Effect of the number of arms on the mechanical properties of a star-shaped cyclic olefin copolymer†
Star copolymers of norbornene and 1-octene with various numbers of arms were synthesized from a single linear boronic acid chain end-functionalized copolymer by varying an amine additive, which accelerated the dehydrotrimerization of boronic acids. The average number of arms of the star polymers, evaluated from the change in hydrodynamic volume, increased with the number of secondary amino groups contained in the additives and reached eight when the polymer was treated with triazine-based triamine 4c. The glass transition temperature of the copolymer was almost independent of the star assembly, whereas its mechanical strength was doubled. Moreover, the terminal relaxation time drastically increased for a star polymer with triamine 4c. These changes in mechanical properties were accompanied by an increased number of arms.