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Communications to the Editor Vol. 75 and 51% styrene when initiated by typical free radical catalysts such as benzoyl peroxide at 60°; for typical cationic initiators such as SnCh a copolymer containing more than 99% styrene is obtained; for typical anionic initiators the copolymer contains more than 99% methyl methacrylate. Thus, the composition of the copolymer formed from /3-ray initiation indicates that a free radical mechanism is operative.By polymerizing pure styrene at 30°with various concentrations of 2-azobisisobutyronitrile, we have shown that the monoradical line3 at 30°is given by: 1 /D.P. = 2.0 X 10-6 + 69.2 Rp (Rp = rate of polymerization in moles liters-1 sec.-1). For the /3-ray induced polymerization the value of Rp is 4.16 X 10-7 (see Table 1) and the D.P. is 1.07 X 104. This point falls on the monoradical line, which constitutes another proof of the free radical mechanism.The rate of initiation of polymer chains for the /3-ray induced polymerization can be computed from Rp and the slope of the monoradical line.2 It is equal to 2.40 X 10-11 mole liter-1 sec.-1 A similar calculation was made for methyl methacrylate.If the /3-ray induced polymerization, which we have proved to proceed via radicals, occurs homogeneously throughout the medium, and if the energy to produce the initiating radicals be estimated at 50 kcal./mole, we compute that 0.19% of the absorbed radiant energy is effective in producing initiating radicals in the case of styrene and 2.3% of the energy is effective in producing initiating radicals in methyl methacrylate.

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