its removal or recycling would be unwarranted for many applications. Additionally, various organic reducing agents (derivatives of hydrazine and phenol) are used to continuously regenerate the Cu(I) activator in activators regenerated by electron transfer (ARGET) ATRP. Controlled polymer synthesis of acrylates (Mw/Mn < 1.2) is realized with catalyst concentrations as low as 50 ppm. The rational selection of suitable Cu complexing ligands {tris[2-(dimethylamino)ethyl]amine (Me6TREN) and tris[(2-pyridyl)methyl]amine (TPMA)} is discussed in regards to specific side reactions in each technique (i.e., complex dissociation, acid evolution, and reducing agent complexation). Additionally, mechanistic studies and kinetic modeling are used to optimize each system. The performance of the selected catalysts/reducing agents in homo and block (co)polymerizations is evaluated.

Scheme 1.

Proposed mechanism for ATRP processes.

Fig. 1.

Kinetic plot (Left) and molecular weight (Lower Right) and M_w/M_n (Upper Right) as a function of conversion in the ICAR ATRP of St with 50 ppm and 1 ppm Cu. St/EtBrIB/CuCl₂/Me₆TREN/AIBN = 200/1/0.01/0.01/0.1; St/EtBrIB/CuCl₂/TPMA/AIBN = 200/1/0.0002/0.1/0.1; [St]₀ = 5.82 M; 60°C; 50% anisole by volume (entries 1 and 11 in Table 1).

Fig. 2.

predici simulation of kinetic plots for ICAR ATRP of St employing TPMA, PMDETA, or bpy with 50 ppm Cu and AIBN as the free radical initiator.

Fig. 3.

Kinetic plot (Left) and molecular weight (Lower Right) and M_w/M_n (Upper Right) as a function of conversion in the CuCl₂/TPMA-mediated ARGET ATRP of BA, with variable hydrazine (N₂H₄) reducing agent. [BA]₀/[EtBrIB]₀/[CuCl₂]₀/[TPMA]₀/[N₂H₄]₀ = 200/1.28/0.01/0.1/0.05 or 0.1; [BA]₀ = 5.88 M; 60°C; 20% anisole by volume (entries 8 and 9 in Table 2).

Fig. 4.

GPC traces after each step in synthesis of block copolymer PSt-b-PBA. Experimental conditions for ICAR ATRP of St with 50 ppm Cu catalyst (solid line): St/EtBrIB/Cu^II/TPMA = 200/1/0.01/0.1; [St]₀ = 5.82 M; 110°C; 50% anisole by volume. Experimental conditions for ARGET ATRP of BA with 50 ppm Cu catalyst (dashed line): BA/PSt/Cu^II/Me₆TREN/PhNHNH₂ = 400/1/0.02/0.1/0.1; [BA]₀ = 4.90 M; 60°C; 50% dimethylformamide by volume.