Living coordination polymerization of allene derivatives in protic solvents: Remarkable acceleration of polymerization and increase of 1,2-polymerization selectivity

The allylnickel-catalyzed living coordination polymerization of allene derivatives was carried out in protic solvents such as EtOH to figure out their effect on the polymerization behavior. For example, the coordination polymerization of methoxyallene (2a) by [(allyl)NiOCOCF3](2) (1)/PPh3 in EtOH proved to proceed in a living fashion giving a narrowly dispersed polymer in a high yield. In this case, a remarkable acceleration of the polymerization and the higher selectivity of the 1,2-polymerization were observed compared to the case of the polymerization in aprotic solvents such as toluene. That is, the polymerization of 2a (50 equiv, [2a](0) = 1.0 M) was complete within 2 h in EtOH to give a polymer in 95% yield (M-n 4400, M-w/M-n = 1.10, 1,2-: 2,3-polymerizations = 63:37), while it required 12 h in toluene to obtain a polymer having higher content of the 2,3-polymerized unit in a 99% yield (M-n = 4100, M-w/M-n = 1.07, 1,2-:2,3- polymerizations = 27:73). In protic solvents, the actual propagating species was supposed to be a cationic pi-allylnickel complex on the basis of its H-1 NMR and F-19 NMR spectra. The use of protic sources such as pyrrole, indole, and phenol was also found to be effective to accelerate the polymerization and to increase the 1,2-polymerization selectivity.