Polymerization is used to make a whole range of materials but understanding exactly what happens during synthesis when it involves free radicals is difficult. Now, New Zealand chemists have uncovered important clues by following the rates of reaction and the termination steps involved.
Radical polymerization is commonly used to make half the world’s polymers and now Greg Russell and his colleagues at the University of Canterbury have investigated the kinetics of the process that shuts off polymerisation, the termination rate coefficient, for some very common reactions. They have revealed that the diffusion behaviour of short polymer strands, oligomers, in the reaction system is the critical factor.
“The majority of chemists simply try to bring about reactions by mixing different chemicals together under different conditions,” Russell explains. “However it is also important, especially for those who make chemical products on a large scale, to have precise quantitative descriptions of the speeds at which reactions occur. Chemical kinetics is the field of work that develops such descriptions. It is therefore an area where chemistry and mathematics intersect.
He adds that the most difficult problem in radical polymerization is the termination step that stops the polymer growing longer. Diffusion of the growing polymers is affected by their size, concentration, temperature and so on, which makes termination a complicated process to study, one that even after 60 years of intensive investigation is still not fully understood.
Russell, who is on sabbatical at the University of Goettingen, Germany, worked with Philipp Vana there and have found that although highly specialized techniques for measuring termination rate coefficients under precisely controlled conditions have been employed the results they found on attempting to replicate earlier studies were inconsistent. “I have taken this information and attempted to see whether it is consistent with systems where many different termination reactions occur at once, as is the case in commercial processes,” Russell says. “For the monomer styrene [used to make polystyrene] I find there is consistency, but for methyl methacrylate [used for polymethylmethacryalate, PMMA] there is not.”
In trying to explain this result, he eliminated most of the conventional views, and came to the conclusion that the answer lies with the oligomers in each system, which seem to have slightly different diffusional behaviour.
Macromol. Chem. Phys. 2010