Precise control over polymer microstructure can enable the molecular tunability of material properties and represents a significant challenge in polymer chemistry. Stereoblock copolymers are some of the simplest stereosequenced polymers, yet the synthesis of stereoblock polyesters from prochiral or racemic monomers outside of "simple" isotactic stereoblocks remains limited. Herein, we report the development of irreversible chain-transfer ring-opening polymerization (ICT-ROP), which overcomes the fundamental limitations of single catalyst approaches by using transmetalation (e.g., alkoxide-chloride exchange) between two catalysts with distinct stereoselectivities as a means to embed temporally controlled multicatalysis in ROP. Our combined small-molecule model and catalytic polymerization studies lay out a clear molecular basis for ICT-ROP and are exploited to access the first examples of atactic-syndiotactic stereoblock (at-sb-st) polyesters, at-sb-st polyhydroxyalkanoates (PHAs). We achieve high levels of control over molecular weight, tacticity, monomer composition, and block structures in a temporally controlled manner and demonstrate that stereosequence control leads to polymer tensile properties that are independent of thermal properties.