The properties of polymer materials are largely a result of the local and long-range order of polymer chains and systems of polymer chains. The ability to tune polymer chain architecture at the monomer and chain levels through controlled synthesis is therefore a powerful tool for manipulating its properties. Perhaps, the most widely used synthetic means to manipulate polymer properties is copolymerization, where more than one monomer is simultaneously polymerized. For living anionic copolymerization, the statistics of comonomer incorporation have long been known to be dependent on the solvent, temperature, and initiator. Here, we leverage solvent dependence in the anionic copolymerization of styrene and isoprene to tailor the compositional profile along the polymer chain. Copolymerization of styrene and isoprene is conducted with varied quantities of a polar modifier (triethylamine), and the conversion is monitored by in situ attenuated total reflectance Fourier transform infrared spectroscopy. Monomer conversion profiles are used to extract reactivity ratios as a metric for examining the change in the compositional drift as the solvent composition is varied. Increasing triethylamine content leads to a continuous flattening of the compositional profile from the extreme nearly pure diblock structure for synthesis in cyclohexane to an essentially flat compositional profile in 50/50 (vol./vol.) cyclohexane/triethylamine. The ability to continuously tune compositional drift, as shown here, between these two extremes is a powerful synthetic tool for preparing copolymers and block copolymers with tunable properties.