Synthesis and Characterization of Polymers with Controlled Microstructure using Living Radical Polymerization in Miniemulsion
Background:
In recent years, a variety of "living" radical (LRP) polymerization systems have been developed. These systems differ from conventional free radical systems in that most of the polymer chains continue to grow throughout the polymerization. In conventional systems, chains may terminate by one of several mechanisms including bimolecular radical termination and transfer to monomer or other species, typically giving mean chain lifetimes of ~0.1-1 s. In living radical polymerization, irreversible bimolecular termination reactions are significantly suppressed, although the chains are reversibly terminated. Living radical polymerizations do experience some irreversible termination reactions, albeit at a low rate, and therefore are not strictly "living". However they exhibit many of the features of living systems, including improved control of the molecular weight distribution and the ability to synthesize polymers with tailored and/or complex microstructure, including for example block copolymers and star polymers. Equilibrium exists between inactive (dormant) chains associated with a nitroxide and active chains (Scheme 1). As the polymerization progresses, the molecular weight continually increases as the chains continue to grow (Scheme 2).
Scheme 1. Reversible termination of active polystyrene radical by TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) to give dormant chain.
Scheme 2. Gel permeation chromatograph showing evolution of molecular weight distribution with increasing monomer conversion in nitroxide-mediated styrene polymerization (molecular weight increases from right to left).
Relatively little research has been done on the use of nitroxide-mediated living radical polymerizations in emulsion-based systems, although they could have significant scientific and industrial importance. Emulsion polymerization is a technology widely used to manufacture a variety of polymer products, especially paints, coatings and adhesives. It is currently an active area of research because it is a water-based, solvent-free process. A major thrust of new research is the replacement of solvent-borne coating systems by latex (water-borne) systems. My research focuses on furthering the fundamental understanding of LRP systems in miniemulsion (Scheme 3), and the development of commercially feasible routes to making advanced polymeric materials such as block copolymers.
Scheme 3. Miniemulsion droplets (50-200 nm) stabilized using a surfactant and a co-stabilizer (hexadecane)
Research:
We are currently investigating several aspects of living radical polymerization in dispersed systems, including:
- fundamental reaction kinetics of SFRP and ATRP
- rate enhancement using additives
- block copolymer synthesis
- modelling and simulation
- determination of polymer "livingness" through fluorescence measurements
- SFRP in emulsion
- ATRP in miniemulsion
Click here for a recent review of living radical polymerization in dispersed systems.
"If we knew what we were doing it wouldn't be research"
- Albert Einstein
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