Emulsion Polymerisation
Emulsion polymerisation is the commonest means of making a wide variety of industrial polymers, such as paints, adhesives and tyre rubber. It is a complex process involving many simultaneous and separate processes and where historically only a few types of data were available. The complexity and the limited data types meant that conflicting assumptions could be forced to agree with experiment: there was no proper understanding of the process. Gilbert developed and applied mathematical and experimental tools whereby the effects resulting from individual processes could be isolated for the first time.
As with unimolecular reactions, the keys to the qualitative and quantitative understanding of the many processes in emulsion polymerisation are the rate coefficients of the individual steps. These steps are initiation (how quickly a growing chain starts), propagation (how quickly individual monomer units are added), radical loss processes (the termination and transfer of radical activity), and particle formation (nucleation). With Prof D Napper, Gilbert applied equations that he had solved in gas-phase chemistry to the area of emulsion polymerisation. This opened the way for him to develop—initially in collaboration with Napper—new theoretical and experimental methods for extracting the rate coefficients of elementary processes. He produced targeted data using these methods, particularly the time evolution of reaction rates and molecular-weight and particle-size distributions. This included novel types of systems, such as γ-radiolysis relaxation, in which events such as radical loss can be separated from radical propagation and growth.
Gilbert's mathematical treatments and experimental techniques revealed the fundamentals controlling these steps by enabling each of the processes to be effectively studied in isolation. His advances allowed rate coefficients to be measured for virtually any process in emulsion polymerisation, values of these rate coefficients for simple systems to be predicted, and the reliability of new measurements to be checked from theory. He used data from applying these methods to obtain the dependence of rate coefficients on controllable quantities, such as initiator concentration. Thus, he tested existing models, developed new tests—some of which refuted extant models—and refined the older models that withstood his tests. At last, it was possible to achieve consistency between supposed microscopic events and experiment, and, for the very first time in the field, to refute postulated models authoritatively.
Using these data, he quantified radical loss from particles, showing that simple diffusion theory could explain the results. Gilbert and his coworkers then revealed the mechanism for initiation in emulsion polymerisation by the entry of radicals into particles—in terms of fundamental thermodynamic and kinetic precepts—in a theory that clarifies the process as being through production of surface-active species in the water phase. This model produced various qualitative predictions. One prediction, that of the independence of the entry-rate coefficient of the size and surface properties of particles, was widely seen as counterintuitive because of the deep-rooted belief in models that he had shown to be wrong. Subsequently, this prediction was experimentally verified by Gilbert and others. He used the understanding from this knowledge to develop a priori models for particle formation and molecular-weight distribution.
These developments led to a deep understanding of basic processes in free-radical polymerisation—the commonest industrial process. For the propagation reaction, Gilbert led an international team that produced a methodology that overcame the long-standing problem of obtaining reliable rate coefficients for this process. He showed that the Arrhenius parameters for different types of monomer take different classes of values, and developed qualitative and quantitative understanding of these classes from fundamental transition-state theory and quantum mechanics. These new methods were based on those that he had developed in his work on unimolecular gas-phase processes. For the termination reaction, his data and models led to the qualitative and quantitative understanding of this process as diffusion-controlled.
Thirty years ago there was neither real predictability nor qualitative understanding of the dominant mechanisms in emulsion polymerisation. Mechanisms had been ‘proved’ by comparing model predictions with experimental data. The data field was limited and the models had many adjustable parameters, or else fitting parameters had values that were subject to wide uncertainty: it was possible to choose values that could suit any model. It was not uncommon to find two papers claiming that quite different mechanisms were dominant in the same system, a result of not being able to isolate the individual steps. As a result of Gilbert’s work, all individual processes in emulsion polymerisation, one of the commonest ways of making everyday products, are now qualitatively and quantitatively understood. It is now possible to polymerise simple systems and to predict the molecular architecture that will be formed under chosen conditions, while for more complex conditions, trends can be semiquantitatively predicted and understood. The international scientific and technical community in this field now uses the mechanistic knowledge that he obtained as the key to understanding current processes and creating new processes and products. His work has put this industrially important field on a rigorous scientific footing.
Gilbert and others have used this knowledge and understanding to develop means of creating new materials. One major example includes his role as leader of a collaborative project that has led to a new generation of surface coatings. He developed the first practical means to implement on industrially significant scales Dr E Rizzardo’s reversible addition-fragmentation chain transfer (RAFT) method of controlled radical polymerisation.
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