Constructing A Green Cascade Reaction For Direct Transformation Of Epoxide To Polythioether

Polythioether is a class of sulfur-rich polymers with superior electrical,mechanical,and optical properties,which rendered it promising applications in storage,optical device fabrication,rechargeable batteries,etc.However,Conventional synthetic routes to polythioether almost involve environmental pollution,high energy consumption,and demanding requirements for reaction equipment which impeded the further advancement of polythioether.In this paper,a novel green cascade reaction（O-S ER/AAROP）was constructed to precisely synthesize the well-defined polythioethers using epoxides and commercially available thiocyanates as raw materials.This method involved the oxygen-sulfur exchange reaction（O-S ER）between epoxides and thiocyanates,followed by aqueous anionic ring-opening polymerization（AAROP）of the in-situ generated episulfides.In addition,high-yield and high-molecular-weight polythioethers can be obtained by adding organic ammonium salts as catalysts or utilizing water-soluble epoxides as monomers.Finally,a series of polythioethers with diverse side chain structures were prepared through O-S ER/AAROP reaction,by designing and synthesizing various monomer structures.The structure of polythioethers were characterized by nuclear magnetic resonance(¹H NMR,¹³C NMR),elemental analysis（EA）and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry（MALDI-TOF）.The reaction conditions and process of O-S ER/AAROP reaction were investigated by in situ nuclear magnetic resonance spectroscopy,pH value detection and comparison experiment,and then the reaction mechanism was reasonably speculated.The main research content and results are presented as follows:A green cascade reaction O-S ER/AAROP reaction was developed for the precise synthesis of well-defined poly（hydroxyl thioether）.This reaction was conducted under an air atmosphere and in an aqueous medium,employing epoxide with hydroxyl functional groups as the monomer and thiocyanate as the sulforeagent.The monomers and polythioethers were characterized by NMR spectra,IR analysis,GPC measurements and elemental analysis to determine their structural properties.The reaction mechanism of O-S ER/AAROP reaction was thoroughly analyzed and studied by closely monitoring the dynamic reaction process using ¹H NMR spectroscopy.The mechanical properties of PpHMPT were preliminarily investigated through stress-strain tests,suggesting its potential as a novel soft thermoplastic elastomer.Additionally,relying on the hydrogen bonds between the side chains,the polythioether exhibited exceptional thermal stability as confirmed through TGA and DSC analysis.Based on the constructed O-S ER/AAROP reaction,the polythioether with high molecular weight（13.3 kDa）and yield（90%）was prepared by adding organic ammonium salt as catalyst to improve the polymerization efficiency.The impact of different structures of organic ammonium salts on the O-S ER/AAROP reaction was investigated using a controlled variables approach.It was observed that organic ammonium salts serve two functions in the O-S ER/AAROP process.Firstly,they act as phase transfer catalysts to enhance monomer solubility and consequently increase polymer yield.Secondly,they provide large-volume cations to augment the nucleophilicity of S^-anions,thereby improving polymerization efficiency.By O-S ER/AAROP reaction,a linear polyhydroxythioether with side hydroxyl group was synthesized using commercially available water-soluble epoxides（glycidyl）as monomer,KSCN as thioreagent and initiator.Its molecular weight was up to 58.6kDa,which is the largest molecular weight obtained by anionic polymerization in aqueous phase at present.The structure of the poly（hydroxyl thioether）was characterized using NMR,GPC,and elemental analysis,confirming the successful synthesis of linear linear poly（hydroxyl thioether）.Through end-capping reaction and synthesis of block copolymers,the initiating effect of thiocyanate anions and the sulfur anion structure at the growing chain active end were confirmed.With the utilization of in situ nuclear magnetic resonance hydrogen spectroscopy,the rationality of the mechanism for O-S ER/AAROP reaction was further verified.Finally,based on the structural characteristics of the resultant poly（hydroxyl thioether）,its adhesive properties were investigated,and it was observed that this polymer exhibited excellent adhesion to glass,wood,and metal substrates.Notably,due to the presence of sulfur atoms in the polymer’s main chain,its adhesion to metals stood out prominently and surpassed that of hydrogel adhesives with similar structures.By reducing the reaction temperature and precisely controlling the oxygen-sulfur exchange stage of O-S ER/AAROP,hydroxyl-containing cyclic sulfide intermediates were isolated and employed as monomers to investigate the optimal reaction conditions for AAROP.Comparative experiments demonstrated that the presence of a strong alkaline environment or the addition of organic ammonium salts could activated the C-S bond in episulfide,thereby facilitating SCN^-initiated AAROP reaction.Based on this finding,a more comprehensive understanding of the reaction characteristics of O-S ER/AAROP has been achieved,further elucidating the AAROP reactions mechanism of episulfide.After elucidating the mechanism of O-S ER/AAROP reaction,a series of epoxide monomers with diverse functional groups（including pyrenyl,ethoxy,carboxyl,and imino amino）were synthesized,and then investigated their propensity for undergoing O-S ER/AAROP reactions.It was observed that in the absence of hydroxyl groups in the epoxide monomers,organic ammonium salts needed to be added to activate the C-S bond and promote polymerization.Through various analytical techniques such as nuclear magnetic resonance(¹H NMR,¹³C NMR),mass spectrometry,infrared spectroscopy（IR）and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry（MALDI-TOF）,confirmed that polythioethers with different functional groups has been prepared,which created the conditions for realizing the functional application of polythioethers.