Stoichiometric and Fractionation‐Based Assessment for Substitution and Heterogeneity in Sodium Carboxymethylcellulose Produced by Solvent‐Free Reactive Extrusion

Sodium carboxymethylcellulose (NaCMC), a natural agent for low-impact paper systems, is commonly produced via heterogeneous etherification, yet evaluating its degree of substitution (DS) and substitution dispersity remains challenging and method-dependent. Here, technical-grade NaCMC samples were synthesized by high-solids, solvent-free reactive extrusion at varying NaOH/sodium monochloroacetate (SMCA)/cellulose ratios, yielding materials with different substitution levels and heterogeneity. A Stoichiometric Data-Reconciliation (SDR) method was developed as an alternative to direct DS measurements with a stoichiometry-driven workflow, enabling indirect DS estimation (DSSDR) from reaction mass balances closed using independently quantified by-products (sodium glycolate and residual SMCA). To resolve substitution heterogeneity, centrifugation-based fractionation was implemented to isolate high-, intermediate-, and low-substitution populations, providing a mass-weighted DS (DScent). Both DSSDR and DScent showed good agreement with experimental DS values (DSexp) obtained by ATR-FTIR, across eight NaCMC formulations. Near-equimolar NaOH/SMCA maximized DS, whereas higher reagent excess (≥3:1 to cellulose) reduced substitution efficiency. Fractionation revealed dominant high-substitution populations with progressive enrichment of lower-substitution fractions upon sedimentation. Complementary morphological and rheological analyses were consistent with the fractionation outcome. Collectively, the combined SDR and fractionation strategy provides an analytically flexible distribution-aware toolkit for technical NaCMC, enabling improved process-structure-property correlations in solvent-free reactive extrusion.