Synthesis by ATRP and Properties of Liquid Crystalline Three-Armed Star Polymers Containing Azobenzene Chromophores

Abstract

Atom Transfer Radical Polymerization (ATRP) has been applied to synthesize liquid crystalline methacrylic polymers having three-arms star structure, starting from a central core of C₃ symmetry. By just changing the duration of the polymerization process, macromolecules having distinct average chain lengths with low polydispersity values and well defined end-groups have been obtained. All the obtained star-shaped polymers, characterized by differential scanning calorimetry and polarized optical microscopy, exhibit smectic and nematic liquid-crystalline phases on heating, with transition temperatures strongly dependent on the average polymerization degree, and high clearing points. The comparison of these systems with the analogous linear derivatives pointed out higher glass transition temperature values, although they are characterized by lower average polymerization degree. Such behavior can be attributed to the stiffness of the aromatic core, which limits the free movement of chain ends.

Keywords:

• atom transfer radical polymerization
• azobenzene-containing polymers
• photochromic polymers
• side-chain liquid crystalline polymers
• star polymers

Acknowledgments

The financial support by MIUR (PRIN2004) and Consortium INSTM (FIRB2001 “RBNE01P4JF”) is gratefully acknowledged.

Notes

^a Calculated as (g of polymer/g of monomer) · 100.

^b  = Conversion · (M _M6A /M _BMPB ) · MW _M6A  + MW _BMPB .

^c Determined by SEC in THF at 25°C.

^a Obtained from the second DSC thermal cycle.

^b Reference [16].