Living-Radical Polymerization -- A New Way to Do Chemistry.
Polymeric materials are ubiquitous in our daily lives. They can be found in the soles of the shoes we wear, the gum we chew, the biocompatible materials that go into artificial hearts and so on. The list is never ending. Yet, as we find more and more sophisticated applications for polymers, the demands on the performance of these materials increase proportionally. The polymer chemist is on a never-ending quest to discover and synthesize new and improved polymeric materials. One way to do this is to develop new ways of performing existing polymerizations.
Probably the most popular industrial polymeric process is the free radical polymerization process. Hundreds of millions of pounds of plastics and vinylmonomer- based polymers are manufactured each year by this process. The free radical polymerization is inexpensive to perform, is compatible with a wide range of monomers, does not require extensive purification of monomers and can be performed under bulk, solution or emulsion conditions. However, in spite of its many attributes, it has limitations. Since free-radical propagating chains have an inherent ability to react with each other and terminate they produce dead polymer chains. The chains terminate throughout the course of the polymerization causing the distribution of the lengths of the polymer chains, referred to as molecular weight distribution (MWD), to he large. Theoretical calculations have shown that the narrowest MWD that is possible for a free radical polymerization is 1.5, although in practice this value has never been achieved in a conventional free radical polymerization process. Typically, MWDs range from 2 to about 7. (When the lengths of the polymers in a solution are all the same the MWD is 1 Only Mother Nature at present is capable of producing polymers with a MWD of 1.)
At the Xerox Research Centre of Canada we have a particular interest in polymeric materials. Consider, for example, that a typical toner may be made up of 85 per cent of a specially designed polymer that provides just the right melt and flow properties to enable the toner to function as it does. As part of our continuing quest to produce new materials for future Xerox materials applications, we were intrigued with the idea of developing a new way of performing free-radical polymerizations. In short, we were interested in developing a living-radical polymerization process, one in which no dead chains are produced, even though when we started this program many believed it would be an impossible task.
Our approach to making the free-radical polymerization process behave in a living manner is based on the concept of having the propagating polymer chain exist in equilibrium with a dormant species that under appropriate conditions can regenerate the propagating chain (Figure 1). Commercially available stable free radical nitroxides, such as TEMPO, are used to react reversibly with the propagating chain to produce the dormant polymer chain. Since the C-O bond between the polymer chain and the nitroxide is labile, by continued heating the bond breaks to regenerate the active polymer chain that can go on to add more monomer and grow. The nitroxide that is produced reacts with another polymer chain to form a dormant chain. Under these conditions the number of propagating chains that exist in solution at any given time is reduced to the point where the probability of two chains colliding with each other and terminating is almost eliminated.
To ensure a narrow molecular weight distribution that enables controlled and predictable molecular weights, it is necessary to have all the polymers chains start growing at the same time. This is achieved by operating at a temperature at which the initiator half-life is a few minutes or less. In the case of benzoyl peroxide, this is about 135[degrees]C.
When all of this is put together a living-free radical polymerization process, in which the molecular weight is predictable and MWDs are as low as 1.08. is achieved. And, since the polymer chains are living, they can be further reacted to give materials with complex architecture, the original goal of this research. To date, random, block, and tapered copolymers, as well as, hyperbranched and dendritic structures, have all now been made using the living-radical polymerization process.
Scientists at the Xerox Research Centre of Canada are continuing to develop this new branch of polymer chemistry for their material needs. But, since this is a generic way of making polymers it can be applied to a wide range of applications, well beyond the needs of Xerox. As a consequence, the area of living-radical polymerization is presently one of the most popular areas of polymer research. Since the first demonstration by the Xerox scientists that living-radical polymerizations are feasible, two new ways of performing this type of polymerization have been introduced. Many very competent and creative scientists worldwide are now workings in this field ensuring a very bright future for living-radical polymerizations.
Michael Georges is a principal scientist at the Xerox Research Centre of Canada, Mississauga, ON, with 15 years experience in polymer research. It was the initial paper in 1993 by Michael and his group at Xerox that initiated the explosion in living-radical polymerization research.
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|Author:||Georges, Michael K.|
|Publication:||Canadian Chemical News|
|Date:||Jun 1, 2000|
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