CHAPTER-5 5.1. Anion Effect In the Reaction of Methylene Blue with Polymer Radicals So far we have discussed the reaction of methylene blue chloride with various polymer radicals* It would be desirable to investigate if the anion has any influence on the reaction of MB+ ion with polymer radicals. In the electron transfer oxidation of polymer radicals by metal ions it was observed that if the metal Ion Is associated with such ligands as OIT, Cl and the rate of oxidation Is increased many fold. Table-25 reproduces some selected literature data supporting this 131 point. For example, the work of George et al showed that each of the species Fe(DMF)gGl2*, Fe(DMF)^Clg+ and FeCl^" is very reactive towards the polystyryl radical while the corresponding rate constant for the FeUMF)6 ion Is lower by two orders of magnitude. In dimethyl sulfoxide (DMSO) solvent a parallel situation exists*32. George et al131,132 concluded from these results that "the presence of co-ordinated chloride Ions in the complexes of Iron (III) facilitates the electron transfer process in a way that is relatively unaffected by the number of co-ordinated chloride Ions present
even if a statistical correction is applied, by the net charge carried by the ion or by the geometry of the complex, octahedral [Fe(BMF)gCl2*, Fe(DMF)4Clg* and W w Fe(DM$0)gCl2 j or tetrahedral FeGl^*. Similarly, Cavell 133 and Gilson reported that termination of polyacrylamide radicals in aqueous solution by Fe(III) species is facilitated in the presence of co-ordinating ions such as Cl* and 2-7Q SO4. Dainton et al observed that the rate of oxidation of polyacrylamide radical by FeOH is about 6 times as great as that for Fe 3+ aq Similar kind of anion effect on the rate of electron exchange reaction between Fe(II) and Fe(III) Is well 134-137 established both in aqueous and nonaqueous solutions Anions such as OH, Cl, S04, facilitate the electron transfer reaction. No evidence was observed for a specific perchlorate,ion effect. The active anions mentioned above form inner sphere complex on coordination and help electron transfer by a bridged activated complex mechanism. The coordinated anion may be regarded as acting as an electron! conductor during the electron transfer process. 72 Kochi from an extensive investigation of oxidation of small carbon radicals by metal ions concluded from product analysis data that oxidation of carbon radicals by metal ions
-t 156 t- in presence of ligands such, as halides, azide, thiocyanate and cyanide ion respectively result in ligand transfer from the metal ion to the radical - On. the other hand, the aquated ion in the absence of the above mentioned ligands brings about electron transfer. The rate of ligand transfer oxidation is very much higher than the electron transfer.oxidation (vide section 1.7). The difference in rate is very great for carbon radicals.havings electron:withdrawing alpha substituents, for example, the <X-eyanoisopropyl radical is effectively oxidized by CuClg 0 ^-chloroisobutyronitrlle while cupric 84 acetate under the same circumstances is ineffective. Kochi explained this difference in behaviour in the following way. * It should be noted in this connection that George et al could not detect any chlorine in low molecular weight polystyrene samples which are formed due to an interaction between polystyrene radical and Fe(DMP)gCl2+ or FeCDMF^Clg* or FeCl4. From these results George et al concluded that the reaction essentially proceeds via electron transfer mechanism. This is in sharp contrast w^lvthe observations made by Kochi.
% 157 s- The electron transfer mechanism requires the formation of carbenium ion in the transition state. If the radical has an electron withdrawing alpha H a+m(n-1)*] M (n-l) + + carbenium products substituent the formation Of the carbenium ion will be energetically unfavourable and therefore the. energy of the transition state would be considerably large. On the other hand, in.the ligand transfer mechanism the direct transfer of an atom or radical from the metal oxidant to the carbon radical moderatesthe development of positive charge on the * carbon moiety in the transition state so that the energy of the transition state becomes considerably lower than that of the electron transfer process * R* + Mn+X X Mn* -----Ri-X + M (n-l) + The fact that leuco.mb is one of the products of the reaction between MB+ and polymer radical (vide section 4.8.6) suggests that the reaction proceeds via electron transfer
-s 158 s- mechanism. In this chapter the effect of methylene blue perchlorate In the polymerization of two monomers e«g. acrylonitrile and methyl acrylate respectively has been studied and the results compared with those obtained for methylene blue chloride. * * Table-25 Rate constant data for the oxidation of polymer radicals by Fe(III) species it ' 1 3 ' i Radical9 Fe(III) species Temp. og Solvent Rate constant 1 raole" ^ -1 sec Reference PSt ' Fe(DMF)63+ 60 BMF 847 131 PSt P41 Fe(DMF)5Gl 60 BMF 4.15xl04 131 PSt Fe(IMF)4Cl2+ 60 BMF 4 6.55x10 131 PSt FeCl4 60 BMF 3.14X104 131 PSt Fe<DMS0)63* 60 bmso 229 132 PSt Fe(DMS0)5Gl2+ 60 DMSO 7.18xl04 132 (contd...)
159 : Table-25 (Contd...) Fe(III) species Tea^. C Solvent Rat constanj; 1 mole*a Radicala Reference sec PAA 3+ Fe aq 26 V 2.2 + 0.3xl03 76 PAA FeOH2*, 25 HgO 1.4 + 0.3xl04 76 PAA FeGl2*.5Hg0 26 V 18.9xl04 133 PAA Fe304+.4^D 25 v 7.98xl04 133 a - PSt ss polystyrene, PAA?= PolyC acrylamide)
t * -s 160 5.2. Results 5.2.1. Polymerization of Acrylonitrile (AN) in ------, --------------------------------------- --------------- ------ - ------ -,i Presence of MBCIG4 The percent conversion vs.time plot for the polymerization of acrylonitrile,at a fixed concentration of AIBN and monomer but at varying dye concentration are shown in the figure-37. The concentrations of reagents in mole/1 units are as follows : 2xK}-3 mole 1 (ah] =2.27 mole 1 mbcioj -4-1 = varied from 7.5x10 to 30x10 mole 1 As in the case of methylene blue chloride (MBG1) (vide section 4.3) polymerization was found to be preceded by induction period which is caused by some unidentified impurity. After the induction period follows an acceleratory rate period followed by the steady state rate period. The rates of polymerization in the steady state period were calculated from the graph and the data are presented in the t able 26.
* t * % CONVERSION *10 TIME IN MINUTES Fig-37. Plot of percent conversion vs. time for w^.jiuiile polymerization in the presence or absence of MBC10* in EHF at 60 C. QttBNj = 2xl0"3; tafll 3 3. 2? mole l'r. ybcio, concentration for curves 1 through 4 are 0, 3 4-4 -4-1 7.5x10, 22.5x10, 30x10 mole 1 respectively.
161 Table- 26 Hate St polymerisation of acrylonitrile in the presence of methylene blue perchlorate in DMF at 60 C [aibn] = 2xl0~3 mole l"1, [In] a 2.27 mole jmbgk>4 x 104 mole/1 Induction period in min 5 Bp x 10 steady state -1 rate mole 1-1 sec, -4 1/Rp x 10 1 mole" sec 0 0 3.03 3.33 7.5 6 2.34 4.28 22.5 7.5 1.80 5.54 30 14 1.51 6.62 The data are analysed in the section 5.3 and the rate constant for the reaction with MBCIO4 evaluated.
I * -I 162 5.2*2. Polymerization of Methyl Acrylate (MA) in Presence of MBG104 The percent conversion vs.time plots for the polymerization of methyl acrylate at a fixed concentration of AIBN and monomer but at varying dye concentrations are shown in the figure 38. The concentrations of reagents used in mole/1 units are as follows 3xl0~ mole 1 Jma] = 1.63 mole 1-4 -4-1 * varied from 4.08x10 to 12.24x10 mole 1 Here again the percent conversion vs.time curves are similar to those found with MBC1 as retarder. There is an induction period after which follow an acceleratory rate peripd, then a steady state period. The steady state period (marked AB in the figure ) is of short duration after which > follow a deceleratory rate period. The acceleratory rate period following induction period arises due to the depletion of the impurity present in the system. For the purposes of determination of rate constant forthe reaction of MB(C104 ) with PMA radical the rate of polymerization data in the steady
% CONVERSION %
I -J 163 instate period gtarked AB in the curves are used. The decele- ratory rate period following the steady state period is probably due to the product of the reaction bringing about a stronger retardation of polymerization. This aspect has been dealt with in section 4.8.7. The steady state rate of polymerization at different dye concentrations is recorded in table-27 and the data are analysed in section 5.3 and the relevant rate constant evaluated. Table-27 Rate of polymerization Of methyl acrylate-in presence of Methylene Blue perchlorate in IMF at 60 C. JaIBN] = 3x10 ^ mole l**1; Jma =1.63 mole 1 ^. Jmbcio^J x io4 mole l ^ Induction period in min Rp x 104 - steady state^ rate mol l sec"1 1/Rp x 10~ 3-1 1 mole sec 0 0 13*2 7.58 4.08 25 9.06 11.08 8.16 26 6.29 15.9 12.24 32 4.45 22.4
-s 164 l- 6*3. Evaluation Of the Hate Constant In the discussions presehtecbin sections 4.8.5.1 and 4.8.5.3 we have seen that the rate equation 90 fits the polymerization rate data very well for acrylonitrile (AW) and methyl acrylate (MA) respectively with methylene blue chloride as the retarder. We shall now see if the polymerization rate data obtained for methylene blue perchlorate retarder can b.e quantitatively accounted for by equation 90. 5.3.1. Acrylonitrile-fffiClQ4 System Figure-39 shows a plot of 1/Bp vs. ^MBCIQ^J based on^ the rate of polymerization data presented in table-26. It Is evident from the figure'that equation 90 accounts, for the results very well. From the slope of the line drawn by the least square treatment of the data we obtain kx 7 ---- = 1.045 X 10-1, -1 from which was calculated to be 1320 1 mole sec at 60 C. The following values of B^ and k_ were obtained from literature
Fig-39. Test of eouati,on 90. Plot of 1/Rp vs [MBCKQ or Acrylonitrile polymerization in presence or MEClo^ in ^ DMF at 60 C. [If =2.27 mole l 1, [AIBlft = 2xl0_3rrole 1.
Pig-40. Test of equation 90. Plot of 1/Rp vs. [MBCIO4J for. Methyl acrylate oolymerization in presence of MBC10,. in DMF at 60UG. 1.63 m61e l" {AIBHJ = 3x10' mole I 1
I 165.1-1 kp = I960 1 mole sec (Ref.79) -5 -i.1.1 Rj = 1.42x10 IAXBWJ mole 1 sec (Ref. 125) Prom a comparison of the value of k^ obtained for MBCl (section 4.8.5.1) and MBC104 it would be evident that MBCl is a more efficient chain terminator for polyacrylonitrile radical than MBC104. This result indicates that Cl ion is assisting the eleetrion transfer oxidation of PAH radical by MB+ ion. 5.3.2. Methyl Acrylate~MBC104 System Figure-40 shows a plot of 1/Rp vs [MBCloJ based on the data presented in the table-2?. Here again it is evident that the rate equation 90 applies as was observed with MBCl retarder (vide section 4.8.5.3). From the slope of the line in figure 40 drawn by the least square fitting of the data we obtain kx 7 - --------- = 1.21x10 Rl^p [M] -1-1 o from which *x was found to be 1640 1 mole sec at 60 C in comparison to the value of 2120 determined for MBCl
( -s 166 s- Cvide section 4.8.5.3). The following % and kp values were obtained from the literature as before Bi = 1.33X10*3 1.IBn] mole f^see 1 (Ref.79) -1-1 kp = 2090 1 mole sec (Ref. 103) It Is thus clear that as in the case of polyacrylonitrile radical the rate of oxidation of poly(methyl acrylate) radical by MB+ ion is increased in presence of chloride ion. 5.3.3. Discussions From the above results (compiled in Table-28) it is evident that the rate of electron transfer oxidation of polymer radicals by MB ion is somewhat enhanced In presence of chloride counter ion in comparison to the rate in presence of perchlorate counter Ion. As has been discussed in the introduction section of this chapter this kind of anion effect Is quite common in the electron transfer reaction involving metal ions as one of the reactants. One possible explanation attributes the rate enhancement to the formation of inner sphere complex from the metal Ion and the counter ion. The co-ordinated anion acts as an electron conductor facilitating the electron transfer process.
I 167 Table-28 Effect of anion on rate constant of oxidation reaction of Methylene blue towards polymer radicals in BMP at 60 C P olymer radical -1 - kjj. 1 mole sec.1 Methylene Blue chloride Methylene Blue perchlorate Acrylonitrile 5,,800. 1320 Methylacrylate 2,120 1640 ' In the present case formation of the kind of complex envisaged in case of metal ion is out of question. However, if the ions form ion pairs In the medium, the accelerating effect of MB+C1 (chloride ion acting as electron conductor) may be understood. In the dielectric constant of the medium ( (' of DMF = 37.6) and the low concentration of the dye under consideration here no significant amount of ion pairing is expected between the large MB ion and the counter ions Cl and CIO4 respectively. The cause of the anion effect on the rate remains unanswered.