Syntheses of Triblock Bottlebrush Polymers Through Sequential ROMPs: Expanding the Functionalities of Molecular Brushes This manuscript is dedicated to the 75th birthday of Professor Bob Grubbs for his life-long extraordinary achievement in research and education. Lu Su, 1,2,3 Gyu Seong Heo, 1,2,3 Yen-Nan Lin, 1,2,3,4 Mei Dong, 1,2,3 Shiyi Zhang, 1,2,3 Yingchao Chen, 1,2,3 Guorong Sun, 1,2,3 Karen L. Wooley 1,2,3,5 1 Department of Chemistry, Texas A&M University, College Station, Texas 77842 2 Department of Chemical Engineering, Texas A&M University, College Station, Texas 77842 3 Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77842 4 College of Medicine, Texas A&M University, Bryan, Texas 77807 5 Laboratory for Synthetic-Biologic Interactions, Texas A&M University, College Station, Texas 77842 Correspondence to: K. L. Wooley (E-mail: wooley@chem.tamu.edu) or G. Sun (E-mail: guorong.sun@chem.tamu.edu) Received 23 March 2017; accepted 25 April 2017; published online 20 May 2017 DOI: 10.1002/pola.28647 KEYWORDS: block copolymer; functionality; grafting through; nanoparticles; self-assembly; sequential ring-opening metathesis polymerization INTRODUCTION During the past decade, functional bottlebrush polymers, or molecular brushes, have attracted increasing atten- tion, due to their unique physiochemical properties and supra- molecular assembly behaviors, 1–3 as well as demonstrated applications in lithography, 4–6 nanomedicine, 7–9 and photon- ics. 10,11 Three approaches, namely “grafting through,” 4–15 “grafting onto,” 16–18 and “grafting from,” 19–23 have been devel- oped to achieve well-defined bottlebrush polymers. While each synthetic tactic holds its own advantages, it has been recognized that the “grafting-through” strategy can provide greater feasibil- ity on ensuring quantitative grafting efficiency and on tuning the chemical compositions, the functions, and the relative con- centric and lengthwise dimensions, throughout the entire mac- romolecular framework. Serving as a leading pioneer in this regime, Grubbs and coworkers developed a highly active, air and moisture stable, and broadly functional group tolerant ruthenium-based catalyst (Grubbs G3 catalyst) that realized ring-opening metathesis polymerization (ROMP) of norbornene- terminated macromonomers (NB-MMs) with molecular weights up to tens of kiloDaltons into well-defined bottlebrushes. 24,25 That seminal contribution expands ROMP as a powerful and universal toolbox for the synthesis of functionalized bottlebrush polymers and significantly promotes the continued exploration of material and biomedical applications of molecular brushes. To date, NB-MMs containing backbone or pendant ester, ether, and amide functionalities, synthesized from ring-opening polymerizations (ROPs), controlled radical polymerizations, or postpolymerization couplings, have been utilized to generate a multitude of bottlebrush structures with different properties, diverse functional groups, and well-controlled dimensions, through “grafting-through” ROMPs. It is noteworthy that, although various homo- and diblock molecular brushes have been well documented, there are limited reports on the synthe- ses of triblock bottlebrush copolymers through sequential ROMPs of NB-MMs, particularly for macromonomers bearing hetero functionalities. Moreover, recent research revealed that the NB anchoring groups could affect the propagation rate of ROMP-based “grafting-through” polymerizations, due to a com- bination of steric and electronic variations between different polymerizable norbornenyl groups. 26 With an interest in multifunctional bottlebrush polymers that are capable of supramolecular assembly as biologically active nanomaterials, we explored the living nature of ROMP via sequentially “grafting through” a series of functional NB-MMs towards functional triblock brushes, which composed a hydro- philic and stealth segment, a hydrophobic segment that allows for orthogonal self-assembly, and a DNA delivery-addressable functional segment. The corresponding NB-MMs with different NB anchor groups were synthesized by amidation-based cou- plings, ROP, and reversible addition-fragmentation chain transfer (RAFT) (co)polymerization, respectively. Aqueous assemblies of the resulting amphiphilic triblock bottlebrush copolymers with Additional Supporting Information may be found in the online version of this article. V C 2017 Wiley Periodicals, Inc. 2966 JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2017, 55, 2966–2970 RAPID COMMUNICATION WWW.POLYMERCHEMISTRY.ORG JOURNAL OF POLYMER SCIENCE