Light-Induced RAFT Single Unit Monomer Insertion in Aqueous SolutionToward Sequence-Controlled Polymers

First report on the sequential, visible light-initiated, single unit monomer insertion (SUMI) of N,N-dimethylacrylamide (DMAm) into the reversible addition fragmentation chain transfer (RAFT) agent, 4-((((2-carboxyethyl)thio)carbonothioyl)thio)-4-cyanopentanoic acid (CTA(1)), in aqueous solution is provided. The specificity for SUMI over formation of higher oligomers and/or RAFT agent-derived by-products is higher for longer irradiation wavelengths. Red light provides the cleanest product (selective SUMI), showing a linear pseudo-first order kinetic profile to high (>80%) conversion, but also the slowest reaction rate. Blue light provides a relatively rapid reaction, but also gives some by-products (<2%) and the kinetic profile displays a conversion plateau at >65% conversion. Higher specificity with red light is attributed to CTA(1) absorbing at longer wavelengths than the SUMI product, which allows selective excitation of CTA(1). The use of a higher reaction temperature (65 degrees C vs ambient) results in a higher reaction rate and a reduction in oligomer formation.