The generation of polymer brushes by surface-initiated controlled radical polymerization (SI-CRP) techniques has become a powerful approach to tailor the chemical and physical properties of interfaces and has given rise to great advances in surface and interface engineering. Polymer brushes are defined as thin polymer films in which the individual polymer chains are tethered by one chain end to a solid interface. Significant advances have been made over the past years in the field of polymer brushes. This includes novel developments in SI-CRP, as well as the emergence of novel applications such as catalysis, electronics, nanomaterial synthesis and biosensing. Additionally, polymer brushes prepared via SI-CRP have been utilized to modify the surface of novel substrates such as natural fibers, polymer nanofibers, mesoporous materials, graphene, viruses and protein nanoparticles. The last years have also seen exciting advances in the chemical and physical characterization of polymer brushes, as well as an ever increasing set of computational and simulation tools that allow understanding and predictions of these surface-grafted polymer architectures. The aim of this contribution is to provide a comprehensive review that critically assesses recent advances in the field and highlights the opportunities and challenges for future work.

Over the past 10 years, the grafting of polymers from the surface of cellulose nanocrystals (CNCs) has gained substantial interest in both academia and industry due to the rapidly growing number of potential applications of surface-modified CNCs, which range from building blocks in nanocomposites and responsive nanomaterials to antimicrobial agents. CNCs are rod-like nanoparticles that can be isolated from renewable biosources and which exhibit high crystallinity, tunable aspect ratio, high stiffness, and strength. Upon drying, the abundance of surface hydroxyl groups often leads to a degree of irreversible aggregation, as a result of strong hydrogen bonding. Moreover, their relatively hydrophilic character renders CNCs incompatible with hydrophobic media, e.g., nonpolar solvents and polyolefin matrices. By grafting macromolecules from their surface, CNCs can be imparted with surface characteristics and other physicochemical properties that are reminiscent of the grafted polymer. This has allowed the design of nanoscale building blocks whose readily tunable properties are useful for the formation of both colloidal dispersions and solid state materials. In this Perspective, we provide an overview of the morphology and surface chemistry of CNCs and detail various techniques to manipulate their surface chemistry via polymer grafting from approaches. Moreover, we explore the most common polymerization techniques that are used to graft polymers from the surface and reducing end groups of CNCs, including surface-initiated ring-opening polymerization (SI-ROP), surface-initiated free (SI-FRP), and controlled (SI-CRP) radical polymerization. Finally, we provide insights into some of the emerging applications and conclude with an outlook of future work that would benefit the field.