Close inspection of the NMR spectrum reveals minor aryl CH resonances at 7.5 and 7.7 ppm alongside the major set of resonances corresponding to the main Poly(esteralt-Sx) sequence. These can be assigned to arylthioesters connected to ring-opened PO tail positions. Integration shows that 9% of polymer linkage are thioesters compared to 4% at higher sulfur loading. Formation of thioester links can be ascribed to co-occuring PTA/PO ROCOP (vide infra). As doubling the sulfur loading (compared to run #1) again decreases thioester errors (run #3), sulfur loading also appears to not only enhance reaction rate but also linkage selectivity. Increasing the reaction temperature (run #4 to #6) to 100°C results in a substantial improvement of reaction rate so that full conversion is achieved within minutes, albeit somewhat at the expense of molecular weight. Increasing the PTA to LiOBn loading (run #8 and #9) increases the final obtained molecular weights to a maximum of Mw = 43 kg/mol at a linkage selectivity of 97%. Conversely at higher catalyst loadings (run #10 and 11) decreased molecular weights of Mw = 6 kg/mol are observed, demonstrating some level of control over the obtained degree of polymerisation with our methodology. Note that this is lower than theoretical molecular weights as previously observed in co and terpolymerisation of S8 as sulfur-sulfur polymer linkages act as chain-transfer sites. Confirming that lithium acts as a true catalyst, we performed runs with NaOBn and KOBn in place of LiOBn (run #12 and #13) resulting in slower rates and low elemental sulfur incorporation. The structure of the terpolymers let us suggest a propagation mechanism as depicted in Figure Isopropyl and ethyl groups groups at the chain ends were employed for computational simplicity. S3 groups were exemplarily employed for oligosulfide groups Sx. For computational details see ESI Section S6.