Page 201 - Carrahers_Polymer_Chemistry,_Eighth_Edition

164 Carraher’s Polymer Chemistry

The new soluble stereoregulating polymerization catalysts require the following three features:
• A metal atom (active) site
• A cocatalyst or counterion
• A ligand system
While the major metal site is zirconium, other metals have been successfully used including Ti,
Hf, Sc, Th, and rare earths (such as Nd, Yb, Y, Lu, and Sm). Cyclopentadienyls (Cp) have been the
most commonly used ligands, though a number of others have been successfully employed, includ-
ing substituted Cp and bridged Cp. The most widely used metal-ligand grouping is zirconocene
dichloride (zirconocene dichloride has a distorted tetrahedral geometry about Zr).
Methylalumoxane (MAO) (Equation 5.52) is the most widely utilized counterion. MAO is an
oligomeric material with the following approximate structure.
H C CH 3 CH 3
3
\ | /
Al−O−[−Al−O−] −Al
n
/ \ where n = 4−20 (5.52)
H C CH 3
3
Methylalumoxane

It is believed that MAO plays several roles. MAO maintains the catalyst complex as a cation, but
doing so without strongly coordinating to the active site. It also alkylates the metallocene chloride,
replacing one of the chloride atoms with an alkyl group and removing the second chlorine, thus
creating a coordinately unsaturated cation complex, Cp MR . As an olefi n approaches the ion pair
+
2

containing the active metal, a metallocene-alkyl-olefin complex forms. This complex is the inter-

mediate stage for the insertion of the monomeric olefin into a growing polymer chain.
The structure of the catalyst complex controls activity, stereoselectivity, and selectivity toward
monomers. The catalyst structure is sensitive to Lewis bases such as water and alcohols encouraging
the use of strongly oxyphilic molecules, such as MAO, to discourage the inactivation (poisoning)
of the catalyst.
These soluble catalysts are able to give vinyl polymers that have increased stereogeometry with
respect to tacticity as well as allowing the growing chains to form more precise helical structures.
Further, the homogeneity of the catalytic sites also allows for the production of polymers with nar-
row molecular weight “spreads.”
The summation of these affects is the production of polymers with increased strength and ten-
sile properties. For PE, the use of these soluble catalysts allows the synthesis of PE chains with less
branching compared to those produced using solid-state catalysts such as the Ziegler-Natta catalysts
(ZNCs). PE produced employing soluble catalysts also show increased properties in comparison to PE
produced by solid catalysts. Table 5.2 gives some comparisons of the PEs produced using the ZNCs
with those produced with soluble catalysts.
Values of M /M of 2 or less are common for the soluble catalyst systems, whereas values of
n
w
4–8 are usual for ZNC systems. The soluble catalyst systems also are able to polymerize a larger
number and greater variety of vinyl monomers to form homogeneous polymers and copolymers in
comparison to solid-catalyst systems.
The active site is a cationic metallocene-alkyl generated by reaction of a neutral metallocene
formed from reaction with excess MAO or other suitable cocatalysts such as a borane Lewis acid.
This sequence is shown in Figure 6.1 employing MAO with ethylene to form PE. Initiation and
propagation occur through precoordination and insertion of the ethylene into the alkyl group-
polymer chain. Here termination occurs through beta-hydride elimination producing a zirconium

hydride and a long-chain alpha olefin. These long-chain alpha olefins can form linear HDPE or can

9/14/2010 3:39:01 PM
K10478.indb 164
K10478.indb 164 9/14/2010 3:39:01 PM