Polymerization with Radioactive Sources and Electron Beams

Initiation of Polymerization with Radioactive Sources and Electron Beams 

Different radioactive sources can initiate free-radical polymerizations of vinyl monomers. They can be emitters of gamma rays, beta rays, or alpha particles. Most useful are strong gamma emitters, like to or Sr. Electron beams from electrostatic accelerators are also efficient initiators. The products from irradiation by radioactive sources or by electron beams are similar to, but not identical to, the products of irradiation by ultraviolet light. Irradiation by ionizing radiation causes the excited monomer molecules to decompose into free radicals. Ionic species also form from initial electron 

Once the initiating radical is formed, there is competition between addition to the monomer and all other possible secondary reactions. A secondary reaction, such as a recombination of fragments, can be caused by the cage effect of the solvent molecules. Other reactions can take place between a radical and a parent initiator molecule. This can lead to formation of different initiating species. It can, however, also be a dead end as far as the polymerization reaction is concerned. 

After the initiating radical has diffused into the proximity of the monomer, the capture of the free radical by the monomer completes the step of initiation. This is a straightforward addition reaction  

The unpaired electron of the radical is believed to be in the pure p-orbital of a planar, sp carbon atom. Occasionally, however, radicals with sp configuration appear to form.  

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