Page 200 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Ionic Chain-Reaction and Complex Coordination Polymerization 163

Most vinyl monomers give a predominance of the isotactic product. Typically, the more exposed
the catalytic site, the less the stereoregularity of the resulting polymer. Isotactic-PP is produced
using this technique as is HDPE.
The versatility of such stereoregulating systems is demonstrated in the polymerization of 1,3-
butadiene, where all four of the potential structures- isotactic-1,2-, syndiotactic-1,2-, trans-1,4-, and
cis-1,4, can be synthesized in relatively pure form using different catalysts systems.
Molecular weight is regulated to some degree by control of the chain transfer with monomer and
with the cocatalyst, plus internal hydride transfer. However, hydrogen is added in the commercial
processes to terminate the reaction because many systems tend to form longer chains beyond the
acceptable balance between desired processing conditions and chain size.
The stereochemistry of the products is often controlled through control of the reaction tempera-
ture. For instance, use of low temperatures, where the alkyl shift and migration is retarded, favors
formation of syndiotactic-PP. Commercial isotactic-PP is produced at room temperatures.
High-density polyethylene is typically produced using some stereoregulating catalysts. Much of
it is produced using a Phillips catalyst system such as chromia catalyst supported on silica. Some
HDPE and PP are commercially produced employing a Ziegler-Natta catalyst. This initiator is also
employed for the production of polybutene and poly(-4-methyl-pentene-1) (TPX). TPX has some-
o
what high-melting point of about 300 C but because of the presence of the bulky butyl groups, a
relatively low-specifi c gravity of 0.83. The percentage of polymer that is not soluble in n-hexane is
called the isotactic index for some polymers, where the atactic and syndiotactic forms are hexane
soluble.

5.6 SOLUBLE STEREOREGULATING CATALYSIS

The 1940s was a time of studying the kinetics and mechanism of production of vinyl polymers
that took “center stage” in the 1950s. The 1950s incubated the solid-state stereoregulating catal-
ysis that spawned a chemical revolution with the synthesis of stereoregular vinyl polymers in
the 1960s. The 1980s and early 1990s served as a foundational time for soluble stereoregulating
catalysis spawning another revolution related to the production of vinyl polymers with enhanced
properties.
The solid-state stereoregulating catalysts “suffered” from at least three problems. First, while ste-
reoregular polymers were formed with good control of the stereogeometry, polymer properties still
fell short of predicted (upper limit) values. This was probably due to the presence of the associated
solid-catalyst structure that accompanies the active catalytic site. This “excess baggage” restricts
the motion of the growing chains so that while stereoregular control was good, the tendency to form
good secondary structures was interrupted.
Second, in many cases the solid-state catalysis were incorporated, as contaminants, within the

growing polymer, making an additional purification step necessary in the polymer processing to rid
the polymer of this undesired material.
Third, many solid-state catalysts offered several “active polymerization sites” due to differences
in the precise structure at and about the active sites. This resulted in an average stereoregular prod-
uct being formed.
The new soluble catalysts offer a solution to these three problems. First, the “smaller” size of
the active site, and associated molecules, allows the growing chains to “take advantage” of a nat-
ural tendency for the growing polymer chain to form a regular helical structure (in comparison to
polymers formed from solid-state catalysts).
Second, the solution catalysts allow the synthesis of polymers that contain little or no catalytic
agents, allowing the elimination of the typical additional “clean-up” steps necessary for polymers
produced from solid-state catalysts.
Third, the newer soluble catalytic sites are homogeneous, offering the same electronic and ste-
reostructure and allowing the synthesis of more stereoregular-homogeneous polymers.

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