Ring-opening polymerization of benzocyclopropene and naphtho[b]cyclopropene

Abstract

Benzocyclopropene and naphtho[b]cyclopropene have been prepared and subjected to polymerization in the presence of radical and cationic initiators, or thermally, under various conditions. The properties of the resulting polymers have also been investigated. Benzocyclopropene was polymerized with BPO in benzene solution at 80$^\circ$C to give white powdery polymer. Polymerizations also proceeded successfully with boron trifluoride etherate in methylene chloride solution below room temperature. The conversions of radical polymerizations were higher than those of thermal polymerizations at all conditions. The number average molecular weights of polymers lay in the range of 10000-20000 and 2000-3000 in the case of radical(or thermal) polymerization and cationic polymerization, respectively. All the polymers were readily soluble in common organic solvents. The NMR and IR spectra of the polymer confirmed their structure to be the poly (methylene-1,2-phenylene). Naphtho[b]cyclopropene exhibited similar behaviors of polymerization with benzocyclopropene to produce poly(methylene-2,3-naphthylene). However, there was little difference between polymer yields of radical polymerizations and those of thermal polymerizations. The molecular weights were about 2000. These results implied that the polymerizability of naphtho[b]cyclopropene was inferior to that of benzocyclopropene. In the postulated mechanism of radical polymerization, a radical attacks the fused cyclopropene ring to open three-membered ring forming a stable benzyl radical which reacts with another monomer to propagate. In the case of thermal polymerization, the polymerization is initiated by 1,3-diradicals resulting from the homolysis of the single bond. This was supported by the fact that the polymerization of vinyl monomer such as MMA was initiated by benzocyclopropene and naphtho[b]cyclopropene. Quantitative yield of PMMA was produced when MMA inhibited with 10 ppm of MEHQ was polymerized with 1 mole \%-benz...