For a general responsive polymer exhibiting a thermoresponsive transition in a solvent at the upper critical solution temperature (UCST), the interaction between polymer chains and solvent molecules and the interaction among polymer chains are of vital importance. Here, a new thermoresponsive transition in liquid crystalline poly(11-(4-((E)-4-butylstyryl)phenoxy)undecyl methacrylate) (PMAS) is found. The liquid crystalline PMAS containing mesogen moieties of stilbene in the side chains with a controlled molecular weight and narrow molecular weight distribution is synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization, and it can undergo a sharp and reversible insoluble-to-soluble transition in the solvents toluene, 1,2,4-triethylbenzene, and 1,4-dioxane at an UCST ranging from 26.3 degrees C to 52.3 degrees C. Why and how PMAS undergoes a thermoresponsive solubility transition in solvents are revealed, and the conclusion that crystallization drives the thermoresponsive solubility transition is made. It is believed that the crystallization strategy may be utilized to design new thermoresponsive polymers.