Green Cationic Polymerization Based On Boron-Based Initiation System And It Mechanism

Cationic polymerization is an important branch of polymer synthesizing method,especially in the molecular design of polymers such as polyisobutylene and polyvinyl ethers.It has an irreplaceable position.Traditional cationic polymerization has been limited by factors such as water and oxygen intolerance of Lewis acids.It often requires extremely harsh conditions to achieve controlled/living polymerization.In recent years,green cationic polymerization methods,such as aqueous phase and ionic liquids have been emerging,giving new vitality to traditional cationic polymerization.Triphenyltetrafluoroborates（[Ph₃C][B（C₆F₅）₄]）and B（C₆F₅）₃ are resistant to water and oxygen,making them suitable for initiating green cationic polymerization.This thesis focuses on the study of vinyl ether cationic polymerization using these two types of boron-based initiators for vinyl ether cationic polymerization,as described below:This thesis investigated the cationic polymerization characteristics of[Ph₃C][B（C₆F₅）₄]-initiated isobutyl vinyl ether（IBVE）by dual active centers and explored the polymerization mechanism.The green controlled cationic polymerization of IBVE with aqueous system was achieved under room temperature and air atmosphere.Firstly,the activation energy of the polymerization reaction was calculated to be negative by changing the polymerization reaction temperature.Thus the mechanism of[Ph₃C][B（C₆F₅）₄]-initiated isobutyl vinyl ether was verified to be consistent with the cationic polymerization.Next,the molecular weight and molecular weight distribution of the polymer obtained under different polymerization atmospheres were investigated.It was found that unlike conventional cationic polymerization in which water causes chain transfer of the active center to H₂O molecule,the initiation system was mainly influenced by the reversible equilibrium between[Ph₃C][B（C₆F₅）₄]and H₂O.Within this reaction system,IBVE is initiated by double active centers.As the water content increases,it drives the reversible equilibrium to the right,causing a large exothermicity leading to the occurrence of severe chain transfer reactions.The existence of the reversible equilibrium was verified by ¹H-NMR,¹³C-NMR,MALDI-TOF-MS and with the aid of density functional theory（DFT）.The mechanism of the polymerization of IBVE initiated by the double active centers was also verified.Controlled cationic polymerization of the aqueous system under air atmosphere was achieved by adjusting different polymerization conditions and adding a third component to the reaction system to suppress the reactivity of carbon cations.In the second part,the mechanism of[Ph₃C][B（C₆F₅）₄]-initiated cationic polymerization of styrene and its derivatives was investigated.The polymerization mechanism was investigated by varying the reaction time,reaction temperature,solvent polarity and other factors.It was found that the mechanism of[Ph₃C][B（C₆F₅）₄]-initiated cationic polymerization of styrene and its derivatives was different from that of[Ph₃C][B（C₆F₅）₄]initiated IBVE with two active centers.When the cationic reactivity of the monomer is lower than that of styrene,the stable tertiary carbon cation[Ph₃C⁺]cannot initiate the polymerization of the monomer.Only[H⁺]initiates the polymerization.The p-methoxystyrene,p-tert-butylstyrene and p-methylstyrene side groups are electron-donating groups with high cationic reactivity,which will initiate the cationic polymerization with the dual active center simultaneously.Controlled cationic polymerization was achieved when acetonitrile（CH₃CN）was chosen as the reaction solvent.The terminal structure of the polymer was analyzed by ¹H-NMR and infrared spectroscopy.The dual active centers of[Ph₃C][B（C₆F₅）₄]were proposed to initiate the cationic polymerization of styrene and its derivatives.In the third part,aqueous-phase cationic polymerization of 2-hydroxyethyl vinyl ether（HEVE）and diethyleneglycol monovinyl ether（DGVE）and aqueous soap-free emulsion cationic polymerization of 4-hydroxybutyl vinyl ether（HBVE）were successfully achieved using a cull alcohol（Cum OH）/tris（pentafluorophenyl）boron（B（C₆F₅）₃）/ethyl ether（Et₂O）initiation system.The effects of reaction time,polymerization temperature and monomer concentration on the polymerization reaction were investigated.As the polymerization kinetics chain length increased,the active centers were embedded in the polymer chains,forming dead polymer chains with acetal ends,resulting in a"critical polymerization degree"effect.However,when the monomer is added to the system again,the hydroxyl-terminated polymer,which remains in the dormant state,complexes with B（C₆F₅）₃·Et₂O to form the active center.This active center is reversibly activated by the monomer and triggers the polymerization of the monomer once again.The terminal structure of the polymer was analyzed by ¹H-NMR.The mechanism of cationic polymerization of hydroxyvinyl ethers in aqueous solution was proposed.