Synthesis of eight- and star-shaped poly(epsilon-caprolactone)s and their amphiphilic derivatives

Spirocyclic tin dialkoxides are unique initiators for the ring-expansion polymerization of lactones leading to complex, but well-defined macromolecular architectures. In a first example, epsilon-caprolactone (epsilon CL) was polymerized, followed by the resumption of polymerization of a mixture of epsilon CL and epsilon CL alpha-substituted by a chloride (alpha Cl epsilon CL), so leading to living eight-shaped chains. Upon hydrolysis of the alkoxides, a four-arm star-shaped copolyester was formed, whose each arm was grafted by conversion of the chloride units into azides, followed by the Huisgen's [3+2] cycloaddition of alkyne end-capped poly(ethylene oxide) (PEO) onto the azide substituents. The complexity of this novel amphiphilic architecture was increased further by substituting the four-arm interconnecting PCL by an eight-shaped PCL. In a preliminary step, epsilon CL was polymerized followed by a few units of epsilon CL alpha-substituted by an acrylate. The intramolecular photo-crosslinking of the acrylates adjacent to the tin dialkoxides was effective in stabilizing the eight-shaped polyester while preserving the chain growth sites. This quite unusual tetrafunctional macroinitiator was used to copolymerize epsilon CL and alpha Cl epsilon-CL, followed by hydrolysis of the alkoxides, conversion of the chloride units into azides and grafting of the four arms by PEO (see above). This architecture reported for the very first time is nothing but a symmetrical four-tail eight-shaped copolyester macromolecule.