Mid-Chain Radical Migration in the Radical Polymerization of n-Butyl Acrylate

Abstract

The occurrence of intramolecular transfer to polymer in the radical polymerization of acrylic monomers has been extensively documented in the literature. Whilst it has been largely assumed that intramolecular transfer to polymer leads to short chain branches, there has been some speculation over whether the mid-chain radical can migrate. Herein, by the matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometry (MS) of poly(n-butyl acrylate) synthesized by solution polymerization under a range of conditions, it is shown that this mid-chain radical migration does occur in the radical polymerization of acrylates conducted at high temperatures, as is evident from the shape of the molecular weight distribution. Using a mathematical model, an initial approximation of the rate at which migration occurs is made and the distribution of branching lengths formed in this scenario is explored. It is shown that the polymerizations carried out under a low monomer concentration and at high temperatures are particularly prone to radical migration reactions, which may affect the rheological properties of the polymer.

Keywords: acrylates; mid-chain radical; radical polymerization.

Scheme 1

Reaction pathway in synthesis of alkyl acrylates.

Figure 1

MALDI-TOF mass spectra for poly (butyl acrylate) obtained by solution polymerization at 60 °C, 100 °C or 140 °C and at 20 wt % or 30 wt % in p-Xylene. Enlargements in the mass range between 1700 and 2100 Da are shown in the right panel. The bottom panel shows the assigned structures of species observed in the solution polymerization of butyl acrylate in p-Xylene.

Figure 2

Example of hopping on molecular weight distribution. The hopping process leads to a distribution of mid-chain radicals, which after β-scission generates an initial distribution of chain lengths of radicals that can subsequently propagate (left). The final chain length distribution (bold black line, right) is given by the sum of the chain length distribution of each individual initiating species (colored lines, right).

Scheme 2

Radical migration steps starting from the initial tertiary radical position in close proximity to the end chain.

Figure 3

Experimental and simulated MALDI-TOF spectra at 20% (upper panel) and 30% (lower panel) and 140 °C. The experimental MALDI spectra show the complete experimental data (lines) and are overlayed with the assigned peaks using symbols corresponding to structures shown in Figure 1.

Figure 4

Comparison of instantaneous chain length distribution with distribution of number of radical migration steps at varying points of solution polymerization of butyl acrylate ([M]₀ = 3.9 M) at 60 °C, 100 °C and 140 °C.