Challenges in Synthesis and Analysis of Asymmetrically Grafted Cellulose Nanocrystals via Atom Transfer Radical Polymerization.

Gwendoline Delepierre,Ilkka Kilpeläinen,Katja Heise,Tetyana Koso,Justin O Zoppe,Emily D Cranston,Alistair W T King,Leena Pitkänen,Eero Kontturi,Mauri A Kostiainen,Christoph Weder,Kiia Malinen

doi:10.1021/acs.biomac.1c00392

Abstract

When cellulose nanocrystals (CNCs) are isolated from cellulose microfibrils, the parallel arrangement of the cellulose chains in the crystalline domains is retained so that all reducing end-groups (REGs) point to one crystallite end. This permits the selective chemical modification of one end of the CNCs. In this study, two reaction pathways are compared to selectively attach atom-transfer radical polymerization (ATRP) initiators to the REGs of CNCs, using reductive amination. This modification further enabled the site-specific grafting of the anionic polyelectrolyte poly(sodium 4-styrenesulfonate) (PSS) from the CNCs. Different analytical methods, including colorimetry and solution-state NMR analysis, were combined to confirm the REG-modification with ATRP-initiators and PSS. The achieved grafting yield was low due to either a limited conversion of the CNC REGs or side reactions on the polymerization initiator during the reductive amination. The end-tethered CNCs were easy to redisperse in water after freeze-drying, and the shear birefringence of colloidal suspensions is maintained after this process.

Highlights

Cellulose nanocrystals (CNCs) are highly crystalline, rod-like nanoparticles with an anisotropic shape
The first reaction route leading to CNC-RE-g-bromoisobutyryl bromide (BiBB)-1 was based on a one-step pathway in which the amino-terminated atom-transfer radical polymerization (ATRP)-initiator-1 was linked to the reducing end-groups (REGs) via a direct reductive amination in water, at 70 °C and acidic pH (4.5), using 2-picolineborane (2-PCB) as reductant
Two pathways to selectively end-modify CNCs with ATRPinitiators were studied: (1) a direct pathway using reductive amination to attach an amino-terminated ATRP-initiator, and (2) a two-step pathway using reductive amination followed by NHS-mediated coupling of the polymerization initiator

Summary

Introduction

Cellulose nanocrystals (CNCs) are highly crystalline, rod-like nanoparticles with an anisotropic shape. The additional partial substitution of the CNC surface with sulfate half-ester groups (−OSO3−) results in charged particles that display a remarkable colloidal stability in water.[2,3] Owing to their interesting physicochemical properties, CNCs are highly attractive for biobased or biocompatible applications in fields ranging from packaging to biomedical devices, cosmetics, nanocomposites and rheological modifiers in industrial liquids.[4−7]. The physicochemical properties of CNCs can be substantially influenced by functionalizing their surface hydroxy groups.[8−10] Otherwise immiscible in hydrophobic matrices and nonpolar solvents, CNCs can be rendered more compatible by, for instance, surface-grafting of polymer chains, that are tailored toward the respective system.[10,11] this approach imparts the physicochemical properties of the grafted polymer to the CNC surface and further creates building blocks for functional colloidal dispersions and solid CNC-based materials

Methods

Results

Conclusion