Capture of Free Radicals by Monomers

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, as shown above, can be caused by the cage effect of the solvent molecules [41], Other reactions can take 

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, subject to steric effects  

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 [42 4]. 

Using quantum chemical calculations it was demonstrated that nucleophilic and electrophilic alkyl or aryl radicals attack alkenes following a tetrahedral trajectory [45, 46]  

This means that only substituents Y at the attacked olefinic carbon exert large steric effects [47]. In addition to the steric effects, the rates of addition of strongly nucleophilic or electrophilic radicals are governed mainly by polar effects of the substituents R, Y, and Z [48]. In borderline cases, however, the stabilities of the adducts and products tend to dominate [47]. 

---