Metallocene-initiated polymerization

Other Lewis acids have been considered as alternatives to MAO for two reasons (1) one might avoid the cost of the large excess of MAO required to activate the metallocene and (2) simpler systems, which allow isolation of the product(s) from reaction of metallocene and coinitiator, would be useful to obtain a better understanding of metallocene-initiated polymerizations. 

Traditional Ziegler-Natta and metallocene initiators polymerize a variety of monomers, including ethylene and a-olefins such as propene, 1-butene, 4-methyl-1-pentene, vinylcyclo-hexane, and styrene. 1,1-Disubstituted alkenes such as isobutylene are polymerized by some metallocene initiators, but the reaction proceeds by a cationic polymerization [Baird, 2000]. Polymerizations of styrene, 1,2-disubstituted alkenes, and alkynes are discussed in this section polymerization of 1,3-dienes is discussed in Sec. 8-10. The polymerization of polar monomers is discussed in Sec. 8-12. 

Cyclopentene yields mixtures of ROMP and double-bond polymerization with some Ti and V initiators. ROMP occurs exclusively with molybdenum and tungsten initiators, as well as Re, Nb, and Ta initiators. The relative amounts of cis and trans structures vary with the initiator and temperature [Dall Asta et al., 1962 Pampus and Lehnert, 1974]. Metallocene initiators polymerize cyclopentene through the double bond, but the polymer structure consists of cis 1,3-placement (Coates, 2000 Kaminsky, 2001 Kelly et al., 1997]. 

Perfluorinated phenylboranes and perfluorinated phenylborates are well-established activators in the metallocene-initiated polymerization of olefins. With the increasing commercial importance of metallocene technology for the polymerization of ethylene and the copolymerization of ethylene and 1-alkenes, perfluorinated phenylboranes and perfluorinated phenylborates became more readily accessible. As a consequence, a few studies on the influence of these highly fluorinated activators on Nd-catalysis are available in literature. 

Not all syndioselective polymerizations proceed with polymer chain end control. Some metallocene initiators yield syndioselective polymerization through catalyst site control (Sec. 8-5). 

Termination is even more complicated than described, especially when polymerizations by metallocene initiators are considered [Beach and Kissin, 1986 Kissin et al., 1999b Lehmus et al., 2000 Liu et al., 2001b Resconi et al., 2000 Rossi et al., 1995, 1996 Thorshaug et al., 1998 Weng et al., 2000 Zhao et al., 2000]. A variety of different unsaturated end groups have been found in different polymerizations. Isomerization of propagating chain ends followed by P-hydride transfer (Eq. 8-41) results in trisubstituted double bond end... 

Titanocene and zirconocene dichlorides (Cp2MtCl2 with Mt = Ti, Zr) were the first metallocenes studied [Breslow and Newburg, 1957 Natta et al., 1957a], The metallocene initiators, like the traditional Ziegler-Natta initiators, require activation by a Lewis acid coinitiator, sometimes called an activator. AIRCI2 and A1R3 were used initially, but the result was initiator systems with low activity for ethylene polymerization and no activity in a-olefin polymerization. The use of methylaluminoxane (MAO), [A1(CH3)0] , resulted in greatly improved activity for ethylene polymerization [Sinn and Kaminsky, 1980], The properties of MAO are discussed in Sec. 8-5g. MAO has two functions alkylation of a transition metal-chloride bond followed by abstraction of the second chloride to yield a metallocenium... 

The positive charge on the transition metal in XXVII is a consequence of the tetravalent oxidation state of the transition metal in Cp2TiCl2. The active sites in traditional Ziegler-Natta polymerizations may be neutral because the transition metal is trivalent in those initiators (Secs. 8-4e, 8-4h-l). The group 3 metallocene initiators have neutral metal centers because those metals are trivalent. 

Polymerization with oscillating metallocenes is complicated because solvent fractionation of the polymer product shows separate fractions—highly atactic, mostly isotactic, and isotactic-atactic stereoblock. The mechanism of this phenomenon is not clear. It may result from the initiators not being perfectly single-site initiators. There is some evidence that a metallocene initiator may consist of more than one species, and that each species produces a different stereochemical result (Sec. 8-5g-l, 8-5h-l). 

MAO is needed in large excess relative to the metallocene initiator, usually IO2 IO4 1, to achieve high activities and stable kinetic profiles. MAO is usually added first in a polymerization system, and a portion may actually serve the function of destroying deleterious impurities prior to the addition of the metallocene initiator. Otherwise, the impurities would destroy the metallocene if the metallocene were added first. 

Cydoalkenes undergo facile polymerization because ring strain is relieved on polymerization. Polymerization occurs using both traditional Ziegler-Natta and metallocene initiators [Boor, 1979 Coates, 2000 DalTAsta et al., 1962 Ittel et al., 2000 Kaminsky, 2001 Natta... 

Cyclohexene does not polymerize by either route except when it is part of a bicyclic structure as in norbornene. Stereochemistry in the ROMP of norbomene is complicated since the polymer, LXVI in Sec. 7-8, has possibilities of isomerism at both the ring and the double bond. Most polymerizations by the typical ROMP initiators yield cis stereochemistry at the cyclopentane ring with varying amounts of cis and trans placements at the double bond [Ivin, 1987]. Metallocene initiators yield predominantly double-bond polymerization with 1,2-placement [Janiak and Lassahn, 2001]. 

---