Visible Light Photoiniferter Polymerization for Dispersity Control in High Molecular Weight Polymers

Radical Deamination of Primary Amines for Initiation of Controlled Polymerization

Selectively initiating controlled polymerizations using common functional groups is a powerful route to synthesizing advanced polymer architectures. Amines are one of the most common functional groups in small molecules, pharmaceuticals, and biomolecules, and thus are valuable substituents to use for initiating controlled polymerizations. In this study, we present the facile initiation of a controlled radical polymerization from the α-carbon of a primary amine via an electron donor-acceptor (EDA) complex-triggered radical deamination. Through this method, polymers were successfully grafted from a variety of amino acid derivatives. The resulting polymers had good matching between theoretical and experimental molar masses, narrow molar mass distributions (Đ ∼ 1.1-1.2), and exceptional α-chain end fidelities. This method was trialed on a model dipeptide, demonstrating the viability of EDA-RAFT in the synthesis of peptide-polymer conjugates.

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 (Mn ≈ 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.

Acid-Enhanced Photoiniferter Polymerization under Visible Light

Photoiniferter (PI) is a promising polymerization methodology, often used to overcome restrictions posed by thermal reversible addition-fragmentation chain-transfer (RAFT) polymerization. However, in the overwhelming majority of reports, high energy UV irradiation is required to effectively trigger photolysis of RAFT agents and facilitate the polymerization, significantly limiting its potential, scope, and applicability. Although visible light PI has emerged as a highly attractive alternative, most current approaches are limited to the synthesis of lower molecular weight polymers (i.e. 10,000 g/mol), and typically suffer from prolonged reaction times, extended induction periods, and higher dispersities when high activity CTAs (photoiniferters), such as trithiocarbonates, are employed. Herein, an acid-enhanced PI polymerization is introduced that efficiently operates under visible light irradiation. The presence of small amounts of biocompatible citric acid fully eliminates the lengthy induction period (21 hours) by enhancing photolysis, rapidly consuming the CTA, and accelerating the reaction rate, yielding polymers with narrow molar mass distributions (Ð ~1.1), near-quantitative conversions (>97 %), and high end-group fidelity in just two hours. A particularly noteworthy aspect of this work is the possibility to target very high degrees of polymerization (i.e. DP=3,000) within short timescales (i.e. less than five hours) without compromising the control over the dispersity (Ð ~1.1). The versatility of the technique is further demonstrated through the synthesis of well-defined diblock copolymers and its compatibility to various polymer classes (i.e. acrylamides, acrylates, methacrylates), thus establishing visible-light PI as a robust tool for polymer synthesis.

Low-Viscosity Route to High-Molecular-Weight Water-Soluble Polymers: Exploiting the Salt Sensitivity of Poly(N-acryloylmorpholine)

We report a new one-pot low-viscosity synthetic route to high molecular weight non-ionic water-soluble polymers based on polymerization-induced self-assembly (PISA). The RAFT aqueous dispersion polymerization of N-acryloylmorpholine (NAM) is conducted at 30 °C using a suitable redox initiator and a poly(2-hydroxyethyl acrylamide) (PHEAC) precursor in the presence of 0.60 M ammonium sulfate. This relatively low level of added electrolyte is sufficient to salt out the PNAM block, while steric stabilization is conferred by the relatively short salt-tolerant PHEAC block. A mean degree of polymerization (DP) of up to 6000 was targeted for the PNAM block, and high NAM conversions (>96%) were obtained in all cases. On dilution with deionized water, the as-synthesized sterically stabilized particles undergo dissociation to afford molecularly dissolved chains, as judged by dynamic light scattering and 1H NMR spectroscopy studies. DMF GPC analysis confirmed a high chain extension efficiency for the PHEAC precursor, but relatively broad molecular weight distributions were observed for the PHEAC-PNAM diblock copolymer chains (Mw/Mn > 1.9). This has been observed for many other PISA formulations when targeting high core-forming block DPs and is tentatively attributed to chain transfer to polymer, which is well known for polyacrylamide-based polymers. In fact, relatively high dispersities are actually desirable if such copolymers are to be used as viscosity modifiers because solution viscosity correlates closely with Mw. Static light scattering studies were also conducted, with a Zimm plot indicating an absolute Mw of approximately 2.5 × 106 g mol-1 when targeting a PNAM DP of 6000. Finally, it is emphasized that targeting such high DPs leads to a sulfur content for this latter formulation of just 23 ppm, which minimizes the cost, color, and malodor associated with the organosulfur RAFT agent.