Study On The Activation Effect Of The Hydrogen Bonding Interaction On Monomers And Controlled Radical Polymerization

Controlled radical polymerization(CRP) has been widely used for its advantages in synthesize polymers with controlled molecular weight, well-defined structure and high chain-end functionality. Hydrogen bonding is one of the most important intermolecular weak interaction and has been used in controlled radical polymerization for the dual control of molecular weight and tacticity or dual control of molecular weight and sequence, which has made the synthesis of precisely-controlled macromolecules possible. The hydrogen bonding has been formed between the polar groups of monomers and the polar protic solvent. It has been reported in many reports that many monomers has realized CRP in fluoroalcohols. Our research group reported the Cu(0)-mediated radical polymerization of 4VP in 1,1,1,3,3,3-hexafluoro-2-propanol(HFIP) and ascribe the good control of polymerization to the hydrogen bonding formed between 4VP and HFIP.In this article, based on the results of Cu(0)-mediated polymerization of 2-vinylpyridine, we studied the strong hydrogen bonding between HFIP and 2VP by careful 1H NMR analysis and computer simulations. Then, we employed the hydrogen bonding to the polymerization of other vinylpyridine monomers. Further more, we tried to employ the hydrogen bonding to the RAFT polymerization of less activated monomer N-vinylpyrrolidone(NVP). The detailed researches were summarized as the following:(1)The Cu(0)-mediated control radical polymerization(CRP) of 2-vinylpyridine(2VP) was realized using ethyl-2-chloro-2-phenylacetate(ECPA) as initiator in different kind of solvents with the absence of ligand at room temperature. Experimental result shows that in fluoroalcohols the polymerization conveyed reasonably controlled features, especially with HFIP as the solvent. 1H NMR analysis and computer simulations reveal the strong hydrogen bonding between HFIP and 2VP and the transfer of the electron density of the vinyl bond. The polymerization behavior of Cu(0)-catalyzed 2VP under different temperatures shows that with the increase of temperature, polymerization rate decreased and the polymerization conveyed uncontrolled manner. 1H NMR clearly shows that with the increase of temperature, the hydrogen bonding between 2VP and HFIP is somewhat destroyed. The hydrogen bonding between 4VP,3VP, 2VPM, 2VPZ and fluoroalcohol was also analyzed by 1H NMR and undertake polymerization in fluoroalcohol. Results also show the influence of hydrogen bonding to the polymerization. Thus we prove that the hydrogen bonding activated the 2VP monomer by the electron-withdrawing inductive effects and thus change the polymerization behavior of 2VP.(2) The radical polymerization of N-vinylpyrrolidone(NVP) was realized using 2-Cyanoprop-2-yl-1-dithionaphthalate(CPDN) as the chain transfer agent and 2,2-Azobisisobutyronitrile(AIBN) as initiator in different kind of solvents at 60 oC. Experimental result shows that polymerization rate was higher in fluoroalcohols than in common solvents(e.g. isopropanol) or bulk polymerization. With HFIP as solvent, the polymerization conveyed reasonably controlled features with controllable molecular weights and relatively narrow molecular weight distributions. However, the molecular weight distribution was broader(Mw/Mn>1.40) in other solvents. 1H NMR analysis and computer simulations reveal the strong hydrogen bonding between HFIP and NVP and the transfer of the electron density of the vinyl bond at 60 oC. The polymerization behavior of CPDN-mediated NVP in HFIP shows that at lower conversion(conversion<60%), the polymerization progressed in a controlled manner. However, control of polymerization in other solvents does not show any improvement. The chain end functionality calculated from the 1H NMR spectrum was 75%, indicating that the PNVP was end-capped by CPDN species with high fidelity. The above polymerization results clearly demonstrate that the hydrogen bonding activated the NVP monomer through electron-withdrawing inductive effects and thus change the polymerization behavior of NVP.