(203c) Ab Initio Emulsion ATRP Assisted By Surfactant Ligand Design

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

wenjunwang@zju.edu.cn

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