142 Advances in Polymer Science Editorial Board: A. Abe. A.-C ...

Dendrimers and Dendrimer-Polymer Hybrids 205
Scheme 11
lar weight of the polymer with each generation and comparison between the apparent
MW from the universal calibration and absolute MW from SEC/MALLS
proves the highly branched nature of the final product.
Truly polymeric dendrimers require an end-standing branch point with a
specific functionality. This has been realized in the case of poly(ethylene oxide)
[97]. Starting from a trifunctional anionic initiator a three-arm star is formed.
The living potassium alcoholate end groups are reacted with 2,2-dimethyl-5ethyl-5-tosyloxymethyl-1,3-dioxolane
(see Scheme 11). Hydrolysis of the 1,3-dioxolane
ring and activation of the hydroxyl groups with diphenylmethylpotassium
initiates the second generation ethylene oxide. The resulting polymer has
nine branches. The Gaussian chain shrinkage factor [98] g=/ lin=0.605
for this polymeric dendrimer. This value is intermediate between that of a regular
three-arm star (g=0.778) and a regular nine-arm star (g=0.309). Experimentally,
g'=[h]/[h] lin »0.5 has been found from SEC, assuming universal calibration
to be valid. This ratio seems low because in lightly branched regular stars g'»g 1/2
[81]. An alternative method has recently been used to make higher generation
polymeric dendrimers [99]. A true copolymeric dendrimer consisting of a central
six-arm polystyrene star and 12 peripheral PEO arms has also been reported
[100].
Similar polymers, slightly less perfect, are the umbrella star copolymers
[101]. These polymers are based on a central polystyrene star with 25 arms. An
average of five polybutadiene or poly(2-vinylpyridine) branches are grafted onto
the end of each arm. Since these polymers are models for block copolymer micelles
their properties have been studied in selective solvents. In particular, the
PBd-PS umbrella-star copolymers are monomolecularly dissolved in non-solvents
for the core-forming polystyrene.