Bob A. Howell

Center for Applications in Polymer Science

Central Michigan University Mt. Pleasant, Michigan 48859

Over the past several years attempts to moderate the reactivity of the propagating species in radical polymerization have been multifold. The focus of these efforts has been the elimination of unwanted side reactions (radical coupling, disproportionation, radical transfer). In the absence of such reactions which lead to polydispersity broadening and the formation of polymer defect structures the polymerization should be much more well-behaved than traditional radical polymerization.

Success has been achieved to a greater or lesser degree depending on the exact approach used. However, none have been completely successful, i.e., none has provided an ideal polymerization. All of these processes involve the presence of a capping agent (nitroxyl radical, halogen atom, organometal moiety, or other molecular fragment) to react reversibly with the propagating radical to generate a non-radical species which is unreactive. In this bound state the propagating radical is stable or dormant and unable to participate in termination or chain transfer processes. Chain extension is controlled by the position of equilibrium between the dormant and active or free propagating radical. Under appropriate conditions very good mediation of the radical polymerization may be achieved and some of the characteristics of living polymerization may be observed, e.g., ready chain extension to form block polymers, narrow polydispersity, molecular weight control (reflecting the monomer to initiator molar ratio for systems in which preformed initiators/mediators are used), end-group purity, etc. However, none of these reactions meet rigorous criteria for living polymerization.

These include:

1. polymerization proceeds until all the monomer has been consumed - further addition of monomer results in continued polymerization
2. the number average molecular weight, M_n (or X_n, the number average degree of polymerization), is a linear function of conversion
3. the number of polymer molecules (and active centers) is a constant, which is sensibly independent of conversion
4. the molecular weight of the polymer can be controlled by the stoichiometry of the reaction
5. narrow-molecular-weight distribution polymers are produced
6. block copolymers can be prepared by sequential monomer addition
7. chain-end functionalized polymers can be prepared in quantitative yield
8. linearity of a kinetic plot of rate of propagation as a function of time
9. linear dependence of the degree of polymerization as a function of time.

To fairly be termed "living" a polymerization must meet all these criteria since some are sensitive to certain characteristics of living polymerization while others are sensitive to other features. Because none of the reported mediated radical polymerizations may be legitimately described as living but, at the same time, have some of the characteristics of living polymerization, a great variety of terminology has sprung up to describe these processes including pseudo-living, quasi-living, etc. Such descriptions often reflect a desire to include living in the descriptive terminology rather than a regard for accuracy in description or a precision of language. More recently specific descriptors such as nitroxyl mediated radical polymerization (NMRP) or atom transfer radical polymerization (ATRP) have been used in an attempt to introduce a greater degree of precision in the description of the polymerization reaction. This has largely been successful. Neither of these processes is living but both possess certain characteristics of living polymerization which vary with the exact nature of the polymerization and the reaction conditions. Since these processes have been rather widely explored and their scope and limitations largely established, the terms NMRP or ATRP are functionally descriptive, i.e., it is well understood to what they refer. Neither contains any direct reference to the extent of livingness of the process which it describes - in either case, this is variable depending on the reaction conditions, is reflected experimentally, and is implicit in the descriptor. For both these cases and for others in which a mediating agent is present, the propagating radical is neither free nor living. However, because of the presence of the mediating species, unwanted side-reactions are suppressed (to a greater or lesser degree depending on reaction conditions) and the resulting polymer is more nearly ideal than that which can be obtained in the absence of the mediating agent. Therefore, it is perfectly appropriate to refer to these processes as mediated radical polymerization (MRP) rather than controlled radical polymerization (the control isn’t complete in any case and is variable from case to case) or living radical polymerization (all mediated radical polymerizations exhibit some features of living polymerization but none display all such characteristics). The term mediated radical polymerization (MRP) is not only descriptive of a wide range of radical polymerizations carried out in the presence of agents to modify the reactivity of the propagating species but avoids the confusion associated with the terms living free radical, living radical, controlled radical polymerization and the like.

The goal of any nomenclature/terminology should be clarity. To broaden the definition of living to include a variety of mediated radical; processes serves only to destroy the utility of the term living. That term should be reserved for processes which truly meet the criteria for livingness.

To avoid confusion, to introduce clarity, and to maintain mechanism as the basis for descriptive terminology, processes such as those described above should best be described as mediated radical polymerization (MRP).

First published: ACS Division of Polymeric Materials: Science and Engineering (PMSE), 83, 578 (2000).