Hyperbranched Poly(ethylene glycol) Copolymers: Absolute Values of the Molar Mass, Properties in Dilute Solution, and Hydrodynamic Homology

Abstract

Hyperbranched poly­(ethylene glycol) copolymers were synthesized by random anionic ring-opening multibranching copolymerization of ethylene oxide with glycidol as a branching agent, leading to poly­(ethylene glycol) structure with glycerol branching points. Extending the available range of molar masses by novel synthesis strategies, a limited extent of control over the degree of polymerization was achieved by variation of the solvent in this copolymerization. Generally, absolute molar mass characterization of hyperbranched polymers still represents an unresolved challenge. A series of the hyperbranched poly­(ethylene glycol)-<i>co</i>-(glycerol) copolymers (<i>hb</i>PEGs) of a wide range of molar masses (1400 < <i>M</i> < 1 700 000 g mol^–1), degree of branching (DB) = 0.04–0.54, and moderate polydispersity (<i>M</i>_w/<i>M</i>_n) ≈ 2.1 ± 0.2 were studied, in both water and dimethylformamide by the methods of molecular hydrodynamics. Analytical ultracentrifugation, intrinsic viscosity, translational diffusion measurements, and SEC were combined. Molar masses of <i>hb</i>PEGs were estimated from the comparison of the velocity sedimentation and translational diffusion coefficients, i.e., applying the Svedberg relationship. It was demonstrated that the use of linear PEG for the SEC calibration results in the significantly underestimated values of the molar masses of <i>hb</i>PEGs. The largest <i>hb</i>PEG samples exhibited a hydrodynamic radius of ≈14 nm in aqueous solution. The obtained Kuhn–Mark–Houwink–Sakurada scaling relations show linear trends in all range of molar masses. The detected scaling indexes virtually correspond to the homologous series characterized by a direct proportionality between the molar mass and the volume of the macromolecules that make up this series. The effect of branching on the molecular dimensions and on the hydrodynamic characteristics is discussed, and the corresponding contraction factors have been estimated