An in-depth mechanistic study into the solution based initiator-free UV-induced radical copolymerization of 1,3-butadiene with acrylonitrile is reported. The light induced constant radical flux leads to moderate monomer conversions within 4 to 24 h. The number-average molecular weights of the prepared nitrile butadiene rubber (NBR) range from 2500 to 50 000 g mol(-1) (1.7 <= PDI <= 2.4), while the achievable monomer conversion ranged from close to 7 up to 31% depending on the polymerization temperature, reaction time and UV light intensity. The rate coefficient for the generation of primary radicals, determined as the coupled parameter k(1)*k(3), showed a dependence on the UV light intensity with values between 6.0 s(-2) and 34.6 s(-2) deduced for the UV light intensity range of 280 to 700 W. The estimated values of the lower limit average termination rate coefficient displayed no dependence on the UV light intensity, with lower limit values between 2.6 x 10(8) L mol(-1) s(-1) and 6.3 x 10(8) L mol(-1) s(-1) for the UV light intensity range of 280 to 700 W. The deduced values for the average termination rate coefficient were above the expected values for comparable average termination rate coefficients.