RAFT-photomediated PISA in dispersion: mechanism, optical properties and application in templated synthesis

Abstract

Diblock copolymer nanoparticles were prepared without initiator by visible-light photopolymerization-induced self-assembly (“photo-PISA”) in dispersion. Both copolymer blocks were synthesized via an original photomediated reversible deactivation radical polymerization mechanism combining reversible addition–fragmentation chain transfer (RAFT) and reversible photolysis of the C–S bond of the chain-transfer agent (CTA). The photosensitive CTA was a commercially available trithiocarbonate, namely cyanomethyl dodecyl trithiocarbonate. The first hydrophilic poly(hydroxyethyl acrylate) block was synthesized in solution, then used as a macro-CTA for the dispersion polymerization of styrene in methanol/water. Spherical nanoparticles with a narrow size distribution and an average diameter of 20–40 nm were achieved. The copolymers were characterized by size-exclusion chromatography (SEC) in DMF, which confirmed the control of the reaction, with relatively narrow molecular weight distributions (M_w/M_n ≤ 1.30). Two regimes can be discerned in the RAFT-mediated dispersion photo-PISA: a slow particle nucleation (4–15% conv.) followed by particle growth. A steady number of particles was obtained when steric stabilization was adequate. Study of the optical properties by UV-Vis spectrophotometry enabled to separate the respective contributions of absorption and scattering during the PISA process. Finally, we demonstrated that the copolymer nanoparticles could act as soft templates for the synthesis of mesoporous carbon materials.