Page 34 - Engineering Plastics Handbook

8 Introduction

Polycondensation (step-growth) Chain-growth (addition)
Long reaction times are needed to complete Very fast reaction times are needed to
macromolecular polymerization. complete macromolecule polymerization.
Reaction between different molecular Chain growth is entirely the addition
species can occur independently. of monomers to a growing chain.
Monomers disperse quickly to form Faster polymerization rate increases
oligomers. yield, but not average MW.
Oligomer sizes increase to termination During polymerization, mixture
of step-growth process. consists of macromolecules and
unreacted monomer with few
actively growing chains because
polymerization rate is so fast.
Reaction mixture consists of different
size oligomers—dimers, trimers,
tetramers, and higher oligomers—in a
distribution that is calculable by f(δs/δt)
with oligomer size s at a given time t.

Chain-Growth Polymerization
(Addition Polymerization)
Chain-growth polymerization produces thermoplastics such as polyac-
etal, polyethylene, polypropylene, polystyrene, methylmethacrylate,
and PVC. Molecular weight can range from 25,000 to millions of grams
per mole. Unlike in polycondensation polymerization, no by-product
such as H O, HCl, CH OH, NaCl, or any other by-product is produced.
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Also unlike in polycondensation polymerization, chain-growth poly-
merization is extremely rapid and oligomers do not exist very long, if at
all. Complete polymers can form in less than 0.1 s.
Three events are involved with chain-growth polymerization: catalytic
initiation, propagation, and termination [3]. Monomers with double bonds
(––C==C––R R ––) or sometimes triple bonds, and R and R additive groups,
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initiate propagation. The sites can be anionic or cationic active, free-rad-
ical. Free-radical catalysts allow the chain to grow when the double (or
triple) bonds break. Types of free-radical polymerization are solution free-
radical polymerization, emulsion free-radical polymerization, bulk free-rad-
ical polymerization, and free-radical copolymerization. Free-radical
polymerization consists of initiation, termination, and chain transfer.
Polymerization is initiated by the attack of free radicals that are formed
by thermal or photochemical decomposition by initiators. When an organic
peroxide or azo compound free-radical initiator is used, such as t-butyl per-
oxide, benzoyl peroxide, azo(bis)isobutylonitrile, or diazo- compounds, the
monomer’s double bonds break and form reactive free-radical sites with
free electrons. Free radicals are also created by UV exposure, irradiation,
or redox initiation in aqueous solution, which break the double bonds [3].