Page 55 - Chemical and process design handbook

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POLYMERIZATION

Polymerization is a process in which similar molecules (usually olefins)
are linked to form a high-molecular-weight product; such as the formation
of polyethylene from ethylene

nCH CH → H–( CH CH ) –H
2 2 2 2 n
The molecular weight of the polyethylene can range from a few thousand
to several hundred thousand.
Polymerization of the monomer in bulk may be carried out in the liquid
or vapor state. The monomers and activator are mixed in a reactor and heated
or cooled as needed. As most polymerization reactions are exothermic, pro-
vision must be made to remove the excess heat. In some cases, the polymers
are soluble in their liquid monomers, causing the viscosity of the solution to
increase greatly. In other cases, the polymer is not soluble in the monomer
and it precipitates out after a small amount of polymerization occurs.
In the petroleum industry, the term polymerization takes on a different
meaning since the polymerization processes convert by-product hydrocar-
bon gases produced in cracking into liquid hydrocarbons suitable (of lim-
ited or specific molecular weight) for use as high-octane motor and
aviation fuels and for petrochemicals.
To combine olefinic gases by polymerization to form heavier fractions,
the combining fractions must be unsaturated. Hydrocarbon gases, particu-
larly olefins, from cracking reactors are the major feedstock of polymer-
ization.

(CH ) C=CH → (CH ) CH C(CH )=CH
3 2 2 3 3 2 3 2
(CH ) CH C(CH )=CH → C H
3 3 2 3 2 12 24
Vapor-phase cracking produces considerable quantities of unsaturated
gases suitable as feedstocks for polymerization units.
Catalytic polymerization is practical on both large and small scales and
is adaptable to combination with reforming to increase the quality of the

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