On the mechanism of polymerization of caprolactam catalyzed by phosphoric acid. II

Abstract

In the presence of water or catalysts giving off water, caprolactam is polymerized by several reaction mechanisms taking place simultaneously. It was investigated whether the formation of polyamide takes place according to a polycondensation or polyaddition mechanism in the presence of phosphoric acid. In order to prove the polycondensation mechanism, the formation at 250°C. of a mixed anhydride from the carboxylic acids and the orthophosphoric acid was studied. N‐phosphoryl aminocaproic acid has also been synthesized. No catalytic effect was exerted by this compound; on the basis of this fact it can be stated that in the case catalysis by orthophosphoric acid, the increase of the polyamide chain does not take place according to a polycondensation mechanism. The adduct of caprolactam and orthophosphoric acid is able to catalyze the polymerization of caprolactam. Polymerizations carried out at temperatures ranging from 170 to 200°C. proved that the acid—amide bond of the caprolactam ring is weakened by the phosphoric acid, thereby rendering possible the polymerization to take place at a lower temperature. At elevated temperatures (exceeding 160°C.), the ammonium salts of phosphoric acid undergo dissociation; thus, free orthophosphoric acid is always present in the mixture undergoing polymerization. Due to the dissociation at higher temperatures, the phosphoric acid yields a product having higher molecular weight than at lower temperatures. It has been established that the rate of polymerization is directly proportional to the square root of the molar concentration of the free orthophosphoric acid. The activation energy of the polymerization catalyzed by phosphoric acid was 46 kcal./mole.

Cationic Ring Opening polymerization of ε-caprolactam by a Montmorillonite Clay Catalyst

“…Mechanism studies showed that monomer inserted into the growing chains with the acyloxygen bond scission rather than the break of alkyl-nitrogen bond. FTIR, 1 H NMR, and 13 C RMN spectroscopic studies did not indicate any significant difference in the structure between the obtained polymer and that obtained by strong acids. DSC showed a slight improvement in thermal stability compared to what is reported in the literature.…”

“…Measurements of 1 H and 13 C NMR spectra molecular weight of polymer reach maximum values around 240-250 °C. On the other hand, with the increase in the reaction temperature above 250 °C the molecular weight of the obtained polymer and monomer conversion decrease progressively suggesting the possible occurrence of thermal degradation.…”

“…Chemical shifts of the remaining aliphatic CH2 protons were observed around 1.35 ppm ( 4 H) and 1.1 ppm ( 2 H) [34]. 13 C NMR spectra gave a peak at 176.93 ppm corresponding to the carbon of the amido group. The peaks at 39.90 and 35.83 ppm correspond to the carbon atoms next to the amido groups (i.e.…”

“…Results of 1 H and 13 C NMR indicate that the monomer inserts into the growing chains with the acyl-nitrogen bond scission rather than the break of alkyl-oxygen bond (Scheme 2).…”

“…The cationic polymerization of ε-caprolactam and its reaction mechanism has been studied for many years in the presence of cationic initiators [6][7][8][9][10][11]. Various strong protonic acids able to promote the polymerization of ε-caprolactam have been reported [12][13][14][15][16][17][18]. The separation of the initiators from the polymer is not always possible.…”