High pressure RAFT of sterically hindered ionic monomers. Studying relationship between rigidity of the polymer backbone and conductivity

Abstract

The synthesis of poly(ionic liquid)s (PILs), a new class of polymers with numerous possible applications and tailored thermorheological properties, is quite a challenging task. To achieve that goal, different strategies have been proposed and developed. However, in the majority of cases macromolecules with relatively low molecular weights and high dispersities were produced, probably due to the strong intermolecular coulombic interactions that determine the behavior of monomeric ionic liquids. In this paper, we proposed a completely new approach that relies on the pressure-induced reversible addition fragmentation chain transfer polymerization (RAFT) to produce PILs of desired properties. For this purpose, a series of model imidazolium-based ionic monomers, with different lengths of aliphatic side chains as additional steric hindrances, have been successfully polymerized under high pressure (p = 250, 500 and 800 MPa). In contrast to results obtained at ambient pressure, all monomers yielded high molecular weight polymers (degrees of polymerization DPn ≤ 10 000) with narrow dispersities (Ð∼1.10). From the kinetic data obtained at various thermodynamic conditions, the rate of polymerization, Rp, and overall activation volumes, ΔV, were estimated, which in the limit of low pressures varied as follows −16.7, −18.1, −32.6 and −35.6 cm mol−1 for [MVIM][NTf2], [EVIM][NTf2], [BVIM][NTf2] and [OVIM][NTf2], respectively. An unexpected significant jump in ΔV can be correlated with the nanostructure organization that, accordingly to the literature, starts to dominate in the latter two monomers. It was also demonstrated that below p = 500 MPa, the termination reaction is almost completely suppressed, independently on the sample. On the other hand, above that pressure both the polymerization rate and the control over the reaction decreased due to the high viscosity preventing diffusion of the monomers. Moreover, taking advantage of the high pressure polymerization, we had a unique opportunity of exploring and better understanding a correlation among molecular weight, Mn, the glass transition temperature, Tg, and the dc conductivity, σdc, for a very wide range of Mn (up to 430 kg/mol) polymers of various backbone rigidity. We observed that the evolution of Tg with Mn follows a typical Fox-Flory relation. Nevertheless, Tg decreases with an increase in the size of the monomer. Additionally, a similar chemical structure dependence was observed for the dc conductivity, which seems to strongly depend on the rigidity of the produced polymers. It can be seen that the higher rigidity of the polymer (longer alkyl chain of side group), the higher σdc. We believe the results reported herein offer an easy alternative way to synthesis of well-defined polyelectrolytes of moderate/high conductivity in shorter reaction times, and expand our knowledge of their properties and the correlations among them.