Polymerization of 1 , 4-dinitrosobenzene : Kinetics and Submicrocrystal Structure

In this study, the structure of the polycrystalline poly(l,4-phenyleneazine N,N-dioxide) was investigated by Scanning Electron Microscopy and related with the previously calculated molecular structure. Kinetics of polymerization of 1,4-dinitrosobenzene, prepared by both cryogenic UV photolysis and in vacuo deposition were measured by time-resolved FT-IR spectroscopy. Acquired data was analyzed by curve fitting with the standard Avrami-Erofeev and two-step consecutive reactions models. Activation parameters were calculated from Arrhenius and Eyring-Polanyi equations, for both models. The results obtained by using the two models, along with the goodness-of-fit parameters, were compared. It was shown that both bulk-based and reaction-based models can be used to adequately describe solid-state chemical reaction kinetics. Furthermore, a two-step consecutive reactions model is a suitable alternative to the most commonly used AvramiErofeev model.


INTRODUCTION
[3] They exist in two different forms, as nitroso compounds (monomers), and Eor Zazodioxy compounds (dimers or polymers). [4,5]In the solid state, most aromatic C-nitroso compounds appear in the azodioxy form, but can be converted to stable or metastable monomeric forms by dissolution in organic solvents, heating, or by UV irradiation at cryogenic temperatures. [5]n important feature of the nitroso/azodioxy system is that the two forms can be easily distinguished by spectroscopic methods (UV / Vis, NMR, IR, etc.), which facilitates kinetic measurements. [4],4-Dinitrosobenzene has been investigated because it is, next to 1,3-dinitrosobenzene, the simplest aromatic Cdinitroso compound capable of polymerization.It has been found that by deposition of 1,4-dinitrosobenzene vapor onto the surface cooled to 12 K produces a solid layer consisting only of the nitroso monomer.[6,7] Warming the sample to around 150 K causes fast polymerization.Recently, it has been found that the formation of polymers under cryogenic condition is not a simple process.The reaction on the cold surface follows the multistep mechanism.The first intermediate observed by IR spectroscopy is the Z-azodioxy isomer, but further warming causes the irreversible rearrangement to the E-isomer, and finally the formation of the stable polymer.Besides the use of cold surface deposition, 1,4-dinitrosobenzene can also be prepared photolytically.UV irradiation of the polymer encapsulated in a KBr pellet at 13 K yields monomers.However, these monomers dimerize and oligomerize exclusively to the azodioxy products with E-stereochemistry.
The polymerization mechanism as well as the stability of the polymer depends on the topotactic solid-state conditions. [3]It appears that monomers are more randomly distributed after cryogenic deposition, while after photolysis of the polymer the orientation of monomer molecules is more conserved, which is convenient for A repolymerization. [1,2]Also, repolymerization after photolysis depends on the regularity of the crystal structure of the starting polymer. [1]In this work, we present a morphological investigation of the polymeric material, complemented with isothermal determination of activation energies for the polymerization under cryogenic conditions.
For the analysis of isothermal polymerization, the standard Avrami-Erofeev (AE) approach is used along with the two-step consecutive reaction model (2CR).The reactions in the 2CR model can be written as: where R, R* and P are the reactant, the activated reactant and the product, respectively.The activated reactant R* can be thought of as a transient intermediate having a property (for example, spatial arrangement) which makes the reaction facile.In phase transitions, the activated reactants can be thought of as Avrami's germ nuclei, which might or might not form into active growth nuclei.[8-10]  The motivation for using 2CR is that it is a reaction-order based model, meaning that it can give additional mechanistic insights about the underlying processes, while the AE model only yields information about the overall transformation.More generally, the question is: Can reaction-order based models be used for the description of solid-state chemical reactions?

Microscopy
Zeiss electron microscope Gemini Supra installed at Laboratory for Microscopy and Microanalysis, University of Pretoria, RSA was used for obtaining high resolution SEM images.In Lens detector was employed at 1.7 mm.Due to very sensitive material, working voltage was set just to 600 V.

Kinetic Measurements
Samples for kinetic measurements were prepared either by photolysis of standard PNND doped KBr pellets (m(PNND) / m(KBr) ≈ 1 / 100) or by cryogenic deposition of 1,4-dinitrosobenzene vapors on a cooled KBr window.KBr pellets were secured on the cryocooler cold finger between two KBr discs and irradiated with 254 nm UV low-pressure Hg lamp (UVP R-52G).Samples were irradiated until no further changes in the IR spectra were noticeable.
Cryogenic conditions were achieved by Leybold-Heraeus ROK 10-300 cyclic helium cryostat.Isothermal kinetic measurements were performed at desired temperature after warming the samples from 13 K and holding the temperature constant.Temperature was control by Scientific Instruments model 9700 connected to a Si diode and a heater unit.Multiple measurements at different temperatures were conducted for the same sample to obtain activation parameters of repolymerization.FT-IR spectra were recorded on a PerkinElmer SpectrumTwo spectrometer, averaging 10 scans per spectrum.The background spectrum was obtained by observing ambient air.Spectra were recorded every ~ 65 s and the reactions were monitored over a 2-4 h period.Solid-state repolymerization rates were measured following the change in absorbance of the -N=O stretching at 1521 cm -1 , E-ONNO stretching at 1264 cm -1 , or Z-ONNO stretching at 1387 cm -1 .Baseline corrections for the spectra were performed using adaptive iteratively reweighted penalized least squares algorithm.Amount of the compound of interest was calculated according to the following equation: where Y ' ( ) x t is the normalized amount of compound Y present in time t, AY(tstart) and AY(tend) are absorbances at the start and end of the measurement, respectively.Therefore, the normalized amount of the product is 0 at tstart and 1 at tend.

Avrami-Erofeev Model
The description solid-state kinetics has proven to be a difficult problem for which a variety of solutions has been devised, with the AE model being the most prevalent. [12,13]n often used form of the AE model is: Here, ∞ ' x and ' 0 x are used as fitting parameters, along with the rate constant k and the dimensionality coefficient m.This implementation of the Avrami equation often gives better variances than the standard implementation: despite having a larger number of parameters. [6]15][16]
x , and P 0 ' x are the starting amounts of the reactant, activated reactant and product, the following equation can be used to calculate the amount of the product at time t:

Calculation of Rate Constants and Activation Parameters
Data fitting was accomplished using numerical integration and nonlinear least-squares with the default simplex algorithm implemented in the Wolfram Mathematica 10.3.
In the following section, we present results from the 2CR along with AE model.x , k1 and k2).The activation energies for both models were calculated according to the Arrhenius equation, while Gibbs energy of activation, enthalpy of activation and entropy of activation were calculated from Eyring-Polanyi equation.

Morphology of the Submicrometer Particles of PNND
The freshly sublimed polymer was studied by SEM microscopy under different magnifications (Figure 2).The polymeric material has interesting morphology.By going to higher magnifications, (Figure 2A-2C) the regularity of the (poly)crystalline structure can be inferred.All of the particles are structured in layers, while some of them are also folded (Fig. 2C).The less folded crystals can be seen in figures with the largest magnification (Figure 2D-2E).In our previous paper the molecular geometries of oligomers (up to octamers) were calculated.The molecule is helical and moderately folded.From the length of the octamer, which is 5.1 ± 0.1 nm, it is possible to estimate that the number of monomeric units (1,4-dinitrosobenzene molecules) in the largest arc in Fig. 2C might be around 5×10 3 .In the smallest edge of the non-folded crystal in the middle of the Fig. 2F this number might be less than 4×10 2 .The observable folding which appears in larger crystalline particles is expected from the extent of molecular folding found in PNND by calculation. [6]Such a robust structure is a consequence of regular molecular arrangement and as such could be a suitable starting material for the study of the photochemical and thermochemical reaction mechanisms in the solid state.

Photodissociation and Re-dimerization
According to previous investigations, UV irradiation can be used to induce the dissociation of the azodioxy bond and formation of nitroso monomers, as evidenced by IR spectra. [6]The azodioxy bonds reform upon warming the samples.This is indicated by the decrease of the monomeric -N=O signal (1521 cm -1 ) and the simultaneous increase of the E-ONNO signal (1264 cm -1 ).
Figure 1.Schematic representation of 1,4-dinitrosobenzene polymerization after deposition onto a cold window and after cryogenic UV photolysis.
The optimal temperature range for kinetic measurements of 1,4-dinitrosobenzene polymerization using IR spectroscopy is found to be between 120 and 150 K.
The repolymerization curve features a rapid growth phase, followed by significantly slower roughly linear growth, which resembles some types of aggregative processes. [13]Figure 3a shows fitted kinetic curves for 2CR (black) and AE models (orange), while residuals of the fits are shown in Figure 3b.On average, the residual sum of squares of the AE fits are 18 % larger when compared to 2CR (Table 1).As shown in Figures 3a  and 3b, the difference is especially pronounced at the beginning of the curve, resulting in larger residuals for the AE model.For the AE model the Arrhenius and Eyring plots are easily obtained, as there is only a single rate constant.However, in case of the 2CR model there are two rate constants, k1 and k2.In Figure 4, it is shown that the slope of the Arrhenius plot for the 2CR k2 constant (black) is very similar to the one obtained for the Avrami k constant.Moreover, activation enthalpies and entropies calculated from the Eyring-Polanyi equation are independent of the used model.These results suggest that the k2 constant of the 2CR model corresponds to the AE k constant, giving similar activation parameters, as shown in Table 2.
Important contributions of the 2CR model are the time plots of the reactant, activated reactant and product.As shown in Figure 5, some of the activated reactant is already present at the beginning of the measurement.This might be due to the imperfections in the crystal lattice of the monomer, although a possible facile interconversion between the reactant and the activated reactant cannot be ruled out.10] These results, along with the Arrhenius plots, suggest that the formation of the activated reactant (by k1) could correspond to a nucleation-type process, while the k2 constant might characterize the chemical reaction.

Polymerization (Dimerization) of the in vacuo Deposited Monomers
The IR spectra of the deposited samples, when compared to the photolyzed ones, show that different processes take place upon warming.In the deposited sample three parallel processes, displayed in Figures 6 and 7, can be noticed: At 130 K the nitroso → Z-azodioxy (orange) conversion is dominant, but a relatively slow, parallel nitroso → E-azodioxy (green) reaction also occurs.However, by warming the sample to 150 K, a very fast, seemingly irreversible formation of the E-azodioxy compound takes place at the expense of the Z-isomer (blue).Because these three processes cannot be     1) after cryogenic UV photolysis.

Figure 2 .
Figure 2. SEM images of the polycrystalline PNND sample at different magnifications.

Figure 3 .
Figure 3. (a) 2CR (black) and AE (orange) fitted curves for repolymerization reaction at 140 K of the photolyzed sample; (b) 2CR (black) and AE (orange) residuals of the fitted curves for repolymerization reaction at 140 K of the photolyzed sample.

Figure 4 .
Figure 4. Arrhenius plots for the 1,4-dinitrosobenzene polymerization analyzed by 2CR model (black) and AE model (orange).The 5 rate constants were obtained from a total of 422 data points.

Figure 6 .
Figure 6.The qualitatively observed reactions in the deposited monomers.

Figure 7 .
Figure 7. Kinetic profiles for the repolymerization reaction of the deposited sample at 130 and 150 K. Intensities of -N=O, E-ONNO and Z-ONNO stretching vibrations are shown in orange, black, and green, respectively.Temperature is indicated by the blue line.

Figure 8 .
Figure 8.(a) 2CR (black) and AE (orange) fitted curves for repolymerization reaction at 210 K of the deposited sample; (b) 2CR (black) and AE (orange) residuals of the fitted curves for repolymerization reaction at 210 K of the deposited sample.

Table 2 .
The calculated activation parameters for the AE and 2CR models.