Page 33 - Engineering Plastics Handbook

Chemistry of Polymerization 7

glycol (1,4-butanediol) with terephthalic acid or dimethyl terephthalate.
Polycondensation polymerization provides effective use of the by-products
CH OH and tetrahydrofuran. Higher MW is obtained with solid-state
3
polycondensation.
Polycarbonates are produced by a number of polycondensation poly-
merization methods including interfacial polycondensation. Solid-state
polymerization of polycarbonate is the subject of industrial and aca-
demic projects to produce semicrystalline and high-MW oligomers. One
method uses supercritical CO at temperatures down to 90°C (194°F) [8].
2
Thermotropic LCPs can be produced by phenyl esterification of the car-
bonyl group, and typically in two-phase polycondensation polymerization.
Methods to produce polyphenylene-ether by polycondensation polymer-
ization include oxidation of 2,6-dimethylphenol (2,6-xylenol) with H O by-
2
product [4], or displacing the halogen from 4-halo-2,6-disubstituted
phenols, to form the ether linkage. Polysulfone can be produced by poly-
condensation polymerization of a disodium salt of bisphenol-A and p-
dichlorodiphenyl sulfone monomers. PAEK can be self-condensation
polymerized, or polymerized through intermediates.
Solution polycondensation polymerization is used when the temper-
atures needed to melt the monomer are too high or when bulk (mass)
polymerization is too exothermic and hazardous. With solution poly-
merization, monomers and polymerization initiators are dissolved in a
nonmonomeric liquid solvent at the start of polymerization. The solvent
is usually a solvent for the polymer and copolymer product.
Interfacial polycondensation polymerization is an alternative to bulk
polycondensation polymerization when bulk polymerization would
require excessively high temperatures or generate high exothermic tem-
peratures. Interfacial polycondensation polymerization is carried out at
the boundary of two immiscible solutions. The monomer in one solvent
at the interface reacts with the monomer in the other solvent at the
interface of the two solvents. One solvent can be aqueous, and the other
solvent is organic. When an emulsion is formed, the polymerization rate
is determined by the diffusion rate and emulsion “capsule” surface area.
Polymerization is very rapid.
Certain polymers such as poly(arylene-ether)s are produced by nucle-
ophilic aromatic substitution, which involves the addition of a nucleophile
during polymerization. Nucleophiles typically have negative ions (anions)
that are attracted to, and attach to, a positive charge. A nucleophile
(nucleus-friendly) is an electron-rich ion or molecule that donates electrons
to, and reacts with, an electron-poor specie. The positive nuclear charge
of an electron-poor specie is an electrophile (electron-friendly) [16, 17].
Electrons always go from a nucleophile to an electrophile. The reaction
forms a new covalent bond [16, 17].
The following table compares polycondensation with chain-growth
polymerization [2].