We discuss the effects of local fluctuations in monomer composition during chain copolymerization reactions on sequence distributions and phase behavior. In particular, we show that near critical mixtures of type A and B monomers (or macromonomers) with equal reactivity ratios can polymerize to form very blocky random copolymers. We further argue that this blockiness can be sufficient to induce liquid-liquid phase separation or mesophase separation within the reaction medium. Applications to bulk and emulsion polymerizations are briefly discussed.