Rational photocatalyst selection for efficient radical-polar crossover phosphonylation of -hydroxyphthalimide ester-containing polymers

Abstract

The photoredox-catalyzed postfunctionalization of polymers bearing N-hydroxyphthalimide (NHPI) ester moieties via radical-polar crossover (RPC) is a promising strategy for introducing phosphonate groups into polymer backbones, as we previously reported. However, the degree of functionalization (DOF) in the phosphonylation of methyl acrylate copolymers has been limited to 15% owing to competition between the RPC process and hydrogen atom transfer. In this study, we demonstrate that the rational selection of an organic photocatalyst on the basis of three key features, i.e., oxidation power, photostability, and excited-state lifetime, substantially improves the RPC efficiency in the phosphonylation of methyl acrylate copolymers containing NHPI ester moieties. Systematic screening of eight organic photocatalysts revealed that 2,4,5,6-tetrakis(3,6-di-tert-butyl-9H-carbazol-9-yl)isophthalonitrile (4CzIPN-tBu), operating via a reductive quenching mechanism, affords a DOF of 28%. The approach was extended to various phosphite nucleophiles and NHPI estercontaining polymer substrates, all of which afforded higher DOFs than those reported in our previous work. In addition, computational studies revealed for the first time the role of neighboring group participation in RPC-based polymer reactions: the ester groups of the methyl acrylate units form stable cyclic intermediates with the carbocation intermediates generated via RPC, effectively suppressing undesired back electron transfer and enabling efficient phosphonylation even from inherently unstable secondary carbocations. This study establishes rational design principles for enhancing RPC efficiency in postfunctionalization reactions, thereby contributing to the development of a broader range of functional polymer architectures.