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Ionic Chain-Reaction and Complex Coordination Polymerization 161

that observed with cationic polymerization. For soluble anionic initiators at low temperatures, syn-
diotactic formation is favored in polar solvents, whereas isotactic formation is favored in nonpolar
solvents. Thus, the stereochemistry of anionic polymerizations appears to be largely dependent on
the amount of association the growing chain has with the counterion, analogous with the cationic
polymerizations.
The stereochemistry of diene polymerizations is also affected by solvent polarity. For instance,
the proportion of cis-1,4 units is increased by using organolithium or lithium itself as the initia-
tor in the polymerization of isoprene or 1,3-butadiene in nonpolar solvents. A polymer similar to
natural hevea rubber is obtained using the anionic polymerization of isoprene under these condi-
tions. In more polar solvents employing sodium and potassium initiators, the amount of cis-1,4 units
decreases and trans-1,4- and trans-3,4 units predominate.

5.4 STEREOREGULARITY
As noted in Chapter 2, there exists stereogeometry and stereoregularity in polymers. These differ-
ences have profound effects on the physical and, to a lesser degree, on the chemical properties of
the polymers produced from the same monomer. Three possible units can be formed from the poly-
merization of butadiene as shown in Equation 6.36.

R R R R R
CH 2
R (5.47)
H C H C
2
2
1,3-Butadiene 1,2- Cis-1,4- Trans-1,4-

For isoprene, there are four possible units formed (Equation 5.48).
As shown in Figure 2.5, there are three possible stereoregular forms for mono-substituted vinyl
polymers. These are isotactic—all of the pendent groups are on one side of the chiral carbon;
syndiotactic—the pendent groups appear on alternate sides of the chiral carbon; and atactic—some
mixture of geometries about the chiral carbon.
CH 3 CH 3
H C R R R R R R
2
R H C R
H C CH 3 H C 3 (5.48)
2
H C 3
C 2
H 2
Isoprene 1,2- 3,4- Cis-1,4 Trans-1,4-

It is important to realize that polymer configuration and conformation are related. Thus, there is
a great tendency for iostactic polymers (configuration) to form helical structures (conformation) in

an effort to minimize steric constrains brought about because of the isotactic geometry.

5.5 POLYMERIZATION WITH COMPLEX COORDINATION CATALYSTS
Before 1950, the only commercial polymer of ethylene was a highly branched polymer called high-
pressure polyethylene, where extremely high pressures were used in the polymerization process. The
technique for making linear polyethylene (PE) was discovered by Marvel and Hogan and Banks in
the 1940s and 1950s and by Nobel Laureate Karl Ziegler in the early 1950s. Ziegler prepared high-
density polyethylene (HDPE) by polymerizing ethylene at low pressure and ambient temperatures

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