Rare Earth Metal-Initiated Living Polymerizations of Polar and Nonpolar Monomers

Abstract

This review article shows that by using the versatile functions of rare earth metal complexes we can polymerize both polar and nonpolar monomers in living fashion to obtain monodisperse high molecular weight polymers at high conversions. A typical example is the polymerization of methyl methacrylate with [SmH(C5Me5 2)]2 or LnMe(C5Me5)2 (THF) (Ln = Sm, Y, Lu), which leads quantitatively to high molecular weight syndiotactic polymers (M n > 500 000, syndiotacticity > 95%) at low temperature (- 95 °C). The initiation mechanism was discussed on the basis of X-ray analysis of the 1:2 adduct (molar ratio) of [SmH(C5Me5)2]2 with MMA. Living polymerizations of alkyl acrylates (methyl acrylate, ethyl acrylate, butyl acrylate) were also made possible by using LnMe(C5Me5)2(THF) (Ln = Sm, Y), with the results: poly(methyl acrylate) M n = 48 × 103, M w/M n = 1.04, poly(ethyl acrylate) M n = 55 × 103, M w/M n = 1.04, and poly(butyl acrylate) M n = 70 × 103, M w/M n = 1.05. By taking advantage of the ABA triblock copolymerization of MMA/butyl acrylate/MMA, it was possible to obtain rubberlike elastic polymers. Lanthanum alkoxide(III) has good catalytic activity for the polymerization of alkylisocyanates (M n > 106, M w/M n = 2.08). Monodisperse polymerization of lactones, lactide, and oxirane was also achieved by polymerization with rare earth metal complexes. C1 symmetric bulky organolanthanide(III) complexes such as SiMe2[2(3),4-(SiMe3)2C5H2]2LnCH(SiMe3)2 (Ln = La, Sm, Y) show high activity for linear polymerization of ethylene. Organolanthanide(II) complexes such as racemic SiMe2[2-SiMe3-4-tBu-C5H2]2Sm(THF)2 as well as C1 symmetric SiMe2[2(3),4-(SiMe3)2C5H2]2 Sm(THF)2 were also found to have a very high activity for polymerization of ethylene. Thus, polyethylene of M n > 106 (M w/M n = 1.60) was obtained by using SiMe2[2(3),4-(SiMe3)2C5H2]2 Sm(THF)2. 1,4-cis Conjugated diene polymers of butadiene and isoprene became available by the efficient catalytic activity of C5H5NdCl/AlR3 or Nd(octanoate)3/AlR3. The Ln(naphthenate)3/ AliBu3 system allows selective polymerization of acetylene in cis fashion to take place at high yield. Considering the fact that rare earth metal-initiated living polymerization can be achieved for both polar and nonpolar monomers, attempts have been made to block copolymerization of ethylene with MMA or lactones yielding polyethylene derivatives having high chemical reactivity.