pH Responsiveness of Dendrimer-like Poly(ethylene oxide)s

Abstract

Poly(ethylene oxide) (PEO) and poly(acrylic acid) (PAA), two polymers known to form pH-sensitive aggregates through noncovalent interactions, were assembled in purposely designed architecture -a dendrimer-like PEO scaffold carrying short inner PAA chains-to produce unimolecular systems that exhibit pH responsiveness. Because of the particular placement of the PAA chains within the dendrimer-like structure, intermolecular complexation between acrylic acid (AA) and ethylene oxide (EO) units-and thus macroscopic aggregation or even mesoscopic micellization-could be avoided in favor of the sole intramolecular complexation. The sensitivity of such interactions to pH was exploited to generate dendrimer-like PEOs that reversibly shrink and expand with the pH. Such PAA-carrying dendrimer-like PEOs were synthesized in two main steps. First, a fifth-generation dendrimer-like PEO was obtained by combining anionic ring-opening polymerization (AROP) of ethylene oxide from a tris-hydroxylated core and selective branching reactions of PEO chain ends. To this end, an AB(2)C-type branching agent was designed: the latter includes a chloromethyl (A) group for its covalent attachment to the arm ends, two geminal hydroxyls (B(2)) protected in the form of a ketal ring for the growth of subsequent PEO generations by AROP, and a vinylic (C) double bonds for further functionalization of the interior of dendrimer-like PEOs. Reiteration of AROP and derivatization of PEO branches allowed us to prepare a dendrimer-like PEO of fourth generation with a total molar mass of 52,000 g x mol(-1), containing 24 external hydroxyl functions and 21 inner vinylic groups in the interior. A fifth generation of PEO chains was generated from this parent dendrimer-like PEO of fourth generation using a "conventional" AB(2)-type branching agent, and 48 PEO branches could be grown by AROP. The 48 outer hydroxy-end groups of the fifth-generation dendrimer-like PEO obtained were subsequently quantitatively converted into inert benzylic groups using benzyl bromide. The 21 internal vinylic groups carried by the PEO scaffold were then chemically modified in a two-step sequence into bromoester groups. The latter which are atom transfer radical polymerization (ATRP) initiating sites thus served to grow poly(tert-butylacrylate) chains. After a final step of hydrolysis of the tert-butyl ester groups, double, hydrophilic, dendrimer-like PEOs comprising 21 internal junction-attached poly(acrylic acid) (PAA) blocks could be obtained. Dynamic light scattering was used to determine the size of these dendrimer-like species in water and to investigate their response to pH variation: in particular, how the pH-sensitive complexation of EO and AA units affects their overall behavior.

Versatile Route to Synthesize Heterobifunctional Poly(ethylene glycol) of Variable Functionality for Subsequent Pegylation

“…PEG (6) and PEG (7). In order to avoid this dimer formation and to synthesize PEG with protected thiol end groups, a two-step synthesis was adopted.…”

Phase-Segregated Dendrigraft Copolymer Architectures

“…This includes copolymers with polystyrene cores and a corona of poly(ethylene oxide) [50] or poly(tert-butyl acrylate)/poly(acrylic acid) [51,52] chains, copolymers derived from poly(ethylene oxide) cores with poly(tert-butyl acrylate)/poly(acrylic acid) segments in their corona [53], as well as more complex structures such as dendrimer-like poly(ethylene oxide) containing up to 48 poly(ethylene oxide) segments on the outside and 21 side-chains of poly(tert-butyl acrylate)/poly(acrylic acid) covalently bonded to the branch junctions of the preceding generations [54]. The self-assembly of some of the tert-butyl acrylate copolymers was investigated by the Langmuir force balance technique [51,52], while dynamic light scattering measurements in aqueous solutions showed that the copolymers with poly(acrylic acid) segments grafted at the branch junctions displayed pH-dependent size variations depending on the degree of ionization of the polyelectrolyte segments [54].…”

Introduction: Opportunities and Challenges of Real Time Monitoring on Membrane Processes

“…dendrimers [21]. These molecules can be made in different sizes, even nano-sizes, and they can be spherical or have other 3D structures.…”