The synthesis and living ring-opening metathesis polymerization (ROMP) of substituted cyclobutenes were investigated with the functional group tolerant initiators (PCy3)(2)Cl2Ru=CHCH=CPh2 (1) and (PCy3)(2)Cl2(R)u=CHPh (2). Synthetic methodology was developed for the synthesis of a wide variety of 3-functionalized cyclobutenes containing ether, ester, alcohol, amine, amide, and carboxylic acid substituents. Coordination of these functional groups to the propagating carbene was observed, resulting in the formation of a chelated propagating species with concomitant loss of one phosphine ligand from Studies aimed at understanding this chelation and its effect on the polymerization On the basis of these results, the synthesis of a series of functionalized cyclobutenes was accomplished which minimized this chelation and allow-ed for living polymerizations. A new class of functionalized poly(butadiene)s were synthesized and their thermal properties analyzed by thermogravimetric analysis and differential scanning calorimetry.