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Polylactide (PLA) is a commercial and sustainably sourced aliphatic polyester but its applications have been limited by its low toughness. The insertion of a rubbery segment within the PLA backbone is among the promising strategies to enhance the mechanical properties of PLA while retaining sustainability. Herein, we disclose a catalytic stereoretentive ring-opening metathesis polymerization process to access high molar mass (Mnexp up to 127.9 kg/mol) all-cis telechelic polycyclooctene (PCOE) at low catalyst loadings. The use of cis-1,4-diacetoxy-2-butene as a chain-transfer agent in the presence of stereoretentive dithiolate Ru carbenes afforded precise control over the cis content, the molar mass, and the introduction of acetoxy chain ends. Subsequent hydrolysis of the acetoxy motifs followed by chain extension via ring-opening polymerization of D,L-lactide yielded high molar mass (Mnexp up to 105.0 kg/mol) all-cis PLA ABA triblock copolymers. The influence of the molar mass of the all-cis PCOE over the thermal and mechanical properties of the ABA triblock was investigated.