Hyperbranched Poly(ethylene glycol) Copolymers: Absolute
Values of the Molar Mass, Properties in Dilute Solution, and Hydrodynamic
Homology
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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<sup>–1</sup>), degree
of branching (DB) = 0.04–0.54, and moderate polydispersity
(<i>M</i><sub>w</sub>/<i>M</i><sub>n</sub>) ≈
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