The chain microstructure (cis/trans ratio) in ring-opening metathesis polymerization (ROMP) of norbornene and methyltetracyclododecene (MTD) using the Schrock-type initiator Mo(N-2,6-i-Pr2C6H3) (=CHCMe2Ph)(OCMe3)(2) is shown to be a strong function of monomer concentration, providing a convenient means for tuning the average trans content in the resulting polymers. Moreover, since this is a "living" ROMP initiator, chains formed in batch polymerizations show a continuous gradient of trans content down the chain, with the trans content increasing with conversion. This gradient can be quite substantial; for a typical norbornene polymerization, trans content varies from 30% at one end of the chain to nearly 80% at the other end. The results are explained based on a literature kinetic description for the behavior of this initiator, where cis and trans units are added to the chain by syn and anti rotamers of the active site, respectively. A quantitative mathematical description is developed for the chain microstructure, and associated kinetic parameters are measured for norbornene and MTD at room temperature. The model accurately describes both the variation in average trans content with starting monomer concentration, and the gradients in trans content measured via samples taken at different conversions throughout the polymerization. In contrast, ROMP of MID using the first-generation Grubbs' initiator Ru(=CHPh)Cl-2(PCY3)(2) shows no down-chain gradient in trans content. (C) 2010 Elsevier Ltd. All rights reserved.