Polycyclotrimerization of diynes, a new approach to hyperbranched polyphenylenes

Polycyclotrimerization of diynes was explored as a new route to hyperbranched polymers in this investigation. Polymerization of terminal diynes of 1,8-nonadiyne and 1,9-decadiyne was studied using TaCl₅, NbCl₅, Mo(CO)₄(nbd) and [Mo(CO)₃cp]₂ as catalysts (where nbd = 2,5-norbornadiene, cp = cyclopentadiene). A soluble polymer was obtained when the polymerization of 1,9-decadiyne was initiated by TaCl₅ at low temperature (0°C). The polymer, however, became partially soluble after purification, possibly due to the postpolymerization-induced crosslinking. NbCl₅-catalyzed polymerization of 1,9-bis(trimethylsilyl)-1,8-nonadiyne gave a completely soluble polymer. Soluble polymers were also obtained from the polymerization of 3,9-dodecadiyne initiated by NbCl₅, Mo(CO)₄(nbd), [Mo(CO)₃cp]₂, PdCl₂-ClSiMe₃ and Pd/C-ClSiMe₃. IR, UV, and NMR spectroscopic analysis revealed that different catalysts gave polymers with different structures, ranging from linear polyenes to hyperbranched polyphenylenes. The polymers absorb UV light at around 250 nm and emit fluorescence at 340 nm when they are excited at 248 nm.