Radiochemical studies of free‐radical vinyl polymerizations. Part I. The nature of the initiation and termination processes in methyl methacrylate and styrene polymerizations using C¹⁴‐labeled initiators

Abstract

Abstract C 14 ‐Labeled initiators, viz., di‐( ar ‐C 14 )‐benzoyl peroxide, di‐( carboxyl ‐C 14 )‐benzoyl peroxide, and C 14 ‐azobisisobutyronitrile (AZBN), have been used to initiate the polymerization of methyl methaerylate at 60°C. in benzene. Determination of the rates of polymerization, and the molecular weights ( M̄ n ) and C 14 ‐contents of the resultant polymers permits the following conclusions to be made. (1) Rates of initiation by benzoyl peroxide and AZBN are given by the equations, R i = 5.15 × 10 −6 [Bz 2 O 2 ] mole l. −1 sec. −1 and R i = 1.17 × 10 −5 [AZBN] mole l. −1 sec. −1 , respectively. (2) The initiator efficiency, f , for AZBN is 0.50; f for benzoyl peroxide cannot be evaluated in the absence of a reliable k d value for this compound in the present system. (3) Initiation is solely by addition of initiator radicals, R·, to the monomer; in the case of benzoyl peroxide, 43% of the radicals R· are phenyl and 57% are benzoyloxy under the reaction conditions used. (4) The average number of C 14 ‐initiator fragments per polymer molecule, n , is 1.27, indicating that bimolecular termination of polymethyl methacrylate radicals is 57.5% by disproportionation and 42.5% by combination.