The reactivities of a series of 1-substituted cyclobutene derivatives (carboxylate esters, carboxamides, and carbinol esters) were investigated as substrates for ring-opening metathesis polymerization (ROMP) with [(H(2)IMes)(3-Br-pyridine)(2)(Cl)(2)Ru=CHPh]. Both the secondary amides of 1-cyclobutenecarboxylic acid and the esters of 1-cyclobutene-1-methanol undergo polymerization. The secondary amides provide translationally invariant polymers (E-olefins). Although the carbinol esters yield stereo- and regiochemically heterogeneous polymers, the 1-cyclobutenecarboxylic acid esters and tertiary amides undergo ring-opening metathesis (ROM) but not ROMP. The regio- and stereochemical outcomes of these ROMP and ROM reactions were analyzed at the B3LYP/6-31G* and LANL2DZ levels of theory. Calculations suggest that the regiochemistry and stereochemistry of the addition to the propagating carbene to form the metallocyclobutane intermediate depend on both charge distribution and steric interactions.