Functional Polymers - American Chemical Society

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Chapter 11

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

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Hajime Yasuda, Eiji Ihara, Yuu Nitto, and Takamaro Kakehi, Masakazu Morimoto, and Mitsufumi Nodono Department of Applied Chemistry, Faculty of Engineering, Hiroshima University, Higashi-Hiroshima 739, Japan

This article deals with the rare earth metal initiated living polymerization of polar and nonpolar monomers to give monodisperse high molecular weight polymers at high conversion. A typical example is seen in the polymerization of methyl methacrylate with [SmH(C Me ) ] or LnMe(C Me ) (THF)] (Ln=Sm,Y, and Lu), giving high molecular weight syndiotactic polymers (M > 500,000, syndiotacticity > 95%) quantitatively at low temperature (-95 °C). Synthesis of high molecular weight isotactic poly(MMA) with very low polydispercity was also achieved by the efficient catalytic action of [(SiMe ) C] Yb. Living polymerizations of alkyl acrylates were successfully carried out by the excellent catalysis of LnMe(C Me ) (THF) (Ln=Sm and Y). By taking advantages of the living polymerization ability, we attemted the ABA triblock copolymerization of MMA/butyl acrylate/MMA to obtain rubber-like elastic polymers. Organo rare earth metal complexes such as LnOR(C R ) or LnR(C R ) (Ln = Sm and Y, R = Me) conducted the living polymerizations of various lactones such as ß-propiolactone, δ-valerolactone andε-caprolactone,and also conducted the block copolymerizations of methyl methacrylate with various lactones. C symmetric bulky organolanthanide(III) complexes such as SiMe [2(3),4-(SiMe) C H ] LnCH(SiMe ) (Ln = La, Sm, and Y) show high catalytic activity for linear polymerization of ethylene. Organolanthanide(II) complexes such as racemic SiMe [2-SiMe -4tBu-C H ] Sm(THF) as well as C symmetric SiMe [2(3),4(SiMe ) C H Sm(THF) were found to show high catalytic activity for the polymerization of ethylene, giving M > 10 with M /M = 1.6. Utilyzing the high polymerization activity of rare earth metal complexes towards both polar and nonpolar monomers, block copolymerizastions of ethylene with polar monomers such as methyl methacrylate and lactones were realized for the first time. 5

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Highly Stereospecific Living Polymerization of Alkyl Methacrylates It is very significant to find ways which synthesize highly syndiotactic or isotactic polymers with Af > 500,000 and M /Af < 1.05. Although various living n

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©1998 American Chemical Society In Functional Polymers; Patil, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

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polymerization systems have been reported, no type of anionic (1), cationic (2), Group Transfer(3), and metal carbene initiated rxdymerizations (4) has ever achieved this end. In this connection, it is remarkable that high molecular weight poly(MMA) having an unusually low polydispersity has been synthesized by the use of organolanthanide(III) complexes (Fig. 1)(5). The relevant complexes include lanthanide hydrides, bulky alkyl lanthanide, trialkylaluminum complexes of alkyllanthanides, and simple alkyl complexes synthesized from LnCl(C5Me5>2 (Fig. 2). Most of them have been isolated and well characterized by single X-ray analysis. The results from the polymerization of MMA with [SmH(C5Me5)2h initiator at different temperature are summarized in Table 1. The most striking is M /Af =1.021.04 forAf > 60 x 10 - Remarkably, SmH(C5Me5)2l2 complex gives high conversion (polymer yield) in a relatively short period, and allow the polymerization w

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Living Polymerization of Alkyl Acylates In general, living polymerization of alkyl acrylates is difficult because the chain transfer or temination occurs preferentially, owing to a high sensitivity of the acidic a-proton to the nucleophilic attack. Exceptions are the living polymerization of a bulky acrylic ester catalyzed by the bulky alkyllithium/inorganic salt (LiCl) as well as the Group Transfer polymerization of ethyl acrylate with Znl as the catalyst Porphyrinatoaluminum initiator systems also induced the living polymerrization of tDownloaded by UNIV OF ROCHESTER on June 18, 2013 | http://pubs.acs.org Publication Date: May 8, 1998 | doi: 10.1021/bk-1998-0704.ch011

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Fig. 12. Typical divalent organolanthanide complexes with Sml2 (29). Table 7 shows the results from the olefin polymerization with divalent samarium complexes. It is seen that the mesotypecomplex has the highest activity for the polymerization of ethylene, but the molecular weights of the resulting polymers are the lowest On the other hand, the racemic and C i symmetric complexes produce much higher molecular weight polyethylene but the activity is

In Functional Polymers; Patil, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

159 rather low. Particularly, the very high molecular weight polyethylene (M > 100 x 10 ) obtained with Ci complex deserves attention. For the polymerization of aolefins, only the racemic divalent complex showed good activity at 0 °C in toluene: poly(l-hexene) Af = 24,600, MJM = 1.85; poly(l-pentene) Af = 18,700, M^IM - 1.58. Thus, we see that the reactivity of divalent organolanthanide complexes depends on their structure. The poly(l-alkene) obtained revealed highly isotactic structure (> 95%) when examined by C NMR. The dihedral angles of Cp'-Ln-Cp of racemic and mesotypedivalent complexes were 117 and 116.7°, respectively; these values are much smaller than those of Sm(C5Me5) (THF) (136.7°) (32) and Sm(C5Me5)2 (140.1°) . Therefore, it can be concluded that the complexes having smaller Cp'-Ln-Cp angles are more active for the polymerization of ethylene and ccolefins. In the initiation step, an a-olefin inserts between two racemic lanthanide(II) species, but the insertion hardly occurs between two Ci-symmetric lanthanide(II) complex due to steric reasons. On the other hand, meso-type lanthanide(II) should easily take place the ^-elimination to result in lanthanide(III)-allyl complex. 1,5Hexadiene was polymerized smoothly by the catalytic action of rac-Me2Si(2-SiMe34-tBu-C5H2)2-Sm(THr )2 to give poly(methylene-13-cyclopentane) at a ratio of cis/trans = 55/45. n

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Fig. 13. Cyclic polymerization of 1,5-hexadiene Polymerization of Styrene Recent study showed that the single component initiator [(tBuCp)2LnCH3]2 (Ln = Pr, Nd, Gd) can initiate the polymerization of styrene at relatively high temperature, 70°C, with a conversion of 96% and the Af of 3.3 x 10 for [(tBuCp) NdCH3] , though stereoregularity was very poor(30). Styrene polymerization was also performed successfully using the single component initiators, [(Me3Si)2N] Sm(THF) , [(Me3Si) CH] Sm, and La(C5Me )[CH(SiMe3)2]2(THF) 4

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Fig. 14. Styrene polymerization by La(C5Me5)[CH(SiMe3)2h at 50 °C in toluene without addition of any cocatalyst. The resulting polymers had Af = 1.5 - 1.8 x H^and MJM = 1.5 - 1.8, and show atactic property (23). Thus no success has yet been achieved in synthesizing syndiotactic polystyrene with rare earth metal complexes, in contrast to the synthesis of highly syndiotactic polystyrene with TiCl3(C5Me5)/(AlMe-0-) (syndiotacticity > 95%) (31). n

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Block Copolymerization of Ethylene with Polar Monomers Block copolymerization of ethylene or propylene with polar monomers is yet to be attained in polyolefin engineering. The success of this type of block copolymerization should give hydrophobic polymeric materials having remarkably high adhesing, dyeing, and moisture adsorbing properties. The following is the first example of a well-controlled block copolymerization using the unique dual catalytic

In Functional Polymers; Patil, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

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function of LnR(C5Me5>2 (Ln = Sm, Yb, and Lu; R = H, Me) complexes toward polar and nonpolar olefins (32). Block copolymenization was carried out by the homopolymerization of ethylene (17-20 mmol) with SmMe(C5Me5)2(THF) or [SmH(C5Me5)2]2(0.05 mmol) at 20 °C in toluene under atmospheric pressure,

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Fig. 15. Block copolymerizations of ethylene with MMA followed by sequential addition of MMA (10 mmol) (Table 8). The initial step proceeded very rapidly, completed in 2 min, and gave a linear polymer of Af = ca. 10,100 and M /Af = 1.42 - 1.44. However, the second step was rather slow, with the reaction taking 2 h at 20 °C. The resulting polymer was soluble in 1,2dichlorobenzene and 1,2,4-trichlorobenzene at 100 °C but insoluble in THF and CHCI3. This fact indicates quantitative conversion to the desired linear block copolymer. Repeated fractionation in hot THF did not change the molar ratio between the polyethylene and poly(MMA) blocks, though poly(MMA) blended with polyethylene can easily be extracted with THF. With the copolymerization, the elution maximum in GPC shifted to a higher molecular weight region, with its initial unimodal pattern unchanged. The relative molar ratio of the polyethylene and poly(MMA) blocks was controllable at will in the range of 100:1 to 100:103 if the Af of the initial polyethylene was fixed to ca. 10300. H and C NMR spectra for the copolymers as well as their IR absorption spectra were superimposable onto those of the physical mixtures of the respective homopolymers. The molar ratio of the poly(MMA) and polyethylene blocks, however, decreased as the Af of the prepolymer increased, especially when it is exceeded ca. 12,000 at which value polyethylene began precipitating as fine colorless particles. It is noteworthy that smooth block copolymerization of ethyl acrylate or methyl acrylate to the growing polyethylene chain (Af = 6,600 - 24,800) can be realized also by the sequential addition of these two monomers. The above work was extended to the block copolymerization of ethylene with lactones. 8-Valerolactone and E-caprolactone were incorporated to the growing polyethylene end at ambient temperature and the expected AB type copolymers (100:1 to 100:89) were obtained in high yield. Reversed addition of the monomers (first MMA or lactones and then ethylene) induced no block copolymerization at all, even in the presence of excess ethylene, and only homo-poly(MMA) and homopoly(lactone) were produced. The treatment of the resulting block copoly(ethylene/MMA) (100:3, Af = 35,000) and block copoly(ethylene/ecaprolactone) (100:11, Af = 12,000) with dispersed dyes (Dianix AC-E) made them deeply dye with three primary colors,blue, red and yellow, though polyethylene itself was inert to these dyes. Hence, these copolymers can be said to have a very desirable chemical reactivity. More recently, Yang et al. have examined a new approach in which a reactive functional group was introduced into polyolefins using methylenecyclopropane. Thus, ethylene (1.0 atm) was copolymerized with methylenecyclopropane (0.25 - 2.5 mL) using [LnH(CsMe5)2r2 (Ln = Sm, Lu) in toluene at 25 °C, and it was shown that 10-65 of exo-methylenes were incorporated per 1000 -CH - units. The resulting polymer had a Af of 66-92 x 10 . Yet its Af /M was larger than 4.0. n

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In Functional Polymers; Patil, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

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Table 8. Block Copolymerization of Ethylene with Polar Monomers a

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Polar Polyethylene block Polar polymer block Unit monomer Afn/10 ratio Afn/10 Af /Afn M /Afn MMA 10.3 1.42 1.37 100:103 24.2 26.9 100:13 1.37 1.39 12.8 100:12 40.5 1.90 1.40 18.2 100:71 MeA 6.6 1.40 1.36 15.0 100:4 24.5 1.66 2.01 3.0 EtA 100:85 10.1 1.44 2.74 30.8 100:21 24.8 3.84 1.97 18.2 VL 100:20 10.1 1.45 1.44 7.4 100:5 1.97 24.8 1.97 4.7 CL 100:89 6.6 23.9 1.76 1.40 100:7 2.01 24.5q 2.01 6.9 a) Determined by GPC using standard polystyrene, b) Determined by l H NMR. Polymerization was carried out at 0 °C. w

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In Functional Polymers; Patil, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.