Contribution à l'étude des polymérisations amorcées par des rayonnements nucléaires
Authors: Chapiro A. , Cousin C. H. , Landler Y. , Magat M. 
Published: September 2010

It was shown in a previous publication that it is possible to initiate the polymerisation by irradiation with slow neutrons. The primary mechanism of creation of free radicals is the so‐called Szilard‐Chalmers effect (expulsion of the neutron capturing atom by the recoil energy of capture‐γ's). It appeared however that the free radicals created by the primary process could be responsible only for a small fraction (less than 10%) of the polymer formed, the rest of the polymer being initiated by secondary centers. These secondary centers are created by the capture and desintegration γ's, by desintegration β's and by collisions between the expelled atoms of high kinetic energy with molecules present. The relative contribution of these various factors could now be determined. The contribution of the various γ and β rays together to the total number of free radicals formed is of the same order as that of the primary Sz.‐Ch. effect. By far the largest part of the radicals is therefore formed by the impact of expulsed atoms.

In the course of this investigation we were obliged to study the polymerisation of pure monomers by the γ‐rays of Ra. We found that the polymerisation of pure styrene proceeded much more slowly than the polymerisation of acrylonitrile in aqueous solutions, previously investigated by Dainton and that the rate was proportional to the square root of the intensity of the source, whilst Dainton found a proportionality with the intensity. This induced us to investigate the question more closely.

The rate of polymerisation induced by γ‐rays varies greatly with the monomer. It is lowest for butadiene and styrene and highest for vinyl‐acetate. Addition of small amounts (up to 20%) of various organic compounds usually increases the rate of polymerisation.

For styrene the largest increase is produced by addition of CCl4, alcohols and amines, the smallest by benzene. It is probably due to the ability of most liquids to give more free radicals than styrene when bombarded by secondary electrons. For higher dilutions of monomers the liquids added fall into three classes: in good solvents there is a continuous rate increase up to fairly high dilutions and then a decrease. For strong precipitants (alcohols) the decrease of polymerisation rate begins shortly before the precipitation point but, after passing a minimum, the rate increases very abruptly for concentrations of precipitant higher than 80%. The rate increase is related to an increase of the molecular weight and cannot be explained by the larger number of free radicals formed.

It is suggested that this rate increase may be due to a change in the rate of the termination process, which, while still remaining mutual, becomes diffusion controlled. Poor solvents (ether) and soft precipitants (heptane) behave in an intermediate way between good solvents and strong precipitants. There are some indications that the dependence on the source intensity becomes different for very high concentrations of alcohols, where the dependence on the irradiation time ceases to be linear. The polymerisation rate increases in such cases continuously with time, as it does for pure vinylacetate. The polymerisation initiated by X‐rays and by electrons proceeds in a similar way.

The number of free radicals formed by irradiation could be evaluated by the benzoquinone addition technique, suggested by Burnett and Melville. It is of the same order as the number of ion‐pairs estimated in the usual way.

Possible kinetics mechanisms are suggested.