An approach to core-shell-type architectures in hyperbranched polyglycerols by selective chemical differentiation

In contrast to dendrimers, hyperbranched polymers show no distinguishable i nterior and periphery. Hyperbranched polyglycerol, however, possesses two t ypes of OH functionalities (arising from linear and terminal glycerol units ), which can be chemically differentiated. To generate a core-shell-type ar chitecture in hyperbranched polyglycerol (M-n = 5000, M-w/M-n = 1.5), the 1 ,2-diols of the terminal glycerol units have been selectively converted int o the corresponding acetals or ketals. This allows one to distinguish betwe en interior (closer to the focal unit) and periphery (distant from the foca l unit) of the macromolecule, since the remaining linear glycerol units rem ain unaffected by this transformation. In subsequent reactions, the linear units were functionalized with alkyl halides, such as allyl chloride or ben zyl chloride, under phase transfer conditions to obtain the corresponding p olyether polyketals. Selective deprotection of the 1,2-ketals was achieved with an acidic ion-exchange resin to give "core"-functionalized polyglycero ls. By this procedure, hyperbranched polymers can be selectively tailored t o contain hydrophobic substituents in the interior or in the periphery. Dis tribution coefficients (CHCl3/H2O), unusual thermal behavior, and spectrosc opic properties of these macromolecules corroborate their core-shell-type a rchitectures. Structural inversion of the architecture (apolar periphery wi th polar core vs polar periphery with apolar core) leads to striking change s in physical properties. Furthermore, the presented synthetic methods perm it the preparation of a fully alkylated polyglycerol, which was obtained by phase transfer alkylation of polyglycerol. To demonstrate that the linear "core" units of polygylcerol are suitable for further selective functionali zation, a polyglycerol pseudo-dendrimer, containing exclusively dendritic a nd terminal units, has been prepared by selective transformation of all lin ear into dendritic units.