3-Substituted cis-cyclooctenes (3RCOEs, R = methyl, ethyl, hexyl, and phenyl) were synthesized and polymerized, and the polymers therefrom were hydrogenated to prepare model linear low density polyethylene (LLDPE) samples. The ring-opening metathesis polymerization (ROMP) of the 3RCOEs using Grubbs' catalyst proceeded in a regio- and stereoselective manner to afford polyoctenamers [poly(3RCOE)] exhibiting remarkably high head-to-tail regioregularity and high trans-stereoregularity. The overall selectivity increases with the increasing size of the R substituent. Hydrogenation of poly(3RCOE)s afforded precision LLDPEs with R substituents on every eighth backbone carbon.