Synthesis of Block Copolymers from Cellulose Nanoparticles via Atom Transfer Radical Polymerization

Abstract

Cellulose by far is the most abundant natural polymer that exists on this planet and presents scientists with the advantage to utilize it as an inexhaustible source of raw material in the synthetic development of environmentally friendly and biocompatible products. Due to its availability and low cost, cellulose and its derivatives are extensively used in industries consisting of textiles, plastics, wood and paper products, coatings, and pharmaceuticals among others. Its structural framework consists of extensive intra and intermolecular hydrogen bonding that makes it completely insoluble in normal aqueous solvents and solutions. Cellulose fibrils contain highly crystalline regions that co-exist with amorphous regions, which has a capacity of holding relatively large amounts of water, thus making it a very hygroscopic molecule. These crystalline regions can be conveniently separated from the low order regions to form rod-like cellulose microcrystallites. The rod-like particles can be coupled with various synthetic polymer structures forming hybrid copolymer blocks that display properties of amphiphiles. Atom Transfer Radical polymerization, a controlled radical polymerization process, is used an effective tool to bridge this carbohydrate molecule with a synthetic macromolecule to generate a hybrid amphiphilic copolymer block that can aggregate in aqueous environments to form micelles.

Rod-like nano-crystals were prepared by acid hydrolysis of fibrils in filter paper powder through a heterogeneous acid diffusion process that resulted in breaking of β-glycosidic linkages in cellulose fibrils. By controlling the strength of sulfuric acid, reaction time, and temperature, the crystalline regions were separated from the amorphous regions giving individual rod-like crystals that contained the non-reducing end on one side and the reducing end on the other.

Cellobiose, a basic repeat unit of cellulose, was used as a model study in polymerization of styrene to create cellobiose end-capped polystyrene polymer. The reducing end aldehyde group was reacted with 4-vinylaniline forming a derivative compound (CB-d-4AS) that was used as a functionalized initiator for ATRP. Among the various solvents tested, owing to solubility reasons, DMF gave considerable yields of CB-d-4AS. The reaction of cellobiose derivative compound with 1-(bromoethyl)benzene in presence of Cu(I)Br, pentamethyldiethylenetriamine (PMDETA) catalyst system produced an efficient initiating system in the ATRP of styrene to give CB end-capped PS polymer.

With the intention of following the model study, hydrolyzed cellulose crystals were also derivatized with 4-vinylaniline creating a compatible derivatized initiator for ATRP of styrene. Amphiphilic diblock copolymer (Cellulose-block-Polystyrene) was successfully synthesized by ATRP using C-d-4AS, 1-(bromoethyl)benzene in presence of Cu(I)Br, PMDETA catalyst system. FT-IR, 1H NMR, and TGA measurements compliment the results of model study and confirm the formation of diblock (Cell-PS) copolymer. The appearance of PS protons in the 1H NMR spectra and the absence of cellobiose and cellulose protons indicated the formation of aggregates in CHCl3 solvent, which is good selective solvent for polystyrene and not for cellobiose and cellulose.