Cationic Polymerization Of Vinyl Monomers In Ionic Liquids And Their Polymerization Mechanism

Ionic liquid (IL) is a new kind of molten salt, which is composed of anion and cation. They have many unique physical and chemical properties and serve as clean, green and environmentally friendly solvents. Now ILs are alternative to traditional solvents to be applied to various fields, where ILs show their unique advantages. However, the cationic polymerization in ILs is limited to styrene, and their polymerization mechanism is still vague. In this thesis, cationic polymerizations of typical vinyl monomers in ILs were studied.The cationic polymerization mechanism was proposed by the density functional theory (DFT) and the chain end structures of polymers. In addition,aiming at the weakness of the concentrated emulsion templating whose monomer is single, and porous monoliths synthesized by which are hard and brittle, the porous elastomeric polyurethane monolith with pressure-sensitive conductive was synthesized by concentrated emulsion templating. The work expands the monomers and opens a new application field of the concentrated emulsion templating. Based on the obove contents, the main research content is as follows:(1) The solubility of monomer in IL is the key problem whether cationic polymerization could proceed successfully in IL medium. In the second chapter, the solubilities of three representative monomers in cationic polymerization of p-methylstyrene (p-MeSt), isobutyl vinyl ether (IBVE), and isobutylene (IB) are calculated using conductor-like screening model for real solvents (COSMO-RS). To understand the solubilities behavior of monomers in ILs in detail, the distribution of the screening charge density on the surface and monomer-IL interaction energies were analyzed. Thus, the guiding principles of the selection of ILs medium for the vinyl monomers of cationic polymerization were proposed. Firstly, less polar cation family of IL has higher van der Waals energy (EvdW) and misfit energy and has higher monomer solubility in IL. Secondly, the alkyl chain of cation is longer, the nonpolarity and Evdw are stronger, thus resulting in the higher solubility of monomer in IL.Thirdly, the anion alkyl chain is longer, the nonpolarity and Evdw are stronger,and thus the monomer solubility in IL is higher. Forthly, the regularity of the monomer solubility in IL with different nature anions is not obvious; yet the anion with smaller polarity has resultant higher monomer solubility in IL.(2) In the third chapter, cationic polymerization of p-MeSt with a CumOH(2-phenyl-2-propanol)/BF3OEt2 initiating system in imidazolium-based [NTf2-1] ILs was investigated, where the ILs were 1 -butyl-3 -methylimidazolium bis(trifluoromethanesulfonyl)imide([Bmim] [NTf2]), 1 -hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([Hmim] [NTf2]),1 -octyl-3 -methylimidazolium bis(trifluoromethanesulfonyl)imide([Omim][NTf2]). The polymerization in ILs was compared with that in the traditional solvent CH2Cl2. Moreover, the interactions between ILs and propagating carbocation were performed by DFT. The results showed that the inert IL did not participate in the reaction and only acted as the solvent in the whole polymerization process. The p-MeSt cationic polymerization in ILs proceeded in a milder exothermic manner and lower polymerization rate than in CH2Cl2. ILs could interact with carbocationic species and stabilize the propagating carbocationic species through the moderation or delocalization of the positive charges, thus leading to a low polymerization rate. The p-MeSt cationic polymerization in [Bmim][NTF2] with the CumOH/BF3OEt2/DTBP(2,6-di-tert-butylpyridine) initiating system proceeded in a controlled manner,thus affording polymers with a number-average molecular weight (Mn) of about 3×104 g·mol-1 and a molecular weight distribution (Mw/Mn) of 1.4～1.8.According to the above results, we proposed a mechanism for the polymerization of p-MeSt in the NTf2-1-based ILs using the CumOH/BF3OEt2 initiating system.(3) In fourth chapter, cationic polymerization of IBVE in 1 -octyl-3 -methylimidazolium tetrafluoroborate ([Omim][BF4]) was thoroughly examined at 0 ℃. A variety of coinitiators were used in conjunction with IBVE-HCl adduct as a cationogen. The polymerization in[Omim][BF4] was compared with that in the traditional solvent CH2Cl2. The relationship between time and conversion, the first-order plot of monomer consumption, and the effect of the third group on the polymerization were investigated. It was found that the cationic polymerizations in [Omim][BF4]proceeded smoothly and there existed long-lived species in this system. The polymerization of IB VE in [Omim][BF4] could proceed in a controlled fashion by addition of DTBP, thus affording polymers with Mn of about 3 × 104 g·mol-1 and Mw/Mn of 1.4～1.8. The DFT results showed that [Omim][BF4] could interact with carbocationic species and could stabilize the propagating carbocationic species through the moderation or delocalization of the positive charges, which led to a lower stereoregulation. Finally, we proposed the corresponding elementary reactions of IBVE cationic polymerization in[Omim] [BF4].(4) In fifth chapter, the cationic copolymerization of IB and p-MeSt was carried out in [Hmim][NTf2]. The effects of the initiating systems, the monomer ratio in the feed, reaction time, the type and polarity of solvent on the copolymerization were studied. The results showed that the content of IB in the comonomers increased with the IB ratio in the feed, up to 40 mol%. The copolymerization was uncontrollable because of the chain transfer reaction.The ionic environment, the high polarity, and high viscosity of ILs probably were the main reasons for this uncontrollability. The products characterized by H1 NMR, DSC were random copolymers. The molecular weight need to further improve.(5) In sixth chapter, via chosening a reactive surfactant, the concentrated emulision templating technique was successfully applied to fabricate polyurethane foam whose pore size was controllable; when the acid-treated multi-walled carbon nanotubes (MWCNTs) were introduced, the polyurethane foam with pressure-sensitive conductive was obtained. Moreover, the effects of the composition, the surfactant, the volume fraction of dispersed phase,castor oil, and the amount of MWCNTs on the morphologies of porous PU were investigated. The results showed that the reaction between surfactant and oil phase could form the viscous barrier layer between the water phase and oil phase. This barrier layer was the key to a successful preparation of polyurethane by concentrated emulision templating. When the colloidal silica particles which could further stabilize the emulision were introduced, a more uniform and finer porous structure could be achieved. A decrease of two orders of magnitude was observed when MWCNTs was introduced; a compression strain of about 40% could cause a reduction in resistance by two orders of magnitude.