Page 32 - Engineering Plastics Handbook

6 Introduction

5. Chain terminators, end capping
6. Nucleating agents for semicrystalline polymers
7. MW and MWD
8. Solvent removal and by-product removal for polycondensation poly-
merization such as vacuum removal of H 2 O.

Polycondensation Polymerization
(Step-Growth Polymerization)
With polycondensation polymerization, two monomers join to produce
dimers, trimers, tetramers, and higher oligomers; and finally polymers,
O. HCl, NaCl, CH OH, and other by-
plus a by-product which is usually H 2 3
products can be formed instead of water. When a single monomer is used
instead of two monomers, the reaction is self-polycondensation polymer-
ization. Stoichiometry is monitored and controlled when two different
monomers are polymerized, but stoichiometry is ensured with self-
polycondensation. Most engineering thermoplastics can be produced by
polycondensation polymerization: ABS, PET, PBT, polyamides (nylons),
polycarbonate, PAI, LCP, TPI, PEI, PPS, PPE, PSU, and PAEK.
Cyclic, linear, branched, and crosslinked polycondensation polymers
are produced with polyfunctional monomers, such as trifunctional
monomers to produce branched polymers. Bifunctional monomers yield
cyclic and linear polymers. When the ring has fewer than 5 atoms or more
than 7 atoms, the polymer can be converted to linear [3]. Polycondensation
polymerization can be an endothermic or exothermic reaction. MW is
usually about 2000 to 25,000, lower than that of chain-growth polymer-
ization [2]. Polymerization can take up to several hours and longer. The
rate of the polymerization reaction is increased when the by-product is
water, by carrying out polymerization in a vacuum to remove water.
The following examples illustrate specific polycondensation poly-
merizations: Polycondensation polymerization of ethylene glycol plus
dimethyl terephthalate (DMT) to produce PET yields CH OH by-product.
3
Crystallization is facilitated with organic and inorganic nucleating agents,
such as organic nucleating agents, monocarboxylic acid salts and poly-
carboxylic acid, sodium chlorobenzoate, disodium terephthalate, dipheny-
lamine, and epoxides; and inorganic nucleating agents, mica, silicate, clay,
titanium oxide, magnesium oxide, and antimony trioxide. Nucleating
agents include an array of fillers and reinforcing fibers as well as resid-
ual polymerization catalysts. Quick-quenching and solid-state polymer-
ization (SSP) produce clear amorphous blow-molded, bottle-grade PET.
Higher MWs are obtained by solid-state polymerization of pellets.
Solid-state polymerization involves removing polycondensation by-prod-
uct. PBT is typically the product of polycondensation of tetramethylene