Cationic Polymerization Of Isobutylene In Ionic Liquids And Their Polymerization Mechanism

Low molecular weight(Mn=500-5000 gmol-1)olefin end functional PIB is an intermediate for the preparation of motor oil and fuel additives(such as surfactants or dispersants).High Reactivity polyisobutylene(HR PIB)with high exo-olefinic end-group content(>60%,or>75%much better)is mainly used to produce polyisobutenyl succinimide ashless dispersants.In recent years,the synthesis of HR PIB has attracted wide attentions.The synthesis of HR PIB is usually carried out in organic solvents or solvents containing chloroalkanes,however,these solvents are volatile,toxic and polluting the environment.Ionic liquids have many unique properties which are different from molecular solvents.They are widely used in organic synthesis,catalysis and macromolecular engineering,and are called "green solvents".In this thesis,the cationic polymerization of isobutylene(IB)was investigated by using ionic liquids as solvents.Based on polymer end group structures analysis and molecular simulation results,the polymerization mechanism of HR PIB in ionic liqiuds was proposed.The main work is as follows:1.The interactions between IB and eight ionic liquids based on imidazolium cations and isobutylene were simulated by density functional theory(DFT)at the GGA/PW91/DNP3.5 level.Based on the interaction of[AlCl4]-ionic liquids and IB,the suitable ionic liquids for IB cationic polymerization were preliminarily screened.According to the dissolution mechanism of small molecular compounds in ionic liquids,the dissolution mechanism of isobutylene in ionic liquids was proposed.The results show that the anions play a key role in the interaction between ionic liquids and IB,and the potential[PF6]-,[BF4]-and[Tf2N]-ionic liquids for IB polymerization are first screened out.On this basis,the dissolution mechanism of isobutylene in ionic liquids is proposed,that is,IB is filled in the larger internal pore of ionic liquids formed by small angular rearrangements of the anions.Confined by the free volume in ionic liquids nano-networks and the volume of isobutene,the solubility of isobutylene in ionic liquids is limited.2.On the basis of molecular simulation,2,4,4-trimethyl-2-chloropentane(TMPCl)/titanium tetrachloride(TiCl4)as an initiator system,were used to investigated the polymerization of IB in[Bmim][PF6],[Bmim][BF4]and[Bmim][Tf2N]ionic liquids at-10℃.[Bmim][PF6]was the best ionic liquid for IB polymerization.The characteristics of cationic polymerization of IB in traditional solvents were compared,and the regularity of IB cationic polymerization in[Bmim][PF6]was emphatically investigated.The polymer products were analyzed by 1H NMR,13C NMR and GPC.The results show that,compared with the traditional polymerization characteristics of CH2Cl2,the content of exo-olefin terminal group of polyisobutylene obtained in[Bmim][PF6]reached to 80%.The polymerization reaction is heterogeneous and stirring conditions have a great influence on the polymerization.The bimodal distribution in GPC of PIB products indicates that there are two kinds of initiation systems,namely TMPCl/TiCl4 and H20/TiCl4.3.The cationic polymerization of IB in[Bmim][PF6]was studied systematically.The effects of initiator concentration,concentration ratios of initiator to co-initiator,reaction time and types of initiation systems(including TiCl4,BC13 and ethyl sesquialuminium(Al2Et3Cl3)on the cationic polymerization of IB were investigated.Based on the conductor-like screening model(COSMO)and the calculation level of GGA/VWN-BP/DNP4.4,the effects of ionic liquids polarity and anion/cation structure on the stability of the initiation systems and the growth of carbocations were calculated by DFT method.The results reveal that the highly reactive polyisobutylene(HR PIB)with a high exo-olefin end group content(aoubt 80%)was synthesized using H20/TiCl4 initiation system in[Bmim][PF6].The polymerization is proceeded in a gentle exothermic manner at the interface between the ionic liquid particles and isobutylene droplets;[PF6]-anions played an important role in promoting ionization of initiation system and stabilizing the caborcation active center.The possible mechanism for the synthesis of HR PIBs was proposed by taking account of the microstructure of ionic liquids.4.Density functional theory was used to investigate the regulation mechanism of IB polymerization by[Bmim][PF6].The investigation involves three aspects,that is,the stability of the initiator and co-initiator,the isomerization of the growth of carbocation and the selectivity of polymerization products.The results show that[Bmim][PF6]structure is the main reason for the significant difference in the polymerization of isobutylene in CH2Cl2 and[Bmim][PF6].The nano-network structure of ionic liquid can provide a micro-reaction site for IB polymerization.The polymerization occurs on the interface of ionic liquid particles and IB droplets that are in contact with each other.The[PF6]-anion plays a major role in regulating the polymerization.The growth carboations is highly stable and ionic liquids inhibit the occurrence of various isomerization reactions.The exo-olefin structure has a lower solubility in[Bmim][PF6]than other polymer end group structures,which protects it from ionic liquid solvents.The regulation of elementary reaction of IB cationic polymerization by ionic liquid was through the electrostatic interaction of[PF6]-anion with initiators,co-initiators,growth carbocations,polymeric chain cations,and polymerization products.