Shaghayegh Hamzehlou

Analyzing the discrepancies in the activation energies of the backbiting and β-scission reactions in the radical polymerization of n-butyl acrylate

The formation of midchain radicals (MCRs) and their subsequent fate heavily influences the radical polymerization of acrylic monomers. This article aims to shed light on the discrepancies in the activation energies of the formation of MCRs by backbiting and their reaction by beta-scission in the radical polymerization of n-BA determined by both experimental and theoretical methods. For this purpose, bulk and solution batch polymerizations of n-BA at different solids contents and over a wide range of nominal temperatures of 60, 100 and 140 °C initiated by a thermal initiator were carried out and the kinetics, branching density and average molar masses as well as the macromonomer density were measured by means of 13C NMR, 1H NMR and size-exclusion chromatography with multi-angle light scattering (SEC/MALS). A detailed model was used to predict the experimental data and to estimate the activation energies for the backbiting and β-scission reactions. The estimated values for the backbiting activation energy are closer to the upper range of values previously reported and, on the other hand, the estimation shows that in the whole range of temperatures, the β-scission rate coefficient should be higher than the accepted values reported in the literature which failed to predict the properties in the wide range of temperatures used in this work.

On the Termination Mechanism in the Radical Polymerization of Acrylates.

In a recent publication, Nakamura and co-workers studied the termination mechanism in the radical polymerization of acrylates. Contrary to conventional thinking, their conclusion is that termination is overwhelmingly by disproportionation. This finding impacts on a large body of the previous work in the polymerization of acrylic monomers which this work seeks to address. Analysis of the molecular weight distribution of acrylic polymers obtained under different polymerization conditions shows that termination by combination is the more probable mechanism for mutual termination of secondary radicals. It is proposed that in the experiments conducted by Nakamura and co-workers, backbiting plays a key role and their experimental data are reinterpreted, showing that they are more revealing with respect to the mode of termination of the midchain radical produced by backbiting, than to bimolecular termination of secondary radicals.

Morphology of Composite Polymer Latexes: An Update on Synthesis and Applications, Modeling, and Characterization

Polymer latexes are used in a wide range of technological and industrial products. Polymer–polymer and polymer–inorganic composite (hybrid) latexes have found specific applications in coatings, adhesives, impact modifiers, and medical diagnostics, among others. Control of latex particle morphology is a key factor to achieve a desired application in structured polymer latex particles. Fundamental knowledge of the effects of different synthetic routes and reaction parameters such as temperature, glass transition temperature, and functional groups is vital to obtain the required morphology of polymer–polymer and polymer–inorganic hybrids. Subsequently an unambiguous characterization of particle morphology is essential, which requires determining the shape of the particles, the surface composition, and the internal composition. In this review, we focus on the most recent developments (of the last 5 years) in the synthesis and application of composite (hybrid) latex particles, including polymer–polymer and polymer–inorganic latex systems. Furthermore, we discuss the most recent modeling efforts to simulate the development of particle morphology in composite polymer latexes synthesized by (mini)emulsion polymerization and the latest advances and improvements in characterization techniques to determine the morphology of composite polymer latexes.