A thorough understanding of how molecular weight influences mixed transport remains nascent. The most comprehensive study thus far was that of BBL, which demonstrated significant enhancements in both µ and µC* as the molecular weight increased from 4.9 kDa to 51 kDa. The stronger π-π interactions and higher crystallinity of larger BBL chains led to improved charge transport and therefore performance within the tested OECTs. The relevance of these finding for more common OMIEC motifs, such as polythiophenes, is questionable, since BBL shows high rigidity and volumetric capacitance, and exhibits minimal passive swelling. To address this gap, we investigate regioregular poly(3-[2-(2-methoxyethoxy)ethoxy]ethylthiophene-2,5-diyl) (P3MEEET), a simple prototypical polythiophene-based OMIEC with glycolated side-chains. Kumada catalyst transfer polymerization (KCTP) was used to obtain polymers with molecular weights ranging from 13.9 to 32.5 kg mol -1 , or Xn from 60 to 145 (Figure ); this range includes the important structural transition from chain-extended crystals to interlinked crystalline lamellae and amorphous regions for polythiophenes. As OMIECs operate through volumetric ingress of ions, the relationship between OECT performance and molecular weight were investigated within two different prototypical electrolyte compositions, NaCl and KTFSI (Figure ). We demonstrate that there is a molecular weight dependence on mixed transport within glycolated polythiophenes, however, this relationship is electrolyte dependent. Elevated OMIEC performance was observed beyond an intermediate molecular weight threshold within both the KTSFI and NaCl electrolytes, however, both µC* and gm were relatively enhanced in KTSFI compared to NaCl at higher molecular weights, primarily due to a molecular weight-dependent increase in mobility. While exsitu characterization is a great tool for rapid screening, it's inherently limited to probing material properties in the dry state. OMIECs in particular, experience significant structure modulation upon electrolyte exposure and under electrochemical bias; in-situ and operando characterization techniques allow for analysis conditions better resembling the dynamic behavior of OMIECs within OECTs. To that extent, thin films comprising P3MEEET with differing molecular weights were investigated using in-situ grazing-incidence wide-angle X-ray scattering (GIWAXS) and electrochemical quartz crystal microbalance with dissipation monitoring (E-QCMD), to understand the role of electrolyte composition on the molecular weight dependence of mixed conduction. Operando analysis suggested that the enhanced mobility of P3MEEET at higher molecular weights may be negated by significant swelling when operated in NaCl due to disruption of intercrystallite charge percolation. The role of molecular weight was revealed through operation in KTFSI electrolyte, as doping occurs through cation expulsion, preventing the determinantal swelling otherwise observed at higher molecular weights in NaCl, thereby maintaining percolative pathways. The highest performing batch of P3MEEET was at an intermediate molecular weight (Mn = 22.4 kg mol -1 ) when operated within KTFSI (µC* = 332 F cm -1 V -1 s -1 ), making it the highest performing glycolated polythiophene homopolymeric OMIEC to-date. These findings demonstrate the importance of both molecular weight and electrolyte composition to enhance OECT performance of glycolated polythiophenes. We chose poly(3-[2-(2-methoxyethoxy)ethoxy]ethylthiophene-2,5-diyl) (P3MEEET) as a model system to investigate the role of molecular weight on mixed transport due to its relatively highperformance, structural simplicity, and accessibility through well-established Kumada catalyst transfer polymerization (KCTP). Synthesis via KCTP is conceptually attractive as the existence of a long-lived p-complex via ring walking enables low dispersity (Đ = Mn/Mw) and control over molecular weight unlike traditional step-growth mediated C-C coupling polymerization processes (Stille, Suzuki). Each P3MEEET batch was polymerized following previously reported procedures using [1,3-bis-(diphenylphosphino)propane]dichloronickel(II) (NiCl2(dppp)) as a catalyst and tert-butyl magnesium chloride as a Grignard reagent (Figure ). Molecular weight control is enabled in KCTP through modifying [Ni]/[M]0, where higher molecular weight is afforded with lower catalyst loading as there are fewer propagating chains. By systematically lowering the relative loading of catalyst, batches of P3MEEET were synthesized with number average molecular weights (Mn) ranging from 13.9 to 32.5 kg mol -1 (Figure ). Further decreasing the catalyst loading did not produce polymer with higher molecular weight (Figure ); the Đ ~ 1.4 for each batch of material suggests incomplete 3MEEET activation, an observed phenomena when polymerizing hygroscopic monomers, leading to premature termination and therefore limited molecular weight. Even with these shortcomings, KCTP was used to synthesize multiple batches of P3MEEET with relatively narrow dispersity, well below the Fluory-Shultz distribution afforded by alternative step-growth systems, with systematically increasing Mn in ~ 5 kg mol -1 increments from P3MEEET-1 to P3MEEET-5. The molecular weight distributions of each batch were obtained with high resolution as an advanced polymer chromatography (APC) system was utilized (See Supporting Information); the unimodal and relatively smooth profile of each batch under high resolution APC indicates a particularly high purity of each batch. The higher molecular weight of P3MEEET-3 -P3MEEET-5 compared to previously reported glycolated polythiophenes synthesized by KCTP (Table ) allowed for the systematic study of molecular weight on performance within an organic electrochemical transistor (OECTs) for this class of material. We first studied the performance of each P3MEEET batch in OECTs operating in conventional 0.1 M NaCl electrolyte (Figure , Table ). OECTs operated within NaCl display a slight decrease in threshold voltage (|Vth|) with increasing molecular weight, consistent with previous reports. 25 The µC* of P3MEEET shows varies depending on the molecular weight, which is primarily due to variations in µ, as there was no observed relationship between C* with molecular weight (Figure ). In particular, P3MEEET-3 performed significantly better than other materials in the series due to its increased mobility of 0.14 cm 2 V -1 s -1 , indicating that optimal performance is reached at an intermediate molecular weight (Figure ). Unlike the behavior of polythiophenes in organic field-effect transistors (OFETs) which typically first show enhancements in carrier mobility with increased molecular weight followed by a leveling off at a given threshold, there seems to be an indirect relationship between molecular weight and mobility within an OECTconsistent with a previous report of p(g2T-T). The high µOECT for P3MEEET-3 is notable as it is significantly (~ 1 -2 orders of magnitude) larger than other reports of similar glycolated polythiophenes operated within the same electrolyte and is comparable to previously reports of P3MEEET operated in alternative electrolytes (Table ).