Glass Transition Temperature of Cyclic Stars

Cyclic homo- and diblock copolymers with different topologies were synthesized using a combination of "living" radical polymerization and "click" coupling reactions. The topologies included 2- and 3-arm stars, with the arms consisting of either cyclic or linear polystyrene. In addition, a diblock consisting of a cyclic polystyrene and a cyclic poly(acrylic acid) was also made. The topologies by imposing topological constraints due to the presence of cyclic polymers and branch points had a marked influence on the glass transition temperature (T_g). It was found that for the polystyrene topology series, the T_g increased above the glass transition temperature at infinite molecular weight for a linear chain (i.e., T_g^∞) and correlated to the more compact nature of cyclic polymers. For the cyclic diblock of polystyrene and poly(acrylic acid), the T_g increased significantly due to separation of the blocks into their pure phases. This resulted in significant stretching of the chains and thus loss of conformation entropy.