Date of Award
Doctor of Philosophy
Materials Science and Engineering
Jimmy W. Mays
Bin Zhao, Joseph E. Spruiell, Roberto S. Benson
Conventional free radical polymerizations were carried out in a variety of room temperature ionic liquids (RTILs). Generally, methyl methacrylate (MMA) and styrene (St) were used as typical monomers to compare the polymerization behavior both in RTILs and in common volatile organic compound solvents (VOCs). In most cases, it was observed that both yields and molecular weights are enhanced in the RTIL. While we believe the “diffusion-controlled termination” mechanism makes the termination of the radical propagating chains difficult due to the highly viscous nature of RTIL, other researchers have suggested that the rapid polymerization rates are due to the high polarity of these reaction media.
By employing more than a dozen RTILs with a wide range of anions and cations, we attempted to correlate the viscosity and polarity of the RTILs with the molecular weights and polymerization rates. This correlation was not successful, suggesting that other parameters may also play a role in affecting the polymerization behavior.
Nitroxide-mediated living radical polymerizations of St and MMA were attempted in 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM]PF6). Neither the bimolecular initator system: benzoyl peroxide (BPO)+2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) or a unimolecular initiation system: 2,2,5-trimethyl-3-(1-phenylethoxy)-4-phenyl-3-azahexane (TMPPAH) gave a controlled polymerization, i. e: polydispersities were large and molecular weights could not be stoichiometrically controlled. Slow degradation of TEMPO at elevated temperatures and slow diffusion of the stable radical away from propagating chain may explain this non-living polymerization behavior.
One of RTILs investigated: trihexyl-tetradecyl-phosphonium bis(2,4,4-trimethylpentyl) phosphinate ([H3TDP][(PM3)2P]), forms a redox pair with a radical initiator such as BPO. The generated free radicals can efficiently initiate the polymerization of MMA at room temperature through a conventional free radical polymerization mechanism. The possible mechanism is the “ion cavity” formed through the ionic interaction and steric effects caused by both ions that probably enhance the reducing nature of the cation. This “ion cavity” can trap BPO inside and form the redox pair. High viscosities also contribute to the higher yield and molecular weight as compared with reaction in benzene under the same conditions (except at 70°C) due to diminished chain termination.
The formation of PSt-b-PMMA by sequential monomer addition through the standard free radical polymerization mechanism, using BPO as initiator, can be realized in [BMIM]PF6 due to the insolubility of polymerized first block – PSt in [BMIM]PF6. The macroradicals wrapped inside the chain coils have prolonged lifetimes because of the diminished termination, which allow some of these radicals to initiate polymerization of MMA at room temperature to form diblock copolymer.
Solvents effects on reactivity ratios for free radical statistical copolymerization have been observed on comparing reactivity ratios in [BMIM]PF6 to those in common organic solvents such as benzene. The calculated reactivity ratios of St and MMA (rSt=0.381±0.02 and rMMA=0.464±0.02) in [BMIM]PF6 by a non-linear method (CONTOUR computer program) are significantly different from those (rSt=0.54±0.04 and rMMA=0.50±0.04) in benzene at 60°C. The “boot-strap” model, polarity of the solvents, interactions between solvent and monomers (e. g. solvent-monomer complex), viscosity and system heterogeneity all possibly contribute to the difference of the reactivity ratios in RTILs and in benzene. These results demonstrates that RTILs have the potential to facilitate the synthesis of statistical copolymers having different monomer sequences, and thus different properties, as compared to copolymers made in conventional solvents.
Zhang, Hongwei, "Free Radical (Co)Polymeirzation of Methyl methacrylate and Styrene in Room Temperature Ionic Liquids. " PhD diss., University of Tennessee, 2004.