Impact of Alkyl Core Substitution Kinetics in Diaryl Dihydrophenazine Photoredox Catalysts on Properties and Performance in O-ATRP.

Abstract

Organocatalyzed atom transfer radical polymerization (O-ATRP) is a controlled radical polymerization method mediated by organic photoredox catalysts (PCs) for producing polymers with well-defined structures. While N,N-diaryl dihydrophenazine PCs have successfully produced polymers with low dispersity (Đ < 1.3) in O-ATRP, low initiator efficiencies (I* ~ 60-80%) indicate an inability to achieve targeted molecular weights and have been attributed to the addition of radicals to the PC core. In this work, we measure the rates of alkyl core substitution (AkCS) to gain insight into why PCs differing in N-aryl group connectivity exhibit differences in polymerization control. Additionally, we evaluate how PC properties evolve during O-ATRP when a non-core-substituted PC is used. PC 1 with 1-naphthyl groups in the N-aryl position resulted in faster AkCS (k 1 = 1.21 ± 0.16 × 10-3 s-1, k 2 = 2.04 ± 0.11 × 10-3 s-1) and better polymerization control at early reaction times as indicated by plots of molecular weight (number average molecular weight = M n) vs conversion compared to PC 2 with 2-naphthyl groups (k 1 = 6.28 ± 0.38 × 10-4 s-1, k 2 = 1.15 ± 0.07 × 10-3 s-1). The differences in rates indicate that N-aryl connectivity can influence polymerization control by changing the rate of AkCS PC formation. The rate of AkCS increased from the initial to the second substitution, suggesting that PC properties are modified by AkCS. Increased PC radical cation (PC•+) oxidation potentials (E 1/2 = 0.26-0.27 V vs SCE) or longer triplet excited-state lifetimes (τ T1 = 1.4-33 μs) for AkCS PCs 1b and 2b compared to parent PCs 1 and 2 (E 1/2 = 0.21-0.22 V vs SCE, τ T1 = 0.61-3.3 μs) were observed and may explain changes to PC performance with AkCS. Insight from evaluation of the formation, properties, and performance of AkCS PCs will facilitate their use in O-ATRP and in other PC-driven organic transformations.