Reactivities of monomers toward the 2-cyano-2-propyl radical by NMR study of end groups in copolymers

Copolymerizations of styrene (S) with methyl methacrylate (MMA) and methacrylonitrile (MAN) at 60°C were performed with azoisobutyronitrile as initiator, enriched in its methyl groups with ¹³C. From the NMR spectrum of a copolymer it is possible to compare the numbers of initiator fragments attached to the two types of monomer unit; this information can be used to study the competition between the monomers for capture of the radicals formed from the initiator. The velocity constants for addition of the 2-cyano-2-propyl radical to S, MMA, and MAN are in the proportions 1:0.56:0.34; it is concluded that polar effects are of some importance in the reactions of the radical.