(Arylimido)niobium(V)–Alkylidenes, Nb(CHSiMe₃)(NAr)[OC(CF₃)₃](PMe₃)₂, That Enable to Proceed Living Metathesis Polymerization of Internal Alkynes

Abstract

Living polymerizations of internal alkynes (2-hexyne, 3-hexyne, 4-methyl-2-pentyne, and 1-phenyl-1-propyne) have been demonstrated at 50 °C by (arylimido)­niobium­(V)–alkylidene complex catalysts, Nb­(CHSiMe3)­(NAr)­[OC­(CF3)3]­(PMe3)2 [Ar = 2,6-Me2C6H3 (4a), 2-MeC6H4 (4c), and 2,6-Cl2C6H3 (4d)], in the presence of PMe3, which plays an essential role to proceed the living polymerization without certain catalyst deactivation. The living nature of 4a was preserved even at 80 °C in the 2-hexyne polymerization under optimized conditions, and 4c and 4d showed higher activity than 4a. The living polymerizations of 4-methyl-2-pentyne by 4c,d and of 3-hexyne by 4c have also been achieved in the presence of PMe3 at 50 °C, and the effect of the internal alkynes toward the activity by 4c is in the order 2-hexyne > 4-methy-2-pentyne > 3-hexyne > 4-octyne ≫ 5-decyne (negligible). The living polymerization of 1-phenyl-1-propyne has also been demonstrated by 4d (at 25 and 50 °C) in the presence of PMe3.

Ring Opening Metathesis Polymerization (ROMP) of Norbornenes by (Arylimido)Niobium(V)–Alkylidene Catalysts, Nb(CHSiMe₃)(NAr)[OC(CF₃)₃](PMe₃)₂

“…The rate of the ROMP is first order dependent on the monomer concentrations 21),22), 31) , but the TON values under low catalyst loading seemed to increase (runs 43, 44 and 47) due to different TCD conversion after 5 min. Although we do not have any clear explanation for the effect of the arylimido ligand between NBE and TCD, similar observations were seen in the metathesis polymerization of internal alkynes 33) . The dichlorophenylimido analogue (3) showed higher catalytic activity than the ortho-tolyl analogue (2) for the polymerization of 1-phenyl-1-propyne, with the opposite trend for the polymerizations of 1-hexyne, and 4-methyl-2-pentyne.…”

“…of PMe3, whereas no such effect was observed with 1 and 2. The ob-served activity (TOF 236 s -1 ) of the 3-PMe3 catalyst system at 50 °C is much higher than those of the (imido) niobium _ alkylidene catalysts (this report) 31), 33) as well as other Nb complex catalysts reported previously (TOF 4 h -1 by catalyst B, Scheme 1) 29),30), 32) , and is higher than the activity of the analogous vanadium( 22) . The observed effect of the arylimido ligand on the activity was different from effect on the vanadium _16),21) 23), 25) , molybdenum _7), 14) alkylidene catalysts, in which introduction of the electron withdrawing substituents lead to higher activity.…”

“…The resultant polymers prepared by 2 and 3 pos- 33) sessed high molecular weights, but the PDI values (Mw/ Mn) were rather large with no significant time course dependences toward the Mn values. Clearly these ROMPs did not proceed in a living manner as observed for 1 (as well as by the adamantylimido analogue) 31) , although the reason is not understood at present.…”

“…This increase in the activity was not observed for 1 (and by 2), so was probably as the result of improvement of stability of the catalytically active species by coordination of PMe3, as observed in the metathesis polymerization of internal alkynes (2-hexyne, 4-methyl-1-pentyne etc.) using 3 33) .…”

“…Toluene (anhydrous grade, Kanto Chemical Co., Inc.) for polymerization was transferred into a bottle containing Na _ K alloy in the drybox and was passed through short alumina column prior to use. 33) , 2,6-Cl2C6H3 (3) 33) ] were prepared according to the published procedures. Norbornene (NBE, Tokyo Chemical Ind.…”