Isoselective polymerization

Variation in the substituents at the nitrogen donor atom has also been examined,126 and in one case isoselective polymerization of propylene was described (mmmm pentad = 56% using (31)).127 Syndioselective propylene polymerization with an rr triad content of 63% has been reported using (32)/MAO, although residual Me3Al must be removed from the MAO in order to suppress chain transfer to aluminum.128... 

While the properties and applications of isotactic polymers have been extensively studied, those of syndiotactic polymers received less attention until relatively recently. The reason is the relative ease of forming isotactic polymers. Syndioselective polymerizations were less frequently encountered or proceeded with less efficiency compared to isoselective polymerizations. But the situation is changing fast as initiators and reaction conditions have been developed for syndioselective polymerizations. In the case of polypropene, the properties of the syndiotactic polymer have been examined [Youngman and Boor, 1967]. Syndiotactic polypropene, like its isotactic counterpart, is easily crystallized, but it has a lower Tm by about 20°C and is more soluble in ether and hydrocarbon solvents. 

There have been efforts to enhance stereoselectivity in radical polymerization by using fluoroalcohols or Lewis acids that complex with monomers such as MMA and vinyl acetate [Isobe et al., 2000, 2001a Okamoto et al., 2002], In almost all instances the effects are nil or very small. For example, the use of perfluoro-t-butyl alcohol as solvent instead of toluene changes (rr) from 0.89 to 0.91 in the polymerization of MMA at —78°C. An exception is in the polymerization of acrylamide in the presence of some rare-earth Lewis acids such as ytterbium triflate. The polymer is atactic at 0°C, (m) = 0.46, in the absence of the Lewis acid, but significantly isotactic, (m) — 0.80, in the presence of the Lewis acid. The reason for this effect is unclear. More highly isoselective polymerization occurs in some radical polymerizations of MMA (Sec. 8-14b). 

For polar monomers, heterogeneity is seldom a requirement for isoselective polymerization with traditional Ziegler-Natta initiators syndiotactic polymers are obtained only with the soluble initiators. Styrene and 1,3-dienes are intermediate in behavior between the polar and nonpolar monomer. These monomers undergo isoselective polymerization with both homogeneous and heterogeneous traditional Ziegler-Natta initiators. 

Various mechanisms have been proposed to explain the stereoselectivity of Ziegler-Natta initiators [Boor, 1979 Carrick, 1973 Corradini et al., 1989 Cossee, 1967 Ketley, 1967a,b Tait and Watkins, 1989 Zambelli and Tosi, 1974]. Most mechanisms contain considerable details that distinguish them from each other but usually cannot be verified. In this section the mechanistic features of Ziegler-Natta polymerizations are considered with emphasis on those features that hold for most initiator systems. The major interest will be on the titanium-aluminum systems for isoselective polymerization, more specifically, TiCl3 with A1(C2H5)2C1 and TiCLt with A1(C2H5)3—probably the most widely studied systems, and certainly the most important systems for industrial polymerizations. 

Fig. 8-11 Mechanism for catalyst site control model of isoselective polymerization. After Cossee [1964] (by permission of Academic Press, New York and Elsevier, Oxford).

The driving force for isoselective propagation results from steric and electrostatic interactions between the substituent of the incoming monomer and the ligands of the transition metal. The chirality of the active site dictates that monomer coordinate to the transition metal vacancy primarily through one of the two enantiofaces. Actives sites XXI and XXII each yield isotactic polymer molecules through nearly exclusive coordination with the re and si monomer enantioface, respectively, or vice versa. That is, we may not know which enantio-face will coordinate with XXI and which enantioface with XXII, but it is clear that only one of the enantiofaces will coordinate with XXI while the opposite enantioface will coordinate with XXn. This is the catalyst (initiator) site control or enantiomorphic site control model for isoselective polymerization. 

The enantiomorphic site control model attributes stereocontrol in isoselective polymerization to the initiator active site with no influence of the structure of the propagating chain end. The mechanism is supported by several observations ... 

The C2-symmetric ansa metallocenes possess a C2 axis of symmetry, are chiral, and their two active sites are both chiral. The two sites are equivalent (homotopic) and enantioselective for the same monomer enantioface. The result is isoselective polymerization. C2 ansa metallocenes are one of two classes of initiators that produce highly isotactic polymer, the other class being the C ansa metallocenes (Sec. 8-5e). C2 ansa metallocenes generally produce the most isoselective polymerizations. 

Styrene is slightly polar compared to ethylene and a-olefins. The lack of a strongly polar functional group allows styrene to undergo highly (>95-98% ) isoselective polymerization... 

The stereoselectivity of polymerization depends on the transition metal and the structure of the initiator. Syndioselective polymerization is more common than isoselective polymerization. Some titanium phenoxy-imine initiators yield highly syndioselective polymerization by chain end control. For example the initiator with R2 = R3 = t-butyl yields polypropene with (rr) = 0.92 [Tian and Coates, 2000]. The initiator with R2 = t-butyl and R1 = C6Fs yields polypropene with (rr) = 0.98 [Saito et al., 2001 Tian et al., 2001], Moderately isoselective polymerization is obtained with some zirconium and hafnium phenoxy-imine initiators [Saito et al., 2002]. 

The isoselective polymerization of acetaldehyde has been achieved using initiators such as zinc and aluminum alkyls, Grignard reagents, and lithium alkoxides [Kubisa et al., 1980 Pasquon et al., 1989 Pregaglia and Binaghi, 1967 Tani, 1973 Vogl, 2000], The isoselectivity is high in some systems with isotactic indices of 80-90%. Cationic initiators such as BF3... 

Isoselective polymerization of one enantiomer or the other of a pair of enantiomers results in an optically active polymer [Ciardelli, 1987 Delfini et al., 1985 Pino et al., 1963]. For example, polymerization of (5)-3-methyl-l-pentene yields the all-.S polymer. The optical activity of the polymer would be maximum for the 100% isotactic polymer. Each racemic placement of the S-monomer decreases the observed optical activity in the polymer. 

The isoselective polymerization of a racemic mixture of monomers can proceed in two ways depending on initiator, monomer, and reaction conditions. Racemate-forming enantiomer-differentiating polymerization involves both the R and 5 monomers polymerizing at the same rate hut without any cross-propagation [Hatada et al., 2002]. A racemic monomer mixture polymerizes to a racemic mixture of all-5 and all-5 polymer molecules [Pino, 1965 ... 

Sigwalt, 1976, 1979 Tsuruta, 1972]. This is consistent with the mechanism for isoselective polymerization that attributes steric control to the initiator. The initiator contains R and 5 enantiomeric polymerization sites in equal numbers such that R sites polymerize only R monomer and S sites polymerize only S monomer. If the isoselectivity of R and S sites is less than complete, there is some cross-propagation of the two enantiomeric monomers and a decrease in the overall isotacticity of the reaction product. 

As discussed in Sec. 8-14a, an optically active polymer sample, composed of either all-5 or all-5 polymer molecules, can be synthesized by isoselective polymerization of a pure enantiomer, the pure R or pure 5 monomer, respectively. The direction of optical rotation of the polymer is usually the same as the corresponding monomer. 

More sophisticated experimental and theoretical analysis of isoselective polymerizations have been performed by using a two-site model for propagation [Inoue et al., 1984 Wu et al., 1990]. The polymer product is fractionated into the highly isotactic, insoluble and atactic,... 

Ward, B.D., Bellemin-Laponnaz, S., and Gade, L.H. (2005) C3 chiraUty in polymerization catalysis a highly active dicationic scandium(III) catalyst for the isoselective polymerization of 1-hexene. Angewandte Chemie International Edition, 44, 1668. 

Isoselective Polymerization Catalyst site control Error 1 MM M M 1 1 1 1 Chain end control Error 1 1 ... 

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