Models for molecular weight estimation using diffusion‐ordered NMR on poly(γ‐benzyl‐ l ‐glutamate) with low degrees of polymerization

Diffusion‐ordered NMR provides a unique opportunity to obtain valuable information regarding polymer molecular weight, purity, polydispersity and physical information in one characterization method through the self‐diffusion coefficient. The diffusion coefficients of low molecular weight poly(‐benzyl‐ l ‐glutamate) (<11 000 g mol −1 ) commonly used in biomedical research were obtained in two solvents: deuterated dimethyl sulfoxide (DMSO‐d 6 ) (between 0.52 × 10 −10 and 1.21 × 10 −10 m 2 s −1 ) and deuterated chloroform (CDCl 3 ) (1.27 × 10 −10 to 2.18 × 10 −10 m 2 s −1 ). Four mathematical models for estimating the molecular weight through the diffusion coefficient were compared using the percentage change based on the molecular weight obtained from 1 H NMR end‐group analysis and matrix‐assisted laser desorption/ionization time‐of‐flight (MALDI‐TOF). The models are (I) the unmodified diffusion coefficient–molecular weight relation, (II) a small molecule model (SEGWE), (III) a species comparison model and (IV) a power law model. The unmodified diffusion coefficient–molecular weight relation proved to be linear, and this model/regression produced the lowest percentage changes of −4.04% in DMSO‐d 6 and 15.84% in CDCl 3 ; in other words, this model proved to be the most accurate in estimating the molecular weight from diffusion coefficients. The power law model yielded data that deviated significantly from the correct molecular weight, specifically in DMSO‐d 6 , with percentage changes of 78.76% in DMSO‐d 6 and −16.25% in CDCl 3 . The SEGWE model and the species comparison model produced rather inconsistent results. Moreover, polydispersity indices were estimated in both solvents and were found to be in excellent agreement (within ±10%) with those obtained by MALDI‐TOF. Lastly, concentration‐based diffusion‐ordered spectroscopy experiments confirmed the onset of chain entanglement for poly(‐benzyl‐ l ‐glutamate) in chloroform which agreed with intrinsic viscosity results.