Four alternating AB copolymers have been prepared through ring-opening metathesis polymerization (ROMP) with Mo(NR)(CHCMe2Ph)[OCMe(CF3)(2)](2) initiators (R = 2,6-Me2C6H3 (1) or 2,6-i-Pr2C6H3 (2)). The A:B monomer pairs copolymerized by 1 are cyclooctene (A):2,3-dicarbomethoxy-7-isopropylidenenorbornadiene (B), cycloheptene (A'):dimethylspiro[bicyclo[2.2.1]hepta-2,5-diene-2,3-dicarboxylate-7,1'-cyclopropane] (B'), A:B', and A':B; A':B' and A:B' are also copolymerized by 2. The >90% poly(A-alt-B) copolymers are formed with heterodyads (AB) that have the trans configuration. Evidence suggests that one trans hetero C=C bond is formed when A (A or A') reacts with the syn form of the alkylidene made from B (syn-MB = syn-MB or syn-MB') to give anti-MA, while the other trans C-C bond is formed when B reacts with anti-MA to give syn-MB. Cis and trans AA dyads are proposed to arise when A reacts with anti-MA in competition with B reacting with anti-MA.