Synthesis of Star Polymers with Ultrahigh Molecular Weights and Tunable Dispersities via Photoiniferter Polymerization

Simultaneous control over macromolecular chain topology, molecular weight, and dispersity is an important synthetic goal in polymer chemistry. The synthesis of well-defined poly(methyl acrylate) star polymers with ultrahigh molecular weights (>10⁶ g mol^–1) and tunable dispersities is realized for the first time via blue light-controlled photoiniferter polymerization using a tetrafunctional switchable RAFT agent (SRA₄). The spectroscopic properties and polymerization activity of SRA₄ can be reversibly tuned by addition of acid/base. For example, protonation of SRA₄ with 4-toluenesulfonic acid (TsOH) leads to enhanced UV–visible light absorption, a faster polymerization rate, and a lower dispersity for the resulting star polymer. Star polymers were prepared with predicted molecular weights (M_n ≈ 80–1550 kg mol^–1) and tunable dispersities (Đ ≈ 1.8–1.2) when targeting degrees of polymerization in the range of 1000–20000 in the presence of varying amounts of TsOH. High end-group fidelity for such star polymers was confirmed by one-pot chain extension experiments, which afforded a series of pseudoblock copolymers with controlled dispersities. Finally, rotational rheology was used to examine the effect of molecular weight, dispersity, and chain topology (whether linear or star-shaped) on solution viscosity.