Ab initio Emulsion ATRP Assisted by Surfactant Ligand Design
Yipeng Wei1, Yanyu Jia1, Wen-Jun Wang,1 Bo-Geng Li1 and Shiping Zhu2
1College of Chemical & Biochemical Engineering, State Key Laboratory of Chemical Engineering, Zhejiang University, Hangzhou P.R. China 310017;
2Department of Chemical Engineering, McMaster University, Hamilton, Canada L8S 4L7
Abstract The main challenge of conducting an ab initio emulsion ATRP lies in the design of the catalyst-ligand complex structure. In emulsion ATRP, highly hydrophobic ligands such as dNbpy and BPMODA are commonly used to retain Cu catalyst in the oil phase, resulting in good control over the polymerization. However, a careful examination revealed that these systems cannot be classified as ab initio emulsion ATRP. The highly hydrophobic ligands remain in the monomer droplets, which generate osmotic pressure between particles. This in turn prevents the migration of monomer from the droplets into the micelles/particles, causing majority of the polymerization to occur inside the monomer droplets. Therefore, these systems behave more or less like miniemulsion ATRP or so-called aqueous dispersed ATRP, but not the ab initio emulsion polymerization based on micellar nucleation mechanism.
In ab initio emulsion ATRP, the ligand molecules are ideally hydrophobic and stay in oil phase but do not mainly reside in the monomer droplets. The persistent ligand in monomer droplets generate osmotic pressure that could prevent monomer transfer across the aqueous phase. In order to solve this challenge, we introduced a new concept of "surfactant-ligand" (SL), which is a molecule that has both surfactant and ligand functionalities. As the name suggests, SLs molecules play dual roles in the ab initio ATRP, as a ligand and as a surfactant. These molecules form micelles and stabilize polymer particles, and they also help to stabilize the monomer droplets. The ligand groups of SL reside inside micelles/particles and could not come out to aqueous phase. The SL chemical structure is illustrated in Scheme 1.
The experiment results showed the micelles were converted to polymer particles with both water-soluble and oil-soluble initiators. The micellar nucleation mechanism was confirmed by the growth of polymer particles during polymerization, in comparison to a control experiment without the addition of SL. It was also found that diffusion-controlled deactivation inside particles occurred (Scheme 1), which broadened the polymer molecular weight distribution (PDI>2). Adding a small amount of free ligand (dNbpy at a 1:10 mole ratio of dNbpy to SL) was found to improve the polymerization control. [i]
Scheme 1 A) SL chemical structure and B) the concept of SL-mediated emulsion ATRP