Comparing SG1 and TEMPO for NMP of n-butyl acrylate in miniemulsion to optimize the average particle size for rate and molecular control

Abstract

Nitroxide mediated polymerization (NMP) in aqueous miniemulsion potentially offers both control over chain length and livingness, while reducing the reaction time compared to the NMP in bulk. However, the identification of the optimal average particle diameter (d_p) to achieve these benefits remains elusive. In this work, for the NMP of n-butyl acrylate (nBuA), a deterministic population balance model, including 4D Smith–Ewart equations, is developed to find the optimal d_p. It distinguishes nanoparticles according to their numbers of four radical types to calculate the monomer conversion and livingness and is coupled to moment equations to calculate average molar masses in a miniemulsion polymerization. This analysis is carried out for the first time for the two most important free nitroxides, being (N-(2-methyl-2-propyl)-N-(1-diethylphosphono-2,2-dimethylpropyl)-N-oxyl) (SG1) and (2,2,6,6-tetramethylpiperidine-1-oxyl) (TEMPO), using measured temperature-dependent free nitroxide partition coefficients, and accounting for backbiting and β-scission side reactions. This detailed and validated model reveals up to seven kinetic regimes (based on relative changes in rate acceleration and deceleration) in a wide d_p range from 5 to 350 nm, featuring different NMP rates and levels of molecular control. It is shown that the SG1-based NMP needs a lower d_p compared to TEMPO-based NMP to establish kinetic regimes different from the pseudo-bulk regime, hence, the so-called segregation effect (less termination in separate particles) and confined space effect (faster deactivation in small particles) are only active if d_p is sufficiently decreased. It is further shown that the temperature needs to be sufficiently low to achieve a good balance between polymerization rate and control over average molar mass and livingness. A more industrially attractive higher solids content (e.g. 30%) can be employed if d_p values below 120 nm for NMP with SG1, and below 150 nm for NMP with TEMPO, are aimed at. Higher TCL (targeted chain lengths, e.g. a TCL of 1000) reveal the beneficial effect of miniemulsion compared to bulk conditions specifically when employing a d_p of ca. 50 nm for the SG1 case. Overall, the model enables the fine-tuning of reaction time, dispersity, and livingness, enabling the precision synthesis of a poly(n-butyl acrylate) latex with enhanced solids content and TCL.