Facile Method to Determine the Molecular Weight of Polymer Grafts Grown from Cellulose Nanocrystals

One of the main challenges associated with the modification of cellulose nanocrystals (CNCs) with polymers by surface-initiated polymerization is the characterization of the resulting products, notably the molecular weight of the grafts. The solid nature of the (modified) CNC nanoparticles limits the possibility to apply solution-based characterization techniques, and the cleavage of the macromolecules from the surface of the CNCs to enable their characterization using solution-based techniques is intricate. Here, we report that ¹H NMR spectroscopy of the supernatant of the heterogeneous reaction mixture can be used to approximate the molecular weight of poly(hexyl methacrylate) grafts grown from the surface of CNCs via surface-initiated atom transfer radical polymerization. This was achieved using ¹H NMR spectra to determine the monomer conversion from the change of the relative ratio of monomer and solvent signals in the ¹H NMR spectra, which in turn allowed determining the weight of PHMA produced. The number-average molecular weight of the grafted polymer was then estimated by assuming that standard atom transfer radical polymerization kinetics are at play and using the initiator concentration on the CNC surface determined by elemental analysis. The method was validated by comparing the results with the gravimetric data and the data of free polymers that were synthesized with a sacrificial initiator.