Living cis-1,4-Selective Polymerization of 1,3-Dienes by Enamino-Oxazolinate Rare-Earth Metal Complexes

Abstract

A series of enamino-oxazolinate rare-earth metal complexes were successfully synthesized and structurally characterized. The alkyl elimination of enamino-oxazolinate proligands [R1 = H, R2 = CF3 (L1-H); R1 = Me, R2 = CF3 (L2-H); R1 = i Pr, R2 = CF3 (L3-H); R1 = i Pr, R2 = Ph (L4-H)] with an equimolar amount of Ln­(CH2SiMe3)3(THF)2 (Ln = Y, Lu, Sc) smoothly afforded the desired rare-earth metal complexes [L­(1–4)-Ln­(CH2SiMe3)2]­(THF). Upon activation by a borate, all of these complexes, especially 1-Y, exhibited high catalytic activities for the living polymerization of 1,3-dienes (isoprene, β-myrcene, and β-farnesene) to afford highly cis-1,4-regular polymers (up to 99.6% for polyisoprene, 99.6% for polymyrcene, and 99.9% for polyfarnesene). Noteworthily, the presence of the polar −CF3 group in the ligands had a positive effect on the rare-earth metal complexes to realize satisfactory catalytic performance.

Binuclear Enamino-Oxazolinate Rare-Earth Metal Complexes: Synthesis and Their Catalytic Performance in Isoprene Polymerization

“…This produced PIP with high molecular weight and narrow molecular weight distribution ( M n = 8.6 × 10 4 g mol –1 , M w / M n = 1.12, entry 1 in Table ). 1 H NMR and 13 C NMR spectra indicated that the generated polymer had highly cis- 1,4 microstructure (96.1%), which was slightly lower than the polymer generated by the corresponding mononuclear yttrium complex (99.6% cis- 1,4) . The reason might be attributed to the steric repulsion of the two enamino-oxazolinate-metal units in 1-Y , as reported in previous literatures .…”

Synthesis of bis(oxazoline)-based rare-earth metal complexes and their catalytic performance in the polymerization of isoprene and polar ortho-methoxystyrene

“…5 d As its analogous enamino-oxazolinate-based yttrium catalytic system, 1-Y /[Ph 3 C][B(C 6 F 5 ) 4 ]/TIBA also promoted the isoprene polymerization in a living manner. 14 The molecular weight of the obtained PIP increased linearly with both the conversion and monomer feed increasing (Fig. 5 and entries 3, 6–9 in Table 1), and the molecular weight distributions remained narrow ( M w / M n = 1.09–1.10).…”

“…Noticeably, the catalytic activity and cis -1,4-selectivity of 1-Y /[Ph 3 C][B(C 6 F 5 ) 4 ]/Al i Bu 3 were comparable to those of its β-diketiminate and enamino-oxazolinate analogues. 5 d ,14 Adding more Al i Bu 3 such as 10 equivalents, the catalytic performance of 1-Y /[Ph 3 C][B(C 6 F 5 ) 4 ]/Al i Bu 3 had no obvious difference to that adding 5 equivalents. Moreover, the 1 H NMR spectrum of mixed 1-Y and 1 equiv.…”

“…This suggested the high thermostability of the 1-Y /[Ph 3 C][B(C 6 F 5 ) 4 ]/Al i Bu 3 ternary system, which is rather different from its enamino-oxazolinate analogue. 14…”

“…13 Very recently, we have reported rare-earth metal complexes bearing special enamino-oxazolinate ligands featuring a backbone similar to that of β-diketiminate ligand, and the yttrium-based catalytic system exhibited high catalytic activities for the living polymerization of isoprene in the presence of a borate, affording highly cis -1,4-regular polyisoprene (up to 99.6%). 14 However, the initial trials of these enamino-oxazolinate rare-earth metal catalytic systems in o MOS polymerization revealed that only atactic polymers were isolated with low activity. Considering the importance of C 2 symmetry of β-diketiminate ligand in yttrium-catalyzed isotactic polymerization of o MOS, 9 a we supposed that the rare-earth metal complexes with a C 2 symmetric bis(oxazoline) (BOX) ligand would exhibit special catalytic performances in the stereoselective polymerization of o MOS and also the cis -1,4-polymerization of isoprene.…”