Hydrolysis rate constants and activation parameters for phosphate- and phosphonate-bridged phthalonitrile monomers under acid, neutral and alkali conditions

Hydrolysis data for Bis(3-(3,4-dicyanophenoxy)phenyl) phenyl phosphate and Bis(3-(3,4-dicyanophenoxy)phenyl) phenylphosphonate under pH 4, 7 and 10 are presented. Conversion/time plots collected by HPLC analysis, typical chromatograms and NMR spectra of the reactions products are given. Pseudo-first order rate constants are determined for both substrates at 25, 50 and 80 °C. Activation parameters were calculated from Arrhenius equation.

The data is useful for estimation of application possibilities for phthalonitrile phosphoric and phosphonic triesters as monomers for heat-resistant thermosets.

Data
Phthalonitrile phosphoric (1) and phosphonic (2) triesters were recently introduced as very prospective monomers for thermosetting matrices for carbon fiber reinforced plastics (CFRP) manufacturing due to untypically low melt viscosities (for phthalonitriles) and increased thermo-oxidative stability [1]. Phthalonitrile thermosets are known as the most heat resistant polymers possessing heat deflection temperatures 4400°C and stable up to 520°C [1][2][3][4][5]. Development of low-melting phthalonitrile resins suitable cost-effective technologies for CFRP manufacturing could widely extend applications of composite materials in aerospace (for production of complex-shaped parts, e.g. jet engine blades) and other applications such as high-temperature composite tooling. However, due to the presence of organic esters of phosphonic or phosphoric acids in monomer structure a question of hydrolysis resistance of 1 and 2 naturally came up. Hydrolysis of triesters of phosphoric and phosphonic acids is barely reported in literature [6,7]. Here the data on hydrolysis kinetics of 1 and 2 in three different pH values 4, 7 and 10 is presented including rate constants and activation parameters calculated from Arrhenius equation.
Tables Table 1 and Table 2 contain experimental data describing hydrolysis of phthalonitriles 1 and 2.

Materials
Monomers 1 and 2 [1] and resorcinol derivative 3 [8] were obtained according to the literature methods. Phenol was obtained from Sigma Aldrich and purified by vacuum sublimation. Acetonitrile (HPLC grade) was obtained from Sigma Aldrich and used as received. Buffer solutions with pH 4, 7 and 10 were purchased from Panreac Applichem.

Sample preparation and hydrolysis study
Monomers 1 and 2 were diluted in dry acetonitrile to concentration 2 mg/ml. The initial solution (10 ml) of monomer in acetonitrile was heated to a desired temperature (25, 50 or 80°С) and poured  to equal volume of preheated buffer solution (10 ml) under stirring with magnetic bar. After that flask with the reaction mixture was heated on water bath and 1 ml aliquots were withdrawn with a pipette in certain time periods, placed to 1.5 ml glass vial, sealed with septum head and placed into liquid nitrogen to stop all the chemical processes. Samples were unfrozen immediately before HPLC analysis.
In the case of hydrolysis study under pH 10 withdrawn samples were poured to 10 μl of concentrated HCl and shaken to prevent further hydrolysis and then treated as described above.

HPLC-analysis
Retention times for all the pure compounds were determined before measurements (Fig. 2). Series of solutions of 1, 2 and 3 with concentrations 1.0, 0.05, 0.01, 0.005 and 0.001 mg/ml were prepared for each monomer by consequent dilution of initial solutions with acetonitrile for LC calibration. Concentrations of the investigated compounds were determined based on calibration by automatic analysis with Agilent ChemStation software.
Several assumptions were made for the results interpretation. First of all hydrolysis of monomers 1 and 2 was considered as a pseudo-first-order reaction because water concentration in experimental  conditions was sufficiently higher than substrates concentrations. It was suggested and then proven that hydrolysis passed only by the first stage (Figs. 3 and 4).
The 48 h samples were acidified with HCl and then solvents were removed from samples under reduced pressure and solid residues were dissolved in DMSO-d6 to provide NMR-study. In 31 P NMR spectra in both cases new single signals were observed indicating formation of only one phosphoruscontaining compound in each case which is in accordance with the suggested reaction path (Fig. 1). In the case of 1 product with chemical shift -12.12 ppm was detected while phosphorus resonance in the initial phthalonitrile was at -18.54 ppm [1] indicating formation of the disubstituted arylphosphate. For the phthalonitrile 2 only resonance at 11.60 ppm was observed besides the residual peak of initial monomer 2 at 12.54 ppm [1] (Figs. 5 and 6).