Nitroxide Mediated Polymerization: Microemulsion of N-Butyl Acrylate and the Synthesis of Block Copolymers
Li, Wing Sze Jennifer
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Living radical polymerization has proved to be a powerful tool for the synthesis of polymers as it allows for a high degree of control over the polymer microstructure and the synthesis of tailored molecular architectures. Although it has great potential, its use on an industrial scale is limited due to environmental and economical aspects. Nitroxide mediated polymerization is explored to bring this technology closer to adoption in commercial applications. One of the obstacles encountered using nitroxide mediated polymerization in microemulsion systems is the difficulty in controlling both the particle size and target molecular weight. Due to the nature of the formulation, a decrease in the target molecular weight is coupled to an increase in the particle size. For many applications, it is important to be able to design polymer particles with both specifications independently. Strategies to decouple these two properties and processing conditions required for targeting a range of particle sizes and molecular weights for n butyl acrylate latexes are presented. Furthermore, in an attempt to reduce the large amounts of surfactant typically used in microemulsions, these methods were explored at low surfactant to monomer ratios (0.2 to 0.5 by wt.) in order to reduce the costs associated with excess surfactant and post processing steps for surfactant removal (high surfactant levels also give poor water-resistance in coatings). Stable nanolatexes with particle sizes <40 nm have been obtained by other groups using NMP in microemulsions with SG1 but have done so by using much higher surfactant to monomer ratios (~2.5 by wt.) and at much lower solids content (6 10 wt. %). In this work, molecular weights of 20,000 to 80,000 g∙mol-1 were targeted and stable, n-butyl acrylate microemulsions with particle sizes ranging from 20 120 nm were prepared at a solids content of 20 wt. % using much lower surfactant concentrations. Although numerous studies have shown the effects of process parameters on particle sizes and methods to control the molecular weight, the decoupling of the molecular weight and particle size effect in NMP microemulsions under these conditions has not been done to this extent. In copolymer systems, nitroxide mediated polymerization also provides an efficient method to synthesize well defined block copolymers. Random copolymers are widely used as protective colloids, but the use of block copolymers for these applications has not been well studied. It is unclear what effects do the importance of a narrow molecular weight distribution and purity of block copolymers have on their performance as protective colloids. In order to investigate this, a range of block copolymers with different properties would need to be synthesized for systematic analysis. The direct synthesis of polystyrene b poly(acrylic acid) copolymers of varying lengths and compositions was successful by use of nitroxide mediated polymerization in bulk and solution polymerization. The characterization of these amphiphilic block copolymers was explored by titration and nuclear magnetic resonance spectroscopy.