Investigations into the Propagation and Termination Kinetics of the Radical Polymerization of Polar Monomers in Aqueous Solution

Jens Schrooten

doi:10.53846/goediss-3860

Abstract

Propagation and termination rate coefficients of radical polymerizations in aqueous solution were determined from pulsed-laser induced polymerizations and chemically initiated polymerizations. Pulsed-laser induced polymerizations of N,2‑dimethylprop‑2‑enamide, N,N‑dimethylprop‑2‑enamide, 2‑methylprop‑2‑enamide, and prop‑2‑enamide were performed. Subsequent analysis of the products by size-exclusion chromatography provided access to propagation rate coefficients. The obtained variation of the propagation rate coefficients with monomer concentration may be explained by the degree to which internal rotations and vibrations in the transition state of the propagation step are hindered. The dependence of the degree of hindrance on monomer concentration may be ascribed to an increase of intermolecular interactions of the transition-state structure with solvating molecules towards higher monomer content. Pressure and temperature were varied from ambient pressure to 2 000 bar and from 10 °C to 80 °C, respectively, to allow for determination of volumes of activation and Arrhenius activation energies for propagation. The activation energy and the absolute value of the volume of activation in case of 2‑methylprop‑2‑enamide are larger compared to N,N‑dimethylprop‑2‑enamide. This finding may be ascribed to the circumstance that N,N‑dimethylprop‑2‑enamide lacks an α‑methyl group. Both activation parameters in case of N,2‑dimethylprop‑2‑enamide are close to the ones of N,N‑dimethylprop‑2‑enamide. This is unexpected because N,2‑dimethylprop‑2‑enamide exhibits an α‑methyl group. Differences between both monomers regarding the conformation of the carbon–carbon double bond relative to the carbon–oxygen double bond may explain this observation. Initiation of a polymerization by a single laser pulse and subsequent detection of monomer-to-polymer conversion by time-resolved near-infrared spectroscopy was used to measure termination rate coefficients. The time resolution has been improved to 0.33 μs. Repetitive application of laser pulses in conjunction with near-infrared spectroscopic analysis yields termination rate coefficients as a function of the degree of monomer conversion. Investigations into the termination kinetics of prop‑2‑enamide, 2‑methylprop‑2‑enamide, N,2‑dimethylprop‑2‑enamide, N,N‑dimethylprop‑2‑enamide, and 1‑vinylpyrrolidin‑2‑one were performed at 2 000 bar because of the better signal-to-noise ratio at high pressure. Additional measurements were conducted at pressures of 500 bar, 1 000 bar, and 1 500 bar in case of most prop‑2‑enamides. The so-obtained volumes of activation may be used to estimate termination rate coefficients at ambient pressure. The volumes of activation amount to 12.4 cm3·mol−1 and 14.3 cm3·mol−1 for the termination rate coefficients of N,2‑dimethylprop‑2‑enamide and prop‑2‑enamide, respectively. The volume of activation in case of N,N‑dimethylprop‑2‑enamide (4.9 cm3·mol−1) is lower than expected. Termination rate coefficients of 1‑vinylpyrrolidin‑2‑one could be obtained for a large range of initial monomer concentrations and degrees of monomer conversion. This information allows for a detailed analysis of the parameters used to describe the monomer-conversion dependence of the termination rate coefficient. Within the monomer-conversion ranges under investigation the termination rate coefficient is assumed to be controlled by segmental, translational, and reaction diffusion. In case of N,N‑dimethylprop‑2‑enamide and 1‑vinylpyrrolidin‑2‑one, dynamic viscosities of monomer–water mixtures were determined at ambient pressure to assist the understanding of termination kinetics. A large set of termination rate coefficients in dependence on initial monomer concentration was obtained for these two monomers by pulsed-laser induced polymerization. Chemically initiated polymerizations of prop‑2‑enamide were conducted to determine termination rate coefficients at ambient pressure as a function of the degree of monomer conversion. The obtained data are in good agreement with data determined by pulsed-laser induced polymerizations. Investigations into the binary copolymerization of 1‑vinylpyrrolidin‑2‑one and sodium acrylate revealed a strong incorporation of sodium acrylate into the copolymer, as was measured by nuclear magnetic resonance spectroscopy. Monomer reactivity ratios were determined by means of the Mayo–Lewis equation.

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