The copolymerization behavior of 7-methylene-2-methyl-1,5-dithiacyclooctane (MDTO) with a range of common comonomers has been investigated and found to be highly unusual. The resulting copolymers contained blocks of homo-poly(MDTO), even at low MDTO feed ratios. Changes in temperature and monomer concentration have a significant effect on the copolymer composition of methyl methacrylate-MDTO and styrene-MDTO mixtures and therefore on the apparent reactivity ratios. The magnitude of the concentration effect is independent of the solvent. A new copolymerization mechanism is proposed in which the addition of an MDTO radical- to the comonomer is reversible while all other propagation reactions are effectively irreversible. To our knowledge, this is the first observation of such a mechanism. Parameters for MDTO-M-2 (M-2 = STY,MMA) according to this model are reported as (r(MDTO), r(2), k(-12)/k(22)) = (0.06, 7, 17; M-2 = STY) and (0.38, 3.2, 2.5; M-2 = MMA) at 60 degreesC.