| Metallocenes
have well-understood structures and only one kind of active site,
producing very uniform polythene. Since Kaminsky's
discovery an estimated $3bilion worth of research has focused on
modifying and improving this family of catalysts.
Catalyst composition and geometry can be
varied systematically to produce extremely uniform homo- or co-polymers
designed to have the desired properties. Not just the length of
branches is controlled but also their distribution.
One poly(ethene) is even better than Kevlar
for making bullet-proof vests. It can stop bullets because it
has a molecular mass of between 6,000,000 and 7,000,000, much
higher than Ziegler
catalysts can give. Other polymers can be made with long chain
branches, distributed as desired by the manufacturer, either uniformly
branched or completely amorphous.
What limits their application is the
cost, many times the cost of traditional Ziegler
catalysts. But they are more productive giving 10 to 100 times
more polymer per kg of catalyst. As development continues costs
are expected to decrease.
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