The living Cl polymerization of sulfoxide ylides initiated by organoborane is one of the few methods for controlling the molecular weight, polydispersity, and topology of simple hydrocarbon polymers. However, the synthesis of linear hydrocarbon polymers (polymethylene) with molecular weights >50 kDa via this method often results in some erosion of polymer polydispersity (PDI 1.3-2.0). In the absence of known chain transfer or termination steps, the origin of the PDI erosion remained a mystery. Here, we report that the PDI erosion can be attributed to small quantities of a borinic ester (R2BOR) impurity that arises from the oxidation of the trialkylborane initiator/catalyst (R3B) by trace oxygen. The propagation rate of R2BOR is substantially lower than that of R3B. Since the oxidized initiator/catalyst would produce very little polymer during the course of polymerization, the low reactivity of R2BOR alone could not account for the increased PDI. However, we have found that during the course of the polymerization (10 min) R2BOR will complex with ylide and undergo 1,2-oxygen migration. The resulting species R2BCH2OR is a more reactive initiator/catalyst than its borinic ester precursor R2BOR. The catalyst transformation introduces reactive growing polymer chains into the system after initiation and throughout the remaining polymerization, contributing to the formation of lower MW polymer. These results are supported by a computational study of the activation energies of the rate-limiting steps. The introduction of a less oxygen-sensitive amine borane complex initiator/catalyst minimizes this complication and provides a method for synthesizing high-MW, low-PDI polymethylene.