Nicholas Ballard

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.

The underlying mechanisms for self-healing of poly(disulfide)s.

Recently, self-healing polymers based on disulfide compounds have gained attention due to the versatile chemistry of disulfide bonds and easy implementation into polymeric materials. However, the underlying mechanisms of disulfide exchange which induce the self-healing effect in poly(disulfide)s remain unclear. In this work, we elucidate the process of disulfide exchange using a variety of spectroscopic techniques. Comparing a model exchange reaction of 4-aminophenyl disulfide and diphenyl disulfide with modified reactions in the presence of additional radical traps or radical sources confirmed that the exchange reaction between disulfide compounds occurred via a radical-mediated mechanism. Furthermore, when investigating the effect of catalysts on the model exchange reaction, it could be concluded that catalysts enhance the disulfide exchange reaction through the formation of S-based anions in addition to the radical-mediated mechanism.

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.

Equilibrium orientations of non-spherical and chemically anisotropic particles at liquid-liquid interfaces and the effect on emulsion stability.

The effective stabilization of emulsions by solid particles, a phenomenon known as Pickering stabilization, is well known to be highly dependent on the wettability and the adhesion energy of the stabilizer employed at the liquid-liquid interface. We present a user-friendly computational model that can be used to determine equilibrium orientations and the adhesion energy of colloidal particles at interfaces. The model determines the free energy profile of particle adsorption at liquid-liquid interfaces using a triangular tessellation scheme. We demonstrate the use of the model, using a variety of anisotropic particles and demonstrate its ability to predict and explain experimental observations of particle behaviour at interfaces. In particular, we show that the concept of hydrophilic lipophilic balance commonly applied to molecular surfactants is insufficient to explain the complexity of the activity of colloidal particles at interfaces. In addition, we show the importance of the knowledge of the free energy adsorption profile of single particles at interfaces and the impact on overall free energy of emulsification of packed ensembles of particles. The delicate balance between optimization of adhesion energy, adsorption dynamics and particle packing is shown to be of great importance in the formation of thermodynamically stable emulsions. In order to use the model, the code is implemented by freely available software that can be readily deployed on personal computers.

Polymerization kinetics of a fluorinated monomer under confinement in AAO nanocavities

In this work we show for the first time the kinetic study of the radical polymerization of a fluorinated acrylic monomer (MFA) in the confinement of anodic aluminum oxide (AAO) nanocavities. AAO templates with different pore sizes were used as nanoreactors and polymerization kinetics were studied in situ by Raman spectroscopy and in bulk by differential scanning calorimetry (DSC). Afterwards, a mathematical model that describes the effect of nanoconfinement on the polymerization kinetics was derived. Furthermore, similar nanostructures were observed by SEM when in bulk polymerized PFA was infiltrated into the AAO nanocavities. Superhydrophobic surfaces were achieved with the water contact angle of 159°, much higher than its analogous non-nanostructured PFA, 114°. The “lotus effect” was observed in the superhydrophobic surface which has a low sliding angle of 8°.

Determining the effect of side reactions on product distributions in RAFT polymerization by MALDI-TOF MS

Reversible Addition–Fragmentation chain Transfer (RAFT) polymerization has emerged as one of the most versatile reversible deactivation radical polymerization techniques and is capable of polymerizing a wide range of monomers under various conditions. One of the most important factors governing the success of a RAFT polymerization is the fraction of living chains at the end of the reaction, which can be maximized by using a low amount of initiator. From the point of view of the process, it is tempting to perform the polymerization in solution, which allows a better mixing. However, in this work it is shown that this choice may be negative for the quality of the polymer. Detailed analysis using Matrix Assisted Laser Desorption Ionization Time of Flight Mass Spectrometry (MALDI-TOF MS) of poly(n-butyl acrylate) (pBA) obtained at high conversion in the RAFT solution polymerization revealed that in addition to the polymer chains, formed by the RAFT mechanism, there were two distinct populations resulting from chain transfer to solvent and transfer to polymer followed by β-scission. Complementary results from Size Exclusion Chromatography coupled with Multi Angle Light Scattering detector (SEC/MALS), quantum chemical calculations, and a mathematical model that predicts product distributions, were also used to further confirm the assigned structures. The results highlight the scope and limitation on the living fraction of chains due to chain transfer events using RAFT polymerization and reversible deactivation radical polymerizations in general, and furthermore, yielded information about the fate of midchain radicals formed by intramolecular transfer to polymer.

Redox Active Compounds in Controlled Radical Polymerization and Dye-Sensitized Solar Cells: Mutual Solutions to Disparate Problems.

Controlled radical polymerization (CRP) and dye-sensitized solar cells (DSSCs) are two fields of research that at an initial glance appear to have little in common. However, despite their obvious differences, both in application and in scientific nature, a closer look reveals a striking similarity between many of the compounds widely used as control agents in radical polymerization and as redox couples in dye-sensitized solar cells. Herein, we review the various redox active compounds used and examine the characteristics that give them the ability to perform this dual function. In addition we explore the advances in the understanding of the structural features that enhance their activity in both CRP and DSSCs. It is hoped that such a comparison will be conducive to improving process performance in both fields.

Adsorption and desorption behavior of ionic and nonionic surfactants on polymer surfaces

We report combined experimental and computational studies aiming to elucidate the adsorption properties of ionic and nonionic surfactants on hydrophobic polymer surface such as poly(styrene). To represent these two types of surfactants, we choose sodium dodecyl sulfate and poly(ethylene glycol)-poly(ethylene) block copolymers, both commonly utilized in emulsion polymerization. By applying quartz crystal microbalance with dissipation monitoring we find that the non-ionic surfactants are desorbed from the poly(styrene) surface slower, and at low surfactant concentrations they adsorb with stronger energy, than the ionic surfactant. If fact, from molecular dynamics simulations we obtain that the effective attractive force of these nonionic surfactants to the surface increases with the decrease of their concentration, whereas, the ionic surfactant exhibits mildly the opposite trend. We argue that the difference in this contrasting behavior stems from the physico-chemical properties of the head group. Ionic surfactants characterized by small and strongly hydrophilic head groups form an ordered self-assembled structure at the interface whereas, non-ionic surfactants with long and weakly hydrophilic head groups, which are also characterized by low persistence lengths, generate a disordered layer. Consequently, upon an increase in concentration, the layer formed by the nonionic surfactants prevents the aprotic poly(ethylene glycol) head groups to satisfy all their hydrogen bonds capabilities. As a response, water molecules intrude this surfactant layer and partially compensate for the missing interactions, however, at the expense of their ability to form hydrogen bonds as in bulk. This loss of hydrogen bonds, either of the head groups or of the intruding water molecules, is the reason the nonionic surfactants weaken their effective attraction to the interface with the increase in concentration.

Synthesis of poly(methyl methacrylate) and block copolymers by semi-batch nitroxide mediated polymerization

In order to successfully implement the commercial synthesis of polymers by using reversible deactivation radical polymerization (RDRP) techniques in solution, semi-batch processes should be developed. However, the low monomer concentrations involved present a challenge for RDRPs. This is especially the case in the nitroxide mediated polymerization of methacrylates where, historically, even in batch control over the polymerization has been difficult. Here, we show that the alkoxyamine, 3-(((2-cyanopropan-2-yl)oxy)(cyclohexyl)amino)-2,2-dimethyl-3-phenylpropanenitrile, which was recently shown to successfully control the polymerization of methyl methacrylate and styrene, can also be used to control the polymerization of methacrylates under semi-batch conditions. High instantaneous conversions, whilst maintaining the polymer end groups are observed and the limits of control over the polymerization and the potential to synthesize block copolymers under semi-batch conditions are explored.

Novel alkoxyamines for the successful controlled polymerization of styrene and methacrylates

The design of alkoxyamine/nitroxide species capable of mediating the polymerization of both styrene and methacrylic monomers has been notoriously difficult. Herein, the nitroxide-mediated polymerization of styrene using a series of readily obtained alkoxyamines that were recently shown to control the polymerization of methyl methacrylate is presented. The robustness of the system is tested towards different temperatures and molecular weights. The influence of the substitution pattern in the nitroxide adducts on the polymerization of styrene is monitored through kinetic analyses. An alkoxyamine that can successfully mediate the homopolymerization of styrene and methyl methacrylate is presented. This alkoxyamine is subsequently utilized for the synthesis of PMMA-b-PS and PS-b-PBMA copolymers.