PPS with the simplest repeat unit structure was synthesized based on the polycondensation of p-dichlorobenzene and sodium sulfide in a polar solvent under heat. The chemistry is described. Though the chemistry seems simple, there are four principal steps involved in practical processes to obtain the good quality products: (1) preparation of sodium sulfide from aqueous sodium hydrosulfide and caustic in a polar solvent; (2) dehydration of sodium sulfide stream; (3) production of PPS from dichlorobenzene and sodium sulfide; and (4) recovery of polymer and removal of by-product, NaCl.
However, this polymerization does not seem to exhibit classic condensation polymerization behavior. That is, polymers of higher molecular weight than predicted by the Carother’s equation are produced at low conversions314 and even at unequal p-dichlorobenzene to Na2S ratios. This behavior was rationalized by Fahey and Ash as a natural consequence of the sensitivity of the SNAr mechanism to the substituents on the aryl chloride, and to the nucleophilicity of the sulfur anion for each individual growth step. Most of the growth steps have rate constants higher than that for the initial condensation of the monomers. More specifically, the enhanced activating ability of a para sulfur substituent, relative to chlorine, provides the driving force for developing high-molecular-weight polymer at low initial monomer conversions.
Since the starting materials are relatively inexpensive, commercialization of this polymer became feasible. Polymer produced in this process was a linear polymer with a molecular weight in the range of 15 000–20 000, resulting in a moderate degree of mechanical strength. The polymerization process was further refined at Phillips and the product later was sold under the tradename of Ryton Polyphenylene Sulfide. It was discovered by the same group of researchers that the as-produced moderate molecular weight polymers could be converted into a much tougher material by simple thermal treatment in oxygen or air. When the molten polymer was subjected to heat in the presence of air, the melt became dark and gelled and solidified very quickly, resulting in a crosslinked material that was insoluble in all organic solvents tested, even at elevated temperatures. Although the mechanism of this curing process has not been completely understood, obviously there are both polymer chain extension and crosslinking taking place in the curing process, which results from several possible reactions such as oxidative crosslinking, disproportionation of phenylene sulfide, and thermal crosslinking. The cured polymers were much stronger, tougher, and easier to process by injection or compression molding than the as-produced virgin polymers, but were no longer thermoplastics.
