Harnessing the power of the Fenton reaction in RAFT polymerization technique
AffiliationChemical and Biomolecular Engineering
Document TypePhD thesis
Access StatusThis item is embargoed and will be available on 2021-07-29.
© 2019 Dr. Amin Reyhani
Polymer scientists have been attempting to synthesize tailored polymers with predetermined molecular weight, composition, architecture, and molecular weight distribution by moving from traditional free radical polymerization (FRP) to reversible deactivation radical polymerization (RDRP) techniques, i.e. atom transfer radical polymerization (ATRP), reversible addition-fragmentation chain transfer (RAFT) polymerization, and nitroxide-mediated polymerization (NMP). Redox-activated FRP initiated by a classical chemical reaction (i.e., Fenton reaction) has been extensively used for the synthesis of different monomers and fabrication of various polymeric materials. Fenton reaction describes the reduction process of hydrogen peroxide (H2O2) by ferrous ions (Fe2+), generating highly reactive hydroxyl radicals. Despite its unique features such as high reaction speed and cheap reagents, Fenton chemistry was not employed for the initiation of RDRP methods. This thesis proposes the application of the Fenton reaction for initiating the RAFT technique at ambient conditions. The presentation is organized by the manipulation of H2O2 and Fe2+ sources and type of monomers. This work leads to the synthesis of well-controlled linear homo- and co-polymers with different polymer chain lengths. In the 1st part, the Fenton reaction was introduced to the RAFT process for the first time, termed as Fenton-RAFT polymerization, as an “on-demand” chain growth method. The ultra-fast Fenton–RAFT technique resulted in well-defined hydrophilic polymers with high monomer conversions (~ 75%) within 1 min at room temperature. The study on the polymerization rate and polymers’ characteristics in the presence of air, showed oxygen-tolerance of the Fenton-RAFT system with good control over polymers’ size. The 2nd part of this study directly addressed the drawbacks of Fenton-RAFT process (i.e., non-affordable full conversions and presence of metal ions) by replacing synthetic H2O2 and inorganic source of Fe2+ (i.e., ammonium ferrous sulphate) with two proteins, i.e. glucose oxidase (GOx) and hemoglobin (Hb). Biologically activated Fenton-RAFT polymerization was termed as Bio-Fenton-RAFT. Since Bio-Fenton-RAFT successfully led to the synthesis of well-defined polymers with full conversion values in either water and biological media, a RAFT polymerization catalyzed by real red blood cells was attempted. The Bio-Fenton-RAFT and blood-catalyzed Fenton-RAFT systems showed excellent tolerance towards oxygen and control over polymer chain lengths. In the 3rd part, we took advantage of the capability of the catalyst system (i.e., GOx/Hb) via the Bio-Fenton-RAFT process in supplying initiating radicals to make ultra-high molecular weight (UHMW) polymers. This technique led to the synthesis of unprecedented large, well-controlled UHMW polymers with a molecular weight as 20 × 106 g mol-1. The amount of GOx-generated H2O2 was detected to be ~ 3 mM in reaction solution within 2 h. To reduce the reaction time, Hb was replaced with ammonium ferrous sulphate. Synthetic H2O2 was carefully and gradually injected and added to the reaction solution by using a syringe pump, leading to well-defined UHMW polymers after 2 h. Such controlled production of initiating radicals offers unique access to predefined UHMW polymer materials via other RAFT processes. In the 4th part, I attempted to address the challenge (i.e., catalyst residuals) I faced in all previous Fenton-RAFT processes. To do so, I developed heterogeneously catalyzed Fenton-RAFT technique by using Fe2+ metal-organic framework (MOF) particles, termed as MOF-Fenton-RAFT. In this methodology, synthetic H2O2 was used, and ammonium ferrous sulphate, commercial Hb, and native Hb were replaced with heterogeneous catalysts, Fe2+ MOF particles. The obtained results demonstrated that MOF-Fenton-RAFT is pH-dependent, and acid-bearing RAFT agents play an important role in the rate of RAFT polymerization.
KeywordsFenton reaction; RAFT polymerization; reversible deactivation radical polymerization; polymer synthesis; redox reaction
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