Polymerization of 2-Ethyl-2-oxazoline Using Di-, Tetra-, and Hexafunctional Ruthenium Tris(bipyridine) Metalloinitiators

Ruthenium reagents with two, four, and six chloromethyl functionalities, [(bpy) n Ru{bpy(CH 2 Cl) 2 } 3 - n ](PF 6 ) 2 ( n = 0−2), were employed as multifunctional metalloinitiators for the cationic polymerization of 2-ethyl-2-oxazoline to generate orange, glassy polymers with narrow molecular weight (MW) distributions. MWs were determined by GPC vs PMMA standards and for selected samples by GPC with multiangle laser light scattering (MALLS) detection. In-line diode array UV/vis spectroscopic analysis coupled with GPC MW determination confirms the presence of [Ru(bpy) 3 ] 2+ chromophores in the eluting polymers. Polymers are luminescent, and they exhibit thermal properties analogous to the metal-free PEOX counterparts ( T g ∼ 54 °C; onset of thermal decomposition at 365−385 °C). Unlike reactions run with a labile hexafunctional Fe initiator, reactions wherein MWs > 100K and degrees of polymerization (dp) >200 are attainable, Ru-centered polymers reach an upper MW limit of ∼25K regardless of the number of functionalities on the initiator (difunctional, dp = ∼125; tetrafunctional, dp = ∼63; hexafunctional, dp = ∼42 per initiator site). High monomer loadings and concentrations and elevated reaction temperatures were explored to surmount this barrier, and control experiments using different combinations of components found in typical Ru reaction mixtures are also described. Though M n vs percent conversion plots are roughly linear for the Ru initiators, observed MWs are lower than expected values based on monomer/initiator loading. This may be due to poor correlation of polyoxazolines with linear PMMA standards and/or to competing chain transfer side reactions during the polymerization. Linear first-order kinetics plots were obtained. Comparison of rate constants obtained from the slopes of these plots reveals the trend expected for the targeted structures: hexa > tetra > di.