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Ring-opening metathesis polymerization/cross-metathesis(ROMP/CM)of cyclooctene (COE) or 3-alkyl-substituted COEs (3R-COEs, R = ethyl, n-hexyl) using several trialkoxysilyl monofunctionalizedalkenes as chain-transfer agents (CTAs; vinyl trimethoxysilane (1), allyl trimethoxysilane (2), and 3-(trimethoxysilyl)propylacrylate (3)) and various Ru–carbene–alkylidenecatalysts afforded several trialkoxysilyl mono- and difunctionalizedpolyolefins. The formation of α-monofunctional (MF), α,ω-difunctional(DF), isomerized α-monofunctional (IMF), linear nonfunctional(LNF), isomerized linear nonfunctional (ILNF), and cyclic nonfunctional(CNF) PCOEs is rationalized by a two-stage mechanism. First, formationof monofunctionalized (MF) and nonfunctionalized (LNF, CNF) macromoleculestakes place through a ROMP/CM along with RCM (ring-closing metathesis)process. Subsequently, CC isomerization (ISOM) combined witha second CM process give isomerized (ILNF, IMF) and difunctionalized(DF) macromolecules. The nonfunctionalized polymers (CNF, LNF, andILNF) were formed in minor quantities compared to the trialkoxysilyl-functionalizedpolymers (MF, IMF, and DF), as evidenced by NMR and MALDI-ToF MS analysesand fractionation experiments. The rate and selectivity of the reactionvaried with the nature of the CTA, COE substituent, catalyst, andto 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-freeprocess with selective formation of MF polymers. The reactive Grubbs’second-generation catalyst (G2) afforded the best compromisein terms of productivity, reactivity, and selectivity. Under optimizedconditions favoring the formation of MF/DF, i.e., in CH2Cl2at 40 °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 1and 3selectively gave mixturesof MF and DF, allyl CTA 2resulted in a mixture of IMF,MF, and DF. [ABSTRACT FROM AUTHOR]