There are several major factors that influence the reaction rate and product quality in the above polycondensation process, such as dehydration of sodium sulfide, catalyst, and solvent. It is important to eliminate the water of the hydrated sodium sulfide to certain degree for producing a proper ‘sulfidizing’ agent. Incomplete dehydration results in a lower reaction rate and more undesired by-products, while too little water would lead to a high reaction rate with unfavorable side reactions. In practice, the optimal water content is 0.8–1.2 mole per mole of alkali metal sulfide throughout the polymerization. Another problem that is encountered during the dehydration process is that hydrogen sulfide, an environmentally harmful substance, escapes from the reaction system. Besides environmental demands to capture the escaped harmful H2S, the loss of H2S causes a problem in that it destroys the stoichiometric balance between the alkali metal sulfide and the dichloro aromatic compound since the amount of the alkali metal sulfide varies in an uncontrolled manner. Therefore, the amount of H2S produced in the dehydration step should be analyzed precisely to fully define the amount of sulfur present in the reaction vessel. In addition, the polymerization rate can be accelerated by the proper choice of catalysts. Suitable polymerization catalysts are organic metal carboxylates. Considering the cost and efficiency, sodium acetate is a preferred catalyst since it is cheap and moderately soluble in the polymerization system.317 Other types of catalysts include organic sulfoxide compounds, cyclic amine compounds, and N-heteroaromatic compounds. In addition, achieving a high-molecular-weight polymer also depends on the use of NMP as the reaction medium. In fact, the ability of the NMP solvent to facilitate SNAr processes by dissociating ion pairs is well documented.318–320 The special qualities that NMP brings to this process go well beyond that of a conventional solvent. Fahey and Ash313 reported that a chemical reaction occurs between hydrated Na2S and NMP at elevated temperatures, yielding an isolable product with the empirical formula Na2S·NMP·H2O. It was clearly revealed by NMR that NMP ring is hydrolytically opened to yield a mixture of sodium 4-(N-methylamino)-butanoate (SMAB) and sodium hydrosulfide in equal mole. SMAB is considered to both solubilize NaSH and act as a proton acceptor.

A new synthetic process was developed later by Campbell,321 in which high-molecular-weight PPS (Mn = 35 000–65 000) were obtained directly during polymerization without the postcuring process. The new process involved the use of an alkali metal carboxylate as a polymerization modifier and eliminated the need of curing process in the above-mentioned conventional process to obtain high molecular weight. It has been reported that an soluble polymer with even higher molecular weight of 200 000 could be prepared by the incorporation of a very small amount of a 1,2,4-trichlorobenzene as a comonomer into the polymerization recipe. The mechanism of Campbell’s reaction was traditionally considered to be a nucleophilic substitution. Instead, Heitz et al.12,322 have proposed a one-electron-transfer process, with radical cations as reactive intermediates. However, the general characteristics and responses to reaction variables for this mechanism have not yet been firmly established.