Controlled/“Living” Radical Polymerization. Atom Transfer Radical Polymerization of Acrylates at Ambient Temperature

TLDR: In this article, the authors used a multidentate amine ligand, tris[2-(dimethylamino)ethyl]amine (Me6TREN, Figure 1).

Abstract: Significant progress has been made in controlled/ “living” radical polymerization in recent years.1 Several new methodologies have been developed to provide control over both molecular weight and molecular weight distribution. One of the approaches is the transition metal catalyzed atom transfer radical polymerization (ATRP).2-8 Of all the transition metals investigated, copper appears to be the most promising in terms of price and versatility. A variety of monomers, including styrene9 and its derivatives,10 (methyl)acrylic esters,11,12 and acrylonitrile,13 can be polymerized in a well-controlled fashion with degree of polymerization (DP) predetermined by the ratio of the change in monomer concentration and initial initiator concentration (DP ) ∆[monomer]/[initiator]o) with polydispersities (Mw/Mn) as low as 1.05. Recently we have reported the use of multidentate amines as the ligands for copper mediated ATRP.14 Multidentate amines are less expensive, the copper complexes with these ligands generate less color to the system, and rates of polymerization are faster compared to complexes with 2,2′-bipyridine and its derivatives. In this paper, we report an improved ATRP system for the polymerization of acrylates using a new multidentate amine ligand, tris[2-(dimethylamino)ethyl]amine (Me6TREN, Figure 1). The new system allows for the wellcontrolled polymerization of acrylates at ambient temperature with very fast polymerization rates. Me6TREN can be easily prepared in a one-step synthesis using the literature procedure from commercially available tris(2-aminoethyl)amine (TREN).15 When methyl acrylate (MA) was polymerized using ethyl 2-bromopropionate (2-EBP) as the initiator and CuBr/Me6TREN as the catalyst (1.0 equiv to initiator) in a sealed tube at room temperature, the polymerization reached 85% conversion within 15 min to yield poly(methyl acrylate) (PMA) of Mn,SEC ) 21 600 (Mn,Cal ) 17 000) and Mw/Mn ) 1.18. The polymerization was exothermic, causing the solution inside the tube to boil. Better heat transfer was achieved when the polymerization was carried out in a water bath. The representative results for the ATRP of acrylates using CuBr/ Me6TREN as the catalyst are summarized in Table 1. The data in Table 1 indicates that controlled polymerizations were achieved with different catalyst-to-initiator ratios and that all the polymerizations showed high initiator efficiencies (initiator efficiency f ) Mn,Cal/ Mn,SEC) and low polydispersities. Significantly, a catalystto-initiator ratio of 0.1 was enough to prepare PMA with a polydispersity as low as 1.09. With a decrease of the catalyst-to-initiator ratio, the rate of polymerization decreased and the initiator efficiency slightly increased. The relatively lower initiator efficiency at higher catalystto-initiator ratio was likely due to the higher radical concentration at the beginning of the polymerization which resulted in the termination of some of the propagating chains by radical coupling reactions. Other acrylates, such as butyl acrylate (Table 1, entry 5), were also polymerized under similar conditions to yield welldefined polymers with low polydispersities. The employment of unmethylated TREN as the ligand resulted in the uncontrolled polymerization of MA with a much higher than predicted molecular weight and high polydispersity (Table 1, entry 6). When polymerization of MA was carried out using 2-EBP as the initiator and 0.2 equiv of CuBr/Me6TREN to initiator at 22 °C, the reaction solution quickly became viscous, and the conversion of MA reached 80% within 2 h. The reaction mixture was light green and homogeneous at the beginning, but turned heterogeneous at higher conversions. The heterogeneity was likely due to the low solubility of copper(II), generated by irreversible chain termination reactions, in the polymerization solution. A linear plot of ln([M]o/[M]) vs time was observed throughout the polymerization, indicating a constant number of growing chains (Figure 2). The molecular weight of the resulting PMA increased linearly with conversion, and the measured molecular weights were close to the calculated values Figure 1. Structure of Me6TREN.