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Synthesis and Solubility of (Mono-) End-Functionalized Poly(2-hydroxyethyl methacrylate-g-ethylene glycol) Graft Copolymers with Varying Macromolecular Architectures

Comblike, side-chain graft copolymers, also called molecular bottle brushes, are branched macromolecules composed of an array of side chains attached to a main chain. Such a motif exists in biology; for example, aggrecan macromonomers which provide the compressive stiffness of cartilage tissue, have a protein core backbone and chondroitin sulfate glycosaminoglycan polysaccharide side chains spaced 2-4 nm apart.1 Recently, there has been a large interest in theoretically studying such systems,2 as well as preparing analogous synthetic macromolecules3 due to their ability to exhibit a broad array of unique conformations4 and nanomechanical properties5,6 including, in many cases, stimulus responsiveness.7 Property control can be achieved via the choice of side chain and main chain chemical structures, as well as the macromolecular architecture (e.g., side chain graft density, length, and distribution along the main chain) to tailor entropic, excluded volume, electrostatic, and other noncovalent interactions. Precise design for particular applications will require the synthesis of well-defined structural libraries in conjunction with theoretical and experimental studies of intraand intermolecular interactions, thus facilitating a detailed understanding of fundamental nanoscale structure-property relationships. Recently, we reported the synthesis, characterization, and single macromolecule elasticity measurements of a neutral (mono-) end-functionalized graft copolymer, thiol-terminated poly(2-hydroxyethyl methacrylate-gethylene glycol) or SH-poly(HEMA-g-EG) with a 1% PEG (poly(ethylene glycol)) graft density6 via the atom transfer radical polymerization (ATRP) method8 with protecting group chemistry.9 (Mono-) end-functionalization provides the ability to chemically end-graft these polymers to surfaces6 and small structures (e.g., nanoparticles, nanofibers, etc.) in a well-defined manner. Expanding upon this methodology, the first goal of this research was to synthesize a library of water-soluble SH-poly(HEMA-g-EG) graft copolymers with varying macromolecular architecture; i.e., PEG and PHEMA (poly(2-hydroxyethyl methacrylate)) molecular weights, EG/HEMA mole ratios, and PEG side chain graft density. PEG and PHEMA were chosen as the model systems since they are each individually known to exhibit unique interactions in aqueous environments.10,11 For example, PEG exhibits a hydrogen-bonded, waterbound, contracted, helical trans-trans-gauche local conformation,12 and PHEMA has been suggested to have a significant amount of bound structured water.13 Hence, both are technologically important materials used extensively for biomedical engineering applications, since such unique and strong interactions with water are likely to be a major contributor to bioinertness.14 Aside from the potential to create surfaces with improved biocompatibility, PEG/PHEMA graft copolymers also have potential for applications in areas such as single macromolecule nanomechanical design (control of molecular elasticity) and as nanoreactors for nanoparticle preparation.6,15-20 The second goal of this study was to establish the relationship between macromolecular architecture and composition with aqueous solubility, as quantified by the cloud point temperature in dilute solution, thus providing insight into the nature of macromolecular intraand intermolecular interactions.