Ring-opening metathesis polymerization/crossmetathesis (ROMP/CM) of cyclooctene (COE) or 3-alkyl-substituted COEs (3R-COEs, R = ethyl, n-hexyl) using several trialkoxysilyl monofunctionalized alkenes as chain-transfer agents (CTAs; vinyl trimethoxysilane (1), allyl trimethoxysilane (2), and 3-(trimethoxysilyl)propyl acrylate (3)) and various Ru-carbene-alkylidene catalysts afforded several trialkoxysilyl mono- and difunctionalized polyolefins. The formation of a-monofunctional (MP), alpha,omega-difunctional (DF), isomerized alpha-monofunctional (IMF), linear nonfunctional (LNF), isomerized linear nonfunctional (LLNF), and cyclic nonfunctional (CNF) PCOEs is rationalized by a two-stage mechanism. First, formation of monofunctionalized (MF) and nonfunctionalized (LNF, CNF) macromolecules takes place through a ROMP/CM along with RCM (ring-closing metathesis) process. Subsequently, C=C isomerization (ISOM) combined with a second CM process give isomerized (ILNF, IMF) and difunctionalized (DF) macromolecules. The nonfunctionalized polymers (CNF, LNF, and ILNF) were formed in minor quantities compared to the trialkoxysilyl-functionalized polymers (MF, IMF, and DF), as evidenced by NMR and MALDI-ToF MS analyses and fractionation experiments. The rate and selectivity of the reaction varied with the nature of the CTA, COE substituent, catalyst, and to a lesser extent of the solvent. The use of 1,4-benzoquinone (BZQ) as additive allowed inhibiting completely the ISOM process. Alternatively, steric hindrance in 3-RCOEs substituted monomers resulted in an ISOM-free process with selective formation of MF polymers. The reactive Grubbs' second-generation catalyst (G2) afforded the best compromise in terms of productivity, reactivity, and selectivity. Under optimized conditions favoring the formation of MF/DF, i.e., in CH2Cl2 at 40 degrees C for 24 h with [COE](0)/[CTA 3](0)/[G2](0)/[BZQ](0) = 2000:20-200:1:100, the polymerization was rather well-controlled. While CTAs 1 and 3 selectively gave mixtures of MF and DF, allyl CTA 2 resulted in a mixture of IMF, MF, and DF.