Poly(methyl methacrylates) with Ruthenium Tris(bipyridine) Cores via NiBr₂(PR₃)₂-Catalyzed Atom Transfer Radical Polymerization (ATRP)

Abstract

Ruthenium(II) tris(bipyridine) complexes derivatized with six α-bromoester functional groups were used as multifunctional initiators for the bulk and solution (toluene) polymerizations of methyl methacrylate (MMA) and methyl acrylate (MA) using NiBr2(PPh3)2 and NiBr2(PBu3)2 as atom transfer radical polymerization (ATRP) catalysts. Both tertiary (1) and secondary (2) halide metalloinitiators were prepared for investigation. The resulting orange-colored polymers were characterized by gel permeation chromatography (GPC) in chloroform. The molecular weight distributions of the poly(methyl methacrylates) (PMMAs) were narrow (toluene, Mw/Mn = 1.1−1.3; bulk, Mw/Mn < ∼1.3 at <50% conversion). The presence of [Ru(bpy)3]2+ in the eluting polymers was verified by in-line, diode-array UV/vis spectroscopic analysis. For polymerizations of MMA in toluene at 80 °C using the tertiary initiator and NiBr2(PPh3)2 as the catalyst, Mn vs percent conversion and pseudo first-order kinetics plots were linear. Thus, these conditions were used to prepare a series of Ru-centered PMMA stars with molecular weights ranging from 2800 to ∼350 000. The linear polymer arms were cleaved from the Ru cores using NaOMe. Polydispersities of cleaved linear PMMA arms were low (∼1.10−1.24) and Mnstar/Mnarm ≈ 6, further confirming the efficiency of initiation and controlled nature of these reactions. As the percent toluene relative to monomer was reduced, control diminished. Likewise, all other polymerizations carried out with varied combinations of initiator (secondary or tertiary), monomer (MA or MMA), Ni catalyst (Ph or Bu), and reaction conditions (bulk vs solution) were not “living.” Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) of materials revealed no evidence of chain transfer and termination processes.