Effect of Very High Charge Density and Monomer Constitution on the Synthesis and Properties of Cationic Polyelectrolytes

Abstract

Abstract:The free-radical homopolymerization of 1,3-bis(N,N,N-trimethylammonium)-2-propylmethacrylate dichloride (di-M) and 1,3-bis(N,N,N-trimethylammonium)-2-propylacrylate dichloride (di-A) in aqueous solution yields cationic polyelectrolytes (PEL) with theoretical/structural charge spacing of only «0.12 nm. The high charge density causes condensation of «82% of the chloride counterions. The high level of counterion condensation reduces the ionic strength in the polymerizing batch when the monomer molecules connect to PEL chains. This has the consequence that the hydrodynamic and excluded volume of the PEL molecules will change. Studies of the free radical polymerization revealed non-ideal polymerization kinetics already at low conversion and additionally autoacceleration above a certain monomer concentration and conversion. Similar autoacceleration was not observed for monomers yielding PEL with charge spacing of 0.25 or 0.5 nm. Coulomb interactions, monomer association, steric effects, and specific features of the monomer constitution have been evaluated concerning their contributions to the concentration dependence and conversion dependence of kinetic parameters. The different backbone constitutions of di-M and di-A not only influence the polymerization kinetics but also equip poly(di-M) with higher hydrolytic stability. The experimental results confirm the impact of electrochemical parameters and the necessity to reconsider their inclusion in kinetic models.

Experimental and Modeling Investigation of Radical Homopolymerization of 2‐(Methacryloyloxyethyl) Trimethylammonium Chloride in Aqueous Solution

“…Thus, the extent of counterion condensation increases with conversion and total monomer content, in agreement with observations made in a study of poly(vinylbenzyltrialkylammonium) that revealed a significant effect of both polyelectrolyte concentration and polymer chain length on polyion–counterion interaction 39. Recent studies by Ahmadloo et al on the radical homopolymerization of two doubly charged cationic monomers in aqueous solution concluded that high charge density results in high counterion condensation (≈82%), thereby greatly reducing the ionic strength of the polymerizing batch 31. The level of condensation (≈30%) is much lower for 10 wt% TMAEMC in solution, an indication that the influence of free counterions remain important throughout the polymerization.…”

“…While previous studies provide insights on the effects of counterions on the final properties of PELs in dilute solution,31,40 little is known about the solution properties at the higher concentrations of PELs associated with polymerization 35. Thus, conductivity and counterion concentrations of non‐reacting solutions were measured at ambient temperatures to investigate the changes that occur during transition from a monomer to a polymer solution in batch polymerization.…”

“…[39] Recent studies by Ahmadloo et al on the radical homopolymerization of two doubly charged cationic monomers in aqueous solution concluded that high charge density results in high counterion condensation (≈82%), thereby greatly reducing the ionic strength of the polymerizing batch. [31] The level of condensation (≈30%) is much lower for 10 wt% TMAEMC in solution, an indication that the influence of free counterions remain important throughout the poly merization. With increasing conversion, the concentration of free monomer decreases at a faster rate than the rate of Cl counterions, as only partial condensation occurs.…”

“…In addition, the viscosity, counterion activity, pH, and conductivity are measured for various monomer/polymer mixtures (in the absence of reaction) to obtain a better understanding of how the physical system changes with conversion. Leveraged with insights provided by previous PEL characterizations,31,35,39–41 the investigation enables the development of a mechanistic model to describe this homopolymerization system, accounting for the reaction complexities. The capabilities of the model are also tested through comparison to a smaller set of batch and semi‐batch reactions conducted in a stirred lab reactor.…”

“…The value of 〈k t 〉 can also be estimated from k p /〈k t 〉 0.5 , the coupled rate coefficient available from continuously initiated experiments, and compared to values determined from SP-PLP-EPR studies of CLD termination kinetics or SP-PLP-NIR produced k p /〈k t 〉 values. [19] While both k p and k t of TMAEMC increase with increasing initial monomer content and ionic strength, the combined effect on rate of monomer conversion (proportional to k p /〈k t 〉 0.5 ) in a batch system is difficult to predict, as has been noted in homopolymerization studies of other permanently charged cationic [31,32] and ionizable anionic [33,34] monomers. Moreover, the presence of counterions in solution, which diffuse freely prior to the start of polymerization and begin to condense on the polyions with polymer formation, may further complicate development of a comprehensive understanding of the system.…”