Perturbation of the oscillatory BZ reaction with non-ionic polymers: experiments and model calculations

a a a b b bR. Lombardo , C. Sbriziolo , M.L. Turco Liveri , K. Pelle , M. Wittmann , Z. NoszticziusaDipartimento di Chimica Fisica “F. Accascina” - Università di Palermo

bCenter for Complex and Nonlinear Systems and the Department of Chemical Physics - Budapest University of Technology and Economics

IntroductionMacromolecules play a more and more important role in investigations of various phenomena in nonlinear chemistry like oscillations, chaos, waves and pattern formation.Polymers are widely used incontinuously fed unstirred reactors (CFURs) to establish a convection free inert medium for reaction diffusion systems. Chemical waves and later on Turing patterns were studied in such reactors. Nevertheless, it was soon realized that such media were not completely inert: e.g. the gel was able to change the velocity of a propagating chemical wave or interact with the activator species during the formation of a Turing pattern.Other examples of the application of polymeric reagents in nonlinear chemistry is the creation of a self-oscillating gel by coupling chemical oscillations with osmotic swelling. The syntesis of a polymer itself can be conducted in an oscillatory chemical medium with interesting results.The above examples demonstrate that perturbation of nonlinear chemical systems with polymeric reagents can lead to various new dynamic phenomena thus it is worth to study the chemical mechanism of these perturbations. When investigating the chemical mechanism of a perturbation with a polymer it is an important question whether it affects the reaction system by its reactive side groups only or the polymeric nature of the reagent also matters. In order to answer this question we have conducted parallel experiments with the polymer polyethylene glycol and with low molecular weight reagents containing the same reactive side group, methanol and ethylene glycol. The dynamics of BZ reaction was investigated in a stirred batch reactor by measuring CO evolution rate and Ce(IV) absorbance changes. The 2

experimental results have been compared with model calculations applying a latest model of the BZ reaction, the Marburg-Budapest-Missoula (MBM) mechanism (1).

References

(1)Hegedüs, L.; Wittmann, M.; Noszticzius, Z.; Shuhua, Y.; Sirimungkala, A.; Försterling, H-D.; Field, R.J. Faraday Discussions 2001, 120, 21.(2)Harris, J.M.; Poly(ethylene glycol) Chemistry; Plenum Press: New York and London, 1992 (3) Lombardo, R.; Sbriziolo, C.; Turco Liveri, M.L;Pelle, K.; Wittmann, M.; Noszticzius, Z.;.ACS Symposium series - Nonlinear Dynamics & Polymeric Systems, in press.(4)Pelle, K.; Wittmann, M.; Noszticzius, Z.; Lombardo, R.; Sbriziolo, C.; Turco Liveri, M.L. J. Phys. Chem. A, in press.

Polyethylene glycol (PEG): a versatile perturbant

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.It strongly influences the dynamics of the BZ systemit is widely used and commercially available in various molecular weightsit gives good chance to compare the polymer - monomer or the polymer backbone - endgroup effects with model calculations

information is available about the possible reactions of primary alcohols in a BZ systemethylene glycol (EG), the monomer of PEG, is a stable compound thus perturbation experiments are possible with the monomerthe ratio of the reactive endgroups to the polymer backbone can be varied varying the molecular weightperturbation effect of the polymer backbone can be studied separately using metoxylated PEGs (MPEGs)

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Model calculations

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To simulate the response of the oscillatory reaction for polymer perturbations qualitatively we applied the latest model of the BZ reaction the MBM mechanism (1) and added

the perturbation reactions of the polymer backbone (P2)-(P4)the perturbation reactions of the alcoholic endgroup (P5)

estimated rate constants were used for the polymeric reactions (P2)-(P5)

the semiquantitative calculations prove that the assumed mechanism of the polymeric perturbations is realistic

qualitative prediction of the observed perturbation effects due to the polymers

polymer backbone reaction (Figure 4B) causes a shortening of the induction period in accordance with the experimentsperturbation due to the alcoholic endgroups (Figure 4C) lengthens the induction period and shortens the time period of the oscillationsFinally a perturbation by both the polymer backbone and alcoholic endgroup (Figure 4D) reactions causes some shortening of the induction period (this is a result of the two opposite perturbation effects) and a shortening of the time period (again a result of opposite effects but now it is the effect of the alcoholic endgroup, which dominates)

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Experimental conditions?

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-3 [MA] = 0.1 mol·dm

- -3[BrO ] = 0.03 mol·dm3

-4 -3[Ce(IV)] = 4·10 mol·dm

-4 -3 -2 -31.87·10 mol·dm £ [additive ] £ 3.75·10 mol·dm

t = 20.0 ± 0.5 °C

Premixing experimentsPremixing of one the perturbants with all the BZ reagents, except for the Ce(IV), for two hours

BrMA accumulates in the systemon addition of the catalyst

in the case of PEG-2000 the system immediately started to oscillatein the case of both the other two PEGs used a decrease in the IP was observedin the case of MPEG-2000 the IP slightly decreasedin the case of EG the IP was unaffected

Premixing of both EG and MPEG with all the BZ reagents, except for the Ce(IV), for two hours

on addition of the Ce(IV) the oscillations immediately began

Premixing of the reactants practically has no influence on the time period and amplitude of the oscillations

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Effects of the polymer chainThe effects of the perturbant at low concentration can be explained by means of the following reactions (2,3):

HBrO production would enhance the positive 2

feedback loop increasing the IPHOBr production would decrease the IP since HOBr brominates the malonic acidpathway related to k dominates on that occurring via 1

k 2

the extent of the decrease on the IP depends on how easily the given PEG's radicals are formed

PEG-400 radicals being formed very slowly, do not contribute to the negative feedback loopthe other two PEG's radicals significantly do

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(P2)

(P1)

(P3)

(P4)

Poly + BrO2· Poly· + HBrO2

Ce(IV) + Poly Poly· + Ce(III) + H+

Poly· + BrO2· Poly-BrO2 k1

k2

Poly-BrO2

P1 + HOBr

P2 + HBrO2

Effects of the terminal -OH groupsThe effect of the perturbant at high concentration can be explained by means of the following reactions (4):

The overproduction of the autocatalytic intermediate BrO · will require the system to accumulate a larger 2

amount of BrMA in order to “turn off” the autocatalytic reaction

To reach that higher [BrMA] requires more time, CRIT

i.e. the induction period becomes longerIn the oscillatory regime the inflow of the autocatalytic intermediate will lengthen the autocatalytic and shorten the inhibitory phase

In the present BZ system the inhibitory phase is much longer than the autocatalytic one, the result is a shortening of the total time period

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(P5)

(R6)

(R7)

HBrO2 + HBrO3 Br2O4 + H2O Br2O4 2BrO2·

PEG + H+ + BrO3- P-CHO + HBrO2 + H2O