A reversible on/off switch to control chain growth in a living anionic polymerization is a meaningful challenge for its profound potential in both polymerization mechanisms and the preparation of advanced materials. Herein, we report a thermally controlled on/off switch for chain propagation in an anionic polymerization that is realized with a 1-cyclopropylvinylbenzene (CPVB) monomer. Interestingly, the specific ring-opening manner and thermal sensitivity are shown in the addition reaction of CPVB with the initiator or living chains. The ring opening of the cyclopropyl group in CPVB is observed to be a possible anion migrated ring-opening mechanism in the addition reactions. Additionally, the polymerization of CPVB occurs only at high temperature, and the chain growth constant k(CC) is near 0 at 20 degrees C, 0.0039 (L/mol)(1/2) min(-1) at 50 degrees C, and 0.016 (L/mol)(1/2) min(-1) at 60 degrees C. Thus, based on its unique characteristics, an ingenious design of "heat (60 degrees C)-cool (20 degrees C)" cycles is employed to achieve a thermally controlled on/off switch for chain growth in an anionic polymerization. It is expected that this finding can provide new insights into both controlling the monomer or multiblock sequence and raise a novel technique to stage control the chain growth with temperatures in living anionic polymerization.